Phase-shifting transformer system

• US 5,166,597 A
• Filed: 08/08/1991
• Issued: 11/24/1992
• Est. Priority Date: 08/08/1991
• Status: Expired due to Fees

• Current Assignee: Electric Power Research Institute
• Original Assignee: Electric Power Research Institute
• Inventors: Miller, Nicholas W., Larsen, Einar V.
• Primary Examiner(s): Stephan, Steven L.
• Assistant Examiner(s): BERHANE, ADOLF D
• Application Number:
  US07/742,859
• Time in Patent Office: 474 Days
• Field of Search: 323/215, 323/216, 323/218, 323/255, 323/257, 323/258, 323/340, 323/342
• US Class Current: 323/215
• CPC Class Codes: G05F 1/20 semiconductor devices only
• CPC Class Codes: H02J 3/1814 wherein al least one reactive element is actively controlled by a bridge converter, e.g. unified power flow controllers [UPFC]
• CPC Class Codes: H02J 3/1878 using tap changing or phase shifting transformers
• CPC Class Codes: H02P 13/06 by tap-changing; by rearranging interconnections of windings
• CPC Class Codes: Y02E 40/10 Flexible AC transmission systems [FACTS]
• CPC Class Codes: Y02E 40/30 Reactive power compensation

Abstract

A faster, more controllable phase-shifting transformer system is provided, along with a method of retrofitting and upgrading a preexisting system for faster operation, and a method of applying a desired phase shift to the voltage on a polyphase transmission line. A phase-shifting transformer set has series and regulating transformers coupled together by a switching network, An optional modulator modulates the phase shift voltage about a nominal level established by the switching network. The modulator may be a variable susceptance switched inductor or a switched capacitor, or a variable source device, such as a voltage source inverter device or a synchronous condenser. A thyristor augmented switching network may have a pair of thyristor valves augmenting a reversing switch, or a thyristor augmented load tap changer for varying the phase shift discreet step magnitude. Continuous control over the effective phase shift is obtained by partial conduction control of a thyristor valve with a harmonic filter in parallel with each excited winding of the series transformer, and by including a harmonic filter across the ends of regulating winding of the regulating transformer.

Claims

1. A phase-shifting transformer system for applying a desired phase shift to the voltage on a transmission line, comprising:

a series transformer with mutually magnetically coupled series and excited windings, the series winding for coupling in series with the transmission line;

a regulating transformer with mutually magnetically coupled exciting and regulating windings, the exciting winding for receiving power from the transmission line;

a switching network coupling together the regulating winding of the regulating transformer with the excited winding of the series transformer, for establishing a nominal phase-shifting voltage level on the series transformer series winding; and

a modulator for modulating the voltage of the series transformer series winding about the nominal phase-shifting voltage level provided by the switching network.

2. A phase-shifting transformer system according to claim 1 wherein the modulator comprises a variable susceptance device coupled in parallel with the series transformer series winding.

3. A phase-shifting transformer system according to claim 2 wherein the variable susceptance device comprises a switched capacitor.

4. A phase-shifting transformer system according to claim 2 wherein the variable susceptance device comprises a switched inductor.

5. A phase-shifting transformer system according to claim 1 wherein the modulator comprises a variable source device coupled in series with the series transformer excited winding.

6. A phase-shifting transformer system according to claim 5 wherein the variable source device comprises an inverter.

7. A phase-shifting transformer system according to claim 5 wherein the variable source device comprises a synchronous machine.

8. A phase-shifting transformer system according to claim 1 further including a coupling transformer which couples the modulator with the series transformer.

9. A phase-shifting transformer system according to claim 1 wherein the series winding has a center tap, and the regulating transformer exciting winding is coupled with the transmission line through the center tap of the series transformer series winding.

10. A phase-shifting transformer system for applying a desired phase shift to the voltage on a transmission line of a power system, comprising:

a series transformer with mutually magnetically coupled series and excited windings, the series winding for coupling in series with the transmission line;

a regulating transformer with mutually magnetically coupled exciting and regulating windings, the exciting winding for receiving power from the transmission line, and the regulating winding having two opposing ends with plural taps therebetween; and

a thyristor-augmented switching network coupling together the regulating winding of the regulating transformer with the excited winding of the series transformer, with the switching network having a load tap changer for selectively coupling one of the plural taps of the regulating winding with a neutral ground potential, and an openable reversing switch for selectively coupling either end of the regulating winding with the series transformer excited winding, with at least one of the load tap changer and the reversing switch being augmented by at least one thyristor valve for conducting full load current therethrough as required by the power system during application of the desired phase shift and at least until a new steady-state condition is reached, positions of the load tap changer and reversing switch establishing a nominal phase-shifting voltage level on the series transformer series winding.

