The Research of Intelligent Substation Time Synchronization System and the Influence of Its Fault to Relay Protection

doi:10.4236/epe.2013.54B090

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Energy and Power Engineering, 2013, 5, 468-473

doi:10.4236/epe.2013.54B090 Published Online July 2013 (http://www.scirp.org/journal/epe)

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

Email: 863227870@qq.com

Received February, 2013

ABSTRACT

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breaker has been replaced by fiber optics.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

[2].

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-

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

()(

TT TT

t 

)

(2)

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-

C.-B. XU ET AL.

470

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-

ror.

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

Compass

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

Self-maintain

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

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

Protection

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.

C.-B. XU ET AL.

472

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

-voltage

count

Medium

-voltage

Count

Low

-voltage

count

Note

18:19

:29.000838 3998 3997 3997

Losing synchroniza-tion,

no hoppoing