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Journal of International Council on Electrical Engineering Vol. 2, No. 1, pp. 14~19, 2012
A Self-healing Scheme of Single-phase-to-ground Fault in Smart Power
Distribution System
Shen-Xing Shi† and Xin-Zhou Dong*
Abstract – The paper presents a self-healing scheme of single-phase-to-ground fault in smart power
distribution system. The scheme consists of three parts: single-phase-to-ground fault detection, faulted
feeder selection and faulted feeder isolation. Fault detection is based on power-frequency voltage
measured by a voltage transducer. A single-phase-to-ground fault is detected when measured voltage is
greater than threshold. A fault generated current traveling waves are used to identify faulted feeder. The
feeder with different initial traveling wave’s polarity to other feeders is selected as faulted feeder. When
faulted feeder is identified, the interconnection switch between different bus-bar is switched on firstly,
then the switch at the load side of faulted feeder is switched off to isolate the load from the fault, the
switch at the power source side of faulted feeder is switched off to isolate the source from the fault lastly.
Simulation studies show that the scheme can meet the requirements of isolating single-phase-to-ground
fault without affecting power supply to the load.
Keywords: Smart power distribution system, Single-phase-to-ground, Transient traveling waves, Fault
isolation
1. Introduction
Fault self-healing is the key to smart power system[1-3].
As far as power distribution system is concerned, fault selfhealing means to isolate the faulted equipment without
affecting power supply to the load. Unfortunately, until now,
there has been no fault self-healing technology. Any fault in
power system should be detected and processed by
protective relaying, which will trip the fault as soon as
possible when the fault is confirmed. And when the fault
in power distribution system is isolated, power supply to
the load has to be interrupted [4-6].
In power distribution system, the power grid is radial and
feeders are connected with load directly to supply power.
The reliability of power supply to the load depends on safe
and reliable operation of feeders. However, the fault in the
feeders is inevitable. When a fault occurs, the faulted feeder
will be isolated and power supply to the load will be
interrupted. And most of the fault in power distribution
system is single-phase-to-ground. According to the statistics
from power utilities in china, more than 70% fault in power
distribution system is single-phase-to-ground. In order to
improve the reliability of power supply to the load, the
†
Corresponding Author: Dept. of Electrical Engineering, Tsinghua
University, China (shishenxing@tsinghua.edu.cn)
*
Dept. of Electrical Engineering, Tsinghua University, China (xzdong
@tsinghua.edu.cn)
Received: December 6, 2011; Accepted: December 28, 2011
neutral in power distribution system is always ungrounded
or grounded through Peterson Coil in china. In such power
distribution system, single-phase-to-ground fault can’t
affect power supply to the load instantly for phase-to-phase
voltage is still symmetrical and there is no short circuit
current flowing in the system. At the same time, singlephase-to-ground fault generated current is so small that
over-current protection relays used in power distribution
system can’t detect the fault, which will lead to phase-tophase fault. When a phase-to-phase fault occurs, overcurrent protection will trip the circuit breaker as soon as
possible to isolate the fault. To reduce the time of power
supply recovery, two or more feeders to one load are built
in typical urban power distribution system in china.
Focusing on the fault in urban power distribution system
with neutral none-effectively grounded, much technology
has been proposed and used in the field. Feeder automation
technology has been presented to respond to short-circuit
fault like phase-to-phase fault; while faulted feeder selection technology has been proposed to process single-phaseto-ground fault. Feeder automation technology detects
short-circuit fault based on over-current and trips the
corresponding circuit breaker to isolate the faulted feeder.
Single-phase-to-ground fault feeder selection technology
selects the faulted feeder when the fault occurs. And the
faulted feeder is isolated by operating staffs. Whatever the
fault is, has power supply to the load to be interrupted
before the backup power supply is switched on. Therefore,
Shen-Xing Shi and Xin-Zhou Dong
the existed technology corresponding to the fault in power
distribution system can’t meet the requirements of fault
self-healing, the key to smart grid.
The paper discusses a novel single-phase-to-ground fault
processing scheme and presents a fault self-healing scheme
in smart power distribution system. The scheme comprises
single-phase-to-ground fault detection, faulted feeder
selection and single-phase-to-ground fault isolation. The
paper proposes basic principle at first, then presents scheme
implementation, verifies the scheme by simulation studies,
and draws a simple conclusion at last.
2. Basic Principle
The neutral in power distribution system is always noneffectively grounded, including neutral ungrounded, neutral
grounded through Peterson coil or neutral grounded through
high resistances. In such a power distribution system, when
a single-phase-to-ground fault occurs, it is not necessary to
trip the faulted feeder instantly for limited fault current.
And it provides the possibility to achieve fault self-healing.
In order to achieve fault self-healing function, fault
detection and fault isolation are indispensable. Therefore,
the scheme presented will detect the fault, then determines
the faulted feeder and isolates the faulted feeder without
affecting power supply to the load. The basic principle of
fault detection, faulted feeder selection and faulted feeder
isolation will be analyzed in details.
