EP3127205A1

EP3127205A1 - Power generation system

Info

Publication number: EP3127205A1
Application number: EP15714614.3A
Authority: EP
Inventors: Robert Seliga; Emil Ernest; Krzysztof PACIURA; Neil Brown
Original assignee: Cummins Generator Technologies Ltd
Current assignee: Cummins Generator Technologies Ltd
Priority date: 2014-04-04
Filing date: 2015-03-25
Publication date: 2017-02-08
Also published as: WO2015150740A1; GB201406098D0

Abstract

A power generation system is disclosed comprising an electrical generating set (16) and a power module (20) for supplying power from a supplementary power source (22). The power module comprises a DC to AC converter (26). The power generation system comprises means (32) for measuring an output of the electrical generating set, and a controller (30) for controlling the DC to AC converter, wherein the controller is arranged to control the DC to AC converter (26) in dependence on a measure of the output of the electrical generating set (16), thereby to improve the quality of the load seen by the electrical generating set. This may improve the efficiency and stability of the generating set.

Description

POWER GENERATION SYSTEM

The present invention relates to a power generation system, and in particular to a power generation system comprising an electrical generating set and a power module for supplying power from a supplementary power source.

Electrical generating sets (gensets) comprise a prime mover coupled to a generator for generating electricity. Generating sets have many applications, including use in situations where there is no power grid, or as an emergency power supply if the grid fails. Generating sets may also be used for more complex applications such as peak-lopping, grid support and export to the power grid.

The prime mover in a generating set is usually an internal combustion engine supplied with fuel in the form of gas, diesel, petrol, biofuel, etc. The generator is usually a wound field synchronous generator. The generator converts

mechanical energy produced by the prime mover to electrical energy and supplies this energy in the form of active power to the load. The generator also needs to produce reactive power which is not converted by the load to useful energy but is needed by the load to be able to convert the active power to another form of energy.

Although synchronous generators are able to supply reactive power, any inductive load will lead to a drop in efficiency due to increased losses in both the excitation and armature circuits. Furthermore, any other reduction in the quality of the power caused by, for example, unbalanced loads and higher harmonics will reduce the efficiency of the generator. In addition, capacitive loads can lead to problems with voltage stability supplying the loads. Recently the commercial availability of alternative energy sources such as wind and solar power has made them attractive not only as grid-tied sources, but also as supplementary power sources for autonomous AC micro-grids powered by electrical generating sets. Increasing fossil fuel cost and demand to reduce generating set fuel consumption has led to solutions whereby the generating set is assisted by a supplementary power source. In such systems a power module is normally provided to convert the output of the supplementary power source to the required AC output. Typically such power modules include a DC/AC converter such as a sine-wave voltage source inverter (SVSI). According to one aspect of the present invention there is provided a power generation system comprising an electrical generating set and a power module for supplying power from a supplementary power source, the power module comprising a DC to AC converter, the power generation system comprising:

means for measuring an output of the electrical generating set; and a controller for controlling the DC to AC converter, wherein the controller is arranged to control the DC to AC converter in dependence on a measure of the output of the electrical generating set, thereby to improve the quality of the load seen by the electrical generating set. The present invention may provide the advantage that a power module which is provided for supplying power from a supplementary power source can also be used to improve the quality of the load seen by the electrical generating set. This in turn may improve the efficiency and/or stability of the generating set. Usually the ideal output of an electrical generating set would be at unitary power factor (i.e. with the voltage and current in phase). Furthermore, the output would ideally be at a single frequency (with a waveform in the shape of a sine wave). In addition, in the case of a multiphase system, all phases would ideally be balanced (have equal loads). Thus by improving the quality of the load it is preferably meant that the power conditioner adjusts at least one parameter of the load seen by the electrical generating set such that the output of the electrical generating set is closer to the ideal output than would otherwise be the case. This may be achieved, for example, by increasing the power factor, reducing one or more higher harmonics, and/or reducing any difference in the loads on different phases as seen by the electrical generating set.

In one embodiment the means for measuring an output of the electrical generating set is arranged to measure the reactive power at the output of the generating set. The controller may then be arranged to control the DC to AC converter in dependence on the measure of reactive power so as to reduce the reactive power supplied by the generating set. This may lead to increased efficiency in the generator and/or increased voltage stability.

Thus the controller may control the amount of reactive power supplied by the DC to AC converter to increase the power factor seen by the generating set, for example, to close to unity.

