# Power Quality Issues and Mitigation for Electric Grids with Wind Power Penetration
## 3 PQ Mitigation Techniques
### Future Mitigation Trends
#### 3.2.1 Energy Storage Technology  
Energy storage technologies can play a role in many aspects such as improvement of PQ and mitigation of voltage variation in addition to the reduction of energy cost. Therefore, WTGs configured with energy storage technology becomes an effective solution to minimize PQ issues when WTGs are integrated into the grid [35]. Several energy storage technologies have been investigated and validated such as flow (zinc bromine) batteries and supercapacitors. Further, flywheel ESS, superconducting magnetic energy storage, and compressed-air ESS in addition to pumped-hydroelectric storage are also advantageous [36]. The battery storage enhances the performance under the disturbances conditions of WPPs and can improve the voltage stability of the system. ESS provides a bulk system storage from wind in about 15-min periods during high power availability. Many types of energy storage technologies are being currently used in WPPs. The flow battery system is different in concept and design from the lead-acid battery. The zinc-bromine battery system offers the lowest cost for both the stored and delivered energy. Additionally, it offers high energy density, size, and weight savings over lead-acid batteries. Such type of batteries maintains their performance even after repeated cycling.  
A study in Ref. [37] presents the sizing of ESS for smoothing voltage fluctuation to help high penetration of WTGs into the grid. The proposed approach determines the minimum capacity of ESS to satisfy the voltage limit based on the unbalanced three-phase interval power flow. The output voltages of power flow can directly demonstrate the voltage fluctuation rate based on their interval forms. Therefore, a feedback mechanism is built between the output voltage fluctuation rate, and the input ESS practical power to determine the minimum capacity of ESS to satisfy the fluctuation limit. The results show a reduction in the fluctuation rate from 5.76% to 3.89% when applying ESS with wind installation. Flywheel ESS is considered in simulating WFs comprising of four fixed-speed WTGs. The aim of choosing flywheel ESS is to reduce the power and voltage fluctuations caused by aerodynamic aspectsof WTGs. Thus, the results illustrate the effectiveness of flywheel ESS for mitigating voltage and power fluctuations [38]. It shows a reduction in the magnitude of power fluctuation leading to a smoother output power of WPPs. Additionally, voltage fluctuation increases to 4.8%. Hence, STATCOM and flywheel ESS are implemented to reduce the fluctuation rate. STATCOM reduces the variation to 3.8% while flywheel ESS minimizes it to 0.6% [39]. The hydrogen-storage application is used to improve the grid PQ by smoothing large and quick fluctuations of wind energy. The control system strategies with battery storage can improve the grid PQ by absorbing or releasing power components. The combination of the battery storage with the control strategies could improve harmonic distortion and provides dynamic voltage control at PCC. However, the economic constraint and specific control strategies of the storage technologies should be studied in detail [36].