Modeling and Comparison of Unbalanced Load Compensation Methods in DFIG Systems Based on Mechanical Constraints

In remote areas, stand-alone wind energy conversion systems (WECSs) are significantly employed to supply the power of local loads. In such conditions, the longevity problem of WECSs is a necessary problem. Therefore, it is necessary to investigate the interaction between electric and mechanical parts of stand-alone WECSs. In the literature, the longevity problem of WECSs has rarely been studied. This paper investigates the interaction between electric and mechanical parts of the doubly- fed induction generator (DFIG)-based WECS in stand-alone mode. In this paper, the matter of compensating a non-linear load from the point of mechanic tension problem on the DFIG shaft has been investigated. To this end, the application of the dynamic voltage restorer (DVR) connected to the stand-alone DFIG is presented to compensate non-linear load. The efficiency of the proposed method is compared with the harmonic rejection technique. The simulation results reveal that the proposed method negates the need for maintenance in the long term and increases the longevity of DFIG.

I. Introduction

In today’s power grids, renewable energy sources (RESs) are responsible for supplying part of grid load power, including places with no full access to the main grid. In comparison with the other RESs, employing wind energy conversion systems (WECSs) in power grids has attracted more attention from researchers in recent years. The WECSs are classified into two types fixed speed and variable speed. Due to the neglect of wind speed, the efficiency of fixed speed WECSs is less than variable speed WECSs. According to the characteristics of WECS, it is clear that the rotor speed must be variable to achieve maximum power [1], [2]. Among the variable speed WECSs, the doubly-fed induction generator (DFIG) is one of the most popular variable speed WECSs due to its advantages such as the reduced converter rating, high energy efficiency, and simple structure etc. [1], [2]. The DFIG can be employed in two modes grid-connected and stand-alone [3], [4]. In both modes, researchers have investigated the control and operation of DFIG to improve the power quality and supply the grid load.