GB2408638A

GB2408638A - Starter arrangement for a generator system

Info

Publication number: GB2408638A
Application number: GB0323682A
Authority: GB
Inventors: John Lyons
Original assignee: Bowman Power Systems Ltd
Current assignee: Bowman Power Systems Ltd
Priority date: 2003-10-09
Filing date: 2003-10-09
Publication date: 2005-06-01
Also published as: GB0323682D0

Abstract

The invention provides a generator system comprising a power conditioning unit (14) for receiving and converting an output from an alternator (12) for providing a generator output, and a starter circuit (50) for driving the alternator as a starter motor during start-up, characterised in that the starter circuit 50 comprises an inverter (52) arranged to receive an output from the power conditioning unit 14 for acting as a motor drive for driving the alternator 12 as a starter motor. The invention also provides a method of operating the generator system comprising normally employing the power conditioning unit 14 for receiving and converting an output from the alternator 12 for providing a generator output, and during start-up driving the alternator 12 as a starter motor by applying an output from the power conditioning unit 14 to the starter circuit to provide a motor drive for driving the alternator 12 as a starter motor.

Description

STARTER ARRANGEMENT FOR A GENERATOR SYSTEM

This invention relates to a generator system and to a method of operating such a generator system. More particularly, the invention concerns a starter arrangement for a generator system and a method of employing such a starter arrangement.

Figure 1 shows an example of a conventional electrical generator system, comprising a prime mover, in the form of a gas turbine engine 10, mechanically coupled to a permanent-magnet poly-phase alternator 12.

Electrically connected to the alternator is a power-conditioning unit 14 for converting the output voltages at the terminals of the alternator to the voltage waveforms required at the output of the generator system.

Power conditioning units contain inverters or converters and are used to supply AC or DC power either as voltage-drives into a load or as currentdrives for example in the AC case into a power distribution network. Most power conditioning units operate in a switched-mode and contain output filters on one or more output lines. The purpose of the output filters is to reduce high frequency components of voltage and/or current at the output terminals produced by the switching action.

More especially, inverters/converters that operate at substantial power ratings are generally required to maintain good efficiency, and are frequently required also to offer good output waveform quality. In this context, it will be understood that the term 'waveform' embraces either periodic AC or continuous DC output. These requirements mean that the inverters/converters operate in a switched mode in order to minimise internal losses and have many switching operations per second so as to permit detailed control of the output waveform. A well-known example of a switched-mode technique for shaping the output waveform is pulse-width modulation (PWM), which typically works with IGBTs as the switched output devices and employs modulating frequencies up to about 15 kHz. The switching frequency is sometimes fixed and sometimes caused to vary with time, as it may for example throughout the cycle of an AC waveform. To achieve high waveform quality it is necessary to include an output filter in each electronically driven output line to remove the majority of the high-frequency ripple harmonics in the output voltage and/or current waveform that are associated with the switching action. A typical output filter for one line comprises a series inductor connected to the appropriate output point of the inverter/converter, followed by a parallel capacitor, with the return end of the capacitor being connected either to a neutral point or to one or both rails of a DC link. The inductor may be a separate inductor for each output line that contains an output filter or there may be a multi-winding combined inductor that provides from a single structure the required inductance per line for all or some of the lines. The design of output filters of the type described is generally well understood.

Such a power-conditioning unit is shown in greater detail in Figure 2 and comprises a rectifier circuit 16 connected across the voltage bus to take the alternator voltages as input and produce a DC link voltage as output.

Connected to the output of the rectifier are a boost converter 18 including an inductor 20 and IGBT 22 (see Figure 4) for boosting the DC link voltage, for example from 500 volts DC to 750 volts DC, and a PWM inverter 24 that takes the boosted DC link voltage as input and produces at its output three-phase voltages. Typically, the PWM inverter 24 comprises three switching channels connected across the voltage bus and each including an upper and a lower switching device in the form of an IGBT 24a and anti-parallel diode 24b (see Figure 4). As mentioned, the output voltages typically contain a large content of high-frequency ripple associated with the PWM action. Therefore, connected to the output of the PWM inverter 24 is a three-phase series inductance/parallel- capacitance (L-C) output filter 26 that removes the majority of the high- frequency ripple. An isolator switch 28 is provided at the output of the output filter 26 between the output filter 26 and an external utility/load.

