CIRED Workshop - Ljubljana, 7-8 June 2018 Paper 0275 INFLUENCE OF INVERTER-INTERFACED DISTRIBUTED GENERATION AND ITS CONTROL ON POWER SYSTEM PROTECTION IN MICROGRIDS Maciej Grebla J.R.A.K. Yellajosula Hans Kristian Høidalen NTNU – Norway MTU – USA NTNU - Norway maciej.grebla@ntnu.no jyellajo@mtu.edu hans.hoidalen@ntnu.no ABSTRACT This paper focuses on interaction of inverter interfaced distributed generation (IIDG) into medium voltage and low voltage networks and its effects on power system protection in microgrids. Major focus of this work is to investigate the dependence of short circuit current levels on control schemes applied to inverters in microgrid, and mode of microgrid interconnection. The importance of considering different inverter control modes while developing microgrid protection schemes is proven by using MATLAB/Simulink model of benchmark microgrid test case. INTRODUCTION Introducing distributed generation into electric power systems is the current trend leading to improvement of reliability in electric power delivery, transmission capacity and reduction of greenhouse gases emission by favorizing renewable sources as distributed generators. Entities called microgrids are believed to be building blocks of the future resilient distribution networks. However, there are certain technical difficulties to overcome, which are not observed in common medium voltage (MV) and low voltage (LV) distribution networks. Traditional distribution network protection schemes were designed to work from the perspective of radial power flow and introduction of power generation into distribution networks will affect the existing schemes. Non-directional overcurrent protection and fuses commonly utilized in these networks are unable to provide required amount of sensitivity and selectivity [1]. INVERTER INTERFACED DISTRIBUTED GENERATORS Traditionally, majority of electric power is produced by synchronous generators, which are rotating masses with high inertia. These generators are well analyzed and its behavior during faults is adequately known to providing significant amount of current during faults (approx. 8 times bigger than nominal). However, introduction of microgrids with renewable sources as distributed generation into MV and LV distribution network, changes operating conditions of these networks [5]. Specification of renewable sources usually does not allow these sources to be connected to the main grid directly, since for instance photovoltaics require inverter interface to transform generated current from DC to AC and wind turbines with their stochastic nature need back-to-back converter in order to meet the grid frequency requirement [2]. Moreover, as investigated in [4], contribution to fault current of inverters during faults is much smaller from rotating machines. It means that for networks like microgrids, where the power generation is dominated by inverter-interfaced sources, protection schemes based on assumption of high currents during fault may not be adequate and other criterions for fault detection are required. Fig. 1 Benchmark microgrid schematic Important aspect jeopardizing protection is full controllability of inverters. [6] performs a basic study upon the influence of inverter control method on a level of the current provided by the inverter under specific controls during the fault. It clearly shows significant influence of inverter controls on power system protection schemes in microgrids. Paper No 0275 Page 1 / 4