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SHUNT HYBRID ACTIVE POWER FILTER
USED FOR COMPENSATION OF
HARMONIC CURRENTS WITH
MODIFIED DQ THEORY
G.Spurthi (P.G.Schlor), CH.SeshagiriRao(Asst professor)
Department of Electrical and Electronics Engineering Sreenidhi Institute
of Science and Technology, Hyderabad
E-mail: spurthigadde@gmail.com, chvseshagiri@sreenidhi.edu.in
Abstract
In these days, power quality has become the serious issue. Hybrid Filter is
used for the reduction of harmonics. In this paper, Shunt Hybrid APF’s
are designed to mitigate harmonic components; a passive filter
combination is used which is tuned for reduction of 5th order harmonics. A
3ɸ, 4wire voltage source inverter is taken, which consists of 6 IGBT’s and
a dc capacitor which acts as active filter and used with the combination of
passive filter. DQ theory with modified Phase Locked Loop is designed to
use as a control algorithm. PI controller is used here for the conversion of
dc component of voltage to direct form of current and to maintain dc-link
capacitor voltage regulation. The reference current can be generated
using DQ theory which is given to hysteresis band controller block, where
the gate pulses are generated, that are given to IGBTs in hybrid filter
which produces the compensating currents. The modified PLL technique is
used to suppress the double frequency, present in –ve sequence of non-
ideal voltage source. The results show the value of harmonics in terms of
THD by taking different types of source voltages i.e., balanced source,
unbalanced source, unbalanced distorted source and distorted source by
taking induction motor as load. This technique is performed in MATLAB
SIMULINK.
Keywords: Harmonics, PQ, SHAPF, PLL, PI controller
1. Introduction
In power systems, power quality is the most major notable factor. In
Electrical power systems, among generation, transmission and
distribution, Distribution plays the vital role regarding power quality [1],
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[2]. Because of the existence of nonlinear varying loads the harmonics are
being injected into the voltages and currents waveforms. Harmonics is
one of the sensitive and important factors. Mostly loads which are present
in distribution side are nonlinear loads. For ex in residential sector air
conditioners, washing machines etc.; in industrial sector compressors,
lasers, furnaces etc.; in telecommunication sector chargers, UPS etc.;
which has power electronic components injecting harmonics into the
system which leads to the distortion of the fundamental wave. The other
loads which are connecting at the same point can be affected due to the
presence of harmonics.
Harmonics leads to numerous problems in the system. Harmonics
results in severe effects like overheating of equipment, overheating
of transformers, malfunctioning of switching devices, interference of
telecommunication lines, insulation failure, unnecessary tripping of
circuit breakers etc. so elimination of harmonics should be done to
avoid these type of problems.
In case of single phase transformer third order harmonics are
present while in three phase transformer third, fifth and seventh
order harmonics are present. Because of the presence of these
harmonics there may be malfunction of breakers and insulation
failure due to overheating of equipment. Due to the presence of third
harmonics, neutral currents will also be increased because of the
sharp rise of zero sequence currents.
In the early days for elimination of harmonics, isolation
transformers, dynamic voltage regulators are used. And then after
passive filters are used only for the elimination of particular order of
harmonics and active filters are efficient when compared to passive
filters. To remove the drawbacks of these types of filters, hybrid
filters are used. Hybrid Filters even has a combined design of active
and passive filters. Passive filter is tuned accordingly to mitigation
of 5th order harmonics. Different control techniques have been
evolved such as discrete Fourier Transforms, Fast Fourier
Transform, direct testing and calculation method etc. here we are
using Synchronous Reference Form (SRF) theory with modified
PLL.
Hybrid active filters are installed at the PCC adjusted near to the
load side. The harmonics at load side are eliminated by counter
harmonics produced by active filter which are of same magnitude
but opposite in direction.
The detailed theory of SHAPF is mentioned below sections as
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follows.
