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132/11kV transformer REF Protection frequent tripping

Mustafa AlKatrani

 This is an issue I've noticed all across Iraq, but I'll give you an example that I'm currently working on. We have a 132/11 kV YNd1 transformer, the primary is fed from a GIS, and secondary has a Zigzag NET with NER and a CT on the neutral point. this CT is used for REF protection.

 when we operated this transformer frequent tripping happens due to REF protection, some people say that this is due to grid instability and frequent voltage variation, others say that it's related to the current entering the neutral point due to instability. I'm not convinced by those claims and I think there's a deeper issue or reason.

 Please note that this is common around Iraq, so it might be related to the grid (in some places REF is disabled), but what I want to know is the reason behind this phenomenon.

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  • Hi Mustafa

    Not my specialist subject but I read from the Network Protection and Automation Guide (available to download *) that the delta winding  /earthing transformer arrangement will cause unwanted relay operation.

    16.10 COMBINED DIFFERENTIAL AND RESTRICTED EARTH FAULT SCHEMES

    Section 16.10.1 Application [When an Earthing Transformer is Connected Within the Protected Zone

    A delta-connected winding cannot deliver any zero sequence current to an earth fault on the connected system, any current that does flow is in consequence of an earthed neutral elsewhere on the system and will have a 2-1-1 pattern of current distribution between phases. When the transformer in question represents a major power feed, the system may be earthed at that point by an earthing transformer or earthing reactor. They are frequently connected to the system, close to the main supply transformer and within the transformer protection zone. Zero sequence current that flows through the earthing transformer during system earth faults will flow through the line current transformers on this side, and, without an equivalent current in the balancing current transformers, will cause unwanted operation of the relays.

    The problem can be overcome by subtracting the appropriate component of current from the main CT output. [ continues]

    Perhaps the 

    If of any use, the NPAG can still be downloaded at:

    https://www.iqytechnicalcollege.com/network-protection-and-automation-guide-book.pdf

    With best regards

    David Hawkins C.Eng, M.I.E.T.

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  • Hi Mustafa

    Not my specialist subject but I read from the Network Protection and Automation Guide (available to download *) that the delta winding  /earthing transformer arrangement will cause unwanted relay operation.

    16.10 COMBINED DIFFERENTIAL AND RESTRICTED EARTH FAULT SCHEMES

    Section 16.10.1 Application [When an Earthing Transformer is Connected Within the Protected Zone

    A delta-connected winding cannot deliver any zero sequence current to an earth fault on the connected system, any current that does flow is in consequence of an earthed neutral elsewhere on the system and will have a 2-1-1 pattern of current distribution between phases. When the transformer in question represents a major power feed, the system may be earthed at that point by an earthing transformer or earthing reactor. They are frequently connected to the system, close to the main supply transformer and within the transformer protection zone. Zero sequence current that flows through the earthing transformer during system earth faults will flow through the line current transformers on this side, and, without an equivalent current in the balancing current transformers, will cause unwanted operation of the relays.

    The problem can be overcome by subtracting the appropriate component of current from the main CT output. [ continues]

    Perhaps the 

    If of any use, the NPAG can still be downloaded at:

    https://www.iqytechnicalcollege.com/network-protection-and-automation-guide-book.pdf

    With best regards

    David Hawkins C.Eng, M.I.E.T.

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