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|LBG?RIG ELSEVIER Electric Power Systems Research 35 (1995) 59-64 An expert system for designing the protection system of a power transformer Liu Lifeng a Gao Zhongde ", Yang Qixun a Bai Zhongmin b Department of Electrical Power, North China University of Electric Power, Baoding, Hebei, China b Electric Power Design Institute, Zhengzhou, Henan, China Received 18 April 1995 Abstract The protection of power system elements has become more complicated and it is difficult for design engineers to design perfect protection systems. Qualified engineers feel they are under much more pressure at work because they are expected not only to be use their old skills and experience, but must also take in new technological information at the same time. However, expert system technology is a promising approach to help to do some or most design tasks. This paper proposes an expert system for designing the protection system of a transformer by making use of a support tool, OPS83, with a graphical menu and a friendly interface. The developed expert system is applied to the design work of Henan Electric Power Design Institute in China (HEPDIC) to check its operation. Keywords: Transformers; Protection system design; Relay protection; Expert systems 1. Introduction As electric energy demand has been increasing year after year, power systems have tended to increase their capacity and scale. It is necessary to install a perfect protection system on each element, not only for the reliable operation of the power system, but also to reduce the damage caused by faults to the lowest level. Currently, most protection system design has to be done manually by protection engineers with a lot of practical experience and the use of design standards from handbooks. Therefore, the results achieved by different engineers may differ in quality. Generally it will take up to ten years to produce a competent designer. These factors may seriously hold up the pro- gress of designing. Fortunately, expert system technol- ogy is just the right way to conquer them because it is good at dealing with rules, experiences, and logical inferences and can achieve the following goals: (a) as an advisor to assist ordinary engineers with their work; (b) as an instructor to help beginners in design to become familiar with their work as soon as possible; (c) at least to provide a large available database for skilled engineers. Hence, there should be a tendency to apply expert systems to design tasks. However, few efforts have 0378-7796/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0378-7796(95)00989-U been made in this field. Expert systems, on the other hand, are very active in several fields concerned with power systems such as power system planning [1,2], designing power plant electrical auxiliary systems [3], designing and testing substation ground grids [4], and so on. With respect to power system protec- tion, only protective relay setting and coordinating for transmission line protection had attracted the re- searchers' attention [5,6] until Kalman [7] proposed a knowledge based system for transformer protection. However, it only played a relatively simple role in suggesting the necessary protective relays for a given transformer. This paper proposes an expert system which is not only able to achieve the function proposed in Ref. [7], but also describes the manner in which the protective components are to be configured in the protection system, clearly expresses the relations between sec- ondary circuits and the primary circuit, and provides an available database to present designs graphically. Dur- ing the development of this expert system we have obtained considerable knowledge from the skilled engi- neers in HEPDIC and the available rules based both on the protection standards of China [9] and design hand- books [10]. The proposed relays can be ordered from Shanghai Relay Company (SHRC) or Xuchang Relay Company (XCRC).

An expert system for designing the protection system of a power transformer

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Page 1: An expert system for designing the protection system of a power transformer

|LBG?RIG

E L S E V I E R Electric Power Systems Research 35 (1995) 59-64

An expert system for designing the protection system of a power transformer

Liu Lifeng a Gao Zhongde ", Yang Qixun a Bai Zhongmin b

Department of Electrical Power, North China University of Electric Power, Baoding, Hebei, China b Electric Power Design Institute, Zhengzhou, Henan, China

Received 18 April 1995

Abstract

The protection of power system elements has become more complicated and it is difficult for design engineers to design perfect protection systems. Qualified engineers feel they are under much more pressure at work because they are expected not only to be use their old skills and experience, but must also take in new technological information at the same time. However, expert system technology is a promising approach to help to do some or most design tasks. This paper proposes an expert system for designing the protection system of a transformer by making use of a support tool, OPS83, with a graphical menu and a friendly interface. The developed expert system is applied to the design work of Henan Electric Power Design Institute in China (HEPDIC) to check its operation.

Keywords: Transformers; Protection system design; Relay protection; Expert systems

1. Introduction

As electric energy demand has been increasing year after year, power systems have tended to increase their capacity and scale. It is necessary to install a perfect protection system on each element, not only for the reliable operation of the power system, but also to reduce the damage caused by faults to the lowest level.

