GB2486426A

GB2486426A - Protection of generator windings

Info

Publication number: GB2486426A
Application number: GB201021128A
Authority: GB
Inventors: Denis Hedley Dalby; Michael John Wright
Original assignee: Cummins Generator Technologies Ltd
Current assignee: Cummins Generator Technologies Ltd
Priority date: 2010-12-13
Filing date: 2010-12-13
Publication date: 2012-06-20
Anticipated expiration: 2030-12-13
Also published as: GB201021128D0; GB2486426B

Abstract

A protection circuit 34 for an electrical generator that comprises a main machine 10 producing a multiphase output and an exciter 18 providing excitation to the main machine 10 comprises means 36 for sensing the voltage of each phase of the multiphase output of the generator, means 38 for comparing the voltage of each phase with a reference voltage, and means for reducing the excitation if the difference between at least one of the sensed voltages and the reference voltage exceeds a threshold. The circuit may include surge protection devices 28, 30, 32 that do not include fuses. The excitation may be reduced by a circuit breaker (fig 2, 44) cutting the power to the exciter. Excitation may be reduced if the difference between only one of the sensed voltages and the reference exceeds a threshold but may be prevented if all of the sensed voltages differ from the reference by a threshold. The circuitry 34 can be part of an automatic voltage regulator and reduces or cuts the excitation in response to an asymmetrical short circuit in less time than in response to a symmetrical short circuit, which allows the use of surge protection devices without fuses.

Description

PROTECTION OF GENERATOR WIN DINGS

The present invention relates to techniques for protecting the windings in an electrical generator, and in particular techniques for protecting stator windings from transent over-voltages. The present invention has particular application in protecting stator windings from short circuits resulting from the failure of surge protection devices.

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.

Surge protection devices (SPDs) are a common way of protecting electrical equipment from damage and degradation due to high-energy transient voltage surges. Surge protection devices are designed to absorb and dbsipate the excess energy of such voltage surges. Thus a surge protection device may be connected directly across the electrical generator, in order to protect its windings.

Typically surge protection devices include a fuse in order to protect the device in the event of a sustained voltage surge. The fuse allows the device to remove itself electrically from the circuit before it is damaged. However surge protection devices without fuses have become available. Fuse-less surge protection devices may have the advantage of reduced circuit resistance and inductance, providing superior voltage clamping performance. Fuse-less surge protection devices are preferably able to withstand a specified fault current in failure mode until a circuit breaker in the system trips.

When using a surge protection device in a generator supplying a load, it would be desirable to connect the device directly across the generator stator windings with minimal bus bar length to earth. However, for a surge protection device without a fuse, this would result in no protective circuit breaker being present at the generator terminals. As a consequence, in the event of failure of the surge protection device, the generator would be short-circuited. Although the generator control system may be equipped with an over-

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excitation monitor which would cut off the excitation, there may be a delay before this comes into operation, potentially resulting in an over-current in the stator wind ings. For this reason it has been assumed that surge protection devices which are connected to a generator need to be either fitted with a fuse, or else located on the other side of a circuit breaker.

According to one aspect of the present invention there is provided protection circuitry for an electrical generator, the electrical generator comprising a main machine for producing a multiphase output, and an exciter for providing excitation to the main machine, the circuitry comprising: means for sensing the voltage of each phase of the multiphase output of the generator; means for comparing the voltage of each phase with a reference voltage; and means for reducing the excitation if the difference between at least one of the sensed voltages and the reference voltage exceeds a threshold.

The present invention provides the advantage that, if one of the generator output phases is short circuited, for example through failure of a surge protection device, this can be detected quickly and the excitation reduced in order to prevent damage to the stator windings. This can allow the fitting of surge protection devices without fuses to the output of the generator.

