# Analysis of Six Active Power Control Strategies of Interconnected Grids with VSC-HVDC
## 4 Analysis of Active Power Control Strategies
### More Detailed Description of Step Control and Ramp-Rate Control Strategies  
There is an N-1 contingency event in area 2 with the given scenario, which is represented in Figure 6. Each country, if it has a different Special Protection System (SPS), as the generator tripping schemes are generally called, commonly commands the specific generators to be tripped so as to balance the network power. Basically, the nine cycles as time delay should be taken with the generator tripping scheme, since a mechanical switch is included. As the electronic power equipment only requires communication delay, the activating time is naturally fast [17]. In this paper, three cycles of communication delay are adopted for \(t_{1}\) time, and the minimum frequency occurred at \(t_{2}\) time, as shown in Figures 6 and 7. Accordingly, various active power control strategies can be applicable to the VSC-HVDC to supply more power to area 2. Given the frequency fluctuation of area 2, six active power control strategies in total are introduced hereafter.  
Figure 5: Different RRSs of ramp-rate control; DB: difference between.  
Figure 8a,b both show step control, with the only difference being a power release time. In (a), the power increased at \(t_{1}\), which is the minimum time to receive an SPS signal as 1.05 s. In (b), the converter changes its power at \(t_{2}\). Benefiting from the fast response of power electronics, (a) and (b) show advantages of being able to control the power step-by-step, unlike the common generator characteristics. Except for (a) and (b), all other control strategies are ramp-rate control strategies with different RRSs and sending times. Figure 8c,d have different RSSs between \(t_{1}\) and \(t_{2}\), which are derived from two different power references. More specifically, the power command in (c) is twice the value of (d); thus, the RRS of (d) is selected to be half. This is to observe the results according to both initial power support speed and amount. Figure 8e,f discuss how the angle stability of each area changed during the recovery stage of area 2. Thus, the two control strategies have the same power reference value, but different RSSs between \(t_{2}\) and \(t_{3}\). For this, the \(t_{3}\) is selected for the control variable to adjust the RSS. In the simulation study, the \(t_{3}\) of (e) is twice the value of (f), so that (e) has an RRS twice that of (f). According to the control characteristic, the six control schemes are defined in Table 1, and specific simulation results are introduced hereafter.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Denomination** & **Control Type** & **RRS** & **Control Time** \\ \hline (a) & Step & - & \(t_{1}\) \\ (b) & Step & - & \(t_{2}\) \\ (c) & Ramp-rate & \(RRS_{max}\) & \(t_{1}\) to \(t_{2}\) \\ (d) & Ramp-rate & \(RRS_{init}\) & \(t_{1}\) to \(t_{2}\) \\ (e) & Ramp-rate & \(RRS_{max}\) & \(t_{2}\) to \(t_{3}\) \\ (f) & Ramp-rate & \(RRS_{init}\) & \(t_{2}\) to \(t_{3}\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Six different active power control strategies.  
Figure 6: Contingency scenario in area 2.  
Figure 7: Frequency drop during a contingency event in area 2.