WO2001061840A1

WO2001061840A1 - Engine management system

Info

Publication number: WO2001061840A1
Application number: PCT/AU2001/000165
Authority: WO
Inventors: Colin James Chambers; Richard Ernest Colin
Original assignee: Powerline Ges Pty Ltd
Priority date: 2000-02-17
Filing date: 2001-02-19
Publication date: 2001-08-23
Also published as: WO2001061820A1; WO2001061838A1

Abstract

An engine management system (26) is operative to control a generator engine (21) in various modes so as to enable the generator (11) to operate when connected to mains or operate in a local cluster. When operating in a local cluster, the engine management system (26) promotes load sharing between the generators by regulating the output of the individual generators based on an output frequency related set point (49, 58) which decreases on increasing sensed load (50, 62). The engine management system is also operative when connected to the mains to operate in a power delivery mode wherein the engine management system (26) is provided a power set point (54, 61) which is controlled through a separate controller.

Description

ENGINE MANAGEMENT SYSTEM The present invention relates generally to an engine management system and more specifically to an engine management system for an internal combustion (IC) engine used for driving an electrical power generating means.

It is well-known to use IC engines to drive electrical generators particularly for back-up or primary power requirements. In that application, the generator may be connected to a mains power supply to supplement the mains power, or operated in a stand alone mode wherein it is isolated from the mains power. In either of these configurations, a single generator may be used or multiple generators may be employed which are connected in parallel to a bus.

The present invention aims to provide an engine management system which is in a preferred form is ideally suited for use with power generating means as back-up or for primary power requirements.

In a first aspect, the present invention provides an engine management system for a generator including an IC engine having a solid, liquid, or gaseous fuel supply, and which drives an electrical power generating means to provide an electrical power output for supplying an electrical load. The engine management system includes first sensing means which is operative to sense a characteristic of the generator relating to its output frequency, load sensing means which is operative to sense the electrical output load of the generating means, and processing means operative to establish a set point for said characteristic based on said sensed output load and to compare said set point to said sensed characteristic, and output means responsive to the sensing of a difference between the actual characteristic and the set point and operative to bias the generator to change its actual characteristic to said set point.

In a particularly preferred form, the processing means establishes the set point from a predetermined range of set points. Each set point within that range has a corresponding electrical load value assigned to it, with decreasing set point in the range being assigned increasing load values.

With this arrangement, the processing means inputs the sensed load value and then uses the assigned set point to that load value as the required set point for the frequency output characteristic of the generator.

The operation of the generator according this preferred aspect of the invention is hereinafter referred to as "droop" mode, in that the generator characteristic "droops" depending on the magnitude of the electrical power delivery from the generating means.

The operation of the generator in droop mode in accordance with the invention provides substantial practical benefit when the generating means is arranged in parallel with another like generating means. In that arrangement, the electrical power output of a cluster of generating means is connected in parallel to a common bus with the IC engines driving each generator means under an engine management system according to the invention with the same droop characteristics. Under these circumstances, the engine management system provides a simple means by which equal load sharing can occur merely by providing the same droop characteristics across each of the generators in the parallel configuration.

The frequency output characteristic of the generator that is sensed and the subject of droop may vary depending on the configuration of the generator. In one form, the generating means comprises an alternator and the characteristic measured is the engine speed. In another form, the generator includes an inverter and, in that arrangement, the characteristic that is used to instigate load sharing is the output frequency. In either arrangement, the engine management system provides a very simple and convenient arrangement for loadsharing across a cluster of generators. In the embodiment where the generating means comprises an alternator, the speed of each IC engine in a cluster is directly related to the output frequency and is constrained by the connection of the alternator to a common bus. Accordingly, under steady state conditions, the output of each generator will be the same because they each have the same speed/load characteristics as determined by the engine management system. If a disturbance occurs in the cluster, for example if the load in one generator drops, the engine management system becomes operative and establishes a higher internal set point speed for that generator. The processing means then establishes that there is a difference between the actual speed and the set point speed and the output means then biases the generator to increase the actual engine speed to the set point speed. As the actual speed of the engine is constrained by the generator being in the parallel arrangement, this causes the generating means to increase its electrical output as the engine tries to increase its speed. This then sets up a new load value for that generating means (as well as the other generating means) which is then processed by the engine management system. The cycle then continues until equilibrium is established across each of the generators, wherein the cluster of engines and generating means return to their steady state condition.

Increasing the actual engine speed is achieved by opening the engine throttle which in turn increases the output power. However as the actual speed is constrained, the method by which the alternator converts this extra engine power to electrical energy for the cluster is not by increasing frequency or voltage, but by increasing current. In a cluster with constant electrical load, the method by which a single alternator increases its current and therefore output power is related to the voltage current phase relationship. The more the current phase is advanced relative to the common voltage of the cluster, the higher the output current, and therefore the output power. Similarly, the cluster will drive an alternator consuming power from the cluster if its current phase lags behind the common voltage phase of the cluster. So although a unit fails in its attempt to change its output frequency (by increasing its actual engine speed) due to droop by opening the throttle, it does influence the current phase, which has the desired effect of changing the output power.