2.1 Single-phase-to-ground fault detection
In power distribution system with neutral non-effectively
grounded, three phase voltages are symmetrical, and zerosequence voltage is close to zero when the system is in
normal operation. However, when a single-phase-to-ground
fault occurs, the voltage in the faulted phase decreases
while the other two phases voltages increase. For a solid
fault, the faulted phase voltage decreases to zero and the
other two phases voltages increase to phase-to-phase
voltage, 3 times phase-to-ground voltage. Therefore, the
single-phase-to-ground fault can be detected by using
voltages. In the paper, the voltage at the open delta side of
voltage transducer in the bus-bas is used to detect the fault.
The voltage setting is set to avoid the maxim asymmetrical
voltage when the system is in normal operation. When the
measured voltage is higher than the setting, the fault is
detected.
2.2 Single-phase-to-ground fault feeder selection
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When a single-phase-to-ground fault occurs in one feeder,
fault generated traveling waves will propagate from fault
point along the feeder to the system. Arriving at the point
where wave impedance changes, for example, bus-bar in
the substation, the traveling wave will be reflected and
refracted. At the instant current traveling waves arrived at
the point, the current traveling waves inflow to the point are
equal to the current traveling waves outflow, because they
satisfy Kirchhoff's current law. As far as the bus bar is
concerned, the current traveling wave in the faulted feeder
is inflow current, while the current traveling wave in the
healthy feeder is outflow current. For the number of faulted
feeder is only one, and the number of healthy feeder is at
least two, the current traveling wave in the faulted feeder is
equal to the sum of current traveling waves in healthy
feeders connected to the bus bar. If the current reference
direction in all feeders is defined as the same, like from the
bus bar to the feeder, the polarity of the current traveling
wave in the faulted feeder is reverse to the wave in the
healthy feeder. Thus, when a single-phase-to-ground fault
occurs, it is the faulted feeder with the highest current
traveling wave and different polarity to the others, which is
the basic principle to identify single-phase-to-ground fault
feeder in power distribution system with the neutral noneffectively grounded [7-10].
2.3 Single-phase-to-ground fault isolation
Fault isolation is the core to fault self-healing scheme. In
a radial power grid, to isolate the fault can be done by
tripping the circuit breaker at the power source side of the
faulted feeder, which will interrupt the power supply to all
load in the downstream of the faulted feeder. In order to
reduce the blackout area the switchgear at the load side of
the faulted feeder can be tripped. To recover the power
supply, the backup power source can be put into operation
by closing the switchgear at the open-ring point. At present,
it is to trip the circuit breaker at the power source side
firstly and then to open the switchgear at the load side to
isolate the fault, which will inevitably interrupt the power
supply to the load. As a single-phase-to-ground fault will
not lead to high fault current and phase-to-phase voltage
change, it is possible to close the switchgear at the openring point when a fault occurs. Therefore, in the new
scheme, single-phase-to-ground fault isolation is achieved
by closing the switchgear at the open-ring point at first,
then opening the switchgear at the load side of the faulted
feeder, opening the switchgear at the power source side of
the faulted feeder at last, which will guarantee the power
supply to the load.
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A Self-healing Scheme of Single-phase-to-ground Fault in Smart Power Distribution System
3. Scheme Implementation
The scheme will be implemented by self-healing system,
which is made up of self-healing devices. Self-healing
devices are installed in a power substation and power
switch stations. Self-healing devices in the power
substation and power switch stations are called master and
slave machines respectively. And the master machine and
slave machines will exchange the decision result by
communication. The self-healing device collects bus-bar
voltages, zero-sequence currents in all feeders and switches’
or circuit breakers’ status information in real time. Based on
the voltage information, the self-healing device determines
whether a single-phase-to-ground fault occurs in the system.
At the same time, the device identifies which feeder the
disturbance is from. When a single-phase-to-ground fault is
determined and the faulted feeder is identified by the device,
the self-healing device will send a signal to the other selfhealing device in the faulted feeder and waits for the signal
from other self-healing device. If the received signal shows
the same faulted feeder as the master self-healing device
determines, the faulted feeder is confirmed. And based on
the switches or circuit breakers status, the self-healing
device installed at the load side of the faulted feeder at first
output the signal to close bus tie circuit breaker, and then
trips the circuit breaker at the load side of the faulted feeder,
which will isolate the fault from the load. After a fixed
delay, the self-healing device in the power substation will
trip the circuit breaker to isolate the fault from the power
source. The whole flowchart is as shown in Fig. 1.
As far as hardware is concerned, the self-healing device
is made up of communication unit, input unit, central
processing unit and output unit. The communication unit
sends the result of faulted feeder determined to the other
device installed at the other terminal of the faulted feeder,
and receives the result of the faulted feeder from the other
devices to confirm whether the protected feeder is faulted.
