On the Use of the Ultimate Current Transformer Ratio in Design and Analysis of the Behavior of Differential Protections of Transformers

A reduced ultimate ratio is used as a generalized parameter for describing current transformers (CT). This parameter is used to formulate the requirements to CT under steady state and transient operating conditions. It is shown that the performance of differential protections under transient conditions of short circuits in the protected zone can be considerably increased by increasing the values of the reduced ultimate ratio of CT. This will also reduce the transient imbalance currents at low through currents that appear, for example, upon startup of a powerful engine.