Analysis on the Problems about the Third Defense Line of Power Systems and the Concept of Dealing with the Separation Relay after Loss of Synchronism

"Three Defence Lines" is a concept of the whole security defence system for power systems being used for a long time but clearly defined in this paper as follows: the first is the protection relay system; the second line the stability and security control in-cluding generator rejection and load shielding and the third line the separation relay after loss of synchronism followed by the fre-quency and the voltage emergency control. The paper argues that there exist indentation and deficiency in the third line and analyzes in detail the following five scenarios. a) After tripping of some major tie lines the oscillation centre may shift into the main part of the power system. b) The remote/reserve protection relay may malfunction caused by the delays of fault tripping. c) Voltage insta-bility of the receiving power systems caused by the power flow redistribution after the tripping of major transmission lines. d) Cas-cading process caused by the serious overloading after the mal-function of the third stage of the distance relay. e) The malfunction of the protection relay of major generator units caused by the in-correct coordination of the protection settings. To these problems countermeasures are proposed. Especially, the construction of a scheme of the synthesized separation control of the interconnected power system is studied and presented which can perform the op-timal separation of faulty areas before the voltage instability occurs, or it may transfer the voltage instability problem that is difficult to remedy into the frequency instability that is relatively easy to rem-edy. This proposal will significantly improve the effectiveness of the third defence line of the special protection scheme of system stability and security.