Unified Control of the Parallel LCC-VSCs Interlinking Converters in a Hybrid AC/DC Network

This paper proposes a control scheme for line commutated converters (LCCs) to operate in parallel with voltage source converters (VSCs) in an AC/DC network. The proposed control strategy uses a droop equivalent control and precautionary measures for the LCC to work along with droop controlled VSCs. The paralleling VSCs with LCCs improves the LCC's operation region beyond its conventional (V-I) graph mode. Also, the LCC's resilience to commutation failure is enhanced. Improving the LCC's control scheme leads to a unified control scheme, in which the AC sub-grid's voltage and frequency and DC sub-grid's voltage can be supported in all operating conditions. The operation principles and the system response to the AC and DC sub-grids' disturbances are investigated. The fault response measures of the proposed system show increased stability and power quality. In the proposed control scheme, proper solutions for the operation limits, such as commutation failure and reactive power compensation are provided. The performance of the proposed control scheme is tested with realtime simulator in different operating conditions, and its stability and accuracy are verified.