PRC-025-1: Generator Relay Loadability

Purpose

To set load-responsive generator protective relays at a level such that  generators do not trip during system disturbances that are not damaging to the generator thereby unnecessarily removing the generator from service.

Applicability

3.1. Functional Entities:

3.1.1 Generator Owner that applies load-responsive protective relays on  Facilities listed in 3.2, Facilities.

3.2. Facilities:  The following Elements of the Bulk Electric System generation Facilities, including those identified as Blackstart Resources in the Transmission Operator’s system restoration plan:

3.2.1 Generating unit(s).

3.2.2 Generator step-up (i.e., GSU) transformer(s).

3.2.3 Auxiliary transformer(s) that supply overall auxiliary power necessary to keep generating unit(s) online. ¹

Background

After analysis of many of the major disturbances in the last 25 years on the North American interconnected power system, generators have been found to have tripped for conditions that did not apparently pose a direct risk to those generators and associated equipment within the time period where the tripping occurred.  This tripping has often been determined to have expanded the scope and/or extended the duration of that disturbance.  This was noted to be a serious issue in the August 2003 “blackout’ in the northeastern North American continent. ²

During the recoverable phase of a disturbance, the disturbance may exhibit a “voltage disturbance” behavior pattern, where system voltage may be widely depressed and may fluctuate.  In order to support the system during this transient phase of a disturbance, this standard establishes criteria for setting load-responsive protective relays such that individual generators may provide Reactive Power within their dynamic capability during transient time periods to help the system recover from the voltage disturbance.  The premature or unnecessary tripping of generators resulting in the removal of dynamic Reactive Power exacerbates the severity of the voltage disturbance, and as a result changes the character of the system disturbance.  In addition, the loss of Real Power could initiate or exacerbate a frequency disturbance.

Requirements and Measures 

 R1. Each Generator Owner shall install settings that are in accordance with PRC-025-1 – Attachment 1:  Relay Settings, on each load-responsive protective relay while  maintaining reliable  protection.  [Violation Risk Factor:  High] [Time Horizon:  Long-Term Planning]

M1. For each load-responsive protective relay in accordance with PRC-025-1 – Attachment 1:  Relay Settings, each Generator Owner shall have and provide as evidence, dated documentation of: (1) settings calculations, and (2) that settings were installed.

Rationale for R1:   Requirement R1 is a risk-based requirement that  requires the responsible entity to be aware of each protective relay subject to the standard and applies an appropriate setting based on its calculations or simulation for the conditions established in Attachment 1.

The criteria established in Attachment 1 represent  short-duration conditions during which generation Facilities are capable of providing system reactive resources, and for which generation Facilities have been historically recorded to disconnect, causing events to become more severe.

The term, “while maintaining reliable protection” in Requirement R1 describes that the responsible entity is to comply with this standard while achieving their desired protection goals.  Refer to the Guidelines and Technical Basis, Introduction, for more information.

PRC-025-1 – Attachment 1:  Relay Settings 

Each Generator Owner that applies load-responsive protective relays shall use one of the following Options 1-17 in Table 1, Relay  Loadability Evaluation Criteria (“Table 1”), to set each load-responsive protective relay according to its application.  The bus voltage is determined by the criteria for the various applications listed in Table 1.

Synchronous generator output pickup setting criteria values are determined by the unit’s maximum seasonal gross Real Power capability, in megawatts (MW), as reported to the Planning Coordinator; and the unit’s Reactive Power capability, in megavoltampere-reactive (Mvar), is determined by calculating the rated MW based on the unit’s nameplate megavoltampere (MVA) at rated power factor.

Asynchronous generator output pickup setting criteria values are determined by the site’s aggregate maximum seasonal gross Real Power capability, in MW, as reported to the Planning Coordinator; and the Reactive Power capability, in (Mvar), as determined by calculating the rated Mvars based on the aggregate MVA at rated power factor and adding the Mvar output of any static or dynamic reactive power devices.  Asynchronous generator criteria also include inverter-based installations.