CONTROLLING THE EXCITER CURRENT OF AN ELECTRICAL GENERATOR

The present invention relates to protection circuitry for an electrical generator, and in particular to circuitry which can protect a generator with an automatic voltage regulator against the effects of a fault in the main switching device in the automatic voltage regulator.

Electrical generators operate by rotating a magnetic field produced by the rotor relative to windings in the stator in order to generate an AC output in the stator windings. In a synchronous generator, the rotor's magnetic field is produced by passing a DC current through windings in the rotor. This DC current may be generated by an exciter mounted on the shaft of the generator. An automatic voltage regulator (AVR) may be provided to control the exciter, thereby to control the current supplied to the rotor windings.

In order to control the level of excitation applied to the exciter field, an AVR will normally include a power control device. The power control device may be, for example, a thyristor or a transistor such as a MOSFET or an IGBT. The power control device is typically controlled by a phase control circuit or pulse width modulation circuit, in order to control the voltage and/or current supplied to the exciter.

If the power control device should fail, it may go into various different states including a short circuit. It may therefore be desirable to protect the generator from the effects of a failure of the power control device. This is usually done by sensing an over-voltage or over-current in the generator output. While this will prevent damage to the generator, in some circumstances the transient over-voltages or over-currents in the generator output may be undesirable.

According to one aspect of the present invention there is provided protection circuitry for an electrical generator, the electrical generator comprising a main machine, an exciter for providing excitation to the main machine, and an automatic voltage regulator, the automatic voltage regulator comprising a power control device for controlling excitation fed to the exciter and a control circuit for controlling the power control device, the protection circuitry comprising:

means for detecting an output of the control circuit; means for detecting an output of the power control device;

means for comparing the output of the control circuit with the output of the power control device; and

means for reducing the excitation if the output of the power control device does not correspond to the output of the control circuit.

The present invention may provide the advantage that, if a fault occurs in the power control device, the excitation can be reduced (for example to zero) more quickly than was previously the case. For example, the excitation may be reduced before a significant over- voltage or over-current has developed in the generator output. This may allow the generator to meet certain voltage specifications which may be demanded in particular applications.

The comparing means may be arranged to detect a non-responsive state of the power control device. For example, the comparing means may detect if the power control device does not respond to a change in the output of the control circuit or if the output of the power control device changes without a corresponding change in the output of the control circuit.

Preferably the comparing means is arranged to detect a short circuit of the power control device. For example, the comparing means may detect when the power control device is conducting without a corresponding signal at the output of the control circuit (that is, a signal which would normally cause the power control device to conduct). This may allow the excitation to be reduced before an over-voltage occurs in the generator output due to the short circuit. Alternatively or in addition the comparing means may be arranged to detect some other non-responsive state of the power control device, such as a continuous open circuit or a continuous resistive state. The excitation may be reduced (for example to zero) when any of the above states are detected.

The power control device may be any device suitable for controlling the amount of excitation which is provided to the exciter. In one example, the power control device is a transistor, for example a bipolar transistor such as an IGBT (insulated gate bipolar transistor), or a field effect transistor such as a MOSFET (metal oxide semiconductor field effect transistor). In another example the power control device is a thyristor. Preferably the control circuit operates using a pulse control technique, such as pulse width modulation or phase control.

Preferably the means for detecting an output of the control circuit and/or the means for detecting an output of the power control device comprise voltage sensors and/or current sensors.

The means for detecting an output of the control circuit and/or the means for detecting an output of the power control device may be arranged to convert a voltage output to a logic signal. For example, the means for detecting an output of the control circuit and/or the means for detecting an output of the power control device may comprise a Schmitt trigger to ensure that its output signal always takes one of two states. This may allow logic gates to be used in the circuitry. Furthermore, this arrangement may allow the protection circuitry be able to detect various different types of failure of the power control device.