Currently, most protection system design has to be done manually by protection engineers with a lot of practical experience and the use of design standards from handbooks. Therefore, the results achieved by different engineers may differ in quality. Generally it will take up to ten years to produce a competent designer. These factors may seriously hold up the pro- gress of designing. Fortunately, expert system technol- ogy is just the right way to conquer them because it is good at dealing with rules, experiences, and logical inferences and can achieve the following goals:

(a) as an advisor to assist ordinary engineers with their work;

(b) as an instructor to help beginners in design to become familiar with their work as soon as possible;

(c) at least to provide a large available database for skilled engineers.

Hence, there should be a tendency to apply expert systems to design tasks. However, few efforts have

0378-7796/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved

SSDI 0378-7796(95)00989-U

been made in this field. Expert systems, on the other hand, are very active in several fields concerned with power systems such as power system planning [1,2], designing power plant electrical auxiliary systems [3], designing and testing substation ground grids [4], and so on. With respect to power system protec- tion, only protective relay setting and coordinating for transmission line protection had attracted the re- searchers' attention [5,6] until Kalman [7] proposed a knowledge based system for transformer protection. However, it only played a relatively simple role in suggesting the necessary protective relays for a given transformer.

This paper proposes an expert system which is not only able to achieve the function proposed in Ref. [7], but also describes the manner in which the protective components are to be configured in the protection system, clearly expresses the relations between sec- ondary circuits and the primary circuit, and provides an available database to present designs graphically. Dur- ing the development of this expert system we have obtained considerable knowledge from the skilled engi- neers in HEPDIC and the available rules based both on the protection standards of China [9] and design hand- books [10]. The proposed relays can be ordered from Shanghai Relay Company (SHRC) or Xuchang Relay Company (XCRC).

Page 2: An expert system for designing the protection system of a power transformer

60 L. Lifeng et a l . / Electric Power Systems Research 35 (1995) 59-64

2. Design of power transformer protection

Faults in transformers are relatively infrequent but they cause damage expensive to repair. The fault clear- ance time greatly affects the extent and cost of removal of the damage. Outage of a transformer may upset the power balance in the power system and cause loss of system stability. Therefore, the quality of the design of its protective system is the key to the whole of the power system. Fig. 1 shows the design procedure of a transformer protective system.

Generally, a protective system for a transformer can be developed according to the factors listed below:

• the characteristics of the given transformer, such as its type, size and ratings;

• the role the transformer plays in the power system, for example, for generator step-up, for power transmis- sion, for distribution, or for equipment;

• the construction of the primary circuit, such as what elements (generators, buses or lines) the trans- former is connected to and the position of circuit breakers (CBs), etc.;

• the operating mode of the transformer (for in- stance, the mode of operation with other transformers, neutral-earth mode, voltage regulating mode) and co- ordination with the protection of adjoining buses or lines;

• the type of the transformer's user which may be an ordinary user or rectifier equipment, or a large trans- former may be equipped with the user, stove type user, etc.

Of course the policy and budget of the operating authority and other factors can also have some impact on the project. Therefore, different protection schemes may be designed for the various actions given above. Perhaps a simple protection set, like a fuse, is enough for a small transformer for equipment, but large trans- formers may be equipped with numerous protection systems in an attempt to cover a great variety of possible faults.

The expert system proposed in this paper can assist engineers with the following problems:

(a) determination of the protection system; (b) choice of protective mode; (c) deployment of power transformers (PTs), current

transformers (CTs) and relays; (d) calculation of current transformer turns ratios

and relay settings; (e) selection of specific relays from relay manufactur-

er's catalogues.

3. Outline of the proposed expert system

The proposed expert system has been developed on a personal computer AT-486/DX2 by making use of

Analysis o f the faults the circuit o f tranaforme~ on

iv prot~lion sys~m Dotennination o f

iv Choice of protection mode

4 r Deployment of CT, PT and relays

( iv

y N

Forming alarge databa~ for and printing orders for protection

,& Drawing design for seoondaty ~,~aits ]

Fig. I. The procedure for designing a protection system.

)

OPS83, a support tool for building up an expert system. OPS83 is very suitable for design work because it is good at the forward inference process, easy to link with other languages, with simple expressions and at high speed. Unfortunately, it cannot easily express the knowledge of a specific field since it is a support tool in general use. However, a representation method, suitable for designing transformer protection, has been devel- oped here.

The expert system shown in Fig. 2 is made up of four main parts: the knowledge base, the inference engine, subroutine programs for setting CTs, PTs, and relays, and the user interface.

