EP1393424A2

EP1393424A2 - A generator system and method of operating such a generator system

Info

Publication number: EP1393424A2
Application number: EP02727727A
Authority: EP
Inventors: Jeremy Mortimer; Martyn Harris; John Lyons
Original assignee: Bowman Power Systems Ltd
Current assignee: Bowman Power Group Ltd
Priority date: 2001-06-01
Filing date: 2002-05-15
Publication date: 2004-03-03
Also published as: AU2002257924A1; WO2002097942A2; GB0113347D0; WO2002097942A3

Abstract

A generator system comprising a local a.c. generator (14) that is operable to be connected to a utility power line (L1, L2 and L3) and a load (12), a current sensor (S1, S2 and S3) for sensing instantaneous current demanded by the load (12) and a controller (15) for varying the output of the local a.c. generator (14) to cause it to supply a pre-determined fraction of the instantaneous current demanded, preferably substantially all of the current demanded.

Description

A GENERATOR SYSTEM AND METHOD OF OPERATING SUCH A

GENERATOR SYSTEM

This invention relates to a generator system and method of operating such a generator system when it is connected to a utility power supply.

In some markets, a.c. generators can be connected to local loads via power cables that are provided by a utility power company, as shown in Figure 1. In this example, the utility company 10 provides a three-phase supply to the local load 12 via three power cables LI, L2 and L3 and a neutral cable N. The generator 14 is connected in parallel to the utility 10 and the local load 12 using each of the utility cables LI, L2 and L3 and N. When connected in this way, the utility company generally requires that the generator 14 does not export power to the utility supply 10, that is, it does not generate more power than the local load 12 requires. In addition, the utility company 10 needs to monitor the usage of utility power by the local load 12 when the power supplied by the generator 14 is insufficient. Conventionally, this is achieved by fitting a power transducer 16 between the utility company and the generator, as shown in

Figure 2.

In the arrangement of Figure 2, the power transducer 16 is connected to each phase line LI, L2 and L3 of the utility supply 10 and monitors the average three phase power flow between the utility 10 and the combination of the generator 14 and the local load 12. In this way, the transducer 16 is able to detect when the generator 14 is generating too much power. When a net transfer of power to the utility 10 is detected, this information is fed back to the generator 14, which varies its output until there is no net power flow out of the local load. Likewise, when power is drawn from the utility 10, the transducer 16 is able to monitor the usage by the load 12.

A disadvantage of the arrangement of Figure 2 is that the power flow in each phase is not monitored or controlled, instead only the average three phase power can be maintained and controlled. Thus, if the power demand of the local load is unbalanced between the phases, this unbalance may be passed on at least in part to the utility. A further problem arises when non-linear loads generate harmonic currents, because the flow of harmonic currents to the utility is effectively not monitored or controlled by the method described, so that the utility 10 has to support at least in part any harmonic currents caused by such non-linear loads. A still further disadvantage is that there is no control of the reactive power, by which is meant the term VIsinφ, where V and I are rms magnitudes of sinusoidal components of phase voltage and current respectively, and φ is the phase angle between them, which corresponds to a component of power that oscillates in direction over the AC cycle when φ is non-zero.

It is known that active filters 18 can be fitted to problem loads to reduce or eliminate harmonic currents seen by the utility 10, as shown in Figure 3. This overcomes some of the problems with the arrangement of Figure 2. However, a disadvantage of the arrangement of Figure 3 is that the power transducer/active filter combination is expensive, the cost typically being a substantial fraction of the overall cost of the power converter for the generator. Furthermore, the transducer/filter combination is awkward and time consuming to fit to generators. An object of the invention is to provide a system that reduces or eliminates reactive and harmonic currents seen by a utility power supplier when a local generator system is connected to the utility power supplier.

According to one aspect of the present invention there is provided a generator system comprising: a local a.c. generator that is operable to be connected to a utility power line and a load, a current sensor for sensing instantaneous current demanded by the load and means for varying the output of the local a.c. generator as a function of the instantaneous current sensed to cause the local a.c. generator to supply a pre-determined portion of the instantaneous current demanded, preferably substantially all of the current demanded.

