The Dedicated Real-Time Data Transmission Network Between Regional and the National Control Centers of EDF

THE DEDICATED REAL-TIME DATA TRANSMISSION N~TWORK BETWEEN REGIONAL AND THE NATIONAL CONTROL CENTERS OF EDF

G. SIMONNET Electricite de France - Energy Transmission Branch A. DUMONT Societe d'Etude de Systemes d'Automation (SESA) France

Abstract

the dedicated Real-Time Data Transmission Network. between the Regional C~ntrol Centers and the National Control Center of Electricite de France is star-structured packet switching network with fast throughout delay. Two types of informations are exchanged on that network : elementary information on the status of generation and energy transmission system and calculation results concerning economic and security assessments on electric network.

Keywords

Data Transmission Network, control centers.

I -

FOREWORD

The electrical energy consumption in France· during the year 1977 has reached 207 TWH. Most of this energy has 'been produced by "ELECTRICITE DE FRANCE" (EDF) , a national companv. ' 1-1- Energy generation and transmission The EDF generation system includes 400 hydro-electric power plants and about lOO thermal or nuclear units. The energy transmission network (EHV and HV lines) involves about 1500 substations connected through 73 000 km of power transmission lines (above 45 kV), among which 30 000 km correspond to 400 kV or 225 kV. 1-2- Controlling the generation and transmission system The "real-time" control of the energy generation/transmission aims at permanently adjusting power generation to power consumption in order to ensure energy supply within the best conditions of availability and cost. ,The decision-making lev~l of this network control function is located in "control centers". There are I National Control Center and 7 Regional Control Centers. At the present time, when a control center decides that a remote device (i.e. circuitbreaker, isolating switches) must be operated upon, instructions are given, by phone, to people on the sites (substations,stations or power plants). These people, in their turn,

• either operate the equipments manually (thermal units, or hydro-plants, or 136 substations). • either use an existing remote control facility. This is the , case for: - 5 hydro control centers controlling 36 hydro-plants, - 100 "grouped cO,ntrol stations", controlling 350 substations, 39 "distibution stations", controlling 400 substations. • either call for some other people to move to the remote substations or hydro-plants and operate the equipments manually. 1-3- Present evolution of the control system The above organization has been set up during the years 1965 - 1970. It has now been decided to set up a new organization in order to obtain higher execution speed in devices operation. This will be made possible through - remote control of hydro-plants - direct remote control of substations by the new control centerswhich are foreseffi from 1980 to 1985. This facility will also be provided to the National Control Center for the 400 kV network. This new organization 'can be put into operation only if a "real-time" data collection and data gathering system (data transmission system) is itself in operation. Such a data transmission system is presently being put into operatio~

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It permits the control c~nters, with a high security adn availability level, to receive status indications and measurements and to send remote control orders. 1-4- Real-time data transmission system At the present time, there exist two distinct data transmission levels : • The remote control networks, centered around the "grouped control stations" (P.C.G.) . • The data gathering networks of the regional control centers. In 1985, it is foreseen that a unique network will remain. Its structure is described below - The first concentration point is the "grouped control station" (P.C.G.). A computer, which may be duplicated, ensures data collection from the substations (P.A.) and transmission to the contr I centers. In the opposite direction, remote control orders received from the control center are sent to the relevant substation. In addition, the system performs some ldcal logging and edition functions. The data transmission rates between P.C.G. and P.A. are 200 b/s most of the time. - The second level is the Regional Control Center. This center is equipped with

- directl y from the main process computers of local control center, - thr()u~h TTR ne t lvOrk , "nm other control center and transmit them to the relevant P.C.G. stations. Also, the C.A.C.Q. computers permit the main process-computers of distinct regions to exchange data, through the T.T.R. network. The transmission lines with P.C.G. stations are duplicated, with speeds ranging from 600 to 1200 b/s. Transmission with the process computers is "short distance", at a rate of 48000b/s. The transmission lines with the data transmission network (T.T.R. network) are duplicated, at a rate of 9600 b/s. - The third level is the National Control Center. This center is connected to the 7 Regional Control Centers, through the T.T.R. data transmission network. It receives status indications and measurements. It transmits remote control orders and various data to the regions. In addition, the packet switching network is provided with extra addressees which will facilitate future connection to foreign european control centers. 1-5- Implemeptation schedule The development of P.C.G. computer systems started in 1973 and is considered as over. About 50 grouped stations are equipped with computer systems.

