Some Control Applications in Electric Power Systems

This thesis presents some control applications in electric power systems. The work consists of the four parts: interaction between DC cables and ship steering autopilots; frequency estimation; computer relaying; field tests using load switching to damp power oscillations. The first three parts are based on four papers that are given in Appendix A-D. The thesis presents supplementary text to these papers. The last chapter is more self contained and presents field tests from a small hydro power station. These tests show that load switching is an excellent method to damp power oscillations. A proof used in stochastic analysis of fault locators is presented in Appendix E. The first paper presents novel results on interaction of DC cables and ship steering autopilots. Simulations with a non-linear model show that a ship using steering autopilot with a magnetic compass can be captured by a DC cable, i.e, the ship tracks the cable. A full scale experimental study has been performed at the Kontiskan HVDC link. Good agreement was obtained between measurements and computer sinulations. The second paper presents a method for frequency estimation in power systems by demodulation of two complex signals. The third and fourth paper presents some improvements of the differential equation algorithm (DEA) that can be used for transmission line protection. The third paper uses the determinant to explain why isolated estimates from the DEA can be very poor. A median filter is proposed to reject these poor estimates. The fourth paper suggests the followinmg improvements to the DEA: fault classification; intermediate filtering and a new algorithm for three phase faults. Simulation results indicate a nominal operation time of 5-7 ms for three phase faults and slightly longer for other fault type

Frequency estimation by demodulation of two complex signals

This paper presents a method for frequency estimation in a power system by demodulation of two complex signals. In power system analysis, the αβ-transform is used to convert three phase quantities to a complex quantity where the real part is the in-phase component and the imaginary part is the quadrature component. This complex signal is demodulated with a known complex phasor rotating in opposite direction to the input. The advantage of this method is that the demodulation does not introduce a double frequency component. For signals with high signal to noise ratio, the filtering demand for the double frequency component can often limit the speed of the frequency estimator. Hence, the method can improve fast frequency estimation of signals with good noise properties. The method loses its benefits for noisy signals, where the filter design is governed by the demand to filter harmonics and white noise. The method has been previously published, but not explored to its potential. The paper presents four examples to illustrate the strengths and weaknesses of the method

Fault classification for distance protection

This paper presents an overview of power system fault classification methods and challenges. It also contains some ideas about structured testing