## 3 Frequency Coordination Control Strategy at LCC Terminal  
### Auxiliary Frequency Controller Design Based on VFF-RLS Algorithm  
Auxiliary frequency controllers can be used to assist in the primary frequency regulation of power systems, with the basic principle being DC active power modulation. The deviation in active power caused by the small-amplitude, short-period, and rapidly changing wind power output fluctuations is denoted as \(\Delta P\). This is equivalent to a load disturbance with the same amplitude and period, and the frequency deviation caused by this disturbance is considered as primary frequency control, adjusted by the governor of the generator unit. Therefore, according to the power balance relation, it is expressed as follows:  
\[\Delta P=\Delta P_{\mathrm{G}}-\Delta P_{D}=K\Delta f, \tag{1}\]  
where \(\Delta P_{\mathrm{G}}\) represents the power increment adjusted by the generator unit based on the droop characteristic, and \(\Delta P_{D}\) represents the load increment adjusted by the load regulation effect. \(K\) is the power-frequency static characteristic coefficient. From (1), the larger the value of K, the smaller the frequency variation caused by power fluctuations in the system. Therefore, the stability of the system frequency can be improved. The AFC controller achieves power regulation by adding a DC current modulation signal to the main controller of the LCC converter station; the specific structure is shown in Figure 2, where \(\Delta\omega\) represents the selected speed difference between generators, \(T_{w}\) is the time constant of the DC-blocking link, \(G_{\mathrm{c}}(s)\) is the controller transfer function, \(\Delta I_{d}\) is the DC current regulation command, \(I\) is the actual measurement of DC current, \(I_{d0}\) is the DC current setpoint, \(K_{1}\) and \(K_{2}\) are parameters of the LCC converter station main controller, and \(\alpha\) represents the firing angle. The specific principle of the AFC controller is summarized as follows:  
When the system frequency fluctuation exceeds the action frequency set by the AFC controller, the AFC controller responds and outputs an additional DC current regulation command \(\Delta I_{\mathrm{d}}\), which is superimposed on the main controller of the LCC converter station, jointly participating in modulating the active power of the DC interconnection line. The rapid increase or decrease in active power in the DC contact line is utilized to alleviate the active power imbalance in the area and to realize the mutual support of power between areas, thus improving the system frequency. The additional DC current regulation command \(\Delta I_{\mathrm{d}}\) is output by the AFC controller, which can be used to obtain the additional DC power regulation command \(\Delta P_{\mathrm{dc}}\) to regulate the DC power. \(K_{\mathrm{dc}}\) is the corresponding frequency regulation coefficient The Equation (1) can be rewritten as follows:  
\[\Delta P=\Delta P_{\mathrm{G}}-\Delta P_{D}+\Delta P_{\mathrm{dc}}=(K+K_{ \mathrm{dc}})\Delta f. \tag{2}\]  
From (1) and (2), it can be seen that the participation of AFC is equivalent to improving the active power-frequency static characteristic coefficient of the system; that is to say, for  
Figure 2: System structure of the AFC controller. the same amount of power variation, the system with AFC participation has a small amount of frequency variation. Thus, the stability of the system frequency is enhanced. To avoid frequent responses of the AFC controller to the active power of the DC line, a dead zone limit link is introduced, and the size of the dead zone limit needs to be coordinated with the primary frequency modulation. In addition, to eliminate the influence of steady-state error on the controller, a time constant \(\mathrm{Tw}=10\) s is added after the dead zone limit link. The recursive least squares (RLS) algorithm is an adaptive filtering algorithm commonly used for system identification.