AC power generating system

• US 6,801,019 B2
• Filed: 12/31/2002
• Issued: 10/05/2004
• Est. Priority Date: 01/28/2000
• Status: Expired due to Term

• Current Assignee: Cummins Generator Technologies Limited (Cummins Incorporated)
• Original Assignee: Newage International Ltd (Cummins Incorporated)
• Inventors: Al-Tayie, Jawad, Dziuba, Robert Pawel, Leonarski, Jaroslaw, Laird, George Williamson, Brown, Neil, Al-Khayat, Nazar, Dalby, Denis Hedley, Haydock, Lawrence, Koczara, Wlodzimierz
• Primary Examiner(s): Ponomarenko, Nicholas
• Assistant Examiner(s): Cuevas, Pedro J.
• Application Number:
  US10/182,529
• Publication Number:
  US 20030107349A1
• Time in Patent Office: 644 Days
• Field of Search: 322/28, 322/17, 322/16, 322/7, 322/4, 290/44, 290/40.C
• US Class Current: 322/17
• CPC Class Codes: H02J 9/062 for AC powered loads
• CPC Class Codes: H02J 9/066 characterised by the use of dynamo-electric machines H02J9/08 takes precedence
• CPC Class Codes: H02M 1/007 Plural converter units in cascade push-pull DC/DC converters with pre-regulator H02M3/3374; DC-AC converters following a DC-DC stage including a high frequency transformer H02M7/4807; DC-AC converters following a DC-DC conversion stage generating periodically varying voltages H02M7/4826
• CPC Class Codes: H02M 5/458 using semiconductor devices only
• CPC Class Codes: H02P 9/04 Control effected upon non-electric prime mover and dependent upon electric output value of the generator
• CPC Class Codes: H02P 9/48 Arrangements for obtaining a constant output value at varying speed of the generator, e.g. on vehicle H02P9/04 - H02P9/46 take precedence

Abstract

An electric power generating set which automatically varies its speed to match load variations at any time includes an engine driven generator G which provides a positive, Ga and a negative, Gb AC output voltage each of which is rectified by a respective rectifier Re3a, Re3b. Each rectifier Re3a,Re3b has one terminal connected to a common neutral line N. The DC output voltage of each rectifier Re3a, Re3b is boosted in a respective booster circuit which also has one terminal connected to the neutral line N. The combined output of the two booster circuits, Va+Vb is fed to an Inverter IN which converts it to the AC power output of the generating set. A DC load current between the booster circuits and the inverter IN is monitored and used to provide a speed demand feedback signal Srs to the speed control of the engine EN. The inverter IN is controlled so that the voltage and frequency of the AC power output of the generating set are reduced in response to a fall in the intermediate DC voltage Va+Vb that is fed to the inverter IN and which is monitored by a voltage sensor Vsab.

Claims

1. An AC power generating system including inverter means which are operable to convert an intermediate DC voltage into an AC power output, means for generating and maintaining the intermediate DC voltage at one level and voltage sensor means operable to monitor the intermediate DC voltage, wherein control means are provided for the inverter means, said control means being responsive to the voltage sensor means and being operable to control operation of the inverter means so that at least one of the voltage and the frequency of the AC power output is reduced in response to a fall in the intermediate DC voltage to a certain level caused by the application of a step load to the AC power output whereby to provide a transient off-loading effect which will give the generating means time to respond and thereby allow the intermediate DC voltage to be restored to said one level.

2. An AC power generating system according to claim 1 wherein a neutral setting of the AC power output is generated and regulated by a controlled division of the intermediate DC voltage.

3. An AC power generating system according to claim 1 wherein the means for generating and maintaining the intermediate DC voltage include generator means operable to generate a variable voltage AC power supply and rectifier means having an output and operable to rectify the variable voltage AC power supply to establish the intermediate DC voltage.

4. An AC power generating system according to claim 3 wherein a neutral setting of the AC power output is generated and regulated by a controlled division of the intermediate DC voltage and said rectifier means are full wave rectifier means.

5. An AC power generating system according to claim 1, wherein said means for generating the intermediate DC voltage include booster means which are operable to establish the intermediate DC voltage.

