Improving Pacific Intertie Stability Using Slatt Thyristor Controlled Series Compensation

This paper discusses control design issues for the existing thyristor controlled series compensation (TCSC) device at Slatt substation on the Slatt–Buckley 500-kV transmission line owned by BPA. The controls were developed at Washington State University (WSU) in a project funded by Bonneville Power Administration (BPA). The project has been carried out in close collaboration between the WSU team and the BPA engineers. The controls are primarily aimed at providing fast transient stabilizing control during the first few swings of critical contingencies. Mid-term small-signal stability enhancement is the secondary control objective. A comprehensive overview of TCSC control schemes can be seen in [1]. Recent papers, [2] and [3], discuss the control design issues of TCSC installations in China and in Brazil respectively. Note that the BPA TCSC installation at Slatt was the first of its kind. Among other considerations, the site location on the Slatt–Buckley 500-kV line was motivated by its proximity to the thermal power plant at Boardman for conveniently testing the subsynchronous resonance issues [4] (see Figure 1). The Slatt–Buckley line is one of several 500-kV lines that feed into the Pacific AC intertie, that is, the California-Oregon Intertie (COI) within a complex network of 500-kV transmission lines in the lower Columbia region. Traditionally , TCSC impedance is varied for the purpose of controlling active power flow through the line in a suitable fashion. Owing to the existence of multiple power flow loops in the vicinity of the Slatt TCSC, a change in active power flow on the Slatt–Buckley line does not result in a corresponding change in active power flow on the critical COI tie lines. Therefore, design of controls for the Slatt TCSC, which provide transient stability support for severe contingencies affecting the COI tie lines, becomes a challenging problem. While the effect of Slatt TCSC variation on COI active power flow is rather poor, we noted in our studies that the Slatt TCSC can provide excellent dynamic reactive power support for the COI lines. Therefore, the TCSC control is formulated like a voltage control device by varying its impedance proportional to a voltage input. Short-term overload capability of the TCSC is utilized for deriving transient stability support during the first few swings after severe contingencies. A detailed description of the technical features of Slatt TCSC can be seen in [4]. Slatt TCSC consists of six identical modules per phase. Each module can operate either bypassed or …