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Earthing for Delta connected networks

Geoff Robinson
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
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    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
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    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
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