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Combined Control of Substation Voltage and Reactive Power Compensation by Microcomputer
Copyright
©
IFA C "",,"er System, alld
Po\,"er Plant COlltrol. Heijing. IQS6
COMBINED CONTROL OF SUBSTATION VOLTAGE AND REACTIVE POWER COMPENSATION BY MICROCOMPUTER Huang Yi-zhuang and Huang Shen-yi Departlllellt of Elatrical ElIgillfl'l"illg, Tsillghua UllilWrsit)', Beijillg, PRC
Abst r act . A regulative rule and its characteristics for decentralized control of substation voltage and reactive power compensation by microcomputer a re introduced. The hardware structure and software design are illustrated as well. The prototype of this system has been operating in Dajiaoting Subs t a tion in Beijing since September 1985. Two trans formers with load tap changing (LTC) and two compensa tion capacitor banks in that station are under control. The operating result so far showed that the combined control rul e of vol t age and r eac tive power compensa tion according to the load compensation method by microcomputer is correct and reasonable. It ca n follow the variation of load to a utomatically determine the reaso nable volta ge that has to be maintained. The accuracy o f voltag e regula tion ca n a chieve 0.5% and that of reactive compensation is 5% . I t s application can not only improve the voltage quality of power system, but al so decrease loss of electric energy. 'rhe economic benefit is obvious . Keywo rds. Microcomputer; power substation control; voltage regulation; reactive power compensation; on-off control. 1. INTRODUCTION two reactive power compensation capacitor banks. These two capacitor banks contain capacity of about 7000 kvars each, and are connected to 10kV bus separately. the r a ted voltage of the transformers is 110± 3x2 .5/38.5±5/11 kV . The c on trol system has been working in closed loop since Sep tember 1985 , and the voltage regula tion and rea ctive compensa tion is und er its combined control . This paper will be organized as follows: Se ction 2 introduces the problems of the existing scheme of substation voltage and reactive power compensation. Section 3 gives the investigated regula tive rule of the combined volt/VAR control system by microcomputer . The hardware structure and its block diagram, and the software design of the control system are described in section 4 and 5 respectively. Finally, section 6 gives a brief conclusion.
Automatic r egula tion for voltage and reactive power of power system can be divided into two types, decentralized a nd centralized regula tion. The term "decentralized regula tion" used here means that by a utoma tic a lly moving the t aps of transformer with load tap changing and controlling rea ctive power compensation installations or other VOltage r egula tion facilities a cert a in power pl an t or substation is taken as a centre to maintain the voltage of this region and rea ctive power flow within a desired r ange when load va ries. Thus, both technical a nd economic performance of system operation can be guaranteed. Centraliz ed regulati on is a regula tion type us ed by some large power systems. The voltages and rea ctlve power fLOWS in each dlspatcn cen~r e of ~he networK a re c ontrolled by a computer to guar an tee ltS vOLtage va rla tion within the desi re d r ange and miniffilze the t ot a l energy Losses of the system . Hut this manner o f regulation needs to do remote measurement and remote control of the VOltage , rea ct ive power, rea l power and the state of operation in each dispa tch centre. 00, it is very complicated in technology and very costly. Therefore, in developing countries it is reasonable and realistic to develop decentralized regulation by using microcomputer. Here such a kind of Volt/VAR control system was developed in order to make use of the abilities of computa tion, logical judgment a nd memory of microcomputer to improve the regulation of substation voltage and reactive power compensation. It was developed in December 1984. After one month dynamic physical simulation, the prototype was installed in Dajiaoting Substation in Beijing. In that substation there are two transformers with LTC and
2. THE EXISTING SCHEME Nowadays more and more substations of 35kV and over are installed with transformer LTCs and capacitor banks for reactive power compensation. Most switching operations for these apparatus are manual. Because the voltage and reactive power of system vary frequently, the workers have to work hard. However , it is still not easy to operate in time and can not ensure the system to work economically and under a good condition. A few kinds of transistor logical circuits were developed to do automatic regulation. This improved the operation a lot, but the regulative rule was limited. It could not satisfy the demand for the principle of I~egulation with a positive slope", i.e. could not increase bus voltage as load increases and voltage drop of tie line increases. The rule of
PSPPC-O *
95
96
Huang i'i- zhuang and Huang Shcll-,i
regulation is me rely load cycle va riation dispatcher. Although way , but th ere exist follows:
based on the daily curve gi ven by the it is a rea listic some disadvantages as
(3)
( 1 ) The peak time and valley time of daily load cycle va riation were got fro m statistics and expe rience s . I t c ould not agree with the everyday a ctua l load va riation, see Fig . 1. (2) Generally speaking , the actual load va ries gradually, but the daily load curve assigned by dispatcher gives a step changing between peak and valley. So the conventional regulation could only keep the voltage a t two limited positions, upper or lower. It could not make full use of their regul a tion capa cities to improve the quality of voltage a t the receiving end .
