Static VAR generator with time-related-error minimizer

• US 4,188,573 A
• Filed: 04/03/1978
• Issued: 02/12/1980
• Est. Priority Date: 04/03/1978
• Status: Expired due to Term

• Current Assignee: Asea Brown Boveri AB (ABB Limited)
• Original Assignee: Westinghouse Electric Company LLC (Brookfield Asset Management Incorporated)
• Inventors: Gyugyi, Laszlo, Brennen, Michael B.
• Primary Examiner(s): Pellinen, A. D.
• Application Number:
  US05/892,804
• Time in Patent Office: 680 Days
• Field of Search: 323/101, 323/102, 323/106, 323/108, 323/109, 323/110, 323/119, 323/124, 323/127, 323/128, 13/12, 13/13, 219/108, 219/110, 219/112, 219/113, 219/114, 219/115, 219/131 R
• US Class Current: 323/210
• CPC Class Codes: H02J 3/1864 wherein the stepless control of reactive power is obtained by at least one reactive element connected in series with a semiconductor switch
• CPC Class Codes: Y02E 40/10 Flexible AC transmission systems [FACTS]

Abstract

A control circuit for a static VAR generator measures appropriate circuit variables during consecutive half cycles of line voltage. This information is utilized to determine the firing angles of the VAR generator thyristors for providing compensating current to the line for keeping the source current in phase with the source voltage, for balancing source current in three-phase electrical systems and for reducing the line voltage fluctuation. A time-related-error minimizer is provided in the control system of the VAR generator to reduce the deviation between the amount of compensating current which is predicted for proper compensation during a given half cycle and the actual amount of compensating current needed at the time of application. The error is related to the differential between the time of the calculation of the value of current needed and the time it is actually applied. In one case, an open loop-type computational technique is utilized to determine the required inductor current. In a second case, a feedback control for reducing error utilizes the desired or predicted current and the actual inductor current; and in a third case, the compensating error signal is indirectly determined by utilizing the difference between the instantaneous AC terminal voltage and its steady state average value. The error signal associated with the latter case is combined with the computed desired inductor current to reduce time related terminal voltage variations.

Claims

1. A VAR generator of the type which is connected to an electrical system for the purpose of supplying reactive power to the electrical system, comprising:

(a) calculating means for determining an estimated amount of reactive power that is to be provided to said electrical system during a given period of time, said calculating means providing an estimated reactive power output signal which is related to said estimated amount of reactive power;

(b) predictor means connected to receive said estimated reactive power output signal from said calculating means for weighing said later signal by a first amount and for adding said weighed later signal to a previously stored estimated reactive power output signal which has been weighed by a second amount to form a predictor means output signal; and

(c) reactive power producing means connected to said electrical system and said predictor means for providing reactive power during said time period in correspondence to said predictor means output signal.

2. A VAR generator of the type which is connected to an alternating current electrical system for the purpose of monitoring the electrical system current and system voltage to thus provide reactive power to the electrical system during successive half cycles of system voltage, comprising:

(a) a thyristor-controlled-inductor connected in circuit relationship with a fixed capacitor, said later circuit combination being connected in appropriate circuit relationship with said electrical system for supplying said reactive power to said electrical system, the amount of said reactive power supplied during each half cycle of system voltage being a function of the thyristor phase back angle (α

); and

(b) control means connected in appropriate circuit relationship with said electrical system to measure system voltage and current for estimating in a given half cycle of system voltage the amount of inductor current which is apparently necessary to provide a value of reactive power to said electrical system in a succeeding half cycle sufficient to maintain said system voltage and said system current in a generally fixed phase relationship within limits during said succeeding half cycle, said control means also being interconnected with said thyristor-controlled-inductor for providing a composite output signal thereto for varying said phase back angle (α

), said composite output signal being related to the sum of said estimated amount of inductor current as weighed by a first predetermined amount and the estimated amount of inductor current determined in a previous half cycle as weighed by a second predetermined amount.

3. A VAR generator of the type which is connected across two conductors of an electrical system for the purpose of supplying reactive power to the system, comprising:

(a) calculating means connected in circuit relationship with said conductors for estimating the amount of reactive power that is to be provided to said electrical system during a given period of time as a function of a circuit variable and for providing an output signal related to the estimated amount of reactive power;

(b) storage means interconnected with said calculating means for storing information related to the amount of reactive power previously provided to said electrical system and for providing an output signal related to the amount of previously provided reactive power;

(c) predictor means connected to said calculating means and said storage means for receiving said output signal from said calculating means for weighing said later signal by a first amount, said predictor means also receiving said output signal from said storage means and weighing said later signal by a second amount, said first weighed signal and said second weighed signal being added to form a predictor means output signal; and

(d) VAR generation means interconnected with said electrical system and with said predictor means for utilizing said predictor means output signal to provide the actual amount of reactive power which is applied to said electrical system during said given period of time.

