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Expert system for designing transmission line protection system
TTERWORTH I N E M A N N
Electrical Power & Eneryy Systems, Vol, 17, No. I, pp. 69-78, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0142-0615/95 $10.00 + 0.00
Expert system for designing transmission line protection system K Kawahara Fukuyama University, Fukuyama-City, 729-02, Japan H Sasaki and J Kubokawa Hiroshima University, Higashi-Hiroshima, 724, Japan M Kitagawa and H Sugihara Chugoku Electric Power Company, Hiroshima-City, 724, Japan
so as to prevent the damage of related facilities and limit the outage area as much as possible. A protective relaying system is composed of relays, potential transformers (PTs), current transformers (CTs) and circuit breakers (CBs). In order to accomplish the above mentioned objective, it is necessary to explore an optimal deployment of relays and coordination of their settings, needless to say, to pursue the best combination with related apparatus. Conventionally, protective relaying systems have been designed by experienced protection engineers based on their wealth of domain knowledge, considering various factors .such as system configuration, fault current, voltage class, etc. In particular, relay setting tasks require highly sophisticated knowledge as conditions for the relay setting are always changing as a result of new installations of power equipment, changes in short-circuit capacity due to operational changes of generators, and increase in load currents. Many studies have been devoted to making computers replace these tasks of protection engineers. One early study was to solve the coordination of relays in off-line 1. However, this is unacceptable to protection engineers since it made use of neither the experience nor the intuition of the domain engineers. To overcome this shortcoming, the utilization of a CAD system and an interactive approach are proposed 2'3. Another approach, in which a computer is used to assist the jobs of protection engineers, dealt with the coordination among line protection relays in a looped system4'5. Neither of these studies exploit the domain knowledge or experience. Stimulated by the recent remarkable progress in knowledge engineering, expert systems have been applied to solve various problems in power systems by integrating the domain specific knowledge. In fact, fault location and
This paper presents a powerful expert system (ES) which makes a basic design of an adequate protective relaying system based on the knowledge of skilled protection engineers. Following the basic design, the ES carries out the relay setting and its validation by means of the integrated power flow and fault calculation programs, Furthermore, the ES has a capability of securing coordination among separa,~ely set relays. It is very flexible in that it is able to reset the relays according to power system changes. The effectiveness has been demonstrated by using a part of the Chugoku Electric Power Company System. Keywords: expert system, transmission line protection, protective relaying system, relay setting, coordination check
I. Introduction Since transmission lines are spread over vast geographical areas, they are more likely to be exposed to various natural stresses such as lightning strokes, storms [typhoons, hurricanes), etc. This means that the rate of fault occurrence on transmission lines is very large compared with that of other equipment. If a prompt and adequate countermeasure is not taken after the incidence of a fault, it may propagate to other apparatus, leading to a system-wide blackout in an extreme case. The main objective of a protective relaying system of transmission lines is to disconnect a faulted line promptly Received 25 November 1993; revised 2 March 1994; accepted 19 April 1994
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Expert system for designing transmission line protection system: K. Kawahara et al.
system restoration have been the most effective application area for expert systems and many production grade systems are already in use. A similar movement has been observed in the field of designing protective relaying systems. In Reference 6, the setting and coordination of distance relays are tackled by expert systems. However, this sort of research is limited to the solving of particular topics such as the relay setting or coordination. So far, there are no such expert systems which can totally assist the tasks of protection engineers which include the combustion of related equipment. The authors have already proposed an expert system which overall supports the design of line protection systems v. The major functions of the proposed system were the determination of a relaying system, effective deployment of relays, and the setting of the chosen relays. Although a simplified relay setting calculation has been incorporated to take into account shunt effects in multi-terminal transmission lines, it is based not on fault calculation results but on the relay setting standards. Also, the ES has no capability for checking the validity of the setting by means of fault calculation results. Another function to be included in an ES of this sort is an ability to confirm the coordination among independently set relays. This paper proposes an extension of the work in Reference 7. In general, tasks associated with line protection may be classified into two parts: (1) the fundamental design of a relaying system, and (2) relay setting and its validation, coordination check and resetting according to changes in the power system configuration. Although some improvements are added in the former, the major contributions of this work are concerned with the latter. As a big step, the ES has been fortified with the ability to call directly power system analysis programs, that is, load flow and fault calculation programs developed by the authors. The fault calculation program is especially powerful in that it can handle not only any kind of single fault but simultaneous faults consisting of different single faults. This has enabled accurate relay settings to be made, validate the setting values, and ensure consistency among independently set relays (coordination). Another contribution worth noting is that the ES has become very flexible: it can reset the relays according to alterations in the power system topology. In the near future, relays to be used for high voltage transmission line protection will gradually be replaced by digital relays. In such circumstances, the method proposed in this paper will be truly valuable for realizing an accurate line protection. The proposed ES has been applied to a part of the 110 kV transmission line system of the Chugoku Electric Power Company. Results obtained are much better than those in Reference 7 owing to the improvement in knowledge representation and the integration of numerical analysis programs.
II. Outline of the proposed system I1.1 Outline of protection tasks The aims of transmission line protection are to determine a suitable protective relaying system and to calculate the setting values. Currently, these tasks are achieved by
skilled protection engineers who have a great deal of experience and knowledge of transmission line protection. The tasks may be summarized as follows: (1) to design an adequate protection system and select the most suitable equipment, (2) to set the equipment for normal operating conditions; (3) to readjust the setting values according to changes in operating states; and (4) to store necessary operation records of the protective system. In the above, tasks (1)-(3) need domain-specific knowledge, and hence they can be good targets of support by expert systems. Concerning the related tasks of line protection, the Chugoku Electric Power Company has established the following two criteria for the purpose of providing protective engineers with guidelines in carrying out the above tasks. (I) Installation criteria The installation criteria which describe the standard relaying systems aim to ensure the safety of equipment to be protected by carrying out the optimal distribution of relays. The standard protective relaying systems are separately prescribed according to voltage class, the number of terminals and the types of transmission. (II) Relay settin9 standards The relay setting standards that describe basic setting policies aid the execution of relay setting appropriately. A standard range of relay setting is provided for various types of protective relays. Here, (I) contains the knowledge associated with issues (1) and (3), whereas (II) has the knowledge about (4). In addition to these oflicial documents, the experiences and domain knowledge of skilled protection engineers are indispensable for constructing an expert system which will be able to truly assist the tasks of protection engineers. For instance, in the deployment of relays, there are decisive factors other than those prescribed in (I), namely the length of a protected line and the loading condition of the other terminal. Concerning the task of relay setting (3), it may be necessary to make coordination with relays installed at the other terminal depending on the case. Furthermore, the configuration of a power system is always changing due to outages by scheduled maintenance works. Hence, in order to carry out the setting successfully, it is necessary to grasp the connecting state of the power system in on-line, this produces a troublesome problem for protection engineers. In this paper, an improved version over the ES in Reference 7 is proposed which can flexibly cope with changes in operating states of a power system. To realize this, the power system to be protected is represented in detail including switching devices such as CBs and disconnecting switches (DSs). The proposed ES checks the on- and off-states of these devices prior to the assistance of protection engineers, thus enabling the flexibility. Figure 1 shows the configuration of the proposed ES together with its processing flow. First, the connecting states of the switching devices are checked based on the nominal system data, which is followed by the fundamental design of a protective relaying system by the ES. The contents of the design are: the determination of a protective relaying system and the
Expert system for designing transmission line protection system: K. Kawahara et al. Informationon System I Configuration I I
I- Knowledge Base
I
I l # Installation Criteria Checking the Connecting I , ~ # RelaySetting Standards [ Statesof the PowerSystemI / ] # Others I"
I-Database
I
---4~:_~°eff.......-4 # Protectivedevices I=''~''~ IX I #Others pNumerical Calculations
~ ~
1
#
#
Design of Protective
- Relaying System # Determination of Protective Relaying System # Deployment of Relays
# Determination of Primary Turns Ratio of CTs
LoadFlowCalculation Fault Calculation
]
_ Settings of Protective Relays and its Validation
# Calculation of :initial setting values # Checking the coordination with adjacent protection intervals.
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rules. The following three factors must be examined regarding the switching devices: (1) there are two transmission lines between substations, (2) switching devices connecting the transmission line and the substation are closed, (3) bus tie breakers are closed in the case of multiple buses. Each factor is translated into a production rule and these rules are stored as the state grasping rules. If the above three rules hold true, the ES understands that the relevant lines are parallel lines. Though the system configuration changes depending on the on- or off-states of the switching devices, the ES can handle versatile situations by using the state grasping rules. However, impossible combinations of the on- or off-states of the switching devices are excluded from the KB of the ES to avoid redundant inference. 11.3 Representation of a p o w e r system In the proposed ES, the following two situations are considered in regard to a power system representation:
Figure 1. The configuration of the proposed expert system and the processing flow
(1) ease in changing the description of power facilities in the database; and (2) ease in grasping the connecting state of a power system based on the states of switching devices.
deployments of relays. During the design process, the inference engine refers to the database, the knowledge base and results of numerical calculation programs through working memory. In the knowledge bases, the installation criteria and the relay setting standards are stored in the form of production rules ( I F - T H E N rules). The database contains the types of protective relays and the classes of transmission lines. In the second place, the setting of the deployed relays and its validation will be carried out via the load flow and fault calculation programs by referring to the setting standards stored in the knowledge base. As these functions are affected by the connecting states of CBs and DSs, they are executed independently of the design work.
