The Online Journal on Power and Energy Engineering (OJPEE)
Vol. (2) – No. (2)
Effect of FACT Devices on Distance Relay
A. Elmitwally, member IEEE
The author is with the Electrical Eng. Dept.,
Mansoura university, Egypt (e-mail: kelmitwally@yahoo.co.uk).
Abstract—In this paper, the effect of installation of
FACT devices on the transmission line distance relay is
investigated under the phase to ground, phase to phase
and three phase faults. This is done by means of
comparing the actual measured impedance at the relaying
point and the ideal tripping characteristics of distance
relay. Three different candidate locations of FACT device
on the transmission line are considered with respect to the
concerned distance relay, i.e. at near end, mid-point, and
far end. Besides, two possible situations for the FACT
device in terms of inclusion or exclusion of fault loop are
analyzed. The impact of FACT device size on the
modified distance relay characteristics is also assessed.
Numerical application is provided and results are
discussed.
Keywords: Protection, Distance relay, FACT devices,
Transmission lines, Faults.
I. INTRODUCTION
The measured impedance at the relaying point is the core of
the distance protection operation. There are several factors
affecting the measured impedance at the relaying point. This
can be categorized into two groups. First group is the
structural conditions, while the second group is the
operational conditions. In addition to the power system
parameters, the fault resistance could greatly influence the
measured impedance [1]- [7].
In the recent years, flexible AC transmission systems
(FACT) devices, which are controlled power-electronics
based devices, are used to increase the transmitting capacity
of the lines and provide the optimum utilization of the system
capability. This is done by pushing the power systems to their
thermal limits. The inclusion of these devices into the power
system will surely modify its characteristics. Therefore, it is
essential to study the effects of FACT devices on the
protective systems, especially the distance protection for
different fault conditions [8], [9].
The most common types of FACT devices are the static
series synchronous condenser (SSSC), the static compensator
(STATCOM), and the unified power flow controller (UPFC).
They are installed at some strategic nodes in the power
system to provide an adequate range of controllability over its
performance. SSSC injects a controlled series voltage to
control the line power flow direction and magnitude.
STATCOM is shunt connected to the line to inject or absorb a
controlled amount of reactive power to adjust the line reactive
Reference Number: W10-0034
power flow. UPFC combines the role of both SSSC and
STATCOM [3], [8].
In the presence of FACT devices, the conventional distance
relay characteristics such as Mho and Quadrilateral are
greatly subjected to mal-operation in the form of overreaching or under-reaching the fault point. Therefore, the
conventional relay characteristics may not work properly in
the presence of FACT devices [9], [10].
In reference [1], the effect of SSSC and STATCOM on
distance relay has been investigated for single phase to
ground faults. In reference [2], the effect of SSSC and
STATCOM on distance relay has been investigated for phase
to phase and three phase faults. In reference [3], the effect of
instrument transformers connection point on measured
impedance by distance relay in presence of SSSC and
STATCOM has been studied. Adaptive distance relay setting
in presence of FACT devices has been studied in [4].
This paper explores the effect of the installation of FACT
devices on the transmission line distance relay under the
phase to ground, phase to phase and three phase faults. This is
done by means of comparing the actual measured impedance
at the relaying point and the ideal tripping characteristics of
distance relay. Three different candidate locations of FACT
device on the transmission line are considered with respect to
the concerned distance relay, i.e. at near end, mid-point, and
far end. Besides, two possible situations for the FACT device
in terms of inclusion or exclusion of fault loop are analyzed.
The impact of FACT device size on the modified distance
relay characteristics is also assessed.
II. MODELING OF SSSC AND STATCOM
SSSC is placed in the group of series connected FACT
devices. As shown in Fig.1, SSSC consists of a voltage source
inverter connected in series through a coupling transformer to
the transmission line. A source of energy is required for
providing and maintaining the dc voltage across the dc
capacitor and SSSC losses compensation [1].
