A Microprocessor-Based Excitation Regulator: Its Design Principle and Test Results

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A MICROPROCESSOR-BASED EXCITATION REGULATOR: ITS DESIGN PRINCIPLE AND TEST RESULTS Chen Xian-ming, Li Nailu and Zhang Zaida Salljillg AII/Ollla/ioll Re.learc/i Iw/i/II/I'. PO Box 323. Salljillg. PRe

Abstract . In the paper the design principle of a developed microprocessor - based excitation regulator (MPER) of generator is introduced and severa l test results on a 50 MW hydrogenerator at a power station in Fujian province are given . The output of MPER is used to fire the SCR converter . It consists of some INTEL OEM single board computers, some transducers of electric quantities, a few modules for digital firing control of SCR, a manual control channel(MCC), etc . The flow charts of the principal application program for operation of generators with MPER are also explained . The 50 MW hydrogenerator controlled by MPER has been operating smoothly and satisfactorily on site since the end of April , 1985. Keywords . Microprocessor (MP), excitation regulator, generator, SCR, PlO control, microprocessor - based. INTRODUCTION day by day. The excitation power is increasing rapidly as the unit power of modern large syn chronous generator increases . The requirements for the excitation need to be stri cter , suc h as higher reliability in opera t io n , superiority in technical and economic perfo r mance and facility in imp l eme nt ation of special con t ro l func tions . However, it will make the excita tion con t ro l more complex. The traditional analogexcitation regulator (AER)hardly meets all of these requirements . lt is well known that a ll functions of an AER are realized by using various printed circuit boards (PCB) . The more functions are implemented for the regu l ator, the more PCBs are used. In that case, the cOO1ponents and the numbers of contacts of plug - in system and of s o l d er poin t s increase immensely . The electric circuit might be extremely complicated and maintenance of the regulator will be more difficu l t . In consequence, the improvement of reliability of generator operation with such kind of regu l ator is scarcel y possible. That is why more advanced excitation con trol technology has to be adopted for lar ge power generators.

IMPLEMENTATION SCHEME OF MPER

PT,

transducers

r.: - - - - - - - - - - - - - - - - - - - - - - - - -I Tr'PE _1-T-! HPER

I

I

Manual Control channel

I L-

uni_ t __ _ _ _digi _ _tal _setting ____

_ ___ _

Field Breaker

Fig . 1 The single-line diagram of a shunt excitation generator with MPER, type WLT - 1 .

All functions of MPER are implemented by software, so that it is entirely different from AER . The addition of a new function wouldn ' t increase any hardware, but only the subroutines will be concerned. In addi tion, some functions which are difficult or even impossible to be completed in AER can be implemented by MPER. For instance, some pa r ameters of regulator can be chan ged automatically according to the opera tion condi t ion of the generator for cer tain requirements . Besides, the MPER pro vides the possibility to adopt modern con trol theory, such as optimal control or adaptive control, which would need some complex calculations .

Fig.1 shows the single line diagram of a shunt excitation system of a generator with MPER, type WLT - 1 . The major part of MP ER includes an INTEL single - board computer iSBC 80/24, a data acquisition module iSBC 711, an input and output module iSBC 556 with optica l iso l a tors . Besides, there are some transd ucers, modules for digital firing control of SCR, a manual control channel, LED disp l ay and its controller, etc. INTER OEM Single Board Computer MPER is used in on-line real - time electric process control . It features high speed closed loop feedback regulation . The CPU

Apart from the technical superiority MPER will offer an economic advantage because the price of microcomputer is decreasing

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Chen Xian-ming, Li Nailu and Zhang Zaida

used should have computing speed as high as possible . Usually a 16 bit MP has faster computing speed than an 8 bit one, but it is more expensive and cannot be fully utilied, so that the iSBC 80/24,8085 CPU with clock frequency 4.84 MHz is selected. Transducers of Electric Quantities There are several electric quantities, i.e terminal voltage uG' active and reactive power P,Q,field current If and the voltage setting value Us of generator etc. to be converted to DC voltage in specific range via respective transducers.