In order to satisfy these requirements, power system components are classified into three levels. The highest level contains power facilities: substations, transmission lines and power station. The second level includes components such as buses, lines, transformers, etc. The lowest rank defines switching devices, CB and DS. Figure 2 shows an example of the knowledge base which represents a substation. The data structures in all the levels are unified. Attribute 'ako' in each level is to identify the power system components on the same level. Attribute 'instance' inherits a name of the power facility from the highest level and can identify to which level the current frame belongs. The connections between two facilities are described only through frames representing switching devices on the lowest level, thus facilitating grasping the connecting state of a power system. In the
11.2 Knowledge representation In constructing an ES, it is important to decide what type of knowledge representation is adopted. As the knowledge base of an ES is continually evolving through the addition and/or modification of knowledge, an appropriate knowledge representation facilitates the maintenance of the knowledge base. Moreover, if the knowledge representation has an easily accessible form to protection engineers who are not accustomed to managing the ES, the ES would be more welcome. The knowledge used in the proposed ES includes many abstract representations because the knowledge has been translated from manuals, ~:he installation criteria and the relay setting standards. For example, the terms 'parallel line' and 'tie line' are trivial to protection engineers, but the ES cannot understand them directly. If there is a parallel line in the system diagram with its receiving end buses disconnected, it should not be identified as the parallel lines from the operational standpoint. In addition, the terms associated with transmission line protection such as 'protective zone' and 'existence of the power source behind' are mostly described as viewed from a CB installation standpoint. Here, let us show how the ES identifies a parallel line by using the state grasping
~
Substation A
Bus 1 r
Bus :
~.t )-
I
~
open close
CB ~-
0
DS -0-
@ J
Substation A
l
l
.+
CBI I DS21
Bus l, Bus 2, Transformer.... DS 1, CB l ....
I
I
name = Substation A ako = substation belong_to= source type = double bus voltage = 110kV
flame = Bus 1 ako = bus belong_to= substation type =A voltage = 110kV
name =CBI state = open ako = CB belong_to= bus from = DSI to = DS2 instance = Substation A
Figure 2. Example of knowledge representation for a substation
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Expert system for designing transmission line protection system. K. Kawahara et al.
Line l
Line 2
Substation A
open -0
close
CB DS
~-
@
O
Substation B
Figure 3. Example which cannot be regarded as a parallel line from an operational standpoint case where some power facility is added or removed, it suffices to add or delete the relevant frames in each level, without making any modifications of other frames. The independence among the data is a great advantage in this approach. As an example, let us assume that a transformer is newly added to substation A. The necessary procedures are (a) to add a new frame in the second level with 'ako=transformer' and enter the necessary data, and (b) to add new frames in the lowest level of CBs and DSs to be connected to the new transformer. The slots of some frames must be modified which correspond to devices connecting the transformer to the system.
III. Design of protective relaying systems II1.1 Determination of protective relaying system Protective relaying systems are classified into short circuit protection types and ground-fault protection types. Each type of protection has both primary and back up. There are three major factors that must be considered in determining a protective relaying system: (1) voltage class for transmission line (500kV, 220 kV,...); (2) importance of transmission lines (important lines or general lines); and (3) the number of terminals. In the above, factor (2) provides classification of transmission lines according to their degree of importance. Transmission lines tying primary substations or lines connecting to thermal or nuclear plants are regarded as important lines. The latter are mainly load leased lines, except for important lines. Factor (3) is the number of tapped lines. Although factors (1) and (3) are uniquely determined from the initial system data, factor (2) is identified by the ES according to the connecting state of the power system. Therefore, factor (2) is clarified after executing the state grasping rules and affects the determination of protective relaying systems. 111.2 Deployment of protective relay Various types of protective relays are used in a power system, depending on the protection objective. Therefore, it is a rather difficult task to distribute those relays correctly. In general, relay types to be distributed are mostly determined by the selected protective relaying system. However, although a protective relaying system
is prescribed for every transmission line unit, actual circumstances differ subject to CB installation points. As an example, let us consider the case in Figure 3. Two-circuit lines connect Substation A with a power source and Substation B. Protection engineers may determine that the transmission lines should be equipped with distance directional relays as short-circuit protection and ground-fault directional relays as ground fault protection, respectively, since they belong to the category of general lines. Though relays at CB1 and CB2 are distributed according to this protective scheme, relays to be connected to CB3 and CB4 should be mainly overcurrent relays since Substation B is a receiving end. In addition, CB4 requires a ground-fault relay since it connects to a transformer. In this ES, a minimum set of relays as required by the system connection is distributed at every CB installation point according to the determined protective relaying system, and then relays corresponding to special considerations are treated by adding individual rules. Figure 4 shows an example of the knowledge and rules written in OPS83 for the deployment of relays for ground fault protection. II 1.3 Determination of primary rated current of CTs Although this is not the main issue of this paper, the following descriptions of CTs are somewhat related to the relay setting. The primary rated current of CTs must take into account the rating of devices, the characteristics of loads, a degree of overcurrent, the structure of the CT and the combination with other devices. The primary currents of CTs are generally selected to 100-120% of the rated line currents. Since the primary operating currents of these CTs are required to be wide enough, it is an important task to select the primary rated currents of CTs adequately. The overcurrent characteristic should be a problem for a certain type of relaying system. The following two factors are regarded to be the most important: (1) thermal capacity of transmission lines; and (2) transformers with maximum capacity at substations. The first factor is mainly determined from the line material and its information is stored in the frames for the power system data. Thermal capacities of these transmission lines are embedded in a database. In the proposed ES, it estimates the primary rated current of CTs to be 1.1 times the thermal capacity of transmission lines which are protected by the relays. In the case of buses connected to a transformer, the primary rated current of CTs is selected as the smaller one of (1) anc[ (2). Then, the nearest turn ratio value is sought from the Rule tsrh 17 {&A(control op =tns; status = active; step = 1); &B(dummy2 kind = O r d i n a r y ; para <> P a r a l l e l ,); &C(equipt n a m e = &B.nback; type = A _ s i n g l e _ b u s ; ako = s u b s t a t ion); & D ( c o n ako = b r a n c h e d _ l i n e ; name = &B.line; type < > U n b a l a n c e d ) ; &E(equipt n a m e = &D.instance; type = T w o _ c i r c u i t _ l ines); -.>
local &i :integer, &.k :integer; &k= l;&i= 1; &RSET[&il.tab[&k].relay =151RI; &RSET[&i].tab[&k+l ].relay = 167GRI; &RSET[&i].tab[&k+2],relay = 164VI; &RSET[&i].tabl&k+3].relay = 167GRTI; &RSET[&i].tab[&k+4],mlay = 127RI; &RSET[&i].tab[&k+5].relay = 127RTI;
Figure 4. A rule for the deployment of relays
Expert system for designing transmission line protection system: K. Kawahara et al.
Calculation of initial setting values # Based on the relay setting standards I~ "" ~"
[ Modify setting [ Ivalues I
Validation of initial setting values for each relay # To operate duty # Not to operate duty
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the protective zone changes according to operating conditions, the setting values of the relay should be modified accordingly. In the proposed ES, we intend to acquire knowledge from the relay setting standards and automatically to modify the setting values corresponding to on- and off-states of the switching devices. Figure 6 shows the flow diagram for calculating initial setting values. Once the connecting states are changed, the state grasping rules for relay settings are fired and the interim results are output to a working memory. The relay setting rules are executed thereafter.
Missettings B
Coordination with foward [ adjacent sections # Reach and time delays
I
Miscoordination [Output of setting values ]
Figure 5. Flow chart of relay setting database and adopted as the best rated current. Let us consider, for example, a three-terminal transmission line that may have a power source at every bus. If the thermal capacity of the main line is different from the tapped line, the ES adopts its smaller value as the primary rated current. The primary rated currents of CTs recommended by the ES are just for information.
IV. Settings of protective relay and its validation Protection engineers spend most of the time on the setting of relays since it is affected by many factors such as the number of terminals, shorl: circuit capacity, load current and zero-phase circulating current. In general, it is processed according to the flow diagram in Figure 5. First initial setting values are calculated, followed by the validation phase that is done separately for each relay. The last step is to check coordination with adjacent protection intervals.