The means for measuring an output of the electrical generating set may be a power measurement circuit, which may be connected to the output of the generating set. For example, the power measurement circuit may comprise a voltage sensor and a current sensor for measuring instantaneous values of voltage and current at the output of the generating set, and means for deriving a measure of active and/or reactive power therefrom. The power module may further comprise a reactive power reference signal generator. In this case the measure of reactive power may be fed to the reactive power reference signal generator. The reactive power reference signal generator may generate a reactive power reference signal, which may vary in dependence on the measure of reactive power. The reactive power reference signal may be used by the controller to control the converter. Thus the reactive power reference signal generator may be arranged to generate a reactive power reference signal for use by the controller to control the DC to AC converter based on the measure of reactive power. For example, the converter may be arranged to produce an AC output having the appropriate phase shift between the output voltage and the output current, in dependence on the reactive power reference signal, in order to reduce the reactive power seen by the generating set.

The means for measuring an output of the electrical generating set may also be arranged to measure the active power at the output of the generating set. In this case the power module may further comprise an active power reference signal generator, which may generate an active power reference signal for use by the controller to control the converter based on a measure of active power. This can allow the power module to supplement the active power output by the generator, for example using the output of a supplementary power source. The power generation system may be a multi-phase system arranged to generate two, three or more output phases. In this case the means for measuring an output of the electrical generating set may be arranged to measure an output of each phase. For example, the active and/or reactive power on each phase may be measured. The controller may then be arranged to control the DC to AC converter so as to reduce an imbalance between the phases. For example, where a different load is applied to each phase, the converter may be used to reduce the imbalance in the loads as seen by the electrical generating set. This may be achieved by separately controlling each phase so that the generator sees effectively the same load. This may help to improve the efficiency of the electrical generating set.

In some circumstances a load which is connected to the output of the power generation system may cause higher harmonics (i.e. harmonics which are above the fundamental frequency of the supply) to be present. This may lead to unwanted currents flowing in the windings of the generator, reducing its efficiency. In another embodiment of the invention, the means for measuring an output of the electrical generating set is arranged to measure the power (active and/or reactive) of at least one higher harmonic in the output. The controller may then be arranged to control the DC to AC converter so as to reduce the higher harmonic.

Preferably the power module is operable in parallel with the electrical generating set for use in supplying power to a load. In this case the power module may be arranged to support the generator, for example during transient conditions. This may help to improve the quality of the power which is supplied to the load. For example the power module can help to ensure that the correct voltage is supplied to the load, and/or help to reduce unwanted harmonics, and/or help to ensure that the phases are balanced. In addition, the power module can facilitate the integration of supplementary energy sources, such as renewable energy sources and/or energy storage systems.

The DC to AC converter may be an inverter, such as a voltage source inverter or a current source inverter. Since an inverter typically has a faster dynamic response than a generator, the inverter may be able to respond to changes in the load more quickly than the generator. This may enable the power module to support the generator during transient conditions, which may help to improve the quality of the power supplied to the load. In a preferred embodiment, the DC to AC converter is a sine wave voltage source inverter, although other types of inverter could be used instead.

The DC to AC converter may be arranged to receive an input from a source of electrical power. In one embodiment the source of electrical power is a supplementary energy source. For example, the source of electrical power may be a renewable energy source, such as a solar, wind, wave or waste heat recovery energy source, or some other energy source such as a fuel cell or an auxiliary generator. In some cases the supplementary energy source may produce an AC output. In this case the AC output may be converted to an intermediate DC for supply to the power module. This may be achieved, for example, using an AC to DC converter such as a rectifier, and optionally a DC to DC converter. In another embodiment the source of electrical power comprises energy storage, such as a battery or a capacitor. The energy storage may be provided on its own or in combination with a supplementary energy source.

Where the source of electrical power comprises energy storage, the converter may be a bidirectional converter, such as a bidirectional DC/AC converter. This can allow the energy storage to be charged, for example from the generating set and/or from a grid, for example during periods of low load.

In one embodiment the power module is part of a back-up power system. In this embodiment the source of electrical power comprises energy storage such as one or more batteries and/or one or more capacitors. This can allow the power module to support the generating set during transients, as well as providing other advantages such as fuel savings and help with overload situations. The energy storage may be charged by the generating set when it is not supplying its full rated power, and used to support the generating set when it is at or near to full power.

In another embodiment, the power module is operable to control the amount of reactive power supplied by the DC to AC converter without receiving an input from a source of electrical power. For example, the power module may be disconnected from any source of electrical power, or it may be connected to a power source which is not supplying any electrical power. For example, in the case of renewable energy sources such as solar or wind power, the supply of power may be intermittent, and there may be periods of time in which little or no power is supplied. In these situations the power module may still be operable to control the amount of reactive power it supplies. This may be achieved, for example, by having one or more suitably sized capacitors connected on the DC side of the DC to AC converter, which may act as temporary storage during different parts of the AC cycle.

The electrical generating set may comprise a prime mover and an electrical generator. The prime mover may be an internal combustion engine such as a diesel engine, or any other source of rotating mechanical energy. The electrical generator may be a wound field synchronous generator, or any other type of generator such as a permanent magnet generator. The electrical generator may produce a 3-phase output, although a single phase output or any other number of phases could be used instead. Preferably a 3-phase, 4-wire topology is used with a neural wire between the generator and the power module.

