INTRODUCTION

The management decided that together with this reconstruction of the operator information system, it would be valuable to establish an intelligent advisory system to support the operators and the plant engineers with special expert information and knowledge. Similar efforts are running in Budapest as well [9].
The idea of applying expert systems to combine human knowledge with computerized tools is not new in the field of electrical energy. Several attempts were done worldwide (for example), and in Hungary (mostly by DYNADATA Ltd.) to use expert knowledge processing in different parts of electrical energy systems. This paper summarizes the problems and the results of our Paks substation project, which is different from all previous efforts, and it presents the substation itself and the different intelligent functions and finally implementation issues will be discussed.

The 400/120 kV Substation

On the top of the figure there are 16 and 120 kV fields and on the bottom right part are the five 400 kV fields. On the bottom left can be seen the transformer field that contains two transformers and an 18 kV feeder. The four, 400 kV fields at the lower right part of the figure receive power from the generators of Paks’ four nuclear units. At the bottom of the figure there are four, 400kV lines which are part of the Hungarian backbone electricity net. The 120 kV fields supply power to cities nearby and can also feed power back to the nuclear units when it is necessary to restart a unit after a shutdown.
The basic switching elements of the substation are the circuit breakers (squares on the figure) and the line and ground disconnectors (circles). The circuit breakers can switch high currents, while the disconnectors are used to isolate portions of the substations from high voltage or securely connect parts of the substations to ground. The operator remotely switches most of these elements but in case of short circuits or other disturbances, protective relays automatically open or close the circuit breakers to keep other equipment safe.

INTELLIGENT FUNCTIONS

• The management wanted an intelligent advisory system to support the operators with the know-how of the engineers, especially in alarm situations.
• To reduce maintenance costs and to assist maintenance design it is valuable to collect the switching information and operational history of all types of equipment (how many times did a circuit breaker switch, and why, when and how did it switch).
• The management wanted a better understanding of the technology to compare theoretical protection sequences with the actual performances of the equipment.
• Plant engineers need effective assistance in better understanding some error/disturbance events in the substation and they need support in recovery and some other design tasks (as for example switching sequence design).

A. Topology analysis with voltage and current map generation, and determination of dangerous topologies
The SCADA system provides measurements of many voltage and current values at various points of the substation. The number of these values allows generating an actual voltage and current map for the operator. This calculation is done periodically and the system also determines whether any parts of the substation are close to a dangerous state or not. Often, the present topology and some secondary signals together can indicate an alarm situation that is very difficult for a human operator to recognize (e.g. oil pressure unstability when only one transformer is on).

B. Equipment diagnostics and maintenance design based on the switching types of the different items of switchgear (closed, open, trip)
Before the new SCADA system was implemented only the catalog data were available; there was no information on actual performance. This function monitors the switching actions of all equipment and calculates various features, e.g. the slowest and the quickest switching time, deviation, average etc. It compares this information with the catalog data and if necessary warns the operator. It also examines the conditions of the switches and classifies them according to the performance of each. The result of the classification is compared with the catalog data to provide maintenance data about the aging of the equipment. At Paks this is very important information because maintenance of substation equipment must be scheduled in coordination with maintenance outages of the nuclear units.

C. Intelligent interlocking system based on the measured voltages and currents
Based on the topology it is possible to determine for each item of equipment whether it is permissible to switch it or not. This function sends updated information to the operator and sends warning messages if conflicts are detected between the topology and the built-in interlocking mechanism. This mechanism works with logical equations.

D. Diagnostics of disturbances, determination of the places and types of short-circuits, and advising in recovery procedures
When an error/disturbance situation occurs the SCADA system sends many error and warning signals to the operator, and it is very difficult for the operator to quickly determine the real reason of those signals. Typically many changes occur within an interval of 10-100 ms. The advisory system detects the changes in proper order, and tries to fit the predefined "protection samples" with a kind of tolerance and finally informs the operator about the best fitting sample. Earlier this information was available only several hours or even days later, when the engineering team analyzed recordings made by the (analog) measurement equipment.
The table below (Table 1.) gives a part of one of these protection samples.
 