TABLE I

Arm nurn b ers

0

conduc.tinR. SCR Y-(Y, ¥o)

I

I

U

I 0 U U U I O°
0

I

u

SCR, SC!!. SCR,

U

I

I

SC!!. SCR< SCR, SCR

0

0

I

SCR .. SCR.

S I I

s" 0 '

0

S I 0

SCR.

SC~

SCR~

SCR,

SCR,

SCR, SCR.

~m

SCR, SCR. SC!!. SCR. SCR,-

From table 1 in reference to the SAB SBC, SCA and Y, it is easy to decide whic~ arms of the three phase fully controlled SCR bridge (TPFCSB) should be conductive. Gate Pulse Generaotr and Amplifier

The static output versus input characteristic of a transducer must be strictly linear. The output of a transducer must be smooth, and its dynamic response time should be as small as possible. Synchronous and Clock Circuit

5.-.&

SStSc.A.

II

SA~_ h

S~~ SCA- " 0 0 orTlllO 0 -j6~ G

JLnJ1SLfl I

h~ _ lllll l (8)

Fig.2 Synchronous and clock circuit(A) and wave shapes of relevant points(B) In the SCR rectifier circuit with phase shift control, synchronous circuit is needed for firing the SCR according to regular sequence. Fig.2(A) shows the synchronous circui t of MPER. The line voltages UAC ' UBC' UBAare applied to the zero-crossing detectors ZClZC3 respectively to form three quadrate wave synchronous signals SAB SBC' SCA(see Fig.2(B». One cycle of voltage curve can be divided into six equal distance areas (60 electric degrees) versus time axis. All these time areas, SAB' SBC' SCA are coded by 101; 100; 110; 010; 011; and 001 in sequence. Monostable triggers MI-M6 produce 6 pulses per cyc~e, which are exactly located at zero-crossl.l1g points of line voltage curves . All these six pulses are logically added in OR gate to get the clock for external interrupt request which is also applied to a flip flop FF to produce a 150 Hz quadrate wave for the use of the gate pulse generator. The range of phase shift of SCR firing 0 angle 1S as of 0 to 180 0 electric degree which is divided into 3 tri§gering zones =0'1..-60 0 , 60~ 120 0 , 120~ 180 denoted by Y=(Yl, Yo),respectively equal to (0,0),(0, 1), (1,0).

Fig.3

Gate pulse generator (A) and wave shapes of relevant points(B)

Fig. 3(A) shows the principal circuit of gate pulse ~enerator . A single phase-locked loop (PLL) 1S used to control the time delay of firing angles of all SCR in TPFCSB. The 150 Hz quadrate wave from synchronous circuit is applied to point A of phase detector, which will produce an output if there is a phase difference between point A and point B of feedback input. This output becomes a DC voltage via filter and adjusts the frequency of voltage-controlled oscillator(VCO), and thereby the counting speed of the counter, so that the turn over time of flip-flop changes.At last the phase difference between points A and B and the output of phase detector both equal zero. The FLL is on locked state. Fig . 3(B) shows the wave shape of point A, B, C in this case. The 8 bit comparator of binary code on Fig.3(A) compares the digit from MP with that from the 8 bit counter in PLL A which is continuously increased until 2° - 1=255. The comparator will produce a pulse at point C when both its inputs are equal. This pulse will be logically ANDed with the arm numbers of TPFCSB produced by MP, to generate gate pulses which are sent to pulse amplifier for firing the powerful SCR converter. Manual Control Channel WLT-l type MPER has a MCC as usual which is independent of automatic control channel (ACC) with MP. The MCC can follow up ACC exactly all the time before change over , so that once changeover from ACC to MCC takes