2. Shunt hybrid Active Power Filter topology
The preferred shunt hybrid active power filter design is shown in Figure
below. It contains the passive power filter & an active power filter in
shunt combination that creates a hybrid active power filter respectively
and a 3-leg converter with two capacitors serves as the APF and which is
designed to connect in parallel with varying loads, in this case
asynchronous machine (induction motor) load is used in 3-Ø, 4-wire
distribution network. Here all elements which are present inside the
inverter are joined to the system with an inductor i.e. utilized to reduce
the ripples present in the inverter. The passive filter is tuned to reduce 5 th
order harmonic frequency i.e. associated in parallel with the electrical
lines at the loads and also low impedance path for the harmonic currents
is created simultaneously, helps to decrease the rating of the APF.
Figure1. Proposed shunt hybrid active filter
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Figure 2: SRF method based control block diagram
3. Proposed control technique
3.1. Modified Phase Locked Loop (PLL) operation
To extract the clear and detailed values of the maximum voltage
at instant phases, in the unbalanced voltage case, it needs to convert
the 3ɸ unbalanced voltage to the two dimensional stationary axis (αβ
axis). In balanced case, if we assume the converted form of voltage
vectors are plotted on a DQ frame then a round vector is formed
which rotates at the speed equivalent to the frequency. In the same
manner, if the converted unbalance voltage vectors are plotted, an
eclipse formation is created as a result of both positive and negative
sequences present in the vector. Considering the frequency & the
phase details that are previously obtained and by taking an
imaginary frame which rotates in the direction of positive sequence
vector with the speed equivalent to frequency. At that point the
positive sequence vector shows up like stationary and negative
sequence vector rotates at the double speed of the frequency. For
secure and predictable working of APF with the unbalanced and
distorted voltage sources, the frequencies and the phase
information’s of +ve sequence elements of the source voltage must
be achieved rapidly & exactly. Regular Phase Lock Loop of DQ
method working strategy was unable to provide adequate solutions at
unbalanced voltage sources due to the presence of the dynamic
characteristics. The improvised Phase locked loop is created in this
project which can be seen in Figure3. Therefore the working of PLL
is to find the positive sequence elements in not optimal voltage
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sources:
Figure 3: modified PLL block
Vsabc= [
𝑉𝑎𝑉𝑏𝑉𝑐
]=[
𝑉𝑎+
𝑉𝑏+
𝑉𝑐+
]+[
𝑉𝑎−
𝑉𝑏−
𝑉𝑐−]+[
𝑉𝑎0𝑉𝑏0𝑉𝑐0
]………… (1)
=𝑉+ [
𝑠𝑖𝑛𝜃
sin[𝜃 −2𝜋
3]
sin[𝜃 +2𝜋
3]
] + 𝑉− [
𝑠𝑖𝑛𝜃
sin [𝜃 +2𝜋
3]
sin[𝜃 −2𝜋
3]
] + 𝑉0……………… (2)
Considering αβ transforms, the voltage vectors are
𝑉𝛼𝛽 = [𝑉𝛼𝑉𝛽] = [𝑇𝛼𝛽]𝑉𝑎𝑏𝑐…………. (3)
Where
[𝑇𝛼𝛽] = 2/3 [1 −1/2 −1/2
0 √3/2 −√3/2]
So,
𝑉𝛼𝛽 = [𝑉𝛼𝑉𝛽] = [ 𝑉+𝑠𝑖𝑛𝜃+ + 𝑉−𝑠𝑖𝑛𝜃−
−𝑉+𝑐𝑜𝑠𝜃+ + 𝑉−𝑐𝑜𝑠𝜃−]……………… (4)
Performing DQ transform
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𝑉𝑑𝑞 = [𝑉𝑑𝑉𝑞] = [𝑇𝑑𝑞]𝑉𝛼𝛽
=[ 𝑐𝑜𝑠𝜃 𝑠𝑖𝑛𝜃−𝑠𝑖𝑛𝜃 𝑐𝑜𝑠𝜃
] [ 𝑉+𝑠𝑖𝑛𝜃+ + 𝑉−𝑠𝑖𝑛𝜃−
−𝑉+𝑐𝑜𝑠𝜃+ + 𝑉−𝑐𝑜𝑠𝜃−]…………….. (5)
=[𝑉+ sin(𝜃+ − 𝜃) + 𝑉− sin(𝜃− + 𝜃)
−𝑉+ cos(𝜃+ − 𝜃) + 𝑉− cos(𝜃− + 𝜃)]
The approx. phase angle=𝜃;
Phase angle of +ve sequence voltages =𝜃+;
Phase angle of -ve sequence voltages =𝜃−;
Let, 𝜃=ωt, successfully the PLL spots the phase at𝜃 = 𝜃+ = 𝜃−.
