Parameter Detection And Suppress Of SSR Using HHT

Large-capacity and long-distance transmission is the necessary demand of the development of our power industry, series compensation is an effective way to en-hance the long-distance transmission ability and the stability of the system. How-ever, if the resonant frequency of series compensated transmission network and the natural frequency of turbine-generator shaft complement each other (the sum is equal to synchronous frequency), it may result in Subsynchronous Resonance (SSR). This thesis is focused on SSR caused by series compensated capacitance. The main work is as follows:(1) Directed towards the weak point of low precision of formerly linearization methods, a time-frequency analysis using EMD and wavelet is proposed to decom-pose SSR oscillation mode. This algorithm uses local maximum and local minimum to calculate the upper and lower envelope mean value of resonance signal, then minus it with original signal; such process is repeated until a satisfied IMF is gained. The emulation result shows that this method can extract signal characteristics and system transient information precisely, decomposing each mode according to the property of signal itself, realizing power system harmonics extraction precisely.(2) Carrying out Hilbert transform on each IMF gained in above process, thus get the analytic expression of IMFs. From the expression, instantaneous amplitude a(t) and instantaneous frequency f(t) which vary with time can be obtained. The EMD, together with Hilbert transform, is called Hilbert-Huang Transform(HHT). This paper detects fundamental wave and subharmonics using a(t) and f(t).(3) Based on the research on equivalent impedance characteristic of Thyristor Controlled Series Capacitors(TCSC) under synchronous frequency, the different ef-fects of TCSC equivalent resistance and equivalent reactance damping SSR is ana-lyzed. With the angle of flow increases, the modulation level of TCSC on SSR volt-age deepens, and the damping capability rests with the composite effect of resistance and reactance.