When using an inverter, the engine speed is not synchronised with the output. Nevertheless the frequency of the output is synchronised by the connection of the generators to the common bus. Accordingly, to achieve the load sharing, the engine management system of each generator includes the same frequency output/load characteristics. Similar to the above arrangement, if a disturbance occurs in a cluster where the load in one generator drops, the engine management system becomes operative and establishes a higher internal set point for frequency output for that generator. The processing means then establishes that there is a difference between the actual frequency and the set point frequency and the output means then biases the generator to increase the frequency to the set point frequency. As the output frequency of the generator is constrained by being in the parallel arrangement, any attempt to increase the frequency output will increase its electrical output. This then sets up the new load value for that generator, as well as the other generators and the processing of the engine management system continues until equilibrium is again established across each of the generators wherein the cluster of generators return to their steady state condition.

A particular advantage of this arrangement is that each or the generator in the cluster operates independently with the interrelationship of the generators to achieve equal load sharing being determined by the droop characteristics in the engine management system rather than by any overriding control system. This has important practical advantages in the reliability of the system as it minimises the impact of a single point failure in a cluster. With the engine management system according to this aspect of the present invention, a single point failure will have no effect on the other generators in the cluster. In contrast, if each of the generators in the cluster were centrally controlled, a single point failure may cause a consequential failure in the other generators leading to failure of the entire cluster.

In the embodiment where the generating means includes an alternator, a further advantage of the engine management system is that each engine is able to operative efficiently merely by its choice of set point speed ranges. In operation in the droop mode, the range of set point speeds may be relatively narrow with the range being preferably less than 15% of a mean speed. Working within this narrow speed range, the engine may be designed to operate at or close to its optimum levels for efficiency purposes.

With this arrangement, the output means is operative to control the throttle setting on the IC engine. Preferably the throttle opening is controlled via an output to a throttle control stepper motor which allows the throttle opening to be maintained at a range of discrete points between a fully opened and a fully closed position. To ensure freedom from oscillation induced by discontinuous behaviour, preferably a speed resolution is incorporated to use the steps available in the throttle. Preferably the speed resolution is in the order of 0.5 rpm. A PID controller may be used as the processing means of the engine management system and generates an electrical output to the throttle control stepper motor to bias the required change in engine speed.

In the embodiment where an inverter is used to produce the output load from the generator, the engine speed is now not dictated by the output frequency. Accordingly, a separate control mechanism may be included whereby the engine set point speed is changed in relation to changes on load of the_. engine. This may be implemented by a look-up table which contains data of which engine speed set point gives the best efficiency for a given engine load. This can be produced by engine testing but, typically, light loads are more efficient at low rpm, and heavy loads at higher rpm.

A second PID controller (in addition to the first PID controller which maintains the engine speed at the set point) may be introduced to adjust the engine speed set point based on the engine mode. Typically this second PID controller will run a lot slower than the first PID controller for stability purposes.

In a further aspect, the present invention relates to a plurality of generators each including an engine management system according to any form described above. In this arrangement, the droop characteristics of each of the generators is substantially the same so as to allow equal load sharing across the generators.

In yet a further aspect, the present invention relates to a method of controlling a plurality of generators connected in parallel, each generator including an IC engine having a solid, liquid, or gaseous fuel supply and an electrical power generating means driven by said IC engine and operative to provide an electrical power output for supplying electrical load, the method including the steps of:

(i) establishing operating characteristics for each generator, the operating characteristics being substantially the same as for each generator and comprising a range of set points for a characteristic of said generator relating to its output frequency, with each set point in that range having a corresponding electrical load value assigned to it, with decreasing set points in the range being assigned increasing load values; (ii) determining a value for each generator for the characteristic of said generator and the electrical output load of that generator; (iii) determining the set point characteristic for said sensed output, and comparing said set point with the sensed characteristic; and (iv) biasing each of the generators to change its sensed characteristic to its said set point characteristic when a difference between the actual characteristic and the set point characteristic is determined.

In a further aspect, the present invention relates to an engine management system which is operative to control a generator including an IC engine driving an electrical generating mean coupled to a mains electrical supply.

In this aspect of the invention, the engine speed is governed entirely by the mains supply itself, with the engine management system seeking to establish a certain power delivery (independent of frequency) to the electrical bus. Preferably, in this aspect, the engine management system includes input means to establish a power delivery set point, load sensing means which is operative to sense the electrical output load of the generating means, and processing means operative to compare said power delivery set point with said sensed output load, and output means responsive to the sensing of a difference between the sensed output load and the power delivery set point and operative to change the output load of the generating means to match said power delivery set point.

The system by which the engine management system is operative to change the output load of the generating means to match the power delivery set point may vary depending on the construction of the generator. In an arrangement where an alternator is used, this may be achieved by changing the engine speed so that the output load of the generating means matches the power deliver set point. In a similar manner to the earlier aspect of the invention, preferably the output means is operative to control the throttle setting on the IC engine. Again, as the actual speed of the engine is constrained, this time by the mains frequency via the electrical generator means, any attempt by the engine to increase its speed increases the load that the generating means will produce. In this way changes in the engine speed will directly effect the power output produced by the generating means.

Preferably again a PID controller is used as a processing means to compare the sensed power delivery with the power delivery set point and then to generate an electrical signal change the output load.

