Study on fundamental impedance characteristics and control strategy for TCSC with dual-TCR branches

In order to satisfy the controllability and security requirement for power systems, the TCSC (Thyristor Controlled Series Capacitor) is installed on the UHV (Ultra High Voltage) line. The UHV and high current become the challenge to TCSC, i.e. with the substantial increase of the rated current of the TCSC, the current flowing through the thyristor branch should also be improved, which may exceed the current limit of thyristor. In this paper, a TCSC scheme with the second TCR branch parallel with the single TCR is proposed, which can diverse the large current. The dual TCRs will inevitably increase the control complexity. Firstly the manufacturing error and measurement error of valve-controlled reactor will make the current between the two TCR branches not equal, which will damage the thyristor in severe conditions, therefore current-sharing control is needed. Secondly, the impedance table is the basis for TCSC control. Considering current-sharing characteristics of dual TCRs, the analytical algorithm of impedance table of TCSC with both TCRs is complicated. On the basis of studying of impedance characteristics of dual TCRs, a simple method is proposed to describe the impedance table.