Research On Characteristics Of The Sub-fault State Of Power Systems

As the development of the power systems, which trends to be extra high voltage, long-distance and high capacity transmission, the large systems are under tremendous potential risks that any minor impact may result in incalculable losses. The previous studies focus too much on the fault-state of the power systems, meanwhile, the improving coherence leads the generators and the grid to more easily in a sub-fault state, which is between the security and fault state. The sub-fault state, although, does not cause the malfunction form immediately, but it is a potentially serious fault of the power system. Due to the relatively simple power system, the sub-fault state rarely appeared, and didn't attract peoples' attention. As the power systems become more complex, as well as the external stimuli increased, the sub-fault becomes more and more obvious. It not only brings many risks to the power systems, but also put forward higher requirements to the coordination of generators and grid. Therefore, it is necessary to study this sub-fault state in-depth.This paper first proposed the concept of the sub-fault of the power systems, and points out the manifestations that may occur, from the perspective of the generators and the grid, and described its impact on the systems. Through the PSASP simulation of the regional Grid A, dig out the sub-fault states appear in the two typical operation modes, and analysis the sub-fault states from four aspects: the over limit of the transient electrical quantities,the non-compliance of the dynamic performance of the excitation system, the misallocation of the over excitation limit, and the weak damping of the system. Mathematical models of the three typical excitation systems in PSASP were studied, and the equations of state were calculated. The paper theoretically explained how the exciter time constant, the power amplifier gain and the feedback coefficient affect the properties on the machine side. The RF and YM generators were selected as samples to try to suppress their sub-fault state, through the optimization of excitation system, and achieved some certain results. It was also pointed out that the over limit of the YM generator's terminal voltage, needs the principle of oscillation to suppress the sub-fault state.