NATIONAL POWER SYSTEMS CONFERENCE, NPSC 2002
112
Effect of Series Capacitor on Line Protection - A
Case Study
Anand Mohan, Vikas Saxena, Mukesh Khanna & V.Thiagarajan
Abstract: Series compensation is a time tested reliable
technology being used successfully to compensate the reactance
of transmission lines.
The advantage of use of series
compensation on long EHV lines is improvement in
Transmission System performance and increase in line
loadability. This has been well established and implemented for
many years world over. For the first time in India fixed series
compensation of 35% has been proposed on a 400kV S/c i.e
Kanpur - Ballabhgarh 400kV S/c to enhance the line loadability
and system stability. The introduction of a series capacitor into
the line affects the line distance protection due to abrupt change
in line series impedance (which is inductive) at the series
capacitor. The paper discusses in detail the effect of series
compensation on distance protection and various Short Circuit
and EMTP studies carried out.
I. INTRODUCTION
Indian Power System, presently with more than 100GW
installed capacity, is being operated as four independent
regional grids - Northern, Southern, Western and Eastern &
North-Eastern Regions. Northern region is one of the largest
grid with an installed capacity of about 27 GW, consisting of
eight states and a Union Territory. Eastern part of Northern
Region has major coal reserves and thermal Power Stations
like Singrauli, Rihand, Anpara and Obra have been developed
to tap this reserve. Power from these stations is transferred to
Western part of the grid, where there is a concentration of
loads, through long distance 400 kV lines via Kanpur/Panki
axis. For transfer of power beyond Kanpur/Panki towards
Western part, presently there are mainly three 400kV AC
lines i.e. Kanpur - Ballabgarh (395km), Panki-Muradnagar
(400km) and Kanpur – Agra (250km) (Figure-1). The
loading on Kanpur - Agra line remains generally higher and
also due to long length of the other two, it has been proposed
to install series compensation on Kanpur-Ballabgarh and
Panki-Muradnagar lines. Under Phase-I, series capacitor is
proposed to be installed on Kanpur-Ballabgarh and under
phase-II, it is proposed to install on Panki-Muradnagar
400kV S/c line.
Application of Series compensation increases the line
loadability, But the integration of series capacitor into the line
makes the line protection complex due to abrupt change in
line series impedance at the series capacitor, which changes
the fault current, voltage and apparent impedance measured
by the line relays. Moreover, the fault current may change
from its normal lagging angle, with respect to voltage, to a
leading angle. The presence of series capacitor on a line can
cause incorrect operation of the relays on that line and relays
on adjacent lines.
Anand Mohan , Vikas Saxena , Mukesh Khanna & V.Thiagarajan
Engineering Division
Power Grid Corporation of India Ltd
B-9 Qutub Instt. Area, Katwaria Sarai , New Delhi-110 016, India
Figure I- Schematic diagram of Series capacitor Protection
This paper discusses in detail the studies carried out to
study the effect of series compensation on conventional
distance relays and remedial action taken.
II. SYSTEM DESCRIPTION
A. Network configuration
Fig. 1 shows the spatial arrangement of proposed Series
Compensation, the series capacitor is proposed to be located
at Ballabgarh end of Kanpur-Ballabgarh 400kV S/c line.
Ballabgarh, other than Kanpur, is connected to Agra and
Jaipur by 400kV Single Circuit line while a 400kV Double
Circuit line connects it to Dadri. Similarly Kanpur bus is
connected to Panki and Allahabad by two numbers of 400kV
lines while it is connected to Singrauli and Agra by 400kV
Single Circuit line. Agra is connected to both Kanpur and
Ballabgarh by 400kV Single Circuit line.
