Energy and Power Engineering, 2013, 5, 463-467
doi:10.4236/epe.2013.54B089 Published Online July 2013 (http://www.scirp.org/journal/epe)
Solution Scheme for Time Synchronization of Current
Differential Protection between Digital Substations*
Bin Duan, Yao Wu
College of Information, Xiangtan University, Xiangtan, China
Received January, 2013
ABSTRACT
In digital substations with power transformers, the key to implementation of line differential protection is synchroniza-
tion of electrical quantities at both two sides. System framework of current differential protection for lines between dig-
ital substations is analyzed briefly, as well as the necessity of demand for time synchronization. Principle of time syn-
chronization based on GPS and IEEE1588 PTP (Precision Time Protocol) are introduced. Time synchronization of cur-
rent differential protection between digital substations based on IEC61850 is discussed. This paper puts forward two
solution schemes of time synchronization, analyzes their advantages and disadvantages respectively, and points out that
the development direction of time synchronization of differential protection between digital substations is IEEE1588
PTP.
Keywords: Differential Protection; Time Synchronization; GPS; IEEE1588
1. Introduction
At present, synchronous clock source based on global
positioning system (GPS)[1] is widely used in electric
power system in China. GPS has its unique superiority
and is the best candidate synchronous clock source in
available clock reference sources. Second pulse statistical
error of its output is 1 μs and has no cumulative error,
which can meet requirements of many application fields
for synchronous clock. Substation receives a standard
Second Pulse signal (Pulse per Second, PPS) from GPS,
then send synchronous Pulse to total IED of all substa-
tions through special cable. This scheme can realize time
tick of multiple IED clocks with ease. However, the de-
velopment trend of digital substation is that secondary
hardwiring in station will be replaced by serial commu-
nication line. The clock system Based on GPS has
showed some limitations.
IEEE 1588[2,3]defines a precision time protocol(PTP)
used in distributed measurement and control system.
IEEE1588 has important significance for construction of
digital substation and provides a good technical options
for realization of IEC61850 [4] T5 level precision clock
as a distributed network time synchronization scheme
[5,6] whose accuracy can achieve submicro second level.
Despite IEEE1588 is still in the test phase, its high preci-
sion network time tick has distinctive characteristics
which indicate its bright application prospect.
This paper briefly introduces system architecture of
current differential protection for lines between substa-
tions [7-9], and respectively analyses synchronous solu-
tions based on GPS and IEEE1588. The aim is to provide
reference for engineering application of data synchroni-
zation of current differential protection between digital
substations in future.
2. The Structure of Protection System and
Requirement Analysis of Data
Synchronization to Current Differential
Protection based on the Information
Exchange between Substations
2.1. The System Architecture of Information
Exchange in Substations，which Supports
the Protection of Current Difference Lines
This diagram includes circuit breaker, current trans-
former, information collector(filter, hybrid circuit, A/
Dconverter), emitter，receiver, remote switching system，
time-delay device and protection device. First, current
transformer measures current value iA, and the current is
converted into right current value which information
collector can capture. Second, information collector of A
collects data and converts analog signal into a digital
signal. Finally through the remote exchange system,
transmitter will sent the collected data of A to B, through
*This research was supported by National Natural Science Foundation
of China (NSFC) (No.61170191). Scientific Research Fund of Hunan
Provincial Education Department (11CY017)．

B. DUAN, Y. WU
464
a time-delay device, the collected data of B which is
captured by the data acquisition unit will realize data
synchronization with the data of A. Then we can get the
iD by compared the data of A with B, and if it is larger
than set value meaning that line has internal fault, both A
and B circuit breaker will disconnect, otherwise they will
be normal operation. In the process of data acquisition
and transmission, the protection device has been up to
protect data without being interfered (If in the ideal case,
the value of iD will be zero).
