Planning Considerations for the Intermountain HVDC Transmission System

Planning of an HVDC transmission system embedded into an existing ac network may require unique control and converter equipment capability in order to satisfy stability requirements of the network. The planning and design of the Intermountain Power Project HVDC transmission system are based on unique control performance features for power transfers during terminal ac faults, rapid power recovery following fault clearing and utilization of exceptionally high transient overload capability of 2 per unit during the post-fault period to maintain system stability. The Department, in its efforts to meet the Western Systems Coordinating Council´s (WSCC) reliability criteria, developed and incorporated into its HVDC specifications very rigorous reliability and availability requirements. One objective of this project was to attain the unusually low bipolar converter system outage frequency goal of one outage in five years to achieve a bipole performance rating equivalent to two ac circuits of similar capability. The reliability goals of this project have provided one of the impetus for revisions to the WSCC Reliability Criteria for System Design. The reliability goals and other features adopted for the Intermountain HVDC transmission system are discussed in this paper and study results are included to demonstrate the effectiveness of various controls implemented as stability enhancement techniques.