In an arrangement where an inverter is used, the engine management system may instruct the inverter to increase the power output to match the power delivery set point.

Inherent in the design of the inverter, it knows the power output in order to control the inverter operation correctly. Preferably the engine management system includes an input to establish a power delivery set point load and it then preferably implements a PID controller on the error between the desired power and the actual power. The output of the controller adjusts the power put into the mains. This adjustment is carried out in the inverter. Specifically when paralleled, the inverter has no control over voltage, so it must use current control of its output drivers. By adjusting the pulse width modulated (PWM) signal that goes to the output drivers, the inverter can adjust the current output. Additionally, the inverter can also control phase relationship between the output current and the mains voltage by adjusting the time reference of the PWM signal that goes to the output drivers. This adjustment in phase achieves the same result as when the throttle opens to push the alternator against the mains in the synchronous alternator approach described above. In this way, the inverter can still achieve full control of the power delivery to the mains. In this mode of operation (which is hereinafter referred to as "constant power" mode) the engine power delivery is requested by an input into the engine management system. In yet a further aspect of the invention there is provided an engine management system which includes two distinct control modes, being the droop mode and the constant power mode. In a particularly preferred form, the engine management system is operative so that smooth switching can occur between the droop mode and the constant power mode.

In accordance with this aspect of the invention, there is provided an engine management system for a generator including an IC engine having a solid, liquid, or gaseous fuel supply, and an electrical power generating means driven by said IC engine and operative to provide an electrical power output for supplying an electrical bus, the generator being operative to be connected to, and isolated from, a mains power supply, wherein the engine management system including first sensing means which is operative to sense a characteristic of the generator relating to its frequency output, load sensing means which is operative to sense the electrical output load of the generating means, and processing means operative to establish a set point for said characteristic or for said output load, and output means which are operative to change the output load of the generating means, wherein the engine management system is operative when isolated from the mains to operate in a droop mode wherein the processing means is operative to compare said set point for said characteristic with said sensed characteristic, and the output means is responsive to the sensing of the difference between the actual characteristic and the set point characteristic and is operative to bias the generator to change its actual characteristic to said set point and, when the generator is connected to the mains, the engine management system is operative to operate in a constant power mode wherein the processing means is operative to compare said power deliver set point with said sensed output load and the output means is responsive to the sensing of a difference between the sensed output load and the power delivery set point to change the output load of the generating means to match the power delivery set point.

The engine management system according to any form above has been designed especially for use as part of an engine control system (ECS) for use as part of an energy generation and supply system (hereinafter referred to as a "GES unit" - general energy system unit). The GES unit is described in more detail below and in the applicant's simultaneously filed co- pending application entitled "An Energy Generating and Supply System", the disclosure of which is to be taken as incorporated herein by this cross reference.

It is convenient to hereinafter describe embodiments of the present invention with reference to the accompanying drawings. The particularity of the drawings and the related description is to be understood as not superseding the generality of the preceding broad description of the invention.

In the drawings:

Figure 1 is a functional block diagram of a first embodiment of the GES unit;

Figure 2 is a functional block diagram of a second embodiment of a GES unit;

Figure 3 is a functional block diagram of an energy generation and supply system comprising a number of GES units in parallel and associated with a mains supply;

Figure 4 is a schematic illustration of an engine control system for the engine of the GES unit of Figure 1 ;

Figure 5 is a logic flow diagram for the engine control system of the GES unit of Figure 1 ; and

Figure 6 is a logic flow diagram of the engine control system of the GES unit of Figure 2. Figure 1 illustrates a functional block diagram of a GES unit 11 installed as part of the energy system 10.

The GES unit includes an internal combustion gas engine 21. The engine is a high reliability long life power plant, specifically designed for waste-heat recovery (liquid-cooled block, and heat exchanger for exhaust heat recovery). The engine employed is a single-cylinder spark ignition unit with a maximum output of 3.75 kW at 3000 rpm. The energy source for the engine is by way of a gas input 22 which is natural gas or LPG. Shut off and safety interlocks are not shown, but are typically implemented to satisfy regulatory requirements.

A waste heat recovery unit 23 is included which delivers waste heat via a circulating coolant flow. The coolant flows through the engine 21 and the exhaust heater exchanger and is delivered to the external coolant loop where upon it gives up thermal energy and re-enters the engine 21 at a reduced temperature. A conventional wax pellet thermostat is used in the engine to ensure stable engine temperatures.

In the embodiment of Figure 1 , a three-phase brushless alternator 24 is used to convert the engine power to electrical power. The excitation of the alternator is controlled by a voltage regulator 25. The voltage regulator determines the appropriate level of alternator excitation whilst operating stand-alone, and also when in parallel with other GES units, or when connected to a mains. A quadrature current droop method of voltage control is employed to obtain good reactive power sharing between machines. A closed-loop reactive power control method is used when in parallel with the second distribution system in order to keep reactive power flow to and from the mains within acceptable limits.