According to another aspect of the present invention there is provided a method of operating a power generation system comprising an electrical generating set and a power module for supplying power from a supplementary power source, the power module comprising a DC to AC converter, the method comprising: measuring an output of the electrical generating set; and

controlling the DC to AC converter in dependence on a measure of the output of the electrical generating set, thereby to improve the quality of the load seen by the electrical generating set. The reactive power supplied by the DC to AC converter may be controlled so as to reduce one or more of reactive power, higher harmonics and phase imbalance as seen by the electrical generating set. According to another aspect of the present invention there is provided a power conditioner for use with an electrical generating set, the power conditioner comprising:

a DC to AC converter;

a controller for controlling the DC to AC converter; and

means for obtaining a measure of reactive power supplied by the generating set;

wherein the controller is arranged to control the DC to AC converter in dependence on the measure of reactive power so as to control a reactive power supplied by the DC to AC converter.

According to another aspect of the present invention there is provided a method of operating a power conditioner which supplies electrical power in parallel with an electrical generating set, the method comprising:

obtaining a measure of reactive power supplied by the generating set; and controlling a DC to AC converter in dependence on the measure of reactive power so as to control a reactive power supplied by the DC to AC converter.

Features of one aspect of the invention may be provided with any other aspect. Apparatus features may be provided with method aspects 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 a typical power generation system ;

Figure 2 shows a possible topology of a hybrid generating system ;

Figure 3 shows an overview of a power generation system in which a power conditioner is provided in conjunction with a generating set;

Figure 4 shows a possible configuration of a power conditioner;

Figure 5 shows a power conditioner integrated as part of a generating set; Figure 6 shows a simplified circuit diagram and control structure of a sine- wave voltage source inverter;

Figure 7 shows an embodiment of a power generation system with reactive power sharing; and

Figure 8 shows a power conditioner in an embodiment of the invention.

Overview of a power generation system

Conventional stand-alone distributed generation power systems are based on a diesel engine driving a wound-field synchronous generator. Depending on the application, this kind of generating set may operate in stand-by mode to supply critical loads in case of main distribution grid failure, or in prime-power mode as the main source of power to utility loads. The engine and generator run at a fixed speed of typically 1500 rpm or 1800rpm to provide the required 50Hz or 60Hz AC voltage to load. However the fixed speed operation requirement leads to low engine fuel efficiency and faster engine wear out, particularly when long-lasting light loads are fed from the generating set. One commonly used solution to reduce those drawbacks is to parallel different size generator sets and to switch them on or off when load demand changes. Although this enables some fuel savings, in most cases alternative solutions would be desirable.

Figure 1 shows a typical power generation system in which a number of power sources and loads are connected together. In the arrangement of Figure 1 , a first generating set 16, a second generating set 17, a load 15 and a utility grid 18 are all connected at a point of common coupling (PCC) 14. However in practice any appropriate mix of AC power sources and loads can be connected together.

Referring to Figure 1 , the generating set 16 comprises a prime mover 10 connected via a coupling 1 1 to a generator 12. The prime mover 10 drives the generator 12 at a fixed speed to produce a fixed frequency AC output. The output of the generator is connected via a circuit breaker 13 to the point of common coupling (PCC) 14.

The generator 12 converts mechanical energy produced by the prime mover 10 to electrical energy and supplies this energy in form of active power, or real power, P (Watts) to the load and/or grid. The generator also needs to produce reactive power Q (VAr) which is not converted by the load to useful energy but is needed by the load to be able to convert the real power to another form of energy. The extent to which a reactive load is present in the system is usually expressed as the power factor. The power factor is the ratio of the active power to the apparent power. The power factor is unity when the voltage and current are in phase. In the case of a reactive load, the power factor can be lagging (inductive load) or leading (capacitive load).

The prime mover 10 is usually an internal combustion engine supplied with fuel in the form of gas, diesel, petrol, biofuel, etc. The engine speed is fixed across the entire load range (no-load to full load). In most industrial applications the generator is a wound field synchronous generator. Although wound field synchronous generators are able to supply reactive power within their design limits, any inductive load will lead to a drop in efficiency due to increased losses in both the excitation and armature circuits. In addition, capacitive loads can lead to problems with voltage stability supplying the loads.

