Current Differential Relays Over IPMPLS Networks

8/10/2019 Current Differential Relays Over IPMPLS Networks

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PERFORMANCE EVALUATION OF CURRENT

DIFFERENTIAL RELAYS OVER A WIDE AREA NETWORK

P. Beaumont*, F. Kawano*, A. Kawarada, T. Kase, H. SugiuraF. Lam

, J. Hurd

, P. Worthington

, D. Richards

, P. Merriman

*Toshiba International (Europe) Ltd, U.K.Toshiba Corporation, JapanAlcatel-Lucent, Canada

[email protected]

Keywords: Ethernet-based-protection, Communications

infrastructure, Next-Generation-Networks, Wide-area-

Ethernet.

Abstract

Progress in the development of communication network

technology continues apace and Ethernet technology, oncelimited to local area network applications, is increasingly

being considered for use as the backbone technology for wide

area communication networks. The technology, most

commonly referred to as Carrier Ethernet, seems likely tobecome the dominant technology within wide area networks

ultimately replacing earlier generation networks based upon

PDH and SDH / SONET platforms. The availability of

Carrier Ethernet and in particular, in terms of the testing

described here, IP/MPLS-based Carrier Ethernet now enablesus to consider the application of Ethernet technology to

teleprotection.

1 Introduction

Since 2007 we have continued to make progress with the

development, manufacture and testing of a gigabit Ethernet

based current differential relay [1, 2]. Although confident that

our internal evaluation had been thorough we were cognizant

of the fact that the communications infrastructure used for our

internal evaluation is a local network facility. Accordingly,

we recently undertook a joint evaluation with Alcatel-Lucent,

in which we were able to prove that a gigabit Ethernetprotection relay system operating over a wide-area network

has the capability to be utilized in a practical, real world

environment.

Teleprotection is an essential technology for operating andmaintaining a reliable, robust and safe electric grid.

Teleprotection devices rely upon a deterministic service

provided by a stable, symmetric, constant delay

telecommunications network for their communicationrequirements. Current differential protection, widely applied

for the protection of HV and EHV feeders due to its inherent

strengths of high sensitivity and selectivity, relies upon the

provision of a relatively high-bandwidth communication

channel and the existing PDH and SDH / SONET networkshave proven to be well suited to this task. In view of the more

demanding performance imposed upon the communications

network, current differential protection was chosen as the

primary teleprotection to lead this investigation.

In order to assess the suitability of Ethernet technology forteleprotection the authors established a demonstration system

to study the operation of current differential relays over

Ethernet. A solution to the fundamental requirement ofestablishing and maintaining synchronisation between relays

was developed using demonstration relays incorporating a

newly developed, dedicated Ethernet interface with an

integrated high-accuracy time control function. Based upon acomprehensive set of tests [3], it was concluded that a

network comprised of IP/MPLS routers will comply with all

of the requirements of the demonstration relays.

2 Current Differential Protection over Ethernet

A current differential protection relay compares locally

measured current data with data from the remote end of the

transmission line which has been transmitted from the remote

terminal via a communication channel. It determines that an

internal fault condition has occurred when a differential

current results from the comparison of the two current data

quantities. In order to compare two current data quantities

correctly, the data must be representative of the same instant

in time. Since a relay cannot compare the current values until

the remote end data arrives, the transmission delay must be

short in order to achieve fast operation of the protection. Datacommunication is vital for the operation of current differential

protection, and the requirement on performance of thecommunication channel is correspondingly high.

The requirements of the current differential relay on thecommunication channel are two-fold as follows:

- The required operating time of the protection relaymust be met

- Synchronisation of the data must be achievable

In order to achieve the required protection relay operating

time, the transmission delay time through the communicationchannel is critical, and generally the required operating time

is shorter at higher system voltage levels.

Each relay requires sampling timing synchronisation. In orderto be able to achieve synchronisation using data


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network and corroborate measurements made by the relays.

An Ethernet Network Emulator was used to introducedifferent network profiles (transmission delay, jitter, and

packet loss) to simulate communication burdens and to insert

data errors in the communication paths. Oscilloscopes were

used to simultaneously display the corresponding timingedges in the relays as shown in Fig. 2.

Figure 2: Oscilloscope for measuring the sampling time offsetand jitter between the two relays.

High-performance, high availability multiservice edge routers

are used to deliver differentiated services. A ServiceAggregation Router (SAR) provides IP/MPLS and pseudo-

wire capabilities and is used to groom and aggregate multiple

media, service and transport protocols onto an Ethernet and

IP/MPLS infrastructure. These routers and the Ethernet

Network Emulator were used together to construct the end-to-

end network experienced by the demonstration relays. To

summarise:

- Ethernet relays (illustrated in Fig. 3): 3 devices

- X.21 relays: 2 devices

- MPLS routers: 5 devices (Route via 3 or 4 routers)

- Network tester: For performance measurement andinsertion of data errors and communication burden,

packet length: 64/512/1518 byte

- Network emulator: Insertion of transmission delay,jitter and packet loss

- Test equipment not shown: For observation andmeasurement of latency, jitter and delay symmetry

- Relay test equipment: to inject relay input quantities

Figure 3: Fascia of line differential relay showing status

information.

