# Power Quality Issues and Mitigation for Electric Grids with Wind Power Penetration
## 1 Introduction
intermittent renewable sources like the wind into the existing power system, several technical aspects must be considered from a PQ (power quality) perspective. These PQ requirements mainly require the WECSs to maintain voltage and frequency within safe operational limits and minimize the harmonic emissions. Adhering to the PQ standards help WECSs to meet the load as well as ensure their reliable and safe integration to the grid. As more power electronic loads are connected to the grid, their impact from a PQ perspective must be examined. Power electronic loads such as variable frequency drives, rectifiers, and inverters are considered nonlinear loads and are found in most residential, industrial, and commercial settings. These loads are highly sensitive to PQ problems which are important issues to consider for wind turbine generators (WTGs) interconnection. As wind power penetrations (WPPs) are integrated into the grid, voltage variations and harmonic distortions are some of the important PQ concerns. WTGs harvest energy from wind which is highly intermittent. For a fixed speed wind turbine, variations in wind cause fluctuations in the mechanical torque. It also varies the electrical output from the generator leading to voltage fluctuations that need to be managed [9]. As a result, PQ issues such as voltage sag, swell, flicker, and harmonics are observed in the transmission and distribution systems. Variable speed WTGs are popular as they can extract maximum power from the wind while ensuring low mechanical stress as compared to fixed speed wind turbines. A variable speed WTG utilizes power electronic converters for mitigating output fluctuations from the generators and is a huge source for harmonic distortion. These power electronic converters can inject harmonic current into the grid and lead to high PQ issues in the power system. To address this problem, new standards and grid codes have been developed for enabling smooth and efficient integration of WPPs into the grid [10; 11]. To ensure harmonic distortions are within acceptable limits to meet the satisfactory requirements by both utilities and consumers, two approaches passive and active filterings are used [8; 12; 13; 14; 15]. Conventional passive filters consist of inductors, resistors, and capacitors and are effective in attenuating harmonic components at a low cost [12]. Active filters are more commonly used as they respond efficiently to the dynamics of the electric grid as compared to the passive filters. This is because the tuning frequency and design of the passive filters cannot be changed to meet the changing system conditions. Active filters are very expensive and complicated to design at medium and high voltage levels [13]. Hybrid filters utilize the characteristics of both active and passive filters in their design and provide better performance to mitigate harmonic resonance in WPPs [15]  
Voltage variation is the degree of deviation from the normal sinusoidal voltage waveform in the power system network. The first concern for integrating WTs into the grid is to ensure that the voltage in the grid is kept within an acceptable range. In the European voltage standard EN 50160, a range of 10% of the 10-min average voltages is acceptable at the end-user to ensure that appliances are not damaged or malfunctioning [16]. Further, IEC Standard 61400-21 stated that the 10-min average of voltage fluctuation should be within 5% of its nominal value for the wind power plant system [17; 18]. This paper discusses short-duration voltage variation namely voltage sag and voltage swell that affect the power system during wind integration. To resolve this issue, several mitigation techniques using flexible AC transmission devices (FACTs) such as dynamic voltage restorer (DVR), and static synchronous compensator (STATCOM), or energy storage system (ESS) are used to mitigate the impact of WPP interconnection in the grid. This paper discusses the impact of PQ issues, namely voltage fluctuation and harmonic propagation, in integrating WPPs into the grid.