6. An AC power generating system according to claim 5, wherein said means for generating and maintaining the intermediate DC voltage include generator means operable to generate a variable voltage power supply and respective control means, and the voltage sensor means provide a feedback control signal to the control means of the generator means whereby to effect variation of the variable voltage power supply and thereby to counter a tendency of the intermediate DC voltage to vary.

7. An AC power generating system according to claim 6, wherein the generator means are driven by a variable speed prime mover, the control means of the generator means comprising speed control means operable to control the speed of the prime mover.

8. An AC power generating system according to claim 7, including current sensor means operable to monitor a DC load current caused by connection of a load across the intermediate DC voltage, the current sensor means being operable to emit a signal which is indicative of the monitored DC load current, comparator means being provided for comparing an output signal from the current sensor means with a reference signal and for emitting a speed correction signal which is proportional to the amount by which the signal from the current sensor means exceeds the reference, the output from the comparator means being supplied to the speed control means of the generator means to effect an increase in the speed of the prime mover above that required for the applied load.

9. An AC power generating system according to claim 6 including brake control means and means responsive to the output of the voltage sensor means that is operable to monitor the intermediate DC voltage whereby to connect the brake control means across the intermediate DC voltage to apply a load thereto when the intermediate DC voltage rises to a certain level.

10. An AC power generating system according to claim 1 wherein said control means for the inverter means are operable to increase the frequency of the AC power output in response to an increase in the intermediate DC voltage to a certain high level.

11. An AC power generating system according to claim 10 wherein said control means for the inverter means are operable to increase the frequency of the AC power output in response to an increase in the intermediate DC voltage to a certain high level which is higher than the level at which the brake control means is connected across the intermediate DC voltage.

12. An AC power generating system according to claim 3, including booster means having an input which is connected to the output of the rectifier means and which are operable to increase the voltage of the rectified output of said rectifier means and thereby establish the intermediate DC voltage, and including electrical energy storage means connected across the intermediate DC voltage so as to be charged by the intermediate DC voltage, wherein bistable switching means and associated control means are provided, said bistable switching means normally being in one state in which they are operable to make the connection of the electrical energy storage means across the intermediate DC voltage to enable the electrical energy storage means to be charged and to interrupt a connection between the electrical energy storage means and the connection between the output of the rectifier means and the input of the booster means, said bistable switching means being operable in its other state to isolate the electrical energy storage means from the intermediate DC voltage and to make said connection of the electrical energy storage means with the connection between the output of the rectifier means and the input of the booster means whereby to enable discharge of electrical energy from said electrical energy storage means into the connection between the rectifier means and the booster means, said control means associated with the bistable switching means being responsive to the monitored intermediate DC voltage and being operable in response to a fall in said intermediate DC voltage to a certain reference level to switch said bistable switching means from its normal said one state to said other state whereby to discharge electrical energy to augment the rectified output of the said rectifier means and thereby to counter the sensed fall in said intermediate DC voltage.

13. An electrical power generating system including generator means operable to generate a variable voltage power supply, booster means having an input which is connected to an output of said generator means and which are operable to increase the voltage of the output of said generator means and thereby to provide a DC voltage, a variable speed prime mover drivingly coupled with the generator means, and speed control means operable to control the speed of the motor, wherein current sensor means are provided, said current sensor means being operable to monitor a current caused by connection of a load across the DC voltage, the current sensor means being operable to emit a signal which is indicative of the monitored current, comparator means being provided for comparing an output signal from the current sensor means with a reference signal and for emitting a speed correction signal which is proportional to the amount by which the output signal from the current sensor means exceeds the reference signal, the output from the comparator means being supplied to the speed control means for the generator means to effect an increase in the speed of the variable speed prime mover, wherein the improvement comprises said current sensor means being operable to monitor the DC load current on the side of said booster means remote from said rectifier means.

14. An electrical power generating system according to claim 13, wherein said generator means are operable to generate a variable voltage AC power supply and rectifier means are provided, the rectifier means having an output and being operable to rectify the variable voltage AC power supply, the booster means being connected to the output of the rectifier means and being operable to increase the voltage of the rectified output of said rectifier means.