Fig. 2 shows the model of SSSC which consists of a series
connected voltage source in series with an impedance. This
impedance represents the impedance of SSSC coupling
transformer. The magnitude of the injected voltage can be
controlled, according to a compensation strategy, but the
phase angle of the injected voltage would be generally
perpendicular to the line current [3].
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The Online Journal on Power and Energy Engineering (OJPEE)
EA
= h e − j
EB
For a single phase to ground fault, the system shown in
Fig.3 will be analyzed into 3- sequences (positive, negative
and zero sequence) as revealed in Fig.4. The apparent
impedance measured by the distance relay ZA for singlephase to ground fault is obtained via circuit analysis of Fig.4
as follows [1].
Voltage Source Inverter
( VSI
)
Energy Source
Figure (1): SSSC configuration
V in r e
V in
-
jγ
Z se
+
Vol. (2) – No. (2)
Vout
Figure(2): Equivalent circuit of SSSC
STATCOM can be modeled as a shunt branch consisting of
an reactive impedance, due to the coupling transformer, in
series with a voltage source. The latter is in phase with the
voltage of its connection point, so it can only inject or absorb
reactive power according to the amplitude of the voltage
source [7].
Z1A = Z1SA + pZ1L
Z1B = Z1SB + (1-p)Z1L
Z0A = Z0SA + pZ0L
Z0B = Z0SB + (1 - p)Z0L
ZΣ = 2
Z Z
Z1A Z1B
+ 0A 0B
Z1A + Z1B Z0A + Z0B
(1)
(2)
(3)
(4)
(5)
C1 =
Z1A Z1B
Z1A + Z1B
(6)
III. MEASURED IMPEDANCE FOR SINGLE PHASE TO
GROUND FAULT
C0 =
Z 0A Z 0B
Z 0A + Z 0B
(7)
In the absence of FACT devices and for zero fault
resistance, the measured impedance by a distance relay only
depends on the length of the line section between the fault
location and the relaying point. In Fig.3, the relay is cited at
A, this impedance is equal to pZ1L, where p is per unit length
of the line section between the fault and the relaying point,
and Z1L is the line positive sequence impedance in ohms.
K 0L =
A
EA
Z1SA
(1-P)Z1L
Den = Z1A he − jδ + Z1B
Z1SB
EB
Rf
Figure (3): Power system configuration for single phase to
ground fault
(8)
(9)
K1d = 1 − he − jδ
(10)
C1d = (Z1d + 3R f )K 1d D
(11)
Z A = pZ1L +
B
pZ1L
Z 0L − Z1L
3Z 0L
3R f
C1d + 2C1 + C 0 (1 + 3K 01 )
(12)
It can be seen that when the fault resistance is equal to
zero, the measured impedance at the relaying point is equal
to the impedance of the line section between the relay and
the fault locations.
Once a FACT devise is installed on the transmission line,
the measured impedance at the relaying point is affected. The
device is installed at the length of i per unit from the relaying
point. In the following, the measured impedance in the
presence of SSSC and/or STATCOM is estimated.
A. Measured impedance in the presence of SSSC
Once SSSC is installed on the transmission line, the
measured impedance will change. The following equations
are introduced due to SSSC presence on the line, independent
of its exclusion or inclusion in the fault loop.
(13)
C1S = Z Se Z1L
Fig. 4. Equivalent circuit of single phase to ground
The operational conditions prior to the fault instance can be
represented by the load angle of the line (δ), and the ratio of
voltage magnitude at the line ends (h), where
Reference Number: W10-0034
C 0S = Z Se Z 0L
(14)
Z1AI = Z1SA + iZ1L
(15)
Z1BI = Z1SB + (1 − i + C1S )Z1L
(16)
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The Online Journal on Power and Energy Engineering (OJPEE)
Z0AI = Z0SA + iZ0L
(17)
Z 0BI = Z 0SB + (1 − I + C 0S )Z 0L
(18)
(i) In the case of SSSC out of the fault loop
Equations (1)-(4) and (9)-(10) should be modified, and
some extra equations are introduced.
