GB2196448A - Generator controller - Google Patents

Abstract

A controller for an induction generator 2 monitors output voltage and varies a resistive dump load 4 connected at the generator's output. This, in effect, varies generator speed and therefore regulates output voltage and frequency. The controller may also monitor output current of a three phase generator to vary shunt capacitance 12 as well as resistive dump load. Load disconnection and alarm signals are generated if the output voltage or frequency exceed preset limits. <IMAGE>

Description

SPECIFICATION Generator controller The present invention relates to electrical generators. Specifically the invention is concerned with stand alone induction generators.

Many generators are connected to an electrical mains supply grid. The reason for this is that surplus electricity can be exported to the grid, and also that reactive power can be imported from it. The reactive power is used on start up and in maintaining the output voltage at the desired level. The grid may be regarded as an infinite bus, thereby maintaining the output voltage and frequency at a constant level, despite varying input power to the generator from the prime mover, for example, an IC engine or water turbine. However, the capital costs involved when connecting to the grid, namely: wiring; import/export metering and protection; and a three phase 10kV transformer are considerable. Therefore, in the small scale power range it becomes uneconomical to connect a generator to the national grid.

Another disadvantage of connecting to the grid is that generally speaking, only three phase electricity of over 10 kW of power is accepted for the grid.

However, there are many situations where a small scale generator would prove very useful.

For example, a farmer may want to use a fast-flowing stream nearby to provide single phase power for his farm, or a small industrial premises may need a standby diesel generator.

At present, the only small scale stand alone generators generally available are of the DC or synchronous types. One disadvantage associated with these types is that they require a continuous DC supply to enable them to function, and thus require a charge battery for start-up. Another disadvantage is that they require maintenance, particularly of the brushes and slip-rings. Further, they are not very robust and their capital cost is high in the small scale power range. Additionally, it will be appreciated that where a DC generator is used to supply AC power such as when used as a standby generator, then extra equipment is needed for conversion from DC to AC.

Because of its characteristics, a stand alone induction generator would apparently overcome these problems. However, it has always been appreciated that using a small scale, stand alone induction generator poses major control problems. Firstly, because it is not connected to the grid, means must be provided to control some of the output parameters such as voltage and frequency so that they stay between desired operational limits.

For example, if the voltage supplied for a television were to vary beyond a desired operational range, the electronic circuits inside may be damaged beyond repair. These voltage and frequency requirements must be adhered to, even in the case of varying input power to the generator (say, a water turbine), and output to the load. Further, a correct value of shunt capacitance must be connected across the stator windings to facilitate self-excitation start-up. And finally, the load must not be connected to the generator output until the generator is up-and-running.

The present invention is directed towards solving these problems by providing a controller for a generator and in particular an induction generator. Additionally, the invention is directed towards providing an efficient stand alone induction generator assembly.

According to the invention there is provided a controller for maintaining the output voltage and frequency of a self-excited induction generator within desired limits comprising: a monitoring circuit for monitoring a generator output parameter; a resistive dump load for connection with the generator at its output; and means for varying the resistive value of the dump load in response to the generator output parameter. preferably the generator output parameter is the output voltage.

In another embodiment, the generator output parameter is output current.

The invention will be more clearly understood from the following description of some embodiments thereof given by way of example only with reference to the accompanying drawings in which: Fig. 1 is a block diagram of an induction generator assembly incorporating a controller according to the invention; Fig. 2 is a partial circuit and block diagram of portion of the induction generator assembly; Fig. 3 is a graph illustrating selected generator output voltage limits; Fig. 4 is a partial circuit and flow diagram illustrating the operation of the controller; Fig. 5 is a graph illustrating generator output voltage as a function of load current for different speeds; Fig. 6 is a graph illustrating output voltage as a function of load current for various values of shunt capacitance and speeds; and Fig. 7 is a circuit diagram of portion of another controller according to the invention.

Referring to the drawings, and initially to Fig. 1, there is illustrated a generator assembly including a controller indicated generally by the reference numeral 1 connected to a three phase induction generator 2. The controller 1, (indicated by interrupted lines) comprises a control unit 3 connected to a dump load 4. The prime mover, in this embodiment, a water turbine 5 drives the induction generator 2 through a gearbox 6. At its output, the induction generator 2 has phase lines 7 connected to a load 8. Both the control unit 3 and the dump load 4 are connected to the phase lines 7. The control unit 3 is also connected to the induction generator 2.

