Upload
rahul-radhakrishnan
View
111
Download
5
Embed Size (px)
Citation preview
Application of Power Electronics in Power SystemsB. G. Fernandes
1/454
EE 660Application of Power Electronics
in Power Systems
B. G. FernandesDepartment of Electrical EngineeringI. I. T [email protected]
Application of Power Electronics in Power SystemsB. G. Fernandes
2/454
Course Outline
• Introduction
• Load Compensation
• Shunt Compensation
• Series Compensation
• HVDC Transmission
TheoryEquipmentTheoryEquipment
Application of Power Electronics in Power SystemsB. G. Fernandes
3/454
Books for Reference• T. J. E. Miller “Reactive power control in Electrical system,” John Wiley & Sons, New York, 1982.
• K. R. Padiyar “FACTS CONTROLLERS in Power Transmission & Distribution,” New Age International (P) Ltd.,” 2007.
• K. R. Padiyar “HVDC POWER TRANSMISSIONSYSTEMS Technology and System Interactions,” New Age International (P) Ltd.,” 1990.
• Hingorani N. G “Understanding FACTS Concepts & Technology of FACTS Systems,” IEEE PRESS, 2000.
Application of Power Electronics in Power SystemsB. G. Fernandes
4/454
Introduction
“Power Electronics has grown as a major & extremely important discipline in Electrical Engg.”
What are major applications of Power Electronics ?
• Major role in Power Transmission & Distribution
Application of Power Electronics in Power SystemsB. G. Fernandes
5/454
• Consumption of Electricity are Demanding Customers
• Loss of Power for single cycle can make computer screen go blank
• Can interrupt sensitive Electronic equipment
Application of Power Electronics in Power SystemsB. G. Fernandes
6/454
• Consumption of Electricity is also
• Transmission lines are being operated close to their limits
• Power is being transmitted through long overhead transmission lines & they are interconnected
Application of Power Electronics in Power SystemsB. G. Fernandes
7/454
• Voltage limit
• Stability limit
SIL
THERMAL LIMIT
Voltage and Stability Constraints
Distance
P
• Thermal limit (depends on ambientconditions)
Application of Power Electronics in Power SystemsB. G. Fernandes
8/454
Type of conductors• Thermal limit No. of Conductors
Ambient conditions
• Voltage limitations• For typical 400 kV line Zc = 300 Ω
SIL = 540 MW• For cable SIL is large
Application of Power Electronics in Power SystemsB. G. Fernandes
9/454
• Voltage profile along the line is flat if P = SIL
• If VS = VR = 1, V ↓ as we move towards the midpoint, if Ps > SIL
P < SIL
P > SIL
P = SIL
VS VR
Application of Power Electronics in Power SystemsB. G. Fernandes
10/454
• Line absorbs reactive power
• V ↑ if PS < SIL
• Voltage swell, line generates ‘Q’
Transmission Lineis iR
VRVs
P, Q P, Q
Application of Power Electronics in Power SystemsB. G. Fernandes
11/454
• To control VR & ↑ power transfer capacity of the line, ‘Q’ generation is required at the receiving end
Application of Power Electronics in Power SystemsB. G. Fernandes
12/454
CXVQ
2
= ↓ As VR ↓
‘Q’ requirement ↑ as VR ↓
• Other limitations
• ‘L’ required during over voltage
• Separate ‘L’ & ‘C ’ are required
Application of Power Electronics in Power SystemsB. G. Fernandes
13/454
• High ‘V’ & high KVar source
• 3-ph inverter can supply Q±
• Requires only ΔP
Application of Power Electronics in Power SystemsB. G. Fernandes
14/454
O/P V => PWM
• 2- level inverter
• Harmonic spectrum depends on switching
frequency (FS)
• PWM Constant FS
Variable FS => Not suitable
Application of Power Electronics in Power SystemsB. G. Fernandes
15/454
• What sort of PWM technique to use ?
• With low switching frequency how to improve the harmonic spectrum
• Do we need to change the power circuit configuration ?
Application of Power Electronics in Power SystemsB. G. Fernandes
16/454
• To have sufficient stability margin max. length of line = 450 km
• Provide shunt reactive power compensation, there by P↑ & maintain V profile.
• Use a mid point compensator
δSinXVV RS=Ρ
Application of Power Electronics in Power SystemsB. G. Fernandes
17/454
VVVV RSm ===
tedUncompensaPSinXVP 2
22 2
=⎟⎠⎞
⎜⎝⎛=δ
It can be shown, for loss- less line
Application of Power Electronics in Power SystemsB. G. Fernandes
18/454
• “If shunt compensation is applied at sufficient close interval, it may be possible to transmit power up to thermal limit of line”
• P transmitted over long lines is limited by series reactance ‘X’
Application of Power Electronics in Power SystemsB. G. Fernandes
19/454
Provide • Series capacitive compensation to cancel a
portion of series ‘X’
δ
Application of Power Electronics in Power SystemsB. G. Fernandes
20/454
• C is not permanently connected in series• During fault condition, Xeff should be
increased• May require ‘L’ also
( ) δSinXK
VP−
=1
2
K = Degree of compensation = XXC
puVV RS 1==
Application of Power Electronics in Power SystemsB. G. Fernandes
21/454
• Is it possible to change the phase angle difference between two ends of the line and there by control the power flow
• “Phase angle regulator” ?
• Inject a voltage in series with the line & proportional to the current flow (voltage should lag the I )
Application of Power Electronics in Power SystemsB. G. Fernandes
22/454
δ
Application of Power Electronics in Power SystemsB. G. Fernandes
23/454
• Injecting V in series with line and with any phase angle with respect to VS
δ
• Both magnitude & phase angle of I has changed
• Both P & Q flow has changed
Application of Power Electronics in Power SystemsB. G. Fernandes
24/454
• Consider an AC network
• Power flow in Line-1 & 2 depends on circuit conditions
• Lower X line may be over loaded• Not possible to set the amount of power that
should flow through a particular line!
Application of Power Electronics in Power SystemsB. G. Fernandes
25/454
• Definite amount of power that should flow through HVDC line can be set
• If power transfer over long distances• Two near by areas having different
frequencies ( Back to Back connection)
Application of Power Electronics in Power SystemsB. G. Fernandes
26/454
• Power flow control through AC lines is not “FLEXIBLE”
• Depending upon the loading, there could be voltage swell or sag as we go towards the mid point
Review
V1 V2
R+jX
Application of Power Electronics in Power SystemsB. G. Fernandes
27/454
• To control the power flow & to maintain voltage profile, provide
• Shunt compensation
• Series compensation
Passive elements with P.E switches or
Inverter
• At Tr. voltage levels PWM with high switching frequency may not be possible
• Modify the existing power circuit
• Can we regulate the power flow by converting AC-DC-AC => HVDC Transmission ?
Application of Power Electronics in Power SystemsB. G. Fernandes
28/454
Load compensation• Loads are unbalanced• P.F is lagging
No compensation of harmonics
• Source should supply only active power &
see a balanced load
Introduction ( contd…)
Application of Power Electronics in Power SystemsB. G. Fernandes
29/454
• Most of the loads are Non-linear
• Harmonics are generated
• Voltage at P.C.C is non sinusoidal
• P.F is lagging
• Circuit to filter the harmonics (on-line) + compensate the loads
Application of Power Electronics in Power SystemsB. G. Fernandes
30/454
P.C.C Point of common coupling→
Application of Power Electronics in Power SystemsB. G. Fernandes
31/454
Current drawn by the load fed from P.E. equipment flows through system impedance. Voltage at P.C.C is non-sinusoidal(We had assumed that 'V' is sinusoidal).
Application of Power Electronics in Power SystemsB. G. Fernandes
32/454
5 7
1
a 02 3 1 1i = I sin t- sin t+ sin t-.............
5 7= 6N , Harmonics
Line Commutated converter causes notches in the source voltage waveform.
Source current has harmonics.
ω ω ωπ
⎡ ⎤⎢ ⎥⎣ ⎦
±⇒ →
→
Application of Power Electronics in Power SystemsB. G. Fernandes
33/454
Effect of harm onics:A. In the Rotating m achine Increases heating.
They produce noise.Torque pulsations.
B. In Transform ers Cu losses .Audible noise & heating.
C. In Cables Additional heating.
D. P.F corre
→→→
→ ↑→
→
ction capacitors. Therm al voltage stress.
E. Electronic Equipm ents Affects control system .M aloperation of relays.
→
→→
Application of Power Electronics in Power SystemsB. G. Fernandes
34/454
• Load compensation + Active filter
• Depending upon the voltage & power level, circuit configuration & control should change
Application of Power Electronics in Power SystemsB. G. Fernandes
35/454
• Load compensator + Active filter to compensate non-linear loads
• Power flow in AC network is determined by circuit conditions
• Power transfer capability can be increased through shunt & series compensation
• HVDC can be used for bulk power transmission & to inter connect the systems of different frequencies
Conclusions
Application of Power Electronics in Power SystemsB. G. Fernandes
36/454
• In ideal power system• V & F should be constant• V should be sinusoidal• P.F = 1
• The above should be independent of size & characteristics of load
• No interference between different loads
Load compensation
Application of Power Electronics in Power SystemsB. G. Fernandes
37/454
• How nearly constant are V & F at the supply point ?
• How near to unity is the P.F ?
• In 3-ph system, degree to which V & I are balanced
Notation of quality of supply
Application of Power Electronics in Power SystemsB. G. Fernandes
38/454
• What are the characteristics of power system & loads which can deteriorate the quality of supply ?
• How to compensate ?
Objectives of load compensation• Power factor correction• Improvement in voltage regulation• Load balancing
Application of Power Electronics in Power SystemsB. G. Fernandes
39/454
• Correct the power factor to unity• Reduce the voltage regulation to an
acceptable value• Balance the load current => not expected to
compensate harmonics in V & I, also will not generate harmonics
• Should consume zero avg. power• Response time = 0
Ideal compensator
Application of Power Electronics in Power SystemsB. G. Fernandes
40/454
• Large no. of uncompensated industrial loads, P.F is less than 0.8 ( they are non linear also)• Arc furnace, induction furnace, steel rolling
mills, large motor loads• ‘S’ rating of the compensator (P=0)
Load requires P.F correction
LLLLL SSQ Φ−=Φ== 2cos1sin
PL
QLSL
ФL
Application of Power Electronics in Power SystemsB. G. Fernandes
41/454
• Which is the most important parameter of the load & supply system affects regulation ?
Voltage regulation
E
V
IS
RS+jXS
Sl = PL +jQL
YL = GL +jBL
IL
Application of Power Electronics in Power SystemsB. G. Fernandes
42/454
VVE
VVE
Vreg
−=
−=
LS IZV =Δ
IL= IS
IS RS
I S X S ΔVX
ΔVR
ΔVV
ENo compensator IL= IS
LLL jQPVI +=*
Application of Power Electronics in Power SystemsB. G. Fernandes
43/454
VjQPI LL
L−
=
( )V
jQPjXRV LLSS
−+=Δ
VQRPXj
VXQPR LSLSSLLS −
++
=
XR VjV Δ+Δ=• Change depends on both active & reactive power of the load
Application of Power Electronics in Power SystemsB. G. Fernandes
44/454
E
V
IS
RS+jXS
IL IC
Replace QL by
( ) ( )222XR VVVE Δ+Δ+=
CLS QQQ +=
)(22
AV
QRPXV
XQPRV SSLSSSLS −−⎭⎬⎫
⎩⎨⎧ −
+⎭⎬⎫
⎩⎨⎧ +
+=
Such that
Adding a compensator in parallel with load
So that VE =
Application of Power Electronics in Power SystemsB. G. Fernandes
45/454
IL
I S R S
jIS X
S
ΔV
V
E
ICIS
Vary QS => ΔV rotates till VE =
Solve (A) with VE =
• There is always a solution for QC for any value of P
Application of Power Electronics in Power SystemsB. G. Fernandes
46/454
• If the compensation is used to make P.F unity then
VPjXPRV LSLS +
=Δ
( )VPjXR L
SS +=• Independent of QL
• Not under the control of compensator• Passive reactive compensator can not maintain constant V & unity P.F at the same time
Application of Power Electronics in Power SystemsB. G. Fernandes
47/454
*
2*
SCSCSCSCSC Z
EEIjQPS ==+=
,SSSC jXRZ +=
SCSC ZZ =*
SCSC
SCSCS SEZR Φ=Φ= coscos
2
SCSC
SCSS SEZX Φ=Φ= sinsin
2
Short circuit at the load bus• Approximate relationship for voltage regulation
ISC → S.C Current
Application of Power Electronics in Power SystemsB. G. Fernandes
48/454
• Change in V influenced by ΔVR
• Neglect ΔVX
VXQPRV SLLS
R+
=Δ
VZQZP SCSCLSCScL Φ+Φ
=sincos
1≈VE
SCLSCLSCR QP
VZ
VV
Φ+Φ=Δ sincos2
SCLSCLSC
QPS
Φ+Φ= sincos1
Assume
E
V ΔVR
ΔVX
Application of Power Electronics in Power SystemsB. G. Fernandes
49/454
• If short circuit resistance of source=0
SC
L
SQ
VV=
Δ
SC
L
SQ
VVE=
−
⎥⎦
⎤⎢⎣
⎡+=
SC
L
SQVE 1
1
1−
⎥⎦
⎤⎢⎣
⎡+=
SC
L
SQEV
⎥⎦
⎤⎢⎣
⎡−≈
SC
L
SQE 1
Slope = -E/SSC
VΔV
QL
=> CosФSC = 0
Application of Power Electronics in Power SystemsB. G. Fernandes
50/454
• Assume all loads are fully compensated for reactive VA
120,120
,0
∠=−∠=
∠=
Lca
Lbc
Lab
VVVVVV
Load balancing
bccac
abbcb
caaba
IIIIIIIII
−=−=−= Vca Vab
Vbc
Application of Power Electronics in Power SystemsB. G. Fernandes
51/454
jXV
RVI caab
a −=
XV
RV
jXV
RV LL 3001200 ∠
−∠
=∠
−∠
=
( )
⎭⎬⎫
⎩⎨⎧
−−=
⎭⎬⎫
⎩⎨⎧ +−=
Xj
XRV
jXR
V
L
L
2231
30sin30cos11
Application of Power Electronics in Power SystemsB. G. Fernandes
52/454
RV
jXV
RV
jXVI LLabbc
b0120 ∠
−−−∠
=−−
=
RV
XVL −∠
=30
)2(2
12
3−−−
⎭⎬⎫
⎩⎨⎧
−−=Xj
RXVL
jXV
jXV
jXV
jXVI LLbcca
c −−∠
−∠
=−
−=120120
)3(
3030
−−−=
−∠−
∠=
XVj
RV
XV
L
L
Application of Power Electronics in Power SystemsB. G. Fernandes
53/454
120∠= cb II
⎟⎟⎠
⎞⎜⎜⎝
⎛+−=−⎟⎟
⎠
⎞⎜⎜⎝
⎛−
23
21
21
23 j
Xj
Xj
RX
XRX 231
23
−=⎟⎟⎠
⎞⎜⎜⎝
⎛−
RX 3=
Application of Power Electronics in Power SystemsB. G. Fernandes
54/454
Review
• Using passive reactive element, it is possible to achieve ΔV = 0
• ΔVX has negligible effect on ΔV
• Determined by ΔVR (≠ iSRS)
IL= IS
IS RS
I S X S ΔVX
ΔVR
ΔVV
E
V ΔVR
ΔVX
E
Application of Power Electronics in Power SystemsB. G. Fernandes
55/454
• Using passive reactive element it is not possible to have ΔV=0 & P.F =1
• Load balancing• All three line currents are balanced if
RX 3=
Contd..
Application of Power Electronics in Power SystemsB. G. Fernandes
56/454
9033
10
210332
121
30332
121
∠=⎭⎬⎫
⎩⎨⎧ +=
∠=⎭⎬⎫
⎩⎨⎧ −−=
−∠=⎭⎬⎫
⎩⎨⎧ −=
RV
RjVI
RV
Rj
RVI
RV
Rj
RVI
LLc
LLb
LLa
• Rule: For the load connected between line a-b, capacitor should be connected between b-c, and Inductor should be connected between c-a
Load balancing (Contd..)
