16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, 2010
480
Impact of STATCOM and SSSC Based
Compensation on Transmission Line Protection
Suresh Maturu, U. Jayachandra Shenoy, SMIEEE
Department of Electrical Engineering
Indian Institute of Science, Bangalore-560012, India
Tel: +91-080-22933357, Fax: +91-080-23600444, E-mail: ujs@ee.iisc.ernet.in
Abstract--In this paper, the performance of distance relays
when applied to transmission system equipped with shunt
FACTS devices such as Static Synchronous Compensator
(STATCOM) and Static Synchronous Series Compensator
(SSSC) is described. The aim of the proposed study is to evaluate
the performance of distance relays when STATCOM and SSSC
are incorporated at the mid point of transmission lines. A
detailed model of these devices and their control strategies are
presented. The presence of these devices significantly affects
apparent impedance seen by the distance relays due to their
rapid response to different power system configurations. The
distance relay is evaluated for different loading conditions and
for different fault locations. The faults are created during various
pre–fault loading conditions. The studies are performed on
400KV and 132KV systems and the results are presented.
Simulation studies are carried out using transient simulation
software, PSCAD/EMTDC.
Index Terms--FACTS devices, distance relay, STATCOM,
SSSC.
I. INTRODUCTION
As the need for electricity is increasing day-by-day, the
existing transmission systems are pushed towards their steady
state stability and thermal limits. This means that security or
reliability of transmission system needs to be continuously
improved [1]. The construction of new transmission rights of
way is difficult due to economical and geographical
constraints in transferring bulk power to the consumers.
FACTS (Flexible AC Transmission System) [2] is a
technology that provides the requisite corrections of
transmission functionality in order to fully utilize existing
transmission system thereby minimizing the gap between
stability and thermal limits. Power system is becoming more
complex due to the increasing use of FACTS devices in the
transmission system which includes STATCOM, SSSC and
UPFC etc. These devices cause further difficulties in power
system protection, in particular transmission line protection.
The concept of FACTS envisages the use of solid state
controllers to achieve flexibility of system operation together
with fast and reliable control. Fast control over the reactive
power can allow secure loading of transmission lines nearer to
their thermal limits, provide voltage regulation and improve
the system damping. Out of all FACTS devices, STATCOM
(STATic synchronous COMpensator), a shunt type FACTS
device, is used for shunt reactive power compensation. The
objective is to control or regulate the transmission voltage at
its reference value by supplying or drawing reactive power at
its connected terminal. The reactive power drawn or supplied
by the STATCOM can be varied by varying the magnitude of
converter output voltage. Ideally the output voltages of the
converter are in phase with the corresponding bus voltages. It
is advantageous to connect STATCOM at midpoint of
transmission line as power transfer and voltage support has
been improved in the transmission system [1]-[4].
The SSSC (Static Synchronous Series Compensator) is a
series connected FACTS device, used to control the power
flow in the transmission lines, improve power oscillation
damping and system stability improvements [2]-[5].
The SSSC is a solid state voltage source inverter that
injects voltage which is nearly sinusoidal and it is of variable
and controllable magnitude in series with the transmission
line. This injected voltage is almost in quadrature with the line
current, a small component in phase with the line current to
replenish the losses in the inverter. The injected voltage can
emulate either an inductive or a capacitive reactance in series
with the transmission line. By controlling the size of this
emulated reactance, the SSSC is able to influence the power
flow in the transmission line.
As distance relays are widely used in transmission line
protection, it calculates the apparent impedance between the
relaying point and the fault point. The apparent impedance
seen by distance relay will be changed under normal loading
and fault conditions due to the presence of STATCOM in the
transmission system [6]-[8].
This paper investigates the impact of STATCOM and
SSSC connected at the midpoint of the transmission line on
distance relay performance. Apparent impedance is calculated
for different loading conditions, L-G fault with different fault
resistances and faults created at different locations of
transmission lines. Simulation studies are carried out in
PSCAD/EMTDC [9].
