NG TGN(E) 181 PDF

Stabilising and loading resistors used in high impedance circulating current protection schemes can be subjected to considerable stress during an internal fault due to the high voltages which can be developed by the current transformers. In many cases, metrosils will have to be fitted to prevent such voltages rising to dangerous levels. It is essential that this relay network equipment is adequately rated to ensure that it does not incur damage as this could either result in a failure of the protection to clear the present fault or impair its performance during future faults on the system.

This TGN sets out a procedure for ensuring that a resistor or metrosil has adequate continuous and shorttime ratings for its particular application. Factors which affect a resistor's capability are continuous rating, the proportion of a tapped resistor in circuit, the knee-point voltage of the current transformers and the characteristic of the metrosil. Factors which affect the metrosil capability are the relay network resistance, the current flowing into this network and the metrosil continuous rating.