The electrical generator of Figures 1 and 2 is commonly referred to as a 'micro turbine-generator', MTG, which is able to provide a relatively small amount of power typically from a few kW to a few MW. An advantage of micro-turbine generators is that instead of having a few generators of very large power rating at fixed locations and transmitting power over large distances for distribution to consumers, the power is generated in a multiplicity of generators of smaller power rating which are spread locally among consumers. These generators may operate to provide power to local loads or to a multiplicity of points in a distribution network or to a parallel combination of local loads and a distribution network. It should be noted that the output characteristic of the generator inverter in this last case is typically current-drive when there is connection to the distribution network, changing rapidly to voltage-drive if this connection is lost.

In this example, the generator has three phases, each of which provides an output to a corresponding one of the power lines L1, L2 and L3 to the customer, and a neutral N (not shown), which is in turn connected to the corresponding neutral line of the load. It is alternatively possible for the generator to have six phases, in which case the illustrated circuits will be supplemented by duplicate circuits providing three further power lines.

In order to provide power to the generator for start-up purposes, a starter circuit is conventionally provided as shown in Figures 3 and 4 for driving the alternator 12 to act as a starter motor to start the gas turbine engine 10. The starter circuit 30 is situated between the load/utility and the alternator 12 in parallel with the power conditioning unit 14 and is arranged by way of a mains switch 32 to receive power from the utility and to control the supply of current to drive the alternator 12 so as to build up the speed of the engine 10 gradually.

Referring to Figure 4, the starter circuit 30 may be seen to comprise a rectifier circuit 34 connected to the mains switch 32 to supply at its output a DC voltage which may typically be of the order of 650 volts. A buck converter 36 comprising an integrator circuit provides a controlled output to an inverter circuit 38 serving as a motor drive to drive the alternator 12, which acts as a starter motor for the gas turbine engine 10. A pair of diodes 40, 42 are connected between the buck converter 36 of the starter circuit 30 and the boost converter 18 of the power conditioning unit 14 in order, during start-up, to begin charging a capacitor 44 of the boost converter 18 and, during normal operation, to isolate the capacitor 44 of the boost converter 18 from the starter circuit 30. A speed sensor (not shown) is provided in order to detect the moment when the gas ignition in the gas turbine engine 10 takes over from the motor drive from the alternator 12, and to generate a signal to switch off the starter circuit 30.

The above arrangement suffers from a number of disadvantages, one of which is that the need for an entirely separate starter circuit, and particularly the start inverter, is costly and ineff cient in terms of manufacture, normal operation and maintenance.

The present invention seeks to overcome these problems by providing an improved starter arrangement.

In particular, the present invention seeks to overcome these problems by combining certain aspects of the starter circuit and the power conditioning unit thereby to reduce the overall number of components.

The present invention also seeks to overcome the above problems by eliminating the need for separate components in some areas of the starter circuit by employing certain components from the power conditioning unit during the start-up mode.

As a result, duplication may be avoided, with consequent advantages in terms of cost and efficiency.

According to the present invention, there is provided a generator system comprising a power conditioning unit for receiving and converting an output from an alternator for providing a generator output, and a starter circuit for driving the alternator as a starter motor during start-up, characterised in that the starter circuit comprises a separate inverter arranged to receive an output from the power conditioning unit for acting as a motor drive for driving the alternator as a starter motor.

Preferably, the power conditioning unit comprises a converter arranged to act as a buck converter during start-up and as a boost converter during normal operation. The starter circuit inverter may then be arranged to receive an output from the buck converter for acting as a motor drive.

