Subsequence Action to Eliminate Blackout after Detecting Islanding using Solid State Transfer Switch Implemented in PSCAD/EMTDC

Abstract

Abstract – One operation circumstance in Dispersed Generation (DG) concepts, an island, is formed when one or more DGs and an aggregate of local loads are disconnected from the main grid and then, remain operational as an islanded entity. This condition of islanding is conventionally undesirable. Consequently, the customers will suffer the blackout. Recently, this blackout phenomenon has been eliminating by facilitating islanded operation. This paper presents a passive method in detecting islanding for DGs by measuring the negative-sequence impedance differences between islanded mode and grid-connected mode at the line frequency. Subsequently, the islanded entity is connected to a backup system by Solid State Transfer Switch (SSTS) in order to smoothly and quickly keep electricity available to customers. Keywords : Islanding detection, DG, SSTS, Blackout, Autonomous operation. 1. Introduction The currently generalized industrial standard requires disconnecting all the DG sources from the islanded entity as fast as possible. For example, under the IEEE 1547 standard, that period is within two seconds [1]. For a smooth transition from a grid-connected mode to an islanded mode of operation, fast islanding detection (ID) is essential to allow DGs to adapt their control strategy to the new operational one. The ID methods can be separated into two basic categories: communication and local one. Local detection methods can further subdivided into active and passive detection schemes [2]. In 2008, C.Wrinch developed a simple control scheme which is easy to accomplish in software as well as hardware [3]. It presents passive ID by using negative-sequence impedance measurement. Information about the step change of actual grid negative-sequence impedance could be used for detecting an islanding situation. That method helps detect an islanding in as fast as a few cycles for the specific settings described afterwards in the simulation in Section 4. Thus, it facilitates an islanding's subsequence actions which fall into two options. The first one is to connect immediately to a backup system. The second option is to regulate control topology of DG which is still in operation inside the islanded system. If subsequent to islanding, the voltage and frequency remain under the limits (which means the DGs can stiffly track islanded power demand with enough active and reactive power), then a proper control topology can maintain the autonomous operation. Nowadays, research approaches are focusing on making the control topology of DG in an islanded entity become perfect. This so-called autonomous operation needs further refinement due to its complexity. Conversely, in this research, a simpler concept is presented which coincides with the first mentioned option after islanding. Distribution systems usually work in radial operation that contains a high number of backup grid terminals. Solid State Transfer Switch (SSTS) conven-tionally transfers the preferred utility to a backup grid in the case of fault occurring at the preferred utility [4]. Instead of mechanical circuit breakers, SSTS can promote the power quality by its superior advantages. Therefore, the proposed solution ignores any blackout to consumers.