Energy and Power Engineering, 2013, 5, 468-473
doi:10.4236/epe.2013.54B090 Published Online July 2013 (
The Research of Intelligent Substation Time
Synchronization System and the Influence of Its Fault to
Relay Protection
Chang-bao Xu1, Han Xiong2, Li-fu He3, Zhong-min Li3, Jun Yang3
1Guizhou Electric Power Research Institute, Guiyang, China
2Wuhan Zhongyuan Huadian Science & Technology CO., LTD, Wuhan, China
3School of Electrical Engineering, Wuhan University, Wuhan, China
Received February, 2013
The intelligent substation realizes the digitization of information in th e whole substation and thus time synchronization
system becomes more and more important. This paper in troduces time synchron ization technology of intelligen t substa-
tion and puts forward the principles for designing intelligent substation time synchronization system. According to
some relay protection malfunction examples caused by time synchronization system fault, analyze the influence of time
synchronization system fault to relay protection and correspondingly put forward some improving measures.
Keywords: Intelligent Substation; Time Synchronization System; Protection Device; Influence
1. Introduction
Compared with the conventional transformer substation,
there exist great changes in intelligent substation struc-
ture. The intelligent substation secondary system com-
monly contains electronic transformer, merger unit (MU),
switch, protection and monitor device, etc. The complex
cable connection of transformers, protection and circuit
breaker has been replaced by fiber optics. The input of
voltage and current sampling values of protection and
control device have transformed from the analog signals
to digital signals. The analog signal sampling of protec-
tion and monitor device is realized by proposed merger
units instead of the device itself. These changes induce
higher request to intelligent substation time synchroniza-
tion system [1].
Conventional transformer substations adopt cluster
sampling. Regardless of the transmission delay of the
primary and secondary electric parameters, relay protec-
tion devices and other automation devices can ensure the
simultaneity of sampling data by sampling corresponding
TA, TV secondary parameters in a certain moment, ac-
cording to its sampling pulse. After using electronic in-
strument transformer, the data acquisition module of re-
lay protection devices and other automation devices
move forwards to merging units. The primary electric
parameters of mutual inductor need the sampling of the
front-end module and the processing of merging unit
afterward. The acquisition and processing of every mu-
tual inductor is inde pendent and has no unified coordina-
tion. And the transmission delay of the primary and sec-
ondary electric parameters brings in time delay. So the
secondary electric parameters have no simultaneity be-
tween different transformers and thus can’t be directly
used for the calculation of the automatic device. High-
accuracy clock synchronization can be used to make sure
that every interval merging unit conducts data acquisitio n
at the same time. Clock synchronization has become an
important part of relay protection, and its performance
affects the normal work of the relay protection directly
According to the demand of intelligent substation , this
paper introduces the intelligent substation time synchro-
nization technology, puts forward the principle of time
synchronization system. Afterwards, the paper analyzes a
substation malfunction examples caused by time syn-
chronization system fault and puts forward the corre-
sponding solu tions.
2. Time Synchronization Technology
Time synchronization technology provides unified time
reference for various protection and control devices,
playing a vital role in power network accident analysis
and guaranteeing the safety of power system operation
[3,4]. IEC61850 has very definite requirement to the ac-
Copyright © 2013 SciRes. EPE
C.-B. XU ET AL. 469
curacy of sampling value synchronization. It defines five
levels of synchronous accuracy, T1-T5. Among them, T1
is the lowest level, with accuracy of 1ms, and T5 is the
highest level, with the accuracy of 1us. At present, the
commonly used time system synchronization technolo-
gies in intelligent substation are network time protocol
(NTP), IEEE1588 Time Synchronization Protocol and
Time Synchronization with IRIG-B.
2.1. Network Time Protocol
Network Time Protocol is now widely used in Ethernet
clock synchronization protocol. It’s applied in the time
synchronization between distributed time server and
customer. NTP is purely based on software. It’s applica-
tion layer protocol running on top of the IP protocol and
UDP protocol [5]. It calculates the time error according
to the time information carried by the packets between
the client and server and eliminates the impact caused by
the uncertainty of network transmission through a series
of algorithms and does dynamic delay compensation.
