[ieee 2012 ieee international conference on power and energy (pecon) - kota kinabalu, malaysia...

Teleprotection Simulation Lab: Understanding the

Performance of Telecommunication Aided

Protection Systems under Impaired

Telecommunication Network Conditions

1Husni Azam Yusof 2Aminuddin Musa

Tenaga Nasional Berhad [email protected]

[email protected]

Ahmad Qisti Ramli

UNITEN

[email protected]

Mohd Iqbal Ridwan

TNB Research Sdn Bhd

[email protected]

Abstract— Protection using telecommunications, such as the

current differential protection schemes, are well known for

their selectivity, sensitivity and fast operating times. Such

advantages make them a desirable choice for the

protection of high voltage transmission lines. However, the

reliability and security of these protection schemes

depends heavily on the performance of the

telecommunication network, which itself is subject to

various form of impairments. Tenaga Nasional Berhad

(TNB), Malaysia’s electrical utility company, depends

heavily on the use of current differential protection scheme

to protect its high voltage transmission lines. Therefore it

is very important to understand how these protection

schemes behave when the telecommunication network is

impaired.

Learning by experience, as several tripping incidents have

shown, can be very costly and time consuming. By building

a teleprotection simulation lab and using it together with a

real-time digital simulator for real load and fault

conditions, the performance of these protection schemes

under an impaired telecommunication network can be

examined and studied in much greater detail under a

controlled lab environment. This will enhance the learning

process and allow proactive measures to be taken before

any unwanted incidents occur.

Keywords- Current differential protection; Simulation lab;

Teleprotection;

I. INTRODUCTION

Protection of high voltage transmission lines is extremely

critical for the stability of the electrical grid system and

ensuring a continuous supply of electricity. Fault occurring in

any of the transmission lines must be cleared quickly and

correctly. Failure to do so may result in the lost of multiple

cascading lines, and may even lead to nationwide blackout.

Transmission grid systems often take the form of a very

complex meshed network, making it a major challenge for the

protection engineers to design a fast, reliable, sensitive and

selective protection system which will trip the correct faulty

line in the fastest time possible.

One of the most effective ways to improve sensitivity and

selectivity of any protection system is by using the aid of a

telecommunications system [1]. A telecommunication aided

protection system would allow protection relays to

communicate and exchange information between one another

in real time, thus making a more accurate tripping decision in

a much faster amount of time. The telecommunication systems

which are used for this purpose is generally referred to as the

teleprotection system, and its implementation in TNB is

described in more detailed in the next section.

In TNB, telecommunication aided protection, in particular

the current differential protection scheme, is used as the main

form of protection for all its high voltage transmission lines.

Current differential protection is used mainly because it has

the advantage of being very selective, sensitive to earth faults,

easy to configure and provides fast tripping decision.

Although the telecommunication aided protection has many

advantages, it has one major weakness. Since its operation

relies purely on the exchange of real time information between

the relays for its correct operation, any failure or impairment

on the telecommunication network will affect the reliability

and stability of the protection system. Thus the reliability of

the protection system is basically as reliable as the

communication link [2].

II. TELEPROTECTION DESIGN IN TNB

A. Special Requirements for Teleprotection in TNB

Teleprotection system carries critical information required

for the correct operation of the protection system. Therefore it

must adhere to very strict performance criteria on reliability

2012 IEEE International Conference on Power and Energy (PECon), 2-5 December 2012, Kota Kinabalu Sabah, Malaysia

978-1-4673-5019-8/12/$31.00 ©2012 IEEE 655

and dependability. TNB in particular follows the IEC 60834

standard [3] on the performance criteria of a teleprotection

which is summarized in Table I:

TABLE I. DEPENDABILITY AND SECURITY REQUIREMENTS

BASED ON IEC 60834-1

Scheme Dependability Security

Blocking >99.9% >99.9%

Permissive Underreach

(PUTT)

>99% >99.99%

Permissive Overreach

(POTT)

>99.9% >99.9%

Intertripping >99.99% >99.9999%

Dependability is defined as the scheme should operate

when it is required to operate, where security is defined as the

scheme should restrain from operating when it is not required

to do so [4]. Using the intertripping scheme as an example, the

requirements based on IEC 60834-1 can be interpreted as

follows [4]:

1. 99.99% dependability is interpreted as the

intertripping scheme should be 99.99% available

at any point of time when it is required or should

have only 0.01% probability of failure at any point

of time.

2. 99.9999% security is interpreted as the

intertripping scheme should have 99.9999%

probability of restraining from operating during

normal (no actual fault) conditions or should only

have 0.0001% probability of mal-operation during

normal conditions.

