GB2494715A - Power control circuit for self-excited electrical generator - Google Patents

Abstract

A power control circuit for controlling excitation in a self-excited electrical generator comprises a normally-on power control device arranged to deliver a rectified residual voltage to an excitation winding. This allows the residual voltage required to initiate the self-excitation process to be reduced, thereby allowing the self-excitation process to be initiated more reliably. The circuit receives an ac supply from the main stator and the thyristor delivers half wave rectified current to the current windings. A freewheel diode D is connected across the excitation winding and conducts during negative half cycles when the thyristor is off. The thyristor is controlled by a normally on depletion mode field effect transistor (FET) Q1 which may be operable in two modes: a first (start-up) mode where the device is used to build up the excitation by producing a residual voltage from the residual magnetism in the generator; and a second (steady state) mode where power to the excitation is controlled.

Description

POWER CONTROL CIRCUIT

The present invention relates to a power control circuit for controlling the excitation in a self-excited electrical generator, and in particular a power control circuit which facilitates S initiation of the self-excitation process.

Electrical generators typically operate by rotating a magnetic field produced.by a rotor relative to windings in a stator in order to generate an AC output in the stator windings.

The rotor's magnetic field may be produced by passing an excitation current through windings in the rotor. In a self-excited generator, the excitation current is derived from the output of the genelator. The excitation current may be applied directly to the rotor windings through slip rings or other means, or alternatively it may be generated by an exciter mounted on the shaft of the generator. An automatic voltage regulator (AVR) may be provided to control the excitation.

Known AVR5 include a power control circuit in order to control the level of excitation. The power control circuit includes a power control device, which may be, for example, a thyristor or a transistor such as a MOSFET or an IGBT. The power control device is controlled by a phase control circuit or pulse width modulation circuit, in order to control the voltage and/or current supplied to the windings.

When a self-excited generator is first started, a small voltage is produced at the generator output due to residual magnetism in the generator. In order to initiate the self-excitation process, the automatic voltage regulator has to deliver a rectified version of the residual voltage to the excitation windings, which in turn will produce more terminal voltage at the generator output. This requires the residual voltage to be sufficiently high to switch on the power control device. For example, known AVRs typically have a residual voltage requirement of 4-6V in order to initiate the self-excitation process.

In practice, the residual magnetism and resulting voltage can vary significantly due to variations in the characteristics of the electrical steel used in the construction of the generator. If the residual voltage does not exceed the minimum requirements for the automatic voltage regulator, the generator will not function. Sometimes magnets have to be fltted to the exciter field or the rotor to guarantee a certain level of residual voltage.

It would therefore be desirable to provide a power control circuit which can reliably initiate the self-excitation process.

According to one aspect of the present invention there is provided a power control circuit for controlling excitation in a self-excited electrical generator, wherein initial power for the excitation is derived from a residual magnetism in the generator, the power control circuit comprising a normally-on power control device arranged to deliver a rectihed residual voltage to an excitation winding.

By arranging a normally-on power control device to deliver a rectified residual voltage to the excitation winding, the residual voltage required to initiate the self-excitation process can be reduced. This can allow the self-excitation process to be initiated more reliably.

The excitation winding may be a winding in the rotor of the generator, or alternatively a winding in a separate exciter. The residual voltage may be rectified by means of a thyristor, a diode, a diode bridge or any other means for converting an ac supply into a dc voltage. Half wave or full wave rectification may be used as appropriate.

The normally on power control device is preferably a device which does not have a turn on threshold. Preferably the device is on (equiva'ent to a closed switch) even when the voltage at a control terminal is zero. For example, the device may be a transistor which is doped such that a channel exists even with zero voltage at its gate or base. The device may conveniently be a depletion-mode field effect transistor (FED, for example, a depletion-mode MOSFET, JFET or IGFET.

The circuit may be operable in a first mode in which the self-excitation process is initiated, and a second mode in which power to the excitation winding is controlled. In this case, the normally-on power control device may be arranged to deliver a rectified residual voltage to the excitation winding in the first mode. Preferably, in the first mode, all of the available residual voltage is rectified and delivered to the excitation winding. This may be achieved by keeping the normally-on power control device in a conducting state in the first mode.

The circuit may be arranged to switch from the first mode to the second mode when the output of the generator reaches a predetermined level. Preferably the predetermined level is a voltage at which normal AVR functions can be supported.

The circuit may further comprise a second power control device arranged to control power fed to the excitation winding in the second mode. This can allow a more standard power control device such as a thyristor, MOSFET or IGBT to be used in the second mode.