Preferably the means for reducing the excitation is arranged to cut the excitation if the output of the power control device does not correspond to the output of the control circuit. For example, the means for reducing excitation may comprise a circuit breaker in the excitation circuit, for example between the power control device and the exciter. This may allow the excitation to be cut quickly in response to a failure of the power control device. Alternatively, the excitation may be reduced to some other level, such as a minimum level for maintaining operation of the generator. This may be achieved by a device other than a circuit breaker which limits the current and/or voltage in the excitation circuit.

Power control devices have a finite response time which depends on various factors such as the nature of the device itself and the size of the generator. In order to ensure that the protection circuitry does not respond during the response time of the power control . device, the circuitry may further comprise a delay circuit between the means for detecting an output of the control circuit and the comparing means. The delay circuit may be an asymmetrical delay circuit which delays an edge of the signal at its input corresponding to an edge of the output of the control circuit which in normal operation causes the power control device to transfer from a conducting to a non-conducting state. This may prevent the protection circuitry from responding during a period in which the power control device is changing state. Thus, this arrangement may help to prevent accidental reduction of the excitation due to the finite response time of the power control device. Depending on the protection circuitry being used, an asymmetrical delay circuit may also be provided to delay the other edge of the signal. A delay circuit (such as an asymmetrical delay circuit) may also be provided between the between the means for detecting an output of the power, control device and the comparing means.

As well as reducing the excitation if the output of the power control device does not correspond to the output of the control circuit, it may be desirable to inform an operator of a possible failure of the power control device. Thus the means for comparing the output of the control circuit with the output of the power control device may be arranged to output an alarm signal. The alarm signal may cause, for example, a visual or audible alarm to be given to the operator. Alternatively or in addition other signals could be generated such as control signals for circuit breakers elsewhere in the generator.

The protection circuitry may conveniently be integrated with an automatic voltage regulator. Thus, according to another aspect of the invention there is provided an automatic voltage regulator for an electrical generator, the automatic voltage regulator comprising:

a power control device for controlling excitation fed to the exciter;

a control circuit for controlling the power control device; and

protection circuitry in any of the forms described above.

According to another aspect of the invention there is provided an electrical generator comprising a main machine, an exciter for providing excitation to the main machine, and protection circuitry or an automatic voltage regulator in any of the forms described above.

According to another aspect of the invention there is provided a method of protecting an electrical generator, the electrical generator comprising a main machine, an exciter for providing excitation to the main machine, and an automatic voltage regulator, the automatic voltage regulator comprising a power control device for controlling excitation fed to the exciter and a control circuit for controlling the power control device, the method comprising:

detecting an output of the control circuit; detecting an output of the power control device;

comparing the output of the control circuit with the output of the power control device; and

reducing the excitation if the output of the power control device does not correspond to the output of the control circuit.

According to another aspect of the present invention there is provided protection circuitry which protects an electrical generator, the electrical generator comprising a main machine, an exciter which provides excitation to the main machine, and an automatic voltage regulator, the automatic voltage regulator comprising a power control device which controls excitation fed to the exciter and a control circuit which controls the power control device, the protection circuitry comprising:

a sensor which senses an output of the control circuit;

a sensor which senses an output of the power control device;

a comparator which compares the output of the control circuit with the output of the power control device; and

a circuit breaker which cuts the excitation if the output of the power control device does not correspond to the output of the control circuit.

Features of one aspect of the invention may be provided with any other aspect.

Apparatus features may be provided with method aspects and vice versa.

Preferred features of the invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows an overview of a generator with an automatic voltage regulator;

Figure 2 shows parts of an automatic voltage regulator in accordance with an embodiment of the invention;

Figure 3 shows one embodiment of a protection circuit;

Figure 4 is a timing diagram of various signals in the protection circuit of Figure 3 during normal operation;

Figure 5 is a timing diagram of various signals in the protection circuit of Figure 3 when a power control device fails;

Figure 6 shows another embodiment of a protection circuit; Figures 7 and 8 show signals in the protection circuit of Figure 6;

Figure 9 is a timing diagram of some of the signals in the protection circuit of Figure 6; and

Figure 10 shows another embodiment of a protection circuit.

