# Analysis of Six Active Power Control Strategies of Interconnected Grids with VSC-HVDC
## 5 Transient Stability Simulation Results with Six Different Active Power Control Schemes  
The interconnected grid configuration for simulation studies is represented in Figure 9, and the three-phase fault event at 345 kV AC transmission occurs in area 2. The detailed system parameters are defined in Table 2. The averaged equivalent circuit of the two-level VSC is used for this feasibility study, and it was supplemented by one on-state switch resistance in each phase, and an equivalent current source at the DC side. Note that the VSC model should be injected by current sources in the PSS\({}^{\text{\textregistered}}\)E program, and the outer and inner current controller parameters are adjusted to achieve the desired system response. Accordingly, the six different power output characteristics of the VSC-HVDC are simulated, as shown in Figure 10. There are two power control start times of \(t_{1}\) and \(t_{2}\). (a), (c), and (d) supply power at \(t_{1}\), and (b), (e), and (f) control power at \(t_{2}\). The RRS and power reference values are all different according to each control characteristic.  
Figure 8: Six active power control schemes: (**a**) step control at \(t_{1}\); (**b**) step control at \(t_{2}\); (**c**) ramp-rate control with high RRS between \(t_{1}\) and \(t_{2}\); (**d**) ramp-rate control with low RRS between \(t_{1}\) and \(t_{2}\); (**e**) ramp-rate control with high RRS between \(t_{2}\) and \(t_{3}\); (**f**) ramp-rate control with low RRS between \(t_{2}\) and \(t_{3}\).  
Figure 9: System configuration for simulation studies.  
As mentioned in Section 2, in order to select PGs when the converter controls its power, the GVIF index of the Korean power system is analyzed. The PGs are selected using Equation (3) with \(\Delta P_{\text{VSC}}\) = 100 MW, and the PGs list is given in Table 3. As a total of 269 generators are connected in the Korean grid at the peak load condition; however, for the sake of convenience, only 22 generators with GVIF are illustrated in this paper.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Grid Parameters** & **Value** & **Converter Parameters** & **Value** \\ \hline Grid frequency & 60 Hz & \begin{tabular}{c} VSC HVDC \\ Rated capacity \\ VSC HVDC \\ DC link voltage \\ \end{tabular} & 1200 MVA \\ Total load & 94,463 MW & \
\begin{tabular}{c} VSC HVDC \\ DC link voltage \\ \end{tabular} & 250 kV \\ Total generation & 95,802 MW &
\begin{tabular}{c} DC capacitance \\ \end{tabular} & 1500 μF \\ AC voltage of each side & 345 kV & \(k_{p1}\), \(k_{p3}\) & 0.5 \\ Short circuit ratio & 25 & \(k_{p2}\) & 0.65 \\ Leakage reactance & 0.07 pu & \(k_{p4}\) & 0.8 \\ Transformer & 345/250 & \(k_{i1}\), \(k_{i3}\), \(k_{i4}\) & 0.01 \\ Voltage ratio & 250/345 & \(k_{i2}\) & 0.1 \\ Transformer rating & 1200 MVA & \(k_{i2}\) & 0.1 \\ \hline \hline \end{tabular}
\end{table}
Table 2: System parameters.  
Figure 10: Six power control schemes of the VSC-HVDC.  
As can be observed in Table 3, the steam turbine (ST) has a zero value of GVIF, since it has a zero droop value, so the active power is not adjusted in response to the frequency variation. On the other hand, the gas turbine has a certain droop value that is more sensitively activated than the steam turbine during the dynamic converter power control. In order to correct the different frequency measurement results, the GVIF index is used to select PGs. We conclude that the power transmitted to area 2 comes from the generators with GVIF values larger than zero. In addition, as shown in Table 3, the most influential PGs are _Yunghung_ #_G3_-_#_G6_ with GVIF = 0.01. With the driven PGs list, the angle spread, which is the difference between the largest and smallest participating machine angles, is analyzed for interconnected areas.