The Analysis of Mal-operation of Protection Relay
for Detecting Disconnection of Neutral Grounding Reactor
Yeonwook Kang, Joosik Kwak, Heeyoung Ryu, Dongjin Kweon and Eungbo Shim
Korea Electric Power Research Institute
103-16 Munji-dong, Yuseong-ku, Deajeon, Korea
Abstract
As the electric system is getting larger to meet the increasing demand for electric power, the rating of power
apparatus is becoming inevitably higher in its working voltage and larger in its capacity. According to KEPCO reports,
power transformers in the KEPCO system have undergone troubles such as winding short insulation breakdowns
every year since 1981. The cause of these troubles was high one line grounding fault currents in KEPCO systems that
had direct grounding systems. KEPCO has installed the NGR(neutral grounding reactor) to lower this fault current
and reduced winding short insulation breakdowns in power transformers. But when a circuit breaker opened a no load
bus, some trips of circuit breakers to protect transformer have occurred due to an incorrect operation of the
59GT(overvoltage ground relay) that detects disconnections of NGR. Therefore, in this paper, we analyzed the cause
and examined the effect of a time delay circuit to prevent wrong operation of 59GT.
Keywords: NGR(Neutral Grounding Reactor), EMTP, Time Constant, Transformer
mal-operation of the 59GT relay by EMTP(Electro-
1 Introduction
High
kV
magnetic transient program). And we also examined
transformers when one line-to-ground fault occurred in
the effect of the time delay circuit to prevent incorrect
KEPCO's distribution line because the system was a
operation of the 59GT.
direct
fault
currents
grounding
flowed
system.
In
into
order
154/23
to
reduce
transformer faults such as winding short insulation, the
2 Background
NGR was installed between the transformer neutral
There has been a large increase in power transformer
point and the ground.
problems in the KEPCO system since 1980. According
If NGR is disconnected and one line-to-ground fault
to the KEPCO survey, 92 transformers (154kV/23kV)
occurs, then there are line-to-line voltages on
have experienced problems from 1981 to 1988 and 62
un-faulted lines that cause insulation problems on
of those were damaged by fault currents that flowed
utilities and loads.
through neutral points of transformers.
Therefore, the 59GT has been used to watch for
To reduce the fault current flowing into the
disconnection of NGR in KEPCO but some trips of
transformer under the condition of a fault, the KEPCO
circuit breaker to protect transformer have occurred
changed the 23kV side of transformer from a direct
through the improper operation of the 59GT, when
grounded system to a neutral grounded system using a
circuit breaker opened only no load and line charged
reactor as shown in Fig 1.
bus.
The value of the reactor L is determined to make one
In this paper, we introduced the background of use of
phase-to-ground fault current equal to 85% that of a
NGR in KEPCO system and analyzed the cause of the
three phase-to-ground fault current like equation (1).
The value is about 6 ohms.
L=
13.2kV/63.5V). The input of 59GT is
2X1 − X 0 − X 2
3
→
---------------(1)
→
→
→
EPT = E A + EB + EC
---------------(2)
where X0 : Zero Sequence Impedance
where EPT : Input Voltage of 59GT
X1 : Positive Sequence Impedance
63.5V
13.2kV
63.5V
EB : Voltage of Phase B of 22.9kV bus x
13.2kV
63.5V
EC : Voltage of Phase C of 22.9kV bus x
13.2kV
X2 : Negative Sequence Impedance
E A : Voltage of Phase A of 22.9kV bus x
VB
N
VC
VA
L
IL
IF
IA
IC
IB
Under normal conditions, the input voltage of the
XC
XC
XC
59GT, EPT is near zero as shown in Fig 3, because the
voltages of each phase of 22.9 kV bus are nearly the
Fig 1. NGR installation to reduce the fault current
same and their vector sum is near zero.
3 Detection of NGR disconnection
B
To reduce this high failure rate(approx. 3%), KEPCO
EB
E A = 63.5 V
E B = 63.5 V
EC = 63.5 V
has inserted the reactor between the neutral point of
the 23kV side of transformer and the ground.
Accordingly, faults in transformers have decreased
N
since 1993.
E PT = 0 V
If NGR is open and one phase-to-ground fault occurs,
then the voltages of the unfaulted phases are increased
A
EA
by 1.73. Therefore KEPCO has used the 59GT to open
C
EC
(a) Phasor diagram
the circuit breakers of the 1st & 2nd side of
transformer and to protect the loads from overvoltages
on the unfaulted phases when the one phase-to-ground
fault occurs under the condition of the disconnection
of NGR, as shown in Fig 2.