Application of Power Electronics in Power SystemsB. G. Fernandes
57/454
• Branch currents of Δ are unbalanced• Reactive power is balanced within Δ• Reactive power generated by C connected
between line b & c = Q is absorbed by L connected between c & a
abL
abL
abL jBGY +=• If the load is
abL
abC BB −=• Compensating susceptance
Comments
Application of Power Electronics in Power SystemsB. G. Fernandes
58/454
• Each branch of Δ will have 3-parallel compensating susceptances
⎟⎟⎠
⎞⎜⎜⎝
⎛ −+−=
⎟⎟⎠
⎞⎜⎜⎝
⎛ −+−=
⎟⎟⎠
⎞⎜⎜⎝
⎛ −+−=
3
3
3
abL
bcLca
LcaC
caL
abLbc
LbcC
bcL
caLab
LabC
GGBB
GGBB
GGBB
Application of Power Electronics in Power SystemsB. G. Fernandes
59/454
Application of Power Electronics in Power SystemsB. G. Fernandes
60/454
• Any linear unbalanced 3-Ф load can be transformed into a equal 3-Ф balanced load
• Net real power is the same• Corresponding elements are purely reactive
Observations
Corresponding to power consumed by the load
As the power varies, X also should change
3RX =
Application of Power Electronics in Power SystemsB. G. Fernandes
61/454
• May not be possible• Most of the loads are non-linear =>
Harmonics + lagging P.F
P.F ≠ cosIV
Application of Power Electronics in Power SystemsB. G. Fernandes
62/454
δsin1
XVVP SC=
IC1
VS VC1
jωLIC1If δ = 0
SC VV >1If
• IC1 is leading VS
Application of Power Electronics in Power SystemsB. G. Fernandes
63/454
• Can be shown that if SC VV <1
⎟⎠⎞
⎜⎝⎛ −
⇒=LVVVIVQ CS
SCS ω1
1
dcC mVV α1
• Ic1 is lagging
Application of Power Electronics in Power SystemsB. G. Fernandes
64/454
• Non ideal case
• Var generated α m α Vdc
IC1 VS jωLIC1
VC1IC1R
δ
IC1jωLIC1
VC1
δ VS
Application of Power Electronics in Power SystemsB. G. Fernandes
65/454
• M => Magnitude of sine wave (not very popular)• Magnitude of space vector• T1 & T2 are to be determined
60sinsin
.60sin
)60sin(
2
1
θ
θ
mTT
TmT
C
c
=
−=
Intelligent controller is required
Application of Power Electronics in Power SystemsB. G. Fernandes
66/454
• Vary Vdc
• Var supplied α Vdc
• Var generated is controlled by varying VC1 & iC1
• O/P voltage of inverter• Indirect current controller Synchronous link converter Var compensator (SLCVC) or STATCOM
Application of Power Electronics in Power SystemsB. G. Fernandes
67/454
Review
• Linear lagging load can be balanced using passive elements
bcLY
abLY
caLY
caCB
abCB
bcCB
• Difficult to realize in real life
• Use V.S.I to supply ‘Q’
Application of Power Electronics in Power SystemsB. G. Fernandes
68/454
• Similar to over-excited Syn. motor on No-load
• Draws only small ‘P’• ‘δ’ is very small
• In V.S.I δ =VC1
VS
• ‘VC1’ is synthesized using PWM
E
Vδ
Contd..
Application of Power Electronics in Power SystemsB. G. Fernandes
69/454
• If space vector PWM is used at the Z.C instant of supply voltage, VS
* should lag by angle ‘δ’
• In sinusoidal PWM technique, fundamental component of VC1 is in phase with modulating wave
Contd..
Application of Power Electronics in Power SystemsB. G. Fernandes
70/454
Harmonic elimination Techniques
Undesirable harmonics can be eliminated and fundamental can be controlled by creating notches at pre-determined angles
If 'n' switchings / cycle⇒ 14
• At the Z.C of supply voltage, modulatingwave should lag by ‘δ’
Application of Power Electronics in Power SystemsB. G. Fernandes
71/454
(n-1) harmonics are eliminated & magnitude of fundamental can be controlled
4 switchings /(1/4) cycle ⇒
⇒
(α1, α2, α3, α4)
α1 < α2 < α3 < α4 < π/2
Application of Power Electronics in Power SystemsB. G. Fernandes
72/454
• 3 significant harmonics = 0• Fundamental can be controlled
• Square wave has quarter wave odd symmetry
• Coefficient of the fundamental & harmoniccomponents are given by
( ) ( )⎭⎬⎫
⎩⎨⎧
−+= ∑=
m
kk
kn n
nb
1cos1214 α
π
Application of Power Electronics in Power SystemsB. G. Fernandes
73/454
• Assume that there are 5 switchings / (1/4) cycle
• 4 harmonics can be made zero• In 3 phase, 3 wire system, triple harmonics
can be ignored
• So harmonics to be eliminated are 5th, 7th, 11th and 13th
3211 cos2cos2cos214 αααπ
−+−=b
cos2cos2 54 αα −+
Application of Power Electronics in Power SystemsB. G. Fernandes
74/454
1 2 3 4
5
1 2 3
4
b = 1-2cos5 +2cos5 -2cos5 +2cos5
-2cos5 = 0
b = 1-2cos7 +2cos7 -2cos7
+2cos7 -
5
7
4 α α α α5π
α4 α α α
7πα 5
1 2
5
1 2
2cos7 = 0
b = 1-2cos11 +2cos11 ........................
-2cos11 = 0
b = 1-2cos13 +2cos13 ........................
11
13
α4 α α
11πα
4 α α13π
5 -2cos13 = 0α
Application of Power Electronics in Power SystemsB. G. Fernandes
75/454
• Non-linear transcendental equations• Solve numerically• Choose required value for b1
⇒ Fundamental component
b1 = 0.986 p.u.• Immediate dominant harmonic ‘V’ gets
amplified
α1 = 10.514, α2 = 23.228, α3 = 29.289, α4 = 46.421, α5 = 50.157
Application of Power Electronics in Power SystemsB. G. Fernandes
76/454
• Var supplied α Vdc
• Var generated is controlled by varying VC1 or iC1
• O/P voltage of inverter
• Indirect current controller Synchronous link converter Var compensator (SLCVC) or STATCOM
Application of Power Electronics in Power SystemsB. G. Fernandes
77/454
( ) tVVtIi mm ωω cos&cos =Φ−=
tItI qP ωω sincos +=
ttItIi qP ωωω cos.sincos2 +=∴
How to calculate Ref. Var ?
( ) tI
tI qP ωω 2sin2
2cos12
+−=
Multiply by cosωt
Application of Power Electronics in Power SystemsB. G. Fernandes
78/454
• Use a low pass filter ⇒ IP/2 ≈ average
• Remaining ⇒ Reactive power
• Limitations: Response time is poor⇒ min. one cycle
Application of Power Electronics in Power SystemsB. G. Fernandes
79/454
• Compensator current is actually sensed & controlled to follow the reference
Controlled current SLCVC
• Source should supply active component of load current + compensate inverter loss
Application of Power Electronics in Power SystemsB. G. Fernandes
80/454
• Reactive component of load current (iqL) should come from inverter
iC = iPC + iqL
iqL ⇒ obtained from Var calculatoriPC ⇒ Accounts for loss
• If there is a mismatch in power supply and consumed ⇒ VdC will change
Application of Power Electronics in Power SystemsB. G. Fernandes
81/454
Control strategy -I
Application of Power Electronics in Power SystemsB. G. Fernandes
82/454
• To ↑iC close S4 & S3 , To ↓iC open S4 & S3
• Response is fast• Switching frequency varies• Var calculator is required
Application of Power Electronics in Power SystemsB. G. Fernandes
83/454
Review
• In harmonic elimination technique, if there are ‘n’ switchings / (¼) cycle, (n-1) harmonics can be eliminated & fundamental can be controlled
⇒ If ‘F’ of pre-dominant harmonic is > 2kHz
at 50Hz, up to 40th harmonic should be absent
⇒ 5, 7, 11, 13, 17, 19, 23, 25, 29, 31, 35, 37
⇒ 12 harmonics should be eliminated
Application of Power Electronics in Power SystemsB. G. Fernandes
84/454
• 13 switchings / (¼ ) cycle• 13 non linear transdential equations to be
solved
• H. S. Patel & R. G. Hoft “Generalized technique of harmonic elimination and voltage control in thyristor inverters,” Part-1 harmonic elimination., IEEE Trans. Ind. Applicat., vol. IA-9, pp 310-317, May 1973.
Contd..
Application of Power Electronics in Power SystemsB. G. Fernandes
85/454
• Compensator current iC = iPC + iqL ⇒sinusoidal if load is linear
• If iqL has the information about the non-linear, ⇒ iC is non
Controlled current SLCVC
Contd..
- sinusoidal
Application of Power Electronics in Power SystemsB. G. Fernandes
86/454
Control strategy -II• Sense source current iS
⇒ Compare with sinusoidal reference current iS*
Application of Power Electronics in Power SystemsB. G. Fernandes
87/454
• iS* is in phase with vS• iS is also in phase with vS• VdC is held constant• All the active power is supplied by the source• Rest (‘Q’ + Harmonic I) supplied by inverter
• iS = iL + iC
⇒ To ↑iS, ↑ iC⇒ To ↓ iS, ↓iC Using inverter switchings
Application of Power Electronics in Power SystemsB. G. Fernandes
88/454
• Once in every cycle• If active power demand of the load has changed
in between +ve Zero crossings• Power is supplied by inverter⇒ VdC will ↓
• VdC > Vm ⇒ peak of VS
⇒ Large size ‘C’ is required
How often iS* is changed ?
Application of Power Electronics in Power SystemsB. G. Fernandes
89/454
• If Inverter iS* is changed in between the cycle
• Source ‘I’ will have a DC component
• Smaller size ‘C’ may be sufficient
Application of Power Electronics in Power SystemsB. G. Fernandes
90/454
• Current control is suitable for low power
• For high power loads switching ‘F’ ↓• Inverter ⇒ Voltage control
• Harmonic spectrum is inferior
• Load current has harmonics
• In addition inverter with voltage control also generates harmonics
Application of Power Electronics in Power SystemsB. G. Fernandes
91/454
• Use two compensators & connect them in parallel
• Var generator ⇒ High power inverter• High V & high I
• Harmonic filter ⇒ Low power inverter• Switching frequency is high• Since low power, use current controlled
PWM technique
Application of Power Electronics in Power SystemsB. G. Fernandes
92/454
Active filter +Var compensator for high power
Application of Power Electronics in Power SystemsB. G. Fernandes
93/454
• Main compensator ⇒ Voltage control mode
• Aux. compensator ⇒ controlled current mode
• Generate iref ⇒ ref. I of suitable magnitude & in phase with source V
• Force iS = iCm + iCx + iL to follow the reference within a hysterisis band
• Error decides the switching instant of aux. compensator devices
Application of Power Electronics in Power SystemsB. G. Fernandes
94/454
• To ↑ iS, ↑ iCx ⇒ close S4 & S3
• To ↓ iS, ↓ iCx ⇒ open S4 & S3
• Now iref = iL(p) + iCm(p)
Where iL(p) = Real component of load IiCm(p) = Real component of the main
compensator current
Application of Power Electronics in Power SystemsB. G. Fernandes
95/454
θδ
∠−∠−
=ZVVi CmS
Cm1
1
( )θ
δδ∠
+−=
ZjKVmVV dCdCS sincos
⎟⎟⎠
⎞⎜⎜⎝
⎛+=
22
2)(1
2)(1
1qpCmrealpCm
Cm
III
Application of Power Electronics in Power SystemsB. G. Fernandes
96/454
Control block diagram
Application of Power Electronics in Power SystemsB. G. Fernandes
97/454
• Var calculator determines Vdc*
Vdc* - Vdc ⇒ determines δ
• µC ⇒ determines iref using Ip, δ, VdC & VS
• Compare iS & iref to generate switching signals for aux. inverter
(‘m’ is constant)
Application of Power Electronics in Power SystemsB. G. Fernandes
98/454
Review
• For low power
Var generator + Active filter
Application of Power Electronics in Power SystemsB. G. Fernandes
99/454
• Used Var calculator to determine ‘Q’ required by the load
• Linear load is assumed
• For high power application
Use high power inverter for Var generation
To compensate harmonics use active filter
Contd..
Application of Power Electronics in Power SystemsB. G. Fernandes
100/454
[v] = [z] [i]
[v'] = [z'] [i']
[v] = [A] [v']
[i] = [A] [i']
[v] = [z] [i]
3-Phase to 2-phase conversion
Application of Power Electronics in Power SystemsB. G. Fernandes
101/454
[A] [v'] = [z] [A] [i'][v'] = [A]-1 [z] [A] [i]
Z'⇒ Inverse should exist
p = i1v1 + i2v2 + i3v3 = [i]t [v]
p' = i1'v1' + i2'v2' + i3'v3'
= [i']t [v']
Application of Power Electronics in Power SystemsB. G. Fernandes
102/454
p = p'
[it][v] = [A] [i'] t [A] [v']
= [i']t [A]t [A] [v']
[A]t = [A-1] or [A] = [A]t-1
[U] ⇒ Unit matrix
Application of Power Electronics in Power SystemsB. G. Fernandes
103/454
iS = K[ia+ ibej2π/3 + ice-j2π/3]Has 2-components ⇒ (α, β)iα = Kd [ia- (1/2) ib – (1/2) ic]iβ = Kq [0 + √3/2 ib - √3/2 ic]i0 = K0 [ia + ib + ic]
• 3-current vectors ⇒ one vector ⇒ space vector
iβ
iα
Vector representation of instantaneous 3-phase quantities
iC
ia
ib
Application of Power Electronics in Power SystemsB. G. Fernandes
104/454
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡−−−
=⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
c
b
a
ddd
iii
KKKKK
KKK
iii
0000
)23()23(0)21()21(
β
α
[C]
[ ]⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
−−−=−
0
0
01
31313131313131032
KKKKKKKK
C
qd
qd
d
Application of Power Electronics in Power SystemsB. G. Fernandes
105/454
If Kd = Kq = 2/3 & K0 =√2/3
[C]-1 = 3/2 [C]t
[ ]⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
−−−=
0
0
0
)23()21()23()21(
0
KKKKKKKK
C
qd
qd
d
t
Application of Power Electronics in Power SystemsB. G. Fernandes
106/454
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡−−−
=⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
c
b
a
vvv
eee
21212123230
21211
32
0
β
α
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡−−−
=⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
c
b
a
iii
iii
21212123230
21211
32
0
β
α
Similarly 3-ph AC voltages ⇒ two phase voltages
Application of Power Electronics in Power SystemsB. G. Fernandes
107/454
p = vaia+ vbib + vcic
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
−−−=
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
02123212123212101
β
α
ee
vvv
c
b
a
p = eαiα+ (-1/2 eα+√3/2 eβ) (-1/2 iα+ √3/2 iβ) + (-1/2 eα-√3/2eβ) (-1/2iα-√3/2iβ)
p = 3/2 (eαiα+eβiβ) ( )ββαα ieie ..23 +=
Application of Power Electronics in Power SystemsB. G. Fernandes
108/454
Instantaneous reactive power compensation
Instantaneous real power
p = vaia+ vbib + vcic
Definition of instantaneous reactive current:
That part of the three phase current can be eliminated at any instant without affecting ‘P’
Application of Power Electronics in Power SystemsB. G. Fernandes
109/454
iβ
iα eα
eβiS VS
φψ
cos
sinS
S
e V
e Vα
β
ψ
ψ
=
=
( )( )
cos
sinS
S
i i
i iα
β
ϕ ψ
ϕ ψ
= +
= +
( ) ( ) 3 cos .cos sin .sin2 S Sp V i ψ ϕ ψ ψ ϕ ψ= + + +
( ) 3 3cos cos2 2S S S SV i V iψ ϕ ψ ϕ= − − =
Application of Power Electronics in Power SystemsB. G. Fernandes
110/454
3 2 sinS Sq V i ϕ=
( )( ) ( )
( ) ( )
3 2 sin
3 2 sin cos cos sin
3 2 cos . sin sin . cos
3 2 3 2
S S
S S
S S S S
V i
V i
V i V i
e i e i e i e iα β β α α β β α
ϕ ψ ψ
ϕ ψ ψ ϕ ψ ψ
ψ ϕ ψ ψ ϕ ψ
= + −
= + − +
= + − +
= − = × + ×
• Can be concluded that 3/2 iS sinφ component of current iS can be eliminated without effecting ‘P’
Reactive power
Application of Power Electronics in Power SystemsB. G. Fernandes
111/454
⎥⎦
⎤⎢⎣
⎡
⎥⎥⎦
⎤
⎢⎢⎣
⎡⎥⎦
⎤⎢⎣
⎡−
=⎥⎦
⎤⎢⎣
⎡
⎥⎦
⎤⎢⎣
⎡⎥⎦
⎤⎢⎣
⎡−
=⎥⎦
⎤⎢⎣
⎡
−
qp
eeee
ii
ii
eeee
qp
1
32
23
αβ
βα
β
α
β
α
αβ
βα
In matrix form
⎥⎦
⎤⎢⎣
⎡⎥⎦
⎤⎢⎣
⎡ −
+=
qp
eeee
ee αβ
βα
βα22
1*32
Application of Power Electronics in Power SystemsB. G. Fernandes
112/454
⎥⎦
⎤⎢⎣
⎡−⎥
⎦
⎤⎢⎣
⎡ −
+=⎥
⎦
⎤⎢⎣
⎡qee
eeeei
i
C
C01.
