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Earthing for Delta connected networks

The generally accepted practice for earthing a network fed from a transformer Delta connection is to use a zig-zag transformer. This will provide a neutral point which can be connected to earth and provide a low impedance path for earth fault currents. I have recently come across some substations in which a Star/Star transformer has been used with the primary star point earthed to provide the earthed neutral point for the MV network. When I look at this arrangement it seems to me that this will provide a high impedance path for earth fault current as there is no flux balance in the core of the earthing transformer. Am I wrong in this assumption? (I would be happy to be wrong on this one). Has anyone come across this arrangement using a Star connected transformer for earthing a Delta system?


thanks

Geoff
  • Hi David,

    Thanks for taking the time to think about htis problem. Yes, the system is very much like the one you have sketched. We have 110/35/10kV YN/yn0/d11 transformers and other substations with 35/10kV or 35/6kV YN/d11 transformers (why they don't just make it simple and use D/yn11 I have not been able to find out). The YN/yn0 "earthnig transformers also provide local substation supply and are earthed on bot sides, the MV side being earthed through a 70 ohm resistance. (I don't really think tht htis is really necessary and solid earth would do). I had not thought about the tank providing the balancing "delta". Referring to your sketch. The earth fault current will divide into three, in phase, currents each equal to the zero sequence current. thus three in-phase fluxes will be developed in the core limbs and flux balance will not be achieved, which is why I came ot the conclusion that this would provide a high resistance path for earht faults and possibly earth faults wouold not be cleared. This would also cause an MV neutral shift at the substation and may overstress the MV windings of the "earthing" transformer and LV equipment in the substation.

    The balancing effect of the steel tank is a good idea. I have not heard anything and neither have I had any liuck consulting "google" to find examples of such a connection. As such I am still very much tempted to recommend to the client that they follow standard practice and use interconeected star for the earthing transformers. 

    If you hear of anything else on this subject I would be pleased to hear.

    regards,

    Geoff
  • HI David,

    Thinking again about the earthing trasformer tank providing the flux path. The flux generated in each limb of the transformer would traverse the oil, essentially a large air gap, this would then be a high reluctance path so I am back to thinking of the star connected earthing transformer providing a high resistance connection to earth.

    regards,

    Geoff
  • Your star-star transformer should be (almost certainly is) a five-limb core transformer, rather than the normal three-limb core.  The extra limbs on the core provide the flux path and allow zero sequence current.  Doesn't the nameplate give the impedances?  Five-limb core Yy transformers are a common alternative to a delta tertiary - it's just not quite so obvious what you've got.


    As to why Yy, the phase shift between voltage levels is usually fixed on a system/national basis to eliminate any risk of paralleling 30deg out of phase.  This quite often leads to transformers having to be Yy when intermediate voltage levels are inserted.


    The earthing resistor is to control the earth fault current - either to reduce it or simply so that you know what you're going to get.  The transformer and network impedance is largely reactive.  The resistor is, well, resistive - so 90deg shifted and dominates.


    Personally I wouldn't be thinking about changing anything on the system that you've described so far.


    John