431

Excitation Regulator place, it will operate smoothly without swing of reactlve power. In shunt excitation of generator, for stable regulation MCC must be connected to form a closed loop with field current If. In order to ensure the smooth change over from ACC to MCC, not only the firing angle must follow up precisely, but also the setting value of If needs to follow up its real value. LED Display and Its Control For convenience of MPER commissioning test and supervision,there is a display control module used to control 8 LED tubes installed on panel of WLT-1. LED display is useful for displaying the contents of 256 pair bytes of RAM selected by eight small switches. MPER is capable of adding in man machine interfaces (CRT, printer and magnetic tape unit etc.) and communicating with higher level computer.

(MMI) program and the regulation program. Fig.4 shows the flow chart of MMI program. I t is used to display eight LED digi ts of hexadecimal code at the panel of MPER. It can also be used for MMI by connecting a CRT to MPER of the running generator. Only two commands are available. One is used to display contents of selected memory area, another to change some regulator parameters during the commissioning test of the excitation system. The other part of MMI program is to determine whether the conditions for starting generator is satisfied and if there is any trouble in ACC and so on. The Regulation Program The regulation program almost includes all application routines of excitation, the normal and abnormal regulation programs.

SAMPLE RATE For a TPFCSB at industrial frequency 50HZ, 300 firing pulses are needed per second. If every pulse is controlled by MPER,there must be 300-time control per second.Apparently, more than that is unnecessary, because once a SCR is conductive, it can not be controlled until the next zero crossing point of line voltages is reached. Of course, it is rather busy for 8 bit MP to do 300--time control per second. By elaborate design of control program the WLT-1 type MPER can properly control TPFCSB 300 times per second satisfactorily. There are a lot of tasks, for instance, data sampling, regulation calculation, output of firing angle and arm numbers of TPFCSB etc,which are needed at each control. But the test has shown that no less than 2S-time regulation calculation of firing angle per second is good enough. So SO-time regulation calculation per second is selected for WLT-1 type MPER, i.e. regulation calculation of firing angle is once per period. The firing angle is the same for all six pulses per period. APPLICATION SOFTWARE

Note :

1- Initialize

2- ACC of liLT-l 3- LED Display HCC /o- LEO Disploy ACC

5-

Heet starting Gen Condition 6- Sampling 7- Any trouble 8- Changeover to HCC 9- LED Display Selected RAM

10-CRT disploy Il-Command Key

12-E Key 13-Execute E commend

l/o-L Key IS-Execute l command

Fig.4 The flow chart of MMI program Application two parts; psppc-o·

software of MPER falls into the man machine interaction

Note: 1- Entrance of interrupt

2- Judging situation 3- flag of peak excit limitation 4- flag of overexcit limitation

5- Flag of underexcit limitation 6- Synch signal 0 0 I 7- Sanpllllg 8- Q ccrnpensation calcul. 9- PlO calcul. IO-Any restnint of firing angle Il-Hodify firing angle 12-Trigging of SCR

13-Q,Jit oC interrupt l/o-Synch signal I 0 1 15-UP 16-SCR arm nunber outpJt 17-11 controlled lS-Synch signal, 1 0 1, 19-calcul foE' return UIi controlled 20-QI. controlled 21-Synch signal, 1 0 1 22-<:.elcul for rebJm U controlled

Fig.S The flow chart of regulation program Fig.S shows its typical flow chart. The part enclosed within broken lines is abnormal operation program, including three limitation programs, i.e time-inversed limiting of peak excitation current (If controlled), time delay limiting of overexcitation (QL controlled), and instantanous limiting of underexcitation (Qc controlled ). The rest of the flow chart is the normal operation program for the generator including the terminal voltage uG control. After the MP acknowledges the external interrupt request, the flow runs into block of judging situation. That means to check if there is a command of start or stop, and if the speed of generator set reaches 90% rated speed, etc. If all conditions for MPER's normal working are satisfied, the program enters into the normal uG control routine unless any flag of limitation is set. First, MP checks if the synchronous signal is 001.If it is true, MP acquires the data concerned, such as uG' P, Q, If etc,through ADC of iSBC 711, then computes the reactive power compensation for generator parallel running with network, compares the setting value Us with measured value of voltage of generator and makes the PlO regulation calculation with the deviation of the voltage. At last the firing angle of SCR in 10 bit binary code

