Ryan Lott

Power system electromechanical oscillatory behavior is an inherent characteristic of the synchronous machines that are interconnected through transmission systems. As the system stress increases during high power transfers or system component outages, these oscillations can become undamped, creating a reliability risk. Inter-area oscillation is a wide-area phenomenon that involves generators in distant regions and affects several transmission paths. For example, north-south inter-area… Show more

Power system electromechanical oscillatory behavior is an inherent characteristic of the synchronous machines that are interconnected through transmission systems. As the system stress increases during high power transfers or system component outages, these oscillations can become undamped, creating a reliability risk. Inter-area oscillation is a wide-area phenomenon that involves generators in distant regions and affects several transmission paths. For example, north-south inter-area oscillation in the Western Interconnection involves generators in Canada and the Pacific Northwest oscillating against generators in the Desert Southwest and Southern California and is manifested in power swings on the California–Oregon Intertie (Path 66), Northwest–British Columbia (Path 3), and Northern–Southern California (Path 26). System planners need to understand the modes of oscillation so they can properly model them in studies and develop operating procedures and mitigation measures.

This document provides an up-to-date quantitative and qualitative description of the major modes of inter-area oscillation in the Western Interconnection. It is an update of the 2014 WECC Modes Report. The analyses in this report are based on two WECC transient stability base cases and many systemwide phasor measurement units (PMU) datasets collected from 2016 through 2019. This report is the most comprehensive and up-to-date assessment of the Western Interconnection’s electromechanical modes. The table below summarizes the properties of the system-wide modes within the Western Interconnection. Our overall understanding of the North-South A (NSA), North-South B (NSB), EastWest A (EWA), and Montana (MT) modes is now well established. This report has especially improved our understanding of the EWA and Montana modes. Better comprehension of the excitability of the EWA mode is still needed. Show less

Recent oscillation events, such as the January 11, 2019, forced oscillation event in Florida that interacted with a natural system mode of the Eastern Interconnection and led to propagation of the oscillation across the Interconnection, have highlighted the need for increased monitoring and consistency in the monitoring of oscillation disturbances. Some of the key recommendations from the report1 on the event included the need for Reliability Coordinators (RCs) and TOPs to utilize real-time… Show more

Recent oscillation events, such as the January 11, 2019, forced oscillation event in Florida that interacted with a natural system mode of the Eastern Interconnection and led to propagation of the oscillation across the Interconnection, have highlighted the need for increased monitoring and consistency in the monitoring of oscillation disturbances. Some of the key recommendations from the report1 on the event included the need for Reliability Coordinators (RCs) and TOPs to utilize real-time oscillation detection tools. RCs and TOPs should have real-time oscillation detection tools in place to identify when oscillations are occurring and determine if the oscillations are limited locally within their footprint or are more widespread. Equally important is to be able to distinguish between forced oscillations, poorly damped natural system modes, or scenarios where forced oscillations may be propagating across a wider area due to resonance conditions. In addition, there was a recognition that RCs should improve their communication with RCs in the event of widespread oscillation disturbances in BPS and when operating procedures could be an effective means of ensuring this coordination upon the identification of an oscillation.

The NERC Synchronized Measurement Working Group (SMWG) was also requested to develop guidance on oscillation analysis methods to encourage consistency in the system quantities that are monitored for oscillation events and the respective thresholds for alarms. The detection and alarming of oscillations and their classification in a consistent manner is critical in ensuring coordinated mitigation of both local and widespread oscillation disturbances in BPS. Show less