II. FACTS DEVICES AND POWER SYSTEM MODEL
A. Power System Representation
Fig. 1 shows the single line diagram of power system
considered for analysis. The interconnected line is modeled
with pi-sections and the relay to be examined is placed at
sending end bus “S”. STATCOM is connected at middle of
the transmission line. Power system is modeled such that fault
location and fault impedance can be varied to carry out
simulation studies for all possible conditions in the
transmission system.
Department of Electrical Engineering, Univ. College of Engg., Osmania University, Hyderabad, A.P, INDIA.
16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, 2010
parallel operation of two or more compensations. Necessary
filters are provided in the control scheme to limit the effects of
system resonance and other harmonics [2]-[3]. The control
strategy for STATCOM model is depicted in Fig. 3.
S
Source
481
Relay
STATCOM
AC Terminal
Load
Fig. 1. Single line diagram of the system under study
B. STATCOM Model
Fig. 2(a) depicts STATCOM model used in the proposed
simulation studies. STATCOM considered in this paper is
modeled with six pulse voltage source converter which
provides a set of 3-phase ac output voltages, each in phase
with and coupled to the corresponding ac system voltage
through small reactance. The current, Idc for the voltage source
converter is obtained using an energy storage capacitor.
STATCOM is modeled as a controlled reactive power
source. It regulates the voltage at mid point of transmission
line by generating/absorbing the reactive power at the point of
common coupling.
The exchange of reactive power between STATCOM and
ac system can be controlled by varying the amplitudes of 3phase output voltage. If Es>Vt, STATCOM generates the
reactive power to the ac system and if Es<Vt, it absorbs
reactive power from the system as indicated in Fig. 2(b).
Vt
PCC
Istatcom (Iq)
Magnetic
coupling
Iq
Supplies Q
Estatcom (Es)
Es>Vt
Eac
Es<Vt
Current
measurement
and
conditioning
K%
Regulation
slope
Vt
Six pulse converter
Iq*K
PWM
Gate Pattern
logic
Vref
PI Controller
Fig. 3. Control system used for STATCOM [6]
D. SSSC Model
Fig. 4 shows the SSSC model used in the proposed
simulation studies. SSSC considered in this paper is modeled
with six pulse voltage source inverter, injects a voltage in
series with the transmission line through the coupling
transformer. SSSC is connected at the midpoint of
transmission line.
Absorbs Q
voltage source
converter
Voltage
measurement
and
conditioning
I
Vq
(b)
Idc
Vdc
Converter
Transformer
dc Energy Source
Voltage source
converter
(a)
Fig. 2. (a) STATCOM diagram (b) Power exchange
Where
Es voltage at the converter output
Vt voltage at the ac terminals
Iq reactive current injected by STATCOM
C. STATCOM Control Strategy
The STATCOM control scheme designed for the study is
necessary to regulate the voltage magnitude at the STATCOM
location. The ability of STATCOM to regulate the voltage
under widely varying operating conditions depends on its
rating. It can be used in conjunction with fixed capacitor
compensation which provides most of reactive power in the
steady state conditions, enabling the reduction of the
STATCOM rating and reduces steady state losses of
STATCOM, thus providing the maximum controllable
compensation during faults [10]. A regulation slope (K) of 3%
is given in the controllable range so that it does not hit
capacitive and inductive limits too often and also facilitates
Vdc
C
Gate pattern
logic
Fig. 4. SSSC diagram
The reactance compensation capability of SSSC can be
made in one of two ways. One way is to inject a reactance
compensating voltage in series with the line whose magnitude
is controlled independent of the line current magnitude. In this
mode of operation the device is not intended to replicate a
specific ohmic value of reactance. Fig. 5 shows the V–I
characteristics of SSSC. The other way is to inject a
compensating voltage whose magnitude is controlled to be
proportional to the line current magnitude. In this mode the
Department of Electrical Engineering, Univ. College of Engg., Osmania University, Hyderabad, A.P, INDIA.