__ Knowledge Base

-1 Data Base I . Facts of ttmmforme~ [

and primm.,y " " [2. Frmae ~mcture o~i~f~ ] ~r-

_ Rule Base

[1. Rules for de " - ~

12. Rules for cona-olUng I l infcteace

Subroutines for I ~ . ~ CT, PT, r ~ , I

OPS83~ Inference engine I

User intatface ]

User ]

Fig, 2. Outline of the proposed expert system.

Page 3: An expert system for designing the protection system of a power transformer

L. Lifeng et al./ Electric Power Systems Research 35 (1995) 59-64 61

{ iF [ Data of fa~]

1 parameters THEN

Transformer is for ] reeti~ng equipment J

ction is deployed for short-ckcuit I~tecfion

Fig. 3. A forward inference process.

3.1. Knowledge base o f the expert system

The knowledge base includes a database and a rule base.

(a) The database is divided into two parts. The first houses the facts and parameters of the transformer and primary circuit, which determine the construction of the protection system. The second holds the data for the results of the protection system formed during the inference process. A frame form is adopted here to describe the result base and its advantages will be explained in the next section.

(b) The rule base is also made up of two groups: a group of rules for protection design and a group for controlling the inferences. The former rules come from the experience of skilled engineers and design standards or handbooks. The latter rules are heuristic and able to control the expert system to search for goal solutions effectively. The rules of the two groups are represented in IF -THEN form which can be offered in a standard statement by OPS83.

3.4. User interface

The expert system provides graphical and helpful features that make the sytem user friendly. Both En- glish and Chinese have been used in the menus that the expert system displays and in the explanations about the inference.

4. Knowledge representation for the protection system

In the proposed expert system, we consider the fol- lowing three requirements in regard to the knowledge representation of the protection system.

(1) It should be able to clearly express the relation- ships between protection components, like the type of protection system, protection of different fault types, protective modes and protective relays.

(2) It must be easy to describe the manner in which the protection system is to be configured into the primary circuits: for instance, what CT should be con- nected with a given relay, where the CT can be in- stalled, or which CB must be tripped by the protection, and so on.

(3) It ought to establish an available database for the graphical presentation of designs.

In order to accomplish these requirements, we use a frame structure to describe the protection system. Here the protection system is classified into five levels of frames, shown in Fig. 4, called the system level, fault- typeP (protection) level, protection mode level, protec- tive relay level, and specific relay (Srelay) level, respectively. Frames in each level have the same struc- ture as shown in Fig. 5 and each frame belongs to some frame in a higher level. The highest level, called the system level, has only one frame housing a group of protections corresponding to a variety of fault

3.2. Inference engine

OPS83 has an engine with plenty of inference mecha- nisms that users can choose for a specific problem. In addition, the engine can deal adeptly with design prob- lems because it is good at the forward inference pro- cess. For example, as shown in Fig. 3, we can get a number of goal parameters from transformer facts dur- ing forward inference.

3.3. Subroutine programs for calculations

OPS83 has good abilities not only in inference but also in calculation. Hence, the ordinary calculations of setting CTs, PTs, and relays can be handled by the OPS83 language itself, and more complex ones are registered as external functions written in FORTRAN.

I Protection system System

level

Fig. 4. Frame structure of the protection system.

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62 L. Lifeng et al . / Electric Power Systems Research 35 (1995) 59 64

name = system explanation = EX[ 1] LV = 10 KV MV = 110 KV HV = 220 KV fatflt- .typel = SCP Ihult-tyoe2 = BP ' fault-type3 = EFP-- fmdt-type4 = OLP'-- fatflt-type5 = GOP-- fault-12/pe6 = BFP--

calls~BpS name = BP belong-to = system ako = fault-typeP explanation = EX[3] model = OVM mode2 = NSCM mode3 = OCUVM loeationl = MV-side location2 = LV-side location3 = LV-side

name = CTR belong-to = OCUVM

I ako = relax, explanafio~n = EX[26] phase = 1 Isa =2.2A time-step = 2 tm~el = 2.5 s time2 = 3 s ~..pl = ICB

p2 = CB,2cB,3cB Sr,day = BL-52/6 -- - 7

calls CRT

name = OCUVM belong-to = BP ako = mode explanatoin=EX[10] relayl = CTR relay2 = UVR coordinate =HV-side rim.= l o / o . m v