Preferably a plurality of surge protection devices are each connected to a respective phase of the generator output. This can allow electrical equipment connected to the generator to be protected against short duration, high energy electrical surges. Preferably the surge protection devices do not include fuses. This can allow the resistance and inductance of the devices to be reduced, improving the voltage clamping performance. However the present invention may also be used with surge protection devices which do include fuses.

Preferably, if the difference between at least one of the sensed voltages and the reference voltage exceeds a threshold, the excitation Is reduced to zero. For example, the means for reducing the excitation may comprise a circuit breaker, which may be arranged to cut the source of power to the exciter. Alternatively, the means for reducing the excitation may be arranged to control the function of a separate excitation control circuit. For example, the means for reducing the excitation may be arranged to control the function of an automatic voltage regulator, which may then reduce the excitation by means of a control signal fed to an output circuit. In this case the excitation may be reduced to zero or some other value.

The means for reducing the excitation may be arranged reduce the excitation if the difference between at least one but not all of the sensed voltages and the reference voltage exceeds a threshold. For example, the circuitry may comprise means for determining whether all of the sensed voltages differ from the reference voltage by more than a threshold, and means for preventing the excitation from being reduced if all of the sensed voltages differ from the reference voltage by more than the threshold. This can allow the excitation to be cut quickly in the event of an asymmetrical short circuit (i.e. when a subset of the total number of phases is short circuited), while aflowing power to be supplied for a longer period of time in the event of a symmetrical short circuit (i.e. when all phases are short circuited). This may improve the low-voltage ride-thiough capability of the generator, while preventing damage to the stator windings which might occur through failure of a surge protection device, or some other asymmetrical short circuit event.

In one embodiment the protection circuitry is implemented as part of an automatic voltage regulator. Thus the present invention extends to an automatic voltage regulator comprising protection circuitry in any of the forms described above. In this embodiment the protection circuitry may share some of the components of the automatic voltage regulator. For example, the integrated protection circuitry and AVR may share one or more of: a power supply; voltage sensors for sensing the main stator voltages; mean value circuits; a processor; and a circuit breaker. This can allow the protection circuitry to be implemented at low additional cost.

The automatic voltage regulator may further comprise an excitation monitor for monitoring the excitation and cutting the excitation if the excitation exceeds a predetermined threshold. Preferably the excitation monitor is arranged to cut the excitation if a symmetrical short circuit occurs, while the protection circuitry is arranged to reduce or cut the excitation if an asymmetrical short circuit occurs. Alternatively, the protection circuitry may be arranged to cut the excitation in either case.

Preferably the protection circuitry and/or automatic voltage regulator is arranged to reduce or cut the excitation in response to an asymmetrical short circuit in less time than in response to a symmetrical short circuit. For example, the time delay between detecting an asymmetrical short circuit and cutting the excitation may be around 2 seconds or less, while the time delay between detecting a symmetrical short circuit and cutting the excitation may be around 10 seconds or less. In each case, the delay may be fixed or variable.

The invention extends to an electrical generator including the protection circuitry or automatic voltage regulator. Thus, according to another aspect of the invention there is provided an electrical generator comprising a main machine for producing a multiphase output, an exciter for providing excitation to the main machine, and protection circuitry or an automatic voltage regulator in any of the forms described above.

The generator is preferably a three phase generator, although any number of phases greater than one may be used. Power for the exciter may be provided from the main stator (self-excitation), or from any other power source, such as a permanent magnet machine, which may be mounted on the same rotor.

In any of the above arrangements, the reference voltage and the threshold may be the same for each phase, or different reference voltages and/or thresholds may be set for each phase. The reference voltage and/or the threshold may be preset, or may be variable.

According to another aspect of the present invention there is provided a protection circuit which protects an electrical generator, the electrical generator comprising a main machine which produces a multiphase output, and an exciter which provides excitation to the main machine, the circuit comprising: a plurality of voltage sensors, each of which senses the voltage of one phase of the niultiphase output of the generator; a plurality of comparators, each of which compares the voltage of one phase with a reference voltage; and a circuit breaker which cuts the excitation if the difference between at least one of the sensed voltages and the reference voltage exceeds a threshold.