15. An electrical power generating system including generator means operable to generate a variable voltage AC power supply, booster means having an input which is connected to the output of the generator means and which is operable to increase the voltage of the output of said generator means and thereby to provide a DC voltage, voltage sensor and control means operable to monitor the DC voltage and to provide a feedback control signal to the generator means whereby to vary the variable voltage power supply so as to counter a tendency of the DC voltage to vary, and electrical energy storage means connected across the DC voltage so as to be charged by the DC voltage, wherein bistable switching means and associated control means are provided, said bistable switching means normally being in one state in which they are operable to make the connection of the electrical energy storage means across the DC voltage to enable the electrical energy storage means to be charged and to interrupt a connection between the electrical energy storage means and a connection between the generator means and the input of the booster means, said bistable switching means being operable in its other state to isolate the electrical energy storage means from the DC voltage and to make said connection of the electrical energy storage means with the connection between the generator means and the input of the booster means, whereby to enable discharge of electrical energy from said electrical energy storage means into the connection between the generator means and the booster means, said control means associated with the bistable switching means being responsive to the monitored DC voltage and being operable in response to a fall in said DC voltage to a certain reference level to switch said bistable means from its normal said one state to said other state whereby to discharge electrical energy to augment the variable voltage power supply and thereby to counter the sensed fall in said DC voltage.

16. An electrical power generating system according to claim 15, in which said generator means is operable to generate a variable voltage AC power supply and rectifier means are provided, said rectifier means having an output and being operable to rectify the variable voltage AC power supply, wherein said connection between the generator means and the input of the booster means is the connection between the rectifier means and the input of the booster means so that the discharged electrical energy augments the rectified output of said rectifier means.

17. An electrical power generating system according to claim 15, which is an AC power generating system in which said DC voltage is an intermediate DC voltage and inverter means are provided, said inverter means being operable to convert the intermediate DC voltage into an AC power output for supplying to an external load.

18. An electrical power generating system including variable voltage power supply generator means, converter means operable to establish a DC link by converting the variable voltage power supply to a DC voltage, voltage sensor means operable to monitor the DC voltage and control means responsive to an output from the voltage sensor means and operable to maintain the DC voltage at one level, wherein the generator means is operable to generate two variable voltage power supplies each connected on one side to a common terminal and having an output voltage terminal on its other side, the output voltage terminal of one of the power supplies being positive and the output voltage terminal of the other power supply being negative such that the two variable voltage

power supplies together comprise the DC voltage, the voltage sensor means comprising two voltage sensors severally responsive to a respective one of the power supplies and the control means comprising two controllers severally operable to maintain the voltage of each of those power supplies at a certain level whereby to maintain the DC voltage at said one level.

19. An electrical power generating system according to claim 18, wherein there are two booster circuits, each connected between a respective one of the output voltage terminals of the two power supplies and the common terminal and each operable to increase the voltage of the respective output potential and thus to increase said DC voltage, each of the controllers being operatively associated with the respective one of the booster circuits.

20. An AC power generating system including variable voltage AC power supply generator means, rectifier means having an output and operable to rectify the variable voltage AC power supply to establish an intermediate DC voltage, inverter means which are operable to convert the intermediate DC voltage into an AC power output, voltage sensor means operable to monitor the intermediate DC voltage and control means responsive to an output from the voltage sensor means and operable to maintain the intermediate DC voltage at one level, wherein the generator means is operable to generate two variable voltage AC power supplies, the rectifier means comprise two full-wave rectifiers, each full-wave rectifier being operable to rectify a respective one of the two variable voltage AC power supplies and each being connected on one side to a neutral terminal and having an output voltage terminal on its other side, the output voltage terminal of one of the full-wave rectifiers being positive and the output voltage terminal of the other full-wave rectifier being negative such that the two variable voltage AC power supplies are severally rectified to produce a positive and a negative potential respectively which together comprise the intermediate DC voltage, the voltage sensor means comprising two voltage sensors severally responsive to a respective one of the positive and negative output potentials of the two full-wave rectifiers and the control means comprising two controllers severally operable to maintain each of those positive and negative potentials at a certain level whereby to maintain the intermediate DC voltage at said one level.

