40 Years Operational Experience and Upgrades/Refurbishment of Square Butte HVDC System

摘要

The Square Butte HVDC transmission system is one of the first thyristor based HVDC systems in the world and went into commercial service in 1977. The bipolar ±250 kV HVDC system and 465-mile overhead transmission line was originally constructed by Minnesota Power and Minnkota Power Cooperative (collectively known as the Square Butte Cooperative) to transfer 500 MW from the Milton R. Young Power Plant to Duluth. Like many HVDC systems, the Square Butte facility has been very reliable and has long been a crucial part of the Square Butte Cooperative's transmission facilities. The design life of the plant at the initial construction was expected to be in the range of 35 to 40 years as was common for most substation equipment at the time. Routine maintenance and selective upgrades have until recently been sufficient to keep the facility operational and functioning at acceptable levels of reliability. Over 40 years of operation, Square Butte HVDC system has encountered relatively few major equipment failures and thus few long-term forced outages. Most of the failures were minor and resulted in one-for-one replacements such as DC bushings and ac and dc filter capacitors. Major upgrades included the DC control and protection system replacement and increased cooling capacity of the converter transformers in 2013 to boost the maximum power to 550 MW. However recent more significant failures such as the failure of a significant number of dc filter capacitors and failure of two converter transformers have focused attention on potential unreliability with respect to equipment nearing is design end of life. Much of the converter station equipment is now approaching its intended design life, and while Minnesota Power's share of the output of the Young plant has declined the importance of the Square Butte facility has not reduced as there are significant amounts of wind power operating and being built in North Dakota which is now the main energy resource to transfer in the future. In view of the continuing importance of the facility to its operations Minnesota Power acquired 100% ownership of the HVDC system in December 2009 and is now planning to upgrade and modernize the Square Butte HVDC converter stations by replacing the thyristor valves, valve cooling and cooling controls, converter transformers, and smoothing reactors. The HVDC controls and protections which were replaced in 2004 will be replaced again as they would need to be modified anyway to interface to the new equipment. These major upgrades of most of the major converter station equipment are expected to restore the HVDC system to near new performance and reliability. This paper summarizes 40 years' operational experience and upgrade/refurbishment history as well as ongoing upgrades to extend the life of this HVDC transmission system.