According to another aspect of the present invention there is provided a method of protecting an electrical generator, the electrical generator comprising a main machine for producing a multiphase output, and an exciter for providing excitation to the main machine, the method comprising: sensing the voltage of each phase of the multiphase output of the generator; comparing the voltage of each phase with a reference voltage; and reducing the excitation if the difference between at least one of the sensed voltages and the reference voltage exceeds a threshold.

Apparatus features may be provided with the method aspect and vice versa. Features of one aspect of the invention may be provided with any other aspect.

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 a protection circuit; Figure 2 shows parts of an automatic voltage regulator; Figure 3 shows parts of a protection circuit; and Figure 4 shows a circuit diagram of a protection circuit.

An overview of a generator with a protection circuit according to the present invention is shown in Figure 1. Referring to Figure 1, the generator comprises a main machine 10 which includes a main rotor 1 2 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. In the example shown, the main stator includes three groups of windings, which produce a three-phase AC output.

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.

In the arrangement of Figure 11 power for the exciter 1 8 is drawn from the main stator 141 via an AVR 26. This k referred to as a self excitation (i.e. the generator provides its own excitation). 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.

Figure 2 shows in more detail parts of AVR 26. Referring to Figure 2, the AVR comprises voltage sensor 36, control circuit 38, output circuit 40, excitation monitor 42, and circuit breaker 44. In operation, the voltage sensor 36 senses the voltage at the output of the main stator. The sensed voltage is fed to control circuit 38. The control circuit compares the sensed voltage to a reference voltage, and produces a control signal which is fed to output circuit 40. The output circuit 40 controls the flow of power from the main stator to the exciter in dependence on the control signal.

Although in Figure 2 a single line is shown from the main stator, if desired all of the output phases, or any subset thereof, may be sensed and/or used to power the exciter.

In an alternative arrangement, power for the exciter is drawn from a separate permanent magnet machine mounted on the shaft 16, rather than from the main stator. In this case, the output circuit controls power from the separate permanent magnet machine, rather than from the main stator.

The AVR shown in Figure 2 includes an excitation monitor 42. The excitation monitor monitors the amount of power which is fed to the exciter. If the amount of excitation exceeds a predetermined limit for a certain period of time, a circuit breaker 44 is opened, thereby cutting off the power to the exciter. For example, if the output terminals are shorted, the excitation will increase in an attempt to maintain the output voltage. The excitation monitor detects the increased excitation, and cuts off the power to the exciter, thereby cutting the excitation. The excitation monitor thus functions to protect the generator from excessive currents in the machine windings. However, generators are often required to maintain output power for short periods of time even if the output terminals are shorted or at low voltage (low-voltage ride-through). Thus the excitation may be maintained for a certain period of time, for example around 10 seconds, before the excitation is cut.

Referring back to Figure 1, a plurality of surge protection devices 28, 30, 32 are provided at the generator output. The surge protection devices are connected directly across the main stator windings. The surge protection devices protect the generator windings from damage and degradation due to short duration, high-energy voltage surges.

Surge protection devices are known in the art, and are available in a wide variety of ratings and packages. They are designed to absorb and dissipate the excess energy of short duration, high-energy voltage surges. They typically comprise Metal Oxide Varistors or Silicon Avalanche Diodes which break down under high voltage in order to short the surge away from the equipment being protected. In the present embodiment the surge protection devices 28, 30, 32 do not include a fuse. Such devices have the advantage of reduced circuit resistance and inductance, which leads to improved voltage clamping performance in comparison to fused surge protection devices.

The surge protection devices 28, 30, 32 are designed such that, should they fail, they do so in a short circuit state. Thus the devices are designed to withstand a required fault current in failure mode until a circuit breaker in the system trips.

