Application of Thermal Models of Digital Protective Relays in Cable Protection

Certain sophisticated digital relays offer the possibility of protecting cables by means of a thermal model with parameter setting using the knowledge of the cable heating time constant and given operating conditions. The principle of the thermal model is based on the theory of heating up a conductor by the flowing current and therefore it is necessary to know to what extent the theory is respected by the relay and what consequences its application brings. The paper presents a comparison between the simulation of heating a real cable by means of a mathematical model and the algorithm for analyzing the cable temperature used in digital relays. It also deals with the definition of the thermal model breaking characteristic as a parametric function of the cable rated current capable of selective setting of the relay and its coordination with the protective system.