B. Series compensation
Theoretically, a transmission system can carry power up to
its thermal loading, in the case of Kanpur-Ballabgarh line
400km built with twin Moose conductor it is 870 MVA. But
loading on a long line is restricted well below its thermal
loading due to angular separation been the two buses, which
as per Transmission Planning Criteria should be within 30
degrees. For example, a 400km long 400kV line with per unit
voltage at both can be loaded upto only 570MW. Provision of
series compensation on the line effectively reduces the line
impedance and hence the angular separation between the
buses. After detailed analysis 35% series compensation has
been proposed on Kanpur-Ballabgarh line. The 35% series
compensation consists of banks of 27% and 8%, has been
considered at Ballabgarh end of Kanpur-Ballabgarh line. The
INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR 721302, DECEMBER 27-29, 2002
8% bank is proposed to be converted into Thyristor
Controlled Series Capacitor in future to improve Dynamic
stability.
113
BALLABGARH BUS VOLTAGE
200000
150000
100000
III. CAPACITOR DESCRIPTION
50000
0
0.04
-50000
A Series Capacitor reduces the inductance in a power
system leading to higher short circuit currents. It is not
feasible to design a series capacitor that has high enough
voltage ratings to withstand such high fault currents. Hence to
protect the capacitor from damage capacitor banks are
bypassed when the current touches the protective level.
0.06
0.08
0.1
0.12
0.14
-100000
-150000
-200000
FAULT CURRENT (NOT TO SCALE)
Exhibit-I- Fault Current & Ballabgarh Bus Voltage
To protect the Series capacitor banks on KanpurBallabgarh line, a combination of MOV triggered spark gap
and Series Capacitor Bypass Circuit Breaker is being
provided. With the inception of the fault, MOV bypasses the
capacitor carrying the fault current. Energy dissipation
capacity of MOV is also limited, therefore triggered spark
gap bypasses the MOV. Schematic diagram of the Series
capacitor along with protection is shown in Figure-II. The
spark gap is triggered within 10msec of MOV conduction,
after which bypass circuit breaker will be closed.
20000
15000
10000
5000
0
0.04
0.05
0.06
0.07
0.08
0.09
0.1
0.11
0.12
0.13
0.14
-5000
-10000
-15000
-20000
The effect of series compensation along with its MOV
during fault conditions and its effect on line protection need
to be studied.
Exhibit-II- Current thru the capacitor
40000
30000
20000
KANPUR
BALLABGARH
10000
0
0.04
8% CAPACITOR 27% CAPACITOR
BANK
BANK
MOV
0.05
0.06
0.07
0.08
0.09
0.1
0.11
0.12
0.13
0.14
-10000
MOV
-20000
-30000
DAMPING
-40000
CKT
Exhibit-III- Current thru the MOV
SPARK GAP
BYPASS CIRCUIT BREAKER
From the above study results, following observations can be
made:
¾
Figure II- Schematic diagram of Series capacitor Protection
IV. EMTP SIMULTAION
In order to study the behavior of series Capacitor along
with its MOV, an EMTP simulation has been carried out. A
detailed system upto second level, both from Kanpur and
Ballabgarh bus, has been modeled in EMTP. All the bus short
circuit has been matched with the expected short circuit level.
The powerflow on the lines have been matched with the
expected value both in magnitude and direction. The MOV
has been modeled in detail as per the V-I characteristics as
provided by the manufacturer.
A fault was created close to the Capacitor terminal on the
line side. The voltage and current monitored are plotted as
below.
¾
¾
¾
Till the voltage across capacitor builds upto protection
level the fault current flows thru the capacitor. (From the
Exhibit-II it can be seen that initially the current flows
through the series capacitor and increases to the
magnitude of about 20kA).
When the protective voltage level of MOV is reached,
MOV bypasses the capacitor bank and fault current
flows thru MOV. (Exhibit-III)
Hence the series capacitor remains in service for a
definite period and its effect on the relays needs to be
studied.
The Ballabgarh bus voltage does not reduces to zero
even for a fault close to bus.
To study the effect of series compensation, till MOV starts
conducting a fault was slided along the Ballabgarh-Kanpur
line and bus voltage of Ballabgarh bus was monitored, MOV
not been modeled. The results are plotted in Exhibit –IV
NATIONAL POWER SYSTEMS CONFERENCE, NPSC 2002
114
lines and locations on which the faults is to be simulated
study has been done on PSS/E
700
600
IV. SHORT CIRCUIT STUDIES
500
400
A. System Modeling
300
200
100
-10
0
0
10
20
30
40
50
60
70
80
90
100
Exhibit-IV- Ballabgarh bus voltage for a sliding fault
For a fault at capacitor terminal, the voltage of Ballabgarh
bus will rise. And the Voltage varies as the fault slides. The
effect of high voltage under fault needs to be studied.