21. An AC power generating system according to claim 20, wherein there are two booster circuits, each connected between a respective one of the output voltage terminals of the two full-wave rectifiers and the neutral terminal and each operable to increase the voltage of the respective output potential and thus to increase the intermediate DC voltage, each of the controllers being operatively associated with the respective one of the booster circuits.

22. An electrical power generating system according to claim 20 wherein the generator means is a permanent magnet generator driven by a variable speed prime mover provided with speed control means operable to control the speed of the prime mover.

23. An AC power generating system according to claim 20, wherein the generator means is driven by a variable speed prime mover provided with speed control means operable to control the speed of the prime mover and current sensor means operable to monitor a DC load current caused by connection of a load across the intermediate DC voltage are provided, the current sensor means being operable to emit a signal which is indicative of the monitored DC load current, comparator means being provided for comparing an output signal from the current sensor means with a reference signal and for emitting a speed correction signal which is proportional to the amount by which the signal from the current sensor means exceeds the reference, that speed correction signal being supplied to the speed control means for the generator as a feed back control signal whereby to effect variation of the variable AC power supply and thereby to counter any tendency of the intermediate voltage to vary.

24. An AC power generating system according to claim 20 including brake control means and means responsive to the output of the two voltage sensors that are operable to monitor the positive and negative output potentials whereby to connect the brake control means across the intermediate DC voltage to apply a load thereto when the potential difference between the monitored positive and negative potentials rises to a certain level.

25. An AC power supply generating system according to claim 24 wherein one pair of capacitors are connected in parallel between the positive output potential and the neutral terminal, another pair of capacitors are connected in parallel between the negative output potential and the neutral terminal and the brake control means are connected across the positive and negative output potentials between the capacitors of each pair.

26. An AC power supply generating system according to claim 23, wherein one pair of capacitors are connected in parallel between the positive output potential and the neutral terminal, another pair of capacitors are connected in parallel between the negative output potential and the neutral terminal and the brake control means are connected across the positive and negative output potentials between the capacitors of each pair, and wherein the current sensor means that are operable to monitor

the DC load current caused by connection of a load across the intermediate DC voltage are connected in one of the positive and the negative connections between the brake control means and the capacitors of each pair that are remote from the two full-wave rectifiers and that are nearer to the inverter means.

27. A method of converting a DC voltage having a positive and negative level of electrical potential into an AC power output in which the positive and negative levels of electrical potential are connected to an AC power output terminal

alternately, wherein each connection of one of the positive and negative levels of electrical potential to the output terminal is separated in time from a connection of the other of the positive and negative levels of the electrical potential to the output terminal by an intermediate period of zero voltage.

28. A method according to claim 27, wherein the source of each of the positive and negative potentials of DC voltage is a respective charged capacitor and the positive and negative levels of electrical potential are connected alternately to the AC power output terminal through an output filter, both sides of the output filter being connected to neutral for said intervening periods.

29. A DC to AC converter including a first source of electrical energy operable to be charged with a positive DC potential, a second source of electrical energy operable to be charged with a negative DC potential, first bistable switching means operable in one state to connect the first electrical energy source to an AC power output terminal through an inductor means of an output filter means and to break that connection in its other state, and second bistable switching means operable in one state to connect the second electrical energy source to the AC power output terminal through said inductor means and to break that connection in its other state, wherein the improvement comprises third bistable switching means which are operable in one state to connect said inductor means to neutral and to break that connection in its other state, and control means operable to control operation of said first, said second and said third bistable switching means so that said first and said second bistable switching means are switched to their said one state alternately and are in their said other state when the other of said first and second bistable switching means are in their said one state and said third bistable switching means is switched to its said one state whilst said first and second bistable switching means are both switched to their said other state between each alternate switching of said first and second bistable switching means to their said one state so that the positive and negative, levels of electrical potential with which said first and second electrical energy sources are charged when said converter is operated are connected to the AC power output terminal alternately and each connection of the one of the positive and negative levels of electrical potential to the output terminal is separated in time from a connection of the other of the positive and negative levels of electrical potential to the output terminal by an intervening period of zero voltage.