The engine itself, and all engine ancillary items are controlled by a microcontroller based engine control system (ECS) 26. Control of the gas valves and all engine protection functions are also carried out by the ECS. The ECS 26 produces spark and throttle position commands to control the engine. Fuelling and ignition are controlled in an optimal manner to provide the best possible fuel efficiency and emission performance. In the configuration where an alternator is used, the primary means of achieving the required performance are through spark timing, throttle control, and mixture tuning. In operation, the engine 21 is run at more or less constant speed at 3000 rpm with varying loads. There are two distinct control modes: droop, and constant power. These two modes are ultimately implemented by means of throttle control, but are quite different in function. In the droop mode, the engine speed "droops" depending on the magnitude of the electrical power delivery from the driven alternator 24 as fed to an ECS input from a power transducer. In the constant power mode, the engine speed is determined by the frequency (via the alternator) in a mains supply, and the engine power delivery is as requested by another input to the ECS.

A programmable logic controller (PLC) 27 acts as a local controller and is provided to manage engine running, stopping and selection of the appropriate engine control mode (droop or constant power) as well as engine control, electrical synchronising and connection with the output electrical bus and electrical trips, are handled by the PLC 27. In addition, the monitored quantities of the electrical load 100 as well as the local thermal load 101 may be brought into the PLC 27, which are then able to be transmitted to a global controller 28 via an external data I/O connection 29. The global controller is also able to gather operating information (including for billing) from the unit 11 , and pass this to an overall server computer for a whole cluster of GES units.

The ECS 26 and the PLC 27 may be provided as separate discrete units within the GES unit 11. Alternatively, the ECS and PLC may form part of an integrated electronic unit.

The alternator 24 output is connected to an output bus 30 in a controlled manner via a contactor 31 with a mechanical isolator (for safety reasons). A synchronisation detector 32 ensures voltage and phase equivalence of the alternator 24 output and the output electrical bus (that is, either side of the switch 31) prior to initiating closure of the switch 31. Suitable frequency match is determined by the ECS 26, although this can also be done by other dedicated means of frequency monitoring. Once closed, the switch 31 stays closed, until requested by the PLC 27 to open (trip).

The electrical output bus 30 of the unit is the common bus of a given installation to which the load is connected, and also the point to which the mains is also connected (in a controlled way) as shown in Figure 3. Therefore, an individual GES input is required to be able to connect to the electrical output bus whatever may be the present sharing this point of common connection. The PLC 27 can also enable connection to a dead bus should no other sources be present on it. As shown in Figure 4, the ECS 26 has various inputs and outputs which enable it to carry out its functions. The ECS monitors the temperature of the coolant used in the engine 21 by way of a coolant temperature sensor 33. The signal for the sensor is used to determine whether an enriched fuel mixture is required during start up, or whether the engine is overheating. The timing of the spark of the engine 21 is facilitated by use of a cam positioning sensor 34. This is a standard Hall effect transducer giving a pulse signal at a particular point of every revolution of the engine. The ECS 26 determines when the spark is to occur in relation to the pulse from the cam position sensor 34 and sends a signal to the ignition module 35 at the appropriate time. The oil pressure of the engine is monitored by an oil pressure switch 36. The engine will shut down if the oil pressure falls below a predetermined level.

The ECS 26 operates several engine components via a high voltage relay box 37 including operation of the engine block heater and operation of the engine starter motor. Throttle opening is controlled via an output to a throttle control stepper motor 38. Using a stepper motor allows the throttle opening to be maintained at a range of discrete points between fully opened and fully closed. Finally, the ECS 26 is controlled by several inputs from the PLC 27. This includes switching between the constant power mode and the droop mode, input of power delivery set point and input via a power transducer of the power output of the electrical generating means. The PLC 27 also controls the operation of the engine and the GES unit will start up or power down according to instructions received from the PLC 27. The control methodology of the ECS 26 is detailed in Figure 5.

The ECS 26 is required to fully manage starting, running and stopping of the engine 21 through the run/stop management module 39. The ECS 26 acts in response to a digital on/off run command 40 provided by the controller 27. The ECS provides a start command to a starter motor control module that provides an output 41 for engine cranking for a limited number of attempts. The ECS 26 issues instructions to open the gas shut off valve at 42, whilst a valid spark is being sensed. As soon as the engine fires, and the sensed speed is above a pre-set value, the starter signal ceases. The engine continues to run whilst the run command is on provided there is no trip conditions. In the event of a trip condition, or the run command being removed the gas supply 22 is shut off by de-energising the gas shut off valve, the throttle closed and the spark disabled thereby stopping the engine. The ECS 26 is also capable of driving an ignition coil 43 (see Figure 4) via a power drive circuit which is preferably part of the ECS itself. The Hall effect position sensor 34 is fitted to the engine 21 such that the ECS 26 can determine some sort of reference angle with respect to real TDC (top dead centre). The ECS adds an advance to the signal in such an amount as to achieve the correct ignition advance angle determined by the PLC 27 and issues instructions for ignition timing as shown at 44. The amount of advanced angle required to achieve the desired ignition advance is calculated by the interval between the latest fly wheel pulse, and the one prior to it. The PLC 27 determines the optimum ignition point by monitoring throttle position and using a look up table. As the operating speed is almost constant, the amount of variation required is not great, with the ability to fine tune this aspect of the engine performance if required. The ECS 26 is supplied for external 50 Hz transformer and raw power supply, delivering 12V and 5V DC to the ECS electronics. As indicated previously, the engine 21 is run from a mains supply of natural gas or LPG. The engine 21 is fitted with a venturi type carburettor that is fed from a zero-pressure gas regulator that effectively allows the engine to draw in the fuel that it needs via a relatively conventional carburettor jet. The ECS 26 provides at 42 instructions to control the shut off solenoid that controls the gas supply 22 (this is a safety and regulatory requirement). Essentially, when the engine is not operating (no spark to be more specific), the gas supply is shut off. The ECS also controls the mixture of the engine during starting by means of an enrichment solenoid.