Consider a simple system shown in Figure-III, power flows
from BUS1 to BUS2 ( Rp>>Xl>>Rl). Under normal
condition, the flow on a line is mostly Real power (MW) as
the load is predominantly Real/Resistive ( Rp>>jXp) and
relay at BUS1 on BUS1-BUS2 line measures a high resistive
load ( as Rp >> Xl).
Under short circuit condition BUS2 voltage becomes zero
and no power will flow thru the load and the entire current
will flow thru the low impedance fault to ground. The relay at
BUS1 on BUS1-BUS2 will measure the actual line
impedance. The line inductance is about 10 times higher than
line resistance ( Xl>>Rl). Hence the relay will measures a
high inductance.
Detailed system up to 132kV level having about 22GW
load has been simulated. The system simulated comprises of
370 nodes, 1000 branches and 70 generating plants. All
generators have been shown behind a Sub transient
Reactance.
B. Study Approach
For a particular fault, the series capacitor affects the input
to the relay only if the location of Series capacitors is
between the relay measurement point and the fault point.
Hence
¾
¾
BUS2
BUS1
( R l + jX l )
(I
+
r jI x )
¾
Relays on Adjacent lines at Ballabgarh end , i.e. DadriBallabgarh 400kV D/c, Dadri-Agra 400kV S/c etc., shall
be affected by the series capacitor for faults beyond
capacitor line terminal only on Kanpur-Ballabgarh line.
¾
For relays on Adjacent lines at Kanpur end i.e. Kanpur –
Allahabad 400kV S/c, Kanpur –Agra 400kV S/c etc,
shall be affected by the series capacitor for fault beyond
capacitor, on Ballabgarh bus and on adjacent lines from
Ballabgarh. As compensation is only for 35%, for faults
beyond Ballabgarh the total impedance from Kanpur to
fault shall always be inductive. This will be added to
actual line impedance, hence the relay operation on
adjacent lines at Kanpur end may be affected only in
Zone-2 & 3. Hence effect of series compensation on
these lines can be ignored.
( Rp + jX p )
Figure III- Normal system
For a fault at the capacitor line terminal, the voltage at
BUS2 does not become zero (rather increases) and hence
power will continue to flow through Rp, at the same time as
Xc is small compared to Rp there will be reactive current
flowing to ground thru Xc. The impedance measured by the
relay at BUS1 on BUS1-BUS2 line will depend on the
position of the fault.
Hence due to installation of series capacitor on Kanpur –
Ballabgarh 400kV line, the relays on adjacent lines on
Ballabgarh bus and on Kanpur–Ballabgarh 400kV line which
needs to be studied for whether there will be any maloperation for a fault at any location, due to series capacitor is
in service. As we are interested when capacitor is conducting,
MOV need not be modeled. The study has been carried out
on
Short
Circuit
package
of
Power
System
Simulator/Engineering (PSS/E), power system software,
developed by Power Technologies Inc, USA. Similar studies
can be carried out with EMTP, but considering the number of
Relay measurement at Ballabgarh end can be done either
at Bus or at capacitor line terminal. In the case of
Kanpur-Ballabgarh the measurement shall be at line side.
With measurement of Relay inputs at Capacitor line
terminal, relay at Ballabgarh end of Kanpur-Ballabgarh
will be unaffected by series capacitors for any fault at
capacitor terminal or fault along Kanpur-Ballabgarh line
and will sense the faults correctly, but will be affected by
the series capacitor for faults behind the capacitor
terminal like fault on Ballabgarh bus or on any adjacent
lines of Ballabgarh.
Relay at Kanpur end of Kanpur –Ballabgarh line will be
unaffected for any fault along Kanpur-Ballabgarh line
upto Capacitor terminal. Kanpur end relays will be
influenced by the series capacitor for fault just after
capacitor, and on adjacent lines terminating at
Ballabgarh.