The input unit inputs the signals from current transducers of
the protected feeders and voltage transducers of the bus-bar,
and the status signals from circuit breakers or switches. The
central processing unit analyzes the signals from the input
unit and determines whether a single-phase-to-ground fault
occurs and which feeder is faulted. The output unit outputs
the signals to trip or close the corresponding circuit
breakers or switches. The hardware of self-healing device is
as shown in Fig. 2.
Fig. 2. The hardware of self-healing device.
4. Simulation Demo
Fig. 1. The scheme flowchart.
A radial power distribution network is described in Fig. 3.
Two transformers SZ9-10000/35 are installed in power
substation with the ratio of 35/10.5kV. And two groups of
capacitances with the capacity 2000kVA are installed.
Feeders are the mix of cable and overhead lines, with the
cable in the power substation and overhead line outside of
the substation. For the length of the cable is so short, it is
omitted in the simulation. When the system is under normal
operation, the bus tie switches both in power substation and
switch station are open. In the system, the overhead lines
are LGJ-70, LGJ-90 and LGJ-120 respectively, with the
length from 1km to 5km. The load current is from 50A to
Shen-Xing Shi and Xin-Zhou Dong
100A. The simulation is based on EMTP. Suppose a singlephase-to-ground fault occurs in Feeder L123, the simulation
results are as shown in Fig. 4-Fig. 7.
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tripped the switch B921, and the fault was isolated from the
switch station and the load supplied by Feeder L123 was
transferred. At T3, when the switch B921 was tripped, the
self-healing device in the switch station sent a signal to the
device in the power substation to trip the switch B123 to
isolate the fault. Thus, the system went back to normal
operation.
3i0 / A
L111
L112
L113
Fig. 3. A simulation power distribution system.
L114
t/
s
a) Busbar I
Fig. 4. Voltages at open delta side measured by voltage
transducers in the substation.
When a phase A-to-ground fault occurs at F1 in Feeder
L123 at T0, the voltage at open delta side of voltage
transducer connecting to bus bar II increases significantly
in power substation, as shown in Fig. 4, and fault generated
traveling waves can be measured by zero-sequence current
transducers installed in Feeder L121—L124, as shown in
Fig. 5. At the same time, the voltage and the current
traveling waves in the switch station have the same changes,
as shown in Fig. 6 and Fig. 7 respectively. Wherever it is in
the power substation or in the switch station, the current
traveling wave in Feeder L123 is reverse to the traveling
waves in other feeders. Therefore, the fault can be detected
by the zero-sequence voltage, and the faulted feeder can be
selected by current traveling waves measured by zerosequence current transducers. The self-healing device in the
power substation selected the faulted Feeder L123, and the
device in the power switch selected the same feeder as the
faulted feeder. The self-healing devices exchanged the fault
result, and confirmed the fault feeder. At T1, the selfhealing device in the switch station closed the bus tie circuit
breaker. At T2, the self-healing device in the switch station
3i0 / A
L121
L122
L123
L124
b) Busbar II
Fig. 5. Current traveling waves measured by zero-sequence
current transducers of feeders in the substation.
Fig. 6. Voltages at open delta side measured by voltage
transducers in the switch substation.
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A Self-healing Scheme of Single-phase-to-ground Fault in Smart Power Distribution System
source terminal will be tripped, which is different to
the traditional feeder automation scheme.
Acknowledgements
The authors would like to thank National Natural Science
Foundation of China under Grant 50937003, Ministry of
Education of China under Grant 20090002120028 and The
Chinese Society for Electrical Engineering under Youth
Foundation 2009 for their supports to finish the work.
a) Busbar I
b) Busbar II
Fig. 7. Current traveling waves measured by zero-sequence
current transducers of feeders in the switch substation.
5. Conclusion
The paper presents a self-healing scheme of singlephase-to-ground fault in smart power distribution system.
The scheme includes fault detection and fault processing.
Some conclusions can be drawn from the study:
1. Single-phase-to-ground fault self-healing is available
in smart power distribution system;
2. Single-phase-to-ground fault self-healing should be
supported by primary equipments in power distribution system. In the scheme, one load can be supplied
power by two or more feeders;
3. Single-phase-to-ground fault self-healing should be
supported by smart protection and control scheme in
power distribution system. In the scheme, the switchgear at the load terminal of the faulted feeder should
be tripped at first, then the switchgear at the power
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Shen-Xing Shi received the B.S. and
Ph.D. degree in electrical engineering
from Northeast Institute of Electrical
Power, Jilin, and Tsinghua University,
Beijing, China, in 1999 and 2006,
respectively. His research interests are
power system analysis and power
system protection.
Xin-Zhou Dong was born in Shanxi
Province, China, in 1963. He received
his bachelor’s degree in 1983, his
master’s degree in 1991 and a Ph.D.
degree in 1996 from the Department of
Electrical Engineering, Xi’an Jiaotong
University, China. He furthered his
post-doctoral research at the Electrical Engineering Station
of Tianjin University from 1997 to 1998. At present, he
serves as a professor of Tsinghua University. His research
interests include protective relaying, fault location and
application of wavelet transforms in power systems. He is
author or co-author of more than 100 journal papers.
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