4. The combination as claimed in claim 3 wherein said given period of time comprises the time span of one-half cycle of alternating electrical voltage between said two conductors of said electrical system.

5. The combination as claimed in claim 4 wherein said previously provided reactive power is provided in the one-half cycle of said alternating electrical voltage immediately preceding said one-half cycle of said given period of time.

6. A VAR generator of the type which is connected across two conductors of an electrical system for the purpose of supplying reactive power to the system, comprising:

(a) calculating means connected in circuit relationship with said conductors for estimating the amount of reactive power that is to be provided to said electrical system during a given period of time as a function of circuit variables and for providing an output current (I_L12,k) related to the estimated amount of reactive power;

(b) storage means interconnected with said calculating means for storing information related to the amount of reactive power previously provided to said electrical system and for providing an output current (I_L12,(k-1)) related to the amount of previously provided reactive power;

(c) predictor means connected to said calculating means and said storage means for receiving said output current from said calculating means and for weighing said later signal by a first amount (b1), said predictor means also receiving said output current from said storage means and weighing said later signal by a second amount (b2), said first weighed current and said second weighed current being added to form a predictor means output current (I_L12,k *); and

(d) VAR generation means interconnected with said predictor means for converting said predictor means output current to the actual amount of reactive power which is applied to said electrical system during said given period of time.

7. The combination as claimed in claim 6 wherein said given period of time comprises the time span of one-half cycle of alternating electrical voltage between said two conductors of said electrical system.

8. The combination as claimed in claim 7 wherein said previously provided reactive power is provided in the one-half cycle of said alternating electrical voltage immediately preceding said one-half cycle of said given period of time.

9. The combination as claimed in claim 6 wherein in said amount of reactive power (VAR) which is applied to said electrical system during said given period of time is defined by the following relationship:

11. The combination as claimed in claim 7 wherein said alternating electrical voltage has a frequency of 60 hz.

12. The combination as claimed in claim 7 wherein said alternating electrical voltage has a frequency of 50 hz.

10. Reactive power is proportional to:

##EQU9##

13. A VAR generator of the type which is connected to an electrical system for the purpose of supplying reactive power to the electrical system, comprising:

(a) calculating means for determining an estimated amount of reactive power that is to be provided to said electrical system during a given period of time, said calculating means providing an estimated-reactive-power-output signal which is related to said estimated amount of reactive power;

(b) measuring means for determining the actual amount of reactive power delivered to said electrical system prior to said given period of time, said measuring means providing an actual-reactive-power-output signal which is related to said measured amount of reactive power;

(c) integrator means connected to receive said estimated-reactive-power-output signal from said calculating means and said actual-reactive-power-output signal from said measuring means for integrating the difference between said later two signals and providing an integrator-means-output signal relative to the integrated difference;

(d) summation means connected to also receive said estimated-reactive-power-output signal and to receive said integrator-means-output signal, said estimated-reactive-power-output-signal being changed by said integrator-means-output signal to provide a summation-means-output-signal; and

(e) reactive power producing means connected to said electrical system and said summation means for providing reactive power during said time period in correspondence to said summation-means-output signal.

14. A VAR generator of the type which is connected to an alternating current electrical system for the purpose of monitoring the electrical system current and system voltage to thus provide reactive power to the electrical system during successive half cycles of system voltage, comprising:

(a) a thyristor-controlled-inductor connected in circuit relationship with a fixed capacitor, said later circuit combination being connected in appropriate circuit relationship with said electrical system for supplying said reactive power to said electrical system, the amount of said reactive power supplied during each half cycle of system voltage being a function of the thyristor phase back angle (α

); and

(b) control means connected in appropriate circuit relationship with said electrical system to measure system voltage and current for estimating in a given half cycle of system voltage the amount of inductor current which is apparently necessary to provide a value of reactive power to said electrical system in a succeeding half cycle sufficient to maintain said system voltage and said system current in a generally fixed phase relationship within limits during said succeeding half cycle, said control means also being interconnected with said thyristor-controlled-inductor for providing an output signal thereto for varying said phase back angle (α

), said output signal being related to the sum of said estimated amount of inductor current and an error, said error being related to the integrated difference between said estimated amount of inductor current and an actual amount of inductor current present in said given half cycle.

