I. Introduction
Recently, high generation level from renewable energy systems, and in particular wind and PV power plants forms a great challenge for maintaining the safe operation of the whole power system. To ensure the reliability and security of the electric power supply in the presence of PV and wind energy sources, these renewable energy systems have to stay integrated to grid and support the grid operation under fault conditions. Many codes for connection of these energy sources into grid have determined various low-voltage ride-through standards that control the integration of PV and wind power plants into electric grid. In many cases, reactive power support from these sources during the occurrence of the fault is required [1], [2]. The control circuit of the grid-side converter, which presents the interface between the grid and the PV and/or wind systems is adjusted to control the exchange of the reactive power between the renewable sources and the grid. Due to the small capacity of the inverter in weak electric networks, a voltage instability problem, which may lead to disconnection of the PV or wind power plants may appear.