Simple Network Time Protocol (SNTP) is a simplified
NTP server and NTP client strategy. Normally the preci-
sion of SNTP varies between 1ms and 50ms, decided by
the synchronization source, network path and other cha-
racteristics [6].
The working principle of NTP is shown in Figure 1.
Firstly, the client sends a NTP package to the server. The
package contains a timestamp 1, the time it leaves the
client. When the server receive the NTP package, it will
adding in order to the p a ckag e th e ti mestamp the p a ckag e
reaches and leaves, 2 and 3
T, and send the package to
the client. The time that the client receives the package is
4. Then, the client can calculate two key parameters of
time synchronization with these four time parameters.
These two key parameters are the package’s roundtrip
delay d and the clock offset between the client and the
time server t.
41 32
()(dTT TT) (1)
21 34
t 
Figure 1. NTP working principle.
In the NTP protocol, these four time labels from 1 to
4 are added in the application layer of the client and the
server. The transmission delay of the message roundtrip
between the customer and the server must keep stable to
guarantee the establishment of formula (2). Transmission
delay contains the delay on the network as well as the
processing time of the computer protocol layer [7]. When
uplink and downlink frame equals in length, the network
transmission delay is thought to be the same. The time
error caused by network protocol processing and opera-
tion system multitask processing can’t be eliminated and
it is millisecond level. So the NTP time protocol is
commonly thought to have millisecond accuracy [8].
NTP clock system has mature technology, simple the-
ory and simple implementation. It is suitable for station
control layer network that has low accuracy demand. But
it can’t satisfy the demand of process layer network
which has higher acc ura cy dema nd.
2.2. IEEE1588 Network Timing
IEEE1588 standard defines Precision Time Protocol
(PTP) to realize the accurate time synchronization at the
measurement and control syste m constituted by network s.
The protocol is designed for distributed measurement and
monitor, is based on the thought of message flow and
timestamp and adopts the method s of the combination of
hardware and software [9].
Compared with the methods of NTP protocol, IEEE-
1588 uses both software and hardware. Time Stamp Unit
(TSU) is located between Ethernet media access control
layer and the Ethernet receiver. It can detect both input
data flow and output data flow to get more accurate tim-
ing synchronization. PTP clock synchronization is
achieved by sending and receiving synchronization mes-
sage. Following is the procession that a master clock and
a slave clock achieve synchronization.
1) The master clock periodically sends a Sync message
to the slave clock, with a general time interval of 2 sec-
onds. The message contains the expected sending time-
stamp, which has some error compared with the actual
sending time. When the slave clock receives the Sync
message, the slave clock will record the accurate receiv-
ing time TS1.
2) The master clock sends a Follow-up message to the
slave clock, which contain the Sync message actual
sending time stamp TM1.
3) The slave clock sends a Delay-Req(Delay Request)
message to the master clock and records the specific
sending time TS3. The interval of Delay-Req message is
set independently and generally longer than Sync mes-
sage time interval. The master clock records timestamp
TM3 when receiving Delay-Req message.
4) The master clock sends a Delay-Resp(Delay Re-
sponse) message to the slave clock and the message con-
Copyright © 2013 SciRes. EPE
tains the time stamp TM3. The slave clock can accurately
calculate the network transmission delay, using TM3 and
the time stamps it records, and adjusts its clock drift er-
The method of the combination of software and hard-
ware overcomes the uncertainty of time delay protocol
stack. It makes that IEEE1588 protocol synchronization
accuracy reaches the microsecond level and provides
solution for intelligent substation process layer time
synchronizati o n [1 0] .
