POWER CONTROL CIRCUIT

The present invention relates to a power control circuit for controlling an excitation field of a generator. The present invention has particular application in a brushless Automatic Voltage Regulator (AVR) controlled generator.

In brushless generators, a main rotor and stator provide the output power, and the excitation for the main rotor is provided by an exciter rotor. An AVR is used to control the level of excitation applied to the exciter field. The AVR responds to a voltage sensing signal derived from the main stator. By controlling the low power of the exciter field, control of the high power requirement of the main field is achieved.

In order to control the level of excitation applied to the exciter field, it is necessary for the AVR to include a power control device. Conventional AVRs use a thyristor as a power control device. The thyristor receives a single phase ac input, and a phase control circuit adjusts the amount of time in each cycle for which the thyristor is switched On', hi this way a half wave rectified dc output is produced, in which the length of the 'on' period is variable in dependence on the required level of excitation.

A disadvantage with using a single thyristor to control the excitation is that the maximum output of the thyristor is a half wave rectified version of a single phase supply. Consequently the maximum output that can be applied to the exciter field is significantly less than the theoretically available output, particularly in a three phase machine.

Various attempts have been made to overcome the shortcomings of the conventional power control device, hi one arrangement two thyristors are controlled so as to provide a variable full wave output from a single phase supply. However this arrangement requires more complex control, which may add to the cost and complexity of the device. Another arrangement uses a three phase rectifier with a controlled transistor. This arrangement can maximise the dc output using a relatively simple control. However this arrangement requires a large dc link capacitor which

increases the cost, and may limit the temperature to which the device can be subjected, hi addition there may be Electro Magnetic Compatibility (EMC) problems since pulses with fast rise and fall times are fed to the generator exciter, which may result in added filter costs.

According to a first aspect of the present invention there is provided a power control circuit for controlling an exciter field of a generator, the circuit having an input for receiving an ac input signal and an output for providing a dc output signal for the exciter field, the circuit comprising: a thyristor for producing the dc output signal; a thyristor control circuit for controlling the thyristor so as to control the dc output signal; and rectification means for selectively providing full wave rectification of the ac input signal; wherein the circuit is operable in a first mode in which the thyristor control circuit controls the thyristor so as to produce a controlled dc output signal, and a second mode in which the ac input signal is full wave rectified and the full wave rectified signal is supplied as the dc output signal.

By providing a first a first mode in which the thyristor control circuit controls the thyristor so as to produce a controlled dc output signal, relatively simple control of the dc output signal can be achieved. By providing a second mode in which the ac input signal is full wave rectified and the full wave rectified signal is supplied as the dc output signal, the dc output signal can be maximised when required. In the second mode there may be no need for control of the dc output signal, since the full wave rectified signal may be simply switched to the output. Thus the present invention can allow both a simple control method to be used, and a large output signal to be provided when required. By using a simple control method, less printed circuit board space may be required, and the cost and complexity of the circuit may be reduced.

The present invention may also avoid the need for a dc link electrolytic capacitor, which may reduce the cost and increase the life of the circuit. Furthermore, the present invention may reduce EMC problems compared to some known techniques.

The circuit may further comprise means for detecting when the generator output exceeds a predetermined threshold, and the circuit may be arranged to operate in the first mode when the predetermined threshold is not exceeded and in the second mode when the predetermined threshold is exceeded. In this way, when a high generator output power is required, the circuit may be switched to the second mode in which the full wave rectified signal is supplied as the dc output signal.

For example, the detecting means may be arranged to detect a generator overload condition, such as a severe load or a short circuit. Alternatively the detecting means may be arranged to detect a substantial load, but not necessarily an overload.

Many generators produce a three phase ac power supply. Thus the ac input signal may be a three phase ac input signal, and the rectification means may be arranged to provide full wave rectification of the three phase signal. This can allow the dc output signal to be maximised when required. However a single phase ac input signal could be used if desired, or if only a single phase supply is available.

The thyristor and the thyristor control circuit may be arranged to provide a variable half wave rectified dc output in the first mode. This can allow a simple control method to be used in the first mode, without the need to control two thyristors as would be the case if a full wave rectified dc output were to be provided.

The thyristor control circuit may be arranged to control the thyristor using phase control in the first mode. The full wave rectified signal may be supplied as the dc output signal without phase control when the circuit is operating in the second mode, which may simplify the control method.