300
[V]
200
100
0
22.9kV BUS
154kV BUS
A
1차 CB
M .Tr
2차 CB
B
: CB Close
-100
: CB Open
-200
∆ Y
NGR
-300
0
10
(file PAPER_04.pl4; x-var t) v:PT_A -PT_G
20
30
v:PT_A -PT_B
v:PT_B -PT_C
40
[ms]
v:PT_C -PT_G
50
(b) Simulation result by EMTP
Fig 3. Voltages at unfaulted condition
59GT
59GT
If phase A is faulted to ground, the ground point
Fig 2. Detection of disconnection of NGR by 59GT
moves to N' from N and voltage of phase A becomes
59GT detects the unbalanced voltage of 22.9 kV bus
zero and voltages to ground in phases B and C
through the PT(potential transformer, Y/open delta,
increase. Therefore EPT does not become zero, but
about 106 V as a result of a simulation under the
assumption that the ground fault resistance is ignored
as shown in Fig 4.
300
[V]
200
100
0
EB B
EB = 78 V
-100
EPT
EC = 78 V
EPT = EB + EC
= 106 V
-200
-300
0
10
(file PAPER_06.pl4; x-var t) v:PT_A -PT_G
N
20
30
v:PT_A -PT_B
v:PT_B -PT_C
40
[ms]
v:PT_C -PT_G
50
(b) Simulation result by EMTP
N'
Fig 5. Voltages at one phase-to-ground fault
A
in disconnection state of NGR
C
EC
In one phase-to-ground fault, as we know, the input
(a) Phasor diagram
voltage of the 59GT is about 106 V in the connected
state of NGR and 190 V in the disconnected state of
NGR. The 190 V value is based on the assumption that
300
[V]
the ground fault resistance is ignored in one
200
phase-to-ground fault and NGR is open. If the ground
100
fault resistance is higher than zero, the voltage is lower
0
than 190 V.
According to a KEPCO report, it is assumed that the
-100
maximum ground fault resistance is 50 ohm in
-200
KEPCO’s system, and that the minimum operating
-300
0
10
20
(file PAPER_05.pl4; x-var t) v:PT_A -PT_G
30
v:PT_A -PT_B
40
v:PT_B -PT_C
[ms]
v:PT_C -PT_G
50
voltage of the 59GT is 120 V. Therefore when the
(b) Simulation result by EMTP
input voltage of the 59GT is above 120 V, the
Fig 4. Voltage at one phase-to-ground fault
transformer is tripped to prevent the overvoltage of the
system and load since one phase-to-ground fault may
If phase A is faulted to ground in the disconnection
occur in the disconnection state of NGR.
state of NGR, the ground point moves to N'(A) from N
like an ungrounded system, and voltages to ground in
4 Mal-operation of the 59GT
phases B and C increase in magnitude by 1.73 times as
KEPCO operates two buses on the second side of
shown in Fig 5.
154/23 kV transformers with section breakers for the
purpose of parallel load shedding. For regular
E BA
E PT
B
provides power to all loads and the other charged bus
E B = 109 V
EC = 109 V
EPT = E B + EC
N
= 190.5 V
C
A
N'
examination of circuit breakers, when one bus
ECA
(a) Phasor diagram
is open as shown in Fig 7, the 1st & 2nd breakers of
the transformer sometimes are tripped by the
mal-operation of the 59GT relay.
After mal-operation of the 59GT relay, input voltages
of the 59GT were measured in the same conditions as
a section breaker opened charged bus without load as
shown in Fig 6. But the voltages were below 120 V for
the operation of the 59GT .
80
[V]
60
40
20
0
-20
-40
0.00
0.02
0.04
0.06
0.08
[s]
0.10
[s]
0.10
[s]
0.10
(file PAPER_C3_5_2.pl4; x-var t) v:XX0070-E_PT
(a) Input voltage of 59GT in M substation
(a) Discrepancy time = 2.8msec
120
[V]
80
40
0
-40
-80
(b) Input voltage of 59GT in Y substation
-120
0.00
0.02
0.04
0.06
0.08
(file PAPER_C3_5_3.pl4; x-var t) v:XX0070-E_PT
Fig 6. Overvoltages at opening no load bus
(b) Discrepancy time = 10msec
150
[V]
When a circuit breaker is opened and closed, it has a
100
discrepancy time among phases as shown in Table 1.
50
The input voltage of the 59GT appears different
0
according to the discrepancy time of the circuit breaker.