Advantageously, the power conditioning unit also comprises an inverter arranged to act as a rectifier circuit during start-up.

According to another aspect of the present invention, there is provided a method of operating a generator system comprising normally employing a power conditioning unit for receiving and converting an output from an alternator for providing a generator output, and during start-up driving the alternator as a starter motor, characterized by applying an output from the power conditioning unit to a starter circuit comprising an inverter to provide a motor drive for driving the alternator as a starter motor.

The invention may thus significantly reduce the number of components required in a generator system for start-up purposes, as well as eliminating the need for certain components altogether, which is advantageous in terms of cost both of manufacture and of maintenance.

The invention is described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of a conventional generator system; Figure 2 is a block diagram of a power conditioning unit of the generator system of Figure 1; Figure 3 is a block diagram showing a start-up circuit for the generator system of Figure 1; Figure 4 is a circuit diagram showing the starter circuit and power conditioning unit of the generator system; Figure 5 is a block diagram of a generator system according to the present invention; Figure 6 is a circuit diagram of the generator system of Figure 5; and Figure 7 is a graph representing operation of a gas turbine engine of the generator system during start-up in accordance with the present invention.

Referring to Figures S to 7, the present invention will now be described. The generator system according to the present invention operates in substantially the same way as the generator system of Figures 1 and 2 during normal operation, and its construction is also similar. Like parts will be designated by the same reference numerals.

Thus, the generator system shown in Figures S and 6 comprises a gas turbine engine 10, an alternator 12 and a power conditioning unit 14 as described above. The power conditioning unit 14 features a rectifier circuit 16 for receiving the three phase alternator voltage as input and for supplying a DC link voltage as output. Connected to the output of the rectifier circuit 16 is a converter 48 arranged as before to boost the DC link voltage, for example from 500 volts DC to 750 volts DC. A PWM inverter 24 receives the boosted DC link voltage and generates as output three phase voltages, which are filtered in an output filter 26 to remove the majority of the high frequency ripple associated with the PWM action. The filtered output is then supplied by way of an isolator switch 28 to an external utility/load.

In accordance with the present invention, the generator system of Figures 5 and 6 also comprises a starter circuit 50, which is connected between the converter 48 and the alternator 12. The starter circuit 50 comprises simply a start inverter 52, including a capacitor 53. The converter 48 is then modified accordingly, by comparison with the boost converter 18 or the buck converter 36 of Figures 1 to 4, in order to act as a boost converter during normal operation and a buck converter during start-up.

More especially, the converter 48 includes components of the boost converter 18 and the buck converter 32 from Figure 4 effectively superimposed on one another to provide the operating characteristics of both in one circuit.

Accordingly, the boost converter 48 comprises on the one hand an inductor 54 in series in the positive supply line of the voltage bus with a parallel connection of an IGBT 56 and a diode 58 so as to act as a buck converter in the start-up mode; and on the other hand the inductor 54 and a parallel connection of an IGBT 60 and a diode 62 arranged across the voltage bus so as to act as a boost converter during normal operation. Thus, in the start-up mode of operation, the converter 48 acts as a buck converter due to operation of the inductor 54 and IGBT 56 and, in the normal mode of operation, the converter acts as a boost converter through operation of the inductor 52 and IGBT 60. In each case, the respective diode 58 or 62 effectively serves to isolate the associated IGBT 56 or 60 during the other operational mode in which the IGBT is not required.

The inverter 52 is connected between the two lines of the voltage bus by way of diodes 64, 66. The arrangement is such that the inverter 52 receives DC voltage from the converter 48 acting as a buck converter during the start-up mode, and is isolated from the converter 48 acting as a boost converter during the power mode.

Operation of the generator system according to the present invention will now be described with reference to Figures 5 to 7.