The rectification means may comprise a rectifier circuit which is arranged to receive the ac input signal and selectively to produce either a discontinuous output signal or a continuous rectified output signal. In this case the output of the rectifier circuit may be supplied to the thyristor. When the output of the rectifier circuit is discontinuous, the thyristor may be controlled using a relatively simple control technique such as phase control. When the output of the rectifier circuit is continuous, this may ensure that the thyristor remains 'on', thereby supplying the full rectified signal to the output. Thus this arrangement can allow both relatively simple control of the dc output in the first mode, and also provide a convenient way of switching the full rectified signal to the output in the second mode.

The rectifier circuit may comprise, for example, a plurality of diodes and at least one further thyristor arranged to be switched 'off in the first mode and switched 'on' in the second mode. Preferably the rectifier circuit comprises two thyristors both of which are switched 'off' in the first mode and switched 'on' in the second mode. This can provide a convenient way of switching the output of the rectifier circuit between a discontinuous and a continuous signal.

The circuit may further comprise a rectifier control circuit for controlling the rectifier circuit so as selectively to produce either a discontinuous output signal or a continuous rectified output signal.

In some arrangements the thyristor itself may be part of the rectification means. This may reduce the total number of components required, since the thyristor used to control the dc output in the first mode may also be part of a rectifier circuit which produces a rectified signal in the second mode. In this arrangement the rectification means may further comprise a second, third and fourth thyristor which are arranged to be switched 'off in the first mode and switched 'on' in the second mode.

In some circumstances, it may be desired to provide a full wave rectified output in the first mode. In order to achieve this, the circuit may comprise a first and a second thyristor for producing the dc output, and the thyristor control circuit may be arranged

to control the first and second thyristors in the first mode so as to produce a single phase full wave rectified output signal.

In the above arrangement, the first and second thyristors may be part of the rectification means. In this way, the two thyristors which are used to control the dc output in the first mode may also be part of a rectifier circuit which produces a rectified signal in the second mode. For example, the rectification means may further comprise a third and a fourth thyristor which are arranged to be switched 'off in the first mode and switched 'on' in the second mode. The first, second, third and fourth thyristors may then form part of a three phase rectifier circuit when switched 'on' . Thus, by switching 'on' all four thyristors, a three phase rectified signal may be supplied to the output. The three phase rectifier circuit may further comprise two diodes, in order to provide a complete three phase rectifier.

In any of the above arrangements the circuit may further comprise a diode connected across the dc output. Such a diode may act as a freewheel diode for the generator exciter field inductive load during 'off periods of the thyristor which produces the dc output.

The present invention also provides an Automatic Voltage Regulator for a generator comprising a power control circuit in any of the forms described above. The present invention also extends to a generator having such an AVR.

According to another aspect of the present invention there is provided a method of controlling the power supplied to an exciter field of a generator, the method comprising receiving an ac input signal and producing therefrom a dc output signal, the method comprising the steps of: in a first mode, controlling a thyristor so as to produce a controlled dc signal, and supplying the controlled dc signal as the output signal; and in a second mode, full wave rectifying the ac input signal and supplying the full wave rectified signal as the dc output signal.

Any of the apparatus features may be provided as method features and vice versa.

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

Figure 1 shows parts of a generating set;

Figure 2 shows parts of a conventional AVR;

Figure 3 shows parts of an AVR according to a first embodiment of the present invention; Figures 4 and 5 show waveforms in the AVR of the first embodiment;

Figure 6 shows parts of an AVR according to a second embodiment of the present invention; and

Figure 7 shows parts of an AVR according to a third embodiment of the present invention.

Figure 1 shows parts of a generating set with which the present invention may be used. Referring to Figure 1, the generator comprises a main rotor 2 and a main stator 3 which provides the output power. Current is supplied to the coils in the main rotor 2 by means of exciter rotor 4, exciter stator 5 and rotating diodes 6. The main stator also provides power for excitation of the exciter field via AVR 7. The main rotor 2, exciter rotor 4 and rotating diodes 6 are all located on shaft 8 which is driven by a prime mover such as a diesel engine (not shown).

In operation the AVR 7 responds to a voltage sensing signal derived from the main stator winding, and controls the level of excitation supplied to the exciter field. By controlling the low power of the exciter field, control of the high power requirement of the main field is achieved through the rectified output of the exciter armature.