(3) " Peak" a nd "va lley" a re rela tive to a daily cycle. Comparing the a ctua l loa d curves from different days , it could be found that for the same substation the load during one day ' s peak time may be lower tha n tha t of va lley time in another day , as shown in Fig . 1. If it is still regulated a ccording to the assigned time scheduling, then obviously the principle of regulation with a positive slope will be violated. For solving the above mentioned problems , we developed a combined control system of substation voltag e a nd rea ctive power by microcomputer . This system continuously measures a nd computes the bus voltage, rea l power a nd rea ctive power of the substa tion. Then a ccording to the principle of voltage regula tion with positive slope and optimization of rea ctive power compensation, the most rea sonable voltage level which should be maintained will automatically be selected , and combined control of the t ap cha nging and capacitor switching will be used to satisfy this aim to guara ntee the qua lity of voltage and economical opera tion.
3 . THE CONTROL RULE The principle of this microcomputer control system of transformer LTCs and capacitor bank switching for reactive power compensation can be illustrated by the simple network in Fig. 2. The basic expressions used are UB -LlUL PLRL
+ QLXL UL
compe nsated by the change of UB .
(1 )
where ULN is the demanded voltage level at user end , which should be kept constant , UB is the bus voltage on the low-tension side of this substation , RL a nd XL are the resistance a nd reactance of the tie line between substation and loa d respectively . Ass ume the natural power f a ctor of load keeps constant or varies in a sma II r ange , i.e . c os ~ ~ const. Substitute PL =SL· cos ~, QL =SL' sin ':P into (3) .
(4) or rewrite it in the fo l lowing form,
(5) where a a nd b are approxima tely constants, and SL is the appa rent power o f load. Thus for keeping voltage ULN constant, U shB ould vary linearly according to the apparent power changing of the loa d. In other words, the regulation with positive slope must be implemented according to the actua l magnitude of apparent power , and then the quality of user end voltag e can be gua ranteed. Of course, the actual system is usua lly more complica ted than that in Fig. 2. There a re a number of feeders in a substation. This means tha t maybe the load increases in one feeder, but decreases in others . Under this situation regulation can only be implemented according to the total load changing of the whole substation. The wa y of voltage regula ting can be considered from the following aspects: (1) From (6)
where Kn is the r a tio of substation tra nsformer with LTC , a nd UA is the bus voltage on the high tension side of this substation, the t ap position is to be cha nged, i. e. Kn is changed to minimize (UL - ULN) . (2) Change reactive power compensation. From Fig . 2 the energy loss on high-tension network (including the loss of transformer) is:
i.e.
(2) The voltage drop AUL from substation to the user end will vary as the load CPL ' QL) changes. For keeping the voltage UL at the user end constant, UB must be regulated, and the variation of UL will be
(7)
Control of Substation Voltage
After switching on reactive compensation kvars Qc :
where Us , Rs and Xs a re the parameters from the network, Pn and QB are the real a nd rea ctive power on the low-tension side of the transformer and RT and XT are the resistance and rea ctance of the transformer respectively. From (7) , it ca n be seen that under the condition of satisfying the voltage quality the reactive power compensation kvars Qc will be switched on and off in accordance with the magnitude of reactive power QB to minimize the kvars exchange (QB - QC) between substation and network. Thus the loss Ss on high-tension network can be minimized . From (6) , (7) and (8) , it can be seen that the variation of voltage caused by load changing can be regulated by controlling the tap position of transformer LTC and reactive compensation kvars comprehensively. This control system can not only guarantee the voltage quality but also minimize line losses to achieve optimal control under the existing high-t ension facilities, According to the above mentioned control rule, different situations possibly occured under various operations of substation were classified, and fourteen control strategies were designed and stored in EPROM of the computer. When the microcomputer control system is operating in closed loop form, the voltage and load of substation will be measured once every two seconds . Then according to the present operating mode the most reasonable voltage that should be kept will be determined automatically . If the bus voltage deviated from the optimal value out of an allowable tolerance or the measured reactive power was not under optimal condition, the control system should search a suitable control strategy according to the present operating mode. After a certain time delay, the regulating command will be given to switch on/off capacitor banks or move up/down the taps of LTC.
