GB2450891A

GB2450891A - Cycloconverter

Info

Publication number: GB2450891A
Application number: GB0713374A
Authority: GB
Inventors: Nazar Al-Khayat; Artur Krasnodebski; Szymon Mroczek
Original assignee: Cummins Generator Technologies Ltd
Current assignee: Cummins Generator Technologies Ltd
Priority date: 2007-07-10
Filing date: 2007-07-10
Publication date: 2009-01-14
Also published as: GB0713374D0

Abstract

An electrical generating apparatus is disclosed, which comprises a generator 10 for generating an AC output. A cycloconverter 12 is provided for converting the output of the generator to a three phase AC output of a different frequency. The cycloconverter 12 has a controllable bridge circuit to switch the output of the generator 10. Three bridge circuits may be used to produce three phase output. An inductor may be located between the bridge circuit and the output (Lf, figure 4). This can allow an AC output signal of a required frequency to be produced using fewer power conversion stages than an inverter generator.

Description

1 2450891

ELECTRICAL GENERATING APPARATUS

The present inventioi relates to an electrical generating apparatus, and in particular to an electrical generating apparatus comprising a generator and an output conditioning circuit for producing an AC output signal.

In a conventional generator set, an engine is used to drive a generator, and the output of the generator is supplied to an electrical load. In such a generator set, the frequency of the AC output is dependent on the rotational speed of the engine and generator. Thus, in order to provide an AC output signal of a fixed frequency, it is necessary to operate the engine and generator at a fixed frequency.

Variable speed integrated generators (VSIGs) have been proposed, in which the AC output of the generator is converted to DC, and the thus produced DC voltage is then converted to an AC output. This can allow the engine and generator to operate at the most efficient speed for a particular load, while still producing an AC output signal of the required frequency and voltage. Such an arrangement is disclosed in WO 01/56133, the contents of which are incorporated herein by reference.

Variable speed inverter generators generally comprise an AC to DC converter which converts the output of the generator to DC, a DC to DC converter which regulates the DC, and a DC to AC converter (inverter) which converts the DC to the AC output. A problem with such an arrangement is that a number of power conversion stages are required, which may add to the cost and bulk of the system, and reduce its efficiency due to the need for double conversion.

According to the present invention there is provided an electrical generating apparatus comprising a generator for generating an AC output, and a cycloconverter for converting the output of the generator to a three phase AC output of a different frequency.

By providing a cycloconverter for converting the output of the generator to a three phase AC output of a different frequency, an AC output signal of a required frequency and voltage may be produced using fewer power conversion stages than an inverter generator.

Preferably the cycloconverter comprises a controllable bridge circuit, which may be used to switch an output of the generator to an appropriate output of the cycloconverter to produce an AC output signal of the required frequency. The bridge circuit may comprise, for example, a plurality of thyristors, or any other controllable switching components such as field effect transistors or bipolar transistors.

In order to produce a three phase output, the cycloconverter may comprise three bridge circuits, and each bridge circuit may be arranged to produce one of the three output phases.

Where the cycloconverter comprises a bridge circuit, an inductor may be located between the bridge circuit and the output. For example, if the cycloconverter comprises a positive bridge and a negative bridge, a first inductor may be located between the positive bridge and the output, and a second inductor may be located between the negative bridge and the output. The bridges may be full bridges or half bridges. The inductors are preferably inductors and preferably of equal value. This arrangement may allow the flow of circulating current between the positive and negative bridges without short-circuiting them.

Where the cycloconverter comprises a positive bridge and a negative bridge, the positive bridge and negative bridge may be operated simultaneously. This may allow the cycloconverter to be operated more efficiently. Providing a first inductor between the positive bridge and the output and a second inductor between the negative bridge and the output may facilitate operation of the two bridges simultaneously, by allowing the flow of circulating current.

Since the input to the cycloconverter is an AC waveform, the output of the cycloconverter may be discrete parts of the input waveform switched to achieve as far as possible the desired output waveform. However the output waveform may include a number of different harmonic components in addition to the base harmonic component at the desired output frequency. The quality of the output waveform can be improved by the addition of an output filter to remove higher order harmonics.

The apparatus may therefore further comprise a filter to filter the output of the cycloconverter.

Preferably the apparatus comprises a control unit for controlling the cycloconverter.