The ECS 26 controls fuelling and the throttle position in different ways depending on whether it is operating in droop mode or constant power mode. The control mode input signal 45 is provided by the PLC 27.

In droop mode, whilst a stand-alone cluster of engines is supplying an electrical mode (without the mains) the throttle position 46 is controlled by means of a PID controller 47 which senses actual speed and compares it with a set point speed. Both the sensed speed 48 and the set point speed 49 are inputted from the PLC 27. The set point speed 49 is biased downwards proportional to the amount of power that the GES unit 11 is delivering. In this way, even load sharing of similar GES units in the cluster will be guaranteed (or will find a similar power equilibrium at a common speed). Specifically, the non-load speed is 3015 rpm, whilst the full load speed shall be 2985 rpm ie. a speed drop of 30 rpm will occur due to a full load delivery, with a proportionally less amount of droop for partial loads. The required amount of speed droop is determined by the ECS from the power sensor input 50 which is fed by an output from an external electrical power transducer (not shown). A speed resolution of 0.5 rpm is employed in order to use the steps available on the throttle, and ensure freedom from oscillation induced by discontinuous behaviour. Whilst not shown, another analog input (0 to 5V) may be provided on the ECS 26 to provide an overall permanent bias over and above the speed droop. This input can be used to trim overall GES cluster frequency if desired, for both a more "ideal" operating frequency, and also to accommodate "ideal" synchronising conditions when initially paralleling with a non-ideal mains bus. This overall speed trim allows variation of the non load speed of the engine in droop from 2965 rpm to 3065 rpm. A fixed internal no load set point of 3015 rpm as described above is acceptable if this option is not available.

In constant power mode, when the cluster of GES units is paralleled to the mains electricity supply, the speed is entirely (forcibly) determined by the mains supply itself. In constant power mode therefore, the throttle position 46 is controlled by means of another PID controller 52 which seeks to establish a certain power delivery (independent of frequency) to the electrical bus. The ECS does this by comparing two analog inputs (0 to 5V) 53. One of these is a power delivery set point 54 (provided by the PLC 27), and the other is an actual power delivery sensed 50 (provided by the electrical power transducer). A digital input 55 (dry contact closure) is provided to the ECS by the PLC 27 to select this constant power mode. When this input is not active, the droop mode shall apply. The initial conditions in both PID loops are co-ordinated in order that smooth transition between the modes is assured. The mode changes are executed in direct response to the digital control input 45 (within 20 m/s).

A "speed OK" window confirmation is produced by the ECS 26 to enable synchronised functions which are handled externally by the PLC system. The window of acceptable set points speed is between 2980 and 3020 rpm (absolute, not with respect to the set point).

Figure 2 illustrates a second embodiment of the GES unit 11. As the second embodiment includes many of the features of the previous embodiment, like features have been given like reference numerals.

In the second embodiment, rather than using an alternator 24 as the electrical generating means, a permanent magnet generator (PMG) 56 is used in combination with an inverter 57.

The ECS 26 is operative to control the PMG/inverter so as to provide the same functionality as the synchronous alternator described above. However in the PMG/inverter arrangement, the engine speed is not limited by the output frequency. Therefore the engine is free to operate independently of the mains with the only limitation being that it must provide enough power to supply the power being used by the inverter from the DC bus 58. Because the throttle 46 can no longer influence the frequency, voltage or phase, the ECS 26 is not longer responsible for implementation of these control parameters, and instead the processor in the inverter affects the output drivers to provide the same result in terms of voltage, frequency and voltage- current phase relationship as in the synchronous alternator embodiment.

The control methodology of the ECS to induce the droop and constant power mode using the PMG/inverter combination is shown in Figure 6. Again the methodology used in the ECS incorporates many of the features of the previous embodiment using an alternator and like features have been given like reference numerals. To operate in the droop mode using the PMG/inverter, the PID controller 47 receives a sensed output frequency 58 from the inverter 57 and compares it to a set point frequency 59 from the PLC 27. The set point frequency is biased downwards proportional to the amount of power that the GES unit 11 is delivering. Therefore in a similar manner to the earlier embodiment, even load sharing of similar GES units in the cluster is achieved. The voltage of the cluster is dependent on all the inverters in the cluster, and the load is dependent on the connected load. Under droop mode, the mains does not set the voltage, and therefore the inverter operates in a voltage-controlled fashion. In order to maintain load sharing, the frequency droop is implemented in the inverters frequency control strategy. The inverter inherently knows its power output, so it reduces its target output frequency as the load increases. Because the frequency of the cluster is dependent on all units, it fails to influence the frequency of the cluster but in attempting to do so, it adjusts the current phase of its output relative to the common voltage phase of the cluster. If it is running in a higher load than the rest of the units in the cluster, it will attempt to droop its frequency, and in doing so, it will reduce its power output until the power output it is delivering results in a frequency that is equal to that of the cluster at which time it reaches equilibrium, and if all units in the cluster have achieved equilibrium the cluster is sharing load equally. If a unit is running lighter loads than the rest of the cluster, it will reduce the droop and try to run at a higher frequency and this has the effect of advancing the current phase relative to the voltage phase and therefore increases the power delivered. As the power delivery increases, the droop increases until the target frequency matches the frequency of the cluster, and equilibrium is achieved. Again, if equilibrium is achieved on all units in the cluster, then equal load sharing is received.