Hybrid generating systems

Recently the commercial availability of alternative energy systems (photovoltaic, wind, fuel cell, energy storage) have made them attractive not only as grid-tied sources, but also as supplementary power sources for autonomous AC micro- grids powered by diesel generating sets. Increasing fossil fuel cost and demand to reduce generating set fuel consumption has led to solutions whereby the generating set is assisted by an alternative power source. In this kind of hybrid generating system additional power may come from, for example, a renewable energy source (e.g. wind, photovoltaic solar, fuel cell), an engine waste heat recovery (WHR) module or a battery storage bank. For example, WO

2012/136964 in the name of the present applicant, the subject matter of which is incorporated herein by reference, discloses a power generation system in which power electronics are used to facilitate waste heat recovery and the connection of external energy sources. A common feature of systems including supplementary power sources is the presence of a DC/AC sine-wave voltage source inverter (SVSI) which is coupled to the wound field synchronous generator (WFSG). In a typical generating set, the amplitude of the output voltage is controlled by an automatic voltage regulator (AVR) and brushless excitation system (BES), while the output voltage frequency is controlled mechanically by the engine speed governor. Such arrangement results in a slow dynamic response. By contrast, an inverter has an inherently fast dynamic operation.

A possible topology of a hybrid generating system is shown in Figure 2.

Referring to Figure 2, a stand-alone generator set 16 comprises a diesel engine 10 connected to a wound field synchronous generator 12. The generator set 16 is assisted by a power module 20 to supply various loads 15 connected to a local autonomous AC-bus configured as a 3-phase, 4-wire micro-grid. The power module 20 consists of a sine-wave voltage source inverter 26 which can be powered by an energy source 22. The energy source 22 may a supplementary energy source such as photovoltaic panels, wind generator, etc. or energy storage such as a battery bank. Although in Figure 2 the energy source 22 is shown conceptually as part of the power module, in practice the energy source may be external to the power module. In this case the power module may comprise terminals for connection to the external energy source. Power conditioner

In embodiments of the invention, a power module such as that described above functions as a power conditioner. The aim of the power conditioner is to adjust the load on the generating set, thereby to improve the quality of the load which is seen by the generating set. In addition, the power conditioner can facilitate the integration of supplementary energy sources, such as solar or wind energy sources, and/or energy storage systems. The power conditioner may use the topology of the power module described above, or some other topology.

Figure 3 shows an overview of a power generation system in which a power conditioner is provided as an add-on to the generating set. Referring to Figure 3, the power conditioner 20 may be connected to an electrical energy source 22. The electrical energy source 22 may be a supplementary energy source, for example from renewable energy such as wind, photovoltaic solar, fuel cells, or engine waste heat recovery. For example, an engine waste heat recovery system such as that disclosed in WO 2012/136964 could be used. The electrical energy source 22 may also comprise energy storage, such as batteries or capacitors, instead of or in addition to a supplementary energy source.

Alternatively, as indicated by the dashed line, the power conditioner 20 may operate without being connected to an electrical energy source.

The other side of the power conditioner 20 is connected at connection point CP1 to the output of the generator 12 via the circuit breaker 13. The connection point CP1 is connected to the point of common connection 14 via isolator/disconnector 24.

Figure 4 shows a possible configuration of the power conditioner 20. The power conditioner 20 in this embodiment comprises a DC/AC converter 26, a filter 28, and a controller 30. The DC/AC converter 26 is a two-level inverter module comprising six insulated gate bipolar transistors (IGBTs) Ti - T₆ distributed in three legs, with an anti-parallel diode connected across each transistor.

Capacitors C_dc are connected across the DC side of the inverter. Each of the transistors is controlled with a control signal from the controller 30. The controller 30 receives inputs 31 from various sensors, such as current and voltage sensors at the output of the DC/AC converter.

One side of the DC/AC converter 26 is connected to an electrical energy source 22 via connectors DC+, DC- and N. In this example, the electrical energy source 22 is a DC energy source such as a set of batteries. The other side of the DC/AC converter 26 is connected to the filter 28, which in turn is connected to the connection point CP1 in Figure 3 via connectors U, V, W and N.

If desired, the converter 26 can be configured as a bidirectional DC/AC converter. In this case, depending on the operation mode, the converter acts as a rectifier or as an inverter. This can allow the energy storage to be charged, for example during periods of light load. Alternatively separate rectifier and inverter circuits could be provided.

In the arrangement shown in Figures 3 and 4, a 4-wire topology is used with a neural wire connected to a neutral point of the filter and to the midpoint of the DC side of the power conditioner. This can allow effective control and simple integration of energy storage with batteries to the generating set system.

Figure 5 shows a similar arrangement in which the power conditioner is integrated as part of the generating set. This can allow the control systems for the generating set and the power conditioner to be integrated into one system.

As shown in Figures 3 and 5, the connection point CP1 is considered

conceptually as part of the generating set 16. In general the concepts described above involve introducing a power conditioner to the generating set as an integrated or add-on module which is transparent from the perspective of the load or utility grid.

Hybrid operation

In embodiments of the invention, a hybrid system as described above can supply loads with the following modes of operation.

