## 6 Example Analysis
### Example 2--Summer Base  
In summer base operation mode, in 0.062 Hz oscillation mode, the energy of some units of the power grid participates in weight \(\eta_{\lambda_{k}i}\), and the transient energy change \(\Delta V_{t_{k}i}\) of the units after being converted according to \(\eta_{\lambda_{k}i}\) is shown in Table 5. It is calculated that the transient energy change of the system under this oscillation mode is \(\Delta V_{t_{k}}=1614\) kW \(\cdot\) h.  
A 2% power disturbance is applied to three DCS at the same time, and the DC modulation sensitivity \(\varphi_{m}\) is obtained by Prony algorithm identification and calculation, and the DC modulation sequence is obtained by sequencing, as shown in Table 6. According to the calculation in Table 6, when \(p=2\), the energy change \(\Delta V_{\text{dc}_{k}}\) used by DC to suppress oscillation is 2876 kW\(\cdot\)h, and it is determined that DC a and DC c participate in modulation.  
\begin{table}
\begin{tabular}{c c c c} \hline DC Modulation Sequence & 1 & 2 & 3 \\ \hline Name & a & b & c \\ DC modulation sensitivity & 0.649 & 0.632 & 0.317 \\ \hline \end{tabular}
\end{table}
Table 4: DC modulation sequence table (summer peak case).  
Figure 5: Frequency curve of power system after an \(N-1\) fault occurred in the AC2 channel (summer peak case). (**a**) a DC modulation; (**b**) a, b, c DC modulation.  
Taking the power disturbance of a and c DC as input, the PI parameters of a DC FLC can be iteratively obtained as K\({}_{1}\) = 1.0096, T\({}_{1}\) = 1.0584, and PI parameters of c DC FLC as K\({}_{1}\) = 1.1063, T\({}_{1}\) = 0.5437 by using the same steps as example 1. When the \(N-1\) fault occurs in AC2, the system frequency change curve is shown in Figure 6.  
It can be seen from Figure 6a that compared to the system without DC modulation, when DC a and DC c participate in the modulation at the same time, the system frequency oscillation is significantly suppressed, which verifies the effectiveness of the proposed strategy.  
In order to further verify the correctness of analyzing the oscillation mechanism of the DC modulation suppression system from the perspective of energy and the ordering of DC modulation sensitivity, three kinds of a DC participate in modulation alone, a and b DC participate in modulation simultaneously, and a and c DC participate in modulation simultaneously are simulated and compared. The frequency change curve of the system is shown in Figure 6b. As can be seen from the figure, when DC a participates in modulation alone, the system oscillation amplitude decreases but the system frequency still oscillates. As a and b DC participate in modulation at the same time, and a and c DC participate in modulation at the same time, the system frequency can be effectively suppressed, but the system frequency oscillation is most effective when a and c DC participate in modulation at the same time.  
\begin{table}
\begin{tabular}{c c c c} \hline DC Modulation Sequence & 1 & 2 & 3 \\ \hline Name & a & c & b \\ DC modulation sensitivity & 0.604 & 0.124 & 0.044 \\ \hline \end{tabular}
\end{table}
Table 6: DC modulation sequence table (summer base case).  
Figure 6: Frequency curve of power system after a \(N-1\) fault occurred in AC2 channel (summer base case).(**a**) a, c DC modulation; (**b**) Two-channel DC modulation.