Condition monitoring by cross-correlation of switching-mode power supplies

1. A method of condition monitoring a switching-mode power supply providing electrical power to a subsea installation by monitoring an output signal of a switching-mode power supply operating at a known switching frequency and disposed at a subsea location, method comprising:

sampling the output signal of a subsea located switching-mode power supply at a sampling frequency above a minimum Nyquist limit to provide a succession of samples;

cross-correlating (a) a reference signal at the known switching frequency and (b) said succession of samples to provide cross-correlation values;

monitoring the cross-correlation values to ascertain a magnitude of a component of the output signal at or near the known switching frequency; and

providing an indication of impending power supply degradation or failure if said ascertained magnitude exceeds a predetermined threshold.

2. A method according to claim 1 in which the cross-correlation values are in, or are converted to, a frequency domain signal, and the monitoring comprises detecting a magnitude of a component or components within a particular frequency range of the frequency domain signal.

3. A method according to claim 2 in which the monitoring employs a plurality of different thresholds for said magnitude.

4. A method according to claim 3 in which the known sampling frequency is at least ten times a highest frequency in the output signal.

5. A method according to claim 1 wherein the reference signal is independent of the output signal.

6. A subsea switching-mode power supply configured to be disposed subsea and to operate at a known switching frequency, said power supply comprising:

a series switch;

a source of a switching signal operating the switch at the known switching frequency;

an inductor in series with the switch;

a shunt diode forming a loop with the inductor and a load;

an output filter;

means for sampling an output signal of the switching-mode power supply at a sampling frequency above a minimum Nyquist limit to provide a succession of samples during subsea operation;

means for cross-correlating (a) to reference signal during subsea operation at the known switching frequency and (b) said succession of samples to provide cross-correlation values;

means for monitoring during subsea operation the cross-correlation values to ascertain a magnitude of a component of the output signal during subsea operation at or near the known switching frequency; and

means for indicating impending power supply degradation or failure during subsea operation if said ascertained magnitude exceeds a predetermined threshold.

7. The switching-mode power supply of claim 6 , wherein said means for indicating is configured to apply a plurality of masks to determine whether said magnitude exceeds at least one of a plurality of different thresholds.

8. The switching-mode power supply of claim 6 , wherein said output filter includes a capacitor of which a condition is represented by said magnitude.

9. A subsea switching-mode power supply according to claim 6 wherein the reference signal is independent of the output signal.

10. A method of condition monitoring a condition of a capacitor of an output filter of a subsea switching-mode power supply supplying electrical power to a subsea installation and operating at a known switching frequency, said method comprising:

sampling an output signal of said subsea switching-mode power supply at a sampling frequency above a minimum Nyquist limit to provide a succession of samples;

cross-correlating (a) a reference signal during subsea operation of the power supply at said known switching frequency and (b) said succession or samples to provide cross-correlation values;

monitoring said cross-correlation values during subsea operation of said power supply to ascertain a magnitude of a component of said output signal at or near the known switching frequency, said magnitude representing the condition of said capacitor, and

indicating impending power supply degradation or failure during subsea operation of said power supply if said ascertained magnitude exceeds a predetermined threshold.

11. A method according to claim 10 wherein the reference signal is independent of the output signal.