Switching Capacitor bank Back-to-Back to Underground …grouper.ieee.org/groups/td/cap/AaronKalyuzhnySwitchingCapacitors... · Switching Capacitor bank Back-to-Back to ... Switching

New Energy Horizons

Switching Capacitor bank Back-to-Back to Underground CablesAaron Kalyuzhny

New Energy Horizons Opportunities and

Challenges

Undergrounding power lines- the great challenge

The Israel Electric Corporation Ltd., Haifa, Israel

Calculation of 1. short-circuit currents. To be revised!

1 1

2. temporary overvoltages.

3. electromagnetic transients

New Energy Horizons


The Israel Electric Corporation Ltd., Haifa, Israel New Energy Horizons Opportunities and

Challenges


Undergrounding power lines

Step-down transformer

n underground cablesStep-down transformer

n underground cables

and the capacitive current switching

transformer

Equivalent V lt

~

yste

m

transformer

Equivalent V lt

~

yste

m

Voltage Source

ibut

ion

syVoltage Source

ibut

ion

sy

A B C

Dis

tri

A B C

Dis

tri

2

A B C

Single Capacitor Bank

A B C

Single Capacitor Bank

New Energy Horizons



Challenges


Types of the capacitive switching

Duty for CBs

Inrush current rate

of rise

Inrush current

peak

Type

Light highVery low

Switching lines/cables

li htllS it hi lightlowlowSwitching single-step

capacitor bankV hhi hhi hB k t b k Very heavyhighhighBack-to-back

capacitor bank

3

New Energy Horizons



Challenges


Traditional approach to switching isolated capacitor bankB b

~LS

Vmaxcos(t)

Bus-bar

Inrush current of isolatedmax ( )

0 0,/peak

V VdiId L

isolated capacitor bank

C,

/peakSS dt LL C

Ipeak = 1.6 kA, di/dt = 2.7 kA/ms

Field tests of isolated 24 kV capacitor bank:I = 7 kA di/dt = 3300 kA/ms

4

Ipeak = 7 kA, di/dt = 3300 kA/ms

New Energy Horizons



Challenges


Di t f b b [k ]V lt h t th

Suggestion: Inrush current is a sum of current surges in the cables

V,pe

ak] 0 0.1 0.2 0.3 0.4 0.5

Distance from busbars [km]

Incidentak]

20

Voltage change at the MV busbars

Feeder

cable

10

Volta

ge [k

V,

voltage

wave (Vin)

Ti [ ]

[kV,

pe 10

1 2 3 4 5 20

-10

VDistance from busbars [km]

0 0.1 0.2 0.3 0.4 0.5

eak]

I id t

Time [sec]Feeder

cable

20

10

olta

ge [k

V,pe Incident

voltage

wave (Vin)MV b sbars

5

Vo

inMV busbars

New Energy Horizons



Challenges


Suggestion: Inrush current is a sum of current surges in the cables

Feeder

cableTime[microsec]

eak] 1 2 3 4 5

eak]


Incident0 0.1 0.2 0.3 0.4 0.5

rren

t [kA

,pe

1.5

1

0.5

I(t)= Iin

rren

t [kA

,p Incident

current

wave (Iin)1

0.5 Iin=Vin/ZW

Cur

2

Capacitor inrush current Feeder

cable

Cur 1

k] 0 0.1 0.2 0.3 0.4 0.5


I(t)

cable

ent [

kA,p

eak

Incident

current

wave (I )

0.5 Iin=Vin/ZW

6MV busbars

Cur

re wave (Iin)1

New Energy Horizons



Challenges


Since measured inrush currents significantly exceed the "inrush current of isolated capacitor bank", their detailed research is necessary for selection of the capacitor bank

Main goals of the research:

necessary for selection of the capacitor bank switchgear.