1 . Genset only operation

In this mode of operation the diesel engine 10 drives the wound field synchronous generator 12, delivering active power P_g and reactive power Q_g to the micro-grid connected loads 15. The diesel engine's governor controller maintains the engine at a fixed speed, and the generator's AVR regulates the AC-bus voltage RMS value, trying to keep it within the limits when loads are changing. Total system output active power P_t is equal to P_g, and output reactive power Q_t is equal to Q_g. In this mode the power module is disconnected from the micro-grid and is in an OFF state as a result of insufficient energy being available from the supplementary energy source.

2. Hybrid load sharing operation

In this mode of operation, both the generating set and the power module are in the ON state and are connected to the micro-grid. Loads are supplied from the generating set 16 and from the power module 20. Both subsystems deliver active and reactive power to the AC-bus. In this mode the total output active power P_t and reactive power Q_t demanded by the loads is shared by both subsystems. This mode is primarily used when the supplementary energy source has a sufficient time energy level E_s to support the generating set in periods of time when high load demand occurs. However, in embodiments of the invention, if the E_s level is low, the power module can still operate but its primary function is to supply reactive power O, to the AC-bus to reduce reactive power Q_g drawn from the wound field synchronous generator by the loads.

3. Power module only operation

With a sufficient E_s level available from the supplementary energy source and light loads connected to the micro-grid, the generating set can be disconnected from the AC-bus and set to the OFF state. In this mode of operation both active P_f and reactive Q_t powers demanded by the loads are delivered by the sine-wave voltage inverter fed from the supplementary energy source. Power module

The main subsystem of the power module / power conditioner 20 is the DC/AC sine-wave voltage source inverter (SVSI). A simplified circuit diagram and control structure of the SVSI is shown in Figure 6. Referring to Figure 6, the SVSI power circuit consists of three individual half-bridges ( T_h T₂), LC filters (L_f, Cf) on the AC output, and a common DC-bus (C_dc) connected to the supplementary energy source. Each pair of ( T_h T₂) and (L_f, Cf) filter components are controlled individually to produce three sinusoidal voltages (U, V, W) with reference to the common neutral (N), shifted by 120 ° respectively. All SVSI output phases can be connected to or disconnected from the micro-grid AC-bus by a 3-pole contactor S₀.

As shown in Figure 6, the output voltages v_f oi each SVSI phase (U, V, W) are controlled individually by means of an inner current loop and an outer voltage loop both with PI regulators to achieve a high quality sinusoidal voltage ½ which feeds the loads connected to the AC-bus.

In power module only operation, the SVSI works as a voltage source and the reference block REF sets the control of voltage p outer loop with its vPI regulator. Then the output from vPI sets the reference to the current i_f inner loop with its iPI regulator. The output of the iPI regulator is fed to the PWM modulator block, which controls the driving circuit DRV of the half-bridge IGBT switches { T T₂) for the relevant phase.

In hybrid load sharing operation, the SVSI works as a current source connected in parallel to the wound field synchronous generator. Reference block REF sets the \P\ regulator to control current />, amplitude and phase with reference to the AC-bus voltage ½ to achieve active power P, and reactive power O, sharing with the generator, with both the SVSI and the WFSG feeding micro-grid loads. To allow synchronised change between different system modes of operation, a phase-lock loop control block PLL is implemented.

Although each phase of the SVSI is regulated independently, they share a common controller CON based on DSP-FPGA architecture with sensing circuits and a CAN (controller area network) bus for external communication.

Reactive power sharing

In embodiments of the invention, the presence of two power sources in the power generation system is exploited to allow reactive power sharing between the sources.

Figure 7 shows an embodiment of a power generation system with reactive power sharing. Referring to Figure 7, the power generation system includes a generating set comprising prime mover 10, coupling 1 1 , generator 12 and circuit breaker 13. The system also includes a power conditioner 20 which comprises bidirectional DC/AC converter 26, filter 28, and controller 30. The power conditioner is connected on its DC side to electrical energy source 22. The parts shown in Figure 7 may be the same as the corresponding parts with the same reference numerals described above with reference to Figures 1 to 6.

In this example, the electrical energy source comprises two battery packs connected in series, with a neutral connection at their mid-point. A control system, such as that disclosed in co-pending UK patent application no.

1403013.4, the contents of which are incorporated herein by reference, may be used to control charging and discharging of the battery packs. In the arrangement of Figure 7, the power at the output of the generator is measured using a power measurement circuit 32. The power measurement circuit 32 includes a voltage sensor and a current sensor, which measure the instantaneous values of the voltage and current for each phase. For each phase, two output signals are produced. The first output signal is a measure of the active power at the generator output for that phase, while the second output signal is a measure of the reactive power at the generator output for that phase. Typically the active power P would be calculated as P = VI coscp and the reactive power Q would be calculated as Q = VI sincp where V is voltage, I is current, and φ is the phase angle between the current and the voltage. The power

measurement circuit 32 may be part of the power conditioner, or the power conditioner may receive measures of active and reactive power from an external power measurement circuit.

