Field Tests and Analysis of Torsional Interaction Between the Coal Creek Turbine-Generators and the CU HVdc System

Recent experience at the Square Butte Electric Coop project in North Dakota indicated that HVdc systems could interact adversely with turbine-generator torsional modes of vibration on adjacent units. A similar system-configuration exists with the new CU project, also in North Dakota. The CU HVdc control system was designed prior to the experience at Square Butte and hence did not include the potential for such an interaction as a design consideration. As a result, studies were initiated prior to the system startup, a torsional protective relay was ordered for the turbine-generator units, and extensive field tests were planned and carried out to measure the torsional interaction with the HVdc system. The tests and subsequent analysis showed that the system could operate without danger of a torsional instability, due in large part to a relatively high first torsional mode frequency of 19 Hz on the Coal Creek Unit. This experience has enhanced the understanding of the HVdc-torsional interaction phenomena, which will contribute to the design of future HVdc control systems to include mitigation of the potential for such adverse interaction. With this as a major design criteria, it is unlikely that such extensive testing will be essential on all future installations but the experience reported in this paper should be of benefit if similar testing is desired