Main goals of the research:

1. Analytical study of the phenomenon

2. Case study of switching 24 kV capacitor banks

7

New Energy Horizons



Challenges


Analytical study

LS RS

cable

ZCWLS RS

cable

ZCW

ZLW~ Vmcost L Cable

t i ti ZLW~ Vmcost L Cable

t i ti ZLW

Ctermination impedance

ZLW

Ctermination impedance

Single phase circuit to analyze the capacitor bank inrush current

8

New Energy Horizons



Challenges


Analytical study

LS RS ZCWLS RS ZCW

L

CW

ZLWLL

CW

ZLWL

LW

-V0 -V0C

LW

-V0 C -V0

Inrush current through the system impedance

Inrush current due to traveling waves in cables

Inrush current through Inrush current due to traveling waves in cables

9

New Energy Horizons



Challenges


Analytical study: inrush current of isolated capacitor bank

LS RS

L

R

C-V0

2( ) sinS

S

R tL

tr peaki t I e t

100 1,/peak

S S

VIL C L C

where

New Energy Horizons



Challenges


Analytical study: inrush current due to the traveling waves

General case: underground cables have unequal lengths

ZCW

LInrush current due to the initial

t

C -V0

current surges

02( )

1CW

V Ci pLCp Z Cp

Inrush current in the Laplace domain

11

1CWLCp Z Cp

New Energy Horizons



Challenges


Analytical study: closed form-expressions of current waveforms

Damping reactor is not applied Damping reactor is applied

2CW WZ Z 2CW WZ Z

( ) 2 sinhti t Ie t ( ) 2 sinti t Ie t 2 2

CW W

where2 2

0

2 2

4, ,

2 24

CW WCW

CW W

Z ZV ZIL LZ Z

WLZC

CCW

ZZn

Characteristic impedance of the

i it f d b

Equivalent surge impedance of n underground

12

circuit formed by inductance L and capacitance C

cables

New Energy Horizons



Challenges


Analytical study: inrush current peak2I

2 22 2

2

peakII

1 ln2

t

is reached when

Limiting case of non-oscillatory Limiting case of oscillatory inrushLimiting case of non-oscillatory inrush current : 0L

Limiting case of oscillatory inrush current :

CZLC n

0peakVI nZ 0Peak

CI VL

13

CZ L

New Energy Horizons



Challenges


Analytical study: maximum value of di/dt

0Vdidt L

dt L

The maximum rate of rise of the inrush current is limited only by the equivalent inductance L. If L includes only stray inductance (damping reactor is not applied) the maximum value of di/dt can be extremely highvalue of di/dt can be extremely high.

14

New Energy Horizons



Challenges


Analytical study: inrush current due to the traveling waves

Particular case: n underground cables have equal length

ZCW

Inrush current due to superposition of the initial current surges and

ZCW

ZLWL

initial current surges and reflected current surges C -V0

02

1

( )1

[( 1)( 2)]

SCW

k k

V Ci pLCp Z Cp

Inrush current in the Laplace domain

15

1

1

[( 1)( 2)]1 4[( 1)( 2)]

k kL

kk

a p p p pp p p p

New Energy Horizons



Challenges


Analytical study: closed form-expression of current waveforms when the initial waves and the first 3 reflected waves are consideredwhen the initial waves and the first 3 reflected waves are considered

1 1 2 32 3( ) ( ) ( )( ) ( ) S i ti i iti tt t( ) 2 sinh ti t Ie t ( ) 2 sinhi t Ie t

11 1 11 1 12 1 1 12 1 1( ) 2 [ sinh( ) sinh( ) cosh( )] ti t Ie a t a t t a t t

( ) i h( ) i h( ) h( )t 22 2 21 2 22 2 2 21 2 2

2 2 2 223 2 2 24 2 2

( ) 2 [ sinh( ) sinh( ) cosh( )

sinh( ) cosh( )]

ti t Ie a t a t t a t t

a t t a t t

t 3

3 3 31 3 32 3 3 31 3 32 2 2 2 3 3

33 3 3 34 3 3 35 3 33 3

( ) 2 [ sinh( ) sinh( ) cosh( )

sinh( ) cosh( ) sinh( )

h( )]

ti t Ie a t a t t a t t

a t t a t t a t t

t t

16

3 336 3 3cosh( )]a t t

New Energy Horizons



Challenges


Case study: the circuit descriptionn feedersT f f 161/24 kV n feedersTransformer of 161/24 kV,