So,
[VdVq]=[
𝑉− sin(2𝜃)
−𝑉+ + 𝑉− cos(2𝜃)]……………….(6)
Now the doubled frequency should be removed off, where this is the
main motto of the design of modified phase locked loop. By elimination
of 2θ frequency, it provides the positive sequence elements. The PLL
network utilizes park and Clarke transformation in which 3ɸ unbalanced
voltages is changed over the synchronous rotating frame voltages to
recognize the maximum voltage of +ve sequence. Then the second order
resonant filters are utilized rather than the respected low pass filter to
eliminate the double(2θ) frequency which are produced due to unbalance
of the system. The rate limiter serves to eliminate the ripples present in
the voltage.
3.2. Generation of reference Currents
In active power filter, the load currents used to be estimated by the use of
the Hall Effect current sensor and transformed to dq0 with the help of
rotational frames that are constant with the positive sequence voltages in
the system. Currents are
[
𝑖𝑑𝑖𝑞𝑖0
] = 2/3 [
sin(𝜔𝑠𝑡) sin(𝜔𝑠 𝑡 − 2𝜋/3) sin(𝜔𝑠 𝑡 + 2𝜋/3)cos(𝜔𝑠 𝑡) cos(𝜔𝑠 𝑡 − 2𝜋/3) cos(𝜔𝑠𝑡 + 2𝜋/3)
1/√2 1/√2 1/√2
] ×
[𝐼𝑙𝑎𝐼𝑙𝑏𝐼𝑙𝑐
]……(7)
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𝜔𝑠𝑡 Is assumed as angle of +ve sequence voltage source and this is
output of PLL.
The observation here is with the help of PLL output the currents are
converted into id and iq form. Then the values of id and iq load
currents are permitted to supply through the LPF to isolate the
alternate & direct current parts thus the LPF allows only DC
components and stops AC components as per its nature and in this
currents the AC parts are responsible for harmonics so, Later the AC
harmonic components will remains as an output because of the
difference operation (subtraction) between the output of LPF and id-
iq currents. The both idAC and iqAC currents that have been filtered
from the below equations are utilized in the production of the exact
reference currents which given as input to the hysteresis loop
controller. The DC components of the currents are removed by the
LPF to produce the harmonic elements which to be referred.
[𝑖𝑑𝐴𝐶𝑖𝑞𝐴𝐶
] = [𝑖𝑑𝑖𝑞] − [
𝑖𝑑𝐷𝐶𝑖𝑞𝐷𝐶
]………….. (8)
The abc reference currents:
[𝑖𝑎𝑟𝑖𝑏𝑟𝑖𝑐𝑟
] = [
sin(𝜔𝑠 𝑡) cos(𝜔𝑠𝑡)sin(𝜔𝑠 𝑡 − 2𝜋/3) cos(𝜔𝑠𝑡 −2𝜋/3)sin(𝜔𝑠𝑡 + 2𝜋/3) cos(𝜔𝑠𝑡 + 2𝜋/3)
] [𝑖𝑑𝐴𝐶𝑖𝑞𝐷𝐶
]………….. (9)
3.3 DC link capacitor voltage control:
In the SAPF, the value of DC link capacitor voltage can be detected by
the senor known as Hall Effect voltage sensor then compares with the
reference DC voltage.