16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, 2010
device directly emulates an ohmic compensating reactance,
with the emulated compensating reactance being the constant
of proportionality.
Imax
I
Vq = VCmax
VC
Fig. 5. V – I characteristics of SSSC
E. SSSC Control Strategy
The SSSC control scheme in this study is to inject an
alternating voltage in series with the line which is lagging or
leading the line current by 900. A positive compensating
voltage emulates a capacitor and a negative compensating
voltage emulates an inductor in series with the line. The
control scheme used for SSSC is shown in Fig. 6. The
simulation studies are carried out using voltage control (Vq
control) method.
Vc*
ABS
()
1
k
Vdc*
PI
Controller
Vdc
SGN
()
) (P2 + Q2 )
2⎫
(3)
2⎫
(4)
⎬ ⋅ (− Δ Q )
⎭
⎬ ⋅ (ΔQ )
⎭
A. System Data
Generator:
Star connected and solidly grounded,
4 × 250MVA, X 1 = X2 = 0.2 p.u, X0 = 0.1 p.u.
Transformer:
15kV/400kV, delta-star grounded,
4×250MVA, X1 = X2 = 0.15 p.u, X0 = 0.135 p.u.
Transmission line:
Line length = 300km
Positive sequence impedance, Z1L = (0.0264+j0.3294) Ω/km
Zero sequence impedance, Z0L = (0.2015+j1.095) Ω/km
Positive sequence shunt suspectance, b1=3.356×10-6 mho/km
Zero sequence shunt suspectance, b0 = 2.646×10-6 mho/km
Zero sequence compensation factor, K0 = (Z0L – Z1L)/Z1L
Source:
Xq*
I
System voltage = 132KV
Star connected and solidly grounded,
Equivalent impedance = 6.9696 Ω
θ ir
Magnitude and Angle
Calculator
Id
Transmission line:
−
Iq
Ia
Ib
π
2
β
d-q
abc
θ
Vb
Vc
(
⎧ 2
2
2
Δ X a = ⎨V . P − Q
) (P 2 + Q 2 )
Where ΔQ represents the difference of reactive power from
sending end between the absence and presence of STATCOM
at the midpoint of transmission line.
From the above two equations, it can be shown that
resistance seen by the relay increases where as reactance
decreases with the reduction in reactance power supplied from
sending end.
Vq = VLmax
Va
(
⎧
2
⎨ 2 ⋅V ⋅ P ⋅ Q
⎩
⎩
VL
Ic
ΔRa =
482
θi
θv
θ vr
Gate Pattern
Logic
To
Inverter
PLL
PLL
B. Calculation of Impedance during Loading Conditions
The impedance is calculated for different loads at the
receiving end of the transmission line. The equivalent circuit
of the system is shown in Fig. 7.
Fig. 6. Control block diagram of SSSC [11]
III.
ANALYTICAL CALCULATION OF IMPEDANCE
As distance relays measure apparent impedance based on
voltage and current signals measured at relaying point, it is
possible to develop the relationship between the apparent
impedance and power flows in the transmission lines:
2
2
2
(1)
Ra = V ⋅ P
P + Q
Xa
(
) (
)
2
2
2
= (V ⋅ Q ) ( P + Q )
Line length = 60km
Positive sequence impedance, Z1L = 0.1397+j0.4016 Ω/km
Zero sequence impedance, Z0L = 0.2651+j0.9880 Ω/km
Positive sequence shunt suspectance, b1 = 2.871×10-6 mho/km
Zero sequence shunt suspectance, bo = 1.764×10-6 mho/km
(2)
Where Ra and Xa are apparent resistance and apparent
reactance seen by distance relay at sending end, V is the
voltage measured at sending end bus and P and Q are real and
reactive power flows from sending end bus to meet load
requirements.