S= 45 MVA, ek =18%

ZC lC ZL lL

Infinite busbar C C ZL lL

QL Single-core,

coaxial cables, XLPE copper

Overhead line ACSR 150/2524 kV

XLPE copper 300/25 cross-section

ACSR 150/25Types of capacitors:

1. Isolated 9MVAR capacitor bank

2 M lti t 3 5 MVAR it b k ( it hi th 1st t )

busbar

17

2. Multi-step 35 MVAR capacitor bank (switching the 1st step)

New Energy Horizons



Challenges


Case study: results of the field tests versus EMTP simulation

S it hi 9 MVAR 24 kV

Measured peak inrush currents

6.7 kAResult of EMTP simulationSwitching 9 MVAR, 24 kV

capacitor bank

Peak inrush currents[kA, peak]

TestNumber

Maximum di/dt = 3600 kA/ms

ITISIR5.05.04.817.14.65.727.24.55.636.63.26.14

Maximum measured

18

Maximum measured

di/dt = 3300 kA/ms

New Energy Horizons



Challenges


Case study: Parametric study of the inrush current peak

Dependence on the number of underground cables

10Ipeak versus number of cables

L 5 H9 MVAR10

Ipeak versus number of cables

L 5 H9 MVAR V

6

8

k [k

A,p

eak]

L= 5 µH9 MVAR

5 MVAR

6

8

k [k

A,p

eak]

L= 5 µH9 MVAR

5 MVAR

0peakC

VI nZ

(proportional to n)

4

6

curr

ent p

eak L= 100 µH

L= 1000 µH

9 MVAR

5 MVAR9 MVAR4

6

curr

ent p

eak L= 100 µH

L= 1000 µH

9 MVAR

5 MVAR9 MVAR

2 4 6 8 10 120

2

Inru

sh µ

5 MVAR

2 4 6 8 10 120

2

Inru

sh µ

5 MVAR

0PeakCI VL

(practically independent of n)

19Number of underground cablesNumber of underground cables

(practically independent of n)

New Energy Horizons



Challenges



D d th i l t i d t 8Dependence on the equivalent inductance, n =8

7Ipeak versus inductance

k]

7Ipeak versus inductance

k]

6

ak [k

A, p

eak

6

ak [k

A, p

eak

Increase of the equivalent

4

5

curr

ent p

ea 9 MVAR

5 MVAR4

5

curr

ent p

ea 9 MVAR

5 MVAR

inductance L leads to reduction of the inrush current peak Ipeak

0 50 100 150 200 2503

4

Inru

sh 5 MVAR

0 50 100 150 200 2503

4

Inru

sh 5 MVAR

20

0 50 100 150 200 250

Total inductance[microhenry]

0 50 100 150 200 250

Total inductance[microhenry]

New Energy Horizons



Challenges


Parametric study of Ipeak – Concept of the minimal length of cable

lMinimal length of cable lmin as the shortest cable length among n cables that makes it possible for the inrush current to reach its peak value Ipeakbefore the reflected wave arrives at the substation busbar

F th b k ith t d i t F th b k ith d i tFor the banks without damping reactor

9 MVAR

1.6lmin versus L

e [k

m]

9 MVAR

1.6lmin versus L

e [k

m]

For the banks with damping reactor

9 MVAR10

12lmin versus L

e [k

m]

9 MVAR10

12lmin versus L

e [k

m]

9 MVAR

5 MVAR1

1.3

engt

h of

cab

le 9 MVAR

5 MVAR1

1.3

engt

h of

cab

le

5 MVAR6

8

10

leng

th o

f cab

le5 MVAR6

8

10

leng

th o

f cab

le

5 7 9 11 13 150.4

0.7

min

imal

le

5 7 9 11 13 150.4

0.7

min

imal

le

50 100 150 200 2502

4

min

imal

50 100 150 200 2502

4

min

imal

21

5 7 9 11 13 15

Stray inductance [microhenry]

5 7 9 11 13 15

Stray inductance [microhenry]Equivalent inductance [microhenry]Equivalent inductance [microhenry]

New Energy Horizons



Challenges


Case study: n underground cables have equal lengthVerification of the derived analytical expressions: lc= 1kmVerification of the derived analytical expressions: lc= 1km,

Q=9 MVAR, L = 5H, n =8

8Inrush current waveform

4

8

[kA

,pea

k]

Numerical

Very good agreement between the analytical waveforms and the EMTP waveforms enables to

4

0

sh c

urre

nt [ Numerical

waveformthe EMTP waveforms enables to accept the hypothesis that the inrush currents occur due to the traveling waves in the alongside

0 10 20 30 408

4

Inru

s

Analytical waveformunderground cables!