For example, where the cycloconverter comprises a plurality of thryistors, the control unit may be arranged to control the firing angles of the thyristors. The control unit may be arranged to control the RMS output of the cycloconverter, in order to produce an output with the required RMS voltage. In addition or alternatively the control circuit may be arranged to control the amount of distortion in the output signal by comparing the output signal to a reference signal. For example, by comparing the output signal to a reference signal, an error signal may be produced, and the error signal may be used to control the firing angles of the thyristors so as to reduce the amount of distortion in the output signal.

The control circuit may include a reference signal generator for producing a reference signal. In general, the reference signal may have a waveform which corresponds substantially to the waveform which it is desired to create at the output. For example, the reference signal may be substantially sinusoidal.

However, in one embodiment, the reference signal may be substantially flat during at least one part of its cycle. Preferably the flat part of the waveform is at the top or bottom part of the waveform, or both. For example, the reference waveform may have a substantially trapezoidal shape. This may allow the efficiency of the cycloconverter to be improved, by allowing better utilization of the input voltage and therefore better input power factor to the generator. The width of the flat part of the waveform may be adjustable so as to improve the efficiency of the cycloconverter, while mmimismg the impact on the output waveform.

In one embodiment of the invention, the speed of the generator is fixed. In this case the prime mover which drives the generator is operated at a constant speed. This arrangement may simplify the control mechanism, but may require a larger generator to support heavy step load applications, and may be less efficient.

In another embodiment of the invention the speed of the generator is variable. This can allow the prime mover which drives the generator to operate at the most efficient speed for a particular load, which may increase the overall efficiency of the apparatus.

In this embodiment, the apparatus may further comprise means for sensing an output voltage and/or current, and means for controlling the speed of the generator in dependence thereon. For example, the speed of the prime mover may be controlled, in order to control the speed of the generator.

The apparatus may thus further comprise a prime mover for driving the generator.

The prime mover may be, for example, a reciprocating engine, a gas turbine, or any other source of power generation.

The generator may be a permanent magnet generator, or a synchronous generator.

The generator preferably has at least four poles, and may have, for example, at least 6, 8, 10, 12, 24 or more poles. The generator may produce a three phase output, or it may produce a two phase output, or an output of any other number of phases.

The apparatus may comprise two cycloconverters connected in series. This may produce better output characteristics.

According to another aspect of the invention there is provided a method of generating an AC electrical power supply, the method comprising generating an AC output in a generator, and converting the output of the generator to a three phase AC output of a different frequency using a cycloconverter.

Any of the apparatus features may be provided as method features and vice versa.

Preferred features of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: Figure 1 shows an overview of an electrical generating apparatus according to the present invention; Figure 2 shows an example of a cycloconverter using a half bridge topology; Figure 3 shows an example of a cycloconverter using a full bridge topology; Figure 4 shows a three phase to one phase half bridge cycloconverter with an output filter; Figure 5 shows a three phase to one phase half bridge cycloconverter with an output filter and two equivalent inductors; Figure 6 shows a three phase to three phase half bridge cycloconverter with output filters and equivalent inductors; Figure 7 shows a three phase double converter; Figure 8 shows a simplified diagram of one phase of a cycloconverter; Figure 9 shows the firing angle modulation of the cycloconverter; Figure 10 is a block diagram of an RMS voltage controller; Figure 11 shows parts of an equivalent current controller; Figure 12 shows parts of a total harmonic distortion controller; Figure 13 is a diagram showing how the circuits of Figures 10, 11 and 12 connect together; Figure 14 shows parts of a variable speed electrical generating apparatus; and Figure 15 shows parts of a cycloconverter which produces a three phase output from a two phase input.

Figure 1 shows an overview of an electrical generating apparatus according to the present invention. Referring to Figure 1, a permanent magnet generator (PMG) 10 is driven by an engine (not shown) to produce a three phase AC output. The generator preferably contains a large number of poles, in order to provide an output voltage of relatively high frequency. For example the generator may have 24 poles.

The output of the generator 10 is fed to a cycloconverter 12. The cycloconverter converts the output of the generator to a three phase output having a frequency which is suitable for supply to a load 14. In this example, the cycloconverter produces a three phase AC output of 50Hz or 60Hz.

Figure 2 shows an example of a cycloconverter using a half bridge topology. In Figure 2, the three phase outputs Ux, Uy, U from the generator 10 are supplied to a bridge circuit consisting of thyristors Thxua, Thyua, Thzua, Thxub, Thyub, Thzub.

Thyristors Th, Tha, Thzija form a positive half bridge, while thyristors ThUb, Thb, Thb form a negative half bridge.