Furthermore, since any delay in the teleprotection system

would directly contribute to a delay in the protection systems

response, the teleprotection system should also have very low

and deterministic propagation delay to ensure a fast and

reliable operation of the protection system. For TNB’s

requirements, the delay introduced by the teleprotection

system must be less than 10ms.

B. TNB’s Teleprotection System Architecture

To achieve the dependability and security requirements in IEC 60834-1, TNB has adopted a cluster of technologies on the three main design aspects of the teleprotection system which are transmission media, telecommunication technology and interface type. Table II describes the technologies chosen for the design aspects and states the reasons of the technology selection.

TABLE II. TNB’S TELEPROTECTION SYSTEM DESIGN ASPECT AND

CONSIDERATIONS

Design

Aspects Adopted Technology Reasons

Transmission

Media Fiber Optics

• Immune against

EMI

• Perfect electrical

isolation

• No crosstalk

between fibers

• Little influence by

atmospheric

conditions

• High bandwidth

• Long distances

possible

Telecommunication

Technology

TDM

(SDH or PDH)

• Fixed data rate

guaranteed

• Deterministic

behaviour

• Mature and proven

technology for

teleprotection usage

• Network resilience

(SDH)

• Network

management

(SDH)

Teleprotection

Interface Type ITU-T 64k G.703

• Internationally

recognized standard

interface

• Simple

implementation (2

pairs of wires only)

• Galvanic isolation

using isolating

transformers (DC

isolation)

• G.703 signal

structure provides

good timing

information

• Long cable wiring

distances possible

In the past, TNB has been using the Power Line

Communication (PLC) as the main communication medium

for teleprotection purposes. The system was used mainly for

inter-tripping and permissive schemes such as the Permissive

Underreach Transfer Trip (PUTT) distance protection scheme.

However, this system has a limited bandwidth capability

which makes it unsuitable for current differential protection.

Furthermore, it is also more susceptible to interference and

noise.

Since the early 1990s, TNB has upgraded its teleprotection

system using the Optical Ground Wire (OPGW) and the All-

Dielectric Self Supporting (ADSS) fiber optics as the main

communication medium. The use of fiber optics allows much

higher bandwidth suitable for current differential applications,

and is also more immune to noise and electromagnetic

interferences.


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The Plesiochronous Digital Hierarchy (PDH) technology

was originally used as the main transmission equipment. The

PDH technology are developed using the Time Division

Multiplexing (TDM) technique, which has deterministic

network behavior suitable for teleprotection requirement [1].

The multiplexing capabilities also allow more than one

application to be used on the same fiber. Therefore other

applications such as SCADA, remote metering, voice and as

well as other corporate applications can be transmitted over

the same fiber. The PDH network was later upgraded to the

Synchronous Digital Hierarchy (SDH) to give higher

bandwidth and better network management.

A dedicated primary multiplexer is then used to interface

to the protection relay. The primary multiplexer operates at

the 2Mbps E1 level and provides a 64kbps ITU-T G.703

interface to the protection relay for current differential

protection schemes or to a teleprotection signaling equipment

for intertripping and permissive schemes.

However, most of the current differential protection relays

in TNB uses various different types of interface, including

proprietary optical interfaces. Therefore an external converter

is normally required to convert the relay’s interface into the

64kbps ITU-T G.703 interface. Figure 1 shows the overview

of a typical TNB’s teleprotection system architecture.

Figure 1. Typical TNB’s Teleprotection System Architecture

III. TELEPROTECTION SIMULATION LABORATORY

TNB has encountered several incidents which involved the abnormal behavior of teleprotection system and its related components. Some of the incidents are:

1. Single ended switching has caused an asymmetrical delay between the send and receive paths. This has resulted in the wrong operation of protective relays.

2. Current differential protective relay mal-operating under a normal (no fault) condition. Investigations revealed an interface converter was faulty and this may have caused a misinterpretation of the corrupted telegram by the current differential relay. However, this finding was inconclusive as it

was not possible to simulate such situation during the investigation.

Therefore, it is imperative for TNB to have a facility to simulate the behavior of teleprotection system which can also be utilized to perform investigations when similar incident occurs.

A. Objectives of Teleprotection Simulation Laboratory

The previous incidents have shown the need for better

understanding on how a protection system performs under the

various types of impairment that may be experienced by the

teleprotection channel. The past experience only gives a small

indication of what could go wrong with the protection system

whenever the network is not behaving as expected.