IV.1 Calculation of initial setting values Initial setting values are dew,ermined based on the contents described in the relay setting standards. In the case of a distance relay for short circuit protection, the first zone is prescribed as follows: (1) 80%-85% of the positive sequence reactance of its own protective zone; (2) 30% of the value of positive equivalent impedance for transformer in the case of two-terminal lines; and (3) 80 85% of impedance seen by the relay for a bus fault at the nearest bus considering shunt effect in the case of multi-terminal lines. Basically, the descriptions in setting standards such as the above should be translated as the knowledge base as correctly as possible. However, descriptions such as 'protective zone' and 'impedance seen by a relay' cannot be identified uniquely since their implications change depending on the connecting states of a power system, and therefore may disturb the inference of the ES. Since
IV.2 Validation of setting values and coordination check After the initial setting values are determined, whether or not the relays can operate correctly for a set of assumed faults must be checked. In general, items to be checked are prescribed according to the types of relays. For example, Table 1 specifies both the 'duty to operate' and 'duty not to operate' conditions of a distance relay for phase to phase protection (called a '44S' relay). For the first zone protection, we translate 'the duty to operate'
Rules for grasping the connecting
['-" state of a power system • ]Checking for operatingstate of / C B s and DSs 1 # Powersource behind | # The numberof terminals [ # Shortcircuitcapacity 1 # Others
Working memory
--]
] / [
~
~_ . I t.nanges m ] ] switchin, devices I I ./I/ ix,¢ i,Iv
Relay settingrules [
# Name of the CBs # Name of next bus # Impedanceof protective zone # Shunteffects # Others
~ [ Resultsof initial v [ settingvalues
Figure 6. The relationship between relay setting process and the rules
T a b l e 1. C h e c k i n g i t e m s for 4 4 S relay
Duty to operate
Duty not to operate
The first zone
To operate within own protective section reliably
Not to operate for the next bus fault
The second zone
To operate for the next bus fault
Not to operate for the bus fault on the next forward section
The third zone
To operate reliably for the bus fault on the next forward section
Not to operate for the maximum load current
74
Expert system for designing transmission line protection system. K. Kawahara et al.
into the following production rules: Rule 1
IF 44S relay has the setting values for the first zone protection, THEN execute a fault calculation for a three phase short circuit at the next bus to obtain the impedance value seen by the relay within the first zone. Rule 2 IF the value of impedance seen by the relay is below the initial setting values, THEN change the setting values. As the fault calculation program as well as the load flow program must be executed on the 'THEN-part' of Rule 1, they are registered as external functions in the ES. The fault calculation has the following features. (1) It can simulate not only a single fault such as a line-to-ground and line-to-line fault but also simultaneous faults with a combination of any types of faults. It can also deal with a fault on any phase with an arbitrary value of fault resistance. (2) The location of an assumed fault is not limited at a bus, but any intermediate point on a line can be specified. (3) Parallel lines and phase shifters are incorporated in the program. (4) All the fault conditions required in the fault calculation can be designated as the arguments of external functions. After the initial setting values are determined, the ES checks the coordination among relays. Necessary points
to be checked in the relay coordination are usually provided according to the types of relays, as in the validation of setting values. For example, the 44S relay which is installed on each phase of a line must be checked whether or not an overreach for leading phases between the first and the third zone protection exists. To provide this, fault calculation is executed for double line-toground faults on the nearest forward adjacent bus. On the basis of the results, whether or not a relay on the unfaulted phase line does operate undesirably is checked. If it does, the ES diagnoses the initial setting values and shows modified values on the CRT display.
V. Execution results and discussions A prototype system of the proposed ES has been applied to the system shown in Figure 7 to demonstrate its effectiveness. This is a part of the 110 kV transmission systems of the Chugoku Electric Company System. The prototype system has 37 rules for grasping system states, 50 rules for designing protective relaying systems, and about 20 rules for the settings of protective relay and its validation. At present, the prototype system can make the settings with respect to only an internal direction type distance relay (44SI) and its backup protection relay (44S); extensions to other types of relays are in progress. Figure 8 shows part of the interim results and relay deployments for the. system configuration shown in Figure 7. In the interim results, information is grouped with respect to each CB. The ES checks whether each switching device is open or closed in the initial system data: first, it identifies bus tie breakers which are
C14 SJ KT
BI~
HM
C15 ,.(~SW ..... Substation HH
Substation H
C37
C3~ .
HS C6
(N C18
C5 C4
C23
Tie Line H
.(N
Substation K
C3 C2
-(N Tie
I
Line KH L25 C25
C1
Substation S Substation KH
Line KH C20 C19
GN
C12
liP
Tapped CLOSE OPEN DS ~ @
CB Figure 7. The example system to test the proposed system
HK
0
C27
Expert system for designing transmission line protection system."K. Kawahara et al.
Open states of b u s - t i e --> B1 Open states of b u s - t i e --> B2 *** C o n n e c t i o n check *** --> OK. Do you change the s y s t e m c o n f i g u r a t i o n *** I n t e r i m results *** A name of CBs C6 C7 C14 C15 C16 C17 C21 C22 C23 C24 C33 C34 C35 C36 C37 *** CB = 1 2 3 4 5 6 7 8 9 i0 ii 12 13 14 15 16 17 18 19 20 21
A kind of bus Source Source Load Load Load Load Source Source Source Source Load Load Consumer Load Load
Protective zone Line Line Line Line Line Line Line Line Line Line Line Line Line Line Line
i0 9 9 i0 12 ii 20 21 13 14 15 16 17 18 19
Results of relay C3 CB = 51S 1 51SB 2 60P 3 60PB 4 51H 5 27H 6 44SI 7 440M 8 67GI 9 67GO i0 64VL ii 64VH 12 27 13 44S 14 44OMB 15 44ST2 16 44ST3 17 67G 18 64V 19 67GT 20 64VT 21 22
? (y/n) n
Importance of lines
Ordinary Ordinary Ordinary Ordinary Ordinary Ordinary Important Important Ordinary Ordinary Ordinary Ordinary Ordinary Ordinary Ordinary
deployment CI0 51S 51SB 60P 60PB 51H 27H 44SI 44OM 67GI 67GO 64VL 64VH 27 44S 440MB 44ST2 44ST3 67G 64VT 67GT 64VT 44S0
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Relaying system Class Class Class Class Class Class Class Class Class Class Class Class Class Class Class
*** CB = C5 1 44S 2 51S 3 44ST2 4 44ST3 5 67G 6 64V 7 67GT 8 64VTI 9 64VT2 CB = C18 1 51H 2 51G
A-4 A-4 B-I B-I B-I B-I A-2 A-2 A-3 A-3 A-3 A-3 B-2 A-3 A-3
Primary turns ratio of CTs i000 i000 50O 500 600 600 8O0 8O0 40O 4OO 600 6O0 i00 600 60O CB = C7 1 44S 2 51S 3 44ST2 4 44ST3 5 67G 6 64V 7 67GT 8 64VTI 9 64VT2 CB = C14 1 51R 2 51G 3 51RXTL 4 51GTLI 5 51GTL2 CB = 1 2 3 4 5 6 7
C15 67G 64V 64VT 67GTLI 67GTL2 51R 51RXT
Figure 8. Execution results 1
in an open state (in this example B1 and B2 are open). Secondly, the states of the DSs between a double-bus and transmission lines are checked. If there is some abnormal state such as a disconnected line, it is shown as an abnormal condition. The contents of information are the kind of bus (sourqze, load . . . . ), names of lines to be protected, the importance of transmission lines (important or ordinary) and selected relaying systems. Deployments of relays are indicated by numbers representing relay functions. These numbers are prescribed in the Standards of Japan Electric Machine Industry Association (JEM) and consistent to the IEC codes. Results for the deployment of relays in Figure 8 are shown only for tie line H, lines AK and HS. Figure 9 shows inference results when the system configuration is changed due to switching operations. State changes in switching devices are at substations A and HM, line K and tapped line S, which are also shown in Figure 9. In these interim results, since line HS is regarded as a parallel line because the bus tie breaker at
substation SJ is closed, the relaying system is specified accordingly. Conversely, as the bus tie device in substation HM is open, the relaying system among substations HH, SN and HM are changed from a parallel to an ordinary type. Figure 10 shows the setting results of 44S type relays among substations HH, K and S before and after changes in CBs (c21, c25 and c28) and DSs (L21 and L25). First, initial setting values before the changes are indicated for several CB installation points. However, for relays installed at c27 and c28, their setting values in the second and third zones are modified by the validation rules for 44S relays. This is because the impedance value seen by the relay exceeds the initial setting values as the result of executing fault calculations at each bus on the forward adjacent line. Next, the initial setting values after the changes are displayed for the relays attached to the newly closed CBs. The initial setting value for the relay installed at CB (c27) is modified as its protective zone has changed from substation HH to substation K. In the validation
Expert system for designing transmission line protection system: K. Kawahara et al.