By utility it is meant any independent supplier of electricity such as a national power supplier or alternatively another third party electrical supplier.

An advantage of this system is that three functions are simultaneously achieved; the local generator provides all or an agreed proportion of the harmonic current and all or an agreed proportion of the instantaneous power per phase demanded by the local load, which further means that it supplies all or an agreed proportion of the reactive power demanded by the local load. The utility effectively maintains the voltage waveform and no instantaneous current is drawn from any line of the utility, except when the local load demand exceeds the local generator capability. Preferably the local generator and the utility each have a plurality of phases in equal number, for example three. A current sensor may be provided on each phase line between the local generator and the load.

The local generator comprises an output inverter, which acts (together with any output filter(s)) as a power conditioner to supply electrical power to the user with appropriate waveform and frequency, and a source of power to supply the inverter. The power source may be a gas turbine driving a permanent magnet alternator, or a fuel cell, or an array of photovoltaic cells, or any other source of electrical power, which may be made suitable to supply the inverter.

Preferably, the local generator is a hysteresis-band bang-bang type current generator or any form of generator with feedback such as to make its characteristics so that output current can be controlled in accordance with a current demand waveform or any other form of generator that achieves the effect of making the output current waveform closely follow the demand waveform.

According to another aspect of the invention, there is provided a method of controlling current supplied to a load from a local a.c. generator that is connected to the load and a utility power line, the method comprising: sensing instantaneous current demanded by the load and varying an output of the local a.c. generator in response to the instantaneous current sensed to generate a pre-determined portion of the current demanded by the load, preferably substantially all of the current demanded. Preferably, the local generator and the utility each has a plurality of different phases in equal number, for example three. Preferably, the step of sensing involves sensing the current demanded in each phase.

According to a yet further aspect of the present invention, there is provided a computer program, preferably on a data carrier or some other computer readable medium, for controlling current supplied to a load from a local a.c. generator, the local a.c. generator being connected in use to the load and a utility power line, the program being operable to: receive from a current sensor a signal indicative of the instantaneous current demanded by the load and provide a control signal that is operable to cause an output of the local a.c. generator to be varied as a function of the instantaneous current sensed to generate at least a pre-determined portion, preferably substantially all, of the current demanded by the load.

Preferably, the local generator and the utility each has a plurality of different phases in equal number, for example three. Preferably, the program is operable to receive a signal indicative of the current demanded in each phase and provide a separate control signal for each phase.

According to a still further aspect of the invention, there is provided a generator system comprising: a local a.c. generator that is operable to be connected to a utility power line and a load, a current sensor for sensing instantaneous current drawn by the load from the utility power line and means for varying the output of the local a.c. generator as a function of the instantaneous current sensed so as to reduce the instantaneous current sensed, preferably to substantially zero.

According to a yet further aspect of the invention there is provided a method of controlling current supplied to a load from a local a.c. generator that is connected in use to the load and a utility power line, the method comprising: sensing instantaneous current drawn from the utility power line, and varying the output of the local a.c. generator as a function of the instantaneous current sensed so as to reduce the instantaneous current sensed, preferably to substantially zero.

According to a yet still further aspect of the invention there is provided a computer program, preferably on a data carrier or some other computer readable medium, for controlling current supplied to a load from a local a.c. generator, the local a.c. generator being connected in use to the load and a utility power line, the program being operable to: receive from a current sensor a signal indicative of the instantaneous current drawn from the utility by the load and provide a control signal that is operable to cause an output of the local a.c. generator to be varied as a function of the instantaneous current sensed in such a manner as to reduce the instantaneous current sensed, preferably to substantially zero.

A system and method in which the present invention is embodied will now be described by way of example only and with reference to the accompanying drawings, of which: Figure 4 is a simplified block diagram of a generator system; Figure 5 is a block diagram of a generator system that includes a gas turbine for providing an input drive;

Figure 6 is a simplified block diagram of another generator system and Figure 7 is a more detailed block diagram of the generator system of

Figure 6.