In the knowledge acquisition phase of the project about 200 different disturbance events were collected (presently only in the 400 kV part of the substation). The table shows a protection sample defined by the technology expert of the substation. It is a meta sample which means that it defines 3 X 3 different samples (3 fields - 400/2, 400/3, 400/5 and the 3 phases - R, S, T). This table describes the logical connections among the signals, the importance of them, and the format of the signals (e.g. 0->1->0). For each sample there are "forbidden" signals defined, which means that if such a signal exists among the collected data then that sample must not fit.
The fitting algorithm first selects those samples for which all "key" signals exist and no "forbidden" signal is present among the collected data. It then classifies the samples as to how well they fit with the other (important, non-important and extra) signals. In the next step the refinement is done, comparing the timing context of the samples and the elements of the collected data set.
Fig. 3. shows the representation of the protection sample given above in G2 as a graph where the relationship among the signals can be seen as well. It is an unsuccessful attempt to reclose a circuit breaker after a short circuit near the other end of a 400kV-transmission line.

In the realization of this function procedural and rule based methods were mixed. The trigger of the function and most of the data processing is coded in procedures but a set of rules should fire to determine which is the most fitted sample.
Each protection sample includes the information about the current peak load (e.g. full, half, quarter or normal) of every circuit breaker that plays a role in the sample. The best-fitted sample defines the maintenance message that is sent to the maintenance design staff.

E. Automatic generation of switching sequences
The operator should plan the switching sequence of the equipment when such a command comes from the nuclear plant operators or from the control center of the Hungarian Electric Board. The user interface of the command selection function was designed for operators unskilled in computer science.
The starting state of the function is the present state of the substation. The final state is automatically calculated from the command and the present state. The switching sequence should be automatically generated using basic electrical rules and the special custom rules followed at Paks. The generated sequence is printed for the operator, who has to check and sign it, then execute it manually. Fig. 4. gives a list of these commands and some examples.

The algorithm defines step by step the sequence of elements to be switched. It uses the SCADA system’s built in interlocking equations and invokes the rules of the experts. The knowledge acquisition process by which the rules given by the experts are represented is done very easily in the G2 [3] code. In the following lines these rules are illustrated-one electrical and two custom rules in switching-off commands:

• for any disconnector D in SWG-LIST if there exists a circuit-breaker in SWG-LIST then remove D from SWG-LIST
• for any 400-line-disconnector LD1 in SWG-LIST if there exists a 400-line-disconnector LD2 in SWG-LIST such that ( the branch of LD2 = the symbol br-ö) and LD1 is not the same object as LD2 then remove LD1 from SWG-LIST
• if there exists a sf6-line-disconnector LD1 in SWG-LIST such that ( is-contained-in-text ("_BS" , the text of the names of LD1 ) = true ) and there exists a sf6-line-disconnector LD2 in SWG-LIST such that ( is-contained-in-text ("_BÖS" , the text of the names of LD2 ) = true ) then remove LD2 from SWG-LIST and conclude that SWG-NXT = the names of LD2

HARDWARE-SOFTWARE ENVIRONMENT

• Its capability of handling very different type of data and information processing methods
• Real-time and object oriented features
• Good, easy to use graphical development tools
• Easy to understand and easy to use, English-like programming language
• Support of cyclic development (e.g. rapid prototypes)
• Relatively easy interfacing to different external systems (e.g. SCADA, SQL)
• More than 6000 industrial references all over the world including power plant sites ([5],[6],[7],[8])
• 5 years experience in design and programming of G2 at SzTAKI

• Server: MS WinNT Server to run FIX SCADA, MS SQL, G2
• Operator Station: MS WinNT WS to run the FIX SCADA Display. The G2 messages and displays are included in the operator’s SCADA station.
• Engineer Station: MS Win NT/95/98, Gensym’s Telewindows

SOME SOFTWARE IMPLEMENTATION ISSUES

CONLUSIONS

ACKNOWLEDGEMENT