To summarize, following studies need to be carried out to
study the effect of Series capacitor:
Response of relay on
9 Kanpur –Ballabgarh line for fault behind capacitor,
on Ballabgarh bus and on adjacent lines of
Ballabgarh
9 Adjacent lines at Ballabgarh end for fault at
capacitor terminal and along Kanpur-Ballabgarh line
INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR 721302, DECEMBER 27-29, 2002
C. Short Circuit Studies:
Relays on adjacent lines at Ballabgarh:
The line in adjacent sections which will get affected due to
series compensation on Kanpur-Ballabgarh/ lines which may
affect the protection of series compensated KanpurBallabgarh line are:
9
9
9
Ballabgarh -Dadri 400kV D/c – 53km
Ballabgarh-Bhiwadi 400kV S/c- 36 km
Ballabgarh-Agra 400kV S/c- 181 km
Effect on Dadri-Ballabgarh relays
A sliding fault was created along Ballabgarh-Kanpur line
and the apparent impedance measured by the relay at Dadri
end of Dadri-Ballabgarh line was monitored, the results are
plotted in Exhibit-V. For a fault at Ballabgarh bus, the relay
at Dadri measures impedance equal to the line impedance,
.008 p.u. But for fault at capacitor terminals due to current infeed effect it measures an impedance of .023-j.095 as against
expected .001-j.02072 p.u. Power flows from Dadri to
Ballabgarh hence resistive component is always positive.
And as the fault moves towards Kanpur the capacitive
impedance measured by the relay reduces and for a fault at
For both the relays the minimum impedance measured is
greater than the actual line impedance( .008p.u).
Effect on Ballabgarh-Bhiwadi relays
Exhibit-7 &8 shows the apparent impedance measured by
the relay at Bhiwadi and Ballabgarh end relays of BhiwadiBallabgarh line. Power flows from Ballabgarh to Bhiwadi.
Hence the Relay at Bhiwadi sees a negative resistance while
the relay at Ballabgarh sees a positive resistance. Only for a
small percentage of line length the voltage at the bus is low
enough that powerflows from Bhiwadi to Ballabgarh.
0.250
0.150
0.050
-0.300
-0.200
-0.150
-0.100
-0.050
-0.0500.000
0.050
-0.250
Exhibit-VII: Apparent impedance (R-X) measured by Bhiwadi end relay
of Bhiwadi -Ballabgarh line
0.250
0.1 50
0.1 00
0.050
0.050
-0.050 -0.0500.000
0.000
0.01 0
0.020
0.030
0.040
0.050
0.060
0.070
0.050
0.1 00
0.1 50
0.200
0.250
0.300
-0.1 50
-0.050
-0.250
-0.1 00
Exhibit-V- Impedance seen by Dadri end relay of Dadri - Ballabgarh
line for fault on Ballabgarh-Kanpur line
about 32% from Ballabgarh end the measured impedance is
nearly resistive. But even this resistive impedance is very
high due to In-feed current, .009p.u, more than 100% of line
impedance. After 32%, the impedance measured by the relay
is inductive
The apparent impedance measured by Ballabgarh end relay
is shown in Exhibit-VI. For a fault at capacitor terminal, the
relay sees an apparent impedance of -.024+j.106 and as the
fault slides the inductive component reduces but even at the
point were it is almost zero the resistive component is -.009,
which is about 100% of line impedance.
0.150
Exhibit-VIII Apparent impedance (R-X) measured by Ballabgarh end
relay of Bhiwadi -Ballabgarh line
For both relays the minimum measured impedance of
Bhiwadi end relay is.035p.u, which is more than twice the
line impedance( .017 p.u).
Effect on Ballabgarh-Agra relays
Exhibit-IX &X shows the apparent impedance measured by
the relay at Agra end and Ballabgarh end of Agra-Ballabgarh
line. The fault current feed for Agra is mainly from Auraiya,
Kanpur and Ballabgarh.
9
Under Normal condition power flows from Agra to
Ballabgarh hence Resistive component is positive for
Agra end Relay.
