Zhihan Xu, Ilia Voloh, Terrence Smith

GE Digital Energy

Principles and applications of series capacitor banks and shunt reactors

Effects of shunt reactors on power systems under normal and fault conditions

Effects of series capacitor banks on power systems – Voltage/current reversion, sub-harmonic transient

Line differential relay application, solution, configuration, communication, and setting recommendation

Reduce voltage increase during light load

Reduce overvoltage on healthy phases during SLG fault

Absorb reactive power

Prevent overvoltage caused by generator self-excitation on capacitive loads

Prevent over-excitation of power transformers caused by overvoltage

Suppress secondary arc with a neutral reactor

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.2

0.4

0.6

0.8

1

1.2

Line Length (pu)

Vo

lta

ge

alo

ng

Lin

e (

pu

)

No Load

SIL

VARabsorbed

=2VARproduced

3-phase short circuit at receiving

0 0.2 0.4 0.6 0.8 11

1.05

1.1

1.15

Line Length (pu)

Vo

lta

ge

alo

ng

Lin

e (

pu

)

No Compensation

With Compensation

Voltage Profile

w/o compensation

Comparison if

compensation applied

Unlikely to experience core saturation compared with transformers under the same overvoltage

Under saturation, the third and fifth harmonics are dominant, but have very small effects and no practical impairment on the performance of protection

Reactor (Red)

V-I Curve

3rd

Fundamental

5th

7th

9th

Voltage

Magnetizing Current

Transformer (Blue)

Harmonics Quantities

Magnetic core reactors is prone to inrush currents during switching-in

Experience core saturation

Generate slightly distorted current, but with a significant dc component

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26

-5

-4

-3

-2

-1

0

1

2

3

Time (s)

Cu

rre

nt

(pu

)

A

B

C

DC offset takes several seconds to decay because of the quite large time constant (higher X/R ratio)

Magnitude of decaying dc component is most influenced by the angle of the voltage when the reactor is energized

Unbalanced three-phase currents result in the zero sequence current in the neutral

Transient can be avoided if three circuit breaker poles are precisely closed at three consecutive phase voltage peaks

After a three-pole tripping, healthy phase(s) of shunt reactor starts resonance with the line capacitance.

0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

-1.5

-1

-0.5

0

0.5

1

1.5

Time (s)

Voltage (

pu)

A

B

C

Resonance frequency is less than the system frequency, can be approximated by

1000

ηffR =

After clearing the fault, the currents in the healthy phases would start oscillating;

Oscillating has the consistent pattern with voltages in terms of frequency and magnitude.

0.1 0.15 0.2 0.25 0.3 0.35 0.4-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Time (s)

Cu

rrent

(pu

)

A

B

C

Due to larger value of the zero-sequence reactor reactance, only absorb negligible amount of the zero-sequence current

Absorb more in the weak system condition

Absorb less if the neutral reactor is connected

)3/(1 _1_0 NLL XXY +=LL XY _1_0 /1=

Improve power transfer capacity

Improve angular stability

Dynamically adjust voltage profile, depending on the load current

The occurrence of voltage inversion depends on the fault location; if the fault point is far away from the capacitor, there may not be a voltage inversion

If there is a voltage inversion, the bus side voltage (VR) has 180 degree angle difference from the source voltage (ET)

If there is a voltage inversion, the line side voltage (VQ) has no inversion

ES VS F VQ X VT ET

VR

0.1 0.15 0.2 0.25 0.3

-1.5

-1

-0.5

0

0.5

1

1.5

Time (s)

Voltage (

pu)

A

B

C

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4-200

-150

-100

-50

0

50

100

150

200

X: 0.3731

Y: 93.2

Time (s)

Angle

(degre

e)

X: 0.08424

Y: -87.12

The occurrence of current inversion depends on the fault location and the total backward reactance

Current inversion is rare for most system configurations

If there is a current inversion, the line side voltage (VQ) would have voltage inversion as well

0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24-30

-20

-10

0

10

20

30

Time (s)

Curr

ent

(kA

)

A

B

C

0.05 0.1 0.15 0.2 0.25 0.3-100

-50

0

50

100

150

200

X: 0.2753

Y: -86.59

Time (s)

Angle

(degre

e)

X: 0.2753

Y: 87.81

With Inversion

Without Inversion

The frequency of sub-harmonic frequency transient current is normally less than the power frequency

The decaying time constant is two times the time constant of fundamental component

The magnitude depends on the source parameters, line parameters, capacitor compensation degree and fault inception angle

The zero fault inception angle would result in the largest transient current

Differential current is equal to the charging currentin the normal operating condition.

