Relaying reliability is generally separated into the two different aspects of dependability and security. The reliability of a protection relay can be improved by carrying out routine maintenance or by including built-in monitoring and self-checking facilities during the design stages. A Markov model is described in this paper which can be used to examine these features. The model also recognizes common-cause failures, temporary and permanent faults and the associated clearing times, operation of back-up protection, and relay mal-trips. Studies have been conducted to illustrate the expected reliability benefits by inclusion of monitoring and self-checking facilities within the relay.

Determination of the optimum routine test and self-checking intervals in protective relaying using a reliability model

A portable electronic unit, such as a defibrillator, includes an autotest system for automatically self-testing various electrical components within the unit during quiescent periods. The autotest system includes an autotest routine that performs an extensive array of rigorous yet time-consuming tests that are impractical to perform during normal modes of operation of the unit. A real-time clock can be set to initiate the autotest routine at a time when the unit is unlikely to be used for its normal mode of operation. The autotest system provides a strip chart printout of the results of the various self-tests performed on the unit, and provides messages on a visual display of any error conditions. If the autotest system detects a needs service malfunction, the operator must acknowledge the error with an input to the unit before the unit will enter into its normal mode of operation. If the autotest system detects a service mandatory malfunction, the unit will discontinue any normal mode of operation until the malfunction is corrected. The autotest system immediately discontinues the autotest routine upon power-up of the unit by an operator for use of the unit in its normal mode of operation.

System for automatically testing an electronic device during quiescent periods.

Protective relay technologies have evolved from single-function electromechanical and static relays to modern multifunction digital relays over the past few years. Protection systems play a vital role in maintaining a desirable level of reliability in power systems. Thus, their own reliability should be evaluated and any cause of their malfunction should be fully comprehended. In this paper, a new reliability model is proposed that associates protection system failures to four main causes: 1) relay hardware, 2) relay software, 3) ancillary equipment, and 4) human error. In addition, this model can take other aspects of the protection system into consideration, such as human error during tests and repair actions, routine inspections effectiveness, level of reliance on self-checking, local backup misoperation, and stuck breakers. These considerations have a significant impact on the reliability of protection systems and have been neglected in the existing research works. Various capabilities of the model are demonstrated in the case study section.

New Considerations in Modern Protection System Quantitative Reliability Assessment

The purpose of this report is to review the progress in the past 20 years or so pertaining to the effect of protection systems on the bulk power systems reliability evaluation. It is the intention of the Task Force that this report defines the state of the art as it exists in the early 1990s and serves as a starting point for those pursuing a further understanding and the development of the subject.

Effect of protection systems on bulk power reliability evaluation

This paper approaches the topic of protection system reliability from an economic point of view by 1) designing an optimal modern protective relay based on a prescribed level of reliability subject to economic constraints and 2) determining optimal routine test intervals by balancing the cost of routine tests and losses due to relay failure. This paper proposes a method that enables the designer to optimally select hardware and software components of a digital protective relay to gain the highest possible overall reliability with a restricted budget. This design can be viewed as a reliability and redundancy allocation problem for which a new easy-to-implement algorithm is proposed. In contrast to the existing literature that views the problem of routine test interval optimization solely from a reliability viewpoint, this paper proposes an optimization procedure for determining optimal test intervals. The optimization objective function consists of three terms: cost of routine tests, losses due to relay unresponsiveness, and losses due to relay maloperation. The value of the objective function is calculated based on a new, simple, and comprehensive Markov model. Illustrative numerical examples clarify the application of the proposed methods and demonstrate their effectiveness in achieving an optimum for both cases.

H.T. Yip

Papers

Reliability evaluation of protection devices in electrical power systems