## 3 Control Strategy of Hybrid Reactive Power Compensation System  
In order to be able to fully consider the response speed of reactive power compensation equipment, and, at the same time, to achieve its economy, a combined reactive power compensation program is proposed. This solution not only has the characteristics of SVG continuous and fast adjustment, but also has the advantages of low-cost and large  
Figure 1: Static Var Generator control system structure diagram.  
Figure 2: Static Var Generator double ring control system structure diagram.  
capacity reactive power compensation, such as parallel capacitors. Its corresponding system structure is shown in Figure 3 below.  
We assume that the capacity of a single group of switchable parallel capacitors is Q\({}_{c}\) and the reactive power capacity required in the wind farm is Q. When \(\text{kQ}_{c}<\text{Q}<(\text{k}+1)\) Q\({}_{c}\), the control system issues a command to switch in the k groups of capacitor banks. At the same time, the system detects the gap in reactive power compensation and issues the input SVG command, in order to further perform reactive power compensation on the system. In this way, the smooth regulation of reactive power and the voltage stability of the grid connection point are always maintained.  
Conventional capacitor banks generally choose the SC type. Usually, to avoid frequent switching of SC capacitor banks, a compensation margin (Y) will be set within the reactive power range. If the power grid system has been put into k groups of capacitor banks as reactive power compensation, then: (1) when \(\text{Q}>(\text{k}+1)\text{Q}_{c}+\text{Y}\), put in the k + 1 group of capacitor banks; (2) when \(\text{Q}<\text{kQ}_{c}-\text{Y}\), exit the k-th capacitor bank. Setting the compensation margin not only avoids the frequent switching of capacitor banks, but also ensures the linear demand of the system for reactive power. The control system flow of the hybrid reactive power compensation is shown in Figure 4 below.  
Figure 3: Hybrid reactive power compensation system topology diagram.  
The cost advantage of the device itself, compared with the SVC type, is that the SVG type reactive power compensation device adopts digital control technology, which has high system reliability, basically does not need maintenance, and can save a lot of maintenance costs. SVG adopts static operation, with no large rotating equipment, no wear, and no mechanical noise. This greatly improves the device's life, and also improves the environmental impact. With the same compensation capacity, the footprint of SVG is reduced by 1/2 to 2/3 compared with that of SVC. Since SVG uses fewer reactors and capacitors than SVC, it greatly reduces the volume and footprint of the device. The reactors in SVC not only have a large volume, but also occupy a large area, considering the installation interval between them.  
In terms of economic analysis of assessment, taking a station as an example, the average monthly assessment was RMB 20,000 before the installation of the reactive power compensation device. After the installation, the assessment was not only exempted, but the monthly reward was stable at more than RMB 5000, and good economic benefits were achieved, as shown in Table 1 below.