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Protection Riddle No.17 – Undervoltage Protection – Minimum Voltage

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  • #204

      I have been reviewing the configuration of some multi-function protection relays. The relay settings file for a Feeder Protection relay shows a Minimum Operating Voltage for Bus Undervoltage protection but it is currently set to 0.00 x VT.

      I know that the Minimum Operating Voltage setting is usually implemented to avoid unwanted tripping while the circuit is de-energised. I think I’m right in saying that, if we did not have this setting then, the relay would trip out whenever the circuit was de-energised and it would then be necessary to either disable the undervoltage protection or energise the circuit (using another feeder) to re-close the relay.

      That all seems reasonably straightforward but I am curious as to why you would ever want to set the minimum voltage to anything greater than 0.00xVT? Please can someone advise?

      I was previously assuming that the control logic for the protection relays would require the voltage to be greater than 0.00 x VT. However, it would make more sense if the measured voltage had to be greater than or equal to 0.00 x VT. This would mean that the 0.00 x VT setting effectively disables the minimum operating voltage requirement.
      If this is correct, can anyone recommend a way of determining a suitable minimum operating voltage value?

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    • #1425

        1-The absolute zero voltage detection is difficult for electronic device. 2-The low accuracy of related measuring VT in zero voltage regions is a basic limitation.

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