Figure 4 shows a utility supply 10 with three power phase lines LI, L2 and L3 and a neutral N that are each connected to a load 12, which may itself be a parallel connection of several diverse loads. Connected in parallel to the utility 10 is a local generator 14. Preferably, the local generator 14 is a hysteresis- band bang-bang type current-generator or a generator with feedback such as to make its characteristics so that output current can be controlled in accordance with a current demand waveform or any other form of generator that achieves the effect of making the output current waveform closely follow the demand waveform.

The local generator 14 has three phases, each of which is connected to a corresponding one of the utility power lines LI, L2 and L3, and a neutral N, which is in turn connected to the corresponding neutral line of the utility. On each phase line connected between the generator 14 and the load 12 is a current sensor SI, S2 and S3. Each sensor SI, S2 and S3 is operable to measure the instantaneous current drawn by the load 12 on a given power line LI, L2 and L3 and is connected to a controller 15 in the local generator 14.

In use, when the local load 12 demand does not exceed the local generator capability, the local generator 14 provides current to the local load in each of the phase lines LI, L2 and L3. When power is being drawn, the current sensors SI, S2 and S3 continuously monitor the instantaneous current in each line LI, L2 and L3. The instantaneous current demanded by the load 12 and the current generated are matched. This is done by feeding back a signal indicative of the instantaneous sensed current to the controller 15 and causing the output of the generator 14 to be varied depending on that value. The feedback could be linear feedback or cyclic feedback, an iterative learning procedure, as taught in co-pending patent application PCT/GBO 1/05003. Alternatively, any other form of feedback that achieves the effect of making the output current waveform of the generator closely follow the demand waveform of the generator could be used.

By ensuring that the current generated matches the instantaneous current sensed, when the waveform of the load current changes, the generator current waveform changes immediately, or within a few cycles, to match it. For example, should the power demand and the load current increase in any one phase, this is sensed by the relevant sensor and the output of the generator 14 is increased accordingly to provide more current in that phase and thereby accommodate the extra power required by the load 12. Hence, no instantaneous power is drawn from the utility 10. Likewise, if the instantaneous current load is decreased in any one phase, this is sensed and the output of the generator 14 is varied to reduce the current in that phase. In this way, no power is exported to the utility nor any current exchanged with it.

It should be noted that since the current in the fourth neutral line N is equivalent to the sum of the currents in the other phase lines LI, L2 and L3, there is no need to monitor or control the neutral current. Instead, the neutral current is automatically altered by the effect of sensing and altering the instantaneous currents in the other non-neutral phase lines.

Figure 5 shows a more detailed example of the generator system of Figure 4, in which a generator that includes a gas turbine engine 26 is connected in parallel to a utility supply 10. In this case, the generator 14 has a controller 15 that includes a microprocessor 20 and a memory 21. Included in the processor 20 is a computer program that controls the overall operation of the system.

The engine 26 drives an alternator 28. that converts mechanical energy from the engine 26 to provide a.c. current. Connected to the alternator 28 is a power conditioner 30, which is connected to a filter 32 and is operable to communicate with the controller 15. Connected to the filter 32 is a contactor 34 that is in turn linked to the controller 15 and to a manually operable isolator switch 36, to which an external load can be connected.

The generator 14 is arranged in this example to provide three phases A, B and C and is connected across the utility power lines LI, L2 and L3, as before. Connected between the controller 15 and the local load 12 are current sensors SI, S2 and S3.

In use, the generator 14 provides current to the load 12 along each of the phase lines LI, L2 and L3. When power is being drawn, the current sensors SI, S2 and S3 continuously monitor the instantaneous current in each line LI, L2 and L3. As before, the current demanded by the load 12 and the current generated are matched at all instants. This is done by feeding back a signal indicative of the instantaneous sensed current from the sensors SI, S2 and S3 to the controller 15 and causing the electrical output of the generator 14 to be varied depending on that value. As before, the feedback could be linear feedback or cyclic feedback. Alternatively, any other form of feedback that achieves the effect of making the output current waveform closely follow the demand waveform could be used.