Referring now to Fig. 2, the induction generator 2 and the dump load 4 are illustrated in more detail. Parts similar to those described with reference to the previous drawings are identified by the same reference numerals. The generator 2 is of the three phase type and has stator windings 1 la, 1 1b and 1 1c and associated phase lines 7a, 7b and 7c, respectively. A shunt capacitance is provided, comprising capacitors 12a, 12b and 12c mounted across the phase lines 7a and 7c; 7a and 7b; and 7b and 7c, respectively. Each capacitor 12a, 12b and 12c has an associated capacitor relay 13a, 13b and 13c respectively. The dump load 4 consists of three sets of resistors R1, R2, R3 and R4 connected to each phase by resistor relays S1, S2, S3 and S4 respectively.The resistors R1, R2, R3 and R4 hold values of 40%, 30%, 20% and 10% respectively of the total dump load resistance per phase so that by using suitable combinations of the resistor relays S1, S2, S3 and S4 discrete variations of 10% of dump load resistance per phase may be obtained.

Load relays 14a, 14b and 14c are connected on the phase lines 7a, 7b and 7c respectively. These enable the load 8 to be disconnected from the generator 2. As shown in Fig. 1, the load relays 14a, 14b and 14c, the resistor relays S1, S2, S3 and S4 and the capacitor relays 13a, 13b and 13c are controlled by the controller 1.

Referring now to Fig. 3, various per phase generator output voltages (Vout) are illustrated. At a value of 300V (Vdang hi) it may be said that the output voltage is dangerously high, and that the generator should be cut out totally by disconnecting the shunt capacitance 12. Above this level the output voltage may be said to be in the "Danger High Zone". The limits of Vout for the desired operational range, "Zone D" are also illustrated. In this case they are 215V (Vlo) and 240V (Vhi).

Intermediate Zone D and the Danger High Zone is "Zone H". There is also a certain low voltage level, below which the load should be tripped out. In this case, this voltage (Vtrip) is 160V and if the voltage is above this level, but below Vio it is in "Zone L" ' and if it is below Vtrip it is in the "Trip Zone".

Referring now to Fig. 4, the circuit of the control unit 3 for each phase is illustrated in more detail, and parts similar to those described with reference to the previous drawings are identified by the same reference numerals. The control unit 3 comprises an isolation circuit 20 for receiving and isolating the per phase output voltage Vout from the remainder of the control unit 3, which mainly comprises logic circuits. The isolation circuit 20 includes a transformer T for isolating Vout and a resistor R1 and capacitor C which form a low pass filter to attenuate any frequencies which are greater than 50 Hz. A resistor R2 and a diode D are provided to remove negative voltages. As stated above, the remainder of the control unit 3 mainly comprises logic circuits, and to provide a clearer illustration, they are only illustrated at flow diagram level.

The logic circuits include four op-amp comparator circuits, each having potentiometers for setting reference input voltages. In this case the input reference voltages are Vtrip, Vlo, Vhi, and Vdang hi and the respective comparator circuits are identified by the reference numerals 21, 22, 23 and 24.

In operation, the induction generator is switched on, either before or after the prime mover 5 has reached a desired speed. The controller 1 is also switched on and the control unit 3 then continuously monitors the generator output voltage Vout. If Vout is in the Trip Zone, the comparator circuit 21 outputs a signal to a latch, not shown, which in turn causes the resistor relays S1 to S4 and the load relays 14a to 14c to open. The comparator circuit 22 will then compare Vout with Vlo, If Vout is found to be in Zone L the resistor relays S1 to S4 are used to reduce the dump load. This is done as often as is required until Vout is in the desired operational range, Zone D.When this is the case, a load switch logic circuit 27 will output Vout to the Vhi comparator circuit 23 if a manual load control switch has been switched by a user.

This is an important feature, as greater protection is given for the load if it can only be switched on if a user has manually instructed the control unit 3 that this may be done. If this manual switch is not closed, no action is taken.

If Vout is in Zone D, the comparator circuit 23 outputs a signal to cause the load relay 14 for that particular phase to be closed,. If Vout is not in Zone D the comparator circuit 24 checks if it is in Zone H,. If it is not, Vout is in the Danger High Zone and alarm relays are activated, the load is disconnected, and the shunt capacitance relays for that phase are disconnected to cutout the generator. If Vout is in Zone H a timing circuit 28 times the duration at which Vout is in Zone H and the dump load is incremented. If Vout is in Zone H for a predetermined time internal, the timing circuit 28 causes alarms to be activated. The comparator circuit 24 includes a clock circuit which delays transmission of a signal indicating that Vout is in Zone H. If Vout drops into Zone D during this interval, the dump load is not incremented. This is an important feature of the invention as it prevents hunting, and therefore, output voltage stability is improved.

Counters are used for controlling dump load variation in discrete steps. It will be appreciated that by varying the generator's electrical power output in this way Vout may be stabilised with the minimum amount of flicker and harmonics.

