Method Of Performing Fast De-Excitation Of A Brushless Synchronous Machine

Abstract

A method of performing de-excitation of a brushless synchronous machine having a stator; and a rotor including: a field winding, an exciter armature, a rectifier having thyristors, the rectifier having input terminals connected to the exciter armature and output terminals connected to the field winding, a field discharge resistor connected in series with the field winding, and a bypass switch connected in parallel with the field discharge resistor, the bypass switch being operable between a closed state in which the field discharge resistor is bypassed, and an open state, wherein the method including: a) controlling the thyristors to fire only during a negative half-cycle of the armature voltage waveforms, and b) controlling the bypass switch to obtain the open state from the closed state to thereby discharge a field winding current through the field discharge resistor.

Claims

1. A method of performing de-excitation of a brushless synchronous machine comprising a stator; and a rotor including: a field winding, an exciter armature, a rectifier comprising thyristors, the rectifier having input terminals connected to the exciter armature and output terminals connected to the field winding, a field discharge resistor connected in series with the field winding, and a bypass switch connected in parallel with the field discharge resistor, the bypass switch being operable between a closed state, in which the field discharge resistor is bypassed, and an open state, wherein the method comprises: a) controlling the thyristors to fire only during a negative half-cycle of the armature voltage waveforms for each electrical phase, and b) controlling the bypass switch to obtain the open state from the closed state to thereby discharge a field winding current through the field discharge resistor.

2. The method as claimed in claim 1, wherein step b) is performed simultaneously with step a).

3. The method as claimed in claim 1, wherein in step a) the thyristors are fired with a firing angle α in the range 90°<α<270°.

4. The method as claimed in claim 1, comprising determining whether a fault condition is present in the brushless synchronous machine, and in case the presence of a fault condition is determined, performing steps a) and b).

5. The method as claimed in claim 4, wherein the fault condition is a stator short circuit fault.

6. The method as claimed in claim 4, comprising controlling the bypass switch to maintain the closed state, to bypass the field discharge resistor, and controlling the thyristors to fire only during a positive half-cycle of the armature voltage waveforms as long as no fault condition is present in the brushless synchronous machine.

7. The method as claimed in claim 1, wherein the bypass switch is an IGBT.

8. The method as claimed in claim 1, wherein the rectifier is a thyristor bridge rectifier.

9. A computer program comprising computer code which when executed by processing circuitry of a control system for a brushless synchronous machine causes the control system to perform the steps of a method including the steps of: a) controlling the thyristors to fire only during a negative half-cycle of the armature voltage waveforms for each electrical phase, and b) contrasting the bypass switch to obtain the open state from the closed state to thereby discharge a field winding current through the field discharge resistor.

10. A brushless synchronous machine comprising: a stator, a rotor comprising: a field winding, an exciter armature, a rectifier comprising thyristors, the rectifier having input terminals connected to the exciter armature and output terminals connected to the field winding, a field discharge resistor connected in series with the field winding, and a bypass switch connected in parallel with the field discharge resistor; and a control system configured to perform a method including the steps of: a) controlling the thyristors to fire only during a negative half-cycle of the armature voltage waveforms for each electrical phase, and b) contrasting the bypass switch to obtain the open state from the closed state to thereby discharge a field winding current through the field discharge resistor.

11. The brushless synchronous machine as claimed in claim 10, comprising a gate control unit, wherein the control system is configured to control the gate control unit to thereby control the firing of the thyristors.

12. The brushless synchronous machine as claimed in claim 10, comprising an exciter stator, wherein the exciter stator is a permanent magnet stator.

13. The brushless synchronous machine as claimed in claim 10, wherein the brushless synchronous machine is a generator.

14. The method as claimed in claim 2, wherein in step a) the thyristors are fired with a firing angle α in the range 90°<α<270°.

15. The method as claimed in claim 5, comprising controlling the bypass switch to maintain the closed state, to bypass the field discharge resistor, and controlling the thyristors to fire only during a positive half-cycle of the armature voltage waveforms as long as no fault condition is present in the brushless synchronous machine.