In the start-up mode, three phase voltages are supplied from the utility through the switch 28 to the inverter 24, which in this instance acts as a rectifier circuit with the anti-parallel diodes 24b serving to rectify the input voltages to provide a DC voltage as output. This DC voltage is supplied to the converter 48, acting as a buck converter, which begins to charge the capacitor 53 of the start inverter 52. The start inverter 52 generates a three phase output for supply to the alternator 12 serving as a starter motor to drive the gas turbine engine 10.

Referring to Figure 7, this state of affairs prevails initially until time To when gas ignition commences, and then continues with the converter 48 acting as buck converter to control the speed of the gas turbine engine until time T2 when the gas ignition takes over completely. At this point, the generator system is now operating in its normal power mode. s

In the power mode, the alternator 12 supplies three phase voltages to the rectifier circuit 16 to produce a DC link voltage as output. The capacitor 44 forming part of the rectifier circuit 16 has already begun charging in the start mode through the action of the converter 48 as buck converter. In the power mode, the diodes 64, 66 serve to isolate the start inverter 52 from this DC link voltage, which is now supplied as input to the converter 48 acting as boost converter through operation of the inductor 54 and the IGBT 60. A smooth boost is thus ensured from the beginning of the power mode. The converter 48 boosts the DC link voltage and supplies this as input to the inverter 24 which generates three phase voltages for output through the filter 26 as is known.

Consequently, the present invention effectively serves to reconfigure a conventional power conditioning unit 14 during start-up so as to employ the alternator 12 as a starter motor for starting the gas turbine engine 10. A completely separate self contained starter circuit or motor drive unit is no longer required, which has the advantage of significantly reducing the costs of manufacture and operation. The main power converter output stage (the inverter 24) is used as a rectifier circuit during start-up, which eliminates the need for a separate start-up rectifier circuit. Further, the boost converter of the power conditioning unit 14 is employed during start-up as a buck converter, which eliminates the need lor a separate buck converter. An added advantage of this reduction in components is that the size of the heat sink normally required for such a generator system can be significantly reduced.

Claims

1. A generator system comprising a power conditioning unit for receiving and converting an output from an alternator for providing a generator output, and a starter circuit for driving the alternator as a starter motor during start up, characterised in that the starter circuit comprises an inverter arranged to receive an output from the power conditioning unit for acting as a motor drive for driving the alternator as a starter motor.

2. A generator system according to claim 1, characterized in that the inverter of the starter circuit is separate from the power conditioning unit.

3. A generator system according to claim 1 or 2, characterized in that the power conditioning unit comprises a converter arranged to act as a buck converter during start-up, and in that the starter circuit inverter is arranged to receive an output from the buck converter for acting as a motor drive.

4. A generator system according to claim 3, characterized in that the converter is arranged to act as a boost converter during normal operation.

5. A generator system according to claim 3 or 4, characterized in that the converter comprises an inductor connected to a first IGBT so as to provide a buck circuit and the inductor connected to a second IGBT so as to provide a boost circuit.

6. A generator system according to any preceding claim, characterized in that the power conditioning unit comprises an inverter arranged to act as a rectifier circuit during start-up.

7. A generator system according to any preceding claim, characterised by rectifier diodes arranged to isolate the starter circuit during normal operation. s

8. A method of operating a generator system comprising normally employing a power conditioning unit for receiving and converting an output from an alternator for providing a generator output, and during start-up driving the alternator as a starter motor, characterised by applying an output from the power conditioning unit to a starter circuit comprising an inverter to provide a motor drive for driving the alternator as a starter motor.

9. A method according to claim 8, characterised by employing a converter of the power conditioning unit as a buck converter during start-up, the starter circuit inverter being arranged to receive an output from the buck converter for acting as a motor drive.

10. A method according to claim 9, characterised by employing the converter as a boost converter during normal operation.

11. A method according to any preceding claim, characterised by employing an inverter of the power conditioning unit as a rectifier circuit during start-up.

12.A method according to any of claims 8 to 11, characterised by isolating the starter circuit during normal operation.