Such a device may have a higher turn-on threshold than the normally-on power control device, but on the other hand may have a higher power rating. This may help to minimise the cost of the circuitry, since normally-on devices with high power ratings may be more expensive.

In one embodiment, the second power control device is a thyristor, and the normally-on power control device is arranged to switch on the thyristor in the first mode. In this embodiment, an output of the normally-on power control device may be connected to the gate of the thyristor. This may allow the thyristor to be switched on using a lower residual voltage than would otherwise be the case. In this embodiment, the normally-on power control device may be arranged to control the thyristor in the second mode. In this way, the same device may be used to control the thyristor in both modes.

In another embodiment, the normally-on power control device is arranged to bypass the second power control device in the first mode. This may be achieved by connecting the normally-on power control device in parallel to the second power control device. By bypassing the second power control device, the normally-on power control device may 23 be able to deliver power to the excitation winding at a lower residual voltage. In this case the normally-on power control device may be arranged to be switched off in the second mode.

In a further embodiment, the normally-on power control device is arranged to control the power supplied to the excitation winding in the second mode, as well as to deliver a rectified residual voltage to the excitation winding in the first mode. This can allow a single power control device to be used in both modes.

Preferably a control circuit is provided for controlling the operation of the normally-on power control device and/or the second power control device.

In another aspect of the invention there is provided an automatic voltage regulator for a self-excited electrical generator comprising a power control circuit in any of the forms described above.

In another aspect of the invention there is provided a self-excited electrical generator comprising a power control crcuit or an automatic voltage regulator in any of the forms described above.

The electrical generator may comprise a main machine and an exciter for exciter for exciting the main machine. In this case the excitation winding may be a winding in the exciter. For example, the excitation winding may be a winding in the stator of the exciter.

Alternatively, the excitation winding may be a winding in the rotor of the generator. In this case, the excitation current may be provided to the excitation winding via a rotating electrical connector such as slip rings or other means.

According to another aspect of the invention there is provided a method of initiating a self-excitation process in an electrical generator. wherein initial power for an excitation winding is derived from a residual magnetism in the generator. the method comprising delivering a rectified residual voltage to the excitation winding using a normally-on power control device.

Features of one aspect of the invention may be applied to any other aspect. Any of the apparatus features may be provided as method features and vice versa.

Preferred embodiments of the invention will now be described, purely by way of exam pie, with reference to the accompanying drawings, in which: Figure 1 shows an overview of a self-excited electrical generator; Figure 2 shows parts of a conventional AVR; Figure 3 shows parts of a power control circuit in a first embodiment of the invention; Figure 4 shows parts of a power control circuit in a second embodiment of the invention; figure 5 shows parts of a power control circuit in a third embodiment of the invention; and FigureS shows parts of a power control circuit in a fourth embodiment of the invention.

An overview of a self-excited electrical generator is shown in Figure 1. The generator comprises a main machine 1 which includes a main rotor2 and a main stator3. The main rotor 2 is located on a shaft 4 which is driven by a prime mover such as a diesel engine (not shown). The main rotor 2 develops a magnetic field, so that rotation of the main rotor 2 relative to the main statar 3 causes a current to be generated in windings in the main stator. In order for the main rotor to develop a magnetic field, an excitation current is passed through windings in the rotor 2.

In the arrangement of Figure 1, the excitation current is generated by a separate exciter S mounted on the same shaft 4 as the main machine 1. The exciter 5 comprises exciter rotor 6, exciter stator 7, and rotating diodes 8. Rotation of the exciter rotor 6 relative to the exciter stator 7 generates an AC output in windings in the exciter rotor. This AC output is converted to DC by the rotating diodes 8, and the DC output of the rotating diodes is fed to the main rotor 2. In this arrangement, the excitation windings are in the exciter stator 7.

Power for the exciter S is drawn from the main stator 3, via an AVR 1 0. This is referred to as a self excitation (i.e. the generator provides its own excitation). The AVR 10 controls the power supplied to the excitation windings in the exciter stator 7. By controlling the relatively low power which is fed to the exciter stator 7, control of the high power in the main rotor 2 is achieved through the rectified output of the exciter rotor.

In an alternative arrangement, the generator does not include an exciter, and the AVR is connected directly to the main rotor windings via a rotating electrical connector such as slip rings. In this arrangement the excitation windings are the main rotor windings.

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 circuit 20 in dependence thereon. Output circuit 20 receives an ac supply from the main stator, which is rectified and controlled so as to produce a dc output for the excitation windings.

The output circuit of Figure 2 includes a power control device in the form of thyristor TV.

