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How to test Differential Stability ?
Just apply 3-phase AC supply (LV probably 400V in India) on HV side of Power transformer before HV C.T.'s differential core.Make a short on all the phases on LV side after LV differential core (as shoen in picture).
Note down the currents in differential circuits (probably at differential relay) for all phases for HV,LV & Spill.Spill is the out-of-balance or so called "operating current" which is responsible for the operation of Differential relay.
In a stabilised condition , the currents at spill should be approximately Zero (0) or some very lower value (which may exists due to operation of transformer on a tap other than Normal (rated) tap or small CT ratio errors.
A high value of current in spill terminals during testing shows a unstable differential protection.The causes of Unstable differential protection can easily be interpreted by Differential Stability Analyzer.