If, therefore, the waveform of the load current changes, the generator current waveform changes immediately, or within a few cycles, to match it. For example, if the load current is increased in any one phase, this is sensed and a signal indicative of this is sent to the processor 20 in the controller 15. The processor software recognises this signal and generates a signal to cause the electrical output of the generator 14 to be increased to provide more current and thereby accommodate the extra power required by the load. In this way, instantaneous power is not drawn from the utility. Likewise, if the instantaneous load current is decreased in any one phase, this is sensed and the software in the controller 15 generates a signal to cause the electrical output of the generator 14 to reduce the current in that phase.

As before, since the current in the fourth neutral line N is equivalent to the sum of the currents in the other phase lines, there is no need to monitor or control the neutral current.

In practice, in order to ensure that no current is exported to the utility at any time, the system may be arranged to provide a pre-determined portion of the current demanded, but not substantially all of it. For example, the system may be adapted to allow a small net in-flow of power and current from the utility at, say, a rate of 2-3% of the demand. Alternatively, the system may be adapted to allow a pre-determined amount of current and power to be provided by the utility. In addition, in cases where the current demanded by the load exceeds the rating of the local generator, the system is arranged so that the local generator shares with the utility the provision of per-phase power and/or reactive power and/or harmonic current in whatever manner is deemed appropriate.

The generator system described with reference to Figures 4 and 5 continuously monitors the instantaneous current drawn by a load in each phase and varies its output to accommodate changes and ensure that the current demanded and the current generated are substantially matched at all times. This means that export of power to the utility can be avoided, and the import of power and/or reactive power and/or harmonics can be controlled as required without the use of expensive components such as a power transducer or active filter. This is advantageous.

Whilst the generator systems of Figures 4 and 5 show the current being sensed at the local load, it will be appreciated that the instantaneous current in each phase line could alternatively be sensed at the connection between the utility and the generator. This is shown in Figures 6 and 7.

The operation of the generator of Figures 6 and 7 is similar to that of Figures 4 and 5, but in this case the current sensed is the instantaneous current drawn from the utility power lines and the output of the generator is varied to ensure that the instantaneous current sensed in each of the three phase lines LI, L2 and L3 is driven to substantially zero. This ensures that the generator is in effect supplying all of the instantaneous current demand by the local load, provided, of course, the required current does not exceed the capacity of the local generator. As before, however, in practice, to ensure that no current or power is exported to the utility at any time, the system may be arranged to provide a predetermined portion of the current sensed, but not all of it. For example, the system may be adapted to allow a small net in-flow of power and current from the utility at, say, a rate that is proportional to the current sensed. Alternatively, the system may be adapted to allow a pre-determined amount of current and power to be provided by the utility. In addition, in cases where the current demanded by the load exceeds the rating of the local generator, the system is arranged so that the local generator shares with the utility the provision of per- phase power and/or reactive power and/or harmonic current in whatever manner is deemed appropriate.

The generator systems of Figures 4, 5, 6 and 7 are typically provided as single units. When the generator system is a micro turbine generator the units are preferably portable, being provided on, for example, a single frame, which frame may be provided on wheels.

Throughout the foregoing disclosure the term 'generator' may be taken to mean a single generator as described, or alternatively two or more generators operating in parallel and working together so as to share the provision of load in accordance with an agreed strategy.

A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the invention. Accordingly, the above description of a specific embodiment is made by way of example and not for the purposes of limitation. It will be clear to the skilled person that minor modifications can be made without significant changes to the operation described above. For example, whilst the system described with reference to Figures 4, 5, 6 and 7 has a three phase supply, it will be realised that the system could be adapted for use with other multiple phase supplies.

Claims

1. A generator system comprising: a local a.c. generator that is operable to be connected to a utility power line and a load, a current sensor for sensing instantaneous current demanded by the load, and means for varying the output of the local a.c. generator as a function of the instantaneous current sensed to cause the local a.c. generator to supply a portion of the instantaneous current demanded, preferably substantially all of the current demanded.

2. A generator system as claimed in claim 1, wherein the local generator and the utility each have a plurality of phase lines in equal number, for example three.

3. A generator system as claimed in claim 2, wherein a current sensor is provided on each phase line between the generator and the load.

4. A generator system as claimed in any one of the preceding claims, wherein the local a.c. generator is adapted to be connected to a power source, which is operable to supply input power.

