Maintenance Testing of Protective Relays

With the increased focus on testing practices for protective relaying systems brought about by the NERC Standard PRC-017-0 – Special Protection System (SPS) Maintenance and Testing, electric utilities are taking measures to ensure that their installed SPS is:

• Properly designed,
• Performing to applicable specifications, and
• Coordinated with the overall protection schemes.

As a result of these measures, an overall maintenance program needs to be documented and implemented, validating the development of the maintenance and testing programs and demonstrating that all operational failures are analyzed and addressed via aggressive corrective maintenance.

Maximize Protection – Minimize Operational Failures

Any relay testing program should be focused on two simultaneous objectives:

• Maximize electric circuit protection, and
• Minimize the risk of operational failure.

Many of the more advanced relays – specifically those currently in vogue in support of electric distribution automation have their own self-testing routines. By properly monitoring their internal self-test diagnostics, 85 percent of the component failures can be detected. Assuming historical trends, these advanced relays have mean time between failure (MTBF) rates of 300 years (i.e. 0.33 percent failures per year). Thus, only a small amount of failures (0.05 percent per year) go undetected.

Best Practice Relay Testing

A best practice in testing relays involves a 4-step approach that, if followed, removes the requirement for periodic testing:

• Verify using checklists, simulations, lab tests, and/or field inspections the performance of the protection system (checking inputs, outputs and settings) at the time of installation.
• Use the Supervisory Control and Data Acquisition (SCADA) or other monitoring system to apply real-time monitoring of the relay self-test alarm, and replace/repair as necessary.
• Continuously monitor the logic inputs, contact outputs, and voltage/current inputs to identify potential relay failures not detected by self-tests.
• Perform root cause analyses on any event reports to verify logic inputs and output contact operation.

Alternatively, electric utilities that do not perform the last 2 steps consistently opt for periodic testing (usually every 10 years) on those portions of the relays not covered by self-tests. These tests are typically limited to verifying the accuracy of relay measurements, asserting inputs, and pulsing output contacts. The actual settings and time-current characteristics are verified at commissioning and do not change. Typically, periodic testing will identify 0.5 percent of the failures that are not identified through self-testing.

NOTE: In installations without SCADA or communications, the self-test alarm is not monitored. In these instances, the testing frequency should be increased to every one to five years, applying the same routine outlined in the previous paragraph.