In the circuit shown in Figure 1, the power for excitation of the exciter field is provided by the main stator (self-excited generator). As an alternative, a separate permanent magnet generator (PMG) may be located on the shaft 8 and may provide the power for excitation of the exciter field (permanent magnet excited generator).

Figure 2 shows parts of a conventional AVR. Referring to Figure 2, an ac supply from the generator main stator is sensed by voltage sensor 12. Comparator 14 compares the sensed voltage with a reference voltage produced by reference voltage generator 16, and produces an error signal. The error signal is fed to phase control circuit 18, which controls output circuitry 20 in dependence thereon. Output circuitry 20 receives a single phase ac supply either from the generator main stator, or from a separate PMG if provided. The ac supply is rectified and controlled so as to produce a dc output for the exciter stator.

Output circuitry 20 comprises thyristor TY connected in series between the ac supply and the dc output. A thyristor is a device which is similar to a diode, but which is turned 'on' by a signal at its gate. Once turned 'on' a thyristor stays on until the current passing through it falls to near zero. In the circuit shown in Figure 2, the phase control circuit 18 supplies a trigger pulse to the gate of the thyristor TY at the appropriate time in the cycle of the ac signal. Once turned on, the thyristor remains on for the rest of the half cycle. By adjusting the point in the half cycle at which the thyristor is turn 'on', the dc output can be controlled.

Output circuitry 20 also comprises a diode D is connected across the dc output. Diode D is a freewheel diode for the generator exciter field inductive load during 'off periods of the thyristor TY.

In the arrangement shown in Figure 2 the maximum output of the output circuitry 20 is a half wave rectified version of a single phase ac supply. Consequently the maximum output voltage is significantly less than the theoretically available output voltage, particularly in a three phase machine.

Figure 3 shows parts of an AVR according to a first embodiment. Referring to Figure 3, voltage sensor 22 senses the voltage of the ac supply from the main stator.

Comparator 24 compares the sensed voltage with a reference voltage produced by

reference voltage generator 26, and produces an error signal. The error signal is fed to phase control circuit 28, which controls output circuitry 30 in dependence thereon.

Output circuitry 30 comprises thyristor TY3 and D5 connected in series. The thyristor TY3 is controlled by phase control circuit 28 to produce the required dc output. As in Figure 2, the diode D5 is a freewheel diode for the generator exciter field inductive load during "off periods of the thyristor TY3.

The AVR shown in Figure 3 also comprises rectifier circuit 32, overload detector 34 and rectifier control circuit 36. Overload detector 34 receives the error signal output from comparator 24, and compares the error signal to a threshold level. When the magnitude of the error signal exceeds the threshold, this indicates an overload condition at the generator output. Such an overload condition may be caused by, for example, a severe load or a short circuit. The overload detector 34 outputs a signal to rectifier control circuit 36 and phase control circuit 28 when an overload condition is detected. Rectifier control circuit 36 performs control of rectifier circuit 32 in response to the overload condition.

Rectifier circuit 32 comprises diodes Dl, D2, D3 and D4, and thyristors TYl and TY2, which are connected in the formation of a three phase rectifier. The circuit 32 receives a three phase ac supply either from the generator main stator, or from a separate PMG if provided, hi normal operation, when no overload condition is detected by overload detector 34, TYl and TY2 are arranged to be in the 'off state. When TYl and TY2 are in the 'off state, Dl to D4 rectify the three phase ac input in such a way that the output voltage is discontinuous. This allows the thyristor TY3 in the power control device 30 to be operated using simple phase control, in a similar way to that described above with reference to Figure 2. In particular, phase control circuit 28 controls the dc output by adjusting the amount of time in each cycle for which the thyristor TY3 is switched on.

When overload detector 34 detects an overload condition, overload control circuit 36 acts to turn 'on' TYl and TY2. This causes the output voltage of the three phase

rectifier (D1-D4 and TYl and TY2) to be continuous. During the overload condition TY3 is held in the 'on' state, thus delivering the three phase rectified output into the generator exciter field. In this way the maximum available voltage is supplied to the generator exciter field during overload conditions.

Once the overload condition is over, TYl and TY2 are switched back to the 'off state, returning the rectifier output to discontinuous voltage. This allows TY3 to resume phase control of the dc output.

Figure 4 shows the waveform at the output of the rectifier circuit 32 when the thyristors TYl and TY2 are in the 'off state. It can be seen from Figure 4 that, in this state, the output of the rectifier circuit returns periodically to zero. Thus, when the waveform shown in Figure 4 is applied to output circuitry 30 in Figure 3, the thyristor TY3 will switch 'off each time the waveform falls to zero. Phase control circuit 28 controls the point in the cycle at which the thyristor TY3 is turned 'on', hi this way a controllable dc output is produced for the generator exciter field.