In the event of failure of one or more of the surge protection devices 28, 30,32, the excitation monitor in AVR 26 will cut off the excitation after a certain period of time, in order to protect the generator windings. However it has been found pursuant to the present invention that, while the excitation monitor may provide adequate protection if all three surge protection devices fail simultaneously, it may not provide adequate protection if only one or two of the surge protection devices fail.

Referring again to Figure 1, the generator is provided with a protection circuit 34. The protection circuit 34 is designed to protect the main stator windings and/or the exciter windings in the event of the failure of one or more of the surge protection devices 28, 30, 32.

Figure 3 shows parts of the protection circuit 34. Referring to Figure 3, the circuit comprises three voltage sensors 46i, 462,463, three mean value circuits 48, 482,483, three comparators 5O, 502, 503, reference voltage generator 52, OR gate 54, delay circuit 56, AND gate 58, and NAND gate 60. Each of the voltage sensors 46, 462, 463 senses the AC voltage of one of the phases at the output of the main stator windings. The sensed voltages are fed to respective mean value circuits 48i, 48»=, 483, each of which produces a mean value of the AC voltage sensed by the voltage sensor (e.g. a root mean square value). The mean values are each fed to one input of a corresponding comparator 501, 50, 503. A reference voltage from the reference voltage generator 52 is fed to the other input of each of the comparators. Each of the comparators produces a signal which is low if the signals at its two inputs are substantially equal, and high if the difference between the two signals is more than a certain threshold. Thus, each of the comparators produces an output signal which is high if the corresponding sensed voltage is lower than the reference voltage set by reference voltage generator 52.

The outputs of the comparators SOi, 502, 503 are fed to OR gate 54. OR gate 54 produces a signal which is high if one or more of the sensed voltages is lower than the reference voltage set by reference voltage generator 52. The output of OR gate 54 is fed via delay circuit 56 to one input of AND gate 58.

The outputs of the comparators SOi, 502, 503 are also fed to NAND gate 60. NAND gate 60 produces a signal which is normally high, but which becomes low if all of the sensed voltages are lower than the reference voltage set by reference voltage generator 52. The output of NAND gate 60 is fed to the other input of AND gate 58.

AND gate 58 produces a signal which is normally low, but which goes high if one or two (but not all) of the sensed voltages is lower than the reference voltage set by reference voltage generator 52. The output of AND gate 58 is used to cut the excitation when the output goes high. For example, the circuit breaker 44 shown in Figure 2 may be arranged to open when the output of AND gate 58 goes high. Alternatively, the output of AND gate 58 may be fed to control circuit 38 shown in Figure 2, and the control circuit may cut or otherwise reduce the excitation by means of the control signal fed to the output circuit 40.

In operation, ii one of the surge protection devices 28,30,32 shown in Figure 1 should fail, its failure mode properties cause it to become short circuit. As a consequence, the corresponding phase of the stator output windings will be short circuited, resulting in an output voltage at or very close to zero. This is detected by the protection circuit 34, which then actsto cut off theexcitation,therebyavoiding damagetothe statorwindings. The delay circuit 56 is designed to provide a short delay to allow external circuit breakers to operate if the short circuit is not caused by failure of the surge protection device, while ensuring that the excitation is cut before the stator windings are damaged. For example, the delay may be less than 2 seconds. I0

The NAND gate 60 in combination with AND gate 58 in Figure 3 prevent the protection device from cutting the excitation if all three phases are short circuited (symmetrical short circuit). In this case the excitation monitor 42 in Figure 4 acts to cut the excitation. The rational for this is that the generator may be able to withstand a symmetrical short circuit for longer than an asymmetrical short circuit without damage to the stator windings.

Furthermore, in some applications, the generator may be required to provide power for a certain period of time such as a few seconds in the case of a symmetrical short circuit. The circuit shown in Figure 3 allows the excitation to be cut relatively quickly when one or two of the surge protection devices fail, while allowing the excitation monitor 42 to control the point at which the excitation is cut if all three devices fail or some other symmetrical short circuit event occurs.