11. A phase-shifting transformer system according to claim 10 wherein the thyristor-augmented switching network further includes thyristor augmentation of the reversing switch comprising first and second thyristor valves, with the first thyristor valve coupling one end of the regulating winding with the series transformer excited winding, and the second thyristor valve coupling the other end of the regulating winding with the series transformer excited winding, one of the first and second thyristor valves conducting the full load current while the reversing switch is open.

12. A phase-shifting transformer system according to claim 10 wherein the thyristor-augmented switching network further includes thyristor augmentation of the load tap changer comprising a thyristor valve coupling a first one of the plural taps of the regulating winding with the neutral ground potential, and wherein the load tap changer comprises a switched load tap changer.

13. A phase-shifting transformer system according to claim 12 wherein the thyristor-augmented switching network further includes a no-load tap changer selectively coupling the thyristor valve with one of the plural taps of the regulating winding, the no-load tap changer being movable from between the plural taps of the regulating winding when the thyristor valve is in a non-conducting state and the switched load tap changer is conducting, the thyristor valve being in a conducting state when the switched load tap changer is in a non-conducting state, so as to vary the magnitude of a discrete step by which the nominal phase-shifting voltage level on the series transformer series winding is changed by positioning the no-load tap changer with respect to the load tap changer during operation.

14. A phase-shifting transformer system according to claim 10 wherein the thyristor-augmented switching network further includes a first thyristor valve and a first filter, each coupled in parallel with the series transformer series winding.

15. A phase-shifting transformer system according to claim 14 wherein the thyristor-augmented switching network further includes:

thyristor augmentation of the reversing switch comprising first and second thyristor valves, with the first thyristor valve coupling one end of the regulating winding with the series transformer excited winding, and the second thyristor valve the coupling the other end of the regulating winding with the series transformer excited winding; and

a second filter located between the two opposing ends of the regulating winding.

16. A phase-shifting transformer system according to claim 10 further including a modulator coupled with the series transformer to modulate the voltage of the series transformer series winding about the nominal phase-shifting voltage level.

17. A method of retrofitting a phase-shifting transformer system to rapidly apply a desired phase shift to the voltage on a transmission line, comprising the steps of:

identifying a phase-shifting transformer system requiring retrofitting which includes;

a series transformer having mutually magnetically coupled series and excited windings, the series winding being coupled in series with the transmission line;

a regulating transformer having mutually magnetically coupled exciting and regulating windings, the exciting winding for receiving power from the transmission line; and

a switching network coupling together the regulating winding of the regulating transformer with the excited winding of the series transformer for establishing a nominal phase-shifting voltage level on the series transformer series winding; and

coupling a modulator with the series transformer to modulate the voltage of the series transformer series winding about the nominal phase-shifting voltage level provided by the switching network.

18. A method of retrofitting according to claim 17 wherein the step of coupling the modulator comprises coupling a variable susceptance device in parallel with the series transformer series winding.

19. A method of retrofitting according to claim 18 wherein the variable susceptance device comprises a switched capacitor.

20. A method of retrofitting according to claim 18 wherein the variable susceptance device comprises a switched inductor.

21. A method of retrofitting according to claim 17 wherein the step of coupling the modulator comprises coupling a variable source device coupled in series with the series transformer excited winding.

22. A method of retrofitting according to claim 21 wherein the variable source device comprises an inverter.

23. A method of retrofitting according to claim 21 wherein the variable source device comprises a synchronous machine.

24. A method of retrofitting according to claim 17 wherein the step of coupling the modulator comprises coupling the modulator with the series transformer using a coupling transformer.

25. A method of retrofitting a phase-shifting transformer system to rapidly apply a desired phase shift to the voltage on a transmission line, comprising the steps of:

identifying a phase-shifting transformer sytem requiring retrofitting which includes;

a series transformer having mutually magnetically coupled series and excited windings, with the series winding coupled in series with the transmission line;

a regulating transformer having mutually magnetically coupled exciting and regulating windings, with the exciting winding receiving power from the transmission line; and

a switching network coupling together the regulating transformer regulating winding with the series transformer excited winding for establishing a nominal phase-shifting voltage level on the series transformer series winding; and

augmenting the switching network with at least one thyristor valve for conducting full load current therethrough at least during the application of the desired phase shift to the voltage on the transmission line.