main process-computers, front-end computers (C.A.C.Q.) The front-end computers (C.A.C.Q.) are in charge of data transmission and data 'handling function at the region level. They are connected to : the grouped control stations (P.C.G.), the main process computers, a computer devoted to "mimic-board" handling, a real-time data tran~mission network (TTR), connecting the Regional Control Centers to eachother and to the National Control Center. The C.A.C.Q. computers receive informations from "P.C.G." systems (Max.24) and gather it to : - the main process computers, - the mimic-board, - the other regions or the National Control Center. In the opposite direction, they receive ' ''remote control" orders:

Development of the front-end computers systems (C.A.C.Q.) started in 1974. These systems are partially installed. They are foreseen to be put in full operation during the first half on 1979. The developments leading to the replacement of the National Control Center computers ' started in 1974. The new system is due to be operational by the end of 1979. Replacement of the process computers in the 7 Regional Control Centers is under way. The first "new" system will be put into service by the end of 1981. The 6 'remaining systems will follow within a delay of 3 years. 11 - PACKET TRANSMISSION NETWORK CHARACTERISTICS (T.T.R.) 11-1- .Network structure The T.T.R. network is a star network using the "packet-switching" technique. Ref. Fig.1. A.central node is connected to the totality of the network addressees.These are:

138

thE' 7 front-end c('mrut (>rs in the regions

11-5- Priorities

(C.A.C.Q.) ,

the National Contro l Cente r, the computer system vf the "Laufenbvurg Control Center" in Switzerland, t he other po ss iblE' addrE'ss ees (max 16).

Distinct levels of priority exist only at the time of packet input into the network. In the network itself, the unique rule, w;,atever the packet, is "first in/first out".

11-2- Meaning of the transmitt ed data

11-6- Transmission delay

One ,'3 n makE' a distinction between two t vp(>s of informati ons :

This time delay is defined as the time between packet input (by a control center) and packet output to its addressee.

on one hand, informations and orders, r e lat ed to th e e ne rgy generation/transmissi,)n contr ,, 1 functi(lO, ,' n rill' oth. ' r hand. i nfo rmat~on s exc lusiv ... ly transmirted be twee n the main process "" mp'ut en "f distinct co ntr,)l centers. In the first type, one finds: - meas urements on the network and remote " dj us l m", n t s, t r a nsmi t ted cyc Hcally ( I 0 s ec .).

For measurements and status indications this delay does not exceed I second, in normal conditions, it should not exceed' 10 s econds. In normal condi dons, the "global transmis sion" of status should not exceed 10 sec. For "high level data" (second type) delay may reach I minute. 11-7-

- info rmations transmitted at random, such as : status indications on various devices in substations (i.e . circuit breakers. alarm signals.etc ... )

Dis ~ urbed

the

condition

If case of lo s s of a packet on the network. the latter ptrforms several attempts (repeat. 11 · , swi tchi ng) . In lhese attempts do -not succeed. this is notified to the packet emitting station.

remote control orders, various countings •... In the second type. one finds - energy accountings. - equivalent networks data, - or the other results of calculations concerning electrical ne twork s security or economic. 11-)- Data transmission volume Ref. Fig. 2 The main flow corresponds to the informations transmitted by the regions to the National Control Center. i.e. :

In case of highly disturbed conditions, or too high traffic, the control centers may provisionnaly eliminate measurements transmission. The loss of any status indication data is d~tected and automatically triggers a "global status collection" procedure. In case of line switching 'between the network and its addressee. no information should be lost. Such a switching takes place automatically after repeated transmission failures on the line. All the network elements are duplicated Nevertheless, it may happen that connection between a center and the network falls do~~. Such an event is detected by the network, as well as by the center concerned, and by its 3ddressees. within 10 seconds.

