## 3 Frequency Coordination Control Strategy at LCC Terminal
### Frequency Coordination Control through AFC and AGC  
The AGC unit responds to the area control error (ACE) signal by outputting reference load adjustment commands to the governor. It adjusts the output of the generator units to maintain the scheduled exchange power \(P_{AB}\) on the tie line while ensuring the stability of system frequency and voltage. The basic control modes of AGC include scheduled tie-line power control, scheduled frequency control, and tie-line frequency deviation control. In this paper, tie-line frequency deviation control is used to maintain system frequency stability and tie-line power at the scheduled value. The calculation formula is as follows:  
\[\begin{array}{l}ACE_{A}=\Delta P_{AB}+\beta_{A}\Delta f\\ ACE_{B}=-\Delta P_{AB}+\beta_{B}\Delta f\end{array}, \tag{17}\]  
where \(\Delta P_{AB}\) represents the tie-line power deviation, \(\Delta f\) represents the deviation of system frequency from the rated value, and \(\beta_{A}\) and \(\beta_{B}\) are the frequency response coefficients for the control area of the units.  
For the wind power DC transmission system, the AGC structure diagram after introducing the AFC controller is shown in Figure 4.  
Figure 4: Diagram of AGC structure.  
AGC corrects the control error in the control area to make the ACE signal zero. Due to the addition of the DC link, the ACE signal is expressed as follows:  
\[ACE=\Delta P_{dc}+\beta(f-f_{0}), \tag{18}\]  
where \(f\) is the actual frequency of the area, \(f_{0}\) is the standard frequency of the area, \(P_{0}\) is the planned power, \(\Delta P_{\text{dc}}\) is the active power modulation command of the DC link, PI represents the \(PI\) section of AGC, and \(\Delta P_{\text{s}}\) is the desired power increment of the generator unit.  
The mechanism of AFC and AGC, working in coordination to assist system frequency regulation, is briefly described as follows.  
For small-amplitude, short-period, and rapid active power fluctuations, the system rapidly adjusts the generator output through the governor, initiating a primary frequency response to counterbalance the active power disturbance in the area. If the resulting frequency change exceeds the dead zone range of the AFC controller, the AFC controller outputs modulation instructions to adjust the DC link power quickly, alleviating the imbalance in active power in this area. If the frequency change remains within a narrow range or returns to stability, AGC does not need to be involved in frequency adjustment.  
For large-amplitude, long-period, and rapid gradient changes causing active power disturbances in wind power output, the resulting frequency deviation cannot be restored solely by primary frequency response and AFC.  
In such cases, once the frequency reaches the action frequency set by the AGC controller, AGC outputs a reference load setpoint adjustment command based on the ACE signal, adjusting the generator output to achieve the desired active power adjustment and restoring the balance of system active power. During the AGC adjustment process, AFC will still respond simultaneously by quickly adjusting the DC link active power, suppressing the ACE signal, and rapidly stabilizing the system frequency.