Research and Summary on Reliability of Relay Protection System

Jingjing Zhang

School of electrical engineering and automation, Hefei University of Technology

Hefei, China

e-mail: dragonzjj@sohu.com

Keywords- relay protection system; reliability; failure model; index

Abstract. In electric power system, relay protection system can powerfully safeguard secure and

stable operation of the first circuit equipment; it’s an important task to ensure reliable operation of

protection system. Research on reliability of it now arises much attention at home and abroad. The

paper carries out detailed research survey on reliability of relay protection system, which contains

AC input circuit, digital relay protection equipment and operation output circuit. In general it also

contains automatic equipment and telecommunication network etc. It belongs to repairable system;

state-space method and fault-tree method are main analysis methods. Each protection has its own

principle and structure, the modeling of which is different. The paper discusses the reliability

modeling and index system of protection system from three aspects, whose failure models are

hardware and software construction, main protection and backup protection construction and

construction with relation to operation mode of power system. With the vigorous development of

DG in smart grid, the prospects of reliability evaluation of wide area adaptive protection system are

presented.

Introduction

Currently secondary circuit system is shifting from old to new comprehensive automatic system at substations within smart grid, research on reliability of it arises much attention at home and abroad, the reliability model and quantitative analysis of relay protection system is to be build and modified, as in [1-13] . The aim of it is to find the specific factor which affects reliability of protection system most and the weakest link, provide guidance for determining optimal time for maintenance and repair, and decrease failure times of protection system.

Each protection has its own principle and structure, the modeling of which is different. The paper discusses the reliability modeling and index system of protection system from three aspects, whose failure models are hardware and software construction, main protection and backup protection construction and construction with relation to operational mode of power system. With the vigorous development of DG in smart grid, the prospects of reliability evaluation of wide area adaptive protection system are presented.

Failure model of hardware and software construction

Failure model of hardware Digital protection equipment is commonly divided into seven function modules in [14], which are shown in figure 1: PSU(Power Supply Unit),CPU(Central Process Unit), DI(Digital Input),DO(Digital Output),AI(Analog Input), CU(Communication Unit) and MMI(Man Machine Interface).

American Military Standard Handbook MIL-HDBK-217E can be consulted to calculate failure rate of each module in the protection equipment. In the process of protection system, there are two failure modes in CPU, DI, DO, AI: non-operation and mal-operation. Suppose each failure rate is 50%, from fault tree of this model, non-operation rate λhj and mal-operation rate λhw of the digital protection is:

2/)(

;2/)(

AIDODICPUhw

PSUAIDODICPUhj

λλλλλ

λλλλλλ

+++=

++++=

(1)

Advanced Materials Research Vols. 433-440 (2012) pp 6755-6759Online available since 2012/Jan/03 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.433-440.6755

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Failure model of software The software of digital protection is an important factor that affects its common operation, for

example arithmetic theory itself may have certain bug which may result in failure of protection. Same as hardware, the reliability of software can be described by probability, but there is much substantial difference between reliability analysis of hardware and software: hardware may age, but reliability of software has a trend to enhance; reliability of hardware lies in analysis of static process, but reliability of software doesn’t. So much attention has been paid by many researchers and many kinds of reliability models arise, such as Duba relibility model, Shooman model, Jelinski-Moraanda model, JohnMusa model etc. Proper model of digital protection software is to be modified, generally failure rate of Software is deemed as a constant value.

Figure 1. Structure of digital protection equipment

Reliability calculation of digital protection equipment

The process of digital protection equipment can be solved by state space method. Suppose: ①

Software failure and hardware failure is independent,②Hardware non-operation or mal-operation

has two modes: self-testable and non-self-checkable;③Non-self-checkable hardware non-operation

failure can be found and repaired by fixed check, software non-operation failure can be repaired by continuous update; the probability of hardware failure self-testable rate is c(constant). The seven state space diagram of digital protection equipment is shown as figure 2. In figure 2, state 0 is normal state; state J is a combined non-operation state, state W is a combined mal-operation state. From figure 2, we can achieve steady probability of above seven state of the digital protection equipment, availability of the protection is:

)](

/[)(

32312132

31323213210

uuuucuuuu

uuuuuuuuuupA

hjhjswsj

hwhj

λλλλ

λλ

−+++

++== (2)

This model is first developed, its method originates from reliability evaluation of primary circuit element of electric power system: splitting digital protection system into two parts: hardware and software, and setting reliability model for them separately, then building reliability model for the whole system using state space diagram, achieving failure rate and availability of the protection in the end. Adding other elements such as current transformer, voltage transformer, circuit breaker and autoreclosure, we can achieve reliability index of the whole protection system. In the calculation, the index mainly depends on the statistical data, so it’s necessary to accumulate initial reliability data of the digital protection equipments, now the work has been carried on.