Furthermore, learning by experience can be very costly and

time consuming. It is much better if the system can be

analyzed methodically under a controlled lab environment.

With this view in mind, TNB Research Sdn Bhd, a

subsidiary of TNB, has decided to develop a teleprotection

simulation lab, in order to facilitate and meet the following

objectives:

1. To simulate real network conditions for accurate

assessment of protection systems under various load

conditions using the Real Time Digital Simulator

(RTDS)

2. To evaluate the performance of protection systems

under impaired telecoms network condition

3. To evaluate various different teleprotection designs,

configurations and interfaces for better performance.

4. To acquire more knowledge and understanding on the

behaviour of the current differential protection

schemes under a safe and secure lab environment

5. To assist in the troubleshooting process for future

incidents

6. To evaluate the use of Ethernet over SDH as the

WAN network technology of choice for the future

IEC 61850-90 standard.

B. Laboratory Design and Architeture

The teleprotection simulation laboratory is designed to

follow the existing teleprotection setup in TNB and is shown

in Figure 2. Two bays are used to represent two remotely

located substations. The SDH equipment is used as the main

telecommunication transmission equipment, with two STM-1

optical links connecting the two bays. The first link is a direct

point to point connection, while the second link goes through a

digital SDH Network Impairment Emulator. The network is

basically configured as a ring network, with the SDH Network

Impairment Emulator simulating a large SDH network. The

SDH is equipped with 16 x 2Mbps E1 interfaces for legacy

TDM circuits and 4 x 10/100Mbps FE interface for Ethernet

over SDH (EoSDH) circuits.


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Figure 2. Architecture of TNB Teleprotection Simulation Laboratory

The primary multiplexer (PMUX) act as the teleprotection interface to the protection relays. The PMUX is equipped with 8 x 64kbps G.703 interface and 4 x IEEE C37.94 optical interfaces for connection to the current differential relays. The Protection Signalling Equipment (PSE) provides 4 x 110/48V I/O port which can be used to connect to the relay’s I/O port for intertrip and permissive signals. Besides connecting the PMUX directly to the laboratory’s SDH equipment, there is also an additional option of connecting the teleprotection equipment directly to TNB’s existing telecommunication network via an E1 circuit whenever required.

At the moment, the laboratory is designed for a two terminal protection system, although a three terminal system can also be created in the future by simply adding a third similar communication bay, if required. The actual picture of the teleprotection system simulator is shown in Figure 3.

Figure 3. Teleprotection System Simulator in TNB

C. Laboratory Function and Features

The teleprotection simulator is able to provide

teleprotection channel via four different interfaces:

1. 2Mbps G.703 E1 interface

2. 64kbps G.703 co-directional interface

3. Nx64kbps IEEE C37.94 optical interface

4. 4 x 110/48V I/O port for teleprotection signalling

interface

In addition, the teleprotection simulator also provides

10/100Mbps FE interface for future testing using IEC61850

over an EoSDH WAN.

The SDH equipment performs basic SDH functions such

as add-drop multiplexing and cross connect functions, as well

as standard network protection features such as the Sub-

Network Connection Protection (SNCP) and 1+1 Multiplex

Section Protection (MSP).

Network delays and impairment are simulated using the

Spirent SDH Network Impairment Emulator. The emulator is

able to simulate several network impairment conditions such

as:

1. Propagation delay up to 600ms in 1us increments

2. Asymmetrical delay up to 600ms in 1us increments

3. Bit Error ranging from 10-12

up to 10-3

4. Loss of Frame

5. Loss of Signal


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IV. APPLICATION OF TELEPROTECTION SIMULATION

LABORATORY

A. Provide actual network delays for protection system

performance analysis

The teleprotection simulation laboratory is located inside the TNB IEC 61850 System Verification and Simulation (SVS) laboratory in TNB Research Sdn Bhd which houses the existing Real Time Digital Simulator (RTDS). The RTDS is used to evaluate the performances of various protection schemes under several load and fault scenarios. Previously, protection relays are connected back-to-back while the various load and fault scenarios are generated by the RTDS as input into the relays. However, performance of telecommunication aided protection scheme depends heavily on telecommunication performance, in particular the propagation delay. The teleprotection simulator will be able to simulate this delay and thus the performance of such protection schemes can be more accurately examined and analyzed.

B. Understanding the performance of different protection

relays under impaired telecommunications condition

There are many different types of relays currently in used in TNB. Past experiences have shown that different relays perform differently when operating under an impaired network condition. By using the teleprotection simulator, various different kinds of relays currently in service can be tested against different telecommunications impairment scenarios to better understand the behaviour of each relays. This will help in the operation and maintenance of existing protection system in TNB.

