Abstract
Penetration of the power grid by renewable energy sources, distributed storage, and distributed generators is becoming increasingly common. Increased utilization of these distributed energy resources (DERs) has given rise to additional protection coordination concerns, particularly where they are utilized in an unbalanced manner or where loading among phases is unbalanced. Digital relays such as the SEL-751 (produced by Schweitzer Engineering Laboratories, Pullman, WA, USA) series have the capability of being set on a per-phase basis. This capability is underutilized in common practice. Additionally, in optimization algorithms for determining relay settings, the time-overcurrent characteristics (TOCs) of relays are generally not treated as variables and are assigned before running the optimization algorithm. In this paper, TOC options themselves are treated as discrete variables to be considered in the optimization algorithm. A mixed integer nonlinear programming problem (MINLP) is set up where the goal is to minimize relay operating times. A genetic algorithm (GA) approach is implemented in MATLAB where two cases are considered. In the first case, the TOC and Time dial setting (TDS) of each relay is set on a three-phase basis. In the second case, per-phase settings are considered. Relay TDSs and TOCs are both considered as simultaneous discrete control variables. Despite the stochastic nature of using per-phase settings for unbalanced systems is found to generally allow for shorter operating times. However, for relatively balanced systems, it is best to use three-phase settings if computation time is of importance.
Highlights
The complexity of the power grid is rapidly increasing with the more common usage of distributed energy resources (DERs) and microgrid structures containing DERs which are capable of both delivering and absorbing energy [1,2]
The main objective of relay coordination is to set the protection to operate as fast as possible for faults when acting as the primary protection and to operate with sufficient delay when acting as a backup
This paper focuses on standard relay curves and only directional overcurrent relays
Summary
The complexity of the power grid is rapidly increasing with the more common usage of DERs and microgrid structures containing DERs which are capable of both delivering and absorbing energy [1,2]. The main objective of relay coordination is to set the protection to operate as fast as possible for faults when acting as the primary protection and to operate with sufficient delay when acting as a backup. In a radial distribution system, the coordination of overcurrent protection is based on the comparisons of the operating times of the relays in response to the various levels of fault currents. If the DERs are single-phase and capable of moderate to high generation levels, traditional relay settings may become invalidated. Both settings based on per-phase considerations when available and three-phase traditional settings need to be considered
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