TPWRD-01028-2015 1 Abstract: Proliferation of distributed generations (DGs) and power electronics-based loads is bringing about more harmonic polluted power signals. While some failures may occur as a consequence of non-sinusoidal current or volt- age waveforms, existing reliability models of protection relays have not taken the harmonic-related failures into consideration. This paper develops a comprehensive Mar- kov reliability model to categorize the possible functional states of a component, say a transmission line, protected by a relay, operating in harmonic polluted environment. In order to make the proposed model practically tractable, it is further simplified in two ways, through merging the states of the same consequence. The first simplified model focuses on the power component reliability assessment; while, the other is built for reliability analysis of a protec- tion system. Calculation of transition rates and how the resultant model is mathematically evaluated are then dis- cussed. Numerical analyses and outcomes are based on the real-world data taken from Canadian substations. Accord- ingly, the results and conclusions drawn in this paper would be interesting to both academia and industry. Moreover, the proposed reliability assessment methodolo- gy is readily applicable in scrutinizing the impacts of har- monic pollution on the protection devices of other power components. Index Terms: Distributed generations, failure analysis, fre- quency balance approach, harmonics, power system reliability, protective relay, stochastic Markov model, transition rates. NOMENCLATURE A, B, C Protected power system components. RA, RB, RC Protective relays. UP Relay or component is in operating condition. DN Component or protection system is failed. ISO Relay tripped and isolated the protected compo- nent. Jakub Jedrzejczak and George Anders are with the Department of Microe- lectronics and Computer Science of the Lodz University of Technology, Lodz, Poland (mr.jak.jed@gmail.com, george.anders@p.lodz.pl). Mahmud Fotuhi-Firuzabad and Hossein Farzin are with the Department of Electrical Engineering, Sharif University of Technology, Tehran, Iran (fotuhi@sharif.edu, farzin@ee.sharif.edu). Farrokh Aminifar is with the School of Electrical and Computer Engineering, College of Engineering, University of Tehran, Tehran, Iran (aminifar@ece.ut.ac.ir). λ1 Line primary protection (RC) related failure rate (FR) - a backup relay (RA) isolates both faulted ‘C’ and healthy ‘A’ objects. λ21 Primary relay (RA) dependability FR. λ22 Primary relay (RA) unavailability FR. λ3 Primary relay (RA) security FR. μ21, μ23 Repair rates of a protected component ‘A’. γ1, γ22, γ3 Automatic switching rates due to main (RA, RC) or backup (RA, RB) protections trip commands. I. INTRODUCTION In recent years, the extent of harmonic components injected in voltage and current waveforms has significantly increased. Although nonlinear loads and devices such as transformers and welding machines have been present in power system since early years, this observation is mainly due to increased application of power electronic converters. The existing trend towards using more renewable energy resources such as wind and photovoltaic generations, which requires cycloconverters and inverters to deliver harvested energy into the power sys- tem, has also been a major cause for the present situation. Harmonic contents have various impacts on power system operation. In contrast to harmonic-related phenomena such as increasing energy losses, reducing available capacity of power equipment and causing harmonic resonance, the studies re- garding the effects on a protection system are limited and this topic has not yet been thoroughly investigated [1]-[4]. Careful review of the prior works reveals that harmonics may have two possible impacts on overcurrent relays [1]-[4] : 1. When harmonic components are present, a relay’s pick up current may change; i.e., the relay may not trip in some fault events and the protected component will carry higher RMS currents than its rated value, and thus will experience extra heat. This situation may, in turn, result in damage to the component and more frequent failure events. 2. Depending on the relay harmonic response characteris- tics and harmonic contents, relay tripping times may change and this undesirable attribute may cause the pro- tection system loss of coordination. For example, while tripping of the primary protection is delayed due to har- monic polluted currents, the backup protection relay may falsely trip and put some additional components, as well as the faulted one out of service. J. Jedrzejczak, G. J. Anders, Fellow, IEEE, M. Fotuhi-Firuzabad, Fellow, IEEE, H. Farzin, and F. Aminifar, Senior Member, IEEE Reliability Assessment of Protective Relays in Harmonic Polluted Power Systems