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.
DANIEL GOMEZ A, K. SHINODA, J.D. PAEZ, F. MOREL, P. DWORAKOWSKI, M. CHEAH-MANE, O. GOMIS-BELLMUNT
Published in CIGRE science & engineering