3.3 Test results and evaluation

a. Ethernet relaysThe test cases for the Ethernet relays and the corresponding

measured data are shown in Table 1.

The transmission delay i.e. the latency introduced by anMPLS router is approximately 20s per router and varies only

slightly. Although the transmission delay increases in

proportion to the number of routers located within a

communication path, this transmission delay is stable. It wasconfirmed that transmission delay has minimal effect upon

sampling timing synchronisation and relay operation.

Test cases Transmission

delay

[s]

Synchronous

accuracy

[s]

Relay

behaviour

(*1)

Normal (via2 routers)

50 -1.0 to +2.0

No

unwanted

operation

Transmissiondelay 5 ms

5073 -0.6 to +2.8

Transmission

delay 10 ms

10073 +8 to +20

Jitter 0.4ms 150 to 187 -24 to +32

Jitter 0.5ms 460 to 700 -120 to +110

Jitter 1.0ms 700 -120 to +120

Transmissionburden 1%

74 to 77 -1 to +6

Transmission

burden 20%

79 to 135 -6 to +21

Transmission

burden 80%

150 to 165 -4 to +12

*1: Observation of relay behaviour under normal load conditions

Table 1: Ethernet relay test cases and measured data.

However, fluctuation in transmission delay i.e. jitter has a

significant effect upon synchronisation control within the

relays and the accuracy of the sampling timing

synchronisation was reduced.

For the test evaluation for jitter, fluctuations were introduced

in the positive direction using the Network emulator. For

example, in the test case for Jitter 0.4ms shown in Table 1,jitter is inserted randomly within the range of 0 - 0.4ms in the

communication path from Master to Slave.

The sampling timing synchronisation control functioncalculates the synchronous error T and the transmission

delay Tdusing Equations (1) and (2) below using the similar

mechanism as Network Time Protocol (NTP) and as in

IEEE1588 [4].

Referring to Fig. 4 below:

Td = ((T2-T1) + (T4-T3))/2. (1)

T = (T2-T1) Td. (2)


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event of network faults well within the target of 50ms and fast

enough to ensure no detrimental effect in the behaviour of thecurrent differential relays.

b. X.21 relays

Test cases for the X.21 relays together with the correspondingmeasured data are shown in Table 2.

Test cases Transmission

delay

[s]

Synchronous

accuracy

[s]

Relay

behaviour

(*1)

Normal (via

2 routers)

2777 -12 to -3

No

unwanted

operation

Transmission

delay 5 ms

7840 -20 to -7

Transmission

delay 10 ms

12843 -21 to -6

Transmission

burden 1%

2903 -28 to -38

Transmissionburden 20%

2900 -28 to -38

Transmission

burden 80%

2903 -28 to -38

*1: Observation of relay behaviour under normal load conditions

Table 2: Test cases and measured data for X.21 relay.

In the MPLS routers, serial data for X.21 communication (64

kbps) is converted to MPLS packets at the transmitting

terminal and these MPLS packets are converted to serial data

at the receiving terminal, thus X.21 end-to-end

communication is realized. Communication data is buffered

prior to converting these packets, and the throughput is, in themain, varied by the setting of the length of the buffer (number

of bytes). This test was carried out using a setting for the

transmission buffer length of 4 bytes and that for thereceiving buffer length of 12 bytes i.e. 1.5ms so that a high

throughput can be obtained. The transmission time delay is

derived from the accumulation of data buffering delays in the

MPLS routers. In the evaluation of transmission burden, weobserved that the transmission delay and sampling timing

synchronisation accuracy were not affected for any of the test

cases.

4 Future benefits

The flexibility and high capacity of Ethernet enables a

number of future benefits to current differential relaying to be

envisaged, such as easier application to circuits of three or

more terminals, the realisation of multi-terminal and wide-

area back-up protection schemes based on current differential

protection. The technology can support adaptive protective

relaying schemes in which the evaluation of relay setting

margins can be performed based upon the quantitative, on-

line, real-time supervision of operating margins against

varying power system conditions [5]. This approach enables

benefits to be gained from settings adaptations that reflect theprevailing power system conditions. In addition Wide Area

Situational Awareness schemes using phasor measurementtechniques can be used for predictive dynamic stability

maintaining systems and phenomenon assumption type

WAMPAC applications. Moreover, the introduction ofredundancy, both of the communication route and of the

transmitted data, can result in improvements in the reliability

of the protection scheme. Some examples of expected future

applications are described below.

a. Multi-terminal current differential protection

One of the advantages of current differential protection is that

it can be applied to multi-terminal applications, and it canperform perfectly in such applications given appropriately

designed communications, as demonstrated by experience

over a long period of time [6]. Where current differential

protection has been applied to circuits of more than three

terminals, it has been most common that a ring

communication architecture has been used, as shown in Fig. 5.