The measures of active power are fed to active power reference generator 34, and the measures of reactive power are fed to reactive power reference generator 36. A separate active power reference generator 34 and reactive power reference generator 36 is provided for each phase. Each active power reference generator 34 generates active power reference signals for that phase, which are fed to the controller 30. Each reactive power reference generator 36 generates reactive power reference signals for that phase, which are also fed to the controller 30. In operation, the measures of active power are used to adjust the active power reference signals produced by the active power reference generators 34. The controller 30 in turn adjusts the amount of active power delivered from the bidirectional DC/AC converter 26. This is achieved through control of the switches in the converter to produce the appropriate phase shifts between the output voltages and the output currents for each phase. This can allow the electrical energy source 22 to supplement the power supplied by the generator 12.

In addition, the measures of reactive power are used to adjust the reactive power reference signals produced by the reactive power reference generators 36. The controller 30 in turn adjusts the amount of reactive power delivered from the bidirectional DC/AC converter 26. This is achieved through control of the switches in the converter to produce the appropriate phase shifts between the output voltages and the output currents for each phase. This can allow an arbitrary amount of reactive power to be delivered by the electrical energy source 22.

With the arrangement shown in Figure 7, it is possible for some of the reactive power which would otherwise be supplied by the generator 12 instead to be supplied by the power conditioner 20 (or the bidirectional DC/AC converter 26 and filter 28). This can allow the power factor seen by the generator to be increased, for example, to a value which is close to unity, which represents the ideal operating condition for a wound field synchronous generator. This can lead to increased efficiency in the generator due to reduced losses in the excitation and armature circuits, as well as increased stability in the output voltage in particular with capacitive loads.

The arrangement shown in Figure 7 can be used to adjust the amount of reactive power delivered from the bidirectional DC/AC converter 26 even if no power source is connected, or when little or no power is available from the power source. This is achieved due to the temporary storage provided by the capacitors C_dc shown in Figures 4 and 6.

The arrangement of Figure 7 can also be used to compensate for other deviations from an ideal three-phase sine wave supply. For example, since each phase of the DC/AC converter 26 is controlled independently, the power conditioner can be used to compensate for unbalanced loads (i.e. where different phases have different loads). In the case where the load is unbalanced, the reference signals produced by the active power reference generators 34 and reactive power reference generators 36 will be different for each phase. The controller 30 can then adjust each phase of the DC/AC converter 26 in order to reduce the difference.

The arrangement of Figure 7 can also be used to reduce higher harmonics.

Where higher harmonics are present in the output of the generator, these will present in the measures of active and reactive power produced by the power measurement circuit 32, and hence in the active and reactive power reference signals produced by the active and reactive power reference generators 34, 36. The controller 30 can then control higher harmonics produced by the DC/AC converter 26 so as to reduce the higher harmonics at the output of the generator 12.

Overall, the aim of the arrangements disclosed herein is to use the existing components in the power module to improve the quality of the load as seen by the electrical generating set, thereby improving the efficiency and the operating life of the generating set.

Possible implementation

Figure 8 shows a possible implementation of a power conditioner in an

embodiment of the invention. In Figure 8, components of the electrical power generating set are shown in diagrammatic form. The generating set includes an engine 10 which is mechanically coupled with a rotor of a generator 12 whereby to rotate the rotor relative to a stator of the generator and thereby to generate a three phase variable frequency and voltage AC output (Vacg) from the generator.

A DC voltage source 22 supplies a DC voltage to DC-AC converter 26. The DC- AC converter is operable to generate a three phase AC power output (Vacc) with a neutral for supply to a load 15. The DC-AC convenor 26 may comprise an inverter which converts the DC voltage into a three phase AC power output

The reference levels for the required value of output frequency and voltage of the DC-AC converter are provided by an amplitude and frequency correction circuit (AFC) which responds to the appropriate reference signals. The reference level signal for each phase is fed to an inverter controller (IC) which provides pulse width modulated signals which control switching of the transistors of the invertor (INV). The corresponding pulse width modulated signals (PWM) for the DC-AC convenor are provided by a current controller 40. Provision of the necessary reference levels to the current controller 40 is described in detail below. Each phase of the three phase AC power output (Vacc) of the DC-AC convertor 26 is fed to the load point of common coupling 14 through a respective LC filter which comprises an inductor 42 and a capacitor 44. A current sensor 46 is connected between each inductor 42 and the respective capacitor 44. A voltage sensor 48 senses the voltage of each phase of the filtered three phase AC power output (Vacc) from the DC-AC convertor 26. The inductor current signal emitted from the current sensor 46 is fed to the current controller 40 and to a current reference generator 50 which generates a reference signal (Irp) which it derives from the inductor current signal. The output voltage signal (Vaca) is also fed to one input of a voltage controller 52. A normally closed transfer switch 54 transmits an output signal (Vinr) from a voltage reference generator 56 to another input of the voltage controller 52 when that transfer switch 54 is in its normally closed state. The voltage controller 52 emits a reference signal linr which it generates from a comparison of the output signal (Vinr) received from the voltage reference generator 56 via the transfer switch 54 and the sensed filtered AC output power voltage (Vaca) sensed by the voltage sensor 48. A second normally closed transfer switch 58 transmits the reference signal (linr) to the current controller 40 when in its normally closed state. The reference (linr) generated by the voltage controller 52 is used as the reference level for the required value of output frequency and voltage emitted by the DC-AC convertor 26 as described above, since the current controller 40 compares the inductor current signal with the reference (linr) to produce the pulse width modulated signals (PWM) for controlling switching of transistors of the DC- AC convertor.

