Protection Strategies to Mitigate Major Power Systems Breakdowns

Doctoral thesis, 2003

This thesis deals with new methods to improve the performance of power system protection in the case of voltage- and transient instability. These methods are designed primarily to mitigate power system breakdown. Relay algorithms are proposed where conventional distance protection is combined with additional relay criteria. In case of voltage instability the criteria are based on the derivative of the voltage whereas the rate of change of the phase angle of the current is used for transient instability. For generator coherency determination a method based on wide area generator speed measurements and Fourier analysis is proposed. Using this method a concept for a System Protection Scheme addressing inter-area events is introduced. Finally, an emergency scheme based on conventional distance relays is proposed to avoid a complete system collapse in the case of severe voltage instability. The performance of these new methods has been compared with conventional methods based on simulations using different test systems. The proposed relay algorithms improve the relay security with respect to voltage instability whereas the reach of the distance protection is not restricted. Neither the line length, different swing frequencies or faults having a slowly decreasing impedance will affect the performance of the proposed schemes as may be the case when using conventional Power Swing Detectors. The fault clearing will also not be blocked as with conventional PSD applications. The proposed method for establishing generator coherency leads to almost identical results if compared with off-line methods as modal analysis or generator speed. Results obtained from phasor measurements however showed deviations. This work also demonstrates that, taking initial transient distortion after a contingency into account, a reliable coherency is faster obtained by the proposed technique than by methods using pure speed or generator voltage angle measurements. It has been demonstrated that the proposed emergency scheme can save part of the system from a voltage collapse. Black-start can then be avoided and the restoration time can be reduced.