In alternative embodiments, the NAND gate 60 and the AND gate 58 may be dispensed with, and the excitation cut when any or all of the surge protection devices fail.

Figure 4 shows a circuit diagram of the protection circuit in an embodiment of the invention. Referring to Figure 4, the voltages from each of the phases of the main stator are indicated as Vi, V2 and V3 respectively. Surge protection devices connected across the output terminals are indicated as SPD1, SPD2 and SPD3. The circuit operates as follows.

Ui A and its surrounding components form a differential amplifier / divider which attenuates the measured stator voltage to a low level for further processing. Ui B and its surrounding components form a precision rectifier. Ri 1 and Ci filter the rectified AC to provide a mean level of generator winding voltage. U2A/B and U3A/B measure the other two phases of the generator in the same way. Schmitt trigger comparator U4AJB,C, compares all 3 stator voltages with the reference voltage V4. OR gate U5A is configured to produce an output if any phase falls below the reference voltage. R37 and C4 provide an adjustable time delay between detection of the fault and excitation cut off. NAND gate U6 allows discrimination between symmetrical / asymmetrical short circuits. NAND gate U6 can be disabled by closing J4. The output of AND gate U7A is used to activate a circuit breaker to cut off the generator excitation.

In an alternative arrangement, the switch i4 may be closed a certain period of time after detection by NAND gate U6A of a symmetrical short circuit. This can allow the NAND gate U6A to be disabled after a certain period of time, for example, 10 seconds. This may allow the protection circuit to provide protection against both symmetrical and asymmetrical short circuits. In this case it may be possible to dispense with the excitation monitor 42 shown in Figure 2.

Although shown in Figure 1 as a separate device, the protection circuit described above may be integrated into the AVR. In this case the protection circuit may share some of the components of the AVR. For example, the integrated protection circuit and AVR may share a power supply, a processor, voltage sensors for sensing the main stator voltages, mean value circuits, and a circuit breaker. This can allow the protection circuit to be implemented at low additional cost.

The protection circuit and AVR described above may be implemented using discrete electrical components, or using a suitably programmed processor, or any combination thereof.