4. HARDWARE STRUCTURE The block dia gram of hardware structure of the system is shown in Fig . 3 . The functions of used circuits are simply described as follows: (1) Input circuit. Voltage and current are taken from voltage transformers and current transformers of the system and transformed into dc voltages by transducers for measurement. (2) Filter circuit. Signals from the transducers contain ac component up to tens of mV. In order to minimize the influence of ac component and other interferences to the precision of measurement, filtering is used to decrease the effective value of a c component down to about 0.1 mV.
9i
(3) Signal processin~. For lowerlng the cos of control system, an 8-bit A/D converter is used. To improve the precision of mea surement, the input voltage and current signals from converter are pretreated, including level-shift and appropriate amplific a tion etc . The va ria tion range of prima ry a na logue qua ntities, which corresponds to the lower a nd upper limi t G o f A/D conversion, is selected reasonably to improve the resolution of signals . A twofold increase in voltage resolution and more than three-fold increase in reactive power resolution a re obtained , so that the precision of voltage regula tion is ± 0. 5% and that of reactive power is ±
5%.
(4 ) Limiter circuit. The limiter circuit is designed for protecting the electronic components in AID converter, multiplex switch etc. And the possibly occuring overvolta ge can be limited within the upper and lower limits of AID converter.
(5) A/D converter. The main functi on of thi s system is to improve the stea dy state performa nce of a power system. So it should avoid to respond to any kinds of transient changes, a nd high noise immunity should be required. Therefore, integra ting type ADC-ET8BC of A/D converter is selected. I t can satisfy the requirment of high immunity. (6) output circuit. The maln functlon of output circuit is to transfer the control commands from the computer to executors through a few decoders and drivers, and make the executors work. (7) Clock circuit. The clock circult gives out clock pulses as interrupt request signals. At the same time, it can be used for multifunctional control such as street light control etc. (8) Dis~lat circuit State dlSP ay unit and normal display unit are included. Each Unit consists of four nixie tubes. The tap position of transformers, on-off state of capa citor banks, and the bus voltage of substation can be displayed normally . The real power, reactive power and reactive power loss of transformers can be displayed whenever necessary.
(9) Microcomputer. A mlcro-c omputer is used as the core of the control system. The bus voltage, current and reactive power of substation are measured routinely by the microcomputer. According to the requirement of voltage regulation with positive slope and the principle of optimal reactive power compensation the most reasonable voltage v~l ue required to be maintained is automatlcally selected. "Up and down" of transformer taps and "on-off" of capacitor banks are controlled in combination.