76
O p e n s t a t e s of b u s - t i e --> B1 O p e n s t a t e s of b u s - t i e --> B2 *** C o n n e c t i o n c h e c k *** --> OK. Do y o u c h a n g e the s y s t e m c o n f i g u r a t i o n ? (y/n) Input n u m b e r s of S w i t c h i n g d e v i c e s = B1 Input t h e s t a t e s (open or c l o s e = close Input n u m b e r s of S w i t c h i n g d e v i c e s = B4 Input the s t a t e s (open or c l o s e = open Input n u m b e r s of S w i t c h i n g d e v i c e s = C27 Input the s t a t e s (open or c l o s e = close Input n u m b e r s of S w i t c h i n g d e v i c e s = L25 Input the s t a t e s (open or c l o s e = open Input n u m b e r s of S w i t c h i n g d e v i c e s = C25 Input the s t a t e s (open or c l o s e = open Input n u m b e r s of S w i t c h i n g d e v i c e s = C22 Input the s t a t e s (open or c l o s e = close Input n u m b e r s of S w i t c h i n g d e v i c e s = L22 Input t h e s t a t e s (open or c l o s e = close Input n u m b e r s of S w i t c h i n g d e v l c e s = L26 I n p u t the s t a t e s (open or c l o s e = open Input n u m b e r s of S w i t c h i n g d e v i c e s = C26 Input the s t a t e s (open or c l o s e = open Input n u m b e r s of S w i t c h i n g d e v l c e s = 0 ***
Interim A name of CBs C6 C7 C14 C15 C16 C17 C21 C23 C24 C33 C34 C35 C36 C37
CB : 1 2 3 4 5 6 7 8 9 i0 ii 12 13 14
results
A kind of bus Source Source Load Load Load Load Source Source Source Load Load Consumer Load Load R e s u l t s of
C7 44S 51S 44ST2 44ST3 67G 64V 67GT 64VTI 64VT2 50S 50G 50GT 64V 27
y
*** Protective zone
Importance of lines
L i n e i0 Ordinary Ordinary Line 9 Ordinary Line 9 Ordinary L i n e i0 L i n e 12 Ordinary L i n e 11 Ordinary Important L i n e 20 L i n e 13 Ordinary L i n e 14 Ordinary Ordinary L i n e 15 L i n e 16 Ordinary L i n e 17 Ordinary L i n e 18 Ordinary L i n e 19 Ordinary relay deployment *** CB = C14 CB = 1 1 50S 2 2 50G 3 3 50GT 4 4 64V 5 5 27 6 7 CB = C15 8 1 50S 9 2 50G i0 3 50GT ii 4 64V 12 5 27 13 14 CB : C17 1 51R 2 51G 3 51RXTL 4 51GTLI 5 51GTL2
Relaying system Class Class Class Class Class Class Class, Class Class Class Class Class Class Class C23 44S 51S 44ST2 44ST3 67G 64V 67GT 64VTI 64VT2 50S 50G 50GT 64V 27
A-3 A-3 B-I B-I B-I B-I A-2 A-3 A-3 A-3 A-3 B-2 B-I B-I
Primary turns r a t i o of CTs i000 I000 500 500 600 600 1200 400 400 600 600 i00 600 600 CB = 1 2 3 4 5
C33 50S 50G 50GT 64V 27
CB = 1 2
C35 51H 51G
CB = 1 2 3 4 5
C36 51R 51G 51RXTL 51GTLI 51GTL2
Figure 9. Execution results 2
of the setting values, it is modified for relays installed at CBs (c22 and c25). Finally, we refer to the execution time of the prototype system. The system is implemented on a personal computer (CPU: 80386 20 MHz), made by NEC. It takes about 15 s to obtain the results of the initial setting values and most of the execution time is spent for firing rules
520 times. Since OPS83 used to construct the proposed ES is a sort of compiler type, the prototype system is able to executing a large number of rules in high speed. For validation of the setting values, the prototype system is able to complete this function within several minutes at the most, though this depends on the number of runs of the fault calculation program.
Expert system for designing transmission line protection system: K. Kawahara et al.
***
44S
CB CB CB CB
= = = =
*** CB CB CB CB
= = = =
c21 C22 c27 c28
initial ZONE1 1.0631 1.0631 1.045 1.045
***
44S = c22 = c25 = c27
***
CB CB CB CB
before changes ZONE3 4.9555 4.9555 7.1521 7.1521
***
Validation of r e l a y s e t t i n g v a l u e s *** c27 Z o n e 2 c h a n g e s its v a l u e s to 2 . 4 8 c28 Z o n e 2 c h a n g e s its v a l u e s to 2 . 4 8 c27 Z o n e 3 c h a n g e s its v a l u e s to 5.33 c28 Z o n e 3 c h a n g e s its v a l u e s to 5.33
*** Change Do y o u c h a n g e Input numbers Input the Input numbers Input the Input numbers Input the Input numbers Input the Input numbers Input the Input numbers
CB CB CB
setting results ZONE2 2.4481 2.4481 1.876 1.876
77
= = = =
the system configuration *** the system configuration ? (y/n) of S w i t c h i n g d e v l c e s = c21 states (open o r close) = o p e n of S w i t c h i n g d e v l c e s = L 2 1 s t a t e s (open o r close) = o p e n of S w i t c h i n g d e v x c e s = c25 states (open o r close) = c l o s e of S w i t c h i n g d e v i c e s = L 2 5 states (open or close) = c l o s e of S w i t c h i n g d e v i c e s = c28 states (open or close) = o p e n of S w i t c h i n g d e v i c e s = 0
initial ZONE1 1.4053 1.5678 0.7027
setting results ZONE2 2.4481 2.7552 1.4053
y
after changes ZONE3 4.9555 6.0423 4.955
Validation of r e l a y s e t t i n g v a l u e s c22 Z o n e 2 c h a n g e s its v a l u e s to c22 Z o n e 3 c h a n g e s its v a l u e s to c25 Z o n e 2 c h a n g e s its v a l u e s to c25 Z o n e 3 c h a n g e s its v a l u e s to
***
***
6.566 13.679 3.672 7.933
Figure 10. Execution results 3
VI. C o n c l u d i n g r e m a r k s This paper presented an expert system for totally assisting protection engineers in mitigating their cumbersome tasks. The major functions realized in the proposed ES are: (1) a basic design of a protective relaying system base on the given power system diagram; (2) deployment of suitable relays by using domain specific knowledge and experience; (3) initial relay setting for the deployed relays; (4) validation check on the setting values by means of fault calculation programs; and (5) coordination check among independently set relays. The specific features of the ES may be summarized as follows. (1) In order to construct the knowledge base, the installation criteria and relay setting standards for protective relaying systems, which are the accumulation of the specialized knowledge of protection engineers, have been translated into production rules. (2) Power system analysis programs, load flow and fault calculation, have been integrated into the ES as external functions so that they can be called directly from production rules. (3) Adequate relay deployment has been realized for various power system topologies. (4) The validation of relay setting values is accurate due to the incorporation of the fault calculation program.
There are, of course, many points to be included or which need further refinement in this ES as this is a prototype system. A construction of a graphical user-interface, inclusions of other types of relays and the reinforcement of coordination capability are typical issues that must be resolved. Nevertheless, the ideas proposed in this paper must be a step towards realizing a flexible protection system in the near future, especially considering that digital relays will sooner or later replace conventional relays in transmission system protection.