To implement these changes in the inverter during droop mode a frequency/phase control signal 60 is issued to the inverter which typically adjusts the time reference of the PWM signal that goes to the output drivers of the inverter. In addition a current/voltage control signal is issued to the inverter to regulate the voltage levels of the output signal. In the constant power mode, the second PID controller 52 compares a power delivery set point signal 61 which it receives from a local controller 64 (through its PLC 27) with the actual power output 62 which is obtained directly from the inverter. The PID controller then issues correction signals to match the desired set point power with the actual power. The output of the controller adjusts the power into the mains. This adjustment is achieved by controlling the current (under a current/voltage control signal 63) issued by the inverter as well as adjusting the phase relationship between the output current and the mains voltage under the frequency/phase control signal 60. Specifically, when paralleled, the inverter has no control over voltage, so it must use current control of its output drivers. By adjusting the PWM signal that goes to the output drivers, the inverter can adjust the current output. Additionally, the inverter can also control phase relationship between the output current and the mains voltage by adjusting the time reference of the PWM signal that goes to the output drivers. In this way, the inverter can achieve full control of the power delivery to the mains.

Because the throttle is not used to implement the output power using the PMG/inverter combination, a separate algorithm is incorporated in the ECS to control the throttle so that the engine is operating at its optimum efficiency. In the illustrated form, this is achieved by incorporating in the ECS 26 a throttle control for constant speed similar to that which was done in the previous embodiment by having a speed set point 64 change with output load (from the power sense input 62).

In this arrangement, a speed set point 64 is established based on the particular power output of the GES unit 11. This set point speed is then compared to the actual sensed speed 48 and a separate PID controller 65 loop is implemented so that the speed set point is caused to match the actual sensed speed. The relationship of the set points speed and the power output is similar to the synchronous alternator with set point speed being biased downwards proportional to the amount of power that the GES unit is delivering. However these can be taken over a wider range to take advantage of the efficiency of running the engine at lower rpms for lighter loads and being able to achieve more power out of the engine at higher engine speeds. Further the throttle control algorithm is independent of the mode of operation of the electrical output.

Figure 3 illustrates the energy system 10 including a number of GES units 11 in parallel and associated with their mains supply 60. In the event that the mains 60 is available and of "unlimited" delivery capacity, the mains is connected to the common bus 30, and the number of GES units 11 that are put online at any one time is discretionary in terms of availability of electrical power to the load 100. However, typically, a governing factor may be the requirement for waste heat 101. If the waste heat requirement is high, most of the GES unit 11 may be put on line, and it will then supply electrical power and waste heat (this is the most economic use of the GES unit). Should the requirement for waste heat be low, only a small number, or none at all need be put online. The global controller 28 can determine the local waste heat requirement, via an input from a local controller 27, and place the appropriate number of GES units 11 online in the cluster. The thermal load is typically monitored by heat meters that monitor the change in temperature and rate of flow of fluid in the thermal distribution system used to fulfil the local thermal load. In this configuration, the net load on the mains equals the cluster delivery minus the load requirement, and the net import, export or a neutral condition with respect to the mains can be achieved.

In the event that the mains 60 is available but of "limited" delivery capacity, this mains is connected to the common bus 30, and the number of GES units 11 that are put online at any one time is determined primarily by the electrical requirements of the load (the GES units augment the limited mains delivery capacity). Furthermore, the quantity of the GES units required over this "essential" level can be governed by the requirement for waste heat 51. If the waste heat requirement is high, more GES units can be put online, and they will supply electrical power and waste heat (the most economic use of the GES unit). Should the requirement for waste heat be low, only the essential number, or none at all need to be put online. The global controller 28 can determine the electrical load requirement and knowing the mains limitation can place the appropriate number of GES units online in the cluster. Also possible is a partial approach which consists of non-essential load shedding in the event that this is preferable to running the GES units 11 without having sufficient use for the waste heat 101. In this configuration, the net load on the mains equals the cluster delivery minus the load requirement, and net import, export or neutral condition with respect to the network power can be achieved.