9
As the fault slides closer to Kanpur, Ballabgarh-AgraKanpur lines act as a parallel path to feed the path hence
after about 80% distance X becomes negative for Agra
end Relay.
9
The minimum impedance measured by the relay is 0.048
p.u, which is higher than the line impedance of .037 p.u
0.100
0.050
0.000
-0.070
-0.250
-0.150
0.1 50
0.000
115
-0.060
-0.050
-0.040
-0.030
-0.020
-0.010
0.000
-0.050
-0.100
-0.150
Exhibit-VI- Impedance seen by Dadri end relay of Dadri - Ballabgarh
line for fault on Ballabgarh-Kanpur line
NATIONAL POWER SYSTEMS CONFERENCE, NPSC 2002
116
9
0.15
0.1
0.05
0
-0.05 0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
-0.1
-0.15
9
-0.2
-0.25
-0.3
Exhibit-IX Apparent impedance (R-X) measured by Agra end relay for
fault on Ballabgarh-Kanpur line
0.35
0.3
0.25
0.2
0.15
Relay at Kanpur End of Kanpur-Ballabgarh Line :
A sliding fault was created on Ballabgarh- Dadri 400kV
line and Ballabgarh-Agra 400kV line and the apparent
impedance measured by Kanpur end relay of KanpurBallabgarh line was monitored. The result of studies are
plotted below :
0.1
0.05
0
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
It is seen that the measured X component of Relay at
Ballabgarh end for fault on Ballabgarh-Dadri line is
always less than its Zone-1 setting. Maximum measured
value is .0287 p.u, which is at the bus. As power flows
from Dadri to Ballabgarh R is always measured as a
negative Value, but the magnitude is small and
Maximum magnitude measured is .0188 p.u
Similar to Ballabgarh-Dadri line, Ballabgarh-Agra line
measures a very small X component, which also
becomes negative for a portion of the line. But the
negative R component magnitude increases very rapidly
and touches up to .13 p.u. Infact, at 10% R is only about
-0.034 p.u, by the time the fault is at 20% the magnitude
of R is about -.068 p.u, greater than the Zone-1 setting of
Kanpur-Ballabgarh line.
-0.05 -0.05 0
-0.1
-0.15
Exhibit-X Apparent impedance (R-X) measured by Ballabgarh end
relay for fault on Ballabgarh-Kanpur line
The Zone setting of Kanpur –Ballabgarh relay is 0.0656 p.u
Relay at Ballabgarh End of Kanpur-Ballabgarh Line :
A sliding fault was created on Ballabgarh- Dadri 400kV
line and Ballabgarh-Agra 400kV line and the apparent
impedance measured by Ballabgarh end relay of KanpurBallabgarh line was monitored to study the effect of series
capacitor on the relay. The result of studies are plotted below:
0.1
0.08
0.06
0.04
0.02
0.035
0
0.03
0
0.025
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.02
0.015
Exhibit-XIII: Apparent impedance (R-X) measured by Kanpur end
relay of Ballabgarh -Agra line
0.01
0.005
0
-0.02
-0.015
-0.01
-0.005
9
0
Exhibit-XI: Apparent impedance (R-X) measured by Ballabgarh end
relay of Ballabgarh -Dadri line
0.15
0.1
0.05
0
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
-0.1
-0.15
-0.2
-0.25
For a fault close to Bus the Kanpur end relay measures a
fault impedance of 0.0544 i.e the net impedance of line
and the capacitor.
9 For any fault on the adjacent lines power shall always
flow from Kanpur to Ballabgarh. Hence the impedance
measured by the Kanpur end relay Resistance is always
positive.
0.08
0.06
0.04
0.02
0
-0.3
0
Exhibit-XII: Apparent impedance (R-X) measured by Ballabgarh end
relay of Ballabgarh -Agra line
The Zone setting of Kanpur –Ballabgarh relay is 0.0656 p.u
9 Under both the cases for a fault on the line close to bus
(at gantry) the relay measures an impedance of .029 p.u ,
the capacitor impedance less than Zone-1 setting
0.005
0.01
0.015
0.02
0.025
0.03
Exhibit-XIV: Apparent impedance (R-X) measured by Kanpur end
relay of Ballabgarh -Dadri line
9
And as the fault slides further the measured impedance
increases. The maximum measured impedance is
0.027+j.077. And upto about 20% distance from the bus
INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR 721302, DECEMBER 27-29, 2002
the impedance is less than Zone-1 setting. And as the
fault approaches Dadri Bus the Fault impedance reduces
again.
