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