A thyristor is a device which, once switched on, will continue to conduct until the current through it is approximately equal to zero. If an AC waveform is applied to a thyristor, the point in the waveform at which the thyristor is switched on is known as the firing angle. If the firing angle is zero, then all of the input voltage is fed to the output. As the firing angle increases, the proportion of the input voltage which is fed to the output decreases. In Figure 2, the thyristors are turned on at the appropriate moments by a firing angle control circuit, in order to produce an AC signal of the required frequency and phase at the output.

Figure 3 shows an example of a cycloconverter using a full bridge topology. In Figure 3, a full bridge converter is formed from two half bridge converters of the type shown in Figure 2. As in Figure 2, the thyristors are turned on at the appropriate moments by a firing angle control circuit, in order to produce an AC signal of the required frequency and phase. The full bridge converter of Figure 3 allows the full cycles of the generator outputs to be used in producing the output waveforms.

Figure 4 shows a three phase to one phase half bridge cycloconverter with an output filter. The output filter is formed from inductor LF and capacitor CF, and is used to filter the output of the cycloconverter so as to reduce the amplitude of the harmonics produced by the cycloconverter.

Figure 5 shows a three phase to one phase half bridge cycloconverter with an output filter and two equivalent inductors. In Figure 5, a first inductor LEI is located between the positive half bridge and the output, while a second inductor LE2 is located between the negative half bridge and the output. The inductors are both inductors of the same value. The inductors are used to allow the flow of circulating current between the positive and negative bridges without short- circuiting them. This can allow the bridges to be operated simultaneously.

Figure 6 shows a three phase to three phase half bridge cycloconverter with output filters and equivalent inductors for each phase. In Figure 6, the three phase cycloconverter is formed from three single phase circuits of the type shown in Figure 5. The three phase outputs Ux, Uy, Uz from the generator are supplied to each of the three single phase circuits. Each single phase circuit produces the AC output signal for one of the output phases Z, Zv, Zw.

Figure 7 shows a three phase double converter comprising two full bridge cycloconverters connected in series. Such a topology may be used to achieve better performance than with a single converter.

Figure 8 shows a simplified diagram of one phase of a cycloconverter. In Figure 8, positive bridge 20 is formed from three thyristors, such as thyristors Thxua, Thyua, Thzua in Figure 2, and negative bridge 22 is formed from three thyristors, such as thyristors Th,,b, Thb in Figure 2. A control signal a1 is applied to the positive bridge 20, and is used to control the firing angle of the thyristors in the positive bridge.

Similarly, a control signal a2 is applied to the negative bridge 22, and is used to control the firing angle of the thyristors in the negative bridge.

Figure 9 shows the firing angle modulation of the positive and negative groups. The continuous bold waveform in Figure 9 shows the modulation of the firing angle of the positive group of thyristors. The waveform is formed by taking the reference signal, which is a sinusoid of unity amplitude and a frequency corresponding to the desired output frequency, and applying the arccosine function. The discontinuous bold waveform shows the modulation of the firing angle of the negative group.

As can be seen from Figure 9, the firing angle of the positive group of thyristors starts irJ2 (900). At this point half of the positive input voltage is fed to the output, since the thyristor will turn on half way through the positive half cycles of the input voltage.

As the angle of the reference signal increases to ir/2, the firing angle decreases to zero, at which point all of the positive input voltage is fed to the output. The firing angle then increases, passing through it/2 and going on to it. At this point the reference signal has the angle 3ir/2, and none of the input voltage is fed to the output. The firing angle then returns to irJ2, and the cycle repeats itself. The firing angle modulation of the negative group of thyristors is the same as the positive group, but phase shifted by it (180 ) so as to control the negative half cycles of the input waveform. By modulating the firing angles of the thyristors in this way, an output signal having a base harmonic component at the frequency of the reference signal is produced.

Figure 9 also shows how the RMS value of the output voltage can be adjusted. The thin lines in Figure 9 show the firing angle modulation with values of the amplitude A equal to 0.8, 0.6, 0.3 and 0.0. As the value of the amplitude A decreases from unity to zero, the shape of the modulation waveform changes, resulting in less of the input voltage being fed to the output, and thereby reducing the RMS voltage at the output.

Figure 10 is a block diagram of an RMS voltage controller, which is used to control the RMS voltage at the output of the cycloconverter of Figure 8. For simplicity, only a single phase is shown in Figures 8 and 10. However it will be appreciated that similar components are also provided for the other phases of the three phase output.