For fault on Ballabgarh – Agra line initially the X
measured by the Kanpur end relay increases rapidly. For
fault at 10% distance from the bus the magnitude of X is
about .084p.u. Since Agra-Ballabgarh is a long line
(180km), as the fault slides from Ballabgarh the voltage
of Ballabgarh bus increase and power flows from Kanpur
to Ballabgarh. Hence measured R value increase heavily,
maximum magnitude is about .14 p.u.
9
V. ATP SIMULATION:
To study the Relay operation with series capacitor and to
see if there is any relay malfunction by transients due to
MOV conduction, capacitor etc an ATP simulation was
carried out. The ASCII values obtained were converted into
COMTRADE format and then fed to distance relays of type
that are existing on concerned lines, through an automatic
relay test kit. Study results are as follows:
9
For a fault on Kanpur-Ballabgarh line the relays on
adjacent lines at Ballabgarh did not maloperate.
9 The Kanpur end Relay tripped in Zone-1 for fault, on
Ballabgarh-Dadri Line, at Ballabgarh end, at 10% and
Dadri end. Similarly the relay tripped in Zone-1 for
fault, on Ballabgarh-Agra Line, at Ballabgarh end. The
relay did not operate for fault at 10%, 50% and Agra
of Ballabgarh-Agra Line
9 The Ballabgarh end Relay tripped in Zone-1 for fault,
on Ballabgarh-Dadri Line, at Ballabgarh end, at 10%,
50% and Dadri end. Similarly the relay tripped in
Zone-1 for fault, on Ballabgarh-Agra Line, at
Ballabgarh end and 10% from Ballabgarh. The relay
did not operate for fault at 50% and Agra end of
Ballabgarh-Agra Line
The test results match with the results obtained through
Short circuit studies for conventional distance relays.
The same tests were also simulated on new generation
numerical distance relays of different makes compatible with
series capacitor. It was observed that these relays are able to
give correct directional discrimination.
117
VI. REMEDIAL MEASURES
The conventional Ballabgarh end relay of Kanpur –
Ballabgarh end line has lost its sense of direction and trips for
behind the bus fault. The Relay has been replaced with new
generation numerical distance relays compatible with series
capacitors. To overcome the over reach of Kanpur end relay
for fault beyond the capacitor following zone settings were
made.
Z1=0.8*(1-k)*ZL
Z2=1.2*ZL
Where k is the degree of compensation
ZL is the uncompensated line impedance
Permissible Over-Reach scheme (POR) has been adopted.
VII. CONCLUSION
This paper presented in detail application of Short Circuit
studies carried out to examine the effect of installation of
series capacitor on Kanpur-Ballabgarh 400kV line on line
protection on principle as well as adjacent lines. The study
results indicate requirement for replacement of conventional
distance relays at Ballabgarh end with numerical relays
compatible with series capacitor and change in Zone setting
of Kanpur end relay and adoption of permissible Overreach
Scheme. While Short circuit studies are helpful in identifying
faults and distance relay locations which are of concern, it is
necessary to make ATP simulation test on actual relays to
ascertain performance and adequacy of protection scheme
provided.
VIII. ACKNOWLEDGEMENT
The authors wish to thank POWERGRID for granting
permission to present the paper. Views expressed in this
paper are of the authors and need not necessarily be of the
management of POWERGRID.
IX. REFERENCES
[1] Application guide on protection of complex transmission network:
working group of CIGRE-34
[2] Series compensation of Power system – P.M.Anderson and F.G Farmer
[3] Limits to conventional distance relaying in series compensated
transmission lines-Case study by V.K.Prashar, Vikas Saxena, P.Jayachandran
& V.Thiagarajan : Integrated Protection, Control and Communication
Experience, Benefits and Trends, CBIP, 2001