Analysis of Restraint Current Forging Algorithms for Digital Differential Protection

Differential protection is one of the most reliable and simple by the principle of relay protection, which has absolute selectivity and performance [1]. The protection measures the vector sum of the currents (a differential current) on all sides of the protected object. In the case of a short circuit in the protection zone, the current rises and the protection turns off the damaged element. With an external short-circuit, the through-currents on the sides of the protected elements according to the first Kirchhoff law together give a current close to zero and called the unbalance current [2]. This current is mainly due to the error of current transformers (CTs). The error of the relaying CT depends on their load, residual induction, primary current and the presence of an aperiodic component. In the steady-state fault mode, the CT error usually cannot exceed 5% or 10%, depending on the accuracy class. However, when the primary current, secondary load or an aperiodic current is exceeded (especially in combination with the residual induction of the CT), the error may increase to 50-80%. The unbalance current increases in these modes. It can cause an incorrect operation of protection during external faults. To prevent incorrect operation of the protection the restraining is applied [3], [4]. The restraining in differential protection is an automatic increase trip current when the current through the CT is increased. Typically, as a restraint current, analog current protections use a current or several sides (legs) [5].