Fig 4: DC link voltage control loop
The DC link capacitor voltage equation is
𝐶𝑑𝑉𝑑𝑐
𝑑𝑡= 𝐼𝑑𝑐………… (10)
Where 𝐼𝑑𝑐is dc-link current
Using LT
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𝐶𝑆𝑉𝑑𝑐(𝑠) = 𝐼𝑑𝑐(𝑠)………… (11)
So
G(S) =𝑉𝑑𝑐(𝑠)
𝐼𝑑𝑐(𝑠)=
1
𝑐𝑠…………. (12)
The output which is the error of the voltage at the nth sampling
moment, the error = 𝑉𝑑𝑐𝑟𝑒𝑓 − 𝑉𝑑𝑐 is fed through to PI controller by
means of this transfer function to create the necessary reference
current.
3.4. Harmonic current generator
Generally, hysteresis comparators are utilized for the fast operation and
proper sinusoidal waveforms tracing ability. However the reference
current𝐼𝑎𝑟𝑒𝑓, and the compensating current 𝐼𝑎𝑐𝑜𝑚are compared and
analyzed. The switching concept is mentioned below:
If the current𝐼𝑎𝑟𝑒𝑓 ≺ (𝐼𝑎𝑐𝑜𝑚) hysteresis band first S1 is OFF and
then S6 is ON of leg (a) in APF.
If the current𝐼𝑎𝑟𝑒𝑓 ≻ (𝐼𝑎𝑐𝑜𝑚) hysteresis band first S1 switch is ON
and then S6 switch is OFF of leg (a) in Active power filter.
Similarly, in leg (b) and (c) the switching process is used with the
help of using hysteresis band controller. The hysteresis bands are
planned by considering the limits of switching frequency of device.
4. Result Analysis
In MATLAB Simulink, the utilization of the 3ɸ, 4wire power system
network blocks with using the shunt hybrid active power filter
combination of passive power filter is done. The induction motor
draws high starting currents which are 6to8 times of rated current
and also reduces within seconds to normal values. The values of
simulation outputs are achieved for the four cases balanced,
unbalanced, distorted and unbalanced distorted sources in the system
network. The utilized parameter values of SHAPF are shown below.
4.1. Balanced voltage source condition
By the consideration of proposed control technique under balanced
voltage source with induction load, it gives effective outputs for
compensation of harmonic currents present in load current. In this case it
reduces the starting high currents of induction motor in source current of
the power system by providing compensation currents and make source
currents in sinusoidal waveform with below 5% of THD according to the
IEEE format as shown in below figures waveforms of Vs, IL,Is and
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THDs of IL and Is.
Fig 5: After injection of compensating currents
a)Voltage source b)Load current c) source current
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Fig 6: a) THD of load current=13.07% b)THD of source
current=2.64%
Fig 7: DC voltage
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4.2. Unbalanced source condition
In unbalanced source case the phases of voltage are uneven,
magnitude of phases are not equal. So the control technique under
this condition is also effective and output waveforms and THDs are
shown below.
.
Fig 8: a) Voltage source, b) Load current, c) source current.
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Fig 9:THD of load current=20.54% THD of source
current=1.31%
Fig10 :DC Voltage
4.3. Distorted source condition
In this case we are injecting 5 th order harmonics in the source
voltage for distortions and observing weather control technique is
satisfactory working or not, the output waveforms and THD are
shown in below figures.
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Fig 11: a) Voltage source, b) Load current, c) Current source.
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Fig 12: THD of load currents=24.40% THD of source
current=1.30%
Fig 13: DC voltage
4.4. Unbalanced distorted source condition
In this case both unbalanced and distortions are present the output
waveforms and THDs are shown below.