32
22αβ
βα
βαβ
α
Application of Power Electronics in Power SystemsB. G. Fernandes
113/454
( )22*
23.
βα
βα ee
qei
C +=
( )22*
23.
βα
αβ ee
qeiC +
−=
[ ]αββα ieieq −=23
Where
Application of Power Electronics in Power SystemsB. G. Fernandes
114/454
Application of Power Electronics in Power SystemsB. G. Fernandes
115/454
• Frequency of eα, iα, eβ & iβ is same as supply frequency
• ‘p’ & ‘q’ are calculated based on instantaneous values
• Assume supply voltages & currents are non-sinusoidal and have few common harmonic components
Application of Power Electronics in Power SystemsB. G. Fernandes
116/454
• Avg. power due to these common harmonic components is finite
• We can not eliminate these frequency components from source i !
• Source ‘i’ is non-sinusoidal
Application of Power Electronics in Power SystemsB. G. Fernandes
117/454
Review
Instantaneous real power
P = vaia+ vbib + vcic
( )ββααϕ ieieIVP SS ..23cos23
+==
Instantaneous reactive current:
That part of the three phase current can be eliminated at any instant without affecting ‘P’
iβ
iα eα
eβiS
VS
φψ
Application of Power Electronics in Power SystemsB. G. Fernandes
118/454
3 2 sinS Sq V i ϕ=
αββα ieie ×+×= 23
• If ‘v’ is sinusoidal, iL is non-sinusoidal
If q=0, then iS will be sinusoidal and in phase with Vs ( since average of the product of fundamental ‘ω’ & higher ‘ω’ term = 0)
⇒
Contd..
∑∞
=
=2
sinsinn
nn tnitvp ωω
Avg. of pn = 0
Application of Power Electronics in Power SystemsB. G. Fernandes
119/454
Contd..
Application of Power Electronics in Power SystemsB. G. Fernandes
120/454
• If ‘v’ is non-sinusoidal & iL is also non-sinusoidal
iS will have component corresponding to common frequency term of voltage & current
⇒
• H. Akagi, Y. Kanzawa, and A. Nabae“Instantaneous Reactive Power Compensators Comprising Switching Devices without Energy Storage Components,” Part-1 harmonic elimination., IEEE Trans. Ind. Applicat., vol. IA-20, No. 3,pp 625-630, May 1984.
Contd..
Application of Power Electronics in Power SystemsB. G. Fernandes
121/454
SS
dtd ωθ
=
⎥⎦
⎤⎢⎣
⎡⎥⎦
⎤⎢⎣
⎡−
=⎥⎦
⎤⎢⎣
⎡
⎥⎦
⎤⎢⎣
⎡⎥⎦
⎤⎢⎣
⎡ −=⎥
⎦
⎤⎢⎣
⎡
s
s
SS
SS
r
r
r
r
SS
SS
s
s
qd
qd
qd
qd
θθθθ
θθθθ
cossinsincos
cossinsincos
θS
dr
qr
dS
qS
ωS
Change of reference frame
Application of Power Electronics in Power SystemsB. G. Fernandes
122/454
( ) ( )( ) ( )
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡+−−−−+−
=⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
cba
qd
ss
ss
r
r
S
S
21212132sin32sinsin
32cos32coscos
32
0πθπθθπθπθθ
3 - phase(St. Frame)50 Hz
2 - phase(St. Frame)50 Hz
2 - phase(rotating. Frame at ωS)D. C
⇒ ⇒
Application of Power Electronics in Power SystemsB. G. Fernandes
123/454
• Let us assume that vS is along dr- axis in the syn. Rotating frame & iS is making an angle φ
ϕcos23
SS IVP =
θS
dr
qr
dS
qS
ωS
φ vS
iSrr dS IV23=
rr qS IVq23
=and
Application of Power Electronics in Power SystemsB. G. Fernandes
124/454
• Transform all the variables to Syn. rotating frame (rotating at ωS)
• Fundamental component of v & i will become dc• Other components will pulsates • Use a filter to eliminate these pulsating
component• (Could have used a filter to eliminate harmonics
from input signal)
Application of Power Electronics in Power SystemsB. G. Fernandes
125/454
• AC filtering ⇒ phase shift• VS is filtered component• iq is made zero
dr
ψ
qr
VSidiq
iSqs
ds
φ
Application of Power Electronics in Power SystemsB. G. Fernandes
126/454
Application of Power Electronics in Power SystemsB. G. Fernandes
127/454
• Information about system frequency is required
• Frequency varies over a narrow range• Should be insensitive to harmonics or multiple
zero crossings
Application of Power Electronics in Power SystemsB. G. Fernandes
128/454
⎥⎦
⎤⎢⎣
⎡⎥⎦
⎤⎢⎣
⎡=
⎥⎥
⎦
⎤
⎢⎢
⎣
⎡
yx
y
x0 -
0 .
.
ωω
Harmonic Oscillator
• Has Eigen values at S = ωj±
• If x(0) = 0 and y(0) =1
ttx ωsin)( = tty ωcos)( =
yx ω=& xy ω−=&
Application of Power Electronics in Power SystemsB. G. Fernandes
129/454
ω
ω−
*
x
∫
∫
*
y
( )x t
( )y t
tyxx nn Δ+=+ ω1
txyy nn Δ−=+ ω1
yt
xx nn ω=Δ−+1
ωxt
yy nn −=Δ−+1
Application of Power Electronics in Power SystemsB. G. Fernandes
130/454
tx ωsin= ty ωcos=
1 3( 120)2 21 3( 240)2 2
a
b
c
v Cos t y
v Cos t y x
v Cos t y x
ω
ω
ω
= =
= − = − +
= − = − −
• Let ea, eb and ec are the 3φ instantaneous system voltages
How to generate 3-phase sinusoids?
Application of Power Electronics in Power SystemsB. G. Fernandes
131/454
acba eeeee23
21
21
=−−=α
cb eee23
23
−=β
βα jeees +=
• Space vector representation of va, vb and vc
32
32 ππ j
c
j
bas evevvv−
++=
Application of Power Electronics in Power SystemsB. G. Fernandes
132/454
)3
2sin3
2)(cos240cos()3
2sin3
2)(cos120cos(cos ππωππωω jtjtt −−++−+=
)23
21)(sin
23cos
21()
23
21)(sin
23cos
21(cos jttjttt −−−−++−+−+= ωωωωω
βαωω jvvtjt +=+= sin23cos
23
• Projection of es on dr and qr
sv• ( is aligned along dr)
Application of Power Electronics in Power SystemsB. G. Fernandes
133/454
)cos( tee sd ωθ −=
tinstes ωθωθ sincoscos +=
)sin( tee sq ωθ −=
teteed ωω βα sincos +=
ttes ωθωθ sincoscossin −=
teteeq ωω αβ sincos −=
Application of Power Electronics in Power SystemsB. G. Fernandes
134/454
Objective• To make the phase and frequency of va, vb ,vc and
ea, eb ,ec same• vs and es are in phase
eq=0
yva = xyvb 23
21
+−= xyvc 23
21
−−=
Application of Power Electronics in Power SystemsB. G. Fernandes
135/454
Review
• In synchronous rotating frame (speed of the frame = ), supply frequency terms will become DC
• If input ‘v’ are unbalanced
+ve sequence terms DC
-ve sequence terms oscillate at 2
sω
→
sω→
Application of Power Electronics in Power SystemsB. G. Fernandes
136/454
• Other higher frequency terms in the synchronousreference frame can be filtered out
• They can also be filtered out in the input side
• Phase shift is introduced – not an issue
• Active filter control
Contd..
Application of Power Electronics in Power SystemsB. G. Fernandes
137/454
To change MI using harmonic elimination PWM technique
10.9091, 23.2907, 29.8505, 46.3408, 50.678110.7120, 23.2678, 29.5761, 46.3867, 50.4260 5, 7, 11, 13 are eliminated and10.5138, 23.2278, 29.2896, 46.4210, 50.1567
• Frequency information is required.• C. Schauder and H. Mehta, “Vector analysis and
control advanced static Var compensators” IEE proc, vol.140, pp. 299-306, 1993
Magnitude of fundamental is
different
Application of Power Electronics in Power SystemsB. G. Fernandes
138/454
• Digitize the sine wave and store in EPROM (1024 part)
• Address the EPROM using 10 bit counter( )
• Use a PLL as a multiplier 1024210 =
Through Hardware
Application of Power Electronics in Power SystemsB. G. Fernandes
139/454
Application of Power Electronics in Power SystemsB. G. Fernandes
140/454
⎥⎦
⎤⎢⎣
⎡⎥⎦
⎤⎢⎣
⎡=
⎥⎥
⎦
⎤
⎢⎢
⎣
⎡
yx
y
x0 -
0 .
.
ωω
Software approach
yt
xx nn ω=Δ−+1
tty ωcos)( =ttx ωsin)( =Harmonic oscillator
ωxt
yy nn −=Δ−+1
ω → Instantaneous frequency
• Input to harmonic oscillator is ω
Application of Power Electronics in Power SystemsB. G. Fernandes
141/454
• 3φ sinusoids which are in phase with supply fundamental component of the supply voltage are required
• Input voltage may have harmonics • ea, eb ,ec input system voltages may have
harmonics + may be unbalanced→
βα jeees +=
Application of Power Electronics in Power SystemsB. G. Fernandes
142/454
• Let va, vb ,vc are the 3φ pure sinusoids
• es should be in phase with vs ωSt vS
eS
yva = xyvb 23
21
+−= xyvc 23
21
−−=
Application of Power Electronics in Power SystemsB. G. Fernandes
143/454
• This voltage waveform can be used as reference current waveform in hystersis current control PWM technique
• Source current follows this reference ‘i’
• Source current is in phase with fundamental component of input voltage
Application of Power Electronics in Power SystemsB. G. Fernandes
144/454
• No zero crossing detection No reference wave• No PLL
Basic Analysis :• Switching frequency is much higher than supply
frequency• Let x(t) be an input to a switch operating at
variable ON and OFF times
One cycle control of 3φ Var compensator and Active filter
generation
Application of Power Electronics in Power SystemsB. G. Fernandes
145/454
• Switching frequency
• Produces switched output with average
= x(t) D(t)
==+ sOFFON TTT
11
dttxT
tyONT
s∫=0
)(1)(
D= duty cycle
Application of Power Electronics in Power SystemsB. G. Fernandes
146/454
• Duty ratio has to be generated as control input based on some reference signal Vref(t)
• If the duty ratio is controlled so that
• Average output
∫∫ =SON T
ref
T
dttVdttx00
)()(
dttVT
tysT
refs∫=0
)(1)(
Application of Power Electronics in Power SystemsB. G. Fernandes
147/454
• Assume that over one cycle Vref(t) is roughly constant
y(t)=Vref(t)• Works for constant switching frequency
• Vref could be a variable feedback signal
• Can be implemented using a simple integrator with reset
Application of Power Electronics in Power SystemsB. G. Fernandes
148/454
• Generate reset pulse at required frequency
• At the start of every cycle switch is turned ONby the reset pulse
• Integrate the input
• When the output of the integrator just exceeds Vref turn OFF the switch
• Start the cycle again after Ts when integratorresets
Application of Power Electronics in Power SystemsB. G. Fernandes
149/454
• A term in the control equation which is being multiplied with duty cycle of the switch has to be passed through a reset integrator and compared with the appropriate reference
Rule to be followed
Application of Power Electronics in Power SystemsB. G. Fernandes
150/454
Assumption:• In one switching cycle input is constant• Vdc is constant and ripple free
1φ AC-DC Active filter + Var generator
Application of Power Electronics in Power SystemsB. G. Fernandes
151/454
S4, S3 ON for DTS:
DCs VVdtdiL +=
S1, S2 ON for (1-D)TS:
DCs VVdtdiL −=
Application of Power Electronics in Power SystemsB. G. Fernandes
152/454
• Assume i(t) is continuous and i(0) = i(Ts)
• Average ‘V’ across L = 0
sSDCSDCs TDVVDTVV )1)(()( −−=+
DVV s
DC 21−=
Application of Power Electronics in Power SystemsB. G. Fernandes
153/454
• is and Vs should be in phaseVs= isRe (Re = Emulated resistance) …..(a)(1-2D)Vdc = isRe
is = (1-2D)Vdc/Re ……(b)• In each switching cycle if the duty ratio D is
controlled in such a way that equation (b) is satisfied , equation (a) also gets satisfied
• Control requirement is (1-2D)Vm = is
Where Vm= Vdc/Re
Aim
Application of Power Electronics in Power SystemsB. G. Fernandes
154/454
• One cycle control
• A term in the control equation which is being multiplied with duty cycle of the switch has to be passed through a reset integrator and compared with the appropriate reference
Review
No PLLNo ZCD
→→
Rule to be followed:
Application of Power Electronics in Power SystemsB. G. Fernandes
155/454
• Generate reset pulse at required frequency
• At the start of every cycle switch is turned ON by the reset pulse
• Integrate the input
• When the output of the integrator just exceeds Vref turn OFF the switch
Contd..
