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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.

  • HV is way outside of my comfort zone, so I probably can't be much practical help, but if I may ask a few silly question (which might even help the discussion along a bit)...

    Is this a conventional setup with the 132V side feeding the 11kV side? (i.e. conventional loads on the 11kV side, rather than backfeeding from an 11kV generator or anything like that)

    How is the transformer wired? As I read the diagram it looks to be delta on the 11kV side and star on the 132kV side - which looks the opposite way around from what I think usually happens in the UK. Is the REF CT (and NER) then between an "artificial N" point and Earth?

    sorry for the dumb questions

       - Andy.

  • in reply to AJJewsbury

    In the UK the 400 and 132kV systems are generally star-wired, though the neutral is not carried on the transmission towers ('pylons')
    reference from ENW .

    However, the 33 or 11kV intermediate network transformers are delta at both ends, though the source end, much as the OP, generates a pseudo neutral by transformer, and earths it - any current in this path intended to indicate line faults on the 11kV network.

    Iraq is not my forte, and to be honest neither is this sort of HV stuff, but the two systems look comparable.

    That said some thoughts come to mind. 

    1) I assume that genuine faults on the 11kV network between lines and true earth have been ruled out ? (the kind of thing those trips are meant to catch)

    2) Are there multiple feeds into one 11kV network ?

    If so there is a current loop between the electrodes of the two transformer stations in whcih any ground currents may flow - sort of ' coming up' one NER and 'going down' the other but not actually caused by anything on the 11kV network.
    Normally in such a case there would be 'busbar blocking' logic, communicating between the two transformers, to distinguish real faults on the outbound network and out of band currents that are false alarms. mentioned here, another doc.

    3) How is the overlap or isolation of the earth electrode structures / meshes of the primary side, where random currents induced in the wires of the 132kV network may flow to ground, prevented form pulling the voltage on the 11kV earth?
    If the earth impedance is not either very low or the two earthing paths are deliberately separated,  surges on the primary side may cause  current flow in the detection transformer, again in effect coming out of the earth.

    Here I can imagine Iraq is not at all like the UK, as the earth proper will be drier and a 'good' earth connection may well be harder to achieve.

    There are others who frequent this forum who have more hands-on knowledge, and it is definitely worth waiting for them to chip in.

    Mike

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