Include reactor in the protection zone:Differential current decreases to the uncompensated charging current when switching reactor in

Differential current is the full charging current when switching reactor out

Exclude reactor from the protection zone:Differential current in the normal condition is always the charging current

A charging current compensation method can be always applied

Include reactor in the protection zone:A fault in the reactor, especially closed to the line side of the reactor, may result in 87L operation

Exclude reactor from the protection zone:87L cannot see any increase of differential current

Reactor protection would trip its breaker only

0.18 0.2 0.22 0.24 0.26 0.28 0.3 0.320

0.5

1

1.5

2

2.5

Diffe

rential C

urrent (k

A)

Reactor Included

0.18 0.2 0.22 0.24 0.26 0.28 0.3 0.320.3

0.35

0.4

0.45

0.5

Time (s)

Diffe

rential C

urrent (k

A)

Reactor Excluded

Fault Inception

Reactor CB Tripped

Line CB Tripped by 87L

Include reactor in the protection zone:Slightly increase ground differential current because of unbalanced inrush current

Exclude reactor from the protection zone:No effect on 87L

Voltage inversion slightly affects current charging compensation

Occurrence of current inversion depends on the fault location

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-100

-80

-60

-40

-20

0

20

40

60

80

100

Fault Location (pu)

Cu

rrent

An

gle

s (

de

g)

Angle at sending

Angle at receiving

S C R

SIR=0.1 Compensation Degree:

50%

Current inversion may cause 87LP fail to trip for an internal fault because the inversed current appears to be through current

0 1 2 3 4 5 6 7 8 9 100

1

2

3

4

5

6

7

8

9

10

Restraint Current (pu)

Dif

fere

nti

al

Cu

rre

nt

(pu

)

SIR=0

SIR=0.05

SIR=0.1

SIR=0.25

Operating Zone

Restraint Zone

Adjust the boundary settings in the percentage plane to avoid failure to operate during the current inversion

Faults located in 0-25% of line section

Assume MOV is not conducted

87LG is more complicated to be analyzed

87LG is designed to detect high-resistive (low fault current) ground faults

0 20 40 60 80 100 120 140 160 180 2000

20

40

60

80

100

120

140

160

Fault Resistance (ohm)

Inv

ers

ion

An

gle

(d

eg

ree

)

High fault resistance increases the resistive part of fault loop impedance, which decreases the inversion angle significantly

May result in the slower operation of 87LTransients are not eliminated in the DFT calculation

Introduce more oscillation in the current magnitude

Longer time constant increases the settling time of the calculated phasor

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Fault Location (pu)

Dela

yed O

pera

tion T

ime (

ms)

Equivalent circuit when MOV conducting.

1 2 3 4 5 6 7 8 9 100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Ipu

(pu)

Equiv

ale

nt

Resis

tance a

nd R

eacta

nce (

pu)

Resistance

Capacitive Reactance

Ipu=2.5~3 Irated

when fault current increases, both equivalent resistance and capacitive reactance decrease, but not to zero before bypassing.

MOV conducting would reduce the possibility of current inversion

MOV conducting has no effect on 87LP

Basically, MOV conducting has limited effect on 87LG

87LG is mostly used to detect low current level ground faults and MOV may not conduct under such condition

High current faults would decrease the capacitive reactance significantly and the 87LP element should operate for such faults

No misoperation during MOV conducting in the studied system

0 1 2 3 4 5 6 7 8 90

1

2

3

4

5

6

7

8

9

Restraint Current (pu)

Diffe

rential C

urr

ent

(pu)

Phase Differential

Ground Differential

87LG Setting

87LP Setting

Fault Location: 0%

Fault Location: 50%

S C R

SIR=0.1 Compensation Degree:

50%

Exclude the shunt reactors from the 87L protection zone

Implement charging current compensation technique

Study system carefully and adjust settings to accommodate current inversion if exists and to avoid possible failure to operate

Apply both 87LP and 87LG since these elements may respond to the different fault conditions, such that the relay dependability can be increased

Configuration 1: protect the whole line using two 87L relays over fiber optic communications

Multiplexers may be required for the long line

Configuration 2: protect each line section using three sets of 87L relays

Breaker-and-a-half Configuration

Two separate channels can be configured to work in parallel to achieve maximum security and dependability

Four settings in the dual slope percentage differential principle.

Pickup Level establishes the sensitivity of the element to high impedance faults

Restraint Slope 1 controls the element characteristic when current is below the breakpoint, where CT errors and saturation effects are not expected to be significant

Restraint Slope 2 controls the element characteristic when current is above the breakpoint, where CT errors and saturation effects are significant

Break Point controls the threshold where the relay changes from using the restraint 1 to the restraint 2.

Used for charging current compensation

Utilizing the synchronized phasors from both terminals in the line differential relay

Positive sequence capacitive reactance: using the positive sequence voltages and differential current under the normal operating condition

Zero sequence capacitive reactance: using the zero sequence voltages and currents during an external ground fault

Estimation Error Xc1 Xc0

Lumped Method 1.4% 2.65%

Distributed Method 0.0054% 0.037%

+

+=

)(2 11

111

rs

rsC

II

VVimagX

Principles and applications of shunt reactors and series capacitor banks

Effects of these apparatus on power systems

Impact on 87L performance under steady and transient conditions

Recommendations for 87L applications

Configurations of the 87L scheme

Arrangement of communication channels

87L settings in the dual-slope percentage differential plane

Simple methods to estimate positive and zero sequence capacitive reactance for CCC setting