Application of Power Electronics in Power SystemsB. G. Fernandes
156/454
• Start the cycle again after Ts when integrator resets
• K. M. Smedley & C. Qiao, “Unified constant-frequency integration control of active power filters –steady –state and dynamics” IEEE Transaction on power electronics, vol. 16, No. 3, May 2001
Application of Power Electronics in Power SystemsB. G. Fernandes
157/454
1φ AC-DC
Control technique
sm iVD =− )21(
e
cm R
VV = → Emulated resistance
Application of Power Electronics in Power SystemsB. G. Fernandes
158/454
s
DT
mi
m idtVT
Vs
=− ∫0
1
ssi
mm iDT
TVV =−
Ti = Integrator time constant
Fs = 1/TS = Switching frequency
• Vm remains constant in one cycle
• If si TT21
= sm iVD =− )21(
Application of Power Electronics in Power SystemsB. G. Fernandes
159/454
TTDVDTVV cc
avgi)1()( −+−
=
Alternate Approach DC-DC Converter
)21( DVc −=
Application of Power Electronics in Power SystemsB. G. Fernandes
160/454
Buck Converter• ‘L’ is small
sc VLiV =+ ω
)21( DVVV csi −==
• ‘Vo’ to be maintained constant
• Compare with referenceand vary D or dependingupon Vs change ‘D’
Application of Power Electronics in Power SystemsB. G. Fernandes
161/454
• Information regarding Vs should be known
• Assume that Vs and is are in phase (required)
• Instead of varying ‘D’ as function of Vs
• Vary ‘D’ as a function of is
• If Vs and is are not in phase chosen values of ‘D’ may not give the desired Vo
• If ‘Vo’ is regulated, our assumption that Vsand is are in phase is valid
Application of Power Electronics in Power SystemsB. G. Fernandes
162/454
Application of Power Electronics in Power SystemsB. G. Fernandes
163/454
Application of Power Electronics in Power SystemsB. G. Fernandes
164/454
• DC link voltage has to be regulated
• Generate fixed frequency clock
• At the rising edge reset the integrator and turn ON the switches S4 and S3
• is ↑
• As t ↑ X ↓ When is = X ; R = 1
• Turn OFF the S4, S3 and Turn ON S1, S2
Application of Power Electronics in Power SystemsB. G. Fernandes
165/454
• For high power applications
• Conventional 3φ Inverter with ‘V’ control
• Switching ‘F’ is low
• ‘F’ of predominant harmonic is low•
•
Inverter topology for high power application
Application of Power Electronics in Power SystemsB. G. Fernandes
166/454
• 2 converters→
→ Var Compensator
Low power inverter for active filtering
• There are only two levels Instead
Application of Power Electronics in Power SystemsB. G. Fernandes
167/454
• Number of pulse should be high for superior harmonic spectrum
• Instead modify the Inverter structure
• More than two levels
• Multi-level inverter
Application of Power Electronics in Power SystemsB. G. Fernandes
168/454
• Consider onlyone leg
• Any time two switches are ON = (n-1)
Diode clamp multilevel inverters
3 Level Inverter:
Application of Power Electronics in Power SystemsB. G. Fernandes
169/454
Switches ON VAX
S1, S2 Vdc
S2, S3
S3, S4
2Vdc
0
• Number of capacitors required = 2 =(n-1)
• Number of switches required = 4/phase = 2(n-1)
Application of Power Electronics in Power SystemsB. G. Fernandes
170/454
Application of Power Electronics in Power SystemsB. G. Fernandes
171/454
Application of Power Electronics in Power SystemsB. G. Fernandes
172/454
• Voltage across each capacitor = Vdc/2 = Vdc/(n-1)
• Number of diodes = 2 ?
4 level Inverter• Number of switches ON = 3 = (n-1)• Number of switches/leg = 6 = 2(n-1)• Number of capacitors = 3 = (n-1)• Voltage across each capacitor = Vdc/3 = Vdc/(n-1)
Application of Power Electronics in Power SystemsB. G. Fernandes
173/454
Review
• In one cycle control ‘iS’ is compared with(1-2D)Vm
• Vm is passed through reset integrator & compared with Vm - RSiS
⇒ RS is sensing resistor
• No reference current waveform generation
Application of Power Electronics in Power SystemsB. G. Fernandes
174/454
• For high power ⇒ Use multi-level inverter
• For 3-level ⇒ VAX = VdC, ½ VdC, 0
Contd..
Application of Power Electronics in Power SystemsB. G. Fernandes
175/454
• At any time 2-devices (n-1) devices are ON• No. of Switches = 2(n-1)• ‘V’ across each ‘C’ = VdC / 2 = VdC /(n-1)
• ‘V’ rating of switch = VdC /2 = VdC /(n-1)
• ‘V’ rating of diode = VdC /2
• No. of diodes = 2 = (m-1)*(m-2)
Contd..
Application of Power Electronics in Power SystemsB. G. Fernandes
176/454
• Bum-Seok Suh and Dong-Seok Hyun “A New N-Level High Voltage Inversion System,” IEEE Trans. Ind. Electron., vol. 44, No. 1,pp 107-115, Feb 1997.
• Nam S. Choi, Jung G. Cho and Gyu H. Cho “A General Circuit Topology of Multilevel Inverter,” in Proc. IEEE Power electron specialist conf. Rec., pp 96-103, 1991.
References
Application of Power Electronics in Power SystemsB. G. Fernandes
177/454
4-level inverter
• Number of switches ON = 3 = (n-1)• Number of switches/leg = 6 = 2(n-1)
Application of Power Electronics in Power SystemsB. G. Fernandes
178/454
S1, S2, S3 ON: ⇒ VAX = Vdc
• ‘V’ rating of each device = Vdc /3
• Number of capacitors = 3 = (n-1)
• Voltage across each capacitor = Vdc /3 =Vdc /(n-1)
Application of Power Electronics in Power SystemsB. G. Fernandes
179/454
S2, S3, S4 ON :
⇒ VAX = 2Vdc /3
S3, S4, S5 ON :
⇒ VAX = Vdc /3
Application of Power Electronics in Power SystemsB. G. Fernandes
180/454
Observations:
• Duty cycle of switch is not the same• Lower switches are ON for longer time• Switch utilization is poor
S4, S5, S6 ON:
⇒ VAX = 0
Application of Power Electronics in Power SystemsB. G. Fernandes
181/454
• ‘V’ rating of DB = 2Vdc/3
• ‘V’ rating of DA = Vdc/3
Application of Power Electronics in Power SystemsB. G. Fernandes
182/454
• ‘V’ rating of diodes is not the same• Number of diodes = (n-1) (n-2) = 6
Application of Power Electronics in Power SystemsB. G. Fernandes
183/454
Voltage space vectors for 3 level inverter
NNP→ NPP → NPN → PPN → PNN → PNP → NNP
• Similar to conventional 2-level inverter
• 6 active vectors and 2 zero vectors
CBA
Large voltage vectors
Application of Power Electronics in Power SystemsB. G. Fernandes
184/454
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
−−−−−−
=⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
co
bo
ao
cn
bn
an
VVV
VVV
211121112
31
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎥⎦
⎤⎢⎣
⎡−−−
=⎥⎦
⎤⎢⎣
⎡
cn
bn
an
qs
ds
VVV
VV
2323021211
Application of Power Electronics in Power SystemsB. G. Fernandes
185/454
( NNP ) ⇒ ( 001 ) ⇒
( PPN ) ⇒ ( 110 ) ⇒
( NPN ) ⇒ ( 010 ) ⇒
( PNP ) ⇒ ( 100 ) ⇒
( NPP ) ⇒ ( 011 ) ⇒
( PNN ) ⇒ ( 100 ) ⇒
0∠dCV
π∠dCV
3/2π∠dCV
3/π−∠dCV
3/π∠dCV
3/2π−∠dCV
Application of Power Electronics in Power SystemsB. G. Fernandes
186/454
C B AO P PP O PP P O
C B AO O PO P OP O O
Small voltage vectors
Application of Power Electronics in Power SystemsB. G. Fernandes
187/454
C B AO O NO N ON O O
C B AO N NN O NN N O
Application of Power Electronics in Power SystemsB. G. Fernandes
188/454
C B A ⇒ O P P
⇒ VAO = VBO = VdC/2, VCO = 0
⇒ Van = VdC /6, Vbn = VdC /6, Vcn = - VdC /3
4321
621
6dCdCdCdC
dVVVVV =−−=
Application of Power Electronics in Power SystemsB. G. Fernandes
189/454
3/2
π∠=∴ dCS
VV
3/42
π∠=⇒∴ dCS
VVPOO
dCdCdC
q VVVV43
3623
=⎥⎦⎤
⎢⎣⎡ +=
OPP
POO
Application of Power Electronics in Power SystemsB. G. Fernandes
190/454
NOO :
VAO = VBO = 0, VCO = -VdC/2
Van = VdC/6, Vbn = VdC/6, Vcn = -VdC/3
,462
3 dCdCds
VVV == dCdCdC
qs VVVV43
3623
=⎥⎦⎤
⎢⎣⎡ +=
3/2
π∠=∴ dCS
VV 3/42
π∠=⇒⇒ dCS
VVONN
Application of Power Electronics in Power SystemsB. G. Fernandes
191/454
OOP :
VAO = VdC /2, VBO = VCO = 0
Van = VdC /3, Vbn = Vcn = -VdC /6
,2dC
dsVV = 0=qsV
02
∠=∴ dCS
VV π∠=⇒⇒2dC
SVVPPO
PPOOON
OOPNNO
Application of Power Electronics in Power SystemsB. G. Fernandes
192/454
NNO :
VAO = 0, VBO = VCO = -VdC / 2
Van = 1/3[0 +Vdc /2 + Vdc /2] = VdC /3,
Vbn = Vcn = 1/3[-2VdC / 2 + VdC / 2] = - VdC/6
,2dC
dsVV = 0=qsV
02
∠=∴ dCS
VV π∠=⇒⇒2dC
SVVOON
Application of Power Electronics in Power SystemsB. G. Fernandes
193/454
OPO :
VAO = VCO = 0, VBO = VdC /2
Van = Vcn = -VdC /6, Vbn = VdC/3
,4dC
dsVV −= dC
dCdCqs VVVV
43
63.6
23
=⎥⎦⎤
⎢⎣⎡ +=
3/22
π∠=∴ dCS
VV 3/52
π∠=⇒⇒ dCS
VVPOP
Application of Power Electronics in Power SystemsB. G. Fernandes
194/454
NON :
VAO = VCO = -VdC /2, VBO = 0
Van = Vcn = -VdC /6, Vbn = VdC/3
,4dC
dsVV −=
dCqs VV43
=
3/22
π∠=∴ dCS
VV 3/52
π∠=⇒⇒ dCS
VVONO
OPONON
POPONO
Application of Power Electronics in Power SystemsB. G. Fernandes
195/454
ONP :
VAO = VdC /2, VBO = -VdC /2 , VCO = 0
Van = VdC /2, Vbn = -VdC /2 , Vcn = 0
,43
dCds VV = dCqs VV43
−=
6/23 π−∠=∴ dCS VV
Medium voltage vectors
Application of Power Electronics in Power SystemsB. G. Fernandes
196/454
NOP :
VAO = VdC /2, VBO = 0 , VCO = -VdC /2
Van = VdC /2, Vbn = 0 , Vcn = -1/2 VdC
,43
dCds VV = dCqs VV43
=
6/23 π∠=∴ dCS VV
Application of Power Electronics in Power SystemsB. G. Fernandes
197/454
NPO :
VAO = 0, VBO = VdC /2 , VCO = -VdC /2
Van = 0, Vbn = VdC /2 , Vcn = -VdC /2
,0=dsV dCqs VV23
=
2/23 π∠=∴ dCS VV
Application of Power Electronics in Power SystemsB. G. Fernandes
198/454
PNO :
VAO = 0, VBO = -VdC /2 , VCO = VdC /2
Van = 0, Vbn = -VdC /2 , Vcn = VdC /2
,0=dsV dCqs VV23
−=
2/323 π∠=∴ dCS VV
Application of Power Electronics in Power SystemsB. G. Fernandes
199/454
Application of Power Electronics in Power SystemsB. G. Fernandes
200/454
Review
3-Level Inverter• No. of large voltage vectors = 6
⇒ VS = VdC
• No. of small voltage vectors = 6
⇒ VS = 1/2VdC
⇒ 12 possible combinations
+ ve or –ve bus
mid point&
Application of Power Electronics in Power SystemsB. G. Fernandes
201/454
Contd..
• No. of medium voltage vectors = 6
⇒ + ve, - ve & mid-point bus
dCS VV 23=⇒
Application of Power Electronics in Power SystemsB. G. Fernandes
202/454
Application of Power Electronics in Power SystemsB. G. Fernandes
203/454
Voltage control
• Space vector PWMDepending upon the position of space vector, switch the corresponding switch
⇒
NPP
NOP
NNPPPPNNNOOO
OPPNOO
OOPNNO
Application of Power Electronics in Power SystemsB. G. Fernandes
204/454
Voltage unbalance between DC-Line capacitance
• Each leg ⇒ 3 possibilities
• There are 27 switching instances are possible
Application of Power Electronics in Power SystemsB. G. Fernandes
205/454
• Unbalances has no effect on load
• Load is connected across the DC bus
• Somewhat effective in reducing voltage unbalance
Application of Power Electronics in Power SystemsB. G. Fernandes
206/454
• C1 supplies the power• C2 does not supply the power• ‘V’ across C2 ↑• For remaining 2 configuration, V across C1 ↑
Application of Power Electronics in Power SystemsB. G. Fernandes
207/454
Load compensation
• Passive elements
• Inverter ⇒ Current control⇒ Voltage control⇒ Main compensator⇒ Aux. compensator
• Instantaneous reactive power theory
• One cycle controlled inverter
• Multi level inverter
Application of Power Electronics in Power SystemsB. G. Fernandes
208/454
Transmission line voltage support
• Provide mid-point compensation⇒ Shunt⇒ Series⇒ Combination of shunt & series
⇒ Combination of series & seriesP < SIL
P > SIL
P = SIL
VS VR
Application of Power Electronics in Power SystemsB. G. Fernandes
209/454
Shunt Compensation :
• Inject current in to the system
• If injected ‘I’ is in phase quadrature with the ‘V’
• Only reactive power transfer
• Else, it has to handle real ‘P’ as well
Series Compensation :• Inject voltage in series with the line
Application of Power Electronics in Power SystemsB. G. Fernandes
210/454
• If ‘V’ is in quadrature with line ‘I’, only reactivepower transfer
Combination of series & Shunt Compensation :
• Inject ‘I’ with the shunt part &
• Inject ‘V’ with the series part
• When combined there can be real powerexchange between the series & shunt controllers
Application of Power Electronics in Power SystemsB. G. Fernandes
211/454
Mid point voltage regulator
• Two machine model
δSinXVV RS=Ρ
XVP
2
max =
⇒ If Vs = Vr = V
Application of Power Electronics in Power SystemsB. G. Fernandes
212/454
• Connect a compensator at the mid point & Vm = Vs = Vr = V
• Whether active power transfer is require ?
• System is loss-less
Application of Power Electronics in Power SystemsB. G. Fernandes
213/454
Application of Power Electronics in Power SystemsB. G. Fernandes
214/454
• Let Vsm & Vmr are fictitious voltages in phasewith Ism & Imr respectively
( )4/. δCosVVV mrsm ==
( ) ( )4/42
4/.2 δδ SinXV
XSinVII mrsm ===
( ) ( )4/.4/4.2
δδ CosSinXVIVPP smsmr ===
( )2/2 2
δSinXV
=
Application of Power Electronics in Power SystemsB. G. Fernandes
215/454
( )4/8 22
δSinXV
=
ccm VIIV ==
( )4/..2 δSinIV sm=
( )( )2/14 2
δCosXV
−=
• Reactive power supplied by the compensator
Application of Power Electronics in Power SystemsB. G. Fernandes
216/454
Application of Power Electronics in Power SystemsB. G. Fernandes
217/454
• Shunt compensator can increase ‘P’
• ‘Q’ demand also ↑
• Can have multiple compensators located at the equal distances
• Theoretically ‘P’ would double for each doubling of the segments
Application of Power Electronics in Power SystemsB. G. Fernandes
218/454
• ↑ the no. of segments results in flat ‘V’ profile
• Expensive
Application of Power Electronics in Power SystemsB. G. Fernandes
219/454
Review
Mid-point shunt compensation
( )2/2 2
δSinXVP =
( )( )2/14 2
δCosXVQ −=
⇒ If Vs = Vr = V
⇒ ‘I’ is injected into the line (in quadrature with ‘v’)
Application of Power Electronics in Power SystemsB. G. Fernandes
220/454
Contd..