16. The combination as claimed in claim 14 wherein said given period of time comprises the time span of one-half cycle of alternating electrical voltage between said two conductors of said electrical system.

18. The combination as claimed in claim 16 wherein said given period of time comprises the time span of one-half cycle of alternating electrical voltage between said two conductors of said electrical system.

19. The combination as claimed in claim 16 wherein said amount of reactive power (VAR) which is applied to said electrical system during said given period of time is defined by the following relationship:

Reactive power is proportional to;

##EQU10##

15. A VAR generator of the type which is connected across two conductors of an electrical system for the purpose of supplying reactive power to the system, comprising:

(a) calculating means connected in circuit relationship with said conductors for estimating the amount of reactive power that is to be provided to said electrical system during a given period of time as a function of a circuit variable and for providing an output signal related to the estimated amount of reactive power;

(b) reactive power producing means connected to said electrical system for providing reactive power thereto;

(c) measuring means interconnected with said reactive power producing means for measuring the actual amount of reactive power delivered to said electrical system prior to said given period of time, said measuring means providing an actual-reactive-power output signal which is related to said measured amount of reactive power;

(d) integrator means connected to receive said output signal from said calculating means and said actual-reactive-power output signal from said measuring means for integrating the difference between said later two signals and for providing an integrator-means output signal related to the integrated difference; and

(e) summation means connected to receive said output signal from said calculating means and to receive said integrator-means-output signal, said output signal from said calculating means being changed by said integrator-means-output signal to provide a summation-means output signal, said summation means being connected to said reactive power producing means for providing reactive power during said time period in correspondence to said summation means output signal.

17. A VAR generator of the type which is connected across two conductors of an electrical system for the purpose of supplying reactive power to the system, comprising:

(a) calculating means connected in circuit relationship with said conductors for estimating the amount of reactive power that is to be provided to said electrical system during a given period of time as a function of a circuit variable and for providing an output current (i₁₂ (t)) related to the estimated amount of reactive power;

(b) reactive power producing means connected to said electrical system for providing reactive power thereto;

(c) measuring means interconnected with said reactive power producing means for measuring the actual amount of reactive power delivered to said electrical system prior to said given period of time, said measuring means providing an actual-reactive-power output signal (i_L12.sup.(a) (t)) which is related to said measured amount of reactive power;

(d) integrator means connected to receive said output current from said calculating means and said actual-reactive-power output signal from said measuring means for integrating the difference between said later two signals and providing an integrator-means output current (ISIG1) related to the integrated difference; and

(e) summation means connected to also receive said output current from said calculating means and to receive said integrator-means-output-current, said estimated-power-output current being changed by said integrator-means-output current to provide summation-means output current (I_L12 *), said summation means being connected to said reactive power producing means for providing reactive power during said time period in correspondence to said summation-means output current.

20. The combination as claimed in claim 17 wherein said alternating electrical voltage has a frequency of 60 hz.

21. The combination as claimed in claim 17 wherein said alternating electrical voltage has a frequency of 50 hz.

22. A VAR generator of the type which is connected to an electrical system for the purpose of supplying reactive power to the electrical system, comprising:

(a) calculating means for determining an estimated amount of reactive power that is to be provided to said electrical system during a given period of time, said calculating means providing an estimated-reactive-power-output signal which is related to said estimated amount of reactive power;

(b) voltage-magnitude-determining means for determining voltage magnitude in said electrical system, said voltage-magnitude-determining means providing a voltage-magnitude-output signal which is related to said voltage magnitude in said electrical system; and

(c) reactive power producing means connected to said electrical system and to said calculating means and to said voltage-magnitude-determining means for providing said reactive power during said time period in correspondence to said estimated-reactive-power-output signal and said voltage-magnitude-output signal.

23. A VAR generator of the type which is connected to an alternating current electrical system for the purpose of monitoring the electrical system current and system voltage to thus provide reactive power to the electrical system during successive half cycles of system voltage, comprising:

(a) a thyristor-controlled-inductor connected in circuit relationship with a fixed capacitor, said later circuit combination being connected in appropriate circuit relationship with said electrical system for supplying said reactive power to said electrical system, the amount of said reactive power supplied during each half cycle of system voltage being a function of the thyristor phase back angle (α

); and

(b) control means connected in appropriate circuit relationship with said electrical system to measure system voltage and current for estimating in a given half cycle of system voltage the amount of inductor current which is apparently necessary to provide a value of reactive power to said electrical system in a succeeding half cycle sufficient to maintain said system voltage and said system current in a generally fixed phase relationship within limits during said succeeding half cycle, said control means comparing the amplitude of said system voltage during said given half cycle with a reference signal to provide an error signal, said control means also being interconnected with said thyristor-controlled-inductor for providing an output signal thereto for varying said phase back angle (α

), said later output signal being related to the difference between said estimated amount of inductor current and a value which is related to said error signal.