Change in resistance, ΔRa and reactance, ΔXa with respect
to change in reactive power demand for constant real power
demand can be calculated as:
(
VS = VR + I L R L + jX L
)
(5)
I S = I L + IC 2
(6)
I L = I Load + I C1
(7)
I Load =
VR
( RLoad + X Load )
I C1 = VR
YC
I C 2 = VS
2
YC
2
(8)
(9)
(10)
The impedance measured by the distance relay, Za at the
relaying point is:
Department of Electrical Engineering, Univ. College of Engg., Osmania University, Hyderabad, A.P, INDIA.
16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, 2010
1.2
VS
Voltage measured at
mid-point of transmission line
(11)
1
IS
VS
IL
IS
Z S + ZT
I Load
RL
XL
I C1
IC 2
YC
YC
2
2
Voltage measured
at sending end
0.8
VR
Voltage (p.u)
Z a = Ra + jX a =
483
0.6
0.4
RLoad
0.2
X Load
0
0
0.3
0.6
0.9
1.2
1.5
T ime (sec)
Fig. 7. Equivalent of the system under study
Fig. 9. Voltages with STATCOM
Where
VS
VR
IS
ILoad
IC1, IC2
IL
ZS, ZT
RL, XL
RLoad, XLoad
Ra
Xa
Apparent resistance and apparent reactance measured by
the distance relay are evaluated using Fourier Full-cycle
algorithm by inputting instantaneous voltage and current
signals and signals sampled at 3 KHz sampling frequency (60
samples per cycle). Table I shows the comparison between
results obtained from theoretical calculations and simulation
results for different loads at receiving end. Loads are chosen
in such a way that real power demand is kept constant and the
load power factor (lagging) selected are 0.9, 0.8 and 0.7.
voltage at the sending end bus
voltage at the receiving end bus
current at the sending end bus
current drawn by load
line charging currents
line current
source and transformer impedances
line resistance and line reactance
load resistance and load reactance
apparent resistance seen by the relay
apparent reactance seen by the relay
IV. SIMULATION RESULTS AND DISCUSSIONS
A. STATCOM
1) Loading Conditions
a) Without STATCOM
Initially a load of 400MW, 0.9 p.f lag is connected at the
receiving end of transmission line. Simulation has been
carried out for duration of 1.5 seconds. Fig. 8 shows the
difference in voltage (p.u) measured at the sending end and at
the mid point of transmission line in the absence of
STATCOM. This is due to fact that there is no reactive power
compensation device to support for the voltage along the
transmission line.
1.2
Voltage measured
at sending end
1
Voltage (p.u)
0.8
Voltage measured at
midpoint of transmission line
0.6
0.4
0.2
0
0
0.3
0.6
0.9
1.2
1.5
TABLE I
COMPARISON OF IMPEDANCE MEASURED BASED ON
THEORETICAL CALCULATIONS AND SIMULATION RESULTS FOR
DIFFERENT LOADS AT RECEIVING END
Load at
receiving
end (MVA)
Without STATCOM
Theoretical calculations
Simulation results
Apparent
Apparent
Apparent
Apparent
Resistance
Reactance
Resistance
Reactance
(Ω)
(Ω)
(Ω)
(Ω)
400+j193.73
449.38
131.41
447.87
128.10
400+j300.00
407.44
234.32
406.89
230.81
400+j408.08
333.95
297.50
334.07
294.30
Table II shows the variation of resistance and reactance
seen by distance relay with and without the presence of
STATCOM in the system. The simulation studies are carried
out for different reactive power demands at receiving end. It
can be observed that the apparent resistance increases with
decrease in reactive power from sending end while apparent
reactance decreases with decrease in reactive power from
sending end. The results are matching with the analytical
study results obtained using (3) and (4).
TABLE II
VARIATION OF APPARENT RESISTANCE AND APPARENT
REACTANCE FOR DIFFERENT LOADING CONDITIONS
Time (sec)
Fig. 8. Voltages without STATCOM
b) With STATCOM
The desired voltage for STATCOM is set at 1.0 p.u. It can
be seen from Fig. 9 that there is an improvement in the voltage
profile during normal loading conditions after the inclusion of
STATCOM, since STATCOM injects necessary reactive
power to maintain midpoint voltage at the desired level.
