Study on reinforcement of longitudinal power system by power electronics

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In this study, we investigated an abnormal voltage by static var compensator (SVC) and a subsynchronous resonance by thyristor controlled series capacitor (TCSC). The results are summarized as follows;1) Study of abnormal voltage by SVC SVC consists of a capacitor and a thyristor controlled reactor (TCR). We derived a theoretical equation for frequency responses of TCR. The response is obtained by superimposing a small voltage of a frequency to a fundamental frequency voltage, and then deriving current change of the frequency. We can caluculate the frequency response very quickly. The responses vary with firing method. For the voltage zero crossing method, TCR conductance takes negative values in some frequency ranges. On the other hand, for the firing method based on the fundamental voltage, the conductance is always positive. Next, we showed that a voltage oscillation mode becomes unstable in a longitudinal power system with SVC. The cause is that TCR conductance is negative. Therefo … More re, if we apply phase-locked-loop (PLL), then the voltage mode becomes stable. Numerical simulations verified this consideration.2) Study of subsynchronous resonance by TCSC Though the structure of TCSC is same as SVC, its action is different. If we superimpose a small voltage of a frequency, many current components flow through TCR. In TCSC, these components circulate the capacitor, and then produce many voltage components. These components influence the current components, again. We derived a linear algebraic equation on these components. We solve this equation to obtain the frequency responses of TCSC. The responses differ with the firing method, again. Next, we calculated a damping torque coefficient of a generator with TCSC impedance. The damping coefficient becomes negative, so TCSC causes SSR, too. However, its value is small compared with a series capacitor, so SSR is mitigated. The peak of the damping coefficient moves with the firing angle in the zero crossing firing method, but its value gets smaller in the firing method based on the fundamental voltage. This shows that the latter method is superior to suppress SSR. Less