To outline the principle behind the operation of the controller 1, take the situation where Vout is in Zone L. The action taken is to decrease the dump load. By doing this, the controller 1 has decreased the electrical power output, causing the generator speed to increase, which, in turn, causes Vout to increase. Referring now to Fig. 5, the graph illustrated shows that Vout also increases according to the generator' s output VI regulation. The desired operational range, Zone D, is indicated by interrupted lines in Fig. 5, and it will be seen that the generator speed is preferably kept to approximately 1500 rpm to help voltage stability. As the generator speed is controlled, the output frequency is also controlled as it is a direct function of generator speed and slip. It will be noted that by continually controlling generator speed, slip is automatically compensated for.For example, as slip increases with increasing power, the difference between actual speed and nominal speed increases to compensate.

The controller 1 is suitable for use with induction generators in most applications. However, problems may arise when controlling a three phase induction generator which receives variable input power and supplies an unbalanced load. The problem with this application is that there may be a large difference in the size of current drawn by the three different phases. For example, one phase could be drawing 6 Amps and another 1 Amp.

Referring now to Fig. 6, Vout is illustrated as a function of load current for different values of shunt capacitance, namely, C1, C2 and C3. It may be taken that these capacitance value are those for each of the capacitors 12a, 12b and 12c in Fig. 2. It will be clearly seen from the graph of Fig. 6 that in the case of an unbalanced load of say, one phase drawing, 1 Amp and another drawing 5 Amps, it will be impossible for the voltage of each of the phases to be simultaneously regulated when each of the capacitors 12a, 1 2b and 1 2c has the same value. Take for example, the situation where an induction generator is running at a speed of 1650 rpm and one phase draws 1 Amp and another draws 5 Amps.In this situation, the phase drawing 1 Amp would require a shunt capacitance of C2, whereas the phase drawing 5 Amps would require the higher capacitance C1 to stay within the desired operational range as indicated by the interrupted lines in Fig. 6. Therefore, the controller 1 must be modified so that it can vary the value of shunt capacitance of an induction generator. To do this, the outputs from the various voltage comparator circuits of the control unit 3 are connected to vary the value of shunt capacitance connected across the phase lines of the induction generator. In this embodiment, a bank of parallel capacitors having switches controlled by a control unit are connected in the place of, or parallel with the capacitors 12a, 12b and 12c of Fig. 2. This capacitor bank will generally form part of a controller.

Referring now to Fig. 7 there is illustrated a portion of an electrical circuit of an alternative construction of control unit according to the invention, indicated generally by the reference numeral 30. The control unit 30 is for use with a controller for operation in the conditions outlined above. The controller 30 comprises voltage comparator circuits similar to these of the controller 1. In addition, the controller 30 includes current conversion circuit 31 for each phase, which converts the output current lout to a voltage signal V which is fed into a dump reset comparator circuit 32 and 95% load comparator circuit 33. The outputs of the comparator circuits 32 and 33 are fed into a control circuit 34 which also receives inputs from voltage comparator circuits similar to those illustrated in Fig. 4.The potentiometer P4 of the dump reset comparator circuit 32 represents 110% of rated current and if the output of the circuit goes high the dump load is accordingly reset to zero. The potentiometer P5 of the 95% load comparator circuit 33 represents 95% of rated current. The control circuit 34 may be regarded as a capacitor and a dump load select controller, in that it receives signals indicating current and voltage levels in each phase and output signals indicating whether the shunt capacitance or the dump load be varied. To determine the value of shunt capacitance which should be connected for each phase, the phase drawing the largest current is always excited by the largest capacitor and thus it may be regarded as the master phase.For example, if the speed is set at 1650 rpm by the master phase, (which is drawing say 5.5 Amps), then any phase drawing less than about 4.6 Amps will need to have a smaller shunt capacitance of say C2 in Fig. 6. Essentially, the control circuit 34 operates as follows: (a) It varies the dump load to regulate speed; (b) It uses the distribution of the dump load to regulate the phase voltage; and (c) where necessary it varies the capacitance to further regulate the phase voltage.

The operation of the control circuit 34 will now be illustrated for various situations. The capacitance values referred to are those of the graph of Fig. 6, in which C1 has a larger value than C2 and C2 has a larger value than C3. If the capacitor C1 is being used and the voltage flags low in a phase then the dump load of this phase should be reduced to zero. If the voltage is still low then the speed of the generator must be increased and this is achieved by reducing the dump load of the other two phases in steps of 10%. Eventually the generator should build up sufficient speed to bring the voltage up sufficiently high in the master phase. If the voltage of either of the other two phases flags high during this procedure, then the shunt capacitance of that phase must be reduced.