2.3. IRIG-B Timing
IRIG time standards are divided into two categories. One
is parallel time code. With time code being parallel, the
transmission distance is near and the code can only be
binary. So parallel time code is not so widely adopted as
the serial code. The other is serial time code. The serial
time code has A, B, D, E, G, H six kind of coding format,
among which IRIG-B code is the mostly widely used
[11]. The frame period of IRIG-B code is 1s, containing
100 code elements of which the period is 10ms [12]. The
IRIG-B code has three kinds of code element. They re-
spectively are binary “0”” 1” and location identification
marking “P” and their pulse width are 2ms, 5ms and 8ms.
The time reference point of each code element is its pulse
front and the reference mark of the frame is made up by a
location identification mark and a adjacent reference
code element. Every ten code elements has a location
identification mark, respectively named as P1, P2, ,
P9, P0. PR is the frame reference point. PTP working
principle is shown in Figure 2.
A time format frame begins from the frame reference
mark. If there are two consecutive 8ms location identifi-
cation mark, the frame begins at the front of the second
mark., the first field of IRIG-B code is the information
about seconds, the second field about minutes, the third
field about hours, and the forth and the fifth field about
Figure 2. PTP working principle.
days, calculated from January 1st. So the time signal of
the frame can be parsed out from the first five fields.
IRIG-B code contains rich time information and neces-
sary monitor information, and is convenient for the
back-end users to use.
IRIG-B time synchronization adopts direct current to
carry information and can be transmitted through special
line. It has long transmission distance high time synchro-
nization precision and rich time information. But IRIG-B
code needs point-to-point transmission and the network-
ing isn’t agile.
3. The Time Synchronization System
Designing Principles
To ensure the reliability of the time synchronization sys-
tem, the designing of in telligent substation time synchro-
nization system should follow the following principles.
3.1. The Redundant Configuration of GPS and
The system supports the redundant clock of GPS and
Compass and the IRIG-B spinning reserve, and is con-
figured with high precision punctuality clock. Time syn-
chronization master clock can access GPS+ Compass
signal and two roads of IRIG-B external synchronizing
signal. Both signals act as spinning reserve to the other.
Time synchronization expansion device can access two
roads of IRIG - B external synchronizing signal, both of
which act as spinning reserve to the other. The redundant
configuration can maintain the h igh reliability and stabil-
ity of the time synchronization system.
3.2. High Stability of Automatic Synchronization
The system adopts rubidium atomic clock to realize high
accuracy synchronization self-maintain. The precision of
the Rubidium atomic clock synchronization self-maintain
can reach 3us/day, which is far better than the standard
requirement of 55 us/day. The system adopts closed-loop
control synchronization self-maintain theory and Kalman
digital filtering technology and uses external time refer-
ence to control and tame rubidium clock. The 1PPS sig-
nal that the system outputs are achieved from frequency
division of internal frequency source. It is synchronized
with the long-term average of 1PPS signal that external
time reference outputs and overcomes the impacts caused
by second pulse signal transitions of external time refer-
ence. The time synchronization signal has very high ac-
curacy and stability, with time accuracy of ±0.1 us.
3.3. The Continuity of Time Hopping
The standard time synchronization master clock and the
Copyright © 2013 SciRes. EPE
C.-B. XU ET AL. 471
timing signal expansion device all have synchronization
self-maintain units. The master clock realizes synchroni-
zation with the external reference signal when the syn-
chronization self-maintain units receive external time
reference signal. Then it keeps a certain level of time
accuracy to guarantee the accuracy of the output time
synchronization signal when the units can’t receive ex-
ternal time reference signal. When the external time ref-
erence signal recovers, the standard time synchronization
master clock and the timing signal expansion device
switch to normal operation condition, and the switching
time is less than 0.5 s. The system takes gradually-
changing steps during the switching pro gress to maintain
the continuity of time hop ping and prevent maloperation.
