Application of UPFC to improve the low-voltage-ride-through capability of DFIG

In this paper, a new controller approach for unified power flow controller (UPFC) is proposed to improve the low-voltage-ride-through (LVRT) capability of a DFIG-based WECS during voltage sag. The shunt and series converters of the UPFC are controlled using a hysteresis current controller (HCC) and proportional integral controllers (PI), respectively. Detailed simulation is carried out using MATLAB/SIMULINK software to highlight the impact of UPFC in improving the overall system performance during grid fault.

I. Introduction

Renewable energy sources have been given more concern due to the harmful effects of conventional energy resources on the environment and utilization of renewable energy sources has been accelerated by proposing a carbon tax in some countries [1]. wind energy is one of the most promising renewable energy sources that has been significantly increased from generating about 93 Gw at the end of the year 2003 to 318 Gw by the end of the year 2013 [2]. The future estimation of the global wind market is very promising as it is expected to grow by more than 13% to reach 536 Gw by the year 2017 [2] and by the year 2020 wind power is expected to supply at least 10% of the global electricity demands [3]. Doubly fed induction generator (DFIG) based wind energy conversion system (wECS) is dominating the market of wind turbines due to its superior advantages over other wind turbine technologies [4]. In DFIG-based wECS, as shown in Fig. 1, the generator stator winding is directly connected to the grid at a point of common coupling (PCC) through a coupling transformer whereas rotor winding is connected to the PCC via the coupling transformer and a back-to-back partial-scale voltage source converter (VSC) that facilitates variable speed operation of the DFIG [5], [6]. During the initial stage of introducing wECS to the electrical energy market, wind turbine generators (wTGs) were allowed to be disconnecting from the grid during fault events in the grid side to avoid any possible damages to wind turbines [7]. Currently, the developed grid codes require wTGs to ride-through intermittent fault conditions to remain connected and support the grid under such events. This will assure sustainable power delivery to the grid during faults and abnormal operating situations [8]. Improving DFIG fault ride through (FRT) capability can be achieved by implementing new control schemes for wECS to make them complying with various grid codes established by transmission system operators as presented in the literature [9]–[13] which is effective for new wECS installation. A cost effective approach to improve the FRT capability of the existing wECS is by connecting flexible ac transmission system (FACTS) device to the PCC [14], [15]. Fig. 1.

Typical configuration of DFIG.