If the voltage flags high in a phase having a shunt capacitance of C1, then the dump load should be increased until if necessary it reaches 100%. Alternatively, if the master phase is in the process of reducing the dump load of the other phases, a smaller shunt capacitance value of say C2 should be se lected instead of increasing the dump load. If this phase has 100% of dump load connected and the voltage is still high, then the speed of the generator will have to be reduced. This is achieved by increasing the dump load of the other two phases in steps of 10%. If the vol tage is still high with the complete dump load connected, then the controller must decide if there is a fault or excess input power.

-If the shunt capacitance C2 or C3 is.being used in a phase and voltage flags low, then the capacitance should be increased. If the voltage flags high then the dump load should be increased. If the voltage remains high or if the phase is being controlled by the master phase, the shunt capacitance should be re duced. The operation of the control circuit 34 will be readily understood from the above de scription. The input signal Hi indicates the out put voltage for a phase is in Zone H, the input signal Lo indicates if it is in Zone L, the EX TERNAL REDUCE indicates that the dump load of the phases other than the master phase should be reduced, and the EXTERNAL IN CREASE signal indicates that the dump load of these phases should be increased. Otherwise the terms used in the circuits refer to the associated phase of the control circuit 34.It will be noted that there will be a separate control circuit 34 and comparator circuit 30 for each phase. The dump load signal outputs from the control circuit 34 are inputted into a dump load controller circuit which controls the combination of dump load resistors required.

It will be noted that as with the control unit 3, if the voltage for a phase enters the Trip Zone the dump load of that phase is reset to zero.

The capacitor signal outputs from the con trol circuit 34 are inputted into a capacitor control circuit (not shown) which includes a selection circuit for selecting values of shunt capacitance. The important point to be borne in mind is that the shunt capacitance for a phase should be kept to a minimum for stable voltage output because generator efficiency decreases with increasirtq shunt capacitance. It will be appreciated that shunt capacitance should therefore, only be varied when a three phase induction generator is operating under the extreme conditions of receiving a variable input power and feeding an unbalanced load.

It will be noted that an important feature of the operation of the above controller is that both voltage and current are monitored.

All of the circuits hereinbefore described have been designed to operate quickly enough to undertake corrective action to avoid loss of output voltage, thus preventing loss of the residual magnetism in the iron of the magnetic circuit of the induction generator. This residual magnetism is necessary for the self-starting of an induction generator.

It will be appreciated that many different circuit arrangements may be used for carrying out the invention. Needless to say, the invention is not limited to the circuit arrangements illustrated and may be varied in construction and detail. Although the dump load variation has been described as occurring in steps of 10%, any other suitable steps may be used instead.

It will be appreciated that the controller according to the invention overcomes all of the induction generator control problems so that it may be used for supplying loads which must receive voltage and frequency between predefined limits.

It will further be appreciated that while the controller of the invention has been illustrated incorporated in an induction generator assembly, the controller may just as easily be used with other types of self-excited generator such as synchronous generator. Further, it is envisaged that any type of prime mover may be used for a generator controlled by the controller, for example, a water turbine, a windmill or an IC engine.

The invention is not limited to the embodiments hereinbefore described but may be varied in construction and detail.

Claims (10)

1. A controller for maintaining the output voltage and frequency of a self-excited induction generator within desired limits comprising: a monitoring circuit for monitoring a generator output parameter; a resistive dump load for connection with the generator at its output; and means for varying the resistive value of the dump load in response to the generator output parameter.

2. A controller as claimed in Claim 1, in which the generator output parameter is output voltage.

3. A controller as claimed in Claim 1, in which the generator output parameter is output current.

4. A controller as claimed in Claim 1 in which the generator output parameter is both output voltage and current.

5. A controller as claimed in any preceding claim, in which there is additionally provided a variable shunt capacitance for connection across the generator output and a capacitance controller for varying the shunt capacitance in response to the generator output parameter.

6. A controller as claimed in Claim 5 in which the capacitance controller only operates when variation of the dump load is insufficient to maintain the generator output voltage and frequency within the desired limits.

7. A controller as claimed in any preceding claim in which there is further provided means for disconnecting the generator output from a load when the controller does not maintain the output voltage and frequency within the desired limits.

8. A controller as claimed in any preceding claim, in which there is further provided means for disconnecting the shunt capacitance from the generator when the controller does not maintain the output voltage and frequency within desired limits.

9. A controller as claimed in any preceding claim, in which there is provided means for emitting a user alarm signal when the controller does not maintain the output voltage and frequency within the desired limits.

10. An induction generator assembly incorporating a controller as claimed in any preceding claim.