2500 measurements (TM) 6500 status indications (TS) The regions also exchange data between themselves (about 200 TM and 500 TS), 11-4- Traffic density The average traffic is coming from the telemeasurements. transmitted every 10 seconds. The highest figures are reached when. in addition to measurements. status indications have to be transmitted, either beacause of device status changes on the energy network ( IS/sec during 20 sec) or, more often. through the use of the "global status collection" procedure.

If this lasts a short time, the waiting messages. measurements excepted. will be kept by the centers and emetted when the link is set up again. Measurements in between are lost. If the failure lasts longer and the storage capacity of the station is exceeded. status indications may be lost and a "global collection" procedure will be automatically set up for recovery). III - PACKET SWITCHING NETWORK DESCRIPTION (T.T.R.) 111-1- Equipments The central node of the network is based upon a duplicated minicomputer.

13S

The computer type is SOLAR 16.40 manufactured by the French Company S.E.M.S. provided with 32 K bits words of semi-conductor memory, At a given time, only one computer is connected to all the data transmission lines and performs its swi tching function. The other one is "waiting", ready to take the service over in case of failure of the other computer. The choice between the two computers,as well as the physical control 0f the data transmissi.on lines connections, are handled by a fully independant "watch-dog system", itself built around a microprocessor system.

The receiving station acknowledges the frame (L.A.P.) and inputs into the network a new packet which is called "global acknowledge". This packet follows the same path in the opposite direction (end to end protocole). A packet has a unique addressee. There is no priority rule inside the network. The only rule is "first in/first out". There is no packet-chaining ~acility. Each packet is considered as an isolated item. Two distinct procedure levels are involved in operational data transmission :

The links between the node and the surrounding "stations" (i.e. control centers) all consist in a set of two independant data transmission lines. The lines are "rented" 4 wires.

- between the stations and the node,frames are transmitted according to the line access procedure (L.A.P.) of X 25 C.C.I.T.T. recommendation

Transmission speeds are 50 Kbi ts i;sec for the lines (short distance)with the Na~ional Control Center, 9,6 Kbits/sec wit~ the Regional Control Centers, and 600 bits/sec with Laufenburg Center (Switzerr"a~).

- between two stations, an end to end protocole is implemented, at packet level. This protocole is similar, to some extent to the "p 25" packet level procedure of X 25 (for permanent virtual circuits).

\

Transmission mode is synchronous, full duplex with HDLC frame .(1). Signals at modem functions are compatible with V 24 and V 35 recommendations. At a given time, the two transmission lines of a link are physically handling by the same computer but only one transmits "operationai data" . .The second one transmits periodically ( 10 sec) dummy informations for test purposes. In case of disturbance on the "operational" line or in case of request by the operator, automatic line switching takes place (no physical switching) and the operational traffic is directed toward the spare line, under controi of the network node.

111-3- Message structure Fig.3 summarizes the structure of the transmitted operational messages. The various "blocks" which contain the relevant data are transmitted by the network in full transparency. IV - SERVICE QUALITY Transmitted data integrity and network access availability are the two main components of service quality. - Transmitted data integrity requires that no data loss can occur without notice and that erroneous data are always eliminated.

111-2- Transmission principles The network operates upon packets of variab le length (max 1000 bi ts). Thos.e packets constitute the "information field" of HDLC frames (ref. ISO!TC 97/SC 6) .. ·TI,e line procedure itself is the "line access procedure" (L.A.P.) of X 25. When the node receives a "correct" frame, it acknowledges it (according to L.A.P.), extracts the packet, finds out its addres~e and builds up ·a new frame which will be transmitted to this addressee, with the packet as information field.

(I) HDLC

High Level Data Link Control

protection against data loss is achieved through the use of the two procedure levels elimination of erroneous data is achieved mainly by : cyclic code redundanc! checks on the lines ( X l6 + Xl2 + X + I) memory parity checks inside the node computers. Network access availability is based upon the duplication of all network elements, maximum independance between the network elements, and automatic switching at several levels. High availability also resumes a fast repair action by maintenance personnel.Maintenance task is handled by LD.F. staff.

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v - CONCLUSION The T.T.R. packet switching network is a specific network which has been implemented for the needs of the power production/transmi ssi on network control function. Its "star" structure comes from the shape of the data flow itself and has been preferred t o mo re complex multipath networks.

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