Case study of dc-MMC interconnecting two HVDC lines with different grid topologies


Dc-dc converters for high voltage direct current (HVDC) applications are interesting devices for the development of multi-terminal and dc grid schemes. They can adapt the differences between the dc systems (voltage level, technology and/or line topology) and provide additional protection, controllability and redundancy services. This paper presents a case study of a non-isolated dc-dc converter interconnecting the NordLink and Cobra cable projects. This interconnection requires a dc-dc converter to adapt the dc voltage and the line topology (the first project being a ±525 kV rigid bipole while the second is a ±320 kV symmetric monopole). The case study has been modelled in Matlab/Simulink and several scenarios are simulated. A first set of 8 simulations change the power flow directions verifying the adequate behavior of the dc-dc converter and HVDC links. Based on the previous simulations, additional 16 cases are simulated to verify the system behavior during faults. Two faults are proposed: a pole-to-ground on the positive pole of the rigid bipole line and another on the positive pole of the monopole. It is shown that the dc-dc converter can interconnect two existing lines without changing their initial control strategy. For the normal operation simulations, a fast power variation through the dc-dc converter is tested proving that the line protections are not triggered. The power perturbations created by the dc-dc converter are small enough to keep the system under normal operation conditions. During the fault simulations, the dc-dc converter is able to isolate the healthy side from the fault disturbance (firewall capability). The healthy dc side observes a power disturbance (loss of power flow through the dc-dc converter) without triggering the protections, allowing the continuous power transmission and regaining steady state conditions. The dc-dc converter employs full bridge submodules to be able to block the fault currents, without the presence of dc circuit breakers.


Published in CIGRE science & engineering