The phase control circuit 18 supplies a trigger pulse to the gate of the thyristorTV at the appropriate time in the cycle of the ac supply. Once turned on, the thyristor remains on for the rest of the half cycle. Thus the thyristor acts to rectify the ac supply to produce a dc output. By adjusting the point in the half cycle at which the thyristor is turn on, the dc output can be controlled. Output circuit 20 also includes a freewheel diode D which is connected across the inductive excitation windings and conducts during negative half cycles when the thyristor is off.

Figure 3 shows parts ofa power control circuit in accordance with a first embodiment of the invention. Referring to Figure 3. the circuit receives an ac supply from the main stator.

A thyristor TV acts as a power control device and in normal operation delivers half wave rectified current to the excitation windings. A freewheel diode D is connected across the excitation windings. The thyristor TY and diode D perform a similar function to the corresponding components shown in Figure 2. Ri,Cl, and R2, C2, are snubber circuits which suppress switching transients and limit the rate of change in voltage.

In the arrangement of Figure 3, the thyristor TV is controHed by a depletion-mode field effect transistor (FED 01. The drain ofOl is connected to the mid point of the R2, C2 snubber circuit by means of a diode Dl and resistor R3. Dl protects Q1 from reverse polarity and R3 limits the current. The source ofOl is connected to the gate of the thyristor TV. R4 is a gate cathode termination resistor forTY to improve immunity to applied rate of change in voltage in the off state.

The transistor Olin the arrangement of Figure 3 is a depletion-mode FET. A depletion mode FEE is one in which a channel exists between the source and drain when no voltage is applied to the gate. In order to turn the device off, a gate-source voltage is applied Thus a depletion mode FF1 is equivalent to a normally closed switch, which can be opened by application of a voltage.

S In the arrangement of Figure 3, the depletion mode FET 01 is operated in two modes. In the first mode (start-up) the device is used to build up the excitation. In this mode, a residual voltage is produced from the residual magnetism in the generator (for example in the exciter field and/or the main rotor). During positive half cycles of residual voltage conditions, field current due to residual magnetism initially flows through R2, Dl, R3 and 01. This current flows into the gate of the thyristorTY until it is triggered, after which time the current is transferred to the thyristor TY. This allows a rectified version of the residual voltage to be delivered to the excitation windings. Once the thyristor is turned on, the charge on the capacitor C2 transiently sustains the gate current to thyristor TY. In this arrangement 01 is used to anode Iire'TY.

Since 01 is a depletion mode device, current will flow even with no voltage applied to its gate. Thus 01 can conduct with a lower level of initial voltage than is required for the thyristor TY. As a consequence, 01 can allow the build up of excitation starting from a lower level of residual voltage than is required forthe thyristorTY. Thus the arrangement of Figure 3 can reduce the residual voltage requirements of the generator, thereby allowing the self-excitation process to be initiated more reliably.

When the generator voltage has reached a predetermined level which is sufficient to support the AVR circuitry, 01 is operated in its second (steady state) mode. In this mode, 01's gate is pulse width modulated with a negative bias in phase control mode by the AVR control circuit 22.

Figure 4 shows parts of a power control circuit in a second embodiment. As in the first embodiment, a thyristor TY acts as a power control device and in normal operation delivers half wave rectified current to the excitation windings. Diode D is a freewheel diode, and RI, Cl and R2, C2, are snubber circuits.

In the arrangement of Figure 4, a depletion mode FF101 is connected in parallel with the main controlling power device TY, via a diode Dl. Dl protects 01 from reverse polarity during negative half cycles. During positive half cycles of residual voltage conditions, held current initially flows through Dl and Qi (which is normally on). This causes the generator's output voltage to increase, thereby initiating the self-excitation process. Since QI is normally on, the self-excitation process can be initiated with a lower residual voltage than would be required by the thyristor TY.

In Figure 4, when the generator's output voltage has reached a level which is sufficient to support the AVR circuitry, the control circuit 24 turns off Qi by taking its gate to a negative bias. The control circuit 24 then drives the gate of the thyristor TY in phase control mode.

In the arrangement of Figure 4,01 is used as a start up (build up) device only and is held off during normal AVR operation.

FigureS shows parts of a power control circuit in a third embodiment. In the arrangement of Figure 5, a depletion mode FET Q1 is used as the main power control device. A full-wave, single-phase bridge circuit consisting of diodes D2 to D5 is used to convert the AC output from the main stator into rectified DC. The depletion mode FETO1 is used to deliver the rectified current to the excitation windings. D is a freewheel diode for the inductive field current which conducts when Qi turns off.

In the arrangement of FigureS, when residual voltage is applied to the lull wave bridge circuit, field current flows through 01 (which is normally on) and causes the generator output voltage to increase. When the voltage has reached a predetermined level which is sufficient to support the AVR circuitry, Cl's gate is pulse width modulated with a negative bias by the AVR control circuit 26.