432

Chen Xian-ming, Li ;\Iailu and Zhang Zaida

is obtained. The most significant 2 bits of this indicate the triggering zone Y.The remaining lower 8 bits are sent to gate pulse generator for producing time delay of 5CR firing angle. In the meantime, MP looks up the number of TPFC5B arm which will conduct soon after, and sends it immediately to gate pulse generator. The limit judging program(LJP) runs at a synchronous signal of 101. Thereafter the arm numbers of TPFC5B are looked up and sent out, just as in the rest time areas of synchronous signal. Before quit of the regulation program there is a self-check signal needed to be sent to watchdog. This denotes that Mp has worked properly this time. THE LIMIT JUDGING PROGRAM

1 1;>~~--------------------------~ NO

tation current If is overridden, there is a necessity to limit it only after the forced peak excitation current is sustained for certain seconds during the network failure. But the forced peak excitation current depending on the situation of failure is indefinite in a shunt excitation generator. So the more peak excitation current is forced to apply, the less time is permissible to be sustained.That is why a time-reversed limitation characteristic is needed.The rise time of peak excitation current is ignored here. From Fig.6 LJP looks for a flag of timcreversed counting when If is higher than allowed value lav. The flag is absent when Ifn is higher than the last sampled value If(n-12. Then lfn is saved in memory AA, instead of If(n-1) for the next comparison. The time-reversed count table is looked up in reference to lfn to find the corresponding count and the latter is sent to counter BB. If the new lfn is no longer higher than the previous one,the flag of time reversed counting will be sent and the content of memory AA will be cleared. If the excitation current holds above the allowed value Iav until the content of counter BB vanishes, that means, the permissible time for that forced peak excitation current is over. The flag of peak excitation limitation will be set and signal of it will be sent out too. Before quit of LJP the flag of time-reversed counting has to be cleared for next use. Abnormal Operation Program

14-Set flag of under e~ci t. limit. IS-Display undere:xcit. limit. 16-Set counter B 17-Fl1!C ** 18-(8B)-1-o 19-5et flag of peak excit. limit. 20-Display peak exci t. I iJni t. 2l-Clear Fl1!C 22-Ij.K>I,t
Note : 1 -Main breaker open. 2

-1.+ >

1.... *

3 -Clear Fl1!C 4 -Qc..y* 7 -(A)-I-01 8 -Set flag of overexcit. limit. 9 -Display overexcite limit IO-Set time delay counter A ll-Lookup Qav tran P Q curve 12-Q> Q"", 13-(B)-I-o * ay means allOW' value 1rl:' me means flag of time-reversed counting

There are 3 kinds (If' QL, Qc controlled) of abnormal operation programs of generator with MPER. Only one of these programs is executed soon after having a flag of limitation set and that regulates the electric quantity to keep below the allowed value. They are almost the same as the normal UG control program, except that the regulated quantity is not uG' but If or Q of generator. Another difference is that the LJP is replaced by a program of condition calculation for return to normal operation. If the condition of return is met, the generator operation will be normal again automatically.