2.2. The Demand for Data Synchronous
For line current differential protection, the most impor-
tant is to compare the current of “the same moment” on
both sides. As can be seen from Figure 1，guide lines can
directly transmit secondary current of short circuit (less
than 10 km) on both sides, which does not exist “differ-
ent moment” problem. However, when it transfers the
current of both sides through the communication channel,
the system should carry digital data acquisition for cur-
rent instantaneous value of each side. Sampling rate of
protection commonly should be 12 - 24 points each
working frequency, and phase difference is 30˚ - 15˚. So
in order to work properly, protection must use the syn-
chronous data on both sides. The “data synchronization”
of both sides contains two meanings: One is the sampling
time of both sides must be strictly same, which is known
as synchronous sampling; The other is to use the sam-
pling data of both sides at the same time to calculate dif-
ferential current, which is called data-window synchro-
nization. However, both ends of the line are apart from
hundreds of kilometers, which are not able to use the
same time to ensure time-unifica- tion and sample syn-
chronization. The technical problem of current differen-
tial protection between digital substations must be solved
to ensure unity of the clock time of two remote substa-
tions and strict synchronization of sampling instant[10].
Figure 1. Current differential line protection.
3. Introduction to Two Synchronous
Technologies
The existing main synchronization technology: one is
based on the global positioning synchronous (GPS) and
the other is based on IEEE1588 network time synchroni-
zation technology[11,12].
3.1. GPS Time Synchronization
GPS is a new generation of space satellite navigation and
positioning system developed in the 1970s, 24 GPS sat-
ellites covered up to 98% of the globe has successfully
launched and emplaced by 1994. One of its functions is
to provide people with high precision time information.
GPS synchronization method is based on this character-
istic. GPS can provide the whole power system with
long-term stable wireless clock source, so far, has been
widely used in PMU, fault location, etc.
Principle of GPS synchronization is that the special
receiver receives the time information from GPS. The
receiver is made up of the receiving module and receiv-
ing antenna. GPS receiver receive time information, then
decode it and make corresponding treatment, which can
get two kinds of information, one is the second pulse
signal (1PPS), Its synchronous error is less than 1 μs be-
tween rising edge and UTC，as shown in Figure 2. The
other is UTC time code (year, month, day, minutes and
seconds). Its corresponding serial port output signal to
differential protection device. So it can provide sample
data with time tag. In order to make pulse rising edge of
the sampling signal synchronization with UTC, what it
need is a stability high crystal oscillator sampling clock
which can produces corresponding sampling pulse. IPPS
synchronize the sampling pulse once per second. When
two ends of line are equipped with this kind of synchro-
nous receptors, they can respectively provide their dif-
ferential protection devices with highly synchronous
sampling pulse (error less than two microseconds).
In the power system, we don’t use precise positioning
technology of GPS, but use its accurate timing informa-
tion to choose the GPS receiver. GPS receiver receive,
decode and process satellite signal, then provide 1PPS
signal, which is international standard signal, and can be
reliably received in around the global. Then these signal
synchronize clocks in power system as the standard clock
source, which ensures that substation clocks will syn-
chronously operate.
Figure 2. GPS receiver clock signal.

B. DUAN, Y. WU 465
3.2. IEEE 1588 Time Synchronization
Technology
The full name of IEEE 1588 is “Standard for a Precision
Clock Synchronization Protocol for Networked Meas-
urement and Control Systems” (PTP for short). Synchro-
nization of PTP system is achieved by exchanging mes-
sages across the communication medium. PTP uses the
following multicast message types: Sync, Delay-Req,
Follow-Up and Delay-Resp.The Principle of IEEE 1588
is as shown in below.
As shown in Figure 3, the master clock sends a Sync
message to all the clocks in the domain at time Ta1,the
slave clock receives this message at time Tb1, the master
may subsequently send a follow-Up message with accu-
rate Ta1 timestamp to the slave, the slave send De-
lay-Req message to the master at time Ta1 after received
the accurate Ta1 timestamp from Follow-up, the master
shall receive Delay-Req at Ta2 and take note of the ac-
curate time Ta2 into Delay-Resq ,which then be sent to
the slave. Once the slave receive the Delay-Resp mes-
sage, the time offset Toff-delay and path delay Tdelay can be
calculated, formulas are listed below:
112 2
-
(-)+( - )
=2
bab a
off delayTT TT
T (1)
112 2-
(-)-(- )-2
=2
babaQdelay
delay TT TTT
T (2)
(TQ-delay stands for forwarding delay quantity.)
If the slave time is assumed to be T sOld right now, then
the new time T sNew can be worked out:
TsNew= TsOld - Toff-delay - TDelay (3)
Accordingly, in the next synchronization period, the
synchronization of the slave clock and the master clock
can be realized by adjusting time offset and the amount
of path delay online. Further more, each clock node link
to power communication network, the synchronization of
all substations in the network can be achieved.
