Tapping on HVDC lines using DC transformers

This paper investigates a new topology for tap terminals on HVDC transmission, which is based on high-power DC/DC converters. The use of DC transformers brings benefits in terms of better resilience from tap system faults, more flexibility in power reversal, an additional control channel and lower costs for main AC/DC converter and transformer. The paper studies both options: using line commutated and voltage source converters as the main AC/DC tap converters. PSCAD simulation results are given for a 0.025 p.u. bidirectional tap on the 1000 MW CIGRE HVDC benchmark system. The simulation responses with a line commutated converter indicate ability to operate the tap terminal at neutral reactive power exchange over wide range of power levels and also excellent resilience to disturbances on the tap AC grid. The simulations with voltage source AC/DC converter also show excellent responses for wide range of inputs but the DC/DC converter becomes more complex. It is concluded that the proposed topology may provide means for wider access to the existing HVDC transmission links.