Load at
receiving end
(MVA)
Without STATCOM
Apparent
Apparent
Resistance
Reactance
(Ω)
(Ω)
With STATCOM
Apparent
Apparent
Resistance
Reactance
(Ω)
(Ω)
400+j193.73
447.87
128.10
451.25
-34.04
400+j300.00
406.89
230.81
475.26
-24.30
400+j408.08
334.07
294.30
505.65
-11.24
Department of Electrical Engineering, Univ. College of Engg., Osmania University, Hyderabad, A.P, INDIA.
16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, 2010
8
Without ST AT COM
7
With ST AT COM
4
With ST AT COM, 400MW laod, 0.9p.f
With ST AT COM, 400MW laod, 0.8p.f
With ST AT COM, 400MW laod, 0.7p.f
120
80
40
0
0
1
2
3
4
5
6
7
8
Apparent resistance (ohm)
Fig. 12. Apparent resistance versus apparent reactance for phase A to ground
fault at various locations during different pre–fault loading conditions
Table III shows the variation of apparent resistance and
apparent reactance for phase A to ground fault, for fault
created at 80% of transmission line with different fault
resistances when the system is loaded.
Fault
resistance,
Rf (Ω)
3
2
1
0
10
20
30
40
50
60
70
Fault location (% of line length)
80
90
100
Fig. 10. Apparent resistance versus fault location curves of transmission line
with and without STATCOM
160
Without ST AT COM
With ST AT COM
Apprent reactance (ohm)
Without ST AT COM
160
120
80
40
0
10
20
30
40
50
60
70
Fault location (% of line length)
80
90
100
Fig. 11. Apparent reactance versus fault location curves of transmission line
with and without STATCOM
The changes in measured values are due to the operation of
STATCOM, which tries to maintain the voltage at the
midpoint to its nominal voltage. During fault conditions,
voltage at the midpoint dips from the nominal voltage.
STATCOM produces the reactive current to boost the voltage
at the midpoint thereby increasing the apparent impedance
measured by the distance relay. This would lead to the under –
reaching of distance relay.
Fig. 12 shows the curves between measured apparent
resistance and measured apparent reactance of distance relay
for phase A to ground fault (A-G fault) created at various
locations of transmission line with different prefault loading
conditions. It can be observed that prefault loading has
Without STATCOM
With STATCOM
Apparent
Apparent
Apparent
Apparent
resistance (Ω) reactance (Ω) resistance (Ω) reactance (Ω)
0.0
6.067
79.971
5.191
105.865
10.0
13.008
81.130
18.746
105.451
40.0
32.587
84.328
50.204
101.650
3) Effect of System Strength
System strength plays an important role in measuring the
apparent impedance between the relaying point and the fault
location. During fault conditions, fault voltages are low in
weak systems. When STATCOM is introduced in weak
systems and if phase A to ground fault occurs on the
transmission line, STATCOM provides more compensation
and therefore, causes more errors in impedance measured by
the distance relay. The results obtained for 132KV
transmission system when a prefault load of (32+j32) MVA
connected at receiving end of transmission line with and
without STATCOM is shown in Fig. 13.
45
Without ST AT COM
40
system capacity =2500MVA
system capacity =5000MVA
35
Apparent reactance (ohm)
Apprent resistance (ohm)
5
0
180
TABLE III
VARIATION OF APPARENT RESISTANCE AND APPARENT
REACTANCE FOR A – G FAULT AT 80% OF TRANSMISSION LINE
WITH 400+J193.73 MVA LOAD
6
0
affected the measurement of apparent resistance and apparent
reactance values during faults.
Apparent reactance (ohm)
2) Simulation Results during Fault Conditions
Phase A to ground fault (A-G fault) is created at different
locations of transmission line and the reference voltage for
STATCOM is set at 1.0 p.u. When fault occurs between the
relaying point and the STATCOM location, there is no effect
on the apparent impedance measured by the distance relay.
