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GE 869 - Reactive Loads and Incorrect Metering

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    GE 869 - Reactive Loads and Incorrect Metering

    Hi everyone,

    Today I was working with an 869 relay which was metering incorrectly.

    The relay was monitoring a breaker and motor fed from a VFD. Although the current and voltage RMS values of the relay were metering correctly the instantaneous values were not.

    The motor was being tested with no load. Therefore, basically all the power was reactive with only some active power being supplied to compensate for mechanical losses.

    Can anyone describe why the relay would not meter correctly?

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    Quote Originally Posted by Bob_Woodward View Post
    Hi everyone,

    Today I was working with an 869 relay which was metering incorrectly.

    The relay was monitoring a breaker and motor fed from a VFD. Although the current and voltage RMS values of the relay were metering correctly the instantaneous values were not.

    The motor was being tested with no load. Therefore, basically all the power was reactive with only some active power being supplied to compensate for mechanical losses.

    Can anyone describe why the relay would not meter correctly?
    It may be the difference between the meter reading True RMS and Fundamental. I had an issue with a SEL-411L with something similar. In the RMS reading, it appeared we had current when we should not have. Did an event trigger and read the event report. The current magnitude was low in this case (mA). What looked like harmonics were summing up and making a larger signal, but again the magnitudes were so low that 100% of 0.001 is?

    Check the manual if the instantaneous is 50/60 Hz or true RMS.

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    Quote Originally Posted by Realtime View Post
    It may be the difference between the meter reading True RMS and Fundamental. I had an issue with a SEL-411L with something similar. In the RMS reading, it appeared we had current when we should not have. Did an event trigger and read the event report. The current magnitude was low in this case (mA). What looked like harmonics were summing up and making a larger signal, but again the magnitudes were so low that 100% of 0.001 is?

    Check the manual if the instantaneous is 50/60 Hz or true RMS.
    Will do. Thanks for the help

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    Quote Originally Posted by Bob_Woodward View Post
    Hi everyone,

    Today I was working with an 869 relay which was metering incorrectly.

    The relay was monitoring a breaker and motor fed from a VFD. Although the current and voltage RMS values of the relay were metering correctly the instantaneous values were not.

    The motor was being tested with no load. Therefore, basically all the power was reactive with only some active power being supplied to compensate for mechanical losses.

    Can anyone describe why the relay would not meter correctly?

    This may not answer your question, but it is still good to be aware of this. We have run into issues when an 869 is being used on VFDs. They tend to trip on unbalance or single face due to their sensitivity (low frequency related sensitivity problems). Your solution would be to block unbalance while you are on the VFD. Has anyone else seen this and found a different solution?

    The other thing to consider is the location of your instrument transformers. Having your PTs on the 60hz side, and your CTs on the opposite side will give you some really inaccurate metering results while in operation.

    Just some food for thought. As i said, this is likely unrelated, but still good to know when it comes to GE869 relays.


    www.taifaengineering.com

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