(19)
Z1AF = Z1SA + PZ1I
Vol. (2) – No. (2)
It can be seen from (41) that in the absence of the fault
resistance, the measured impedance at the relaying point is
not equal to the actual impedance of the line section between
the relay and the fault locations.
B. Measured impedance in the presence of STATCOM
The following equations are applicable due to STATCOM
presence on the line, independent of its exclusion or inclusion
in the fault loop.
(42)
Z1AI = Z1SA + iZ1I
Z1IF = (i − p )Z1L
(20)
Z1A = Z1AF
(21)
(43)
Z1B = Z1IF + Z1BI
Z1BI = Z1SB (1 − i )Z1L
(22)
Z1AF = Z1SA + pZ1L
(44)
Z 0AF = Z 0SA + PZ 0L
(23)
Z 0IF = (i − p )Z 0L
Z1BF = Z1SB + (1 − p )Z1L
(45)
(24)
Z1IF = i = p Z1L
(46)
Z 0A = Z 0AF
(25)
Z 0B = Z 0IF + Z 0BI
Z 0AI = Z 0SA + iZ 0L
(47)
(26)
Z 0BI = Z 0SB + (1 − i )Z 0L
(48)
(27)
Z 0AF = Z 0SA + pZ 0L
(49)
(28)
Z 0BF = Z 0SB + (1 − p )Z 0L
(50)
Z 0IF = i = p Z 0L
(51)
Den = Z1AF h e
K 1d = 1 + r e
jγ
− jδ
(
+ Z1IF 1 + r e
−he
jγ
)+ Z
1BI
− jδ
Then use (11), (12) previously given for the ideal
conditions (without FACT) to calculate the apparent
measured impedance seen by the relay.
(ii) in the case of SSSC in the fault loop
Equations (1)- (4), (9), and (12) should be modified, (10) is
changed to (28), and some additional equations are
introduced [1].
Z1BF = Z1SA + (1 − p )Z1L
(29)
Z1IF = (p − +C1S )Z1L
(30)
Z1A = Z1AI + Z1IF
(31)
Z1B = Z1BF
(32)
Z 0BF = Z 0SA + (1 − p )Z 0L
(33)
Z 0IF = (p − i + C 0S )Z 0L
(34)
Z 0A = Z 0AI + Z 0IF
(35)
Z 0B = Z 0BF
(36)
[ (
)
]
(
Den = Z1AI 1 + r e jγ + Z1IF h e − jδ + Z1BF 1 + r e jγ
)
(37)
K Vse = Z1AI − h e − jδ + Z1BI
(38)
C Vse = −K Vse (Z Σ + 3R f ) r e jγ Den
(39)
C Zse = (C 0S − C1S ) C 0 (1 + 3K 0L ) Z1L
(40)
Use (28) , (11), then substitute in (41)
Z A = (P + C1S ) Z1L +
C ZSe + C VSe + 3R f
C1d + 2C1 + C 0 (1 + 3K 0 L )
Reference Number: W10-0034
(41)
(i) in the case of STATCOM installed at the far end (out of
the fault loop)
Equations (1)- (4) and (9)- (10) should be modified as:
Z1A = Z1AF
Z1B = Z1IF +
(52)
Z Sh Z1BI
Z Sh + Z1BI
(53)
Z 0A = Z 0AF
Z 0B = Z 0IF +
(54)
Z Sh Z 0BI
Z Sh + Z 0BI
(55)
[
Den = Z1BI [Z1AF E Sh + Z1IF ] + Z Sh Z1AF h e − jδ + Z1BF
(
K 1d = Z1BI (1 − E sh ) + Z Sh 1 − h e − jδ
)
]
(56)
(57)
Then use (11) and (12) to calculate the relay measured
impedance. Here, the measured impedance in the case of zero
fault resistance is equal to the impedance of the line section
between the relay and the fault location.