26. A method of retrofitting according to claim 25 wherein:

the identifying step comprises identifying a preexisting phase-shifting transformer system requiring retrofitting which includes the regulating winding having two opposing ends, and a switching network having an openable reversing switch selectively coupling either end of the regulating winding with the series transformer excited winding; and

the step of augmenting comprises augmenting the reversing switch with first and second thyristor valves by coupling the first thyristor valve between one end of the regulating winding and the series transformer excited winding, and by coupling the second thyristor valve between the other end of the regulating winding and the series transformer excited winding so one of the first and second thyristor valves conducts the full load current when the reversing switch is open.

27. A method of retrofitting according to claim 25 wherein:

the identifying step comprises identifying a preexisting phase-shifting transformer system requiring retrofitting which includes a regulating winding having plural taps, and a switching network having a load tap changer selectively coupling one of the plural taps of the regulating winding with a neutral ground potential;

the step of augmenting comprises augmenting the load tap changer with a thyristor valve coupling a first one of the plural taps of the regulating winding with the neutral ground potential, and the method further includes the step of substituting the load tap changer with a switched load tap changer.

28. A method of retrofitting according to claim 27 wherein the step of augmenting comprises coupling a no-load tap changer between the thyristor valve and the regulating winding to selectively couple the thyristor valve with one of the plural taps of the regulating winding, the no-load tap changer being movable from between the plural taps of the regulating winding when the thyristor valve is in a non-conducting state and the switched load tap changer is conducting, the thyristor valve being in a conducting state when the no-load tap changer is in a fixed position coupling one of the regulating winding plural taps with the thyristor valve and when the switched load changer is in a non-conducting state, so as to vary the magnitude of a discrete step by which the nominal phase-shifting voltage level on the series transformer series winding is changed by positioning the no-load tap changer with respect to the load tap changer during operation.

29. A method of retrofitting according to claim 25 wherein the step of augmenting comprises coupling a first thyristor valve and a first filter in parallel with the series transformer series winding.

30. A method of retrofitting according to claim 29 wherein:

the identifying step comprises identifying a preexisting phase-shifting transformer system requiring retrofitting which includes the regulating winding having two opposing ends, and a switching network having a reversing switch selectively coupling either end of the regulating winding with the series transformer excited winding;

the step of augmenting comprises augmenting the reversing switch with second and third thyristor valves by coupling the second thyristor valve between one end of the regulating winding and the series transformer excited winding, and by coupling the third thyristor valve between the other end of the regulating winding and the series transformer excited winding; and

the retrofitting method further comprises the step of coupling a second filter between the two opposing ends of the regulating winding.

31. A method of retrofitting according to claim 25 further including the step of coupling a modulator with the series transformer for modulating the voltage of the series transformer series winding about the nominal phase-shifting voltage level established by the switching network.

32. A method of applying a desired phase shift to the voltage on a polyphase transmission line operating at a system frequency, comprising the steps of:

drawing a polyphase regulating current at a regulating voltage from the transmission line;

establishing a nominal magnitude of a shifting voltage between maximum and minmum values, the shifting voltage being in quadrature with the regulating voltage;

changing the shifting voltage magnitude to any value between the maximum and minimum values within a single cycle of the system frequency to modify the phase shift as desired; and

injecting the changed shifting voltage in series with the voltage on the transmission line.

33. A method of applying a desired phase shift according to claim 32, wherein the changing step comprises the step of modulating the shifting voltage magnitude about the established nominal magnitude with a variable modulating voltage.

34. A method of applying a desired phase shift according to claim 32, wherein the changing step comprises the step of changing the shifting voltage established nominal magnitude at a zero crossing of a half cycle of the system frequency.

35. A method of applying a desired phase shift according to claim 32, wherein:

the changing step comprises the step of chopping the shifting voltage waveform between zero crossings of a half cycle of the system frequency to vary the shifting voltage magnitude about the established nominal magnitude within a half cycle of the system frequency; and

the injecting step comprises the step of injecting the chopped shifting voltage in series with the voltage on the transmission line.

36. A method of applying a desired phase shift according to claim 35, wherein:

undesirable harmonics are generated during the chopping step; and

the method further includes the step of filtering the undesirable harmonics from the chopped shifting voltage prior to the injecting step.