5. SOFTWARE DESIGN The software of microcomputer control system consists of three parts: user's application program, self-checking program , and system supervisory program. For speedlng
98
Huang Yi-zhuang and Huang Shen-yi
operation and saving memory, machine language is used for all programs. (1) User's application program. The block d~agram of user's application program is shown in Fig. 4. Its functions are: (a) to judge the operation mode of substation and turn to appropriate program entry point. (b) to take samples of required input signals (U, I and Q etc.) every two seconds. (c) to compute with sufficiently high precision the reactive power loss of transformers and real time apparent power from AID sampling input. Four bytes data are used to improve the precision of computation. (d) to determine the most reasonable bus voltage of substation, which has to be maintained, on the basis of loads both measured and computed. (e) with high logical ability of judgment to make logic judging about the position of taps, the state and on-off order of capacitor banks; and to give commands after comparing and automatically selecting appropriate strategies. (f) to discriminate interfering signals. (g) to accomplish all the control functions conveniently coordinated by the system supervisory program, and make the system operate easily. (2) Self-checking program. The funct~on of self-checking program is to keep the microcomputer in self-locking state, i.e. in a state that only inputs are sampled, computed and displayed, without any command output, for checking the work of input facilities and those subprograms of sampling, computation and display selection. (3) Control system supervisory program. The task of superv~sory program includes keyboard supervision, command processing~ normal display and state display, and supplying different time intervals required for user's application programs. In brief, the supervisory program serves the application programs and provides conveniences for the operator. Because continuously running of system application is required for monitoring the states of power system. On the other hand, the supervisory program has to do keyboard scanning and normal display. Therefore, these two programs are coordinated with time sharing mode shown in Fig. 5. 6. CONCLUSION From the operation results of the prototype, the following conclusions can be deduced: (1) The design principle of this combined control system is proved out. The user is satisfied with its control functions. (2) The system is able to determine automatically the reasonable voltage value which should be maintained according to the change of load. (3) The adaptability of the microcomputer control system is high. No matter what
kind of operation mode is adopted or how large the load changes, the control program is able to fit it automatically according to load compensation method. (4) The system is reliable with anti-interference capacity against strong electromagnetic disturbances and is capable to aviod to respond to any kinds of transients of the power system.
(5) Results of operation showed that the application of this system not only reduces labor of person on duty and errors in operation, but also improves the voltage quality of electric power supply and decreases the network loss as well. And the economic benifit can be obtained significantly. ACKNOWLEDGEMENTS The authors are grateful to the Bei jing power Supply Bureau, Mr. Wang Yan-zhong, and others who have given helpful suggestions and comments, and have provided Dajiaoting substation as the trial operation site of the prototype of this system.
REFERENCES Bunch, J.B., R.D. Miller, and J.E.lVheeler, (1982). Distribution system integrated voltage and reactive power control. IEEE Trans. Power Apparatus and Systems 101 284-289. Gron~R~1981). Computer-dedicated voltage regulation method for distribution sUbstations. IEEE Trans. Power Apparatus and Systems, 100. 2184-2188. Happ, H.H.] R.A. Fernandes, and K.A. Wirgan, (1980). Optimal reactive power flow for improved system operations, Energy Systems, 2 133-139. Sugiyama, T., S.Kame~a, K.Maeda, A. Kaneda, and T. Goda, (1982). Development and field experience of digital protection and control equipment in power systems, IEEE Trans. power Apparatus and Systems, 101, 4237-4243. Tanaka, K., K. Kanou, Y. Harumoto, T.Mori, K. Suzuki, and T. Gouda, (1980). Application of microprocessors to the control and protection system at substation. IEEE Trans. power Apparatus and Systems, 99,344-35 1 •
99
Control of Substation Voltage
I
Input circu i t
I
Filter
I
Treatment of si gna l
I
Multichannel switch
I
AID converter
1
r---
Microcomputer
--'
1
Selection of operation mode
H
t
RS ,XSI A
I
Clock
't
1 Selection of positive reactive power working value ~
Selection of voltage working range
Cl
J
~
I
U'
J
~
Display Fig. 1, The load curve of transformer No.2 in a certain substation. SI' "." indicates the load curve of 15th, March, 1984. S2: ·x· indicates the load curve of 15th, May, 1984. S3: Equivalent load curve of peak and vallay time assigned by dispatcher.
J J J
Se lection of negat i ve reactive power working value
I
Out put of deco de
l
Buffer ampli fier
I
Output of drive
J J J
BIt
IL ___________ J
Fig. 3. Block diagram of microcomputer control system. Fig. 2. Simplified schematic 1iagram of a substation.
Huang Yi-zhuang and Huang Shen-yi
100
In itie ti on:set user stack pointer, set '~ TC c locking interrupt, clear all internal memory ~its.
Capacity state subprogram
~o rresponding
Run the supervisory program ~--------~ Return
:)ampling. AID conversion subprogram
Sil~nal
of delay time sampling
Discrimination of running mode
Computing subprogram
Sub program for the determination of upper and lo~er limits of voltage and reactive power
Lo~ical
'~ommend
decision subprogram
d iscrimination subprogra'1l
Expcutive command subprogrem
No
Senrt ex ecutive command
L-____________________
~yes
Fi ,'3 . 4 . Block d iagram of user appli cation program
Fig . 5 . Flow chart of combination between supervisory program and application program.