(ii) In the case of STATCOM installation at the near end of
the transmission line (in the fault loop)
Definition of h, δ, Z1SA, and Z0SA are modified as:
Z E + Z1SA E Sh
E ′A = Sh A
Z Sh + Z1SA
(58)
hnew = h E ′A
(59)
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The Online Journal on Power and Energy Engineering (OJPEE)
δ new = δ + ∠ E ′A
Z1SAnew =
Z 0SAnew
Z 1L
(60)
Z Sh Z1SA
Z Sh + Z1SA
(61)
EA
B
pZ 1L
( 1- p )Z 1 L
F
pZ 1L
(62)
Z1B = Z1BF
Figure (5): Power system configuration for phase
to phase fault
Z1SA
A
pZ1L
( 1-p)Z 1L
F
EB
Z Sh
Z Sh + Z1AI
A
Z 1SA
pZ 1L
B
( 1-p )Z 1L
F
Z1SB
EA
EB
Rf
(68)
Den = Z1AI Z1IF h e − jδ + Z1BF E Sh
+ Z Sh Z1AF h e − jδ + Z1BF
]
]
]
Figure (7): Three- phase faults equivalent circuit
(69)
K 1d∆ = Z1AI E Sh − h e − jδ − Z1BI [1 − E Sh ]
(70)
C1d∆ =
(71)
Den
C Sh = Z1IF [C1d∆ + 2C1 (1 − C1A )
+ C 0 (1 − C 0A )(1 + 3K 0L )]
Z A = pZ1L +
Z2SB
Figure (6): Phase to phase faults equivalent circuit
(67)
Z Sh
=
Z Sh + Z 0AI
(Z Σ + 3R f ) K 1d∆
( 1-p)Z 2L
pZ2L
Rf
(66)
[
Z1SB
(65)
Z 0B = Z 0BF
[
B
EA
Z2SA
Z Sh Z 0AI
Z Sh + Z 0AI
[
EB
Z 1 SB
( 1- p )Z 1 L
(64)
Z 0A = Z 0IF
C 0A
A
Z 1 SA
Rf
Z Z
= Sh 0SA
Z Sh + Z1SA
Equations (1)-(4) should be modified, (10) should be
changed to (57), and some new equations are introduced as
follows.
Z Sh Z1AI
Z1A = Z1IF
(63)
Z Sh + Z1AI
C1A =
Vol. (2) – No. (2)
C Sh + 3R f
C1d + 2C1C1A + C 0 C 0A (1 + 3K 0L )
(72)
(73)
It can be seen that in the absence of the fault resistance, the
measured impedance at the relaying point is not equal to the
actual impedance of the line section between the relay and the
fault location.
IV. MEASURED IMPEDANCE FOR PHASE TO PHASE
AND THREE PHASE FAULTS
Fig.5 shows the power system configuration under phase to
phase faults. Fig.6 illustrates the equivalent circuit for that
fault. Fig.7 demonstrates the equivalent circuit for three phase
fault.
Reference Number: W10-0034
Figs. 6 and 7 are similar to Fig. 4. The difference is that the
zero sequence part is removed in Fig.6. Also, both zero and
negative sequence parts are removed in Fig.7. Therefore,
similar equations to (1)- (73) formed for single phase to
ground fault case can be derived for the other two fault types.
However, some changes should be considered as:
 All zero sequence related equations are omitted. As an
example, equations (3), (4), (7) and (8) will be omitted
for the ideal (no FACT device) case.
 The term Z∑ is computed as:
Z Z
Z  = 2 1A 1B
for phase to phase fault
Z 1A + Z 1B
Z =


Z 1A Z 1B
Z 1A + Z 1B
for three phase fault
The term (3Rf ) in any equation is changed into (Rf).
Equation (12) is modified to:
Rf
Z A = pZ 1L +
for phase to phase fault
C1d + 2C1
Z A = pZ1L +
Rf
C1d + C1
for three phase fault
In the same way, the apparent impedance measured by the
distance relay can be calculated when SSSC or STATCOM
are installed. The detailed equations are provided in [2].