Referring to Figure 10, the voltage controller comprises reference signal generator 30, which generates a reference signal UREF representing the desired RMS output voltage. A voltage sensor 32 measures an instantaneous voltage VOLI at the output of the cycloconverter. RMS calculation unit 33 calculates an RMS value UgMS of the output voltage. The RMS value of the output voltage is compared to the reference voltage in comparator 34. The output of comparator 34 is thus an error signal representing a difference between the actual RMS output and the desired RMS output.

The error signal from comparator 34 is fed to P1 (proportional integral) regulator 36.

The P1 regulator 36 processes the error signal in accordance with a proportional integral control function in order to produce a control signal. Although in this example a proportional integral control function is used, other types of control, such as proportional, derivative, or any combination of proportional, integral and derivative, could be used instead.

The output of P1 regulator 36 is fed to firing angle control circuit 38. The firing angle control circuit 38 produces the control signals a1 and a2, which are used to control the firing angles of the thyristors in the positive and negative bridges 20, 22 of Figure 8.

The output of the P1 regulator 36 is used to adjust the value of the amplitude A, as shown in Figure 9, so as to produce an output voltage of the required RMS value.

Figure 11 shows parts of an equivalent current controller, which is used to enable selectively either the positive bridge 20 or the negative bridge 22 of Figure 8.

Referring to Figure 11, the equivalent current controller comprises a current reference signal generator 40, which generates a reference current signal Ipp. A current sensor 42 measures the current at the output of the cycloconverter. The sensed current is compared to the reference current in comparator 44 in order to produce a difference signal. The difference signal from the comparator is fed to bridge activating circuit 46. If the output current is greater than -REF then the positive bridge is activated, and if the output current is less than lp. then the negative bridge is activated. If the output current is less than -I or more than REF then both bridges are activated As an alternative, both bridges may be operated simultaneously. In this case both bridges are driven by modulation of the firing angle and there is no deactivating signal.

Figure 12 shows parts of a TI-ID (total harmonic distortion) controller, which is used to adjust the firing angles of the thyristors in the cycloconverter in dependence on the distortion in the output voltage. Referring to Figure 12, the control circuit comprises a reference signal generator 50 which generates a reference signal having a waveform corresponding to the waveform which it is desired to create at the output of the cycloconverter. The reference signal is fed to the positive input of comparator 52.

The instantaneous value of the output voltage Uo from a voltage sensor (such as voltage sensor 32 in Figure 10) is fed to the negative input of comparator 52. The output of comparator 52 is thus an error signal representing a difference between the actual instantaneous output of the cycloconverter and the desired output.

The error signal from comparator 52 is fed to P1 regulator 54. The P1 regulator 54 processes the error signal in accordance with a proportional integral control function in order to produce a control signal. As in the RMS control circuit of Figure 10, any type of control function, such as proportional, integral or derivative, or any combination thereof, could be used. The output of P1 regulator 36 is fed to firing angle control circuit 38, which produces the control signals a1 and a2 for controlling the thyristors in the cycloconverter. The output of the P1 regulator 54 is used to control the firing angles of the thyristors so as to produce an output voltage with a less distorted waveform.

Figure 13 is a diagram showing how the circuits of Figures 10, 11 and 12 connect together.

In an embodiment of the invention, the reference signal produced by the reference signal generator 50 is substantially trapezoidal. This can allow the efficiency of the cycloconverter to be improved, through better utilization of the input voltage. During the flat part of the reference signal, the value of the firing angle equals zero, which means that the thyristors are triggered at the earliest possible point. This results in a lower lagging of the current drawn from the generator with respect to the voltage, and thus in better efficiency. Although modifying the reference signal in this way may affect the waveform of the output voltage, the consequences of this may be removed by filtering.

The width of the flat part of the trapezoidal waveform may be controllable, in order to achieve optimum efficiency while minimising any impact on the output voltage.

Figure 14 shows parts of an electrical generating apparatus in which the speed of the generator is variable. Referring to Figure 14, engine 60 is mechanically coupled with generator 62 so as to drive the generator, thereby to generate a variable frequency and voltage AC output. The engine has a speed sensor 64 and a speed controller 66 which in this example includes an electronic fuel injection system. The speed sensor 64 produces an output which is indicative of the engine speed, and supplies this signal to speed controller 66. Speed controller 66 controls operation of the fuel injection system so as to control the speed of the engine.

The output of generator 62 is fed to cycloconverter 68. The cycloconverter converts the variable frequency AC output of the generator 68 to a fixed frequency AC output for supply to load 70. The cycloconverter 68 may take any of the forms described above.

Current and voltage sensor 72 senses the current and voltage of the AC output. The sensed current and voltage values are fed to cycloconverter control unit 74. The cycloconverter control unit controls the firing angles of the thryristors in the cycloconverter 68, in order to produce an AC output of the required frequency and voltage.