In the event that the mains 60 has failed, it is then isolated from the common bus 30, and the number of GES units 11 that end up online is determined by the electrical requirements of the load 100, should the requirement for waste heat be low, the global controller 28 can decide to initiate non-essential load shedding, in the event that this is preferable to running the GES unit without having sufficient use for waste heat. In addition, load shedding is initially required down to the level that the number of GES units can provide at the time the mains 60 actually fails. The degree of initial load shedding depends on how many GES units in the cluster are idle. Overall system control priority can be altered to suit loads which require reliable no-break power. Effectively, this is achieved by running a sufficient number of GES units in the cluster to always be sufficient to supply the load 100 if the mains 60 were to fail, and therefore placing a lesser importance on economic usage of waste heat. Once the appropriate number of GES units are online, the load shedding can be scaled back to discretionary level. In this configuration, cluster delivery always equals the load requirement, and there is obviously no import or export with respect to the mains. In the event that the second distribution system returns, and can be reconnected to the common bus 30, without loss of continuity to the load and the situation reverts to either of the two situations indicated above.

A frequency drift method is employed to prevent back feed into the failed mains 60, should the rare occurrence arise where conditions may make it possible for the cluster to support "uncontrolled" loads in the system. In this instance, the system can determine that the second distribution system needs to be isolated, despite the "false" impression that it has not failed. The mains 60 is connected or isolated from the common bus 30 by means of the solid-state contactor 67 in concert with an conventional electro-mechanical contactor. The solid-state contactor 67 provides for fast and clean disconnection of faltered main supplies with little disturbance to the common bus condition. The speed of the disconnection is such that no more than one cycle (20 ms) of outage is experienced on the common bus, even after "hard collapse" failure of the second distribution system. This duration is short enough to ensure ride-through of most commercial equipment (including PCs). The electro-mechanical contactor 67 provides galvanic isolation and maintenance security (in series with the solid-state contactor). Random coincidence style of synchronising is employed upon return of the second distribution system, and the networked power is in again connected to the common bus 30 by means of the solid-state contactor.

A synchronisation detector 68 ensures voltage and phase equivalence of the mains input 60 and the common bus 30 (that is; either side of the switch 67) prior to initiating closure of the switch 67. Suitable frequency and voltage match is also determined by the global controller 28 or separate PLC (not shown via transducers). Once closed, the switch 67 stays closed, until requested by the global controller to open (trip). A local controller 65 manages the electrical synchronising of the common bus 30 with the mains 60, connection to the mains, and gathers other electrical quantities such as the electrical and thermal load demand. These monitored quantities are then available to the global controller 28. In addition, the local controller controls distribution of waste heat from the system, and load shedding which may be required under changed system conditions. However, the decision making functionality for these overall system parameters resides with the global controller 28.

The PLC 27 of the individual GES units may have the capacity to act as the local controller for a particular GES unit cluster. In this configuration, the global controller 28 is able to assign this task to the PLC 27 of a particular GES unit 11 and this may vary at periodic intervals. In an alternative form (not shown) at least some of the functionality of the local controller 65 may be assigned to a separate PLC unit(s) closing depending on the requirements of any particular site. Further information regarding the operation of the global controller and local controllers is described in the applicant's co-pending application entitled "Decentralised Energy Network System", the disclosure of which is to be taken as incorporated herein by this cross reference. The global controller 28 is the gateway to the individual GES units 11 and contains the software to control the overall system 10, including the waste heat distribution and load management.

The mode of operation of individual GES units 11 is dependent upon the mains and the connection conditions. Prior to being connected to the common bus 30 the GES starts off in droop mode. If more than one GES unit in a cluster is to be started, a random coincidence style of synchronising is employed, and the individual unit 11 is then connected to the bus 30 by means of the contactor 31. Once connected to the common bus, the GES unit(s) will either continue in droop mode if isolated or if the network connection via the mains has been isolated due to failure, or it will make a transition to constant power mode if a mains supply is connected to the common bus 30. In droop mode (no mains), the load on a given GES is dependant on the total electrical load, divided by the number of GES units online (in fact the number of GES units placed online is chosen to cause each GES to be loaded to a desired level). In constant power mode, individual GES power delivery is determined by a command request from the global controller 28, however in both droop and constant power mode the GES delivery needs is to be kept at sensibly high levels in order that individual efficiency is not compromised due to under-loading. The global controller 28 takes its sensor and transducer inputs from the waste heat distribution system (not shown) through the local controller 65, as well as electrical transducer information from the mains, the common bus 30 and the load 100. The individual GES unit pass information gathered locally to this global controller through the PLC 27 via the external data I/O connection.

The global controller 28 also contains the communication interface via a further external data I/O connection 66 which allows connection as well as information exchange with the host server computer. In the illustrated embodiment, the service utility retains ownership of each of the GES units 11 and provides the host function which controls the energy network system 10. Further, all billing information is able to be centrally co-ordinated by the host. In view of the embedded nature of the energy system 10, it is not necessary that the energy utility acquire large tracts of land to allow a region to be serviced. Instead, the land occupied by each of the generators 11 can merely be leased, through a reduction in rates, from the owners of the land on which the generators are located.

Accordingly, the present invention provides an energy system which is reliable, efficient and environmentally friendly. The system has minimum impact on the surrounding environment and, due to its embedded and modular nature, is able to be extended easily to additional regions.

It is to be appreciated that various alterations or additions may be made to the parts previously described without departing from the spirit or ambit of the present invention.