©
IFA C "",,"er System, alld
Po\,"er Plant COlltrol. Heijing. IQS6
COMBINED CONTROL OF SUBSTATION VOLTAGE AND REACTIVE POWER COMPENSATION BY MICROCOMPUTER Huang Yi-zhuang and Huang Shen-yi Departlllellt of Elatrical ElIgillfl'l"illg, Tsillghua UllilWrsit)', Beijillg, PRC
Abst r act . A regulative rule and its characteristics for decentralized control of substation voltage and reactive power compensation by microcomputer a re introduced. The hardware structure and software design are illustrated as well. The prototype of this system has been operating in Dajiaoting Subs t a tion in Beijing since September 1985. Two trans formers with load tap changing (LTC) and two compensa tion capacitor banks in that station are under control. The operating result so far showed that the combined control rul e of vol t age and r eac tive power compensa tion according to the load compensation method by microcomputer is correct and reasonable. It ca n follow the variation of load to a utomatically determine the reaso nable volta ge that has to be maintained. The accuracy o f voltag e regula tion ca n a chieve 0.5% and that of reactive compensation is 5% . I t s application can not only improve the voltage quality of power system, but al so decrease loss of electric energy. 'rhe economic benefit is obvious . Keywo rds. Microcomputer; power substation control; voltage regulation; reactive power compensation; on-off control. 1. INTRODUCTION two reactive power compensation capacitor banks. These two capacitor banks contain capacity of about 7000 kvars each, and are connected to 10kV bus separately. the r a ted voltage of the transformers is 110± 3x2 .5/38.5±5/11 kV . The c on trol system has been working in closed loop since Sep tember 1985 , and the voltage regula tion and rea ctive compensa tion is und er its combined control . This paper will be organized as follows: Se ction 2 introduces the problems of the existing scheme of substation voltage and reactive power compensation. Section 3 gives the investigated regula tive rule of the combined volt/VAR control system by microcomputer . The hardware structure and its block diagram, and the software design of the control system are described in section 4 and 5 respectively. Finally, section 6 gives a brief conclusion.
Automatic r egula tion for voltage and reactive power of power system can be divided into two types, decentralized a nd centralized regula tion. The term "decentralized regula tion" used here means that by a utoma tic a lly moving the t aps of transformer with load tap changing and controlling rea ctive power compensation installations or other VOltage r egula tion facilities a cert a in power pl an t or substation is taken as a centre to maintain the voltage of this region and rea ctive power flow within a desired r ange when load va ries. Thus, both technical a nd economic performance of system operation can be guaranteed. Centraliz ed regulati on is a regula tion type us ed by some large power systems. The voltages and rea ctlve power fLOWS in each dlspatcn cen~r e of ~he networK a re c ontrolled by a computer to guar an tee ltS vOLtage va rla tion within the desi re d r ange and miniffilze the t ot a l energy Losses of the system . Hut this manner o f regulation needs to do remote measurement and remote control of the VOltage , rea ct ive power, rea l power and the state of operation in each dispa tch centre. 00, it is very complicated in technology and very costly. Therefore, in developing countries it is reasonable and realistic to develop decentralized regulation by using microcomputer. Here such a kind of Volt/VAR control system was developed in order to make use of the abilities of computa tion, logical judgment a nd memory of microcomputer to improve the regulation of substation voltage and reactive power compensation. It was developed in December 1984. After one month dynamic physical simulation, the prototype was installed in Dajiaoting Substation in Beijing. In that substation there are two transformers with LTC and
2. THE EXISTING SCHEME Nowadays more and more substations of 35kV and over are installed with transformer LTCs and capacitor banks for reactive power compensation. Most switching operations for these apparatus are manual. Because the voltage and reactive power of system vary frequently, the workers have to work hard. However , it is still not easy to operate in time and can not ensure the system to work economically and under a good condition. A few kinds of transistor logical circuits were developed to do automatic regulation. This improved the operation a lot, but the regulative rule was limited. It could not satisfy the demand for the principle of I~egulation with a positive slope", i.e. could not increase bus voltage as load increases and voltage drop of tie line increases. The rule of
PSPPC-O *
95
96
Huang i'i- zhuang and Huang Shcll-,i
regulation is me rely load cycle va riation dispatcher. Although way , but th ere exist follows:
based on the daily curve gi ven by the it is a rea listic some disadvantages as
(3)
( 1 ) The peak time and valley time of daily load cycle va riation were got fro m statistics and expe rience s . I t c ould not agree with the everyday a ctua l load va riation, see Fig . 1. (2) Generally speaking , the actual load va ries gradually, but the daily load curve assigned by dispatcher gives a step changing between peak and valley. So the conventional regulation could only keep the voltage a t two limited positions, upper or lower. It could not make full use of their regul a tion capa cities to improve the quality of voltage a t the receiving end .