VII. R e f e r e n c e s 1 Albrecht,R E, Nisja, W E, Feero, G D, Rockefeller, G D and Wagner, C L'Digital computer protective device coordination program I - General program description' IEEE Trans. Power Appar. Syst. Vol PAS-83 (April pp 402-410
1964)
2
Damborg, M J, Ramaswami, R, Venkata, S S and Posfforoosh, J M 'Computer aided transmission protection system design' IEEE Trans. Power Appar. Syst. Vol 103 No 1 (January 1984) pp 51-59
3
Suzuki, K, Iwamoto, T, Komukai, T and Hitokawa, T 'Interactive computation system of distance relay setting for a large scale EHV power system' IEEE Trans. Power Appar. Syst. Vol PAS-99 No 1 (January/February 1980) pp 165-173
78
Expert system for designing transmission line protection system: K. Kawahara et al. Dwarakanath, M H and Nowitz, L 'An application of linear graph theory for coordination of directional overcurrent relays' SIAM Meeting, Seattle, WA, March 1980, pp 104-114 Bapeswara Rao, V V and Sankara Rao, K 'Computer aided coordination of directional relays: determination of break points' IEEE Trans. Power Deriv. Vol 3 No 2 (April 1988) pp 545-548
Lee, S J, Yoon, S H, Yoon, M C and Jong, J K 'An expert system for protective relay setting of transmission system' IEEE Trans. Power Appar. Syst. Vol 5 No 2 (April 1990) pp 1202-1208 Kawahara, K L, Sasaki, H, Kubokawa, J, Kitagawa, J, Kitagawa, M and Sugihara, H 'Construction of expert system for transmission line protection' Proc. 3rd Symposium on Expert Systems Application to Power Systems (1991) pp 295-299
Electrical Power & Eneryy Systems, Vol, 17, No. I, pp. 69-78, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0142-0615/95 $10.00 + 0.00
Expert system for designing transmission line protection system K Kawahara Fukuyama University, Fukuyama-City, 729-02, Japan H Sasaki and J Kubokawa Hiroshima University, Higashi-Hiroshima, 724, Japan M Kitagawa and H Sugihara Chugoku Electric Power Company, Hiroshima-City, 724, Japan
so as to prevent the damage of related facilities and limit the outage area as much as possible. A protective relaying system is composed of relays, potential transformers (PTs), current transformers (CTs) and circuit breakers (CBs). In order to accomplish the above mentioned objective, it is necessary to explore an optimal deployment of relays and coordination of their settings, needless to say, to pursue the best combination with related apparatus. Conventionally, protective relaying systems have been designed by experienced protection engineers based on their wealth of domain knowledge, considering various factors .such as system configuration, fault current, voltage class, etc. In particular, relay setting tasks require highly sophisticated knowledge as conditions for the relay setting are always changing as a result of new installations of power equipment, changes in short-circuit capacity due to operational changes of generators, and increase in load currents. Many studies have been devoted to making computers replace these tasks of protection engineers. One early study was to solve the coordination of relays in off-line 1. However, this is unacceptable to protection engineers since it made use of neither the experience nor the intuition of the domain engineers. To overcome this shortcoming, the utilization of a CAD system and an interactive approach are proposed 2'3. Another approach, in which a computer is used to assist the jobs of protection engineers, dealt with the coordination among line protection relays in a looped system4'5. Neither of these studies exploit the domain knowledge or experience. Stimulated by the recent remarkable progress in knowledge engineering, expert systems have been applied to solve various problems in power systems by integrating the domain specific knowledge. In fact, fault location and
This paper presents a powerful expert system (ES) which makes a basic design of an adequate protective relaying system based on the knowledge of skilled protection engineers. Following the basic design, the ES carries out the relay setting and its validation by means of the integrated power flow and fault calculation programs, Furthermore, the ES has a capability of securing coordination among separa,~ely set relays. It is very flexible in that it is able to reset the relays according to power system changes. The effectiveness has been demonstrated by using a part of the Chugoku Electric Power Company System. Keywords: expert system, transmission line protection, protective relaying system, relay setting, coordination check
I. Introduction Since transmission lines are spread over vast geographical areas, they are more likely to be exposed to various natural stresses such as lightning strokes, storms [typhoons, hurricanes), etc. This means that the rate of fault occurrence on transmission lines is very large compared with that of other equipment. If a prompt and adequate countermeasure is not taken after the incidence of a fault, it may propagate to other apparatus, leading to a system-wide blackout in an extreme case. The main objective of a protective relaying system of transmission lines is to disconnect a faulted line promptly Received 25 November 1993; revised 2 March 1994; accepted 19 April 1994
69
70
Expert system for designing transmission line protection system: K. Kawahara et al.
system restoration have been the most effective application area for expert systems and many production grade systems are already in use. A similar movement has been observed in the field of designing protective relaying systems. In Reference 6, the setting and coordination of distance relays are tackled by expert systems. However, this sort of research is limited to the solving of particular topics such as the relay setting or coordination. So far, there are no such expert systems which can totally assist the tasks of protection engineers which include the combustion of related equipment. The authors have already proposed an expert system which overall supports the design of line protection systems v. The major functions of the proposed system were the determination of a relaying system, effective deployment of relays, and the setting of the chosen relays. Although a simplified relay setting calculation has been incorporated to take into account shunt effects in multi-terminal transmission lines, it is based not on fault calculation results but on the relay setting standards. Also, the ES has no capability for checking the validity of the setting by means of fault calculation results. Another function to be included in an ES of this sort is an ability to confirm the coordination among independently set relays. This paper proposes an extension of the work in Reference 7. In general, tasks associated with line protection may be classified into two parts: (1) the fundamental design of a relaying system, and (2) relay setting and its validation, coordination check and resetting according to changes in the power system configuration. Although some improvements are added in the former, the major contributions of this work are concerned with the latter. As a big step, the ES has been fortified with the ability to call directly power system analysis programs, that is, load flow and fault calculation programs developed by the authors. The fault calculation program is especially powerful in that it can handle not only any kind of single fault but simultaneous faults consisting of different single faults. This has enabled accurate relay settings to be made, validate the setting values, and ensure consistency among independently set relays (coordination). Another contribution worth noting is that the ES has become very flexible: it can reset the relays according to alterations in the power system topology. In the near future, relays to be used for high voltage transmission line protection will gradually be replaced by digital relays. In such circumstances, the method proposed in this paper will be truly valuable for realizing an accurate line protection. The proposed ES has been applied to a part of the 110 kV transmission line system of the Chugoku Electric Power Company. Results obtained are much better than those in Reference 7 owing to the improvement in knowledge representation and the integration of numerical analysis programs.
II. Outline of the proposed system I1.1 Outline of protection tasks The aims of transmission line protection are to determine a suitable protective relaying system and to calculate the setting values. Currently, these tasks are achieved by
skilled protection engineers who have a great deal of experience and knowledge of transmission line protection. The tasks may be summarized as follows: (1) to design an adequate protection system and select the most suitable equipment, (2) to set the equipment for normal operating conditions; (3) to readjust the setting values according to changes in operating states; and (4) to store necessary operation records of the protective system. In the above, tasks (1)-(3) need domain-specific knowledge, and hence they can be good targets of support by expert systems. Concerning the related tasks of line protection, the Chugoku Electric Power Company has established the following two criteria for the purpose of providing protective engineers with guidelines in carrying out the above tasks. (I) Installation criteria The installation criteria which describe the standard relaying systems aim to ensure the safety of equipment to be protected by carrying out the optimal distribution of relays. The standard protective relaying systems are separately prescribed according to voltage class, the number of terminals and the types of transmission. (II) Relay settin9 standards The relay setting standards that describe basic setting policies aid the execution of relay setting appropriately. A standard range of relay setting is provided for various types of protective relays. Here, (I) contains the knowledge associated with issues (1) and (3), whereas (II) has the knowledge about (4). In addition to these oflicial documents, the experiences and domain knowledge of skilled protection engineers are indispensable for constructing an expert system which will be able to truly assist the tasks of protection engineers. For instance, in the deployment of relays, there are decisive factors other than those prescribed in (I), namely the length of a protected line and the loading condition of the other terminal. Concerning the task of relay setting (3), it may be necessary to make coordination with relays installed at the other terminal depending on the case. Furthermore, the configuration of a power system is always changing due to outages by scheduled maintenance works. Hence, in order to carry out the setting successfully, it is necessary to grasp the connecting state of the power system in on-line, this produces a troublesome problem for protection engineers. In this paper, an improved version over the ES in Reference 7 is proposed which can flexibly cope with changes in operating states of a power system. To realize this, the power system to be protected is represented in detail including switching devices such as CBs and disconnecting switches (DSs). The proposed ES checks the on- and off-states of these devices prior to the assistance of protection engineers, thus enabling the flexibility. Figure 1 shows the configuration of the proposed ES together with its processing flow. First, the connecting states of the switching devices are checked based on the nominal system data, which is followed by the fundamental design of a protective relaying system by the ES. The contents of the design are: the determination of a protective relaying system and the
Expert system for designing transmission line protection system: K. Kawahara et al. Informationon System I Configuration I I
I- Knowledge Base
I
I l # Installation Criteria Checking the Connecting I , ~ # RelaySetting Standards [ Statesof the PowerSystemI / ] # Others I"
I-Database
I
---4~:_~°eff.......-4 # Protectivedevices I=''~''~ IX I #Others pNumerical Calculations
~ ~
1
#
#
Design of Protective
- Relaying System # Determination of Protective Relaying System # Deployment of Relays
# Determination of Primary Turns Ratio of CTs
LoadFlowCalculation Fault Calculation
]
_ Settings of Protective Relays and its Validation
# Calculation of :initial setting values # Checking the coordination with adjacent protection intervals.
71
rules. The following three factors must be examined regarding the switching devices: (1) there are two transmission lines between substations, (2) switching devices connecting the transmission line and the substation are closed, (3) bus tie breakers are closed in the case of multiple buses. Each factor is translated into a production rule and these rules are stored as the state grasping rules. If the above three rules hold true, the ES understands that the relevant lines are parallel lines. Though the system configuration changes depending on the on- or off-states of the switching devices, the ES can handle versatile situations by using the state grasping rules. However, impossible combinations of the on- or off-states of the switching devices are excluded from the KB of the ES to avoid redundant inference. 11.3 Representation of a p o w e r system In the proposed ES, the following two situations are considered in regard to a power system representation:
Figure 1. The configuration of the proposed expert system and the processing flow
(1) ease in changing the description of power facilities in the database; and (2) ease in grasping the connecting state of a power system based on the states of switching devices.
deployments of relays. During the design process, the inference engine refers to the database, the knowledge base and results of numerical calculation programs through working memory. In the knowledge bases, the installation criteria and the relay setting standards are stored in the form of production rules ( I F - T H E N rules). The database contains the types of protective relays and the classes of transmission lines. In the second place, the setting of the deployed relays and its validation will be carried out via the load flow and fault calculation programs by referring to the setting standards stored in the knowledge base. As these functions are affected by the connecting states of CBs and DSs, they are executed independently of the design work.