C. Evaluating different kinds of teleprotection interfaces and

configurations for optimum stability and performance

There are many different ways to design and configure a teleprotection network. An SDH circuit for example can be configured as a bidirectional circuit, unidirectional circuit, non-protected, protected with revertive features or protected with non-revertive features. The teleprotection interfaces can also be of various types, such as the 64k G.703 co-directional, 2Mbps G.703 E1, 64kbps IEEE C37.94 or Nx64kbps IEEE C37.94 interface. Until previously, TNB has no method of gathering empirical data for assessing which teleprotection configuration yields the best result in terms of protection system performance and stability. Using the teleprotection simulator, proper test can be conducted under a controlled lab environment to properly evaluate and assess these different configurations.

D. Providing the WAN connectivity required to evaluate

IEC61850-90

Currently, the SVS laboratory is focused more on testing IEC61850 within a local substation. However, the new released IEC 61850-90-1 has extended the communication over the Wide Area Network (WAN) [6]. One of the ways this communication link can be achieved is by using the “tunneling” approach [5]. By using the Ethernet over SDH features available in the SDH equipment, the teleprotection simulator will be able to provide the “tunneling” required for

the WAN connectivity and test the new IEC 61850-90 configurations for better understanding.

V. PRELIMINARY TEST RESULTS

The teleprotection simulation lab is currently being used to

evaluate the performance of a 2-terminal current differential

protection scheme using the G.703 co-directional 64kbps

communication protocol.

The initial test consists of benchmarking the relay

performance when using an error free G.703 64kbps channel.

Next, the G.703 64kbps channel is impaired by introducing a

high bit error (1.0x10-3

) using the teleprotection network

simulator.

The test uses the RTDS™ equipment to simulate various

different line faults, such as single phase-to-ground faults and

phase-to-phase internal faults. The faults generated by the

RTDS™ equipment is then amplified and fed into the current

differential relays. The preliminary results for a typical phase-

to-phase internal fault are as given below:

Figure 4. Snapshot of the COMTRADE file generated when the Current

Differential relays are tested using an error free G.703 64kbps channel

Figure 5. Snapshot of the COMTRADE file generated when the Current

Differential relays are tested using a high bit error G.703 64kbps channel


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Table III below gives a comparison of the relay tripping

times when it is error free and when subjected to a high bit

error channel:

TABLE III. COMPARISON OF THE RELAY TRIPPING TIMES DURING ERROR

FREE NETWORK AND HIGH BIT ERROR NETWORK

G.703 64k Network Condition Relay Trip Time

Ideal Network (Error Free) 26ms

High Bit Error (1.0x10-3) 89ms

The preliminary tests results indicate that the current

differential relays would still be able to detect an internal fault

and operate correctly, even during a high bit error condition in

the telecommunications network. However the tripping time

becomes much slower when there is a high bit error condition

in the telecommunications network.

VI. CONCLUSION

The teleprotection simulation lab will allow TNB research and development team to comprehensively test and evaluate the performances of various telecommunication aided protection schemes under a controlled lab environment. The data gathered from these test results are important for TNB to better understand the behavior of the existing protection

systems, as well as finding ways to further improve the reliability and performance of TNB’s teleprotection system.

ACKNOWLEDGMENT

The authors would like to thank TNB Research Sdn Bhd for providing the fund for the development of the teleprotection system simulator. The authors would also like to acknowledge PESTECH Sdn. Bhd. for their assistance in the setup and configuration of the teleprotection system simulator

REFERENCES

[1] CIGRE JWG 34/35.11, “Protection Using Telecommunications”,

CIGRE Technical Brochure, 2000

[2] O. Rintamaki and J. Ylinen, “Communicating Line Differential Protection For Urban Distribution Networks”, China International Conference on Electricity Distribution, December 2008

[3] IEC 60834-1 Standard, “Teleprotection Equipment of Power Systems - Performance and Testing - Part 1: Command systems”, Ed. 2, 1999

[4] S.Ward, T. Dahlin and W. Higinbotham, “Improving Reliability for Power System Protection”, 58th Annual Protective Relay Conference, Atlanta, GA April 28-30, 2004.

[5] C. Brunner and F. Steinhauser, “Application of IEC 61850 for Protection Communication between Substations”, PAC World, June 2011

[6] IEC 61850-90-1 Standard, “Use of IEC 61850 for the Communication between Substations”, Ed.1, 2010


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