New Totally-Digitized Control System

IFAC Symposium 1977 Melbourne, 21·25 February 1977

New Totally-Digitized Control System J . TEGAWA , T . HAYASHI Meidensha Electric Manufacturing Co. Ltd ., Tokyo , Japan and

T . KOUDA Tokyo Electric Power Co. , Tokyo, Japan

SUMMARY Achievement of sophistication and modernization in power systems and equipme nt is demanding highDe rform ance contro l . Investigations of various systems indicate that the mo st reasonabl e system is a ; omputer sys t em - - a new total l y- digiti z ed protection and control system . This sys t em picks up al l in f o rmat io n from power systems in digital co de s . Such informatio n is exchanged t hro ugh substations for sys t ematic control of pr otection , adjustment , and oper ation at high speed and accuracy . INTRODUCTION Recent e lec t r ic power systems have been expanded and compl icat ed ext rem e l y . The refo re , it is di fficult to control these systems without utilizi ng a new method other than conventional techniq'.les . In t he pas t one or two years , the development of micro- processors has been rem arkabl e . Mi ni - computer s are also going to be advanced and posi t ively used for power equipme nt . They a re par t ly a dopted for the control of power sys t ems , or us ed as essential equipmen t for the proc ess c ont rols in general industries . They are already rep lacing conventional techniques which have empl oyed much hardware . In this situation we have :-tad a concep t of a new protection an d control sy s t em making use of digi tal processors . Circumsl ances of development of the new system are as i ntroduced be l ow .

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NOTATI ONS Analog Digital Converter CB : Cl rc ui t Breaker DPCD : Digital Potential and Current Device Hi gh- speed Control Section Where i s in charge of operations in t he "m sec " uni t or handling instantaneous sampling values of voltage- current informatio n . Low- speed Cont r ol Section Where is in charge of operations in the " sec " uni t or handoing effec tive value of V. I data . MDP Micro Data Pr Jc essor OSC Sampling Frequency Oscillato r V. I . DATA : Voltage and Current Data for Power Transmission Line

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BASIC CONDITIONS FOR THE NEW TOTALLY-DIGITIZED CONTROL SYSTEM

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For the determination of a new system configuration , we have established the six basic requirements are described below :

When electric power systems a re expanded and complicated, they require mo r e accurate and systematic controls . In order to meet these requirements, we must define and establish the most desirable and essential conditions . Information about system voltages and currents necessary for controls , protection , adjustment, and operation must be processed in the following manner:

(1) High performance must be assured in protective f unctions as well as control functions for the system and facilities (high performance) . (2) Various control functions must be centralized and administered (functional summarization).

(1) Voltage and current information must be accurate th roughout a very wide range . It must be available for any type of contr ol function .

(3) Contr ,)l functions must be highly reliable (high reli abili ty) . (4) Computer systems are easi 1,Y maneuverable and maintenance must be easy (easy operation and maintenance) •

(2) When data are taken from many information sources in a wide area and converted into digital codes , such data sampling should be performed at, the same time . Simultaneous data sampling is very convenient for the whole controls. These data are pr ocessed by the newly developed control system consisting mainl y of mini- computers and micro processors . This is a totally-digitized control system which mainly employs software . In other words the basic concept for the new system is the development of "high-fideli ty transfo r mation system" and the "synchronized sampling system':

(5) When m)dification is attempted for the system , facilities , and control functions , the computer systems must permit easy modification which can be finished in a short time (easy modification) . (6) Overall economical advantages can be expected (economical meri t) .

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CONFIGURATION OF THE NEW CONTROL SYSTEM

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Example of System

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The system configuration for meeting these basic requirements is shown in Figure 1 and Figure 2. It cJntains the two sUbsystems. The power system to be controlled is quite changeable and it is expected that software must be modified frequently. In this respect we have carefully examined various anticipated cases in establishing the system configuration. In any cases the system must not stop for a long time in a wide area. For this purpose "high-speed data bus" and "low-speed data bank" are adopted as the essenti al equipm~nt. The system ensures that there is almost no mutual re l ationship between computers and micro processors. There i s also no relationship between programs of each computer task. 5

SYSTEM CHARACTERISTICS

5.1

High-fidelity Transformer

ation in the sampling frequency depending on both transmission speed and information processing speed. In this system such conditions are taken into considerat ion carefully. As a resul t 600Hz is selected as the optimum sampling frequency. 5.4

Various signals for operation, adjustment, indicat i on, telemtering, recording, setting, etc., are exchanged between high-speed and low-speed control sections. In order to maintain smooth data exchange it i s necessary to install a suitable interface between these sections. This inter face mus t be capable of handling amounts of highspeed data from the high-speed control section and also low-speed data from the low-speed control section. The hi gh-speed data are very different from the low-speed data. The grade of speed conversion and effective combination maintained between the two sections can be an important key to the reliability to be offered by t hi s system. These condi tions also greatly influence economical advantages of the syst em. As shown in Figures 1 and 2, a mini-computer or a micro processor is used in the high-low interface. Data are processed by full us e of software to solve the above-mentioned problems.

It is difficult for conventional transformer to collect a variety of information with high fidelity. To remove such difficulities, sensor output should be converted into digi tal informat i on i mmediately, and information of voltages and currents only should be taken out of it. Since the sensor burden can be reduced extremely in this way, high-fidelity voltage current transformers can be produced. 5. 2

5.5

Automatic Synchroni zat ion

Noi s e Problem

The signal transmissi on speed of thi s system is . very high, amounting to some tens kilo-bits per second. Environmental conditions are not necessarily good for control equipment and transmission paths since they are generally installed close to electric power equipment. Consequently, the problem of radio interference must be solved carefully. In t h i s control sys tem, optical transmissi on system is adopted where i t i s essentially necessary. In this manner intrusion of induction noise into t ransmission paths can be avoided and noise troUble due to ground potential difference can be eliminated. In addition the structure of equipment sJ\eld i s carefully designed and intrusion of noi se from the power source i s perfectly prevented ..

Data are detected in accordance with the data sampling commands. These commands are synchronized throughout the control system. Sampl i ng frequency oscillators installed in respective substati ons are used in this synchronous dri vi ng system. When these osc i llators are driven and synchronized with signals sent from distant SUbstations, there will be a difference in sampling time in each substation. This is due to delay time which i s caused by a difference in transmission distance and hardware configuration. Therefore i t is necessary to measure transmission delay time in each t ransmission system for preliminary correction. However, transmission delay t i me is always changing and the rate of delay correction itself must be compensated continuously and automatically. This control system is based on the assumption ~hat both -way delay time values are identical wi th each other in each transmission system. The result of transmission delay time measurement is always sent to the opposite substation and t i ming for sending out synchronized sampling s ignals is adjusted automatically. 5.3

Combination of CJntrol Sections

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CONCLUSIONS

In Togoshi Substation of the Tokyo Electric Power Co,.:', Inc., field tests have been carried out for s{-inple tri al equipment of the same type has also been operat ed for a large- scale control stati on since July, 1975. Additional fi eld tests will be s t arted in 1977. These t ests are aimed a t prac t ic al admi ni stration of a protective system in 500kV and 275kV power systems. Th vugh the tot.a l system has been introduced in this report, various techni ques included in thi s type of system are independently applicable to existing facilities.

Selection of Sampling Frequency

Sampling frequency must be raised when converting vol tages and currents into digi tal codes wi th high fidelity and realizing prot ective operations at hi gh speeds. On t he other hand, there is a limit-

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