30. A DC to AC converter according to claim 29, wherein the output filter means includes a third chargeable source of electrical energy which is connected on one side to the inductor means and on its other side to neutral, the third bistable switching means being operable to connect the other side of the inductor means to neutral when in its said one state.

31. A DC to AC converter according to claim 29, which is the inverter means of an AC power generating system which includes variable voltage power supply generating means operable to establish an intermediate DC voltage, the inverter means being operable to convert the intermediate DC voltage into the AC power output, there being voltage sensor means operable to monitor the intermediate DC voltage and control means responsive to an output of the voltage sensor means and operable to maintain the intermediate DC voltage at one level.

32. An AC power generating system according to claim 31, wherein the generating means is operable to generate two variable voltage AC power supplies, and two full wave rectifiers are provided, each being operable to rectify a respective one of the two variable voltage AC power supplies and each being connected on one side to a neutral terminal and having an output terminal on its other side, the output terminal of one of the fullwave rectifiers being positive and the output terminal of the other fullwave rectifier being negative such that the two variable voltage AC power supplies are rectified to produce a positive and a negative potential respectively which together comprise the intermediate DC voltage and which are respectively connected across the first and second electrical energy storage means, there being voltage sensor means severally responsive to a respective one of the positive and negative output potentials of the two fullwave rectifiers and control means operable to maintain each of those positive and negative potentials at a certain level whereby to maintain the intermediate DC voltage at one level.

33. A DC to AC converter according to claim 31, wherein there are two booster circuits, each connected between a respective one of the output terminals of the two fullwave rectifiers and the neutral terminal and each operable to boost the respective one of the output electrical potentials, each of the control means being operatively associated with a respective one of the booster circuits.

34. A method of operating an electrical power generating system of the kind which includes an engine driven generator which provides a variable voltage electrical output, booster means operable to boost the voltage of the variable voltage electrical output and thereby to provide an intermediate DC voltage, inverter means which are operable to convert the intermediate DC voltage into an AC power output, means operable to maintain the intermediate DC voltage at one level up to a predetermined speed of the engine driven generator which is less than the maximum speed thereof being provided, wherein the means operable to maintain the intermediate DC voltage at said one level is disenabled when the speed of the engine driven generator rises to at least said predetermined speed and the speed of the engine driven generator and the intermediate DC voltage are allowed to rise so that, when the engine driven generator is running at maximum speed, the electrical power output is allowed to increase until a balance is achieved.

35. A method of operating an AC power generating system including the steps of:

(i) operating a variable speed prime mover driven generator to generate a variable voltage AC power supply;

(ii) rectifying the variable voltage AC power supply to establish an intermediate DC voltage;

(iii) controlling the intermediate DC voltage to maintain it at a substantially constant level;

(iv) operating an inverter to convert the intermediate DC voltage into an AC power output;

(v) monitoring the intermediate DC voltage; and

(vi) controlling operation of the prime mover driven generator by feedback control so as to increase its speed when the monitored intermediate DC voltage falls to the level of the first reference voltage whereby to restore the intermediate DC voltage to said substantially constant level;

characterized by the further steps of

(vii) comparing the intermediate DC voltage with a first reference voltage which is lower than the substantially constant level; and

wherein (viii) the intermediate DC voltage is also compared with a second reference voltage which is lower than said first reference voltage, and (ix) operation of the inverter is controlled so as to reduce the voltage and/or frequency of the AC power output when the monitored intermediate DC voltage falls to the level of the second reference voltage whereby to provide a transient off-loading effect which assists restoration of the intermediate DC voltage to said substantially constant level.

36. A method of operating an AC power generating system according to claim 35, including the further steps of:

a) monitoring a DC load current which results from connection of a load across the intermediate DC voltage

b) comparing the monitored DC load current with a certain current reference level; and

c) further controlling operation of the prime mover driven generator by feedback control when the monitored DC load current exceeds the certain current reference level whereby to further increase the speed of the prime mover driven generator by an amount which is proportional to the amount by which the monitored DC load current exceeds the certain current level.

37. A method of operating an AC power generating system according to claim 35, including the further step of controlling operation of the inverter so as to increase the frequency of the AC power supply when the monitored DC voltage rises to the level of a third reference voltage.