• For each doubling of the segments, transmittable ‘P’ also doubles
• ‘V’ profile is almost flat
• Large no. of shunt compensators ⇒ expensive
Application of Power Electronics in Power SystemsB. G. Fernandes
221/454
• Compensator must remain in synchronism with the ac system under all operating conditionsincluding major disturbances
Summary
• Must regulate the bus voltage
• For the inter connecting two systems, bestlocation is in middle
• For radial feed to a load, best location is at the load end
Application of Power Electronics in Power SystemsB. G. Fernandes
222/454
Methods of controlling Var generation
• Mechanically switched capacitor and/or inductor ⇒ course control
⇒ in-rush current
• Continuously variable Var generation or absorption ⇒ originally over excited syn. motor
• Modern Var generators → use power semiconductor devices/equipment + energystoring elements
Application of Power Electronics in Power SystemsB. G. Fernandes
223/454
Variable impedance type S.V.C
1. Thyristor controlled reactor (TCR):
• T1 & T2 is triggered in the + ve & - ve half cycles respectively
α⇒ Can be measured w. r. t zero crossing or peak of ‘V’
Application of Power Electronics in Power SystemsB. G. Fernandes
224/454
tSinVdtdiL m ω=
( ) ( )tCosCosL
Vti m ωαω
−=∴
απβ −=∴ 2
βα CosCos =
• ‘i’ flows from α to β
i(t) =0 at ωt = β
⇒ β = extinction angle
Application of Power Electronics in Power SystemsB. G. Fernandes
225/454
• ‘i’ is continuous when α = π/2
• ‘i’ is sinusoidal
• No control ⇒ ’L’ is fixed & it is minimum
• As α ↑, all odd harmonics are introduced
Application of Power Electronics in Power SystemsB. G. Fernandes
226/454
⎟⎠⎞
⎜⎝⎛ −−=∴ α
πα
πωα 2sin121)(
LVILF
• As α ↑, L ↑
• VL(MAX) ⇒ Voltage limit
• IL(MAX) ⇒ current limit
• BL(MAX) ⇒ Max. admittance of TCR
⎟⎠⎞
⎜⎝⎛ −−=⇒ α
πα
πωα 2sin1211)(
LBL
Application of Power Electronics in Power SystemsB. G. Fernandes
227/454
2. Thyristor switched capacitor (TSC):
• Small ‘L’ is required tolimit the surge current
• Thyristors are switched when vc = v
• ‘V’ rating of the switch ?
Application of Power Electronics in Power SystemsB. G. Fernandes
228/454
Application of Power Electronics in Power SystemsB. G. Fernandes
229/454
3. Fixed Capacitor, Thyristor controlled Reactor (FC-TCR):
• ‘iL’ is varied by varying ‘α’• iL = iL(max) when α = π/2
• In FC-TCR, for any value of iL, net effect of C ↓
• ‘C’ also provides a low impedance path for harmonics generated by TCR
In TCR
Application of Power Electronics in Power SystemsB. G. Fernandes
230/454
• ‘QC’ is constant
• Net Q = QC when QL = 0 (α = π)
• To ↓ net Q, ↓ α
• Net Q = 0, when QC = QL
• If α is ↓ further, net Q is inductive
Application of Power Electronics in Power SystemsB. G. Fernandes
231/454
• At α = π/2, QL = QL(max)
• Operating V-I region of FC-TCR
Application of Power Electronics in Power SystemsB. G. Fernandes
232/454
STATCOM
• VSI can supply ± Q
• Also known as static synchronous condenser
• Similar to syn. motor
XEVI −
= VX
EVQ .−=
Q ⇒ reactive power received by the source
Application of Power Electronics in Power SystemsB. G. Fernandes
233/454
Control
• ‘Q’ is controlled by M.I & δ⇒ accounts for losses
• Assumed that inverter is capable of injecting ‘Q’demand of the line
Application of Power Electronics in Power SystemsB. G. Fernandes
234/454
• If ‘Q’ demand >Var rating of inverter
• It may fail due to over load
• Have a inner ‘I’ loop
Application of Power Electronics in Power SystemsB. G. Fernandes
235/454
Operating V-I region
Application of Power Electronics in Power SystemsB. G. Fernandes
236/454
Review
T.C.R
• If α = π/2 ⇒ i = imax
• As α ↑ , Leff ↑
• Harmonics
Application of Power Electronics in Power SystemsB. G. Fernandes
237/454
Contd..
T.S.C
• Thyristors are triggeredwhen vc = v
F.C.T.C.R
• T.S.C – T.C.R schemeis also possible
Application of Power Electronics in Power SystemsB. G. Fernandes
238/454
Contd..
• Above schemes are variable impedance types
• Variable source type
STATCOM
XEVI −
=
VX
EVQ .−=
Application of Power Electronics in Power SystemsB. G. Fernandes
239/454
Advantages
• Since voltage profile is maintained (in radial system)
⇒ Voltage instability is prevented
⇒ Improves transient stability
⇒ Damping of power oscillations
⇒ Able to maintain ‘V’ profile
Application of Power Electronics in Power SystemsB. G. Fernandes
240/454
Series compensation
• Reciprocal of shunt compensation
• Shunt compensator : Controlled reactive ‘I’ source connected in parallel with the Tr. Line to control ‘V’
• Series compensator : Controlled reactive‘V’ source connected in series with the Tr. Line to control ‘I’
Application of Power Electronics in Power SystemsB. G. Fernandes
241/454
Series compensation
• Injects voltage in series with the line
• Could be variable ‘Z’ (such as ‘C’ or ‘L’)
• Voltage source
• Effective in controlling the power flow
Application of Power Electronics in Power SystemsB. G. Fernandes
242/454
Concept of series capacitive compensation
⇒ To decrease reactance of the line
δSinXVVP RS ..
=
( )CL XXX −=
Application of Power Electronics in Power SystemsB. G. Fernandes
243/454
LC XXK =
( )CLeff XXX −=
( ) LXK−= 1
⇒ Degree of series compensation
⇒ 0 < K < 1
Application of Power Electronics in Power SystemsB. G. Fernandes
244/454
( ) ( ) Lm XK
SinVVCosIVP−
==1
2.2.2 δδ
( ) ( ) LL XKSinV
XKSinVI
−=
−=
12.2
212. δδ
( ) LXKSinV
−=
1.2 δ
• If VS = VR =V
( ) CL
CC XXK
SinVXIQ .1
2.422
222
−==
δ ( )( ) LXK
KCosV.1
.1.22
2
−−
=δ
Application of Power Electronics in Power SystemsB. G. Fernandes
245/454
⎟⎠⎞
⎜⎝⎛=
2tan max2 δ
sh
se
δmax ⇒ maximum angular difference between the two ends of the line
Application of Power Electronics in Power SystemsB. G. Fernandes
246/454
• If δmax ⇒ 30 - 40o
• Qse = 7- 13% of QSL
• Cost of series capacitor ?
• Location of series capacitor is not very critical
Application of Power Electronics in Power SystemsB. G. Fernandes
247/454
Approaches to controllable series compensation
Objective : Vary VC
1. GTO controlled series capacitor (GCSC)
• GTO is closed when vc = 0
• Open when ‘i’ charges ‘C’
Variable Z type :
• Duality between TCR & GCSC
Application of Power Electronics in Power SystemsB. G. Fernandes
248/454
• GTO is turned ON when vc = 0 for α < ωt < α+γ
( ) ( ) ( )tdtiC
tvt
c ωω
ω
α
.1∫= ( ) tCosIti ω.=∴
( )αωω
SintSinCI
−=
• vc is maximum when ωt = π/2 & vc = 0 when ωt = π-α
Application of Power Electronics in Power SystemsB. G. Fernandes
249/454
• Amplitude of the fundamental
( ) ( )tdtSintvV cc ωωπ
π
..4 2
01 ∫=
⎥⎦⎤
⎢⎣⎡ −−=
πα
πα 221 SinIXc
( ) ( )tdtSinSintSinCI ωωαωωπ
π
..4 2
0
−= ∫
Application of Power Electronics in Power SystemsB. G. Fernandes
250/454
Controlling modes
(a). Voltage compensation mode:
• GCSC ⇒ Should maintain rated compensation voltage when Imin < I < Imax
⇒ Vcomp = Vrated = Imin Xc
⇒ As I↑, ↑ α So that Vcomp is maintained constant
Application of Power Electronics in Power SystemsB. G. Fernandes
251/454
(b). Impedance compensation mode:
cc XI
V=
max
(max)
Protection issues:
• Required to have higher short time rating
• During S.C, ‘I’ could be much higher than Irated
• Ifault > IGTO(rating)
Application of Power Electronics in Power SystemsB. G. Fernandes
252/454
• If it flows through ‘C’, Vc ↑
• ‘V’ across GTO ↑
• Use MOV
Limitations:
• Harmonics are generated
Application of Power Electronics in Power SystemsB. G. Fernandes
253/454
Review
GTO controlled series capacitor (GCSC)
• ‘α’ is measured w.r.t peak of ‘i’
( ) ⎟⎠⎞
⎜⎝⎛ −−= α
πα
πωα 21211 Sin
CX C
α⇒ extinction angle
• ‘VC’ has harmonics
Application of Power Electronics in Power SystemsB. G. Fernandes
254/454
Contd..TCR GCSC
• Switch is series with ‘L’ • Switch is parallel with ‘C’• Supplied from a ‘V’
source• Supplied from a ‘i’
source• ‘α’ (turn-ON delay) is
measured w.r.t peak of ‘v’• ‘α’ (turn-OFF delay) is
measured w.r.t peak of ‘i’
Application of Power Electronics in Power SystemsB. G. Fernandes
255/454
• Control ‘i’ in ‘L’ . Parallel with the source representing variable
admittance to the source
• Control ‘v’ across ‘C’developed by ‘i’ source representing variable
reactance to the source
Contd..
( ) ⎥⎦⎤
⎢⎣⎡ −−=
πα
πα
ωα 221 Sin
LVILF ( ) ⎥⎦
⎤⎢⎣⎡ −−=
πα
πα
ωα 221 Sin
CIVCF
⎟⎠⎞
⎜⎝⎛ −−=⇒ α
πα
πωα 2sin1211)(
LBL ( ) ⎥⎦
⎤⎢⎣⎡ −−=⇒
πα
πα
ωα 2211 Sin
CXC
Application of Power Electronics in Power SystemsB. G. Fernandes
256/454
Thyristor switched series capacitor (TSSC)
• Capacitors are disconnected by turning ONthe thyristors
• They turn OFF naturally (at Z.C of I )
Application of Power Electronics in Power SystemsB. G. Fernandes
257/454
Voltage compensating mode :
• By-pass ‘C’
• Reactance of ‘C’ bank is chosen so as toproduce average rated Vcomp = n XC Imin
(‘n’ is the no. of banks)
• As I ↑ above Imin , ↓ n
Application of Power Electronics in Power SystemsB. G. Fernandes
258/454
Impedance compensating mode :
• Maximum series compensation
max
(max)
IV
nX CC = at rated ‘I’
• TSSC should maintain maximum ratedcompensating reactance at any line current up to Rated current (Imax)
Application of Power Electronics in Power SystemsB. G. Fernandes
259/454
• In FCTCR continuously varying capacitivecompensation is achieved by varying ‘α’of TCR
Application of Power Electronics in Power SystemsB. G. Fernandes
260/454
Thyristor controlled series capacitor (TCSC)
• If ‘V’ is the applied voltage across the TCR
• Fundamental component of ‘I’ for ‘α’(measured w.r.t peak of voltage) is
Application of Power Electronics in Power SystemsB. G. Fernandes
261/454
( ) ∞<< αLL XX
( ) ⎟⎠⎞
⎜⎝⎛
−−=
ααππα
22 SinXX LL
( )( ) ⎟⎟
⎠
⎞⎜⎜⎝
⎛−
−=
CL
LCTCSC XX
XXXα
α.
⇒ Combined ‘Z’ of TCR & fixed ‘C’
( ) ⎟⎠⎞
⎜⎝⎛ −−= α
πα
πα 21211 Sin
XVI
L
Application of Power Electronics in Power SystemsB. G. Fernandes
262/454
& XTCSC = -XC
• When XL(α) = XC XTCSC ⇒ undefined
• When XL(α) < XC XTCSC ⇒ Inductive
At XL(α) = XL ⇒ ⎟⎟⎠
⎞⎜⎜⎝
⎛−
=CL
LCTCSC XX
XXX .
• When α = π/2, XL(α) = ∞
Application of Power Electronics in Power SystemsB. G. Fernandes
263/454
Application of Power Electronics in Power SystemsB. G. Fernandes
264/454
• Continuously varying series capacitor by ‘α’ control
( ) ∞<< αω LXL
( ) ,∞<αLX CXX CTCSC ω1==
• At XL(α) = XC ⇒ parallel resonance, ∞⇒TCSCX
• When
( )αωω >⇒ oLC1=∴ω As L(α) > L
Application of Power Electronics in Power SystemsB. G. Fernandes
265/454
• If XL(α) < XC , There are two operating zones
⇒ Capacitive, ‘i’ leads VC
0 ≤ α ≤ αL(lim) ⇒ XTCSC is inductive
• Not exactly similar to TCR connected in parallel With‘V’ source
• Input ‘V’ is sinusoidal
2(lim) παα ≤≤C
Application of Power Electronics in Power SystemsB. G. Fernandes
266/454
• In TCSC, the ‘V’ is voltage across ‘C’
• Switch is open ⇒ TCR is O.C, ‘i’ flows through ‘C’
• Turn-on TCR at ‘α’(w.r.t peak of ‘v’)
⇒ ‘i’ is +ve & ‘vc’ is -ve
Application of Power Electronics in Power SystemsB. G. Fernandes
267/454
• ‘VC’ gets distorted• In phasor form ‘i’ leads VC
in capacitor zone
Application of Power Electronics in Power SystemsB. G. Fernandes
268/454
• In inductive zone, ‘i’ lags VC
• TCR current is high
⎟⎟⎠
⎞⎜⎜⎝
⎛−
−=
)(.
CTCR
TCRCTCSC XXj
jXjXX
⎟⎟⎠
⎞⎜⎜⎝
⎛−−
=)1( TCRC
C
XXjX
( )CTCRCTCR
CTCR XX
IIXXj
jXi−
=−
−=
1.
)(
Application of Power Electronics in Power SystemsB. G. Fernandes
269/454
• If XTCR = 1.5XC ⇒ Capacitive
• If XTCR = 0.75XC ⇒ Inductive
35.111
1)1(
1=
−=⎟⎟
⎠
⎞⎜⎜⎝
⎛−
=TCRCC
TCSC
XXXX
25.11
1−=
−=
IITCR
CTCR XX 75.0=
Application of Power Electronics in Power SystemsB. G. Fernandes
270/454
375.011
1−=
−=
C
TCSC
XX
475.01
1=
−=
IITCR
• For same magnitude of XTCSC , ITCR in ‘C’zone = (1/2)ITCR in ‘L’ zone
Application of Power Electronics in Power SystemsB. G. Fernandes
271/454
Modes of operation
By pass mode :
• ‘iL’ is continuous & sinusoidal
• Each thyristor conducts for 180o
• XTCSC ⇒ inductive
• Most of the line ‘I’ flow through ‘L’ not ‘C’
• Used to protect ‘C’ against over voltage
Application of Power Electronics in Power SystemsB. G. Fernandes
272/454
Thyristor blocked mode :
• No ‘i’ through ‘L’
• Fixed ‘C’ ⇒ Avoided
Vernier control
• Thyristors are gated and they conductsfor part of cycle
• XTCSC ↑ as conduction angle ↑ from zeroto αC(lim)
Application of Power Electronics in Power SystemsB. G. Fernandes
273/454
Static Synchronous Series Compensation
• Function of series capacitor ⇒ produces anappropriate voltage of fundamental ‘F’ inquadrature with Tr. Line ‘I’
( ) δSinXX
VVPCL
RS
−=
Application of Power Electronics in Power SystemsB. G. Fernandes
274/454
• Instead: Use VSI to inject a voltage inquadrature with ‘i’
( )γqq VjV .±=
Application of Power Electronics in Power SystemsB. G. Fernandes
275/454
• Voltage across ‘L’ ⇒ ( ) qL VVSinV += 22 δ
( )X
VVSinI q+=
22 δ
( ) ( )( )qVVSinVCosP += 22.2 δδ
( )2.2
δδ CosXVV
SinX
V q+=
• If Vq > I.X, power flow will reverse
Application of Power Electronics in Power SystemsB. G. Fernandes
276/454
Review
• Used for vernier control of ‘C’.GCSC also provides this feature
• Cost of GTO > that of thyristor
• Effective capacitive compensation increases as α ↓ from π/2 to αC(lim)
T.C.S.C :
Application of Power Electronics in Power SystemsB. G. Fernandes
277/454
Contd..
