Study Of Hybrid Thyristor Controlled Series Compensation On Damping Sub-synchronous Oscillation

This paper is focused on sub-synchronous oscillation(SSO) suppression characteristic of a new economic phase-imbalance series compensation structure—hybrid thyristor controlled series compensation(HTCSC), where the capacitive compensation on one phase is created by signle-phase TCSC in series with a fixed capacitor and the other two phases are compensated by fixed series capacitors。 Compared with three-phase TCSC, HTCSC only need1/3thyrisotrs and its controller and protection can simply be configured as single-phase, thus it can lower cost and make transmission line reconstructred more conveniently, but little relevant research has been done at home and abroad. This paper puts emphasis on studying three core issues including the exact operating point setting, mechanism of SSO damping and subsynchronous oscillation suppression effect.Topology structure, parameters design and control methods of HTCSC are introduced, and a synchronous voltage reversal(SVR) control algorithm module is created using Fortran language in PSCAD/EMTDC software; system line voltage unbalance rate(LVUR) under unresonable operating point is studied, and the influence of controllable compensation ratio and inductance value of TCSC on phase c is considered as well. Against the voltage imbalance problem caused by improper operating point, an automatic voltage balance control algorithm(AVBC) is proposed, which can quickly determine the right operating point and adaptively meet the three-phase power frequency voltage balance requirements even if the setting value is far from the exact operating point. Design principle and control block diagram of AVBC algorithm are given in the paper and time domain simulation has verified its effectiveness.The general expression of electrical damping coefficient based on single-machine infinite bus system is deduced. On basis of the expression and complex torque coefficient method, the SSO damping characteristic of HTCSC is analyzed, and the mechanism of HTCSC suppressing SSO is explained from the perspective of damping, which is more convincing than that explained from the perspective of amplitude value of forward circular rotating armature magnetic field. The damping analysis results show that HTCSC can not provide enough positive damping for the whole system on its own characteristic, and its inhibition effect is to a large extent dependent on the supplementary damping controller, whose basic principle、input SSO signal selection and parameter tuning methods are detailly given out. The idea of a hybrid modal channel is proposed which has the advatages of single-channel and multi-channel modal schemes at the same time. Prony damping identification and time-domain simulation has proved the effectiveness of supplementary controller designed.Using PSCAD/EMTDC software, SSO related characteristics including damping, suppression result, the minimum controllable compensation ratio and line voltage unbalance rate(LVUR) under symmetric and asymmetric fault are comparatively studied between HTCSC and three-phase TCSC on two systems:IEEE first benchmark model and Huliao/Yifeng system which is complex AC&DC tranmission system containing ten machines. It can be seen from the simulation results that HTCSC with supplementary damping controller can effectively suppress SSO in both test systems and its SSO inhibitory effect is not inferior to three-phase TCSC and even better. Besides, LVUR steady-state values of HTCSC under different types of faults all satisfy the requirement of voltage unbalance limitation, thus it will not adversely affect voltage balance.