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Apparatus for measuring load angle in a synchronous generator is disclosed. The synchronous generator is of a type comprising a main machine, an exciter for exciting the main machine, and a permanent magnet machine for exciting the exciter. The apparatus comprises means for sensing a waveform produc

Apparatus for measuring load angle in a synchronous generator is disclosed. The synchronous generator is of a type comprising a main machine, an exciter for exciting the main machine, and a permanent magnet machine for exciting the exciter. The apparatus comprises means for sensing a waveform produced by the permanent magnet machine, means for sensing a waveform produced by the main machine, and means (20) for comparing the waveform produced by the permanent magnet machine with the waveform produced by the main machine to produce a measure of load angle. The measure of load angle may be used to provide a warning of potential pole slip.

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1. A pole slip indicator for a synchronous generator, the synchronous generator comprising a main machine, an exciter for exciting the main machine, and a permanent magnet machine for exciting the exciter, the pole slip indicator comprising: first sensor arranged to sense a waveform produced by the

1. A pole slip indicator for a synchronous generator, the synchronous generator comprising a main machine, an exciter for exciting the main machine, and a permanent magnet machine for exciting the exciter, the pole slip indicator comprising: first sensor arranged to sense a waveform produced by the permanent magnet machine;a second sensor arranged to sense a waveform produced by the main machine;comparator circuit arranged to compare the phase of the waveform produced by the permanent magnet machine with the phase of the waveform produced by the main machine to produce a signal representing a measure of load angle; anda pole slip indication unit which provides an indication of how close the generator is to pole slip based on the measure of load angle. 2. A pole slip indicator according to claim 1, further comprising a conditioning circuit which changes the frequency of at least one of the waveform produced by the permanent magnet machine and the waveform produced by the main machine. 3. A pole slip indicator according to claim 1, further comprising a conditioning circuit which converts at least one of the waveform produced by the permanent magnet machine and the waveform produced by the main machine to a waveform having at least one of a faster rise time and a faster fall time. 4. A pole slip indicator according to claim 1, further comprising a conditioning circuit which converts at least one of the waveform produced by the permanent magnet machine and the waveform produced by the main machine to a waveform which is substantially a square wave. 5. A pole slip indicator according to claim 1, further comprising a phase shift circuit which phase shifts at least one of the waveform produced by the permanent magnet machine and the waveform produced by the main machine so that the waveforms have a predetermined phase relationship when the generator is at a no-load condition. 6. A pole slip indicator according to claim 1, wherein the pole slip indication unit produces an output signal when the measure of load angle exceeds a predetermined threshold. 7. A pole slip indicator according to claim 1, wherein the pole slip indication unit is arranged to anticipate pole slip based on the measure of load angle, and to output a signal indicating potential pole slip. 8. A pole slip indicator according to claim 7, wherein the pole slip indication unit is arranged to determine a change in the load angle, and to output a signal indicating potential pole slip if the change in the load angle is above a threshold. 9. A pole slip indicator according to claim 8, wherein the pole slip indication unit is arranged to determine a rate of change of the load angle. 10. A pole slip indicator according to claim 1, further comprising an isolating unit which at least partially isolates the generator from a load or a supply network in dependence on at least one of the measure of load angle and signal indicating potential pole slip. 11. A pole slip indicator according to claim 1, further comprising a current limiter which limits the current drawn from the generator in dependence on at least one of the measure of load angle and signal indicating potential pole slip. 12. A synchronous generator comprising: a main machine;an exciter for exciting the main machine;a permanent magnet machine for exciting the exciter; anda pole slip indicator comprising: a first sensor arranged to sense a waveform produced by the permanent magnet machine;a second sensor arranged to sense a waveform produced by the main machine;a comparator circuit arranged to compare the phase of the waveform produced by the permanent magnet machine with the phase of the waveform produced by the main machine to produce a signal representing a measure of load angle; anda pole slip indication unit which provides an indication of how close the generator is to pole slip based on the measure of load angle. 13. Apparatus which provides a warning of pole slip in a synchronous generator, the synchronous generator comprising a main machine, an exciter for exciting the main machine, and a permanent magnet machine for exciting the exciter, the apparatus comprising: a first sensor which senses a waveform produced by the permanent magnet machine;a second sensor which senses a waveform produced by the main machine;a comparator circuit which compares the phase of the waveform produced by the permanent magnet machine with the phase of the waveform produced by the main machine; anda pole slip indicator circuit which provides a warning of pole slip based on an output of the comparator circuit. 14. A method of providing an indication of pole slip in a synchronous generator, the synchronous generator comprising a main machine, an exciter for exciting the main machine, and a permanent magnet machine for exciting the exciter, the method comprising the steps of: sensing a waveform produced by the permanent magnet machine;sensing a waveform produced by the main machine;comparing the phase of the waveform produced by the permanent magnet machine with the phase of the waveform produced by the main machine to produce a signal representing a measure of load angle; andproviding an indication of how close the generator is to pole slip based on the measure of load angle.