Fig.6 the flow chart of LJP LJP (Fig.6) starts to check if the main breaker of the genera tor is closed, and it stops executing soon if the breaker is open. Otherwise, it checks the field current If. If If is less than the allowed value lav' then LJP determines the sign of the reactive power at the time to decide whether to go through the judging routine of overexcitation or underexcitation limit. If the reactive power Q is positive, the allowed value Qav of Q is looked up from P, Q limitation curve according to current P, and it is compared with current Q.The time delay counter A is set to the predetermined value in case Q is smaller than allowed value Oav. In the opposite situation the content of time delay counter A decreases and if it becomes zero, the flag of overexcitation limitation will be set and its signal will be sent out. Otherwise LJP will quit and repeat the same procedure in the next period. In case the

allowed

value

Iav of exci-

The condition of return is very simple.The first is the opening of main breaker of generator. The second is when the reasons causing the electric quantity over its allowed value are eliminated. For instance, the firing angle of If controlled is smaller than that of UG controlled, and if this can last for a few seconds. The return of normal operation can be completed. PlO Regulation Calculation Despite the fact that the modern control theory is developing widely,the traditional PlO regulation is still used in most industrial equipment as it is simple to adjust for engineers. So, it is also used here in MPER. The algorithm of PID is quite simple, nothing needs to be described. Nevertheless, some special considerations must be taken into account in order to get satisfactory results in practical applications. For the sake of obtaining higher degree of precision the addition and subtraction are executed by double precision fixed point operation

Excitation Regulator but the mUltiplication is done by one-byte digit, representing the parameters of PlO, times two-byte signed digit of voltage error or other quantities. The product is of three bytes. In certain circumstances,the highest byte of the product can be ignored. The result of PlO regulation calculation is used to control the firing angle,varied from 0° to 180 ° i.e from OOOH to 2FFH in hexadecimal in TPFCSB. In practice the firing angle ranges from 0° to 150° or OH to 280H in order to prevent commutation failure of SCR. The range of a quantity after 12 bit AOC is from OOOH to FFFH, Suppose that Mn and Sn denote the sampled value and the setting value of a quantity respectively at same period,the range of the difference en=Sn-Mn is from F001H (negative) to FFFH (positive) . In the excitation control, the setting value might be nominal and the sample value of residual voltage is rather low, before the excitation applied during the starting of generator. The voltage difference eo can be very large . ln that case no PlO calculation is required, but the firing angle can be taken as OH after the excitation is applied, the generator is under the forced excitation until the en is small enough,for instance, smaller than OOFFH(positive). Then the PlO calculation is performed. On the contrary M »Sn' the firing angle can be taken as 2g0H and forced de-excitation takes place until en is larger enough, for instance, FFOOH (negative), then the PlO calculation is performed again . (see Fig.7)

433

PREVENTION FROM ELECTRIC NOISE OR INTERFERENCE The MPER can I t work normally if it fails to resist the stronge electric noise or interference at the site of power station. The first thing of MPER is invulnerable to interference, and there are a lot of countermeasures used in MPER . Specially, in order to prevent MP from halting by electric noise, there is a watchdog installed for supervising running of MP. The MP sends a self check signal to watcWog each time the regulation program quits . If the execution of latter stops or goes wrong,the absence of the self check signal renders watchdog to restart MP again.lf MP fails to restart several times, the MPER will change over to MCC automatically. TEST RESULTS A lot of tests of MPER, type WLT were made in the laboratory on a model generator and at a hydro electric station on a 50MW gen erator . The expected result is reached on basis of carefully selecting parameters of PlO and several of them are duplicated as following: Building up of voltage.

Note : 1- Fetch Sn and Mn 2- el1.-S»-M,." 3- el1.I..-OO

4- el'lo)...-Ff

5- e~ is Positive 6- 280~-oi" .

7- PlO calculation 8 - oI H < 0

9- 04. >280. IO-output

,

The practical flow chart of PlO. euh is the significant byte ot double-byte digit en RESOLUTION

If 8-bit AOC is used, the largest value is 28 -1=255. Suppose the largest voltage needed to be measured is 130% nominal voltage, then voltage change relative to one lowest bit is 130%/255 =0.51 % / bit. That means the digit of voltage in binary code lacks resolution, when the voltage change is smaller than ±0.5 %. The precision of regulation is rather poor. It is reasonable to use 10 bit AOC, but some electric quantities, for instance, Q can either be positive or negative. One bit is needed to denote the sign of a quantity.Of course, 12 bit is most attractive and is adopted here. The resolution of firing angle is 180 0 /2FFH=1800/767 bit=0.233°/bit, which is good enough for industrial application.