The sensed current and voltage values from sensor 72 are also fed to speed controller 66. Speed controller 66 controls the speed of the engine 60 in dependence on the power output of the cycloconverter. When the cycloconverter is lightly loaded, the speed of the engine can be reduced, thereby reducing fuel consumption. When the cycloconverter is more heavily loaded the speed of the engine can be increased in order to met the increased power demand. In this way, the engine can be run at the optimum speed for a particular load.

In another embodiment of the invention, two cycloconverters are connected in series to provide better output characteristics.

In another embodiment of the invention, a cycloconverter is connected to a two phase generator and produces a three phase output. Figure 15 shows such an arrangement.

In principle, any number of input phases may be used to produce a three phase output (or an output with any other number of phases). The principles of operation and control remain the same.

The system described above thus comprises a high frequency constant or variable speed permanent magnet generator and a three-phase cycloconverter (direct frequency converter) supplying the load with high quality power of the desired RMS voltage and frequency.

The system may have the following advantages in comparison to VSIG systems: * Low cost * High energy density switch * High power density generating set * Fewer energy conversion stages * Potentially at a market segment which is cost sensitive and can tolerate some THD (total harmonic distortion), and hence may eliminate the need for a filter The system may have the following advantages in comparison to conventional systems: * Variable engine speed -constant output frequency system * Adjustable output voltage and output frequency

Claims

1. Electrical generating apparatus comprising a generator for generating an AC output, and a cycloconverter for converting the output of the generator to a three phase AC output of a different frequency.

2. Apparatus according to claim 1, wherein the cycloconverter comprises a controllable bridge circuit.

3. Apparatus according to claim 2, wherein the bridge circuit comprises a plurality of thyristors.

4. Apparatus according to any of the preceding claims, wherein the cycloconverter comprises three bridge circuits, and each bridge circuit is arranged to produce one of the three output phases.

5. Apparatus according to any of the preceding claims, wherein the cycloconverter comprises a bridge circuit and an inductor located between the bridge circuit and the output.

6. Apparatus according to any of the preceding claims, wherein the cycloconverter comprises a positive bridge and a negative bridge, and a first inductor is located between the positive bridge and the output, and a second inductor is located between the negative bridge and the output.

7. Apparatus according to any of the preceding claims, wherein the cycloconverter comprises a positive bridge and a negative bridge, and the positive bridge and negative bridge are operated simultaneously.

8. Apparatus according to any of the preceding claims, further comprising a filter to filter the output of the cycloconverter.

9. Apparatus according to any of the preceding claims, further comprising a control unit for controlling the cycloconverter.

10. Apparatus according to claim 9, wherein the cycloconverter comprises a plurality of thyristors, and the control unit is arranged to control the firing angles of the thyristors.

11. Apparatus according to claim 9 or 10, wherein the control unit is arranged to control the RMS output of the cycloconverter.

12. Apparatus according to any of claims 9 to 11, wherein the control circuit is arranged to control the amount of distortion in the output signal by comparing the output signal to a reference signal.

13. Apparatus according to any of claim 9 to 12, wherein the control circuit includes a reference signal generator for producing a reference waveform which is substantially flat during at least one part of its cycle.

14. Apparatus according to claim 13, wherein the width of the flat part of the waveform is variable.

15. Apparatus according to any of the preceding claims, wherein the speed of the generator is fixed.

16. Apparatus according to any of claims 1 to 14, wherein the speed of the generator is variable.

17. Apparatus according to claim 16, further comprising means for sensing an output voltage and/or current, and means for controlling the speed of the generator in dependence thereon.

18. Apparatus according to any of the preceding claims further comprising a prime mover for driving the generator.

19. Apparatus according to any of the preceding claims, wherein the generator is a permanent magnet generator.

20. Apparatus according to any of the preceding claims, wherein the generator has at least four poles.

21. Apparatus according to any of the preceding claims, wherein the generator produces a three phase output.

22. Apparatus according to any of claims 1 to 20, wherein the generator produces a two phase output.

23. Apparatus according to any of the preceding claims, wherein two cycloconverters are connected in series.

24. A method of generating an AC electrical power supply, the method comprising generating an AC output in a generator, and converting the output of the generator to a three phase AC output of a different frequency using a cycloconverter.

25. Apparatus substantially as described herein with reference to and as illustrated in the accompanying drawings.

26, A method substantially as described herein with reference to the accompanying drawings.