The functions of the current controller 40, the voltage sensor 48, the transfer switches 54, 56, the voltage controller 52, the voltage reference generator 56, and the current reference generator 50 may provided by a suitably programmed digital microprocessor. This may be incorporated in a power conditioner controller 60, to one input of which is fed the output voltage signal (Vaca) for the purpose of calculating various parameters such as the rms values of output voltage and output current. A switch 62, which is a standard, non-controlled contactor, is provided for connecting the AC power output (Vacc) of the power conditioner 20 to the coupling point CP1 . However before the switch 62 is activated by the power conditioner controller 60 to connect the AC power output (Vacc) of the power conditioner to the connection point CP1 , the grid voltage is monitored by a voltage sensor 64 which emits a voltage reference signal (Vga) which is indicative of the voltage of the grid and which is connected to an input of a phase lock loop system 66. The phase lock loop system 66 produces a sinusoidal signal (Vgrr) which follows the grid voltage very precisely and which is fed to both another controllable switch 68 and to the normally open terminal of the transfer switch 54. The controllable switch 68 is normally open but is closed by an actuating signal (SSyn) fed to it by the power conditioner controller 60 when the latter receives a signal Mo which is for initiating connection of the power conditioner to the power supply grid. The output (Vga) of the voltage sensor 64 which is indicative of the voltage of the grid, the output signal (Vinr) from the voltage reference generator 56 and the output (Vgrr) of the phase lock loop system 68 are fed to the power conditioner controller 60. The normally open switch 68 is closed by the signal (Ssyn) fed to it by the power conditioner controller 60 so that the output (Vgrr) of the phase lock loop system 66 is fed to the voltage reference generator 56. The voltage reference generator 56 reacts to receipt of the signal (Vgrr) by slowly and smoothly changing its output signal (Vinr) towards the output (Vgrr) of the phase lock loop system. This continues until the two signals (Vinr) and (Vgrr) are overlapping in both amplitude and frequency. When the power conditioner controller 60 senses that that overlapping condition has been achieved, the power conditioner controller 60 produces a signal (S2) which actuates a change of state of the transfer switch 54 from its normally closed state to the open state in which the output (Vgrr) of the phase lock loop system 66 is fed to the voltage controller 52 in place of the output (Vinr) of the voltage reference generator 56. Hence the reference (linr) emitted by the voltage controller 52 is derived from a comparison of the output (Vgrr) of the phase lock loop system 66 which is derived from the voltage of the grid, with the voltage of the filtered AC power output of the DC-AC convenor 26. The power conditioner controller 60 then produces a signal (S1 ) which actuates a change of state of the second transfer switch 58 from its normally closed state to its open state which results in disconnection of the reference linr from the current controller 40 and its replacement by another reference (Irp) which is emitted by the current reference generator 50 which receives the output (Vgrr) from the phase lock loop system 66 as well as two reference signals (Pref) and (Qref) from active power reference generator 34 and reactive power reference generator 36, the other reference (Irp) being generated by the reference current generator 50 from the three inputs it receives, namely (Vgrr), (Pref) and (Qref). The active power reference (Pref) and the reactive power reference (Qref) are set based on measures of the active power and reactive power at the generator output, as measured by power measurement circuit 32.

Once the second transfer switch 58 has been actuated to change from its normally closed state to its open state, the power conditioner controller 60 emits an actuating signal (SS) to change the state of the controllable switch 62 to connect the filtered AC power output (Vaca) of the DC-AC converter 26 to the grid.

Active and reactive power transmitted to the grid from the DC voltage source 22 are adjusted and controlled when the power conditioner is controlled by a reference derived from the voltage of the grid. The AC power output voltage (Vacc) of the power conditioner 20 is reoriented into synchronism with the voltage of the grid quickly since the output is controlled by high frequency switching power transistors or other fully controllable power electronic devices and because the energy stored in the LC filter 42, 44 that filters the AC power output (Vacc) of the DC-AC convenor 26 is low. Further, since the connection of the power conditioner to the grid is delayed until the reference fed to the current controller 40 is brought into a overlapping relationship with the grid voltage in both amplitude and frequency, the risk of disturbance of the operation of the load is avoided. When the controllable switch 62 is disconnected or in the event that the voltage of the power supply grid fails, the power conditioner controller responds by actuating instantaneous change of state of the transfer switches 54, 58 from their opened states to their normally closed states. The functions of the phase lock loop system 66, the power conditioner controller 60, the active power reference generator 34, the reactive power reference generator 36 and the switch 68 may be provided by a suitably programmed microprocessor which may be the same microprocessor as provided the functions of the current controller 40, the voltage sensor 48, the transfer switches 54, 58, the voltage controller 52, the voltage reference generator 56 and the current reference generator 50.

