Overview of the CU HVDC Valves and Control System Upgrade Project

The original CU HVDC Scheme was commissioned by ABB in 1978 to transmit 1000MW from the Coal Creek Station generators in North Dakota to the greater Minneapolis area. The system comprised 2×500MW poles operating as a bipole at ±400kV. The 2019 upgrade scope of work included replacement of the original air-cooled thyristor valves with the latest generation of ABB's water-cooled Line-Commutated Converter (LCC) HVDC valves, replacement of the previously-upgraded control system with the latest generation MACH® control system, provision of new water-cooling systems and climate-control for the valve halls, replaced fire detection systems and replacement of some DC switchyard components for improved overall technical performance of the system and renewed lifetime for the replaced components. The upgraded system comprises 2×615MW poles operating as a bipole at ±410kV. The outage duration for the replacement works was 39 days for Pole 1, a 3-day bipole outage and 32 days for Pole 2, including demolition, installation, sub-system and system testing. The bipole outage was needed to reconfigure the DC yard electrode areas and the redundant station service power supplies. This paper reviews the major features of the works, and the possible implications and lessons-learned for other similar projects.