(3) " Peak" a nd "va lley" a re rela tive to a daily cycle. Comparing the a ctua l loa d curves from different days , it could be found that for the same substation the load during one day ' s peak time may be lower tha n tha t of va lley time in another day , as shown in Fig . 1. If it is still regulated a ccording to the assigned time scheduling, then obviously the principle of regulation with a positive slope will be violated. For solving the above mentioned problems , we developed a combined control system of substation voltag e a nd rea ctive power by microcomputer . This system continuously measures a nd computes the bus voltage, rea l power a nd rea ctive power of the substa tion. Then a ccording to the principle of voltage regula tion with positive slope and optimization of rea ctive power compensation, the most rea sonable voltage level which should be maintained will automatically be selected , and combined control of the t ap cha nging and capacitor switching will be used to satisfy this aim to guara ntee the qua lity of voltage and economical opera tion.
3 . THE CONTROL RULE The principle of this microcomputer control system of transformer LTCs and capacitor bank switching for reactive power compensation can be illustrated by the simple network in Fig. 2. The basic expressions used are UB -LlUL PLRL
+ QLXL UL
compe nsated by the change of UB .
(1 )
where ULN is the demanded voltage level at user end , which should be kept constant , UB is the bus voltage on the low-tension side of this substation , RL a nd XL are the resistance a nd reactance of the tie line between substation and loa d respectively . Ass ume the natural power f a ctor of load keeps constant or varies in a sma II r ange , i.e . c os ~ ~ const. Substitute PL =SL· cos ~, QL =SL' sin ':P into (3) .
(4) or rewrite it in the fo l lowing form,
(5) where a a nd b are approxima tely constants, and SL is the appa rent power o f load. Thus for keeping voltage ULN constant, U shB ould vary linearly according to the apparent power changing of the loa d. In other words, the regulation with positive slope must be implemented according to the actua l magnitude of apparent power , and then the quality of user end voltag e can be gua ranteed. Of course, the actual system is usua lly more complica ted than that in Fig. 2. There a re a number of feeders in a substation. This means tha t maybe the load increases in one feeder, but decreases in others . Under this situation regulation can only be implemented according to the total load changing of the whole substation. The wa y of voltage regula ting can be considered from the following aspects: (1) From (6)
where Kn is the r a tio of substation tra nsformer with LTC , a nd UA is the bus voltage on the high tension side of this substation, the t ap position is to be cha nged, i. e. Kn is changed to minimize (UL - ULN) . (2) Change reactive power compensation. From Fig . 2 the energy loss on high-tension network (including the loss of transformer) is:
i.e.
(2) The voltage drop AUL from substation to the user end will vary as the load CPL ' QL) changes. For keeping the voltage UL at the user end constant, UB must be regulated, and the variation of UL will be
(7)
Control of Substation Voltage
After switching on reactive compensation kvars Qc :
where Us , Rs and Xs a re the parameters from the network, Pn and QB are the real a nd rea ctive power on the low-tension side of the transformer and RT and XT are the resistance and rea ctance of the transformer respectively. From (7) , it ca n be seen that under the condition of satisfying the voltage quality the reactive power compensation kvars Qc will be switched on and off in accordance with the magnitude of reactive power QB to minimize the kvars exchange (QB - QC) between substation and network. Thus the loss Ss on high-tension network can be minimized . From (6) , (7) and (8) , it can be seen that the variation of voltage caused by load changing can be regulated by controlling the tap position of transformer LTC and reactive compensation kvars comprehensively. This control system can not only guarantee the voltage quality but also minimize line losses to achieve optimal control under the existing high-t ension facilities, According to the above mentioned control rule, different situations possibly occured under various operations of substation were classified, and fourteen control strategies were designed and stored in EPROM of the computer. When the microcomputer control system is operating in closed loop form, the voltage and load of substation will be measured once every two seconds . Then according to the present operating mode the most reasonable voltage that should be kept will be determined automatically . If the bus voltage deviated from the optimal value out of an allowable tolerance or the measured reactive power was not under optimal condition, the control system should search a suitable control strategy according to the present operating mode. After a certain time delay, the regulating command will be given to switch on/off capacitor banks or move up/down the taps of LTC.