Claims

1. An engine management system for a generator including an IC engine having a solid, liquid, or gaseous fuel supply, and which drives an electrical power generating means to produce an electrical power output for supplying electrical load, the engine management system including first sensing means which is operative to sense a characteristic of the generator relating to its output frequency, load sensing means which is operative to sense electrical output load of the generating means, and processing means operative to establish a set point for said characteristic based on said sensed output load and to compare said set point to said sensed characteristic, an output means responsive to the sensing of a difference between the actual characteristic and the set point and operative to bias the generator to change its actual characteristic to said set point.

2. An engine management system according to claim 1 , wherein the processing means is operative to establish the set point from a predetermined range of set points, each set point within that range having a corresponding electrical load value assigned to it, with decreasing set points in the range being assigned increasing load values.

3. An engine management system according to either claim 1 or 2, wherein the generating means includes an alternator, and the characteristic sensed by the first sensing means is the speed of the engine.

4. An engine management system according to claim 3, wherein the range of set points is less than 50% of the mean speed of the engine.

5. An engine management system according to either claim 3 or 4, wherein the output means is operative to control the throttle opening on the

IC engine.

6. An engine management system according to claim 5, wherein the throttle opening is controlled by controlling a throttle control stepper motor which allows the throttle to be maintained at a range of discrete points between a fully opened and closed position.

7. An engine management system according to claim 6, wherein the processing means includes a PID controller that receives an error signal from the comparison of the set point and the sensed characteristic, and is operative to generate an electrical output to the throttle control stepper motor to bias the required change in engine speed.

8. An engine management system according to claims 1 or 2, wherein the generating means includes an inverter and the characteristic sensed by the first sensing means is the output frequency from the inverter.

9. An engine management system according to claim 8, wherein the processing means includes a PID controller that receives an error signal based on the comparison of the set point signal and the sensed characteristic and generates an electrical output to the inverter to bias the required change in the output frequency.

10. An engine management system according to either claim 8 or 9, further including a second sensing means which is operative to sense the speed of the engine, and wherein the processing means is operative to establish a set point for said engine speed based on the sensed output load and to compare said set point speed to said sensed engine speed, and wherein the output means is responsive to the sensing of a difference between the actual engine speed and the set point engine speed and is operative to bias the engine to change its actual speed to said set points speed.

11. An engine management system according to claim 10, wherein the processing means is operative to establish the engine speed set point from a predetermined range of engine speed set points, each engine speed set point within that range having a corresponding electrical load value assigned to it, with decreasing engine speed set points in a range being assigned increasing load values.

12. An energy generation system, including a plurality of generators each including an engine management system according to any of the preceding claims, wherein the processing means of each generator is operative to establish the set point from a predetermined range of set points, each set point within that range having a corresponding electrical load value assigned to it, with decreasing set points in the range being assigned increasing load values, and wherein the set point/load characteristics of the individual generators is substantially the same.

13. A method of controlling a plurality of generators connected in parallel, each generator including an engine and electrical power generating means driven by said engine and operative to provide electrical power output for supplying electrical load, the method includes the steps of: (i) establishing operating characteristics for each generator, the operating characteristics being substantially the same as for each generator and comprising a range of set points for a characteristic of said generator relating to its output frequency, with each set point in that range having a corresponding electrical load value assigned to it, with decreasing set points in the range being assigned increasing load values;

(ii) determining a value for each generator for the characteristic of said generator and the electrical output load of that generator;

(iii) determine the set point characteristic for said sensed output, and comparing said set point with the sensed characteristic; and (iv) biasing each of the generators to change its sensed characteristic to its said set point characteristic when a difference between the actual characteristic and the set point characteristic is determined.

14. An engine management system for a generator including an IC engine having a solid, liquid, or gaseous fuel supply, and which drives an electrical power generator means to provide an electrical power output for supplying an electrical load, the engine includes input means to establish a power delivery set point, load sensing means which is operative to sense the electrical output load of the generating means, and processing means operative to compare said power delivery set point with said sensed output load, an output means responsive to the sensing of a difference between the sensed output load and the power delivery set point and operative to change the output load of the generating means to match said power delivery set point.

15. An engine management system for a generator including an IC engine having a solid, liquid, or gaseous fuel supply, and an electrical power generating means driven by said IC engine and operative to provide an electrical power output for supplying an electrical bus, the generator being operative to be connected to, and isolated from, a mains power supply, wherein the engine management system includes first sensing means which is operative to sense a characteristic of the generator relating to its frequency output, load sensing means which is operative to sense the electrical output load of the generating means, and processing means operative to establish a set point for said characteristic or for said load output, and output means which are operative to change the output load of the generating means, and wherein the engine management system is operative when isolated from the mains to operate in a droop mode wherein the processing means is operative to compare said set point for said characteristic with said sensed characteristic, and the output means is responsive to the sensing of the difference between the actual characteristic and the set point characteristic and is operative to bias the generator to change its actual characteristic to said set point, and when the generator is connected to the mains, the engine management system is operative to operate in a constant power mode wherein the processing means is operative to compare said power delivery set points with said sensed output load and the output means is responsive to the sensing of a difference between the sensed output load and the power delivery set point to change the output load of the generating means to match the power delivery set point.

16. An engine management system substantially as herein described with reference to the accompanying drawings

17. A method of controlling a plurality of generators connected in parallel substantially as herein described with reference to the accompanying drawings.