5. A generator system as claimed in claim 4, wherein the power source comprises any one of a gas turbine or a photovoltaic cell, or any other source of electrical power.

6. A generator system as claimed in claim 5, wherein the local a.c. generator comprises an alternator, preferably a permanent magnet alternator.

7. A generator system as claimed in any one of the preceding claims, wherein the local generator includes a power conditioner.

8. A generator system as claimed in any one of the preceding claims that is provided as an integral unit, preferably a portable unit.

9. A method of controlling current supplied to a load from a local a.c. generator that is connected in use to the load and a utility power line, the method comprising: sensing instantaneous current demanded by the load and varying an output of the local a.c. generator in response to the instantaneous current sensed to generate a portion of the current demanded by the load, preferably substantially all of the current demanded.

10. A method as claimed in claim 9, wherein the local a.c. generator and the utility each have a plurality of phase lines in equal number, for example three.

11. A method as claimed in claim 10, wherein the step of sensing involves sensing the current demanded in each phase.

12. A computer program, preferably on a data carrier or some other computer readable medium, for controlling current supplied to a load from a local a.c. generator, the local a.c. generator being connected in use to the load and a utility power line, the program being operable to: receive from a current sensor a signal indicative of the instantaneous current demanded by the load and provide a control signal that is operable to cause an output of the local a.c. generator to be varied as a function of the instantaneous current sensed to generate at least a portion, preferably substantially all, of the current demanded by the load.

13. A computer program as claimed in claim 12, wherein the local generator and the utility each have a plurality of phase lines in equal number, for example three.

14. A computer program as claimed in claim 13, wherein the means for receiving the current demanded is operable to receive the current in each phase and the means for providing a control signal is operable to do so for each phase.

15. A generator system comprising: a local a.c. generator that is operable to be connected to a utility power line and a load, a current sensor for sensing instantaneous current drawn by the load from the utility power line and means for varying the output of the local a.c. generator as a function of the instantaneous current sensed so as to reduce the current sensed, preferably to substantially zero.

16. A generator system as claimed in claim 15, wherein the local generator and the utility each have a plurality of phase lines in equal number, for example three.

17. A generator system as claimed in claim 16, wherein a current sensor is provided on each phase line between the generator and the load.

18. A generator system as claimed in any one of claims 15 to 17, wherein the generator is adapted to be connected to a gas turbine, which gas turbine is operable to supply input drive power.

19. A generator as claimed in claim 18, wherein the local generator comprises an alternator, preferably a permanent magnet alternator.

20. A generator as claimed in any one of claims 15 to 19, wherein the local generator includes a power conditioner.

21. A method of controlling current supplied to a load from a local a.c. generator that is connected to the load and a utility power line, the method comprising: sensing instantaneous current drawn from the utility power line, and varying the output of the local a.c. generator as a function of the instantaneous current sensed so as to reduce the current sensed, preferably to substantially zero.

22. A method as claimed in claim 21, wherein the local generator and the utility each have a plurality of phase lines in equal number, for example three.

23. A method as claimed in claim 22, wherein the step of sensing involves sensing the current demanded in each phase.

24. A computer program, preferably on a data carrier or some other computer readable medium, for controlling current supplied to a load from a generator, the program being operable to: receive from a current sensor a signal indicative of the instantaneous current drawn from the utility by the load and provide a control signal that is operable to cause an output of the local a.c. generator to be varied as a function of the instantaneous current sensed in such a manner as to reduce the current sensed, preferably to substantially zero.

25. A computer program as claimed in claim 24, wherein the local generator and the utility each have a plurality of phase lines in equal number, for example three.

26. A computer program as claimed in claim 25, wherein the means for receiving the current sensed is operable to receive the current sensed in each phase and the means for providing a control signal is operable to do so for each phase.

27. A system substantially as described hereinbefore with reference to the accompanying drawings and as shown in Figure 4 or Figure 5 or Figure 6 or Figure 7.

28. A method substantially as described hereinbefore with reference to the accompanying drawings.

29. A computer program substantially as described hereinbefore with reference to the accompanying drawings.