4. HARDWARE STRUCTURE The block dia gram of hardware structure of the system is shown in Fig . 3 . The functions of used circuits are simply described as follows: (1) Input circuit. Voltage and current are taken from voltage transformers and current transformers of the system and transformed into dc voltages by transducers for measurement. (2) Filter circuit. Signals from the transducers contain ac component up to tens of mV. In order to minimize the influence of ac component and other interferences to the precision of measurement, filtering is used to decrease the effective value of a c component down to about 0.1 mV.
9i
(3) Signal processin~. For lowerlng the cos of control system, an 8-bit A/D converter is used. To improve the precision of mea surement, the input voltage and current signals from converter are pretreated, including level-shift and appropriate amplific a tion etc . The va ria tion range of prima ry a na logue qua ntities, which corresponds to the lower a nd upper limi t G o f A/D conversion, is selected reasonably to improve the resolution of signals . A twofold increase in voltage resolution and more than three-fold increase in reactive power resolution a re obtained , so that the precision of voltage regula tion is ± 0. 5% and that of reactive power is ±
5%.
(4 ) Limiter circuit. The limiter circuit is designed for protecting the electronic components in AID converter, multiplex switch etc. And the possibly occuring overvolta ge can be limited within the upper and lower limits of AID converter.
(5) A/D converter. The main functi on of thi s system is to improve the stea dy state performa nce of a power system. So it should avoid to respond to any kinds of transient changes, a nd high noise immunity should be required. Therefore, integra ting type ADC-ET8BC of A/D converter is selected. I t can satisfy the requirment of high immunity. (6) output circuit. The maln functlon of output circuit is to transfer the control commands from the computer to executors through a few decoders and drivers, and make the executors work. (7) Clock circuit. The clock circult gives out clock pulses as interrupt request signals. At the same time, it can be used for multifunctional control such as street light control etc. (8) Dis~lat circuit State dlSP ay unit and normal display unit are included. Each Unit consists of four nixie tubes. The tap position of transformers, on-off state of capa citor banks, and the bus voltage of substation can be displayed normally . The real power, reactive power and reactive power loss of transformers can be displayed whenever necessary.
(9) Microcomputer. A mlcro-c omputer is used as the core of the control system. The bus voltage, current and reactive power of substation are measured routinely by the microcomputer. According to the requirement of voltage regulation with positive slope and the principle of optimal reactive power compensation the most reasonable voltage v~l ue required to be maintained is automatlcally selected. "Up and down" of transformer taps and "on-off" of capacitor banks are controlled in combination.
5. SOFTWARE DESIGN The software of microcomputer control system consists of three parts: user's application program, self-checking program , and system supervisory program. For speedlng
98
Huang Yi-zhuang and Huang Shen-yi
operation and saving memory, machine language is used for all programs. (1) User's application program. The block d~agram of user's application program is shown in Fig. 4. Its functions are: (a) to judge the operation mode of substation and turn to appropriate program entry point. (b) to take samples of required input signals (U, I and Q etc.) every two seconds. (c) to compute with sufficiently high precision the reactive power loss of transformers and real time apparent power from AID sampling input. Four bytes data are used to improve the precision of computation. (d) to determine the most reasonable bus voltage of substation, which has to be maintained, on the basis of loads both measured and computed. (e) with high logical ability of judgment to make logic judging about the position of taps, the state and on-off order of capacitor banks; and to give commands after comparing and automatically selecting appropriate strategies. (f) to discriminate interfering signals. (g) to accomplish all the control functions conveniently coordinated by the system supervisory program, and make the system operate easily. (2) Self-checking program. The funct~on of self-checking program is to keep the microcomputer in self-locking state, i.e. in a state that only inputs are sampled, computed and displayed, without any command output, for checking the work of input facilities and those subprograms of sampling, computation and display selection. (3) Control system supervisory program. The task of superv~sory program includes keyboard supervision, command processing~ normal display and state display, and supplying different time intervals required for user's application programs. In brief, the supervisory program serves the application programs and provides conveniences for the operator. Because continuously running of system application is required for monitoring the states of power system. On the other hand, the supervisory program has to do keyboard scanning and normal display. Therefore, these two programs are coordinated with time sharing mode shown in Fig. 5. 6. CONCLUSION From the operation results of the prototype, the following conclusions can be deduced: (1) The design principle of this combined control system is proved out. The user is satisfied with its control functions. (2) The system is able to determine automatically the reasonable voltage value which should be maintained according to the change of load. (3) The adaptability of the microcomputer control system is high. No matter what
kind of operation mode is adopted or how large the load changes, the control program is able to fit it automatically according to load compensation method. (4) The system is reliable with anti-interference capacity against strong electromagnetic disturbances and is capable to aviod to respond to any kinds of transients of the power system.