• For both region XL < XC (inductive & capacitive)
• In inductive zone, ITCR > ILine and are in phase
• In capacitive zone, ILine is out of phase with ITCR
• ‘V’ across ‘C’ gets distorted
Application of Power Electronics in Power SystemsB. G. Fernandes
278/454
Static Synchronous Series Compensation:
Contd..
• Instead of passive elementsuse VSI
( )2.2
δδ CosXVV
SinX
VP q+=
• Reverse power flow is possible
Application of Power Electronics in Power SystemsB. G. Fernandes
279/454
Control range:
• Voltage compensation mode : SSSC canmaintain the rated capacitive or inductivecompensating ‘V’ for ‘I’ till Iq(max)
• Ideal condition (‘I’ line can not be zero)
• ΔP is required for SSSC
Application of Power Electronics in Power SystemsB. G. Fernandes
280/454
impedance compensation mode :
• Maintain rated XC or XL
up to rated I
Exchange of Active power by SSSC:• Can exchange active as well as reactive power• Some active source should be connected to
DC side
Application of Power Electronics in Power SystemsB. G. Fernandes
281/454
• With series compensation effective ( ) RXX CL − ratio ↓
• Compensation for both reactive and resistivecompensation of series line impedance to keepX/R ratio high (3-10 is desirable)
Application of Power Electronics in Power SystemsB. G. Fernandes
282/454
• X/R ratio in case1 > X/R ratio in case2
• Reactive component of
( )12. ϕδ +== CosIII a
↑
• Real component of
( )12. ϕδ += SinIIq
transmitted to the receiving end decreasescorresponding to R=0
Application of Power Electronics in Power SystemsB. G. Fernandes
283/454
• If VS = VR =V
Per phase power received by the receiving end
( )2.. δϕ −= SinIV
( )ϕδ −+= 290.. CosIVP
( )2.2/2. δϕδ−= Sin
ZVSinV
2/..2/2/.2 2
δϕϕδδ SinCosSinCosSinZV
−=
Application of Power Electronics in Power SystemsB. G. Fernandes
284/454
2/.2/.2/.2 22
δϕδδϕ SinCosCosSinSinZV
−=
( ) δϕδϕ CosCosSinSinZ
V−−= 1..
2
( ) δδ CosRSinXXR
V−−
+= 1..22
2
( )⎭⎬⎫
⎩⎨⎧ −−= δδ Cos
ZRSin
ZX
ZV 1..
2
Application of Power Electronics in Power SystemsB. G. Fernandes
285/454
( )ϕδ −+= 2/90.SinVIQ
( )ϕδδ−= 2/2/2 2
CosZ
SinV
( ) δδ CosXSinRXR
V−+
+= 1.22
2
⇒ Reactive VA associated with the receiving end
Application of Power Electronics in Power SystemsB. G. Fernandes
286/454
• Maximum transmittable active power ↓
Application of Power Electronics in Power SystemsB. G. Fernandes
287/454
Voltage & phase angle regulators
Voltage regulator:• Injection of appropriate in phase
component in series with ac system
• Similar to transformer tap changer
Application of Power Electronics in Power SystemsB. G. Fernandes
288/454
Phase angle controller :• Inject ‘V’ at an angle ±90o
relative to the system ‘V’
• Resultant angular change approx. proportionalto injected ‘V’. Magnitude of ‘V’ is constant
Application of Power Electronics in Power SystemsB. G. Fernandes
289/454
Power flow control :
• Optimal loading of transmission line inpractical system can not always be achievedat the prevailing angle
Occur when ?• Power between two buses is transmitted
over parallel lines of different length, usephase angle regulator (PAR)
Application of Power Electronics in Power SystemsB. G. Fernandes
290/454
PAR : A sinusoidal synchronous ac voltagesource with controllable amplitude and phase angle
rSSeff VVV += SeffS VV =and
Application of Power Electronics in Power SystemsB. G. Fernandes
291/454
• Basic idea is to keep the transmittablepower at the desirable levelindependent of prevailing ‘δ’
Vr
VS> 90o
⇒ angle to be controlledis (δ-σ )
( )σδ −= SinX
VP2
also
Application of Power Electronics in Power SystemsB. G. Fernandes
292/454
• Multi functional FACTS controller :based on back-back VSI with a commonDC-link
• One converter in series (SSSC) and otheris in shunt (SVC) ⇒ unified power flowcontroller (UPFC)
• Both converters are connected in series butin two different lines (Inter line Power FlowController-IPFC)
Application of Power Electronics in Power SystemsB. G. Fernandes
293/454
UPFC :• Able to control simultaneously or
selectively all the parameters affecting thepower flow in Tr. line
Application of Power Electronics in Power SystemsB. G. Fernandes
294/454
• Converter-1 supplies active powerrequired by converter-2
Application of Power Electronics in Power SystemsB. G. Fernandes
295/454
UPFC can fulfill
• Reactive power control
• Series compensation
• Phase angle regulator
• Independently control the reactive powerflow at the point of connection
Application of Power Electronics in Power SystemsB. G. Fernandes
296/454
Application of Power Electronics in Power SystemsB. G. Fernandes
297/454
Control capabilities
Case1 : Voltage regulator
,0=ρ VVpq Δ±=
• Similar to tap changing transformer withlarge no. of steps
Reactance compensator : Series reactive compensator
Vpq = Vq at 90o with I
⇒ Similar to SSSC
Application of Power Electronics in Power SystemsB. G. Fernandes
298/454
Phase angle regulator :
⇒ at any angular relationship w.r.t VSso that desired phase shift is achieved
σVVpq =
Multi functional feature :
σVVVV qpq ++Δ=
Application of Power Electronics in Power SystemsB. G. Fernandes
299/454
ReviewU.P.F.C :
• Two VSI connected back to back with common DC-link
• One connected in series with line and other isconnected across the line
• DC-link ‘V’ is maintained constant by converter-1
Application of Power Electronics in Power SystemsB. G. Fernandes
300/454
Contd..
• Active power required by the system is drawn by converter-1
Can function as
• Voltage regulator ⇒ V+ΔV• SSSC ⇒ injects ‘V’ in quadrature with ‘I’
• Phase angle regulator ⇒ injects ‘ΔV’ inquadrature with ‘V’
Application of Power Electronics in Power SystemsB. G. Fernandes
301/454
Using UPFC
• Active power flow and
• In SSSC : Quadrature injected ‘V’results in increase in power flow
• Reactive power flow can be set
⇒ Magnitude of injected ‘V’ determines ‘P’
⇒ Circuit conditions determines ‘Q’
Application of Power Electronics in Power SystemsB. G. Fernandes
302/454
• Main function : Control the flow of ‘P’ & ‘Q’by injecting a voltage in series with the Tr. line
• Both magnitude & phase angle are varied
• Control of ‘P’ & ‘Q’ allows power flow inprescribed routes
⇒ 2 port representation
Application of Power Electronics in Power SystemsB. G. Fernandes
303/454
⇒ A common DC-link voltage is regulated
( ) 0Re*22
*11 =−+ lossuu PIVIV
• In addition to maintain real power balance,shunt branch can independently exchange reactive power with the system
Application of Power Electronics in Power SystemsB. G. Fernandes
304/454
• Transmitted active power and reactive powersupplied by receiving end
2δjr VeV −=
*
. ⎟⎟⎠
⎞⎜⎜⎝
⎛ −+=−
jXVVV
VjQP rpqSrrr
( )ρδ += 2jpqpq eVV
2δjS VeV =
Application of Power Electronics in Power SystemsB. G. Fernandes
305/454
( ) ( )
⎭⎬⎫
⎩⎨⎧
−−
−−−= +−− ρδδ δδδδ 22 2222 jpqj e
jXV
jXjSinCosjSinCosVVe
( )
⎭⎬⎫
⎩⎨⎧
−= +−− ρδδ δ 22 22 jpqj ejX
VX
VSinVe
( ) ( )ρδδδδ +−−−= jpq ejXVV
jSinCosSinXV .
2222 2
( ) ( ) ( )( )ρδρδδδδ +−+−−= jSinCosjXVV
jSinCosSinXV pq.
22.22 22
Application of Power Electronics in Power SystemsB. G. Fernandes
306/454
( )ρδδ +−=− SinXVV
SinX
VjQP pqrr
.2
( )⎭⎬⎫
⎩⎨⎧
+−− ρδδ CosXVV
SinXVj pq.
22 22
( )ρδδ +−=∴ SinXVV
SinX
VP pqr
.2
( )ρδδ +−=∴ CosXVV
SinXVQ pq
r
.22 2
2
Application of Power Electronics in Power SystemsB. G. Fernandes
307/454
• ‘ρ’ can vary from 0 to 2π
• ‘P’ & ‘Q’ are controllable from
( )XVV
P pq.−δ ( )
XVV
P pq.+δto
⇒ Transmitted real power ( )
XVV
SinX
V pq max2 .
±= δ
Application of Power Electronics in Power SystemsB. G. Fernandes
308/454
Control strategy:
• There are 3 degrees of freedom
• Magnitude and angle of series V
• Shunt reactive current
⇒ Both are VSI
⇒ Series injected ‘V’ can be instantaneouslychanged
Application of Power Electronics in Power SystemsB. G. Fernandes
309/454
⇒ Shunt current is controlled indirectly byvarying output of shunt converter
Series injected ‘V’ control :
• Injected ‘V’ can be split into two components
1. In phase with line ‘I’
2. In quadrature with line ‘I’
• ‘P’ can be controlled by varying series reactanceof the line
Application of Power Electronics in Power SystemsB. G. Fernandes
310/454
• Reactive ‘V’ injection ⇒ similar to series connection of reactance except that injected ‘V’is independent of Tr. Line ‘I’
Shunt current control :• Shunt current can be split into real & reactive
components• Magnitude of real component ⇒ DC link ‘V’
• Magnitude of reactive component ⇒ Bus ‘V’magnitude regulator
Application of Power Electronics in Power SystemsB. G. Fernandes
311/454
FACTS installments in India
• TSC+TCR (400 kV) at Kanpur ⇒ ±240 MVar
• TCSC (400 kV ) at ⇒ Raipur - Rourkela
• TCR (400 kV) at Itarsi ⇒ ±50 MVar
⇒ Gorakhpur - Mazaffarpur
(Double ckt.)
⇒ Kanpur - Ballabhgarh
Application of Power Electronics in Power SystemsB. G. Fernandes
312/454
Kanpur – Ballabhgarh 400 kV line:
Fixed capacitor TCSC
Rated V L-L 420 kV 420 kV
Nominal Var 151.60 MVar 79.87 MVar
Rated continuous‘V’ across ‘C’
42.2 kV 16.6 kV
TCR/ph 4.4 mH-
Application of Power Electronics in Power SystemsB. G. Fernandes
313/454
• Long distance transmission ( Competing technology : AC with FACTS)
HVDC
• Cable transmission (> 40 Km) ⇒ HVDC
• Asynchronous link ⇒ HVDC
• HVDC lines are cheaper than AC lines
• Terminal equipment costs are higher
Application of Power Electronics in Power SystemsB. G. Fernandes
314/454
In India :• Long distance HVDC
• Rihand – Dadri : 1500 MW, ±500 kV
• Chandrapur – Padghe : 1500MW, ±500 kV
• Barsur– Lower Sileru : 200MW, 200 kV
• Talcher – Kolar : 2000MW, ±500 kV
Application of Power Electronics in Power SystemsB. G. Fernandes
315/454
Back to Back :
• Chandrapur – Ramagundam : 1000 MW(Asynchronous link)
• Vindhyachal : 500 MW
• Jeypore – Gajuwaka : 500 MW(Asynchronous link)
• Sasaram : 500 MW
Application of Power Electronics in Power SystemsB. G. Fernandes
316/454
• ‘P’ through DC link can be regulated.
• Power control through firing angle control
Application of Power Electronics in Power SystemsB. G. Fernandes
317/454
• ‘P’ through link can not be regulated
Application of Power Electronics in Power SystemsB. G. Fernandes
318/454
• P1 + P2 can be regulated
• If alternator-1 generates 1000 MW & load 1100 MW
Application of Power Electronics in Power SystemsB. G. Fernandes
319/454
• If alternator-2 generates 1000 MW & load 900 MW
• P1 +P2 has to be -100 MW(frequency of alternator-1 &2 are same)
• P1 + P2 can be set
Application of Power Electronics in Power SystemsB. G. Fernandes
320/454
Types of HVDC system
Two terminal : with DC transmission lineOne rectifier terminal + one inverter terminal
• Two terminals with no DC line ⇒ used forasynchronous link
Back to Back :
Multi terminal : with DC line and several rectifierand/or inverter terminals connected to more thantwo nodes of AC network
Application of Power Electronics in Power SystemsB. G. Fernandes
321/454
Application of Power Electronics in Power SystemsB. G. Fernandes
322/454
Types of links :• Mono-polar
• Bi-polar
Mono-polar HVDC link :
Application of Power Electronics in Power SystemsB. G. Fernandes
323/454
• One conductor (generally –ve)
• Return path ⇒ ground ⇒ Resistance shouldbe low
• Instead metallic return
Application of Power Electronics in Power SystemsB. G. Fernandes
324/454
Bi-polar HVDC link :
• Has two conductors+ve
-ve
Application of Power Electronics in Power SystemsB. G. Fernandes
325/454
• Each terminal has two converters of equalrating ‘V’ connected in series on the DC side
• Junction is grounded
• ‘I’ in two phases are equal
• No ground ‘I’
• Two poles can operate independently
• If one is faulty, then other can operate withground as the return
Application of Power Electronics in Power SystemsB. G. Fernandes
326/454
Review
HVDC
• Asynchronous link
• Back to back
Application of Power Electronics in Power SystemsB. G. Fernandes
327/454
Components of HVDC transmission
Bi-polar HVDC
Application of Power Electronics in Power SystemsB. G. Fernandes
328/454
Converter :
• Perform AC – DC conversionDC – AC conversion
• 12 pulse converter
Transformer with tap changer
Application of Power Electronics in Power SystemsB. G. Fernandes
329/454
Purpose :
• ↓ harmonic voltage & current in DC line
• Prevents ‘I’ from being discontinuous onlight load
• Limit the ‘I’ during S. C in the DC line
Smoothing Reactor : Large value of ‘L’ inSeries with each pole
Application of Power Electronics in Power SystemsB. G. Fernandes
330/454
Harmonic filter :
• Converter generates harmonic currents
• Because of source ‘L’, ‘V’ gets distorted
• Affects the other loads & interferencewith communication network
Application of Power Electronics in Power SystemsB. G. Fernandes
331/454
Reactive power support :
• Both converter & inverter absorbreactive power
• As α ↑ , ‘Q’ requirement ↑
• ‘Q’ source is a must
• If bus is strong, shunt capacitor can be used
• ‘C’ associated AC filter also supply ‘Q’
Application of Power Electronics in Power SystemsB. G. Fernandes
332/454
Basic module of converter :
• 3-ph full bridge
0∠=VVan
120−∠=VVbn
,63 π∠= VVab
240−∠=VVcn
,23 π−∠= VVbc 2103 −∠= VVca
Application of Power Electronics in Power SystemsB. G. Fernandes
333/454
• If α1 is trigger angle for bridge-1
• If α2 is trigger angle for bridge-2
⇒ Neglect idc rdc &Assuming ideal devices
12 απα −=
Application of Power Electronics in Power SystemsB. G. Fernandes
334/454
⇒α = 30o (w.r.t natural commutation)
⇒ corresponding to Z.C ofphase-A α = 60o
or
Application of Power Electronics in Power SystemsB. G. Fernandes
335/454
When T3 is triggered, ‘V’ across T1 = Vab
⇒T1 is turned off at ωt= 30+ (30+120) = 180o
Application of Power Electronics in Power SystemsB. G. Fernandes
336/454
,0180 == SinVa
2360 == SinVb
23−=∴ abV
Application of Power Electronics in Power SystemsB. G. Fernandes
337/454
21210 −== SinVa
190 == SinVb
3−=∴ abV
At ωt = 210o
At ωt = 240o
23,23 =−= ba VV
5.1−=abV
Application of Power Electronics in Power SystemsB. G. Fernandes
338/454
5.1−=∴ abV21,1 =−= ba VV
At ωt = 270o
At ωt = 300o -
0,23 =−= ba VV 23−=∴ abV
At ωt = 300o +, T5 is triggered, ‘V’ across T1 is Vac
23,23 =−= ca VV 3−=∴ acV
Application of Power Electronics in Power SystemsB. G. Fernandes
339/454
5.1−=∴ acV1,21 =−= ca VV
At ωt = 330o
At ωt = 360o
23,0 == ca VV 23−=∴ acV
At ωt = 30o
21,21 == ca VV 0=∴ acV
Application of Power Electronics in Power SystemsB. G. Fernandes
340/454
At ωt = 60o -
0,23 == ca VV 23=∴ acV
⇒ T1 is reverse biased for 210o
What happen when α = 150o
T1 is turned off at ωt = 30+150+120 = 300o
(w.r.t +ve Z.C of Ph- A)
Application of Power Electronics in Power SystemsB. G. Fernandes
341/454
At ωt = 300o
0,23 =−= ba VV 23−=∴ abV
At ωt = 330o
21,21 −=−= ba VV 0=∴ abV
At ωt = 360o
23,0 −== ba VV veVab +==∴ 23
Application of Power Electronics in Power SystemsB. G. Fernandes
342/454
⇒ T2 must attain forward voltage blockingcapability within 30o
Application of Power Electronics in Power SystemsB. G. Fernandes
343/454
αCosVV phdc .34.2=
αCosVLL .35.1=
For α = 30o
Application of Power Electronics in Power SystemsB. G. Fernandes
344/454
Review
HVDC
• Two six pulse convertersconnected in series
12 απα −=
Application of Power Electronics in Power SystemsB. G. Fernandes
345/454
• As α2 ↑, duration for which the devices is reverse biased↓
• When α = 150o, duration for which the devicesis reverse biased = 30o
• As α1 ↑ (AC-DC converter), ‘Q’ requirementalso ↑
Contd..