Failure models of main protection and backup protection construction

No matter what difference in principle and structure design between different protection products, the configure mode of relay protection is always main protection and backup protection. So we can build fault isolation probability model with relation to main protection or backup protection. Using state space theory in [15-17], we suppose fault happens to the protected component, failure rate λ and repair rate µ of the protection system is constant. Planned examining and repair is arranged during component is normal and the protection system is out. Fault doesn’t happen to every protection unit simultaneously, one unit fails, after other unit isolates fault, the whole protection system need to be out, it can come back to good after repair. Don’t consider circuit breaker failure and man-made mistakes.

6756 Materials Science and Information Technology

hwc λ)1( −

hjcλ

2µ

2µ

hwcλ

swλ

3µ

2µ1µ

hjc λ)1( −

sjλ1µ

Figure 2. Seven state space diagram of digital protection equipment

When primary circuit component fails, each protection unit may work correctly or fail to operate. Usually main protection adopts two different theory protections as dual configuration. Taking no account of hardware failure when component fails, the main influencing factor lies in operating theory, protection system model and parameter veracity etc. Different primary circuit component can be configured with different main protection and backup protection, now we suppose the protection system model is two main protections with one local and one remote backup protection. In figure 3, state 1 is normal two main protection with normal spare remote and local backup protection; state 2 is that when one main protection fails, another main protection operates correctly with normal spare remote and local backup protection; state 3 is that when two main protections both fail, local backup protection operates correctly with normal spare remote backup protection; state 4 is that when two main protections and local backup protection fail at the same time, remote backup protection operates correctly; state 5 is that all the main and backup protections fail, the protection system fails. The shifting relation of each state is shown as figure 3, correct fault isolation probability of protection system is:

;2 2

4321dc

dppppA nzur

+

+=+++=

λλµ

〕＋〔 )3)((2)(

);(2

2

2

1

µλµλλµλµ

µλλλ

+++=

+==

znzurzur

zurrn

d

pc

(3)

Different protection configuration has different probability of correct fault isolation, this model can not only calculate correct fault isolation probability of main protection, but also calculate that of remote backup protection. So this method can provide more accurate calculation thought for reliability analysis. But this method neglects mal-operation of protection system and circuit breaker failure, it is to be modified. It also needs large enough statistical data to ensure veracity of parameters.

λ2

µ

λ

nλ

zµznµ

znrµ

rλ

Figure 3. State space diagram of two main protections with one local and remote backup

protection

Failure model partitioned by relation to operational mode of electric power network

Partitioned by relation to operational mode of electric power network in [18], there are two models: first-type failure model and second-type failure model. First-type failure model has no relation to operational mode of power network; it mainly lies in defects of protection design, manufacture and installation etc. Second-type failure model has tight relation to operational mode of power network, fault categories and protection configuration scheme. First-type failure model describes faults in current(voltage) transformer, circuit breaker, relay protection and man-made mistakes, its probability is always calculated by statistical data; first-type non-operation includes open circuit of relay secondary circuit or circuit breaker’s no trip because of fault in itself; other non-operation is called second-type non-operation failure; first-type mal-operation failure is because of accidental turn-on of

Advanced Materials Research Vols. 433-440 6757

secondary trip circuit or circuit breaker fault; other mal-operation is called second-type mal-operation failure. The instantaneous failure probability of protection system is sum of first-type and second-type failure probability, such as:

)(

111)(t

JJetP

λ−−= ； ∏=

=l

i

JJ itPtP1

22 ),()( )()]()([)( 21 tPtPtPtP sJJJ ×+= (4)

where 1Jλ is real time failure rate of first non-operation failure; l stands for protection numbers the

system contains; ),(2 itPJ is non-operation probability of protection i at time t;)(tPs is failure

probability of being protected component at time t, )(1 tPJ is first-type failure probability, )(2 tPJ is

second-type failure probability, )(tPJ is instantaneous non-operation failure probability of protection

system at time t. This model presents instantaneous failure probability of protection systems, which is different

from above two models. But it’s more complex and must consider protection theory, settings and real time operational mode. References [19-21] deduces second-type failure model of stepped current protection, distance protection and differential protection. As it involves too many factors, some important assumptions related to their probability distribution is to be supported by operational data of protection.

Conclusions

The paper discusses the reliability model and index system of protection system from three aspects, whose failure models are hardware and software construction, main protection and backup protection construction and construction with relation to operational mode of power system. They have their own advantages and disadvantages. With more applications of new measurements like photoelectric transformer and PMU, secondary circuit can be simplified; reliability of protection of advanced transformer substation system can be improved greatly. However with the vigorous development of DG in smart grid, its complicated protection system brings new problems to reliability of the whole protection system. How to consider reliability evaluation of wide area adaptive protection system under smart grid environment deserves thorough research, which involves reliability question of telecommunication system and wide area coordination algorithms. With great improvement of computer and communication network technology, reliability research of protection system will make great progress in the near future.

Acknowledgment

This work was supported by the National Natural Science Foundation of China under Grant

50707006 and Science Research Development Foundation of Hefei University of technology

under Grant 2009HGXJ0068. The author thanks.

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Advanced Materials Research Vols. 433-440 6759

Materials Science and Information Technology 10.4028/www.scientific.net/AMR.433-440 Research and Summary on Reliability of Relay Protection System 10.4028/www.scientific.net/AMR.433-440.6755