3.4. Time Signal with Unified Output
Internal time delay occurs when trying to synchronize
local PPS with external reference source. Lead-time de-
lay compensation technology can be used to overcome
the delay. Every output channel has its own delay com-
pensation. Usually a time synchronization system can
realize synchronization with different kinds of ports, like
RS232, RS485, optical port, electricity port and network
port, all of which have different time delay. With differ-
ent lead-time delay compensation output signals of dif-
ferent ports can realize synchronization with the same
external clock source. Multi-port lead-time delay com-
pensation technology can set different delay compensa-
tion according to the internal standard PPS and the
channel signal configurations. It makes that the devices
fully realize synchronization with the external clock
source, overcomes the time deviation between all the
output boards, and enhances the instantaneity and the
stability of the system.
3.5. Redundancy Design of Power Supply
Power module is realized in the form of board, adopts the
redundant backup of double power source and has a wide
range of DC and AC. If one piece of power source fails,
the other can still maintain the power supply, making it
possible to do hot-swap replacement.
4. The Analysis of the Impacts of Time
Synchronization System Fault to Relay
The time synchronization system can provide time and
synchronization information to all kinds of power system
secondary device, for example, dispatching automation
system, microcomputer relay protection device, lightning
location system and etc. The fault of time synchroniza-
tion system may lead to the ineffectiveness of the col-
lected data, the failing of part of the secondary device or
even the maloperation of some protection devices.
4.1. The Analysis of a Relay Protection
Maloperation Accident Caused by the Time
synchronization System Fault
The following is the analysis of a 110 kV substation ma-
loperation caused by the time synchronization system
fault. The substation main connection is shown in Figure
3. The diagram of timing system shows in Fig u r e 4.
The following devices are connected with GPS clock
source expansion board: #2 main transformer medium-
voltage side merging units A/B, #2 main transformer
low-voltage side merging units A/B, 110 kV eastward
line merging unit, #1 main transformer high-voltage side
merging unit B and 110 kV bus merg ing unit A. And the
devices connected with Compass clock source expansion
board are: #2 main transformer high-voltage side merg-
ing units A/B, 110 kV bus merging unit B, 110 kV sec-
tion merging units and #1 main transformer high-voltage
side merging unit A.
At 18:19:24 PM December 28th 2011, 110 kV section
merging unit and #1main transformer high-voltage side
merging unit a lost synchronization. At 18:19:29, #2
main transformer high-voltage side merging units A/B
and 110kV bus merging unit B lost time synchronization.
Figure 3. The diagram of substation main connection.
Figure 4. The diagram of timing system.
Copyright © 2013 SciRes. EPE
At 18:19:30, 110 kV section merging unit and #1main
transformer high-voltage side merging unit A recovered
time synchronization. At 18:19:34.824, #2 main trans-
former high-voltage side merging units A/B and 110 kV
bus merging unit B recovered time synchronization. At
18:19:34.891, #2 main transformer A differential protec-
tion tripped three sides of switches 112, 312 and 012.
After that, the inspection results showed that primary
system devices hasn’t failed and this trip was preliminar-
ily judged to be relay protection malfunction.
In normal operation condition, merging units sample
4000 sampling point per second, counted from 0 to 3999.
Every when the synchronization second pulse arrives, the
merging units reset the sampling value and count the
sampling points from 0. Main transformer differential
protection trips according to the sample count. For ex-
ample, if the sample count of h igh-voltage side sampling
point is 0, the main transformer will compare it with the
medium-voltage side and the low-voltage side sampling
point sampling point whose sample count are 0.
Table 1 shows the contrast of the synchronization of
#2 main transformer merging unit A raw data of three
sides during the accident. When the merging unit lost
synchronization for three seconds, at 18:19:32.824345,
the sample count hopped 705 frames from 3291 to 3996.
At 18:19:34.824335, the merging unit recovered syn-
chronization and the high-voltage side had a gap of 705
frames with the medium-voltage side and the low-voltage
side. When recovered synchronization, the main trans-
former judged that the sampling data was effective,
opened the differential protection, and caused the mal-
function of the differential operation. At 18:19:36.824652,
the system lost synchronization again.