Fig 14: a) Voltage Source, b) Load current, c) Current source
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Fig 15: THD .of load current=32.42% THD of source
current=3.83%
Fig 16: DC voltage
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Table 1.Comparison of THD analysis
Algorith
m
Balanced Unbalanced distorted Unbalanced
distorted
THD(R
L-load)
3.82% 3.16% 3.32% 3.85%
THD(in
duction
motor)
2.64% 1.31% 1.30% 3.83%
Table 2.Simulation parameters
Parameters Values
Source voltage 415v, 50hz
Shunt APF R=1ohm, L=2mh, two DC-link
capacitors=2000UF,
Vdc=700v
Passive power
filter
Fifth harmonic tuned,
R=0.01ohm,L=5mH, C=80UF
Induction motor 5.4HP(4KW),400v,50Hz,
1430rpm
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5. Conclusions
In this paper the main aspect is to improve power quality of a
system with SHAPF. It is achieved by using SRF strategy and a
modified PLL is created which is successfully utilized as per the grid
voltage synchronization by fulfilling all the four conditions. The
Total Harmonic Distortion values of four different conditions are
found below 5%. In this way, to achieve high quality harmonics
compensation and reliable reduction of harmonics with quick action,
the modified SRF method is used.
6. References
1. H.Akagi,(1996), “New trends in active filters for power
conditioning”, IEEE Transactions on Industry Application, Vol.32,
pp1312- 1322
2. S. K. Jain and P. Agarwal, "Design Simulation and Experimental
Investigations, on a Shunt Active Power Filter for Harmonics, and
Reactive Power Compensation," Electric Power Components and
Systems, vol. 31,pp.671-692, 2003
3. Ambrish Chandra, Bhim Singh, B. N. Singh, and Kamal Al-
Haddad,(20000), “An Improved Control Algorithm of Shunt Active
Filter for Voltage Regulation, Harmonic Elimination, Power-Factor
Correction, and Balancing of Nonlinear Loads”Vol.15,No.3,pp.495-
507
4. A. F. Zobaa, "Letter to the Editor: Optimal Sizing of the Passive Filter's
Elements in Hybrid Active Filters," Electric Power Components and
Systems, vol. 35, pp. 483-488, 2007.
5. S. S. Patnaik and A. K. Panda, "Real-time performance analysis and
comparison of various control schemes for particle swarm optimization-
based shunt active power filters," International Journal of Electrical
Power &Energy Systems, vol. 52 pp. 185-197, 2013.
6. Kesler, M., Ozdemir, E.: ‘Synchronous-reference-frame-based control
method for UPQC under unbalanced and distorted load conditions’,
IEEE Trans. Ind. Electron., 2011, 58, pp. 3967–3975
7. Sinha, R.K., Sensarma, P.: ‘A pre-filter based PLL for three-phase grid
connected applications’, Electric Power Syst. Res., 2011, 81, pp. 129–
137
8. Salmeron, P., Litrán, S.P.: ‘A control strategy for hybrid power filter to
compensate four-wires three-phase systems’, IEEE Trans. Power
Electron., 2010, 25, pp. 1923–1931
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9. A. Nasiri, A. E. Amac, and A. Emadi, "Series-Parallel Active
Filter/Uninterruptible Power Supply System," Electric Power
Components and Systems, vol.32, pp. 1151-1163, 2004.
10. De, D., Ramanarayanan, V.: ‘An active shunt compensator for reactive,
unbalanced and harmonic loads under balanced and unbalanced grid
voltage conditions’. 2010 Joint Int. Conf. on Power Electronics, Drives
and Energy Systems (PEDES) & 2010 Power India, 2010, pp. 1–6
11. Lam, C.-S., Wong, M.-C., Han, Y.-D.: ‘Hysteresis current control of
hybrid active power filters’, IET Power Electron. 2012, 5, pp. 1175–
1187
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