A Comparative study of SSR Characteristics of TCSC and SSSC

The advent of series FACTS controllers, Thyristor Controlled Series Capacitor (TCSC) and Static Synchronous Series Compensator (SSSC) has made it pos- sible not only for the fast control of power o w in a trans- mission line, but also for the mitigation of SubSynchronous Resonance (SSR) in the presence of xed series capacitors. While the technology of TCSC using thyristor valves is well established, SSSC based on Voltage Source Converter (VSC) with GTO valves is an emerging controller and has several advantages compared to TCSC (1). This paper presents a comparative study of SSR charac- teristics of TCSC and SSSC based on the analysis of a system adapted from IEEE rst benchmark model (FBM). The anal- ysis is based on D-Q models of the FACTS controllers. It is shown that, while the vernier operation (with constant reac- tance control) of a TCSC leads to damping of the critical tor- sional mode, the constant reactive voltage operation of SSSC has the effect of reducing the electrical resonance frequency, which detunes the SSR. However, this leads to the possibility of a higher torsional mode being destabilized when the series effective resistance is low. This paper also presents a simple, yet an effective method of designing SubSynchronous Damp- ing Controller (SSDC) with input from line current magni- tude. The results from the linear analysis are validated us- ing transient simulation based on detailed nonlinear system model. In this paper, the analysis of SSR of a series compen- sated system with TCSC and SSSC is presented. The analysis of SSR is carried out based on frequency do- main method, eigenvalue technique and transient simula- tion. The results based on linear analysis are validated us- ing transient simulation based on nonlinear system model. A comparative study of the SSR characteristics of TCSC and SSSC is presented with the analysis of a system based on IEEE FBM. The transient simulation considers 3 phase, nonlinear model of the system with the detailed model of the FACTS controller, considering switching ac- tion of thyristors / GTOs. The linear analysis (based on the D-Q model) considers only the dynamic phasors rep- resenting fundamental frequency components in the net- work, ignoring harmonics introduced by the switching ac- tion. The work presented in this paper is also aimed at es- tablishing the validity of D-Q model in the case of SSSC and the application of SSDC for damping the critical tor- sional mode. The design of SSDC (with line current as input) is based on the damping torque analysis. The paper is organized as follows. Section-2 describes the modelling of TCSC and SSSC. Section-3 describes a case study based on IEEE FBM and compares the SSR characteristics of TCSC and SSSC. The design of SSDC and its performance evaluation is presented in section-4. The major conclusions of the paper are given in section- 5.