Checking results showed that 110 kV bus merging unit,
110 kV section merging units, #1 main transformer
merging unit A and #2 main transformer merging unit B
all experienced the lost of synchronization, the hopping
of count, the recovery of synchronization and the lost of
synchronization once again. The hopping amplitudes are
all 705 frames, but the hopping time and the recovery
time were different. They lost synchronization again at
the same time
Field tests showed that #2 main transformer merging
units had no synchronization self-maintain function.
When the Compass system lost satellite signal, the sys-
tem couldn’t switch to GPS time. Meanwhile all the ex-
pansion boards on GPS clock source stopped sending
timing signal, and thus the devices connected to the ex-
pansion boards lost synchronization. According to the
management of the merging units to sampling data dur-
ing the accident, it is obvious that the merging units
connected to the expansion boards on GPS clock all
hopped the same extent o f 705 fram es .
When the sampling lost synchronization, the main
transformer locked the differential protection. When the
sampling recovered synchronization, the main trans-
former opened the differential protection and calculated
the remainder of the sample count divided by 80 to make
sure the synchronization of sampling points from three
sides. On this condition, only time delay caused by net-
work transmission was taken into consideration and the
main transformer couldn’t deal correctly with the cases
that the time difference longer than one cycle.
The direct cause of the accident is the defect of the
main transformer relay protection. When the sample
count of three sides didn’t agree with each other, the dif-
ferential protection was not normally locked and mal-
functioned. The indirect cause is that the system couldn’t
normally switch to GPS clock source and stopped send-
ing timing signal when the system lost the Compass
clock source signal. It directly caused the inconformity of
the sample count of the three sides.
Table 1. The contrast of the synchronization of #2 main
transformer merging units A.
Merging units A
Time High
:29.000838 3998 3997 3997
Losing synchroniza-tion,
no hoppoing
:29.001323 0 3999 3999
:30.001343 0 3999 3999
:31.001343 0 3999 3999
:32.001343 0 3999 3999
:32.824345 3996 3291 3291 Hopping from
3291 to 3996
:32.82534 0 3294 3295
:33.825339 0 3294 3295
:34.824335 3995 3290 3291
Recovering synchro-
niza-tion, no hopping
:34.82534 0 3294 3295
:34.89110 262 3557 3558
:36.824652 3998 3292 3293 Losing synchroniza-tion
Copyright © 2013 SciRes. EPE
Copyright © 2013 SciRes. EPE
4.2. Some Improving Measures to Protection
Devices and the Time Synchronization
On the basis of a detailed analysis of the cause of the
accident, put forward the following suggestions and
measures for protection devices and the time synchroni-
zation system.
1) Some logical criterion should be added to the main
transformer protection: when the sampling synchroniza-
tion clock bit is “FALSE”, the main transformer should
lock the differential protection; when the synchronization
recovers, the sampling synchronization clock bit should
switch to “TURE”. The main transformer protection
should be able to judg e whether the sample counts of the
three sides are consistent; if they aren‘t consistent, the
main transformer protection should lock the differential
protection until the sample counts recover accordance.
2) Eliminate the defects of the timing system, maintain
the normal switch between different clocks, and make
sure that there is no clock hopping during the switch.
3) The three merging units of the three sides of the
same main transformer use the same clock expansion
4) In the case of double configuration of merging units,
the two merging units of the same protection device can’t
use the same clock expansion board.
5) The system should provide corresponding warning
signals to the background.
6) Punctuality function should be added to merging
units to satisfy relevant technical standards.
5. Conclusions
The intelligent substation realizes the digitization of all
the substation information, the network of communica-
tion platform and the standardization of information
sharing. All the devices and subsystems exchange data
through fiber optic Ethernet, putting forward higher re-
quirements to the substation time synchronization system.
This paper introduces several time synchronization tech-
nologies commonly used in practice, gives the design
principles of time synchronization system, analyzes the
impacts of time synchronization system error to protec-
tion devices, and puts forward some improving measures,
which is vital to the reliable operation of the intelligent
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