Claims

CLAIMS1. Protection circuitry for an electrical generator, the electrical generator comprising a main machine for producing a multiphase output, and an exciter for providing excitation to the main machine, the circuitry comprising: means for sensing the voltage of each phase of the multiphase output of the generator; means for comparing the voltage of each phase with a reference voltage; and means for reducing the excitation if the difference between at least one of the sensed voltages and the reference voltage exceeds a threshold.
2. Circuitry according to claim 1, further comprising a plurality of surge protection devices each connected to a respective phase of the generator output.
3. Circuitry according to claim 2, wherein the surge protection devices do not include fuses.
4. Circuitry according to any of the preceding claims, wherein the means for reducing the excitation comprise a circuit breaker for cutting a source of power to the exciter.
5. Circuitry according to any of the preceding claims, wherein the means for reducing the excitation are arranged to reduce the excitation if the difference between at least one but not all of the sensed voltages and the reference voltage exceeds a threshold.
6. Circuitry according to any of the preceding claims, further comprising means for determining whether all of the sensed vottages differ from the reference voltage by more than a threshold, and means for preventing the excitation from being reduced if all of the sensed voltages differ from the reference voltage by more than the threshold.
7. An automatic voltage regulator comprising protection circuitry according to any of the preceding claims.
8. An automatic voltage regulator according to claim 7, wherein the protection circuitry is integrated as part of the automatic voltage regulator, and the integrated protection circuitry and automatic voltage regulator share one or more of: a power supply; voltage sensors for sensing the main stator voltages; mean value circuits; a processor; and a circuit breaker.
9. An automatic voltage regulator according to claim 7 or 8, further comprising an excitation monitor for monitoring the excitation and cutting the excitation if the excitation exceeds a predetermined threshold.
10. Protection circuitry or an automatic voltage regulator according to any of the preceding claims, arranged to reduce or cut the excitation in response to an asymmetrical short circuit in less time than in response to a symmetrical short circuit.
11. An electrical generator comprising a main machine for producing a multiphase output, an exciter for providing excitation to the main machine, and protection circuitry or an automatic voltage regulator according to any of the preceding claims.
12. A method of protecting an electrical generator, the electrical generator comprising a main machine for producing a multiphase output, and an exciter for providing excitation to the main machine, the method comprising: sensing the voltage of each phase of the multiphase output of the generator; comparing the voltage of each phase with a reference voltage; and reducing the excitation if the difference between at least one of the sensed voltages and the reference voltage exceeds a threshold.
13. Protection circuitry substantially as described herein with reference to and as illustrated in the accompanying drawings.
14. A method of protecting an electrical generator substantially as described herein with reference to the accompanying drawings.AMENDMENTS TO CLAIMS HAVE BEEN FILED AS FOLLOWS1, Protection circuitry for an electrical generator, the electrical generator comprising a main machine for producing a multiphase output, and an exciter for providing excitation to the main machine, the circuitry comprising: means for sensing the voltage of each phase of the multiphase output of the generator; means for comparing the voltage of each phase with a reference voltage; and means for reducing the excitation if the difference between at least one of the sensed voltages and the reference voltage exceeds a threshold; wherein the circuitry is arranged to reduce or cut the excitation in response to an asymmetrical short circuit in less time than in response to a symmetrical short circuit.2. Circuitry according to claim 1,further comprising a plurality of surge protection devices each connected to a respective phase of the generator output.3. Circuitry according to claim 2, wherein the surge protection devices do not include fuses. *n s. * *4. Circuitry according to any of the preceding claims, wherein the means for reducing the excitation comprise a circuit breaker for cutting a source of power to the exciter. * r* * . * S. S*:" 5. Circuitry according to any of the preceding claims, wherein the means for reducing the excitation are arranged to reduce the excitation if the difference between at least one but not all of the sensed voltages and the reference voltage exceeds a threshold, 6. Circuitry according to any of the preceding claims, further comprising means for determining whether all of the sensed voltages differ from the reference voltage by more than a threshold, and means for preventing the excitation from being reduced if all of the sensed voltages differ from the reference voltage by more than the threshold.7. An automatic voltage regulator comprising protection circuitry according to any of the preceding claims. a 148. An automatic voltage regulator according to claim 7, wherein the protection circuitry is integrated as part of the automatic voltage regulator, and the integrated protection circuitry and automatic voltage regulator share one or more of: a power supply; voltage sensors for sensing the main stator voltages; mean value circuits; a processor; and a circuit breaker. l09. An automatic voltage regulator according to claim 7 or 8, further comprising an excitation monitor for monitoring the excitation and cutting the excitation if the exdtation exceeds a predetermined threshold.10. An electrical generator comprising a main machine for producing a multiphase output, an exciter for providing excitation to the main machine, and protection circuitry or an automatic voltage regulator according to any of the preceding claims.11. A method of protecting an electrical generator, the electrical generator comprising * 20 a main machine for producing a multiphase output, and an exciter for providing excitation to the main machine, the method comprising: sensing the voltage of each phase of the multiphase output of the generator; * comparing the voltage of each phase with a reference voltage; and reducing the excitation if the difference between at least one of the sensed voltages and the reference voltage exceeds a threshold; wherein the excitation is reduced or cut in response to an asymmetrical short circuit in less time than in response to a symmetrical short circuit.1 2. Protection circuitry substantially as described herein with reference to and as illustrated in the accompanying drawings.13. A method of protecting an electrical generator substantially as described herein with reference to the accompanying drawings.