16. The brushless synchronous machine as claimed in claim 11, comprising an exciter stator, wherein the exciter stator is a permanent magnet stator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The specific embodiments of the inventive concept will now be described, by way of example, with reference to the accompanying drawings, in which:

[0029] FIG. 1 shows schematically shows a brushless synchronous machine;

[0030] FIG. 2 is a flowchart of a method of performing de-excitation of a brushless synchronous machine;

[0031] FIG. 3 shows the energy dissipated in a field discharge resistor for different types of de-excitation processes; and

[0032] FIG. 4 shows de-excitation curves for different types of de-excitation processes.

DETAILED DESCRIPTION

[0033] The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplifying embodiments are shown. The inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description.

[0034] FIG. 1 depicts circuitry of an example of a brushless synchronous machine 1. The brushless synchronous machine 1 may be a generator or a motor.

[0035] The brushless synchronous machine 1 comprises a stator 2, and a rotor 5 configured to electromagnetically interact with the stator 2. To this end, the stator 1 is provided with stator windings 3 and the rotor 5 is provided with a field winding 7.

[0036] The brushless synchronous machine 1 comprises an exciter stator 11. In the depicted example, the exciter stator 11 is a permanent magnet stator. Alternatively, the exciter stator could for example be provided with exciter stator windings.

[0037] The brushless synchronous machine 1 comprises an exciter armature 13. The exciter armature 13 is provided on the rotor shaft of the rotor 5. The exciter armature 13 is configured to electromagnetically interact with the exciter stator 11.

[0038] The brushless synchronous machine 1 comprises a rectifier 15. The rectifier 15 is arranged on the rotor 5. The rectifier 15 has input terminals 15a-15c connected to the exciter armature 13. For example, the exciter armature 13 may comprise three coils, one for each electrical phase, and each input terminal 15a-15c may be connected to a respective electrical phase or coil. The rectifier 15 also comprises output terminals 15d-15e. The rectifier 15 comprises a plurality of thyristors T1-T6. The thyristors T1-T6 are arranged in a multi-phase thyristor bridge configuration. The exemplified rectifier 15 is hence a thyristor bridge rectifier.

[0039] The brushless synchronous machine 1 further comprises a field discharge resistor R and a bypass switch Qi. The field discharge resistor R and the bypass switch Qi are arranged on the rotor 5. The electrical resistance of the field discharge resistor R is typically in the order of single-digit Ohms, however this may depend on the particular application. The field discharge resistor R is connected in series with the field winding 7. In particular, the 3o field discharge resistor R may be connected between the field winding 7 and one of the output terminals 15d-15e of the rectifier 15. The bypass switch Qi is connected in parallel with the field discharge resistor R. The field winding 7 is hence connected to the output terminals 15d-15e of the rectifier 15 via the field discharge resistor R/bypass switch Qi. The bypass switch Qi is configured to be switched between a closed state or conducting state, and an open state or non-conducting state. The bypass switch Qi may for example be a transistor such as an IGBT.

[0040] The brushless synchronous machine 1 comprises a control system 17. The control system 17 is configured to control the bypass switch Qi. The control system 17 is configured to control the thyristors T1-T6. The control system 17 comprises a storage medium including computer code, and processing circuitry, wherein the control system 17 is configured to perform the steps of a method of performing de-excitation of the brushless synchronous machine 1 as disclosed herein, when the computer program is executed by the processing circuitry.

[0041] The control system 17 comprises a controller 19. The exemplified controller is 19 arranged on the rotor 5. The controller 19 is configured to obtain set-point values from a stationary automatic voltage regulator (AVR) 21, which may also form part of the control system 17. The set-points may represent the reference field winding current. The controller 19 is hence configured for wireless communication. The controller 19 is configured to control the gate voltage to the thyristors T1-T6 based on the set-point values. The control system 17 is thereby able to fire the thyristors T1-T6, selectively causing the thyristors T1-T6 to conduct. The controller 19 is configured to control the gate voltage to the bypass switch Qi to set the bypass switch Qi in the open state or in the closed state. Hereto, brushless synchronous machine 1 may comprise a gate control unit (not shown) configured to be controlled by the controller 19 and configured to control the gate voltages to the thyristors T1-T6 and to the bypass switch Qi. The gate control unit may be integrated with the controller 19, or it may be a separate unit.