22

Time [microsecond]

New Energy Horizons



Challenges


Case study: n underground cables have equal lengthVerification of the derived analytical expressions: lc= 5 kmVerification of the derived analytical expressions: lc= 5 km,

Q=9 MVAR, L = 5H, n =8

For n identical long cables the8Inrush current waveform

7 0 kA8

Inrush current waveform

7 0 kA For n identical long cables the maximum value of the inrush current is always achieved at the first peak of the current 2

4

6

kA, p

eak]

6.4 kA7.0 kA

EMTP 2

4

6

kA, p

eak]

6.4 kA7.0 kA

EMTP waveform.

The following crest values of the current waveform are

4

2

0

2

sh cu

rren

t [k waveform

4

2

0

2

sh cu

rren

t [k waveform

smaller than the 1st peak because of the attenuation of the traveling waves.

0 50 100 150 2008

6

4

Inru

s

Analytical waveform

0 50 100 150 2008

6

4

Inru

s

Analytical waveform

23Time [mic rosecond]Time [mic rosecond]

New Energy Horizons



Challenges



Q 9 L 8Dependence on the cable length: Q=9 MVAR, L = 5H, n =8

Variation of the cable length from 0 to lmin results in 10

Maximum crest value versus lc

k]

increase of the maximum current peak from zero to the inrush current peak Ipeak.

6

8

eak

[kA

,pea

k

Imax = 6.4 kA

The following growth of the cable length does not cause increase of the maximum current peak because of the

4

6

um c

urre

nt p

l i 0 7k current peak because of the attenuation of the traveling waves in the cables.

0 0.2 0.4 0.6 0.80

2

Max

imu lmin = 0.7km

24Cable length [km]

New Energy Horizons



Challenges


Case study: Undergrounding the distribution power lines and the inrush current

Total inrush currentTotal inrush current

Inrush current through the transformer

Inrush current through the transformertransformer

Capacitor bank switching back-to back to 8 underground cables

Capacitor bank switching back-to back to 8 overhead feeders

25

New Energy Horizons



Challenges


Switching 170 kV, 200 MVAR capacitor bank back-to-back to underground cables (damping reactor is not applied)underground cables (damping reactor is not applied)

di/dtIpeakCalculation method[kA/ms][kA, peak]

206.8“inrush current of isolated capacitor bank”capacitor bank

900032Switching back-to-back to 6 underground cables

26

New Energy Horizons



Challenges


Operational experience: Analysis of failures of 24 kV, minimum-oil circuit breakers used for switching isolated capacitor banksg p

Handling capacitor inrush currents (Ipeak = 4-6 kA, di/dt = 1000-3000 kA/ms)

Quick oil deterioration; problems with CB mechanical system y

Restrikes during opening CB, Inrush g p g ,currents having Ipeak = 8-12 kA

27

Explosions of CB breaking unit accompanied by a short circuit on the

substation busbar

New Energy Horizons



Challenges


Conclusions

1. Since the inrush current during the capacitor bank switching back-to-back to the underground cablesswitching back to back to the underground cables significantly exceeds the "inrush current of isolated capacitor bank" it should taken into account in the selection of the capacitor bank switchgear.p g

2. It seems that switching capacitor bank back-to-back to the underground cables should be addressed in the standards concerning the capacitive current switching

28

Documents

Switching Capacitor bank Back-to-Back to Underground …grouper.ieee.org/groups/td/cap/AaronKalyuzhnySwitchingCapacitors... · Switching Capacitor bank Back-to-Back to ... Switching