An overview of a generator with an automatic voltage regulator is shown in Figure 1. Referring to Figure 1 , the generator comprises a main machine 10 which includes a main rotor 12 and a main stator 14. The main rotor 12 is located on a shaft 16 which is driven by a prime mover such as a diesel engine (not shown). The main rotor 12 develops a magnetic field, so that rotation of the main rotor 12 relative to the main stator 14 causes a current to be generated in windings in the main stator.

The. main rotor 12 is magnetised by passing a DC current through the rotor windings. This DC current is generated by an exciter 18, which comprises exciter rotor 20, exciter stator 22, and rotating diodes 24. The exciter rotor 20 is mounted on the shaft 16, and rotation of the exciter rotor 20 relative to the exciter stator 22 generates an AC output in windings in the exciter rotor. This AC output is converted to DC by the rotating diodes 24, and the DC output of the rotating diodes is fed to the main rotor 12.

The AVR 26 controls the voltage and/or current supplied to the exciter stator 22. By controlling the relatively low power which is fed to the exciter stator 22, control of the high power in the main rotor 12 is achieved through the rectified output of the exciter rotor.

In the arrangement of Figure 1 , power for the exciter 18 is drawn from the main stator 14, via an AVR 26. This is referred to as a self excitation (i.e. the generator provides its own excitation). In an alternative arrangement, power for the exciter is drawn from a separate permanent magnet (PM) machine mounted on the shaft 16, rather than from the main stator. In this case, the AVR controls power from the separate permanent magnet machine, rather than from the main stator.

Figure 2 shows in more detail parts of AVR 26. Referring to Figure 2, the AVR receives an AC input from either the main stator or a separate PM machine. The AC input is rectified by rectifier 30 and fed to power control device 32. The power control device 32 controls the excitation which is fed to the exciter.

Control of the power control device 32 is by means of voltage sensor 36, reference voltage generator 38, comparator 40 and control circuit 42. In operation, the voltage sensor 36 senses the voltage at the output of the main stator. The sensed voltage is fed to one input of comparator 40. The comparator compares the sensed voltage to a reference voltage produced by reference voltage generator 38, and outputs a difference signal to control circuit 42. The control circuit 42 controls power control device 44 in dependence on the difference signal.

In one example, the power control device is a transistor such as a MOSFET or IGBT. In this case, the output of the control circuit 42 is fed to the gate or the base of the transistor. The output of the control circuit is typically a pulse width modulation signal, in which the width of pulses fed to the gate of the MOSFET determines the amount of excitation fed to the exciter.

Alternatively the power control device may be a thyristor. In this case the thyristor may rectify and control the AC input, and thus the separate rectifier shown in Figure 2 may be dispensed with. The control circuit may be a phase control circuit which controls the point in the AC cycle at which the thyristor turns on. A single thyristor may be used for half wave control, or two thyristors may be used to allow full wave control of the AC input.

Also shown in Figure 2 is protection circuit 44 and circuit breaker 34. Protection circuit 44 senses the output of control circuit 42 and the output of power control device 32. If the protection circuit 44 determines that the power control device 44 is not responding to the output of control circuit 42, it outputs a signal to circuit breaker 34 which causes the circuit breaker to cut the excitation fed to the exciter.

Figure 3 shows one embodiment of the protection circuit 44. The protection circuit of Figure 3 is designed to protect the generator from the effects of a short circuit of the power control device. Referring to Figure 3, the protection circuit comprises voltage sensor 46, inverter 48, asymmetrical delay circuit 50, voltage sensor 52, AND gate 54 and bistable circuit 56. In operation, the voltage sensor 46 senses the output of the control circuit 42 in Figure 2, and converts the output to a logical high or low. The output of voltage sensor 42 is inverted by inverter 48, and the inverted signal is fed to asymmetrical delay circuit 50. Asymmetrical delay circuit 50 delays the positive edge of the signal at its input by a predetermined amount, while the negative edge has substantially no delay. The output of delay circuit 50 is fed to one input of AND gate 54.