RY1

RY2 RY3 RY4 RY5

RY6

Figure 5: Multi-terminal ring communication

On the other hand, if Ethernet is applied it becomes easier to

send the same data to multiple destinations (or terminals) as a

standard function (multi-cast). In other words, the same

hardware with a single port can be applied to multi-terminal

configurations. The general configuration is shown in Fig. 6.

RY1

RY2 RY3 RY4 RY5

RY6

L2 SW L2 SW L2 SW

Figure 6: Multi-terminal current differential protection based

on Ethernet

Needless to say, the relays must be designed so that they can

deal with receiving and processing the data from N-1

terminals within a certain time (where N is the total numberof terminals). In this configuration every relay can calculate

the differential current etc. and operate simultaneously.

b. Wide area current differential back-up systemThe idea for a wide area back-up protection system based on

current differential protection has been in existence for a long

time [7, 8], but has never been applied in practice. According

to this theory, the protection system uses the currentdifferential principle to select the best location for tripping to

occur in order to minimise the area of black out in the event

that the main protection relays or circuit breakers have failed

to clear the fault. This idea has some similarity with the multi-

terminal current differential protection system, but a

configuration of one central unit and multiple terminal units is

more suitable in this case. Local relays send status data for thelocal circuit breakers along with local current data to a central


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unit. The central unit is configured so as to accommodate the

configuration of the network to be protected and can decidethe most appropriate circuit breakers to be tripped and send

commands to the local relays. An additional benefit of this

application is that the complex engineering effort normally

required to design back-up protection coverage can bereduced. Fig. 7 shows the general configuration of wide area

current differential back-up protection.

A S/S

Local Relays

CentralUnit

L2SW L2SW L2SW

L2SW

B S/S

Figure 7: Wide area current differential protection based on

Ethernet

Two types of system can be envisaged, one type in which the

local relays function only as terminal units for gathering and

sending data to the central unit, and the other type in whichthe local relays support independent protection functions and

while only relying on the central unit in case of phenomena

which require a decision based on multi-terminal information.

Various types of network topology can also be considered, theappropriate system configuration being decided according to

the number of devices required, cost, reliability of

communication, importance of the protected network etc.

c. Improvement in reliability by introduction of redundancy

Various types of redundancy can be considered in order to

improve system reliability. In the case of redundancy of the

communication route, data can be transmitted continuously

via two independent routes, and relay operation can be

maintained in the event that a failure occurs in one of the

communication routes. This can be achieved by using Carrier-

class Ethernet, which provides a function to assign thecommunication route as shown in Fig. 8.

L2 SW

Ry-A Ry-BRoute 1

Route 2

Figure 8: Redundant communication based upon Carrier-class

Ethernet

5 Conclusions

Prior to the tests, we had envisaged that the accuracy of

sampling timing synchronisation would deteriorate inproportion to the increase in the number of MPLS routers

within the communication path. We had also envisaged that

constraints in network configuration and system scale would

be the case in the application as a communication facility for

line differential relays.

However we have confirmed that there is almost no effect

from the number of routers; high accuracy sampling timing

synchronisation is maintained with equivalent performance tothat provided by existing systems using dedicated

communication facilities.

This evaluation has proven that we have reached the point atwhich we can now move on to achieve the practical

application of protection relays using a wide-area network.

Furthermore, we have confirmed that exercising the features

already available within communication devices enables us to

enhance the functions and performance of protection relays.

We have also confirmed that existing X.21 legacy relays can

be operated using Ethernet. We believe that this point will

lead to benefits in achieving wider maintainability and

extensibility of the Ethernet protection relays themselves andthe systems in which they are applied.

In future, we plan to undertake further performanceevaluations using different communication configurations. In

addition, towards the realisation of the practical application of

protection relays compliant with wide-area networks, we plan

to consider and evaluate a protection system which exercisesthe performance and features of communication devices such

as communications protocol, security, redundancy control and

priority control.

References

[1] G. Baber, P. Beaumont, F. Kawano, "Current Differential

Protection-over-Ethernet", Cigr Paris 2010

[2] T. Shono et al, "Next Generation Protection System over

Ethernet", IET DPSP 2010

[3] Test Report - Teleprotection over IP/MPLS Networks,

Iometrix 2011

[4] IEEE 1588-2008 Standard for a Precision Clock

Synchronization Protocol for Networked Measurementand Control Systems

[5] F. Kawano et al, "Intelligent Protection System for Smart

Grid", PAC World Conference 2010 Dublin, Ireland

[6] M. Suzuki, et al, "Present State of Transmission Line

Protection Employing Fiber-Optic telecommunication",SC34 Colloquium 1987 June, Turk, Finland

[7] Y. Serizawa, and et al., "Wide-Area Current Differential

Backup protection Employing BroadbandCommunications and Time Transfer Systems", IEEE

Trans. Power Delivery, vol. 13, no. 4, October 1998, pp.

1046-1052.

[8] J. Tang, P.G. McLaren, "A Wide Area Differential

Backup Protection Scheme For Shipboard Application",

IEEE Transactions on Power Delivery, Vol. 21, No.3,

pp.1183-1190. (July 2006)

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Current Differential Relays Over IPMPLS Networks