Application of Power Electronics in Power SystemsB. G. Fernandes
346/454
Harmonic component in converter i/p :
• No even harmonics, only odd harmonics
θθπ
π
π
dCosnIILn .223
30∫
−
=
⎟⎠⎞
⎜⎝⎛=
32.
22
0π
πnSinI
nILn
,.601 IIL π
= 03 =LI
,5
15
LL
II −= 71
7L
LII −=
Application of Power Electronics in Power SystemsB. G. Fernandes
347/454
Phase relationship between phase V & I1
Neglect losses
ϕCosIVIV Lm
dc 10 .2
3 ⎟⎠
⎞⎜⎝
⎛=
00 .336.2
.3 ICosVCosIVm
m απ
ϕπ
=⎟⎠⎞
⎜⎝⎛
αϕαϕ
=∴= CosCos
Application of Power Electronics in Power SystemsB. G. Fernandes
348/454
tdVV ab ωπ
α
α
.26 60
0 ∫+
=
( ) tdtSinV om ωω
π
α
α
.60326 60
+= ∫+
ααπ
CosVCosV dcom ==33
απ
CosVrms233=
αα CosVCosV LLrms 35.134.2 ==
Application of Power Electronics in Power SystemsB. G. Fernandes
349/454
As α ↑ :
• Vdc ↓
• Displacement angle ↑ & P.F ↓
• Q ↑
Effect of source L :
• T1, T2 when conductingT3 is triggered
Application of Power Electronics in Power SystemsB. G. Fernandes
350/454
031 Iii =+
dtdi
dtdi 31 −=
dtdiLV cba
32=
KtCosLVi
c
m +−=∴ ωω23
3
dtdiLtSinV cm
323 =ω
Application of Power Electronics in Power SystemsB. G. Fernandes
351/454
Boundary conditions :
At ωt = α, 0,, 30201 =−== iIiIi
03021 ,,0 IiIii =−==
( )tCosCosLVi
c
m ωαω
−=∴2
33
= α+μ,
At ωt = α+μ, 03 Ii =
( )( )μααω
+−=∴ CosCosLVI
c
m
23
0
Application of Power Electronics in Power SystemsB. G. Fernandes
352/454
22cnbnan
pnVVVV −=
+=∴
dtdiLVV
dtdiLVV
bnpn
anpn
3
1
−=
−=
⎟⎠⎞
⎜⎝⎛ +−+=
dtdi
dtdiLVVV bnanpn
312
Application of Power Electronics in Power SystemsB. G. Fernandes
353/454
mnpn VVV −=∴ 0
cncncn VVV 5.12
−=−−=
Reduction in V0 = (ΔV0) :
( ) tdVVV cnbc ωπ
μα
α
.5.126
0 ∫+
+=Δ
( ) ( ) tdtSinVtSinV mo
m ωπωωπ
μα
α
.25.160326
−++= ∫+
Application of Power Electronics in Power SystemsB. G. Fernandes
354/454
( )( )μααπ
+−= CosCosVm
233
m
cdco
VLIV
32
2 0ω
=
03 ILc
πω
=
0003 ILCosVV c
dc πωα −=∴
Application of Power Electronics in Power SystemsB. G. Fernandes
355/454
Representation of inverter mode of operation in presence of μ
dcdcod IRVV −=− αcos
Application of Power Electronics in Power SystemsB. G. Fernandes
356/454
dcdcod IRVV +−= αcos
dcdco IRV +−= )cos( απ
dcdco IRV += βcos
→α
β
delay angle
Angle of advance→
Application of Power Electronics in Power SystemsB. G. Fernandes
357/454
Converter Inverter
α⇒ delay angle
μ⇒ overlap angle
β = π-α⇒ advance angle
μ⇒ overlap angle
γ = β-μ⇒ extinction angle
γ = π-(α+μ)
Application of Power Electronics in Power SystemsB. G. Fernandes
358/454
)]cos([cos2
μαα +−=Δ dcoo
VV
odcod VVV Δ−=
)]cos([cos2
μαα ++= dcoV
)]cos([cos2
μαα ++= dcod
VVAlso
)]cos()[cos( γπβπ −+−=
Application of Power Electronics in Power SystemsB. G. Fernandes
359/454
)(]cos[cos2
AVdco −−−−−−+= γβ
)]cos([cos2
3 μααω
+−=c
md L
VI
)(]cos[cos2
3 BLV
c
m −−−−−−−−= βγω
)]cos()[cos(2
3 γπβπω
−−−=c
m
LV
Application of Power Electronics in Power SystemsB. G. Fernandes
360/454
m
cmdcod V
LVVV3
33cos ωπ
γ −=
dc
dco ILVπωγ 3cos −=
dcdco IRV −= γcos
m
cd
dco
d
VLI
VV
322cos2 ωγ +=∴
Eq. A+B ⇒
Application of Power Electronics in Power SystemsB. G. Fernandes
361/454
12-pulse converter• Series connection of two 6-pulse converters
3-Φ voltages supplied to onebridge is displaced by 30o
from those applied to 2nd bridge
Application of Power Electronics in Power SystemsB. G. Fernandes
362/454
• DC voltage is doubled
• Harmonic spectrum has improved
12n ± 1 on AC side
12n on DC side
Application of Power Electronics in Power SystemsB. G. Fernandes
363/454
Application of Power Electronics in Power SystemsB. G. Fernandes
364/454
Relation between Ac and DC quantity :With multi phase bridge
bridge
Ldo VTBV ...35.1=∴
Cd XIπ3
Cddod XBICosVVV ..3π
α −==
Corresponding voltage drop :
If ‘Β’ no. of bridges in series
Output
⇒ No load
Application of Power Electronics in Power SystemsB. G. Fernandes
365/454
⎟⎠⎞
⎜⎝⎛−= Cddod XBICosVVπ
α 3..
⎟⎠⎞
⎜⎝⎛−= Cddo XBICosVπ
γ 3..
Application of Power Electronics in Power SystemsB. G. Fernandes
366/454
• Nominal line voltage ⇒ 400 kV
• Maximum line voltage ⇒
Summary of technical data of Padghe
430 kV
• Minimum line voltage ⇒ 380 kV
Total ‘Q’ at both stations ⇒ 800 MVar
⇒ 4*200 MVar
12th harmonic filter ⇒ 2*120 MVar
Application of Power Electronics in Power SystemsB. G. Fernandes
367/454
Power :
• Nominal Power ⇒ 2*750 MW
• 2 hours overload ⇒
• Minimum (single pole) ⇒ 2*75 MW
2*825 MW
• 5 Sec. overload ⇒ 2*1000 MW
24/36 harmonic filter ⇒ 2*80 MVar
Application of Power Electronics in Power SystemsB. G. Fernandes
368/454
Direct voltage :
• Nominal line voltage ⇒ 500 kV
• Maximum line voltage ⇒ 512 kV
• Minimum line voltage ⇒ 488 kV
Application of Power Electronics in Power SystemsB. G. Fernandes
369/454
Direct current :
• Nominal I ⇒ 1500 A
• Maximum I at nominal load ⇒ 1542 A
• Max. I at 2 hour over load ⇒ 1695 A
• Max. I at 5 sec. over load ⇒ 2140 A
Nominal line resistance = 7.5 Ω
Application of Power Electronics in Power SystemsB. G. Fernandes
370/454
Rectifier firing angle :
• Minimum ‘γ’ ⇒ 16o
• Max. ‘γ’ during normal operation ⇒ 18o
• Minimum ‘α’ ⇒ 5o
• Mini. ‘α’ during normal operation ⇒ 12.5o
• Max. ‘α’ during normal operation ⇒ 17.5o
Inverter firing angle :
Application of Power Electronics in Power SystemsB. G. Fernandes
371/454
Basic control :
• DC voltage or I (or power) can be controlled by controlling the internal voltage (Vdcor Cosα)and Vdcoi Cosγ
⇒ Gate control or using tap changing ofconverter transformer
⇒ Gate control is fast
Application of Power Electronics in Power SystemsB. G. Fernandes
372/454
⇒ Tap changing : Slow ( 5-6 sec/step)
⇒ Gate control is used for initial rapidcontrol action
⇒ Followed by tap changing to restore theconverter quantities ( ‘α’ of rectifier & ‘γ’for inverter) to their normal ranges
Application of Power Electronics in Power SystemsB. G. Fernandes
373/454
Basis for selection of control :
Following considerations influences the selectionof control characteristics
• Prevention of large fluctuations of DCcurrent due to variation in AC system
• Maintaining DC voltage near rated value
• Maintaining power factor at the sending &receiving end that are as high as possible
• Prevention of commutation failure in inverter
Application of Power Electronics in Power SystemsB. G. Fernandes
374/454
• Rectifier control ⇒ To prevent largefluctuations in DC current
ciLcr
dcoidcord RRR
CosVCosVI−+
−=
γα
Application of Power Electronics in Power SystemsB. G. Fernandes
375/454
• Denominator is very small
• A small change in Vdcor or Vdcoi cause a largechange in Id
• 25% change either in Vdcor or Vdcoi changes‘id’ by 100%
• If ‘α’ & ‘γ’ are kept constant, Idc can varyover a wide range for small change in i/pAC voltage at either end
Application of Power Electronics in Power SystemsB. G. Fernandes
376/454
• Not acceptable
• Rapid converter control prevents fluctuation of Idc
• For a given power transmitted Vdc profilealong the line should be close to rated values
• It minimizes Id & therefore line loss
Application of Power Electronics in Power SystemsB. G. Fernandes
377/454
• P.F should be as high as possible
• Minimize losses and current rating of equipment in the AC system
• Reduce the voltage drop at the AC terminal as load ↑
• ↓ the cost of reactive power supply to line
Application of Power Electronics in Power SystemsB. G. Fernandes
378/454
• So keep the rated power of the converteras high as possible for a given ‘V’ & ‘I’ ratingof transformer
• P.F depends on ‘α’ & ‘γ’
αmin = 5o (a +ve ‘V’ should appear acrossthe device)
• Normally operate at 15 – 20o, so that Vdcor can be ↑ to control DC power flow
Application of Power Electronics in Power SystemsB. G. Fernandes
379/454
• γ⇒ necessary to maintain a certain minimumextinction angle to avoid commutation failure
• Device should attain forward voltage blocking capability
μ⇒ depends on Id & i/p ‘V’
μβγ −=
= 15o at 50 Hz
Application of Power Electronics in Power SystemsB. G. Fernandes
380/454
Control of HVDC system
Application of Power Electronics in Power SystemsB. G. Fernandes
381/454
ciLcr
dcoidcord RRR
CosVCosVI++
−=
γα
Power at rectifier terminal, Pdr = Vdc .Id
Power at inverter terminal = Vdi .Id
= Pdr-id2 RL
Application of Power Electronics in Power SystemsB. G. Fernandes
382/454
Control characteristics
Ideal characteristics :
• Voltage regulation ¤t regulation
Kept distinct & areassigned to separateterminals
• Under normal operation :
⇒ Rectifier maintains current control (CC) &
⇒ Inverter operates constant extinction angle(CEA)
Application of Power Electronics in Power SystemsB. G. Fernandes
383/454
• Maintains adequate commutation margin
• Vdc ⇒ measured at the rectifier terminals
• Inverter characteristics includes Id.RL drop
( ) dciLdcoid IRRCosVV −+= γ
Application of Power Electronics in Power SystemsB. G. Fernandes
384/454
• Rectifier characteristics can be shiftedhorizontally by adjusting reference currentor current command or current order
• If measured current < current command, controller ↓ α
• Inverter characteristics can be raised orlowered by means of transformer taps
Application of Power Electronics in Power SystemsB. G. Fernandes
385/454
• As taps are changed, CEA regulator quicklyrestores desired γ
• Id changes
• Current regulator of rectifier changes ‘α’and control ‘i’
• Tap changer of rectifier acts to bring ‘α’ inthe desired range (10-20o)
Application of Power Electronics in Power SystemsB. G. Fernandes
386/454
Rectifier firing angle :
• Minimum ‘γ’ ⇒ 16o
• Max. ‘γ’ during normal operation ⇒ 18o
• Minimum ‘α’ ⇒ 5o
• Mini. ‘α’ during normal operation ⇒ 12.5o
• Max. ‘α’ during normal operation ⇒ 17.5o
Inverter firing angle :
Review
Application of Power Electronics in Power SystemsB. G. Fernandes
387/454
Basic control :
• DC voltage or I (or power) can be controlled by controlling the internal voltage (VdcoCosα)and VdcoCosγ
⇒ Gate control or using tap changing ofconverter transformer
⇒ Gate control is fast
Application of Power Electronics in Power SystemsB. G. Fernandes
388/454
⇒ Tap changing : Slow ( 5-6 sec/step)
⇒ Gate control is used for initial rapidcontrol action
⇒ Followed by tap changing to restore theconverter quantities ( ‘α’ of rectifier & ‘γ’for inverter) to their normal ranges
Application of Power Electronics in Power SystemsB. G. Fernandes
389/454
Basis for selection of control :
Following considerations influences the selectionof control characteristics
(a). Prevention of large fluctuations of DCcurrent due to variation in AC system
R ≈ 10 Ω and L =250 mH ⇒ Back to backL =1H ⇒ for long line
τ =20 m.sec ⇒ roughly
Application of Power Electronics in Power SystemsB. G. Fernandes
390/454
(b). Maintaining DC voltage near rated value
(c). Maintaining power factor at the sending &receiving end that are as high as possible
(d). Prevention of commutation failure in inverter
• Simulation study taking line L, R & C inaddition Lfilter is required
Application of Power Electronics in Power SystemsB. G. Fernandes
391/454
• Rectifier control ⇒ To prevent largefluctuations in DC current
ciLcr
dcoidcord RRR
CosVCosVI−+
−=
γα
Application of Power Electronics in Power SystemsB. G. Fernandes
392/454
• γ⇒ necessary to maintain a certain minimumextinction angle to avoid commutation failure
• Device should attain forward voltage blocking capability
μ⇒ depends on Id & i/p ‘V’
μβγ −=
= 15o at 50 Hz
Application of Power Electronics in Power SystemsB. G. Fernandes
393/454
Control of HVDC system
Application of Power Electronics in Power SystemsB. G. Fernandes
394/454
Control characteristics
Ideal characteristics :
• Voltage regulation ¤t regulation
Kept distinct & areassigned to separateterminals
• Under normal operation :
⇒ Rectifier maintains current control (CC) &
⇒ Inverter operates constant extinction angle(CEA)
Application of Power Electronics in Power SystemsB. G. Fernandes
395/454
• Quantities forming the co-ordinates are measured at some common point in the DC line
• Converter terminal can be one such possibility
( ) dciLdcoid IRRCosVV −+= γ
• Has a small –ve slope
Application of Power Electronics in Power SystemsB. G. Fernandes
396/454
• Maintains adequate commutation margin
• Inverter characteristics includes Id.RL drop
Application of Power Electronics in Power SystemsB. G. Fernandes
397/454
• Rectifier characteristics can be shiftedhorizontally by adjusting reference currentor current command or current order
• If measured current < current command, controller ↓ α
• Inverter characteristics can be raised orlowered by means of transformer taps
Application of Power Electronics in Power SystemsB. G. Fernandes
398/454
• As taps are changed, CEA regulator quicklyrestores desired γ
• Id changes
• Current regulator of rectifier changes ‘α’and control ‘i’
• Tap changer of rectifier acts to bring ‘α’ inthe desired range (10-20o)
Application of Power Electronics in Power SystemsB. G. Fernandes
399/454
• Constant current characteristics could be aline parallel to y-axis
• If proportional controller ⇒ slope could be -ve
• Generally current control is given to both the converters
• Ref. current for rectifier > Ref. current forinverter
Application of Power Electronics in Power SystemsB. G. Fernandes
400/454
• Iref(conv) – Iref(inv) = Imargin = +ve
• Assume that power flows in the line to be ↑
