Hybrid Smart Converter Transformer for HVDC with Advanced Grid Support

The HVDC systems are typically utilized for long distance power transmission due to their advantages like minimal-loss bulk power transmission, advanced control features and capability to interconnect asynchronous AC networks. Among the two most common HVDC technologies; voltage source converter HVDC (VSC-HVDC) and line commutated converter HVDC (LCC-HVDC), the former has enhanced control features along with non-dependence upon synchronous machines for commutation. However, it suffers from higher losses due to the involvement of high-speed switching, involves sophisticated gate-driver circuitry, has a weak overload capability, suffers from non-availability in higher ratings and lower reliability due to higher component count. This paper is focused on enhancing the reliability along with reduced control complexity of the highly mature LCC-HVDC technology by converting the inverter-side transformer into smart one through integration of a power electronics-based module between its neutral and converter-station ground. The module enables the conventional converter transformer to perform voltage regulation, harmonics isolation, voltage and impedance balancing. This leads to enhanced robustness of LCC-HVDC against commutation failure, minimization of DC power recovery time and protection against disturbances, such as, solar storms and high-elevation nuclear explosions. The proposed approach is also depicted to introduce power flow control capabilities in AC-tie lines for parallel AC/DC transmission. The PSCAD/EMTDC is utilized to evaluate the proposed approach on the CIGRE benchmark model and the results verify it as a promising solution to multiple HVDC problems.