In order to satisfy these requirements, power system components are classified into three levels. The highest level contains power facilities: substations, transmission lines and power station. The second level includes components such as buses, lines, transformers, etc. The lowest rank defines switching devices, CB and DS. Figure 2 shows an example of the knowledge base which represents a substation. The data structures in all the levels are unified. Attribute 'ako' in each level is to identify the power system components on the same level. Attribute 'instance' inherits a name of the power facility from the highest level and can identify to which level the current frame belongs. The connections between two facilities are described only through frames representing switching devices on the lowest level, thus facilitating grasping the connecting state of a power system. In the
11.2 Knowledge representation In constructing an ES, it is important to decide what type of knowledge representation is adopted. As the knowledge base of an ES is continually evolving through the addition and/or modification of knowledge, an appropriate knowledge representation facilitates the maintenance of the knowledge base. Moreover, if the knowledge representation has an easily accessible form to protection engineers who are not accustomed to managing the ES, the ES would be more welcome. The knowledge used in the proposed ES includes many abstract representations because the knowledge has been translated from manuals, ~:he installation criteria and the relay setting standards. For example, the terms 'parallel line' and 'tie line' are trivial to protection engineers, but the ES cannot understand them directly. If there is a parallel line in the system diagram with its receiving end buses disconnected, it should not be identified as the parallel lines from the operational standpoint. In addition, the terms associated with transmission line protection such as 'protective zone' and 'existence of the power source behind' are mostly described as viewed from a CB installation standpoint. Here, let us show how the ES identifies a parallel line by using the state grasping
~
Substation A
Bus 1 r
Bus :
~.t )-
I
~
open close
CB ~-
0
DS -0-
@ J
Substation A
l
l
.+
CBI I DS21
Bus l, Bus 2, Transformer.... DS 1, CB l ....
I
I
name = Substation A ako = substation belong_to= source type = double bus voltage = 110kV
flame = Bus 1 ako = bus belong_to= substation type =A voltage = 110kV
name =CBI state = open ako = CB belong_to= bus from = DSI to = DS2 instance = Substation A
Figure 2. Example of knowledge representation for a substation
72
Expert system for designing transmission line protection system. K. Kawahara et al.
Line l
Line 2
Substation A
open -0
close
CB DS
~-
@
O
Substation B
Figure 3. Example which cannot be regarded as a parallel line from an operational standpoint case where some power facility is added or removed, it suffices to add or delete the relevant frames in each level, without making any modifications of other frames. The independence among the data is a great advantage in this approach. As an example, let us assume that a transformer is newly added to substation A. The necessary procedures are (a) to add a new frame in the second level with 'ako=transformer' and enter the necessary data, and (b) to add new frames in the lowest level of CBs and DSs to be connected to the new transformer. The slots of some frames must be modified which correspond to devices connecting the transformer to the system.
III. Design of protective relaying systems II1.1 Determination of protective relaying system Protective relaying systems are classified into short circuit protection types and ground-fault protection types. Each type of protection has both primary and back up. There are three major factors that must be considered in determining a protective relaying system: (1) voltage class for transmission line (500kV, 220 kV,...); (2) importance of transmission lines (important lines or general lines); and (3) the number of terminals. In the above, factor (2) provides classification of transmission lines according to their degree of importance. Transmission lines tying primary substations or lines connecting to thermal or nuclear plants are regarded as important lines. The latter are mainly load leased lines, except for important lines. Factor (3) is the number of tapped lines. Although factors (1) and (3) are uniquely determined from the initial system data, factor (2) is identified by the ES according to the connecting state of the power system. Therefore, factor (2) is clarified after executing the state grasping rules and affects the determination of protective relaying systems. 111.2 Deployment of protective relay Various types of protective relays are used in a power system, depending on the protection objective. Therefore, it is a rather difficult task to distribute those relays correctly. In general, relay types to be distributed are mostly determined by the selected protective relaying system. However, although a protective relaying system
is prescribed for every transmission line unit, actual circumstances differ subject to CB installation points. As an example, let us consider the case in Figure 3. Two-circuit lines connect Substation A with a power source and Substation B. Protection engineers may determine that the transmission lines should be equipped with distance directional relays as short-circuit protection and ground-fault directional relays as ground fault protection, respectively, since they belong to the category of general lines. Though relays at CB1 and CB2 are distributed according to this protective scheme, relays to be connected to CB3 and CB4 should be mainly overcurrent relays since Substation B is a receiving end. In addition, CB4 requires a ground-fault relay since it connects to a transformer. In this ES, a minimum set of relays as required by the system connection is distributed at every CB installation point according to the determined protective relaying system, and then relays corresponding to special considerations are treated by adding individual rules. Figure 4 shows an example of the knowledge and rules written in OPS83 for the deployment of relays for ground fault protection. II 1.3 Determination of primary rated current of CTs Although this is not the main issue of this paper, the following descriptions of CTs are somewhat related to the relay setting. The primary rated current of CTs must take into account the rating of devices, the characteristics of loads, a degree of overcurrent, the structure of the CT and the combination with other devices. The primary currents of CTs are generally selected to 100-120% of the rated line currents. Since the primary operating currents of these CTs are required to be wide enough, it is an important task to select the primary rated currents of CTs adequately. The overcurrent characteristic should be a problem for a certain type of relaying system. The following two factors are regarded to be the most important: (1) thermal capacity of transmission lines; and (2) transformers with maximum capacity at substations. The first factor is mainly determined from the line material and its information is stored in the frames for the power system data. Thermal capacities of these transmission lines are embedded in a database. In the proposed ES, it estimates the primary rated current of CTs to be 1.1 times the thermal capacity of transmission lines which are protected by the relays. In the case of buses connected to a transformer, the primary rated current of CTs is selected as the smaller one of (1) anc[ (2). Then, the nearest turn ratio value is sought from the Rule tsrh 17 {&A(control op =tns; status = active; step = 1); &B(dummy2 kind = O r d i n a r y ; para <> P a r a l l e l ,); &C(equipt n a m e = &B.nback; type = A _ s i n g l e _ b u s ; ako = s u b s t a t ion); & D ( c o n ako = b r a n c h e d _ l i n e ; name = &B.line; type < > U n b a l a n c e d ) ; &E(equipt n a m e = &D.instance; type = T w o _ c i r c u i t _ l ines); -.>
local &i :integer, &.k :integer; &k= l;&i= 1; &RSET[&il.tab[&k].relay =151RI; &RSET[&i].tab[&k+l ].relay = 167GRI; &RSET[&i].tab[&k+2],relay = 164VI; &RSET[&i].tabl&k+3].relay = 167GRTI; &RSET[&i].tab[&k+4],mlay = 127RI; &RSET[&i].tab[&k+5].relay = 127RTI;
Figure 4. A rule for the deployment of relays
Expert system for designing transmission line protection system: K. Kawahara et al.
Calculation of initial setting values # Based on the relay setting standards I~ "" ~"
[ Modify setting [ Ivalues I
Validation of initial setting values for each relay # To operate duty # Not to operate duty
73
the protective zone changes according to operating conditions, the setting values of the relay should be modified accordingly. In the proposed ES, we intend to acquire knowledge from the relay setting standards and automatically to modify the setting values corresponding to on- and off-states of the switching devices. Figure 6 shows the flow diagram for calculating initial setting values. Once the connecting states are changed, the state grasping rules for relay settings are fired and the interim results are output to a working memory. The relay setting rules are executed thereafter.
Missettings B
Coordination with foward [ adjacent sections # Reach and time delays
I
Miscoordination [Output of setting values ]
Figure 5. Flow chart of relay setting database and adopted as the best rated current. Let us consider, for example, a three-terminal transmission line that may have a power source at every bus. If the thermal capacity of the main line is different from the tapped line, the ES adopts its smaller value as the primary rated current. The primary rated currents of CTs recommended by the ES are just for information.
IV. Settings of protective relay and its validation Protection engineers spend most of the time on the setting of relays since it is affected by many factors such as the number of terminals, shorl: circuit capacity, load current and zero-phase circulating current. In general, it is processed according to the flow diagram in Figure 5. First initial setting values are calculated, followed by the validation phase that is done separately for each relay. The last step is to check coordination with adjacent protection intervals.