%r= 3000/5A Xre~y = UVR Aloeation =HV-side Ill

name = BL-52/6 I-range = 1.5 - 6 A belong-to = CTR T-range = 0.25 - 9 s ako --- Srelay phase = 1 maker = XCRC co~umplion = 0.5 VA type = ~amistor lime-stop = 2 price = 32 Y IN = 5A *

Fig. 5 shows an example o f the knowledge represen- tat ion f rom the highest level to the lowest. Attr ibute ' ako ' in each level identifies the system components on the same level. By attribute 'belong-to ' , it is possible to know to which higher level frame a componen t belongs. Therefore it is easy to describe or modify the composi- t ion o f the system. Attr ibute 'explanat ion ' can give the user some informat ion about the frame, like the func- tions o f the component , the reasoning behind the frame, and so on.

5. In ference and an e x a m p l e

Although a protect ion system is represented by five levels o f frames, only one system frame is initialized before design inference. First, the informat ion about the t ransformer and pr imary circuit must be input by choosing answers f rom menus. Then the data will be used to drive some rules to reason out the system level frame. On the basis o f the system frame, the second inference step is carried out and fault- typeP level frames are formed. In this way, with each inference step we can get a level o f frames until all o f the specific relays have been selected.

As an example, Fig. 6 shows the pr imary circuit o f a three-winding t ransformer and its parameters. N o w we explain the reasoning process o f the expert system.

Fig. 5. An example of frame representation for a protection system. 5.1. De termina t ion o f the pro tec t ion sys t em

types such as short-circuit protect ion (SCP), earth-fault protect ion (EFP), backup protect ion (BP), over load protect ion (OLP), gas-oil protect ion (GOP), overmag- netization protect ion (OMP), breaker-failure protect ion (BFP), voltage regulator fault protect ion (VRP), etc.

In a single fault- type protection, one or several pro- tective modes may be applied to operate the protection. H-side Thus, a fault- typeP frame should contain some neces- sary protective modes such as the differential mode (DFM), overcurrent mode (OVM), negative-sequence current mode (NSCM), overcurrent /undervol tage mode 1PT (OCUVM), and so on. In addition, the location o f each mode o f protect ion can also be indicated in this level o f frames.

As we know, the protect ion o f each mode m a y be made up o f one or more kinds o f relays. Thus, a mode frame ought to include these relays and the calculated values o f the CTs or PTs connected to the relays. Similarly, each kind o f relay has a frame itself in which some values o f relay settings, relations to pr imary and 3PT secondary circuits, and the name o f the specific relay ~X~ selected are represented. The lowest level frames, called specific relay frames, house some relay parameters, such as the setting ranges, characteristics, normal values, and LH-tide prices provided by the manufacturers.

Before the inference, we initialize the system frame with all o f the fault-type protect ions indicated in the previous section. Then one rule will be matched accord- ing to the informat ion about the given transformer:

20KV M-side

1LS ~ ILS

1CT

~ 220/110/10KV

125MVA

3CT

3CB V I

llqfide : 27alKV 5.30A

3LS M-~le: IIOKV 10110A

L-shk : IOKV 2.~0A 10KV

IIOKV

~ILS 2CB

2CT

~mtce lime Yes 2.5s

No l , ~

No

Fig. 6. The primary circuit system including a power transformer.

Page 5: An expert system for designing the protection system of a power transformer

L. Lifeng et al./Electric Power Systems Research 35 (1995) 59 64 63

Rule system2. IF the HV of the transformer is under 550 kV and there is no voltage regulator installed, T H E N overmagnetization protection and voltage regu- lator protection will be removed

As a result, only six kinds of protection are left in the system frame shown in Fig. 5. The content of EX[I] is:

This is the system frame for transformer protection and the frame is reasoned out by rule system2

5.2. Protective mode choice

With respect to each fault-type protection, backup protection being taken as an example, some suitable protective modes can be determined by rules:

Rule fp5. IF the transformer is to step up voltage and its capacity is above 63 MVA, T H E N the negative- sequence current mode (NSCM) and the overcurrent/ undervoltage mode (OCUVM) should be chosen

R u l e f p l l . IF there is no power source on the MV side of the transformer, T H E N the overcurrent mode (OCM) can be chosen and located on the MV side. Moreover, NSCM and OCUM are installed on the LV side

Then the results are written in a fault-typeP frame, and EX[3] includes the following information:

This is a fault-typeP frame for backup protection and the frame is reasoned out by rules pf5 and pfl 1

5.3. Deployment o f relays, CTs, and PTs

Some necessary relays corresponding to certain pro- tective modes can be suggested in the inference step, for example:

Rule mod6. IF the OCUV mode belonging to the backup protection has been adopted, T H E N choose one overcurrent relay with timer (CTR) and one under- voltage relay (UVR), and coordinate the protection with the adjoining protection on the HV side

In the inference step, one subroutine program calcu- lating settings for the CTs and PTs is called and the settings are recorded in a relay frame.

