# Power Quality Issues and Mitigation for Electric Grids with Wind Power Penetration
## 4 Simulation of Voltage Mitigation
### Results of Selected Case Studies
#### 4.3.2 Distribution System Case Studies  
The IEEE 123-node standard test feeder is selected to be used to present some selected case studies [54]. The feeder involves a mix of three-phase, two-phase, and single-phase line segments. It has four voltage regulators, various balanced and unbalanced spot loads, and four capacitor banks. In the proposed test, the load buses of the IEEE 123 feeders are classified into three categories according to load type viz. residential, industrial, and commercial. The WPP is assumed to be connected to the system at bus \"67\". The size of the wind power plant is chosen to be 50% of the total load of the base case of the feeder. Case 1 System without wind penetration: The base-case load of the IEEE 123 feeder is simulated using the Saudi load profile. Two scenarios have been simulated. In the first scenario, all voltage regulators are de-energized from the feeder. This scenario shows smooth voltage profile variations due to smooth changes in demand. This case shows that to keep the voltage profile time-series within the desired limit of \(V\pm 5\%\), voltage regulators should operate to adapt the variation of the load profiles along the day. Case 2: System study with wind penetration: It is identical to Case 1 but with WPP. The voltage profile time series involves rapid fluctuations due to the intermittency of WPP. The voltages slightly increase and drop back as the wind power generations fluctuate. The result obtained from this case emphasizes the impact of wind power intermittency on electrical power systems. As the voltage regulators are energized, the time-series of the voltage profiles are improved and becomes closer to the allowable limits \(V\pm 5\%\). However, excessive operation of regulator taps could affect the age of the regulator and yet cannot completely handle the fast fluctuations of wind power generation. Case 3 System study with wind penetration and reactive power compensation: It is identical to case 2 but reactive power compensation is included. Firstly, regulators are de-energized, the reactive compensation successfully keeps the voltage within the limits \(V\pm 5\%\). Figure 14 shows that the changes in \(Q_{c}\) so that the voltage is kept within its desired limit. The upper bound of \(Q_{c}\) is the available reactive power \(Q\) from the power plant. The middle of the time axis of Figure 14 shows that as \(Q\) drops, hence, \(Q_{c}\) drops instantaneously. Secondly, when regulators are included in the simulation, they keep the voltage profiles at a slightly higher level. This is due to the setting of the regulator and the delay time required for the regulator to operate. Along the simulation period, more tap movements are performed to keep the voltage within the allowable limits. In general, the intermittent nature of wind power affects the voltage and changes its profile from a smooth pattern to a much-fluctuated pattern. Figure 13: Voltage profiles at PCC for a WPP with reactive power compensation capability, the WPP is connected at the node “30” of the IEEE 30-node. When the voltage magnitude of the regulated bus, at PCC, is within the allowable limits, the compensator will not provide additional \(Q_{c}\) which is opposite to the regulators that are distributed along the feeder and sense different points in the grid. This is exhibited in Figure 15a which shows that \(Q_{c}\) only appears beyond 6 PM whereas the voltage of the regulated bus drops. The amount of \(Q_{c}\) is just enough to regulate the voltage of the PCC as shown in Figure 15a. Hence, the regulators continue to operate to regulate the remaining nodes in the study system. Within the timescale of the simulated series, the regulators operate many times, as shown in Figure 15b, to control and keep the feeder voltages within the preset limits. However, these excessive movements could be avoided by maximizing the role of the reactive power compensation to mitigate the voltage fluctuations.