4. Solutions for Time Synchronization of
Differential Protection between Digital
Substations
4.1. Two Solutions to Time Synchronization of
Differential Protection between Digital
Substations
Solution 1: time synchronization between substations is
implemented by GPS, as shown in Figure 4. Each sub-
station is equipped with GPS receiver, which can output
absolute time serial data and second pulse signal into the
GC within substation, i.e, GPS ensure time synchroniza-
tion between each substation GC. The substation 1-N
stands for station-level system in Figure 4.
Advantages and disadvantages of solution 1:time syn-
chronization of the network between substations is im-
plemented by GPS ,IEEE 1588 is adopted for time syn-
chronization within substation, solution 1 needs hardware
that support IEEE 1588, so the investment would in-
crease ,the total time synchronization error is larger than
the way of single-GPS due to the two-class time syn-
chronization structure formed by GPS and IEEE
1588.The advantages of this solution are as follows: the
time synchronization structure is clear and simple; it can
save serial bus and hard wire that need to be connected to
related devices when single-GPS way is used[13].
Solution2: time synchronization between substations is
implemented by IEEE 1588, as shown in Figure 5. BC in
substation 1-N needs to synchronize with the master sta-
tion’s time. As can be seen from Figure 5, this solution
requires support of the overall communication network.
According to this solution, only one master station is
used to synchronize with the highest class clock of other
substations, in order to complete the redundant configu-
ration. And P2P transparent clock between adjacent sub-
stations is used to receive, regenerate and transmit time
message as a reapter, which can decrease asymmetry
error.
Figure 3. IEEE1588 synchronization process.
N
Figure 4. GPS synchronous scheme.
N
Figure 5. IEEE1588 synchronization scheme.

B. DUAN, Y. WU
466
Advantages and disadvantages of solution 2: IEEE
1588 completes time synchronization between and within
substations, so GPS can be left out. In the perspective of
the whole network, GPS in the master station is nothing
but to provide the absolute reference time. If the satellite
communication network encounter damage or interfer-
ence, the time synchronization system can still keep go-
ing, though the time is no more corresponding to the
standard world’s. IEEE1588 PTP holds a great capability
of fault tolerance and allows exist of time isolated island.
For instance, substation1 loses time message exchange
with the master station, then IEEE 1588 clock in substa-
tion1 would generate a new substation-master-clock to
synchronize other clocks within the substation by the
means of comparing with the best master clock algorithm,
which can meet requirement of synchronization precision
for a short time. If synchronization message can’t be re-
ceived for long, clock skewing would occur, the internal
time would disaccord with standard time, but all internal
devices of substation shall keep a high precision syn-
chronization.The disadvantages are as follows: hardware
equipments that support IEEE 1588 make higher invest-
ment, time synchronization network would enlarge and
need more nodes, the precision of clock must be strictly
tested[14,15]. Meanwhile this solution need support from
long distance communication network, when the size of
cascading substations get larger, the problem that the
time synchronization precision can meet requirement
needs to be verified by practice in future.
4.2. The Necessity and Feasibility of the Solution
At present the main mode is as follows: every substation
has a GPS receiver or other precise clock source to fulfil
synchronization with each substation. However, the syn-
chronization precision between substations is not high
enough, it may cause errors in some cases. For example,
line differential protection is based on both ends’ electri-
cal parameters, which must be high-precision synchroni-
zation with the unified time scale, if the independent
synchronous timing information in each substation isn’t
unified and one end loses synchronous clock, synchroni-
zation error would increase, which lead to incorrect ac-
tion of protector. With development of measurement and
control technology, time synchronization between sub-
stations is more important. IEEE1588 has been mainly
used in local network system, but as the substation
communication network constantly improves, it will be
applicated in larger area network system.
5. Conclusions
GPS synchronization scheme is simple, so it is the first
choice. The application of synchronization scheme based
on IEEE1588 will depend on clock precision of whole
network and wide area communication network which
can support IEEE1588. The key to the normal operation
of transformer substation is precision of time synchroni-
zation. IEEE1588PTP can provide high precision and
unified time information for substation. Scheme based on
IEEE1588 can simplify the method of synchronization
used in the current substation, improve the reliability, has
a good applied foreground, will be dominant mode of
clock synchronization for differential protection in digital
substation in the future.
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