38. A method of operating an AC power generating system according to claim 35 including the further step of connecting a brake controller across the intermediate DC voltage to apply a load thereto when the monitored DC voltage rises to a certain voltage level.

39. A method of operating an AC power generating system according to claim 38 including the further step of controlling operation of the inverter so as to increase the frequency of the AC power supply when the monitored DC voltage rises to the level of a third reference voltage wherein the third reference voltage is higher than said certain voltage level.

40. A electrical power generating system including generator means operable to generate a variable voltage generator output, a variable speed prime mover drivingly coupled with the generator means, speed control means operable to control the speed of the prime mover, converter means operable to establish a DC link by converting the variable voltage generator output into a DC voltage and to derive an electrical power output from that DC voltage for supply to an external load, sensing means operable to monitor the DC link when the load is connected across the DC voltage and to provide a feedback to the speed control means whereby to effect variation of the variable voltage electrical output thereby to counter a tendency of the DC voltage to vary so as to maintain the DC voltage at one level, wherein control means are provided for said converter means, said control means being responsive to the sensing means and being operable to control operation of said converter means so that the voltage of the electrical power output is reduced in response to a fall in the DC voltage to a certain level caused by the application of a step load to the electrical power output whereby to provide a transient off-loading effect which will give the generator means time to respond and thereby allow the DC voltage to be restored to said one level.

41. An electrical power generating system according to claim 40, including inverter means which are operable to convert the DC voltage into an AC power output, said sensing means including voltage sensor means operable to monitor the DC voltage, wherein the transient off-loading effect is provided by said control means being responsive to the voltage sensor means and being operable to control operation of the inverter means so that at least one of the voltage and the frequency of the AC power output is reduced in response to a said fall in the DC voltage.

42. An electrical power generating system according to claim 40 wherein said sensing means includes current sensor means operable to monitor a DC load current caused by connection of a load across the DC voltage, the current sensor means being operable to emit a signal which is indicative of the monitored DC load current, comparator means being provided, for comparing an output signal from the current sensor means with a reference signal and for emitting a speed correction signal which is proportional to the amount by which the signal from the current sensor means exceeds the reference, the output from the comparator means being supplied, to the speed control means of the generator means to effect an increase in the speed of the prime mover above that required for the applied load.

43. An electrical power generating system according to claim 41, including brake control means and means responsive to the output of the voltage sensor means that is operable to monitor the DC voltage whereby to connect the brake control means across the DC voltage to apply a load thereto when the DC voltage rises to a certain level.

44. An electrical power generating system according to claim 41, wherein said control means for the inverter means are operable to increase the frequency of the AC power output in response to an increase in the DC voltage to a certain high level.

45. An electrical power generating system according to claim 41 wherein said certain level is higher than the level at which the brake control means is connected across the DC voltage.

46. An electrical power generator system according to claim 41 in which the variable voltage generator output is an AC power supply and including rectifier means having an output operable to rectify the variable voltage AC power supply, booster means having an input which is connected to the output of the rectifier means and which are operable to increase the voltage of the rectified output of said rectifier means and thereby to establish the DC voltage and electrical energy storage means connected across the DC voltage so as to be charged by the DC voltage, wherein bistable switching means and associated control, means are provided, said bistable switching means normally being in one state in which they are operable to make the connection of the electrical energy storage means across the DC voltage to enable the electrical energy storage means to be charged and to interrupt a connection between the electrical energy storage means and the connection between the output of the rectifier means and the input of the booster means, said bistable switching means being operable in its other state to isolate the electrical energy storage means from the DC voltage and to make said connection of the electrical energy storage means with the connection between the output of the rectifier means and the input of the booster means whereby to enable discharge of electrical energy from said electrical energy storage means into the connection between the rectifier means and the booster means, said control means associated with the bistable switching means being responsive to the monitored DC voltage and being operable in response to a fall in said DC voltage to a certain reference level to switch said bistable switching means from its normal said one state to said other state whereby to discharge electrical energy to augment the rectified output of the rectifier means and thereby to counter the sensed tall in said DC voltage.