[0042] The excitation system 9 may comprise voltage sensors 23 configured to measure the phase-to-phase voltages between the input terminals 15a-15c of the rectifier 15. The excitation system 9 may comprise a current sensor 25 configured to measure the field winding current, i.e. the current flowing through the field winding 7. The voltage sensors 23 may be configured to supply the controller 19 with voltage measurements. The current sensor 25 may be configured to supply the controller with current measurements. The control of the gate voltages to the thyristors T1-T6 and/or to the bypass switch Qi may further be based on the voltage measurements and/or the current measurements, for example based on the difference between the reference field winding current and the measured field winding current.

[0043] A method of performing de-excitation of the brushless exciterless synchronous machine 1 by means of the control system 17 will now be described with reference to the flowchart shown in FIG. 2.

[0044] The condition of the brushless synchronous machine 1 is constantly monitored, e.g. by means of the voltage sensors 23, the current sensor 25 and/or other sensors, to be able to determine whether a fault condition is present in the brushless synchronous machine 1.

[0045] In the event that it is determined that no fault condition in the brushless synchronous machine 1 is present, the controller 19 is configured to control the bypass switch Qi to maintain its closed state. The field discharge resistor R is hence bypassed or shorted, and the field winding current flows through the bypass switch Qi. Furthermore, the controller 19 controls the thyristors T1-T6 to fire only during the positive half-cycle of the armature voltage waveforms present at the input terminals 15a-15c for each electrical phase.

[0046] The control system 17 may hence control the firing angle α to be below 90°. This results in a positive average voltage over the output terminals 15d-15e.

[0047] The determining of whether a fault condition is present may be performed by the controller 19, by the AVR 21, or by another unit which may be comprised in the control system 17.

[0048] A fault condition of the brushless synchronous machine 1 is one which is of the character that requires fast de-excitation of the brushless synchronous machine 1 to minimise damages. Such a fault condition may for example be a stator short circuit fault in the stator 2, a field winding fault, or an external fault, which may be determined according to known methods.

[0049] In case it is determined that a fault condition is present, in a step a) the thyristors T1-T6 are controlled to fire only during a negative half-cycle of the armature voltage waveforms for each electrical phase. In particular, since control is generally performed based on cosine waveforms, this typically means that the thyristors are fired with a firing angle α in the range 90°<α<270°.

[0050] The rectifier 15 is capable of producing a negative voltage at its output terminals 15d and 15e by controlling the firing angle α to be in the range 90°<α<270°. This can be understood by observing the equation for the average DC voltage VDC between the output terminals 15d-15e of the rectifier 15, as shown below.

[00002]$V_{DC}=\frac{3\sqrt{2}}{\pi }{V}_L\cos \alpha$

where V.sub.L is the line voltage, which is an AC voltage.

[0051] In a step b) the bypass switch Qi is controlled to obtain the open state from the closed state. The field winding current is as a result discharged through the field discharge resistor. Step b) is preferably performed simultaneously with step a).

[0052] FIG. 3 shows the energy dissipated in the field discharge resistor for two different types of de-excitation techniques. The curve C1 shows the amount of energy dissipated in a field discharge resistor during de-excitation when only using the field discharge resistor, which corresponds to the case disclosed in the previously mentioned article “High-speed de-excitation system for brushless synchronous machines”. The curve C2 depicts the amount of energy dissipated in the field discharge resistor R when using a negative firing angle α when controlling the thyristors T1-T6 combined with the field discharge resistor R according to the present concept, using the same parameters as in the case shown in the curve C1. It can be seen that there is significantly lower energy dissipation in the field discharge resistor R in the curve C2. Hence, less heat is generated in the field discharge resistor R, which allows for the design of a field discharge resistor R with a smaller footprint than has previously been possible.

[0053] FIG. 4 shows the de-excitation curves for three different types of de-excitation techniques. The curve C3 shows de-excitation where only negative cycle thyristor control is provided with the firing angle α being 180°, without using a discharge resistor, which except for the firing angle corresponds to the case disclosed in U.S. Pat. No. 4,152,636. The curve C4 depicts the case where only a discharge resistor is used for de-excitation, corresponding to the case disclosed in “High-speed de-excitation system for brushless synchronous machines”. The curve C5 shows de-excitation according the present concept using the same parameters as in the cases shown in curves C3 and C4. As can be seen, discharging of the field winding current I.sub.f is considerably faster in this case.

[0054] The inventive concept has mainly been described above with reference to a few examples. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.