Figure 5 shows the waveform at the output of the rectifier circuit 32 when the thyristors TYl and TY2 are in the 'on' state. In this state, the rectifier circuit produces a three phase rectified output which does not return to zero. In this state, once switched on, thyristor TY3 in output circuitry 30 remains on, thereby delivering the three phase rectified output into the generator exciter field.

In the first embodiment shown in Figure 3, a three phase fully rectified ac input is selectively switched to the output of the circuit. As an alternative, a single phase fully rectified ac input could be selectively switched to the output. In this case the diode D4 and the thyristor TY2 in Figure 3 could be dispensed with, and the rectifier circuit would receive a single phase ac input. Thyristors TYl and TY3 would both be kept 'on' when an overload condition was detected.

Figure 6 shows parts of an AVR according to a second embodiment. In Figure 6, voltage sensor 42 senses the voltage of the ac supply from the main stator.

Comparator 44 compares the sensed voltage with a reference voltage produced by reference voltage generator 46, and produces an error signal. The error signal is fed to phase control circuit 48, and overload detector 54. Overload detector 54 compares the error signal to a threshold level, and outputs a signal to overload control circuit 56 and phase control circuit 48 when an overload condition is detected.

Output circuitry 50 receives a three phase ac supply either from the generator main stator, or from a separate PMG if provided, and produces a dc output for the exciter field. Operation of output circuitry 50 is controlled by phase control circuit 48 and overload control circuit 56.

Output circuitry 50 comprises diodes Dl to D3 and thyristors TYl to TY4. During normal operation when no overload condition is detected by overload detector 54, overload control circuit 56 keeps thyristors TY3 and TY4 in the 'off state. Li this state the input to output circuitry 50 is effectively a single phase supply received at the anodes of thyristors TYl and TY2. While thyristors TY3 and TY4 are kept 'off, phase control circuit 48 controls the points in the single phase cycle at which thyristors TYl and TY2 are switched on, with one of the thyristors being switched on in each half cycle. In this way a controlled single phase full wave rectified signal is produced at the output of output circuitry 50.

When overload detector 54 detects an overload condition, overload control circuit 56 acts to turn 'on' thyristors TY3 and TY4. The effect of this is to deliver a three phase rectified signal to the output of output circuitry 50. In this way the maximum available voltage is supplied to the generator exciter field during overload conditions.

Once the overload condition is over, TY3 and TY4 are switched back to the 'off state, returning the output of the power control circuit 50 to a controlled single phase full wave rectified signal.

Figure 7 shows parts of an AVR according to a third embodiment. In this embodiment half wave phase control is used during normal operation, while a three

phase fully rectified output is produced during overload conditions. This embodiment may allow simpler phase control than in the second embodiment.

Referring to Figure 7, voltage sensor 62, comparator 64, reference voltage generator 66 and overload detector 64 act in a similar way to the corresponding parts in Figure 6. Phase control circuit 68 and overload control circuit 76 control the operation of output circuitry 70 so a to produce a single phase half wave rectified output during normal operation, and a three phase fully rectified output during overload conditions.

During normal operation when no overload condition is detected by overload detector 74, overload control circuit 76 keeps thyristors TY2, TY3 and TY4 in the 'off state. While thyristors TY2, TY3 and TY4 are kept 'off, phase control circuit 68 controls the point in the single phase cycle at which thyristor TYl is switched on. In this way a controlled single phase half wave rectified signal is produced at the output of output circuitry 70.

When overload detector 74 detects an overload condition, overload control circuit 76 acts to turn 'on' thyristors TY2, TY3 and TY4. The effect of this is to deliver a three phase rectified signal to the output of output circuitry 70. In this way the maximum available voltage is supplied to the generator exciter field during overload conditions.

Once the overload condition is over, TY2, TY3 and TY4 are switched back to the 'off state, returning the output of the power control circuit 70 to a controlled single phase half wave rectified signal.

The power control circuits of the various embodiments described above may provide one or more of the following advantages compared to conventional control circuits:

• Overall lower cost power topology

• Longer life (no dc link electrolytic capacitor) • Simple control method

• Less printed circuit board space required

• Lower EMC.