Getting Equipped: NERC Targets Transmission Relay Loadability

In mid-February, the North American Electric Reliability Corp.’s board of trustees approved a standard addressing a key recommendation from the U.S.–Canada Joint Task Force on the Aug. 14, 2003, blackout. The standard, PRC-023-1 “Transmission Relay Loadability,” specifies technical requirements for setting protective relays so operators are better able to manage disturbances.

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Upon announcement of the standard, NERC’s director of event analysis and information exchange Bob Cummings said, “Relay loadability has been a contributing factor in major system disturbances since 1965, and was a major contributor to the severity of the 2003 blackout. Following the 2003 event, the industry completed a detailed review of protection system loadability to ensure relays will not prematurely operate during extreme conditions, thereby allowing operators enough time to make system adjustments. This standard makes the improvements made at that time both permanent and enforceable.”

Utility Automation &Engineering T&D sought clarification on the proposed standard from Gerry Adamski, NERC’s vice president and director of standards.

UAE: How exactly does relay loadability contribute to system disturbances? And, more specifically, how did it contribute to the 2003 blackout?

Adamski: Relay loadability is an issue when relays that operate based on the amount of power flowing through a transmission line cause the line to disconnect from the grid when there is no fault on the line. Protective relays cannot directly determine whether power flow over a line is routine or due to a fault. Essentially, this new standard addresses situations where relay settings restrict the line from operating as designed under non-fault, but possibly slightly higher flow conditions than usual. This was a particular issue during the 2003 blackout because several lines tripped out of service due to conservative relay settings, which only allowed the lines to carry slightly more power than intended in usual operations.

UAE: What will the standard PRC-023-1 require of utilities? If approved in the U.S., what will the standard require utilities to do differently now than they have in the past?

Adamski: This standard requires utilities to set their transmission relays so that the relays will not unnecessarily limit power flow through the line when there is no fault. Utilities are required to evaluate how they set their transmission relays to ensure the technical parameters in the new standard are met. The good news is, much of this work has already been done.

UAE: Will this be a particularly difficult standard for utilities to comply with?

Adamski: Not at all. In fact, much of the utility work is already done based on the work of NERC’s planning committee subsequent to the blackout. This new standard codifies the practices that in large part have already been implemented and serves as a benchmark for future relay applications. Utilities have reviewed over 23,000 transmission line terminals over the last five years; about 20 percent of these terminals were found to be out of conformance with the new requirements, and the vast majority of these have already been corrected.

UAE: How about cost? Will coming into compliance with PRC-023-1 require a significant outlay of capital for a utility, or is it a more a case of simply needing to change operating procedures?

Adamski: As much of the work has already been accomplished, there will be little additional cost to utilities other than those situations where the requirements in this standard cannot be met using the existing relay systems. This may require capital improvement to update these systems. Most utilities, for new installations, already use relays with the flexibility to easily meet this standard.

UAE: What is the current status in the U.S. and Canada of the proposed standard?

Adamski: The new NERC standard will be filed for approval with FERC and Canadian regulators in April.

Editor’s note: This interview took place April 4, 2008.

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