Voltage sensor 52 senses the output of power control device 22 in Figure 2, and converts the output to a logical high or low. The output of voltage sensor 52 is fed to the other input of AND gate 54. The output of AND gate 54 is fed to bistable circuit 56. The output of the bistable circuit 56 is fed to the circuit breaker 34 in Figure 2.

Figure 4 is a timing diagram of various signals in the protection circuit of Figure 3 during normal operation. The output of the control circuit is a drive pulse, as shown by signal A. This signal is inverted as shown by signal B. The leading edge of the inverted signal is delayed, as shown by signal C. The output of the power control device is shown by signal D. There will normally be a small time lag before the power control device fully switches on or off in response to a drive pulse, and thus the output of voltage sensor 52 will normally be a delayed version of the output of voltage sensor 46, as shown by signal E. The AND gate 54 compares signal C with signal E. In normal operation, signal C and signal E will never be high at the same time, and so the output of the AND gate will stay low, as shown by signal F. The bistable device 56 is triggered by the output of the AND gate 54, and so in normal operation the output of the bistable device will also stay low as shown by signal G. The asymmetrical delay circuit 50 prevents the AND gate 54 from responding during the turn-off time of the power control device.

Figure 5 is a timing diagram of various signals in the protection circuit of Figure 3 when the power control device fails and presents a short circuit. When the fault occurs, the output of the power control device will go high, as shown by signal D. The output of the voltage sensor 52 (signal E) thus goes high during a period when the output of the asymmetrical delay circuit (signal C) is also high. This causes the output of the AND gate 54 (signal F) to go high. The output of the AND gate 54 triggers the bistable device 56, resulting in a continuous high signal at the output of the protection circuit (signal G). This high signal is fed to the circuit breaker 34, and causes the circuit breaker to cut the excitation supplied to the exciter. The bistable device 56 is also provided with a reset input which allows the protection circuit to be reset once the fault has been removed.

The protection circuit of Figure 3 thus operates to cut the excitation to the exciter if the power control device fails and presents a short circuit. By sensing the input and output of the power control device, the protection circuit can operate quickly in comparison to the time constant of the main field of the generator. For example, the generator main field may have a time constant in the order of 100's of milliseconds on smaller machines, and in the order of seconds on larger machines. Thus the protection circuit can allow the excitation to be cut before a significant over-voltage has developed at the generator output. For example, the voltage output of the generator may be kept within a 130% voltage specification even if the power control device fails.

The output of the bistable device in Figure 3 may also be fed to an alarm circuit, which may provide the operator with a visual or audible warning of a fault.

Figure 6 shows another embodiment of the protection circuit. In Figure 6, the DC input from the rectifier 30 is represented by voltage VI . The exciter field is represented by inductor Ll and resistor R1. D1 acts as a freewheel diode to prevent voltage spikes. The transistor Q1 is the power control device which controls the excitation fed to the exciter field. The gate of the transistor is controlled by control circuit XFG1 via Schmitt trigger U1C. Failure of the power control device is simulated by closing the switch J1. The circuit shown in Figure 6 is common to ground.

In operation, the AVR power control device gate drive signal (AO is inverted by inverting Schmitt trigger Ul B. When a positive edge occurs on the output of Ul B, C2 charges through R4. This causes the positive edge to be delayed by an amount determined by the values of C2 and R4. By contrast, when a negative edge occurs on the output of U 1 B, C2 discharges through D4. Thus the negative edge is substantially un-delayed. The combination of C2, R4 and D4 thus act as an asymmetrical delay circuit. The

asymmetrically-delayed signal is fed to one of the inputs of NAND gate U2A.

The output (BO of power control device Q1 is clamped to 15.7V by R2 and 02, and Inverted by inverting Schmitt trigger U1 A. The combination of R2, D2, V2 and Ul A thus acts as a voltage sensor which senses the output of the power control device and produces a logical high or low signal. The positive edge on the output (D') of Ul A is delayed by R3 and CI. The negative edge on the output of Ul A rapidly discharges CI with D3. CI, R3 ^' and D3 thus act as an asymmetrical delay circuit to delay the positive edge but not the negative edge. The asymmetrically-delayed signal is fed to the other input of N AND gate U2A.

