Setting Load Encroachment and Power Swing Blocking Functions To Comply With Prc-026-1

PRC-026-1 requires that identified protection functions on the bulk electric system do not trip for stable power swings during non-fault conditions. For distance protection elements, PRC-026-1 defines an unstable power swing region in the impedance plane. Any distance protection element whose tripping characteristic is not contained within this defined unstable power swing region and would trip in less than 15 cycles (zone 1, zone 2) requires mitigation. Tripping for a power swing apparent impedance outside of the unstable power swing region is prohibited, while tripping for a power swing apparent impedance that traverses into the unstable power swing region is acceptable. Therefore, an impedance element is inherently compliant if it is completely contained within the PRC-026-1 unstable power swing region, i.e. an impedance element will not trip for a stable power swing if its characteristic is completely contained within the unstable power swing region defined in PRC-026-1. To comply with PRC-026-1, the mitigation solution should either make the distance protection element meet PRC-026-1 Attachment B criteria, or it should exclude the distance protection element from compliance under PRC-026-1 Attachment A criteria. Under PRC-026-1 Attachment A criteria, any non-compliant phase distance element can be excluded from compliance if it is supervised by power swing blocking.

The increasing complexity of the modern grid, with the integration of renewable energy sources and advanced technologies, makes it more difficult to accurately detect and predict power swings to effectively prevent cascading failures in the grid. This makes complying to the PRC-026-1 requirements quite tricky and highlights the need to have efficient algorithms in modern-day numerical relays to perform the task.

Load encroachment will not be an issue if the R-axis value for the inner power swing blocking characteristic (with added margin) does not encroach into the zone 2 characteristic, i.e. there is no part of the zone 2 characteristic that the apparent impedance for a swing can enter without it first being detected as a swing, thereby blocking the zone 2. The R-axis value for the outer power swing blocking characteristic (plus added margin) is inside the minimum load impedance, and the required distance between the inner and outer characteristics has been met. If the above holds true, there is no load encroachment issue, and the inner and outer characteristics will have quadrilateral shape without any cutout for load encroachment.

With the R-axis value for the outer characteristic (plus added margin) inside the minimum load impedance within the defined load area, and the required distance between the inner and outer characteristics having been met, load encroachment becomes an issue when the inner power swing characteristic within the defined load area encroaches within the zone 2 characteristic (the inner and outer power swing characteristics will have quadrilateral shape with + and - R-axis cutouts for both the inner and outer characteristics). If this portion of the zone 2 characteristic falls outside the PRC-026-1 unstable power swing region, PRC-026-1 will not be complied with as the apparent impedance could enter the portion of the zone 2 characteristic outside the unstable power swing region, yet not get blocked by the power swing detection. To mitigate this the load encroachment of the distance protection must also be activated, with its R-axis value inside the load encroachment R-axis value for the inner characteristic of the power swing blocking function, and its load cutout angle larger so it fully encompasses the power swing function’s load cutout area.

This distance between the outer and inner power swing characteristics must satisfy a minimum time that must elapse for the apparent impedance of the first swing to cross between the outer and inner characteristics, at the same time ensuring that a swing up to the maximum required frequency for the first swing is detected as a power swing, as well as satisfy a minimum time that must elapse for the apparent impedance of subsequent swings to cross between the outer and inner characteristics, at the same time ensuring that a swing up to the maximum required frequency for subsequent swings is detected as a power swing. This enables a distinction between first swing vs. slowly evolving faults, and higher frequency subsequent swings.

Very critical from the engineering perspective is the ability to be able to freely set the load encroachment cutout zones to interact as required with the power swing detection techniques. In this paper the following will be discussed in detail – the techniques available to setup modern day protection relay to comply with PRC-026-1 requirements and shed light on what options are available in the market today. Details will also be provided for the reader to visualize the settings for impedance and power swing zones, and the impact of power swing settings on being compliant with the PRC-026-1 requirements.