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<< Click to Display Table of Contents >> Transformer NEC/R Modelling |
  
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<< Click to Display Table of Contents >> Transformer NEC/R Modelling |
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See Also: Transformer Modelling
Equivalent Circuit:
Positive Sequence: |
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Zero Sequence: |
Transformer windings generally provide an open circuit to zero sequence or earth fault currents, except for Auto Transformers and Star-Star (YNyn), however, the tank-delta effect on a YNyn transformer virtually blocks zero sequence current from flowing. The tank-delta effect is accounted for in PowaMaster. |
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Auto Transformer:
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Star-Star (YNyn)
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As per the diagrams shown in the Transformer Impedance Model it can be seen that most transformer windings provide an open circuit to zero sequence or earth fault currents, hence a NEC must be connected to the corresponding transformer primary or secondary to provide a zero sequence path. Normally a Resistance (NER) is added to the Neutral of the NEC to limit high earth fault currents.
In PowaMaster an NEC/R can be modelled as either a single transformer with a Ydn winding option. The n denotes a NEC/R connection to the transformer. Alternatively the NEC/R can be modelled as two separate transformers which is how the system exists in reality. Refer to the Transformer NER.pmd in the Demo directory.
Calculate the required NER for an earth fault current of 600A at 11kV using the p.u. system.
Model the system using PowaMaster and note that for an earth fault on a resistivity earthed system, the Phase-Neutral voltage, on the un-faulted phases increase by approximately 
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Note the following transformations of fault currents through a transformer:
1 Phase – G Fault Reflects as Phase-Phase Condition for Y-D Transformer with NEC
Phase – Phase Fault Reflects as 2:1:1 Distribution for Y-D Transformer
- Three Phase Fault Current.