'I

• ', ,'

':

:

~

I

~

... ,:

~~~~.~~~~'~:' _~·~J.l' :~:L~

11-0-oi'Jo(

Fig.7

'.~

Fig . 8

The oscillogram of voltage building up of the 50MW generator

The oscillogram (Fig.8) of building up a 50MW shunt exciting generator voltage in 50-time regulation calculation per second shows the terminal voltage reached its nominal value smoothly almost without overshoot, thoubh the SCR acted as a rectifier during the voltage building up process . No load uC-f characteristic of the generator Under the nominal voltage at no lOad,the speed of the generator is changed by its prlme mover so that the frequency varied from 45 to 52Hz, the maximum error of voltage is less than 0.1%. The superior characteristic is assured by PLL of gate pulse generator. Abrupt rejection of load

434

Chen Xian-ming. Li Nailu and Zhang Zaida

\"-----

The software of MPER is written in assembly language. It's rather compact.The execution of regulation program, except the MMI program,uses about one fifth of CPU time. The regulation calculation used here is PlO. It is not known yet whether the 8 bit MP can do the job with more advanced modern control algorithm properly. Maybe the more powerful 16 bit MP can be used instead of 8 bit in that case. MPER certainly represents the development trend of excitation regulator and has a lot of advantages such as: a. Flexibility. All the functions of MPER are implemented by software. For the different excitation requirements of generators, it only needs to change software.

Fig . 9

The oscillogram of abrupt rejecting load P=10MW,Q=18MVA, I=1188A of 50 MW generator, voltage overshoot<10%.

The oscillogram(Fig.9) of abrupt rejection 10 MW active and 18MVAR reactive power at nominal field current under 110% nominal voltage of 50MW generator shows that the voltage overshoot was no more than 10% and the transient process attenuated quickly. The quality of dynamic response was proved excellent. Simulation test of peak excitation limiation It's hardly possible to do the test of peak excitation limitation on 50MW generator, connected with network. But it is easy to realize under no-load if the allowed value of field current reduced appropriately.

-.,.or_, "," · • .. · . I · .. ,J:· .·. ~.~'·

_

141 Uti

..

,

Fig.10 The oscillogram of simulation test of the peak excitation limitation. If increased gradua ll y before limitation The oscillogram (Fig,10) shows the peak excitation limitation of 50MW generator under no load. The field current increased gradua lly, and when it was bigger than the allowed val ue, after relevant time delay, the abnormal operation of generator on field current control began,and If reduced to predetermined value. CONCLUSIONS The test results and time-tested operation situations of 50MW generator with MPER, WLT-1 type, indicate that the design of MPER is fully satisfactory. The MP used here is 8 bit, ADC 12 bit, but the resolution of firing angle entirely meets the excitation requirement. The watchdog of MPER has never activated since first day of operation, so prevention of electric interference is succesful.

b. Facility in generalization and standardization. Obviously, the same hardware of MPER, WLT-1 type, can be used in a wide range of generaotrs with different capacities, for instance, from 10MW to 300MW, only the software needs to be revised in each case. c. Ease of maintenance, thanks to less hardware used in comparison with that of AER. In consequence, the operation reliability is improved further. d. Ease of communication with higher level computer when the power station adopts the hierarchical computer control. Aside from the work discussed in this paper on the automatic control of excitation of generators with MPER, ttere are various topics relating to the subject, such as power system stabilizer, optimal control and adaptive control algorithm, self check and self diagnosis program etc, which are in contemplation or in progress in our institute.