Protection of distribution networks with distributed generation

The share of electric power from small scale Distributed Generation (DG) connected to low voltage distribution networks (DNs) is increasing in the electricity market day by day as this technology is not only becoming cheaper relative to other resources but is also environmentally friendly. However, along with these benefits, DG can also have an adverse impact on operation of the power systems. For example, voltage regulation, power system stability and protection coordination can be affected due to connection of DG to DNs. The purpose of this dissertation is to investigate the potential impact of DG on the protection setup of DNs and to suggest solutions to solve these problems for safe integration of DG. The thesis analyzes the potential impacts of DG on DN protection with the help of a case study of DG installation in a typical DN. The issues discussed include increase in the fault current level, blinding of protection, sympathetic tripping, reduction in reach of a distance relay, lack of detection of single-phase to ground (1LG) fault with ungrounded utility side interconnection transformer configuration and failure of a fuse saving scheme. In respect of the case study, practical and effective solutions that rely on conventional protection devices and practices, for example, instantaneous overcurrent relay, zero sequence voltage detection and transfer trip scheme are proposed. A novel protection strategy that combines the characteristics of both the time and instantaneous overcurrent elements along with a simple algorithm for adaptively changing the setting of the latter is proposed to ensure proper recloser-fuse coordination even in the presence of DG. Conventional protection schemes face serious challenges when they are considered for protecting an islanded microgrid having an inverter interfaced DG unit and need major revision in order to detect and isolate the faulty portion, as fault currents are very limited in such systems. Protection coordination issues including lack of sensitivity and selectivity in isolation of a fault can occur in a microgrid, especially in an islanded mode of operation. The thesis presents and critically reviews traditional and state of the art protection strategies in respect of a microgrid protection. Total harmonic distortion (THD) based islanding detection schemes that measure the output current THD of the inverter interfaced DG (IIDG) are discussed and the role of various factors like uncertainties in filter parameters, variations in the utility THD and the utility impedance on the performance of these schemes is investigated. The simulation results show that the THD in the inverter output current may be affected (depending upon the design of the controller) by the variations in the utility THD and the utility impedance. Consequently, the working of THD based schemes may suffer if these factors are not taken into consideration while selecting the trip threshold. A large L2 (the inductor of the LCL filter facing the grid) can make the output current THD of the inverter less sensitive to variations in the utility impedance and, thus, makes the selection of the trip threshold easier. Moreover, through simulations it is shown that electromagnetic compatibility (EMC) capacitors can also affect the islanding detection schemes used for IIDG as they can introduce a large increase in the output current THD of the inverter.