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To obtain the vector correction in a Star-Star-Zero (Yy0) power transformer , the differential protection circuits of HV & LV C.T.s needs to be connected in Delta-Delta configuration.
Any wrong Delta configuration/connection will lead to unstable Differential Protection Circuit , which can cause operation of Differential Protection even for Outer faults
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We can input the details of C.T.s used for Differential Protection along with the capacity of power transformer (see How to check differential protection stability).The C.T. ratios selected as per the voltage ratio & MVA rating of transformer to be protected i.e. for protecting an 132/11KV (Voltage ratio=132/11=12),16MVA transformer , we need 75/0.577A C.T. on HV side & 900/0.577A C.T. on LV side (C.T. transformation =900/75=12).
Any wrong ratio/connection of C.T. will reflect unstable differential protection.
In case of non-availability of proper C.T. ratio , Inter Connecting Transformers (ICT) may be used to balance the C.T. ratios
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We can simulate for the possible causes of unstable Differential Protection Circuit.Some important possibilities may be following:
- C.T. Ratio Halved/Doubled.
- Reversed C.T. Polarity
- Wrong Phase Sequence
- Wrong Delta Configuration
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Differential Stability Analyzer includes a module to save the results of some abnormal condition and we can search saved records in future to get a possible cause of protection unstability by input currents in HV,LV,Spill circuit at site condition.
