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An active damping arrangement for a power conditioning circuit comprising a converter or an inverter and an electronically driven output line having an output terminal for connecting to an external load, the active damping arrangement comprising an inductor-capacitor low-pass output filter (OPF) for

An active damping arrangement for a power conditioning circuit comprising a converter or an inverter and an electronically driven output line having an output terminal for connecting to an external load, the active damping arrangement comprising an inductor-capacitor low-pass output filter (OPF) for connecting between an output of the inverter/converter (PWMVC or BBHCC) and the output terminal; means for sensing current in or voltage across the capacitor or voltage between one end of the capacitor (C) and an effective neutral point; means for multiplying the sensed voltage or current by a coefficient G to provide a damping signal, and means for feeding back the damping signal to an input of the inverter/converter (PWMVC or BBHCC), thereby to damp the output of the inverter/converter.

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The invention claimed is: 1. An active damping arrangement for a power conditioning circuit comprising a converter or an inverter and an electronically driven output line having an output terminal for connecting to an external load, the active damping arrangement comprising: an inductor-capacitor l

The invention claimed is: 1. An active damping arrangement for a power conditioning circuit comprising a converter or an inverter and an electronically driven output line having an output terminal for connecting to an external load, the active damping arrangement comprising: an inductor-capacitor low-pass output filter for connecting between an output of the inverter/converter and the output terminal, means for sensing current in or voltage across the capacitor or voltage between one end of the capacitor and an effective neutral point, means for multiplying the sensed voltage or current by a coefficient G to provide a damping signal, and means for feeding back the damping signal to an input of the inverter/converter, thereby to damp the output of the inverter/converter. 2. An active damping arrangement as claimed in claim 1, wherein the capacitor C is connected in such a manner as to have a return path to a neutral point or to a rail of a DC supply. 3. An active damping arrangement as claimed in claim 1, wherein the capacitor of C is split so as to have a plurality of return paths distributed among two or more neutral points and/or among two or more DC rails. 4. An active damping arrangement as claimed in any one of the preceding claims, adapted for use with a power conditioning circuit that has a plurality of output lines. 5. An active damping arrangement as claimed in claim 4, wherein an inductor-capacitor low-pass output filter is provided for each line, the means for sensing is operable to sense separately the current/voltage in each line, the means for multiplying is operable to act on the sensed voltage or current in each line to provide a separate damping signal for each line and the means for feeding back the damping signal is operable to feed back each signal to an input point for the relevant output line. 6. An active damping arrangement as claimed in either of claim 4 or claim 5, wherein some or all of the capacitance of the capacitor is connected between the output lines. 7. An active damping arrangement as claimed in any one of the preceding claims, wherein the inverter/converter is of a voltage-drive form and the signal sensed by the sensor is current. 8. An active damping arrangement as in any one of claims 1 to 6, in which the inverter/converter is of current-drive form and the signal sensed by the sensor is voltage. 9. A active damping arrangement as claimed in any one of the preceding claims comprising means for applying a demand input waveform at an input of the circuit for power conditioning, a cyclic feedback loop between the input and output, the feedback loop including: means for sampling the output waveform, means for subtracting the sampled output waveform from a pre-determined required waveform to produce an error waveform, means for processing the error waveform in a pre-defined manner to produce a processed error waveform, means for adding or subtracting the processed error waveform from the demand waveform to produce a revised demand waveform, and means for applying the revised demand waveform to the input at a subsequent time as an input demand waveform to produce a new AC output waveform at the output. 10. An arrangement as claimed in claim 9 comprising means for summing previous processed error waveforms, means for processing the sum of previous processed error waveforms to produce a processed cumulative sum of processed error waveforms, and means for adding the processed error waveform to the processed cumulative sum of processed error waveforms to produce a new cumulative sum of processed error waveforms, wherein the means for subtracting are operable to subtract the new cumulative sum of processed error waveforms from the required waveform instead of subtracting the processed error waveform from the demand waveform to produce the revised demand waveform. 11. A power conditioning unit with or without a neutral line including an active damping arrangement as claimed in any one of claims 1 to 10. 12. A method of active damping for a power conditioning circuit comprising a converter or an inverter and an electronically driven output line having an output terminal for connecting to an external load, the active damping method comprising: sensing current in or voltage across a capacitor or voltage between one end of the capacitor and an effective neutral point, the capacitor being an inductor-capacitor low-pass output filter for connecting between an output of the inverter/converter and the output terminal, multiplying the sensed voltage or current by a coefficient G to provide a damping signal, and feeding back the damping signal to an input of the inverter/converter, thereby to damp the output of the inverter/converter. 13. A method as claimed in claim 12, wherein the capacitor C is connected in such a manner as to have a return path to a neutral point or to a rail of a DC supply. 14. A method as claimed in claim 12 or 13, wherein the inverter/converter is of a voltage-drive form and the signal sensed by the sensor is current. 15. A method as claimed in claim 12 or 13, in which the inverter/converter is of current-drive form and the signal sensed by the sensor is voltage. 16. A micro-turbine-generator comprising a gas turbine mechanically coupled to a permanent-magnet alternator, the output of the alternator being electrically connected to a power conditioning unit as claimed in claim 11.