Abstract
The differential current principle of relay protection, based on the comparison of the phases and amplitudes of currents passing through the protected part of the H.T. system, is used in practice in modern line and bus-bar protection. The advantage of differential current protection is the fact that it does not react to external short-circuits and does not require the time lags to be coordinated with the protection of the adjacent sections of the line. This makes it quick-acting. Nor does differential protection react to peak currents caused by overload or swings and it has high sensitivity. The sensitivity of differential protection is limited by the magnitude of the unbalanced current in the presence of external short-circuits. In a number of cases, the pick-up current of differential protection is higher than the normal load current. Operating experience and theoretical investigations confirm the fact that in the presence of external short-circuits, under the influence of aperiodic current components, the cores of the current transformers can be saturated, the result being that considerable unbalance currents appear.
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