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Subsea Power Distribution

  1. Jan 17, 2009 #1
    Gents, this will be a long winded.

    We have a subsea power coupling module that provides isolated 550 VAC to our subsea control pod on a well in the Gulf of Mexico. The power line is actually termed as an umbilical, with the two cores or wires running from our power unit on the platform to the subsea field. The length is in the order of 50 miles or so.

    In our coupling unit we have an Integra power meter which supplies our voltage, current, THD, power factor, apparent power, reactive power, etc, data. We also have a Bender A-ISOmeter IRDH375 that monitors our insulation resistance using Adaptive Pulse Measurement AMP. The ISOmeter has an alarm output contact that opens our contactor in the event of an low (600kOhm) insulation reading. These faults are common in older fields such as this, but this seems to be an exception.

    The power meter documentation for the Integra 1350 says the power factor screen will display any number between 1 and -1, i.e. -.76 which would indicate a power factor of 76 that is lagging (inductive reactance I assume). Our situation is that the power factor is shown to be a .46, which is indicative of a LEADING power factor, and very low at that. The norm for our systems is around -.67 to -.78. I was assuming at first that the power factor was leading due to the large amount of capacitance due to the length of the umbilical. I know that our lines are usually in the negative range in respect to power factor due to the step up transformer in the coupling unit and the step down transformer in the subsea control unit. There are no other components in the line other than the two transformers and 1 large varistor in the coupling unit to suppress transients.

    The problem is a low insulation alarm, and resulting trip in our supply. We cannot seem to correlate this event with any other. The power coupling unit itself is run off a UPS with a stable and conditioned supply. We do see a 20 volt increase in the subsea control unit, followed by a drop of the power factor from .46 to .35, then we will see the ISOmeter go into alarm and trip the circuit.

    Any ideas what could cause such a low, leading power factor? What would be the effects of a low, leading power factor? Would there be any connection between the low power factor and a low insulation resistance, or could the varistor be failing and cause such a low insulation resistance?
  2. jcsd
  3. Jan 18, 2009 #2


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    We have a engineer member here, stewartcs, that is involved with subsea systems. You might want to PM him to see if he might have experienced this situation.
    Yes. I would think so, but I'm no expert here.
  4. Jan 19, 2009 #3
    Thank you sir.

    I will PM.
  5. Jan 19, 2009 #4


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    Are you having an intermittent problem with your isometer tripping or is constantly tripped?

    Have you ran a self-test on the isometer?

    Have you changed the measuring principle from AMP to DC to see if it still trips?

    Have you adjusted the set point for an insulation fault to see if it still trips? (There is usually some leeway with the set point.)

    All the problems I've experienced with subsea cables have been for drilling control systems so they are significantly shorter in length (up to 13,500 feet). However, almost always the isometer faults were due to water ingress in the cable or connector. Unfortunately, it sounds like your cable is not retrievable (perhaps a production unit instead of drilling system) so it may be difficult to troubleshoot. Do you have some more details on the cable (manufacturer and spec) and the control system (e.g. type of load like, manufacturer)?

  6. Jan 19, 2009 #5
    The fault is intermittent, which makes this all the more difficult to define.

    I am not too sure on the operation of the Bender, but I had assumed, possibly incorrectly, that transposing a DC signal may have been a poor choice by introducing DC into the end load, which is an isolation/step down transformer. I am not too familiar with the Bender measurement circuit, but I assume that their unit loaded to prevent any issues with a DC measurement. Sounds like a good idea to try.

    I guess my main question would be regarding the presence of a leading power factor. With the load being a transformer, I would assume that the power factor would resemble that of a normal power distribution circuit, thus being largely inductive in nature. Could the capacitance of a long power line overcome the inductive reactance of the load?
  7. Jan 20, 2009 #6


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    I suppose it is possible. 50 miles is a long cable so the capacitance may very well be that high. But again, I've never dealt with cables that long. Check with the manufacturer for the cable specs. They should be able to tell you what to expect from it.

    Good luck.

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