The measured impedance is almost identical to that of
measured impedance without STATCOM as STATCOM is
not included in the fault loop. However, when fault occurs
beyond the STATCOM location, the apparent resistance
values are smaller and the apparent reactance values are
greater than that of the impedance measured without
STATCOM. Fig. 10 and Fig. 11 respectively show the plots of
apparent resistance and apparent reactance measured at the
relaying point for phase A to ground fault (A-G fault) and the
fault is created at different locations of transmission line with
and without STATCOM.
484
system capacity =10000MVA
30
25
20
15
10
5
0
-2
0
2
4
6
Apparent resistance (ohm)
8
10
Fig. 13. Apparent resistance versus apparent reactance curves for different
system capacities with and without STATCOM for A-G fault; pre-fault load
(32+j32) MVA
Department of Electrical Engineering, Univ. College of Engg., Osmania University, Hyderabad, A.P, INDIA.
16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, 2010
B. SSSC
1) Loading Conditions
The SSSC is connected in series through a transformer at
the midpoint of transmission line. The compensating voltage
of 0.1p.u is set for SSSC. As mentioned earlier, the positive
compensating voltage emulates a capacitor in series with the
line. That means, SSSC injects a voltage which is almost in
quadrature with the line current. Table IV shows the variation
of resistance and reactance seen by distance relay with and
without SSSC in the system.
TABLE IV
VARIATION OF APPARENT RESISTANCE AND APPARENT
REACTANCE FOR DIFFERENT LOADING CONDITIONS
Without SSSC
Apparent
Apparent
resistance
reactance
(Ω)
(Ω)
447.87
128.10
Load at
receiving end
(MVA)
400+j193.73
With SSSC
Apparent
Apparent
resistance
reactance
(Ω)
(Ω)
431.37
85.39
400+j300.00
406.89
230.81
394.26
183.85
400+j408.08
334.07
294.30
321.99
250.72
2) Fault Conditions
Phase A to ground fault (A-G fault) is created at different
locations of transmission line and the reference compensating
voltage for SSSC is set at 0.1p.u. When fault occurs between
the relaying point and the SSSC location, there is no effect on
the apparent impedance seen by the distance relay. However,
when SSSC is included in the fault loop, the apparent
impedance seen by relay gets modified. The corresponding
results are shown in Fig. 14 and Fig. 15.
16
Without SSSC
With SSSC
Apparent resistance (ohm)
14
12
10
8
6
4
2
0
0
10
20
30
40
50
60
70
Fault location (% of line length)
80
90
100
Fig. 14. Apparent resistance versus fault location curves of the system with
and without SSSC for A-G fault
Without SSSC
Apparent reactance (ohm)
80
With SSSC
60
40
485
V. CONCLUSIONS
The results outlined in the proposed simulation studies
using PSCAD/EMTDC software show that the presence of
STATCOM and SSSC in the transmission system significantly
affects the apparent resistance and the apparent reactance
measured by the distance relay under both normal loading and
fault conditions. During normal loading conditions with
STATCOM connected at the midpoint of transmission line,
resistance increases and reactance decreases when compared
to resistance and reactance values measured without
STATCOM. Under fault conditions, resistance decreases and
reactance increases with STATCOM as compared to the case
without STATCOM. In the case of SSSC, connected at the
midpoint of transmission line, under normal loading
conditions, both resistance and reactance decrease as
compared to the values of resistance and reactance measured
without SSSC. During fault conditions, resistance increases
and reactance decreases with SSSC as compared to the case
without SSSC. The results show that there is a need for
distance relay to adjust to new settings in its characteristics
and make it adaptive to system conditions.
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20
0
0
10
20
30
40
50
60
70
Fault location (% of line length)
80
90
100
Fig. 15. Apparent reactance versus fault location curves of the system with
and without SSSC for A-G fault
Department of Electrical Engineering, Univ. College of Engg., Osmania University, Hyderabad, A.P, INDIA.