(5) Results of operation showed that the application of this system not only reduces labor of person on duty and errors in operation, but also improves the voltage quality of electric power supply and decreases the network loss as well. And the economic benifit can be obtained significantly. ACKNOWLEDGEMENTS The authors are grateful to the Bei jing power Supply Bureau, Mr. Wang Yan-zhong, and others who have given helpful suggestions and comments, and have provided Dajiaoting substation as the trial operation site of the prototype of this system.
REFERENCES Bunch, J.B., R.D. Miller, and J.E.lVheeler, (1982). Distribution system integrated voltage and reactive power control. IEEE Trans. Power Apparatus and Systems 101 284-289. Gron~R~1981). Computer-dedicated voltage regulation method for distribution sUbstations. IEEE Trans. Power Apparatus and Systems, 100. 2184-2188. Happ, H.H.] R.A. Fernandes, and K.A. Wirgan, (1980). Optimal reactive power flow for improved system operations, Energy Systems, 2 133-139. Sugiyama, T., S.Kame~a, K.Maeda, A. Kaneda, and T. Goda, (1982). Development and field experience of digital protection and control equipment in power systems, IEEE Trans. power Apparatus and Systems, 101, 4237-4243. Tanaka, K., K. Kanou, Y. Harumoto, T.Mori, K. Suzuki, and T. Gouda, (1980). Application of microprocessors to the control and protection system at substation. IEEE Trans. power Apparatus and Systems, 99,344-35 1 •
99
Control of Substation Voltage
I
Input circu i t
I
Filter
I
Treatment of si gna l
I
Multichannel switch
I
AID converter
1
r---
Microcomputer
--'
1
Selection of operation mode
H
t
RS ,XSI A
I
Clock
't
1 Selection of positive reactive power working value ~
Selection of voltage working range
Cl
J
~
I
U'
J
~
Display Fig. 1, The load curve of transformer No.2 in a certain substation. SI' "." indicates the load curve of 15th, March, 1984. S2: ·x· indicates the load curve of 15th, May, 1984. S3: Equivalent load curve of peak and vallay time assigned by dispatcher.
J J J
Se lection of negat i ve reactive power working value
I
Out put of deco de
l
Buffer ampli fier
I
Output of drive
J J J
BIt
IL ___________ J
Fig. 3. Block diagram of microcomputer control system. Fig. 2. Simplified schematic 1iagram of a substation.
Huang Yi-zhuang and Huang Shen-yi
100
In itie ti on:set user stack pointer, set '~ TC c locking interrupt, clear all internal memory ~its.
Capacity state subprogram
~o rresponding
Run the supervisory program ~--------~ Return
:)ampling. AID conversion subprogram
Sil~nal
of delay time sampling
Discrimination of running mode
Computing subprogram
Sub program for the determination of upper and lo~er limits of voltage and reactive power
Lo~ical
'~ommend
decision subprogram
d iscrimination subprogra'1l
Expcutive command subprogrem
No
Senrt ex ecutive command
L-____________________
~yes
Fi ,'3 . 4 . Block d iagram of user appli cation program
Fig . 5 . Flow chart of combination between supervisory program and application program.