T H E N two time stages are necessary: timel = t ime+ 0.5 = 2.5 s for tripping the CB on the HV side; time2 = timel + 0.5 = 3 s for tripping the CBs on all sides

The expert system compares every parameter with the data provided by the manufacturers:

Rule tel32. IF type = CTR; IN = 5 A; time-stage = 2; phase = 1; /set-range - 1.5-6 A; time-range = 0.25-9 s; maker = XCRC; etc., T H E N choose BL-52/6 relay

The expert system has provided specific relay number BL-52/6 and recorded it and other necessary parame- ters in a relay frame. By the way, if the setting values of a relay do not meet the needs of protection, the infer- ence has to back-track to mode level or fault-type level and try another component. For instance, another UVR (Arelay) has to be added at the HV side of the transformer because, on setting relay2, we find that the UVR chosen had not enough sensitivity as a backul 5 protection for the HV side.

Finally, according to the specific relay number the lowest frames can be filled with parameters provided by the manufacturers. As shown in Fig. 5, the last frame is the specific relay frame for BL-52/6.

6. Conclusions

This paper has presented an expert system for design- ing transformer protection. The system can assist engi- neers to determine the protection system, choose the protective mode, deploy and set relays, CTs, and PTs, and select a specific relay from the range of products of two major manufacturers of protective relays. A frame method is adopted to represent the protection system knowledge. It is not only easy to express the relation- ships among the protective components, but also to describe the relation between the secondary circuit and the primary circuit. Hence, a better library has been created for the graphical presentation of designs. Fur- ther development is being carried out to enhance the system, including more explanations and the capability of presenting new designs graphically. This ensures that it is attractive for protection engineers to adopt the system along with technological progress and changes.

5.4. Setting o f protective relays and selection o f specific relays

Now we take the CTR which the OCUVM frame gives as an example. First, the expert system calls a subroutine program to set the operating values of the relay, like Ise t, etc.; then it calculates the operating time:

Rule rell4. IF the CRT is chosen for OCUVM and it is to be coordinated with the protection on the HV side,

References

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[2] Y.Y. Hsu and J.L. Chen, Distribution planning using a knowl- edge-based expert system, 1EEE Trans. Power Delivery, 5 (3) (1990) 1514-1519.

[3] H.B. Piittgen and J.F. Jansen, Expert system for the design of a power plant electrical auxiliary system, IEEE Trans. Power Syst., 3 (1) (1988) 254-261.

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64 L. Lifeng et al . / Electric Power Systems Research 35 (1995) 59 64

[4] A.V. Machias, E.N. Dialynas and C.A. Protopapas, An expert system approach to designing and testing substation grounding grids, IEEE Trans. Power Delivery, 4 (1) (1989) 234-240.

[5] S.J. Lee, S.H. Yoon, M. Yoon and J. Jang, An expert system for protective relay setting of transmission systems, IEEE Trans. Power Delivery, 5 (2) (1990) 1202-1208.

[6] K. Kawahara, H. Sasaki, J. Kubokawa, M. Kitagawa and H. Sugihara, Construction on expert system for transmission line protection, Proc. 3rd Symp. Expert System Application to Power Systems (ESAPS), Tokyo and Kobe, Japan, 1991, pp. 295-299.

[7] A. Kalam, Knowledge based system for transformer protection, Int. Conf. Advances in Power System Control, Operation and Management (APSCOM), Hong Kong, 1991, Vol. 1, Conf. Publ. No. 348, lEE, Hong Kong, pp. 433-438.

[8] C. Forgy, 0PS83 Official Manual, Personal Media, 1990. [9] The Technological Standards on Relay Protection and Automatic

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[10] Design Handbook on Electric Power Engineering: Secondary Cir- cuits, Northwest Electric Power Design Institute, China, 1990.