U2A is a two-input NAND gate with Schmitt trigger inputs. The NAND gate compares the inverted gate drive signal with the power control device output signal. The delayed positive edges on both signals prevents the NAND gate from responding during the turn- on and turn-off times of the power control device.

Figure 7 shows the gate drive signal (A'), the power control device output signal (EV) and the NAND gate output signal (G') of Figure 6 during normal operation. Figure 8 shows the gate drive signal (A'), the power control device output signal (B') and the NAND gate output signal (G') of Figure 6 when a power control device short circuit event occurs.

Figure 9 is a timing diagram of the signals A', B', E', F and G' in Figure 6. The Schmitt NAND gate U2A compares signals E' and P. For normal operation, the signals are never both high at the same instant, as can be seen from Figure 9. The output of the NAND gate (GO will therefore be permanently high. If the power control device becomes short circuited, the inverted power control device signal (F') becomes permanently high. This results in a pulsed output from the NAND gate. This situation is shown in Figure 8. The output of the NAND gate is used to activate the circuit breaker 34 shown in Figure 2, in order to cut the excitation power to the generator.

In alternative embodiments, devices other than the voltage sensors described above may be used to sense the output of the control circuit or the power control device. For example, current sensors could be used instead of the voltage sensors. If the circuit is not common to ground, or if otherwise desired, optically-coupled isolators or Hall-effect transducers may be used to sense the outputs of the control circuit and the power control device, or level shifting techniques may be used. The embodiments described above are designed to detect a failure of the power control device which results in the device presenting a short circuit. Figure 10 shows another embodiment of the protection circuit 44, which is designed to detect any failure of the power control device 22 which results in the device failing to respond to the output of the control circuit 42. For example, the circuit of Figure 10 is able to detect failure of the power control device resulting in an open circuit or a permanent resistive state.

The circuit of Figure 10 has some parts in common with the circuit of Figure 3. These parts are given the same reference numerals and are not described further. In addition, the circuit of Figure 10 comprises asymmetrical delay circuit 58, inverter 60, AND gate 62, and OR gate 64. In operation, the voltage sensor 46, inverter 48, asymmetrical delay circuit 50, voltage sensor 52 and AND gate 54 function in the way described above to detect a short circuit of the power control device.

In the circuit of Figure 10, the output of the voltage sensor 46 is also fed to asymmetrical delay circuit 58. Asymmetrical delay circuit 58 delays the positive edge but not the negative edge of the signal at its input. The output of asymmetrical delay circuit 58 is fed to one input of AND gate 62. The output of voltage sensor 52 is also fed to inverter 60. The output of inverter 60 is fed to the other input of AND gate 62. The outputs of AND gate 54 and AND gate 62 are fed to the inputs of NOR gate 64. The output of NOR gate 64 is fed to the circuit breaker 34, and may also be fed to an alarm circuit.

In the circuit of Figure 10, if the power control device fails and produces an open circuit, the output of the power control device will be permanently low. The AND gate 62 detects when the output of the power control device is low while the output of the control circuit is high. The asymmetrical delay circuit 58 delays the positive edge of the control signal to prevent the AND from responding during the turn-on time of the power control device.

The voltage sensor 52 in Figure 10 is arranged to convert the output of the power control device to a logic signal having either a high or low state. For example, the voltage sensor 52 may include a Schmitt trigger to ensure that the output signal takes one of two states. Thus, if the power control device fails and presents a continuous resistive state, the output of the voltage sensor 52 will be continuously high or low depending on whether the output of the power control device is above or below the relevant threshold of the Schmitt trigger. In either case this is detected by the protection circuit, either by AND gate 54 or by AND gate 62. The output of the OR gate 64 is therefore a signal which goes high when the power control device fails to respond to the output of the control circuit 42.

It will be understood that various embodiments of the present invention have been described above purely byway of example, and modifications of detail can be made within the scope of the invention. For example, features described in relation to one embodiment may be provided with any of the other embodiments.