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The Online Journal on Power and Energy Engineering (OJPEE)
V. APPLICATION AND RESULTS
A 400 KV transmission line with the length of 300 Km has
been used for calculations. The impedances and other
parameters of the power system shown in Fig.3 are [1]- [3]:
Z1L = 0.01133 + j 0.3037
Ω/km
Z0L = 0.1535 + j 1.1478
Ω/km
Z1SA = 1.3945 + j 15.9391
Ω
Z0SA = 7.4540 + j 27.8187
Ω
Z1SB = 0.6972 + j 7.9696
Ω
Z0SB = 3.7270 + j 13.9093
Ω
h = 0.96
Vol. (2) – No. (2)
(ii) Device at mid-point
In this case, both SSSC and STATCOM are out of the fault
loop for faults on the near half of the line, while it is present
in the fault loop for faults on the far half of the line. Fig.10
shows the effect of leading FACT devices, injecting reactive
power, on the measured impedance.
δ = 16°
A. For single phase to ground fault
In the absence of FACTS devices (ideal case) Fig. 8 shows
the ideal tripping characteristic of the distance relay, which is
the measured impedance at the relying point as the fault
resistance varies from 0 to 200 ohms, while the fault location
moves from the near end up to far end of the line.
Figure (8): Ideal tripping characteristic without FACT
devices under phase to ground fault
(i) Device at the near end
For leading SSSC or STATCOM at the near end, the
calculated relay tripping characteristics is shown in Fig. 9
compared to the ideal characteristics (dashed line). With
SSSC (dotted curve), the measured resistance decreases for
high fault resistances and increases in the case of low fault
resistances. The measured reactance decreases. While with
STATCOM (solid line), the measured resistance variation is
not considerable, while the measured reactance increases.
Figure (10): Tripping characteristic for leading FACT
devices at mid-point under phase to ground fault
----- ideal, .....SSSC, ____ STATCOM
In the presence of SSSC at the mid-point, the tripping
characteristic is split into two parts. The lower part is for
faults on the near half of the line, while the upper part is
corresponding to the faults on the far half. In the presence of
STATCOM, the curve is split similar to the presence of
SSSC, but the two parts are adjoined, i.e. the upper boundary
of the lower part and the lower boundary of the upper part are
the same.
(iii) Device at far end
In this case, SSSC and STATCOM are always out of the
fault loop. Fig. 11 demonstrates the effect of the presence of
leading FACT devices on the measured impedance at the
relaying point compared to the ideal characteristics.
Figure(11): Tripping characteristic for leading FACT
devices at far end under phase to ground fault
----- ideal, .....SSSC, ____ STATCOM
Figure (9): Tripping characteristic for leading FACT devices
at near end under phase to ground fault
----- ideal, .....SSSC, ____ STATCOM
Reference Number: W10-0034
In the case of leading SSSC, the tripping characteristics
shrink but without deformation. In the case of leading
STATCOM, the measured resistance variation is not
considerable while the measured reactance decreases. The
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The Online Journal on Power and Energy Engineering (OJPEE)
Vol. (2) – No. (2)
measured impedance in the case of zero fault resistance is the
actual value.
B. For phase to phase fault
Under phase to phase fault conditions, Figs. 12- 15 are the
counterparts of Figs. 8-11. It is noted that presence of FACT
devices causes less deviation to the ideal tripping
characteristics for phase to phase faults compared to single
phase to ground faults, for the same device size, at all
possible locations of the device. The effect of device on the
distance relay tripping characteristics is much slight for phase
to phase faults when the device is installed at one of the line
ends.
Figure (15) Tripping characteristics with FACT devices at far
end under phase to phase fault
----- ideal, .....SSSC, ____ STATCOM
C. For three phase fault
Under three phase fault conditions, Figs.16-19 are the
counterparts of Figs12- 15. There is a clear similarity of
tripping characteristics as well as FACT device effect
between phase to phase and three phase fault conditions.