In the arrangement of Figure 5,01 is used as both the start up (build up) device and the main controfling power device.

As an alternative to the single-phase bridge rectifier shown in Figure 5, a 6-diode 3-phase bridge rectifier, or other diode combinations, could be used to produce a rectified dc supply.

Figure 6 shows parts of a power control circuit in a fourth embodiment. In the arrangement of Figure 6, a MOSFET or IGBT is used as the main power control device, and a depletion mode FET Qi is used as a start up device. When residual voltage is applied to the full wave bridge input, field current flows through Qi (normally on) and causes the generator output voltage to increase. When the voltage has reached a sufficient level to support the AVA circuitry, Oils turned off by taking its gate to a negative bias. 02's gate is then driven in PWM (pulse width modulation) control mode by the control circuit 28.

Thus, in the arrangement of Figure 6, Q1 is used as a start up (build up) device only and is held off during normal AVR operation.

The arrangement of Figure 6 requires a depletion mode FET with a lower current rating than that of Figure 5. This may reduce costs, particularly where depletion mode FETs at high current ratings are more expensive than their enhancement mode MOSFET, IGBT or similar device courter parts, In any of the above embodiments the depletion mode FET may be any suitable type, such as a depletion mode MOSFET, JEET or IGFET. While an n-channel device is shown in the figures, a p-channel device could be used instead. Furthermore, any type of normally-on power control device could be used instead of a FET.

Claims (1)

1. <claim-text>CLAIMS1. A power control circuit for controlling excitation in a self-excited electrical generator, wherein initial power for the excitation is derived from a residual magnetism in the generator, the power control circuit comprising a normally-on power control device arranged to deliver a rectified residual voltage to an excitation winding.</claim-text> <claim-text>2. A circuit according to claim 1 1wherein the normally-on power control device is adepletion mode field effect transistor.</claim-text> <claim-text>3. A circuit according to claim 1 or 2, wherein the circuit is operable in a first mode in which the sell-excitation process is initiated, and a second mode in which power to the excitation winding is controlled, 4. A circuit according to claim 3, wherein the normally-on power control device is arranged to deliver a rectified residual voltage to the excitation winding in the first mode.5. A circuit according to claim 3 or4, wherein the normally-on power control device is kept in a conducting state in the first mode.6. A circuit according to any of claims 3 to 5, arranged to switch from the first mode to the second mode when the output of the generator reaches a predetermined level.7. A circuit according to any of claims 3 to 6 further comprising a second power control device arranged to control power fed to the excitation winding in the second mode.8. A circuit according to claim 7, wherein the second power control device has a higher tu rn-on threshold than the normally-on power control device, 9. A circuit according to claim 7 orB, wherein the second power control device has a higher power rating than the normally-on power control device.10. A circuit according to any of claims 7 toY, wherein the second power control device is a thyristor, and the normally-on power control device is arranged to switch on the thyristor in the first mode.11. A circuit according to claim 10. wherein the normally-on power control device is arranged to control the thyristor in the second mode.12. A circuit according to any of claims 7 to9, wherein the normally-on power control device is arranged to bypass the second power control device in the first mode.13. A circuit according to claim 12, wherein the normally-on power control device is connected in parallel with the second power control device 14. A circuit according to claim 12 or 13, wherein the normally-on power control device is arranged to be switched off in the second mode.15. A circuit according to any of claims 3 toó, wherein the normally-on power control device is arranged to control the power supplied to the excitation winding in the second mode.16. A circuit according to any of the preceding claims, further comprising a control circuit for controlling the operation of the normally-on power control device and/or the second power control device.17. An automatic voltage regulator for a self-excited electrical generator comprising a power control circuit according to any of the preceding claims.18. A self-excited electrical generator comprising a power control circuit or an automatic voltage regulator according to any of the preceding claims.19. An electrical generator according to claim 18, the generator comprising a main machine and an exciter for exciter for exciting the main machine, wherein the excitation winding is a winding in the exciter.20-An electrical generator according to claim 18, the generator comprising a rotor and a stator, wherein the excitation winding is a winding in the rotor.2 -A method of initiating a self-excitation process in an electrical generator, wherein initial power for an excitation winding is derived from a residual magnetism in the generator, the method comprising delivering a rectified residual voltage to the excitation winding using a normally-on power control device.22. A power control circuit substantially as described herein with reference to and as illustrated in the accompanying drawings.23. A method of initiating a self-excitation process substantially as described herein with reference to the accompanying drawings.</claim-text>