**6. Results and discussion**

**Figure 13** shows the active power generation characteristics of the VSPS unit. Its power generation trends vary according to the characteristics of wind power fluctuations. **Figure 14** shows a comparison of VSPS active power compensation with wind power fluctuations. The comparison takes into account the power fluctuations of the wind farm from its mean value (40.74 MW) and the difference in VSPS production

**Figure 13.** *The active power characteristics of the VSPS.*

#### **Figure 14.**

*The fluctuation compensation characteristics. a) the wind farm power fluctuation from its average value and the compensation power generated and regulated by VSPS system. b) the active power difference between the wind fluctuation and the VSPS compensation.*

#### **Figure 15.**

*The grid frequency response associated with the wind power fluctuation.*

**Figure 16.**

*Evolution of the network in grid frequency response comparing the VSPS active power control with and without droop control during the start-up transient and three-phase faults imposed.*

from its mean value (247.1 MW). The error between the wind farm power fluctuation and the power produced by the VSPS is small. Therefore, VSPS can effectively and quickly compensate for large fluctuations in wind energy. This verifies the ability of VSPS to meet wind power fluctuation compensation requirements. Therefore, as shown in **Figure 15**, a very small deviation in grid frequency is observed, less than 0.002%.

**Figure 16** shows the evolution of the grid frequency response by comparing VSPS active power control with and without droop control during emergency. Compared with the vector control strategy without droop feed control, the deviation of frequency from the nominal value is greatly reduced in the droop feed vector control strategy. The grid frequency response deviation was observed to be 0.04 Hz, while active power was regulated to track wind power fluctuations, start-up transients, and threephase faults. Importantly, the settling time of the droop control scheme is very short. Therefore, the grid frequency dynamics and steady-state response in the droop-fed vector control strategy are quite stable, and the deviation is within an acceptable range.

### **7. Conclusion**

This chapter discusses VSPS systems in wind farm grid-connected systems, which play a vital role in the context of increasing penetration of renewable energy. It also has an important function in real-time applications for balancing generation and demand as it provides fast-response generation. VSPS active power generation can quickly adjust wind power fluctuations, so that the power flow on the grid bus is stable. This chapter proposes a VSPS system based on a droop-fed DPC strategy as a solution. Spectral analysis is used to verify the performance of the proposed control system as well.

The results show that VSPS has important stability control characteristics to respond to different contingencies, such as generator losses, large load changes, faults, *Wind Power Fluctuation Compensation by Variable-Speed Pumped Storage Plant in a Grid… DOI: http://dx.doi.org/10.5772/intechopen.104938*

and start-up transient disturbances. The proposed control strategy can also well regulate grid frequency and AC bus voltage. Spectral analysis method implemented in this chapter is also effective for determining the requirements of wind power fluctuations and stability in large power systems.