Figure(12): Ideal tripping characteristic without FACT
devices under phase to phase fault
Figure (16). Ideal tripping characteristics under
three phase fault
----- ideal, .....SSSC, ____ STATCOM
Figure (13): Tripping characteristic with FACT devices at
near end under phase to phase fault
----- ideal, .....SSSC, ____ STATCOM
Figure (17): Tripping characteristics with FACT devices at
near end under three phase fault
----- ideal, .....SSSC, ____ STATCOM
Figure (14): Tripping characteristic with FACT devices at
mid-point under phase to phase fault
----- ideal, .....SSSC, ____ STATCOM
Figure (18): Tripping characteristics with FACT devices at
mid-point under three phase fault
----- ideal, .....SSSC, ____ STATCOM
Reference Number: W10-0034
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The Online Journal on Power and Energy Engineering (OJPEE)
Figure (19): Tripping characteristics with FACT devices at
far end under three phase fault
----- ideal, .....SSSC, ____ STATCOM
Vol. (2) – No. (2)
Figure (22): Tripping characteristics with leading SSSC
at far end
100
Esh= 1.3
90
Esh= 1
Reactance, X(Ohms)
70
60
50
40
30
20
10
0
0
50
100
150
200
250
300
Resistance, R(Ohms)
350
400
450
Figure (23): Tripping characteristics with leading
STATCOM at near end
90
Esh=1.0
80
Esh=1.1
Esh=1.2
70
Reactance, X(Ohms)
D. Effect of compensator size on tripping characteristic
In the following, the effect of compensator size is studied
for phase to ground fault conditions. Fig.20 depicts the relay
tripping characteristics for different values of r (the series
injected voltage magnitude) when SSSC is working in lagging
mode at near end. Fig. 21 is analog to Fig.20 but for leading
mode. Fig.22 is corresponding to Fig.21 but the SSSC is at
the far end. Figs.23 and 24 show the tripping characteristics
for leading STATCOM with different values of Esh(from 1 to
1.3 p. u) located at near end and far end, respectively. The
compensator size obviously varies the tripping characteristics
as can be seen from Figs.20-23. The form of variation
depends on the operation mode(compare Figs.20 and 21) and
on compensator location (compare Figs. 21 and 22).
Changing STATCOM size results in less skewing to the relay
tripping characteristics compared to changing the SSSC size.
Esh= 1.2
Esh= 1.1
80
Esh=1.3
60
50
40
30
20
10
0
0
50
100
150
200
250
Resistance, R(Ohms)
300
350
400
Figure (24): Tripping characteristics with leading
STATCOM at far end
VI. CONCLUSION
Figure (20): Tripping characteristics with lagging SSSC
at near end
100
Reactance, X(Ohms)
80
r = 0.0
60
r = 0.1
r = 0.2
40
r = 0.3
20
0
-20
0
50
100
150
200
250
300
Resistance, R(Ohms)
350
400
450
Figure (21): Tripping characteristics with leading SSSC
at near end
Reference Number: W10-0034
In this paper, the effect of the installation of FACT
devices on the transmission line distance relay under the
phase to ground, phase to phase and three phase faults has
been investigated. This is done by means of comparing the
actual measured impedance at the relaying point and the
ideal tripping characteristics of distance relay. Three
different candidate locations of FACT device on the
transmission line are considered with respect to the
concerned distance relay, i.e. at near end, mid-point, and far
end. Besides, two possible situations for the FACT device in
terms of inclusion or exclusion of fault loop are analyzed.
The impact of FACT device size on the modified distance
relay characteristics is also assessed. The single phase to
ground fault causes much twisting to the tripping
characteristics compared to other fault types. The effect of
phase to phase and three phase faults on the tripping
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The Online Journal on Power and Energy Engineering (OJPEE)
characteristics are notably similar. Changing size and/or
location of STATCOM results in less skewing to the relay
tripping characteristics compared to the SSSC.
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