Maintenance Testing Program Choices ? TBM, CBM, and PBM

Author: Eric A. Udren, Quanta Technology LLC, USA

The NERC Standard Drafting Team, however, went beyond what FERC ordered and also included time extensions for condition based maintenance (CBM) and performance based maintenance (PBM) programs. In October of 2013, FERC declared PRC-005-2 mandatory and enforceable. The standard includes a companion implementation plan that specifies how protection asset owners are to gradually bring their fleets of protection system components into compliance with the new standard, while not abandoning their prior maintenance programs during the transition. The implementation plan began in April 2014 with a one-year interval during which the owners of the affected protection systems are to develop a Protection System Maintenance Program (PSMP) based on the new standard.

Relationship of Maintenance Types
TBM is the overarching maintenance process common to all of these methods - CBM or PBM provide the basis for extending time intervals between testing sessions by human technicians.
In a TBM program for protection systems, the maintenance time intervals and activities are fixed, and are set by experience and/or by regulatory specifications like NERC PRC-005-2 maintenance tables. For example, a typical protective relay must be tested within an interval of 6 calendar years to be compliant. Depending on the generation or technology of the relay, the testing typically includes a test for calibration or measurement accuracy, along with ability for each protection-critical input or output (such as trip contacts) to operate. The goal is to find failures or drift of hardware, and to validate that settings are as intended. The latter is not saying that the settings must be proven again to be correct for the application - settings should have been validated at last commissioning, and then controlled in a configuration management system.

CBM is a form of TBM in which the maintenance tests are embedded in the on-line operation of the monitored equipment, with time intervals that typically range from milliseconds to minutes rather than months or years. Self-monitoring replaces human testing. Microprocessor relays and data communications systems are typically capable of monitoring the performance of most of the components that are critical for relaying operation. Maintenance testing is reduced to checking of a few items that cannot be monitored (like trip contacts).
PBM is another form of TBM in which the time intervals are adjusted based on good or bad failure experiences in TBM testing. However, as we explain later, the experience must be systematically analyzed via a data mining process to demonstrate the basis for extension of the TBM interval. Unlike CBM, which eliminates maintenance tasks, PBM maintenance activities tend to be the same as for TBM - they are just performed less (or more) often.

TBM, PBM, and CBM may be combined for individual components, or within a complete protection system, to achieve the best overall result. Figure 1 illustrates the relationship among various types of maintenance practices. In the Venn diagram, the overlapping regions show the relationship of TBM with PBM historical information and with the inherent continuous monitoring of CBM. Figure 1 shows:

  • Region 1: The TBM intervals that are increased based on known reported operational condition of indi-vidual components that are monitoring themselves
  • Region 2: The TBM intervals that are adjusted up or down based on results of analysis of maintenance history of statistically significant population of similar products that have been subject to TBM
  • Region 3: Optimal TBM intervals based on regions 1 and 2

Time Based Maintenance
Testing on a periodic time schedule, as the industry has done since its inception with electromechanical and analog solid state relays, is described today as Time Based Maintenance (TBM). Prescribed maximum maintenance or testing intervals are applied for components or groups of components. The intervals may have been developed from prior user or industry experience, or from manufacturers’ recommendations.
The PRC-005-2 tables with intervals and activities are listed according to component type – protective relays, communications systems required for protection (such as unit or pilot line teleprotection channels), voltage and current sources from instrument transformers, trip and control circuits and wiring, and auxiliary dc supply including various types of batteries and chargers. New versions of the standard now in production will add certain critical reclosing relays, and mechanical transformer fault sensing relays. The protection asset owner can test more often than the maximum allowed intervals in the tables; as a practical matter it is necessary to schedule test time intervals considerably less than the maximum allowed intervals to accommodate practical scheduling of work, maintenance emergencies, or personnel errors without slipping into audit findings of non-compliance and financial penalties. For example, an electromechanical or analog solid state distance relay must be tested within six calendar years. A relay tested any time in 2010 must thus be tested by the end 2016 at the latest to be in compliance. The owner may schedule the next maintenance session for 2015, five years after the 2010 test, to stay well within the limit and to have margin for personnel, work rescheduling, or storm induced delays.

Documentation: Requirement 1 of PRC-005-2 calls for the asset owner to document all the implementation specifics of the chosen protection system maintenance program (PSMP) that complies with PRC-005-2. Requirement 3 says that the owner must also document all of the actual maintenance activities in accordance with that PSMP – down to specifics of every test of every relay. Auditors will review the PSMP for compliance with the standard, and will study maintenance records to find specific evidence that the PSMP is being carried out. The standard of performance is 100% - failing to perform properly documented tests in accordance with the PSMP and within the PSMP scheduled time limits, for even a single relay, is a failure of compliance. Requirement 5 requires that the owner keep records of device failures or issues that are not corrected during the test, and show diligent efforts to correct each such unresolved maintenance issue. A separate standard section calls for retention of maintenance records for the entire interval between audits.

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