• αconv ⇒ takes the value of αmin
• Incase Id approaches Iref(conv), then
⇒ rectifier is working under constant ignition control
⇒ Inverter is working under constant extinction control
Application of Power Electronics in Power SystemsB. G. Fernandes
401/454
• After some time, tap changer changes the tap
⇒ ‘α’ of the converter ↑ to attain its normaloperating value (12- 17o)
Application of Power Electronics in Power SystemsB. G. Fernandes
402/454
Actual characteristics :
• Rectifier maintains constant ‘I’ by changing ‘α’
• ‘α’ can not be < αmin
• Once αmin is reached, no further ↑‘V’ is possible
• Rectifier will operate constant ignition angle(CIA)
Application of Power Electronics in Power SystemsB. G. Fernandes
403/454
• Constant current characteristics may not betruly vertical
⇒ Depends on the current regulator
• Therefore rectifier characteristics has twosegments (AB & FA)
• With proportional control C.C characteristics has – ve slope
Application of Power Electronics in Power SystemsB. G. Fernandes
404/454
( ) dcrorderd IRKKIV +−=
[ ]dorderdco IIKCosV −=∴ α
dcrd IRV +=
( ) dcrd IRKV Δ+−=Δ
( )crd
d RKIV
+−=ΔΔ
∴ ⇒ (with PI it is vertical)
Iord ⇒ current order
Application of Power Electronics in Power SystemsB. G. Fernandes
405/454
• At normal voltage , characteristics is defined by FAB
• At reduced ‘V’, it shifts down ⇒ F1 A1 B1
• CEA characteristics of the inverter intersect at ‘E’ for normal ‘V’ condition
Application of Power Electronics in Power SystemsB. G. Fernandes
406/454
• At reduced ‘V’, it does not intersect F1A1B
• A big reduction in rectifier ‘V’ would cause Id & ‘P’ ↓
⇒ System could shut down
Application of Power Electronics in Power SystemsB. G. Fernandes
407/454
• Inverter Iord < rectifier Iord
Iord(R) – Iord(I) ≈ 0.1 Irated
• In order to avoid the problem, inverter isprovided with current control
Application of Power Electronics in Power SystemsB. G. Fernandes
408/454
• Under normal condition
• Rectifier ⇒ C. C
• Inverter ⇒ CEA
• When i/p ‘V’ ↓⇒ rectifier ‘V’↓
⇒ Operating point E1
• Changes from one mode to another is knownas mode shift
Application of Power Electronics in Power SystemsB. G. Fernandes
409/454
• When inverter is on current control
dcidLddoi
doidcidLd
IRIRVCosVCosVIRIRV
+−=+−=
γγ
( ) dcrdLddord IRIRVIIK +−=−−
With proportional controller
Application of Power Electronics in Power SystemsB. G. Fernandes
410/454
( ) ( ),drefdoi IIKCosV −Δ−=Δ γ
( )ciLdd RRIV −Δ−Δ=
( ) dciLd
d IKRRIV
+−=ΔΔ
( ) dciLd IKRRV +−Δ=Δ
K >1
⇒ Slope is +ve⇒ ↑ Vdor to ↑ id
⇒ ↓ Vdoi to ↑ id
Application of Power Electronics in Power SystemsB. G. Fernandes
411/454
When does change over take place ?
• Current order is given to both the converters
Iref(C) > Iref(I)
Iref(C) > Iref(I) - Imargin ⇒ +ve (assume)
Imargin = 0.1 – 0.15 Irated
Application of Power Electronics in Power SystemsB. G. Fernandes
412/454
• Assume that i/p AC has dipped due to fault,Idc ↓ ,
αconv ⇒ αmin
and with this new value of ‘α’, Idc is ↓
• If Idc < (Iref(C) - Imar), inverter takes over thecurrent control & converter is working underC.I.A, after some time tap changer changes the tap
Application of Power Electronics in Power SystemsB. G. Fernandes
413/454
Review
Rectifier characteristics
Constant current by ‘α’ control
Constant ignition angle control
C.CCan have a –ve slope
Can be parallel to Y-axis
Application of Power Electronics in Power SystemsB. G. Fernandes
414/454
• Current control is given to both converters
But Iref(R) > Iref(I)
Iref(R) - Iref(I) = Imargin ≈ 0.1Irated
• Current control loop of inverter is inactivewhen current ≈ Iref(R)
Contd..
Application of Power Electronics in Power SystemsB. G. Fernandes
415/454
⇒ ‘γ’ should be decreased
• ‘e’ is –ve, ‘K’ is +ve ⇒ Iact should be ↓Iact > (Iref – Imar)
⇒ o/p of PI is zero
⇒ selector switch selects γmin
Application of Power Electronics in Power SystemsB. G. Fernandes
416/454
⇒ ‘γ, should be ↑ , so that Iact ↑, ‘K’ is +ve,o/p of PI starts increasing
• ‘e’ is +ve, Iact < (Iref – Imar)
⇒ Selector switch selects maximum of two inputs
Application of Power Electronics in Power SystemsB. G. Fernandes
417/454
• Due to line fault or during low i/p AC voltagecondition Vdco(R) will drop
⇒ Assume Vdco(R)Cosαmin < Vdco(I)Cosγ
• If there is no current control by the inverter , id will ↓ and eventually becomes zero
Application of Power Electronics in Power SystemsB. G. Fernandes
418/454
• Operate at E' till tap changer changes the tap
What happen If Imar is –ve ?
⇒ Rectifier is trying to control Iref(R)
⇒ Inverter is trying to control Iref(R)+ Imar
• In order to avoid this situation inverter is alsoprovided with current control
Application of Power Electronics in Power SystemsB. G. Fernandes
419/454
Inverter side :
• Id can be ↑ by ↑ ‘γ’
• As γ↑ , Id↑, but rectifier controller tries to ↓ the current (Iref(R) < Iref(I) )
• Since Id is ↑ due to increase in γ ,rectifier controller ↑ α to reduce Id
α⇒ towards 90o
γ⇒ towards 90o
Application of Power Electronics in Power SystemsB. G. Fernandes
420/454
⇒ New operating point could be ‘D'’
⇒ Correct sign to Imar is very important
• Imar should not be too smallbecause there could be measurement error
Application of Power Electronics in Power SystemsB. G. Fernandes
421/454
Mode stabilization :
• Intersection of αmin characteristics of converterand inverter CEA may not be well defined
• There could be multiple crossings
• Instead change the slope of theinverter characteristics near the crossing
Application of Power Electronics in Power SystemsB. G. Fernandes
422/454
Alternative inverter γ control
• Instead of regulating ‘γ’ (CEA)
• Maintain a constant DC voltage at a desiredpoint
• Could be sending end
• Required inverter voltage to maintain the abovevoltage is estimated by computing I.R drop
Application of Power Electronics in Power SystemsB. G. Fernandes
423/454
• ‘V’ profile is flat
• Constant ‘γ’ characteristics has droopingcharacteristics
γ ≈ 18o in voltage control mode
Application of Power Electronics in Power SystemsB. G. Fernandes
424/454
Constant ‘β’ control :
γμβ +=
μ⇒ function of id & Vac
⇒ Choose ‘β’ for worst case
⇒ At low loads additional security againstcommutation failure
⇒ As id ↑, minimum ‘γ’ may be encountered
Application of Power Electronics in Power SystemsB. G. Fernandes
425/454
• Vdcoi Cosβ remains constant
• As id ↑, Vd = VdoiCosβ + (RL+Rci)Id also ↑
Application of Power Electronics in Power SystemsB. G. Fernandes
426/454
• Use either constant Vdc or constant β control
Application of Power Electronics in Power SystemsB. G. Fernandes
427/454
Max. short term current = (1.2 -1.3) Irated
Minimum current limit : if id ↓ below acertain limit due to finite ripple in I,current will become discontinuous
• 12-pulse converter
• 12 times in one cycle current become zero(current interruption)
Current limit
Maximum current limit :
Application of Power Electronics in Power SystemsB. G. Fernandes
428/454
• There could be lightly damped oscillations(smoothing L & line C)
• Over voltage across the device
• Simulation study is required
• Ensure Imin in DC link
Application of Power Electronics in Power SystemsB. G. Fernandes
429/454
Voltage depend current-order limit (VDCOL)
• Under L.V condition it may not be desirableor possible to maintain rated current
• Commutation failure
• At one converter end Vac has ↓
↓∴ αCosVdco
Application of Power Electronics in Power SystemsB. G. Fernandes
430/454
• To maintain the current, voltage at the otherend of the line is adjusted
• Either ‘α’ or γ ↑
• Vac has ↓, ‘Q’ supplied by ‘C’ or filter also ↓
• Above problems can be addressed usingvoltage dependent current order limit
• Reactive power demand ↑
Application of Power Electronics in Power SystemsB. G. Fernandes
431/454
• VDCOL characteristics could be a function ofAC voltage or DC voltage
Application of Power Electronics in Power SystemsB. G. Fernandes
432/454
Review
Rectifier characteristics
Constant current by ‘α’ control
Constant ignition angle control
• Inverter ⇒ Constant extinction angle control
• Current control is given to both converters
Application of Power Electronics in Power SystemsB. G. Fernandes
433/454
But Iref(R) > Iref(I)
Iref(R) - Iref(I) = Imargin ≈ 0.1Irated
• Current control loop of inverter is inactivewhen current ≈ Iref(R)
Contd..
Application of Power Electronics in Power SystemsB. G. Fernandes
434/454
Imar should +ve :
• If Imar is –ve, reversal of power takes place(only academic interest)
Contd..
Application of Power Electronics in Power SystemsB. G. Fernandes
435/454
Mode stabilization :
Contd..
• Intersection is not well defined
⇒ Change the slope Constant Vdc
Constant ‘β’
Application of Power Electronics in Power SystemsB. G. Fernandes
436/454
Contd..
Current limit :
⇒ Imax = (1.2 -1.3) Irated
⇒ Imin ⇒ Should not be allowed to go intodiscontinuous
• There could be lightly damped oscillations
Application of Power Electronics in Power SystemsB. G. Fernandes
437/454
Voltage depend current-order limit (VDCOL)
• Under L.V condition it may not be desirableor possible to maintain rated current
• Commutation failure
• At one converter end Vac has ↓
↓∴ αCosVdco
Application of Power Electronics in Power SystemsB. G. Fernandes
438/454
• To maintain the current, voltage at the otherend of the line is adjusted
• Either ‘α’ or γ ↑
• Vac has ↓, ‘Q’ supplied by ‘C’ or filter also ↓
• Above problems can be addressed usingvoltage dependent current order limit
• Reactive power demand ↑
Application of Power Electronics in Power SystemsB. G. Fernandes
439/454
• VDCOL characteristics could be a function ofAC voltage or DC voltage
Application of Power Electronics in Power SystemsB. G. Fernandes
440/454
Rectifier inverter V-I characteristics
• Power transfer over the line can be controlledby varying Imar
• Signals are transmitted throughtelecommunication lines
• Communication may fail or DC line fault ⇒ Reverse power flow may occur
⇒ Inverter is provided with min. α limit ≈ 95- 110o
Application of Power Electronics in Power SystemsB. G. Fernandes
441/454
Application of Power Electronics in Power SystemsB. G. Fernandes
442/454
Summary of basic control principle :
• HVDC system is basically current control
⇒ To limit over current
⇒ To prevent the system from running down due to fluctuations in AC voltage
Application of Power Electronics in Power SystemsB. G. Fernandes
443/454
Significant aspects of basic control :
Rectifier Current control
‘α’ limit
• In current control mode closed loop regulatorcontrols the firing angle to regulate Id at Iord
• Tap changer control of the converter brings ‘α’within 10-20o
Application of Power Electronics in Power SystemsB. G. Fernandes
444/454
• Inverter is functioned with CEA control and acurrent control
• Inverter control could have constant ‘β’ control
• Under normal operation rectifier is in currentcontrol & inverter is on CEA control mode
• In CEA mode, γ is regulated at around 15o
Application of Power Electronics in Power SystemsB. G. Fernandes
445/454
• If there is a ↓ in AC voltage, ‘α’ of rectifier ⇒ αmin (CIA mode)
• If current falls to a certain limit, inverterwill assume C.C
Valve blocking & by passing :
• If one bridge is to be taken out of service
⇒ Only blocking will not extinguish the currentthat was flowing through the thyristor pair
Application of Power Electronics in Power SystemsB. G. Fernandes
446/454
⇒ Inject AC voltage in the link
⇒ There could be ‘V’ & ‘I’ oscillations due tolightly damped circuit
⇒ Transformer feeding the bridge is also subjectedto DC magnetization
⇒ By pass the bridge when the devices (valves)are blocked
Application of Power Electronics in Power SystemsB. G. Fernandes
447/454
⇒ Achieved using by pass valve and by pass switch
⇒ Assume T2 & T3 are conducting & blockingcommand is given
Application of Power Electronics in Power SystemsB. G. Fernandes
448/454
⇒ Commutation for T2 to T4 is in usual manner
⇒ But incoming device T5 is prevented by nottriggering T5. When T1 get F.B (VAB +ve )trigger T1
⇒ Current by pass pair is shunted by closing S1& open S
Application of Power Electronics in Power SystemsB. G. Fernandes
449/454
⇒ Current is first diverted from S1 to bypass pair
⇒ S1 will generate arc voltage
⇒ Trigger bypass pair
• For energization of blocked bridge
Application of Power Electronics in Power SystemsB. G. Fernandes
450/454
Modern techniques
• HVDC using line commutated converters
• Requires AC voltage for commutation
• DC link is equivalent to a current source
• Requires reactive power
• ‘V’ can reverse but ‘I’ can not reverse
• Devices should be able to block –ve voltage
• Not suitable for weak grid
Application of Power Electronics in Power SystemsB. G. Fernandes
451/454
• Instead use VSI
• ‘I’ could be in phase with ‘Vi’
• Inverter devices are self commutated
Application of Power Electronics in Power SystemsB. G. Fernandes
452/454
• No AC voltage is required for commutation
• Conversion at UPF is possible
• DC link is voltage source
Application of Power Electronics in Power SystemsB. G. Fernandes
453/454
• ‘V’ can not reverse, but ‘I’ can reverse
• Devices should be able to carry ‘I’ inboth directions
Application of Power Electronics in Power SystemsB. G. Fernandes
454/454
Thank you