In another embodiment, the inductor current signal may be fed to the voltage reference generator 56 rather than to the current reference generator 50, so that the reference voltage (Vinr) that is received at the other input of the voltage controller 52 via the transfer switch 54 when the latter is in its normally closed state would be derived from the inductor current signal. In the above description, preferred features of the invention have been described with reference to various embodiments. It will be appreciated that features of one embodiment may be used with any other embodiment. Furthermore, it will be appreciated that the invention is not limited to these embodiments, and variations in detail may be made within the scope of the appended claims.

Claims

1 . A power generation system comprising an electrical generating set and a power module for supplying power from a supplementary power source, the power module comprising a DC to AC converter, the power generation system comprising:

means for measuring an output of the electrical generating set; and a controller for controlling the DC to AC converter, wherein the controller is arranged to control the DC to AC converter in dependence on a measure of the output of the electrical generating set, thereby to improve the quality of the load seen by the electrical generating set.

2. A power generation system according to claim 1 , wherein the means for measuring an output of the electrical generating set is arranged to measure the reactive power at the output of the generating set.

3. A power generation system according to claim 2, wherein the controller is arranged to control the DC to AC converter in dependence on the measure of reactive power so as to reduce the reactive power supplied by the generating set.

4. A power conditioner according to claim 2 or 3, wherein the controller is arranged to control the amount of reactive power supplied by the DC to AC converter to increase the power factor seen by the generating set.

5. A power conditioner according to any of claims 2 to 4, further comprising a reactive power reference signal generator arranged to generate a reactive power reference signal for use by the controller to control the DC to AC converter based on the measure of reactive power.

6. A power generation system according any of the preceding claims, wherein the means for measuring an output of the electrical generating set is arranged to measure the active power at the output of the generating set.

7. A power generation system according to claims 6, further comprising an active power reference signal generator arranged to generate an active power reference signal for use by the controller to control the converter based on the measure of active power.

8. A power generation system according to any of the preceding claims, wherein the system is a multiphase system and the means for measuring an output of the electrical generating set is arranged to measure an output of each phase.

9. A power generation system according to claim 8, wherein the controller is arranged to control the DC to AC converter so as to reduce an imbalance between the phases.

10. A power generation system according to any of the preceding claims, wherein the means for measuring an output of the electrical generating set is arranged to measure the power of at least one higher harmonic in the output, and the controller is arranged to control the DC to AC converter so as to reduce the higher harmonic.

1 1 . A power generation system according to any of the preceding claims, wherein the power module is operable in parallel with the electrical generating set for use in supplying power to a load.

12. A power generation system according to claim 1 1 , wherein the power module is arranged to support the electrical generating set during transient conditions.

13. A power generation system according to any of the preceding claims, wherein the DC to AC converter is an inverter.

14. A power generation system according to any of the preceding claims, wherein the power module is arranged to receive an input from a source of electrical power.

15. A power generation system according to claim 14, wherein the source of electrical power is a supplementary energy source.

16. A power generation system according to claim 15, wherein supplementary energy source produces an AC output which is converted to an intermediate DC for supply to the power conditioner.

17. A power generation system according to any of claims 14 to 16, wherein the source of electrical power comprises energy storage.

18. A power generation system according to any of the preceding claims, wherein the DC to AC converter is a bidirectional DC/AC converter.

19. A power generation system according to any of the preceding claims, wherein the power module is part of a back-up power system.

20. A power generation system according to any of the preceding claims, wherein the power module is operable to control the amount of reactive power supplied by the DC to AC converter without receiving an input from a source of electrical power.

21 . A power generation system according to any of the preceding claims, wherein the DC to AC converter comprises a capacitor connected on its DC side.

22. A power generation system according to any of the preceding claims, wherein the electrical generating set comprises a prime mover and an electrical generator.

23. A method of operating a power generation system comprising an electrical generating set and a power module for supplying power from a supplementary power source, the power module comprising a DC to AC converter, the method comprising:

measuring an output of the electrical generating set; and controlling the DC to AC converter in dependence on a measure of the output of the electrical generating set, thereby to improve the quality of the load seen by the electrical generating set.

24. A method according to claim 23, wherein the reactive power supplied by the DC to AC converter is controlled so as to reduce one or more of reactive power, higher harmonics and phase imbalance as seen by the electrical generating set.