IV.1 Calculation of initial setting values Initial setting values are dew,ermined based on the contents described in the relay setting standards. In the case of a distance relay for short circuit protection, the first zone is prescribed as follows: (1) 80%-85% of the positive sequence reactance of its own protective zone; (2) 30% of the value of positive equivalent impedance for transformer in the case of two-terminal lines; and (3) 80 85% of impedance seen by the relay for a bus fault at the nearest bus considering shunt effect in the case of multi-terminal lines. Basically, the descriptions in setting standards such as the above should be translated as the knowledge base as correctly as possible. However, descriptions such as 'protective zone' and 'impedance seen by a relay' cannot be identified uniquely since their implications change depending on the connecting states of a power system, and therefore may disturb the inference of the ES. Since
IV.2 Validation of setting values and coordination check After the initial setting values are determined, whether or not the relays can operate correctly for a set of assumed faults must be checked. In general, items to be checked are prescribed according to the types of relays. For example, Table 1 specifies both the 'duty to operate' and 'duty not to operate' conditions of a distance relay for phase to phase protection (called a '44S' relay). For the first zone protection, we translate 'the duty to operate'
Rules for grasping the connecting
['-" state of a power system • ]Checking for operatingstate of / C B s and DSs 1 # Powersource behind | # The numberof terminals [ # Shortcircuitcapacity 1 # Others
Working memory
--]
] / [
~
~_ . I t.nanges m ] ] switchin, devices I I ./I/ ix,¢ i,Iv
Relay settingrules [
# Name of the CBs # Name of next bus # Impedanceof protective zone # Shunteffects # Others
~ [ Resultsof initial v [ settingvalues
Figure 6. The relationship between relay setting process and the rules
T a b l e 1. C h e c k i n g i t e m s for 4 4 S relay
Duty to operate
Duty not to operate
The first zone
To operate within own protective section reliably
Not to operate for the next bus fault
The second zone
To operate for the next bus fault
Not to operate for the bus fault on the next forward section
The third zone
To operate reliably for the bus fault on the next forward section
Not to operate for the maximum load current
74
Expert system for designing transmission line protection system. K. Kawahara et al.
into the following production rules: Rule 1
IF 44S relay has the setting values for the first zone protection, THEN execute a fault calculation for a three phase short circuit at the next bus to obtain the impedance value seen by the relay within the first zone. Rule 2 IF the value of impedance seen by the relay is below the initial setting values, THEN change the setting values. As the fault calculation program as well as the load flow program must be executed on the 'THEN-part' of Rule 1, they are registered as external functions in the ES. The fault calculation has the following features. (1) It can simulate not only a single fault such as a line-to-ground and line-to-line fault but also simultaneous faults with a combination of any types of faults. It can also deal with a fault on any phase with an arbitrary value of fault resistance. (2) The location of an assumed fault is not limited at a bus, but any intermediate point on a line can be specified. (3) Parallel lines and phase shifters are incorporated in the program. (4) All the fault conditions required in the fault calculation can be designated as the arguments of external functions. After the initial setting values are determined, the ES checks the coordination among relays. Necessary points
to be checked in the relay coordination are usually provided according to the types of relays, as in the validation of setting values. For example, the 44S relay which is installed on each phase of a line must be checked whether or not an overreach for leading phases between the first and the third zone protection exists. To provide this, fault calculation is executed for double line-toground faults on the nearest forward adjacent bus. On the basis of the results, whether or not a relay on the unfaulted phase line does operate undesirably is checked. If it does, the ES diagnoses the initial setting values and shows modified values on the CRT display.
V. Execution results and discussions A prototype system of the proposed ES has been applied to the system shown in Figure 7 to demonstrate its effectiveness. This is a part of the 110 kV transmission systems of the Chugoku Electric Company System. The prototype system has 37 rules for grasping system states, 50 rules for designing protective relaying systems, and about 20 rules for the settings of protective relay and its validation. At present, the prototype system can make the settings with respect to only an internal direction type distance relay (44SI) and its backup protection relay (44S); extensions to other types of relays are in progress. Figure 8 shows part of the interim results and relay deployments for the. system configuration shown in Figure 7. In the interim results, information is grouped with respect to each CB. The ES checks whether each switching device is open or closed in the initial system data: first, it identifies bus tie breakers which are
C14 SJ KT
BI~
HM
C15 ,.(~SW ..... Substation HH
Substation H
C37
C3~ .
HS C6
(N C18
C5 C4
C23
Tie Line H
.(N
Substation K
C3 C2
-(N Tie
I
Line KH L25 C25
C1
Substation S Substation KH
Line KH C20 C19
GN
C12
liP
Tapped CLOSE OPEN DS ~ @
CB Figure 7. The example system to test the proposed system
HK
0
C27
Expert system for designing transmission line protection system."K. Kawahara et al.
Open states of b u s - t i e --> B1 Open states of b u s - t i e --> B2 *** C o n n e c t i o n check *** --> OK. Do you change the s y s t e m c o n f i g u r a t i o n *** I n t e r i m results *** A name of CBs C6 C7 C14 C15 C16 C17 C21 C22 C23 C24 C33 C34 C35 C36 C37 *** CB = 1 2 3 4 5 6 7 8 9 i0 ii 12 13 14 15 16 17 18 19 20 21
A kind of bus Source Source Load Load Load Load Source Source Source Source Load Load Consumer Load Load
Protective zone Line Line Line Line Line Line Line Line Line Line Line Line Line Line Line
i0 9 9 i0 12 ii 20 21 13 14 15 16 17 18 19
Results of relay C3 CB = 51S 1 51SB 2 60P 3 60PB 4 51H 5 27H 6 44SI 7 440M 8 67GI 9 67GO i0 64VL ii 64VH 12 27 13 44S 14 44OMB 15 44ST2 16 44ST3 17 67G 18 64V 19 67GT 20 64VT 21 22
? (y/n) n
Importance of lines
Ordinary Ordinary Ordinary Ordinary Ordinary Ordinary Important Important Ordinary Ordinary Ordinary Ordinary Ordinary Ordinary Ordinary
deployment CI0 51S 51SB 60P 60PB 51H 27H 44SI 44OM 67GI 67GO 64VL 64VH 27 44S 440MB 44ST2 44ST3 67G 64VT 67GT 64VT 44S0
75
Relaying system Class Class Class Class Class Class Class Class Class Class Class Class Class Class Class
*** CB = C5 1 44S 2 51S 3 44ST2 4 44ST3 5 67G 6 64V 7 67GT 8 64VTI 9 64VT2 CB = C18 1 51H 2 51G
A-4 A-4 B-I B-I B-I B-I A-2 A-2 A-3 A-3 A-3 A-3 B-2 A-3 A-3
Primary turns ratio of CTs i000 i000 50O 500 600 600 8O0 8O0 40O 4OO 600 6O0 i00 600 60O CB = C7 1 44S 2 51S 3 44ST2 4 44ST3 5 67G 6 64V 7 67GT 8 64VTI 9 64VT2 CB = C14 1 51R 2 51G 3 51RXTL 4 51GTLI 5 51GTL2 CB = 1 2 3 4 5 6 7
C15 67G 64V 64VT 67GTLI 67GTL2 51R 51RXT
Figure 8. Execution results 1
in an open state (in this example B1 and B2 are open). Secondly, the states of the DSs between a double-bus and transmission lines are checked. If there is some abnormal state such as a disconnected line, it is shown as an abnormal condition. The contents of information are the kind of bus (sourqze, load . . . . ), names of lines to be protected, the importance of transmission lines (important or ordinary) and selected relaying systems. Deployments of relays are indicated by numbers representing relay functions. These numbers are prescribed in the Standards of Japan Electric Machine Industry Association (JEM) and consistent to the IEC codes. Results for the deployment of relays in Figure 8 are shown only for tie line H, lines AK and HS. Figure 9 shows inference results when the system configuration is changed due to switching operations. State changes in switching devices are at substations A and HM, line K and tapped line S, which are also shown in Figure 9. In these interim results, since line HS is regarded as a parallel line because the bus tie breaker at
substation SJ is closed, the relaying system is specified accordingly. Conversely, as the bus tie device in substation HM is open, the relaying system among substations HH, SN and HM are changed from a parallel to an ordinary type. Figure 10 shows the setting results of 44S type relays among substations HH, K and S before and after changes in CBs (c21, c25 and c28) and DSs (L21 and L25). First, initial setting values before the changes are indicated for several CB installation points. However, for relays installed at c27 and c28, their setting values in the second and third zones are modified by the validation rules for 44S relays. This is because the impedance value seen by the relay exceeds the initial setting values as the result of executing fault calculations at each bus on the forward adjacent line. Next, the initial setting values after the changes are displayed for the relays attached to the newly closed CBs. The initial setting value for the relay installed at CB (c27) is modified as its protective zone has changed from substation HH to substation K. In the validation
Expert system for designing transmission line protection system: K. Kawahara et al.