25. The combination as claimed in claim 23 wherein said given period of time comprises the time span of one-half cycle of alternating electrical voltage between said two conductors of said electrical system, said first interval of time comprises the time span of the one-half cycle of alternating electrical voltage between said two conductors immediately preceding said one-half cycle of said given period of time, said second interval of time comprising the time span of a plurality of consecutive one-half cycles of said alternating electrical voltage immediately preceding said one-half cycle of said given period of time.

27. The combination as claimed in claim 25 wherein said given period of time comprises the time span of one-half cycle of alternating electrical voltage between said two conductors of said electrical system, said first interval of time comprises the time span of one-half cycle of alternating electrical voltage between said two conductors immediately preceding said one-half cycle of said given period of time, said second interval of time comprising the time span of a plurality of consecutive one-half cycles of said alternating electrical voltage immediately preceding said one-half cycle of said given period of time.

28. The combination as claimed in claim 25 wherein said amount of reactive power (VAR) which is applied to said electrical system during said given period of time is defined by the following relationship:

Reactive power is proportional to;

##EQU11##

24. A VAR generator of the type which is connected across two conductors of an electrical system for the purpose of supplying reactive power to the system, comprising:

(a) calculating means connected in circuit relationship with said conductors for estimating the amount of reactive power that is to be provided to said electrical system during a given period of time as a function of a circuit variable and for providing an estimated-power-output signal related to the estimated amount of reactive power;

(b) short range voltage amplitude determining means connected to said electrical system for measuring the amplitude of the voltage between said two conductors for a first interval of time which is prior to said given period of time, said short range voltage amplitude determining means providing a short range output signal which is related to said maximum amplitude of said voltage during said first interval of time;

(c) long range voltage amplitude determining means connected to said electrical system for measuring the amplitude of the voltage between said two conductors for a second interval of time which is also prior to said given period of time and which is substantially longer than said first interval of time, said long range voltage determining means providing a long range output signal which is related to said amplitude of said voltage averaged over said second interval;

(d) integrator means connected to receive said short range output signal and said long range output signal for integrating the difference between said later two signals and for providing an integrator-means-output signal related to the integrated difference;

(e) summation means connected to receive said estimated-power-output signal and to receive said integrator-means-output signal for providing a summation-means-output signal related to the difference therebetween; and

(f) reactive power producing means connected to said electrical system and said summation means for utilizing said summation-means-output signal to provide the actual amount of reactive power which is applied to said electrical system during said given period of time.

26. A VAR generator of the type which is connected across two conductors of an electrical system for the purpose of supplying reactive power to the system, comprising:

(a) calculating means connected in circuit relationship with said conductors for estimating the amount of reactive power that is to be provided to said electrical system during a given period of time as a function of a circuit current and for providing an estimated-power-output current (I_L12,k) related to the estimated amount of reactive power;

(b) short range voltage amplitude determining means connected to said electrical system for measuring the amplitude of the voltage between said two conductors for a first interval of time which is prior to said given period of time, said short range voltage amplitude determining means providing a short range output voltage signal (v₁₂ (t)) which is related to said maximum amplitude of said voltage during said first interval of time;

(c) long range voltage amplitude determining means connected to said electrical system for measuring the amplitude of the voltage between said two conductors for a second interval of time which is also prior to said given period of time and which is substantially longer than said first interval of time, said long range voltage determining means providing a long range output voltage signal (V_ST) which is related to said amplitude of said voltage averaged over said second interval;

(d) integrator means connected to receive said short range output voltage signal and said long range output voltage signal for integrating the difference between said later two voltage signals and for providing an integrator-means-output signal (IS1G2) related to the integrated difference;

(e) summation means connected to receive said estimated-power-output current and to receive said integrator-means-output signal for providing a summation-means output signal (I_L12 *) related to the difference therebetween; and

(f) reactive power producing means connected to said electrical system and said summation means for utilizing said summation-means-output signal to provide the actual amount of reactive power which is applied to said electrical system during said given period of time.

29. The combination as claimed in claim 26 wherein said alternating electrical voltage has a frequency of 60 hz.

30. The combination as claimed in claim 26 wherein said alternating electrical voltage has a frequency of 50 hz.