# Power Quality Issues and Mitigation for Electric Grids with Wind Power Penetration
## 2 Power Quality Issues of WPP Integration
### Harmonic Issues of WPP Integration
#### 2.1.1 Effect of Harmonics in WPP  
Harmonic emissions are a major issue in WPPs connected to the grid. The harmonic emissions from WPPs are different from arc furnaces. Case studies are presented to analyze the harmonics emitted from Type 1 and Type 3 WTGs based WPP.  
Type 1 WTGs based WPP: Type 1 WTGs are also known as FSIG turbines. Compensation capacitors are used to compensate for the reactive power consumption of the induction generators from the grid. The capacitor bank consists of mechanically switched capacitors which result in the occurrence of resonance [20]. Generally, even and triplen harmonics are not present in WPPs and thus only 5, 7, 11, and 13 harmonic orders are considered in the study. It is shown that for the same size of reactive power there occur two resonance frequencies [2]. When the reactive power requirement of the WTG matches these critical values, resonance occurs at high harmonic frequencies. The resonance frequency can vary with increased wind speed or increased size of shunt capacitance. 7 th harmonics may be observed at lower wind speed which corresponds to low wind power and lower shunt capacitance. For high wind speed and high power, 5 th harmonics may be encountered [2].  
Type 3 WTGs: In [20], three WPPs with type 3 WTGs and a total capacity of 185 MW are considered for a power quality study. Large voltage and current harmonics are observed at 18 th, 19 th, and 20 th harmonic orders during certain operating conditions of the WPPs. When the three WPPs are operated at 80% of nominal capacity and the power output of the third WPP (WPP3) is reduced from 80% to 0%, severe harmonic injection is seen. It is inferred that at zero output power from WPP3, the unloaded cables and transformer inductances behave like an LC circuit. This results in the generation of large harmonic currents. This harmonic current continued to flow until the WPP3 is disconnected from the power network. Large resonance is still observed at the 19 th harmonic order even after WPP3 is disconnected from the network.  
Resonance in the DC link of the back to back VSC converter can also lead to the occurrence of inter-harmonic frequencies. Ref [14] presents a case study on the PQ analysis performed on three 600 kW WTGs with a full power converter in Northern Sweden. PQ measurements performed at the transformer feeding the WTGs indicated contamination from 7, 11, 13, and 17 th order harmonics. Harmonic spectrum over the entire frequency range up to 4 kHz revealed a flat maximum at around 700 Hz or nearly equal to the 13 th harmonic order and at 4 kHz (80 th order). In all the measured observations, the 13 th harmonic order shows dominance and the 11 th harmonic order has the overall highest presence. In addition to the discrete harmonics, inter-harmonic orders such as 12.5 and 13.5 are also observed to be the dominant ones. Literature indicates that the waveform distortion is very high at low power output from WTGs [21]. THD is one of the metrics used to measure distortion. High THD is observed when the WTG produces low power output.