Adaptive Protection for Renewable-Based AC Microgrids in Low-Voltage Radial Networks

Abstract

By placing clusters of renewable-based AC microgrids in low-voltage radial distribution networks, emergent energy demands can be met. However, this leads to fundamental changes and challenges in the topology and protection coordination performance. Protection challenges such as bi-directional current flow, sympathetic tripping, protection blinding, unwanted islanding, system stability, nuisance tripping and prohibition of unsynchronised reclosing may occur. This is due to the complex operation and control of the embedded renewable-based microgrid, causing increased penetration of dynamic fault currents in the radial distribution network. The aforementioned protection challenges prove that existing South African low-voltage protection philosophy standards are unreliable and need to be re-evaluated. It is expected that by optimally sizing and selecting renewable distributed generators, alongside using communication-based intelligent electronic devices and adaptive protection strategies, a hybrid adaptive protection scheme can be outlined and developed for embedded renewable-based AC microgrids in South African low-voltage radial distribution networks.

The developed adaptive hybrid protection philosophy should be able to successfully clear high- and low-impedance faults in a selective, sensitive, speedy and reliable manner. This investigation looks into the various short-circuit fault current levels experienced in a low-voltage radial distribution network, (namely three-phase, phase-phase, phase-to-ground, and phase-phase-to-ground) when embedded renewable-based AC microgrids are integrated. Three various types of renewable energy sources (namely solar photovoltaic, wind generation and energy storage systems) are modelled and designed to in accordance with the South African grid code requirements for renewable power plants and SANS 10142-1: Wiring of premises standards, and NRS 097-2-1: Grid interconnection of embedded generation. The renewable energy sources and proposed adaptive protection schemes are simulated and validated in a low-voltage radial distribution network with the use of DIgSILENT PowerFactory 2017. The results obtained are used to outline and develop an adaptive hybrid philosophy standard to assist in maintaining the protection coordination between the embedded renewable-based AC microgrid and inline protection intelligent electronic devices. Additionally, an optimisation function is used to optimise the renewable-based AC microgrid’s capacity and power flow, in-turn reducing losses, fault limits, bi-directional power flow, initial cost investments and increasing the voltage profile of the low-voltage radial distribution system. Through this implementation and investigation network efficiency and protection selectivity, sensitivity, stability and reliability of the existing radial low-voltage distribution network, will improve when renewable-based AC microgrids are integrated. This will correct major limitations in the South African low-voltage protection philosophy standards for the development and future introduction of independent power producers in low-voltage radial networks in the utility grid.