-50
For the analysis of the mal-operation of the 59GT relay,
-100
it is assumed that the discrepancy time of the circuit
-150
breaker may be longer than the standard time. Because
the section breaker open for the regular examination
-200
0.00
0.02
0.04
0.06
0.08
(file PAPER_C3_5_4.pl4; x-var t) v:XX0070-E_PT
after several years, the section breaker remains closed
(c) Discrepancy time = 20msec
for a long time in a state of current flow that may
Fig 7. Input voltages of 59GT by discrepancy times
make the contacts of circuit breaker stronger due to
When the discrepancy time is 2.8msec, the input
Joule's heat.
voltage of the 59GT becomes about 50V. However the
Table 1. Discrepancy time (Below 72.5 kV, KEPCO)
discrepancy time becomes longer than 2.8msec, and
Classify
Time
the input voltage of the 59GT is higher. As shown in
3 phase open discrepancy time
Below 1/6 cycle
Fig 7, when the discrepancy time is 20msec, the
3 phase close discrepancy time
Below 1/4 cycle
voltage becomes greater than 180V that can operate
the 59GT relay sufficiently.
In this paper, it is assumed that discrepancy times of
section breakers are 2.8, 10, 20msec for the simulation
5 The improved circuit of the 59GT relay
of input voltage of the 59GT by EMTP, when a section
The transformer was tripped several times through the
breaker opens a charged bus without load. The result is
mal-operation of the 59GT relay, when a section
shown in Fig 7.
breaker opened a charged bus without load. Therefore
the 59GT relay should be improved in order to prevent
mal-operation. The improved circuit of the 59GT relay
Ground
is shown in Fig 8. The input voltage of the 59GT is
Transactions on Industry Applications, vol. 36,
rectified by diodes and then the time for operating
pp.16-21, 2000.
excitation coil is delayed by the R-C time constant
[3] Garry E. Paulson, "Monitoring Neutral-Grounding
until the voltage of the coil becomes 24V. It is helpful
Resistors ", IEEE, 1999.
to prevent mal-operation within about 2 cycles as
[4] John P. Nelson and Pankaj K. Sen, "High
shown in Fig 9.
Resistance Grounding of Low Voltage Systems: A
Differential
Protection
Relays",
IEEE
Standard for the Petroleum and Chemical Industry",
IEEE, 1996.
59GT
R1
R2
V (t )
Biographies
R
C
VIN
Fig 8. Improved circuit of 59GT
150
[V]
100
50
0
-50
-100
-150
-200
0.00
0.02
0.04
0.06
(file Paper_C3_5_4.pl4; x-var t) v:XX0070-E_PT
0.08
[s]
0.10
v:RY_T -RY_G
Fig 9. The operating voltage of 59GT relay at
abnormal condition
6 Conclusion
In this paper, we analyzed the cause of mal-operation
of the 59GT relay and appropriateness of the improved
circuit of the 59GT relay. When the circuit breaker has
not done operation with closed state for a long time,
the discrepancy time of it may become longer than the
standard
time.
And
it
sometimes
causes
the
mal-operation of the 59GT relay to trip the 1st & 2nd
circuit breakers of transformers. To prevent this
mal-operation, 59GT relay should be improved. In this
paper, we analyzed that the improved 59GT is
adequate to prevent it by EMTP simulation.
REFERENCES
[1] "A study for the reduction of the power transformer
failures", KEPRI Final Report, 1992.
[2] Peter E. Sutherland, "Application of transformer
Yeon wook Kang : He received B.S.
and M.S. degree from Chungnam
University, Korea in 1988 and 1990,
respectively. He has been worked for
Korea Electric Power Research
Institute since 1993. His research
interests include lightning outage rate calculation and
insulation coordination.
Joo sik Kwak : He received B.S. and
M.S.
degree
from
Chungbuk
University, Korea in 1994 and 1996,
respectively. He has been worked for
Korea Electric Power Research
Institute since 1996. He is interested in
the field of analysis of power system overvoltages and
lightning protection.
Hee young Ryu : He received B.S.
from Dankook University, Korea in
1995. He has been worked for Korea
Electric Power Research Institute
since 1995. His research interests
include insulation coordination and
substation protection.
Dongjin Kweon : He received his
BSEE from Seoul National Industry
University in 1986. And he received
his MSEE and Ph. Doctor from
Soong-sil University in 1995. He has
been worked for Korea Electric Power
Research Institute since 1995. He is interested in the
field of diagnostic techniques and maintenance for
power equipment.
Eung bo Shim : He received B.S.
degree from Hanyang University,
Korea in 1982. He has been worked
for Korea Electric Power Corporation
since 1982. He is working at Korea
Electric Power Research Institute. His
research interests include the analysis of power system
overvoltages and insulation design.