76
O p e n s t a t e s of b u s - t i e --> B1 O p e n s t a t e s of b u s - t i e --> B2 *** C o n n e c t i o n c h e c k *** --> OK. Do y o u c h a n g e the s y s t e m c o n f i g u r a t i o n ? (y/n) Input n u m b e r s of S w i t c h i n g d e v i c e s = B1 Input t h e s t a t e s (open or c l o s e = close Input n u m b e r s of S w i t c h i n g d e v i c e s = B4 Input the s t a t e s (open or c l o s e = open Input n u m b e r s of S w i t c h i n g d e v i c e s = C27 Input the s t a t e s (open or c l o s e = close Input n u m b e r s of S w i t c h i n g d e v i c e s = L25 Input the s t a t e s (open or c l o s e = open Input n u m b e r s of S w i t c h i n g d e v i c e s = C25 Input the s t a t e s (open or c l o s e = open Input n u m b e r s of S w i t c h i n g d e v i c e s = C22 Input the s t a t e s (open or c l o s e = close Input n u m b e r s of S w i t c h i n g d e v i c e s = L22 Input t h e s t a t e s (open or c l o s e = close Input n u m b e r s of S w i t c h i n g d e v l c e s = L26 I n p u t the s t a t e s (open or c l o s e = open Input n u m b e r s of S w i t c h i n g d e v i c e s = C26 Input the s t a t e s (open or c l o s e = open Input n u m b e r s of S w i t c h i n g d e v l c e s = 0 ***
Interim A name of CBs C6 C7 C14 C15 C16 C17 C21 C23 C24 C33 C34 C35 C36 C37
CB : 1 2 3 4 5 6 7 8 9 i0 ii 12 13 14
results
A kind of bus Source Source Load Load Load Load Source Source Source Load Load Consumer Load Load R e s u l t s of
C7 44S 51S 44ST2 44ST3 67G 64V 67GT 64VTI 64VT2 50S 50G 50GT 64V 27
y
*** Protective zone
Importance of lines
L i n e i0 Ordinary Ordinary Line 9 Ordinary Line 9 Ordinary L i n e i0 L i n e 12 Ordinary L i n e 11 Ordinary Important L i n e 20 L i n e 13 Ordinary L i n e 14 Ordinary Ordinary L i n e 15 L i n e 16 Ordinary L i n e 17 Ordinary L i n e 18 Ordinary L i n e 19 Ordinary relay deployment *** CB = C14 CB = 1 1 50S 2 2 50G 3 3 50GT 4 4 64V 5 5 27 6 7 CB = C15 8 1 50S 9 2 50G i0 3 50GT ii 4 64V 12 5 27 13 14 CB : C17 1 51R 2 51G 3 51RXTL 4 51GTLI 5 51GTL2
Relaying system Class Class Class Class Class Class Class, Class Class Class Class Class Class Class C23 44S 51S 44ST2 44ST3 67G 64V 67GT 64VTI 64VT2 50S 50G 50GT 64V 27
A-3 A-3 B-I B-I B-I B-I A-2 A-3 A-3 A-3 A-3 B-2 B-I B-I
Primary turns r a t i o of CTs i000 I000 500 500 600 600 1200 400 400 600 600 i00 600 600 CB = 1 2 3 4 5
C33 50S 50G 50GT 64V 27
CB = 1 2
C35 51H 51G
CB = 1 2 3 4 5
C36 51R 51G 51RXTL 51GTLI 51GTL2
Figure 9. Execution results 2
of the setting values, it is modified for relays installed at CBs (c22 and c25). Finally, we refer to the execution time of the prototype system. The system is implemented on a personal computer (CPU: 80386 20 MHz), made by NEC. It takes about 15 s to obtain the results of the initial setting values and most of the execution time is spent for firing rules
520 times. Since OPS83 used to construct the proposed ES is a sort of compiler type, the prototype system is able to executing a large number of rules in high speed. For validation of the setting values, the prototype system is able to complete this function within several minutes at the most, though this depends on the number of runs of the fault calculation program.
Expert system for designing transmission line protection system: K. Kawahara et al.
***
44S
CB CB CB CB
= = = =
*** CB CB CB CB
= = = =
c21 C22 c27 c28
initial ZONE1 1.0631 1.0631 1.045 1.045
***
44S = c22 = c25 = c27
***
CB CB CB CB
before changes ZONE3 4.9555 4.9555 7.1521 7.1521
***
Validation of r e l a y s e t t i n g v a l u e s *** c27 Z o n e 2 c h a n g e s its v a l u e s to 2 . 4 8 c28 Z o n e 2 c h a n g e s its v a l u e s to 2 . 4 8 c27 Z o n e 3 c h a n g e s its v a l u e s to 5.33 c28 Z o n e 3 c h a n g e s its v a l u e s to 5.33
*** Change Do y o u c h a n g e Input numbers Input the Input numbers Input the Input numbers Input the Input numbers Input the Input numbers Input the Input numbers
CB CB CB
setting results ZONE2 2.4481 2.4481 1.876 1.876
77
= = = =
the system configuration *** the system configuration ? (y/n) of S w i t c h i n g d e v l c e s = c21 states (open o r close) = o p e n of S w i t c h i n g d e v l c e s = L 2 1 s t a t e s (open o r close) = o p e n of S w i t c h i n g d e v x c e s = c25 states (open o r close) = c l o s e of S w i t c h i n g d e v i c e s = L 2 5 states (open or close) = c l o s e of S w i t c h i n g d e v i c e s = c28 states (open or close) = o p e n of S w i t c h i n g d e v i c e s = 0
initial ZONE1 1.4053 1.5678 0.7027
setting results ZONE2 2.4481 2.7552 1.4053
y
after changes ZONE3 4.9555 6.0423 4.955
Validation of r e l a y s e t t i n g v a l u e s c22 Z o n e 2 c h a n g e s its v a l u e s to c22 Z o n e 3 c h a n g e s its v a l u e s to c25 Z o n e 2 c h a n g e s its v a l u e s to c25 Z o n e 3 c h a n g e s its v a l u e s to
***
***
6.566 13.679 3.672 7.933
Figure 10. Execution results 3
VI. C o n c l u d i n g r e m a r k s This paper presented an expert system for totally assisting protection engineers in mitigating their cumbersome tasks. The major functions realized in the proposed ES are: (1) a basic design of a protective relaying system base on the given power system diagram; (2) deployment of suitable relays by using domain specific knowledge and experience; (3) initial relay setting for the deployed relays; (4) validation check on the setting values by means of fault calculation programs; and (5) coordination check among independently set relays. The specific features of the ES may be summarized as follows. (1) In order to construct the knowledge base, the installation criteria and relay setting standards for protective relaying systems, which are the accumulation of the specialized knowledge of protection engineers, have been translated into production rules. (2) Power system analysis programs, load flow and fault calculation, have been integrated into the ES as external functions so that they can be called directly from production rules. (3) Adequate relay deployment has been realized for various power system topologies. (4) The validation of relay setting values is accurate due to the incorporation of the fault calculation program.
There are, of course, many points to be included or which need further refinement in this ES as this is a prototype system. A construction of a graphical user-interface, inclusions of other types of relays and the reinforcement of coordination capability are typical issues that must be resolved. Nevertheless, the ideas proposed in this paper must be a step towards realizing a flexible protection system in the near future, especially considering that digital relays will sooner or later replace conventional relays in transmission system protection.
VII. R e f e r e n c e s 1 Albrecht,R E, Nisja, W E, Feero, G D, Rockefeller, G D and Wagner, C L'Digital computer protective device coordination program I - General program description' IEEE Trans. Power Appar. Syst. Vol PAS-83 (April pp 402-410
1964)
2
Damborg, M J, Ramaswami, R, Venkata, S S and Posfforoosh, J M 'Computer aided transmission protection system design' IEEE Trans. Power Appar. Syst. Vol 103 No 1 (January 1984) pp 51-59
3
Suzuki, K, Iwamoto, T, Komukai, T and Hitokawa, T 'Interactive computation system of distance relay setting for a large scale EHV power system' IEEE Trans. Power Appar. Syst. Vol PAS-99 No 1 (January/February 1980) pp 165-173
78
Expert system for designing transmission line protection system: K. Kawahara et al. Dwarakanath, M H and Nowitz, L 'An application of linear graph theory for coordination of directional overcurrent relays' SIAM Meeting, Seattle, WA, March 1980, pp 104-114 Bapeswara Rao, V V and Sankara Rao, K 'Computer aided coordination of directional relays: determination of break points' IEEE Trans. Power Deriv. Vol 3 No 2 (April 1988) pp 545-548
Lee, S J, Yoon, S H, Yoon, M C and Jong, J K 'An expert system for protective relay setting of transmission system' IEEE Trans. Power Appar. Syst. Vol 5 No 2 (April 1990) pp 1202-1208 Kawahara, K L, Sasaki, H, Kubokawa, J, Kitagawa, J, Kitagawa, M and Sugihara, H 'Construction of expert system for transmission line protection' Proc. 3rd Symposium on Expert Systems Application to Power Systems (1991) pp 295-299