# Calculation of Main Circuit Steady-State Parameters for Capacitor Commutated Converter System
## 5 Case Study
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \(P_{\text{dR}}\)/MW & \(U_{\text{dR}}\)/kV & \(I_{\text{d}}\)/kA & \(\gamma_{\text{app}}\)\({}^{o}\) & \(\gamma_{\text{real}}\)\({}^{o}\) & \(\mu_{\text{I}}\)\({}^{o}\) & \(Q_{\text{I}}\)/Mvar & Tap \\ \hline  
\hline \hline \end{tabular}
\end{table} 
Table 7: Main circuit parameters at the inverter side (data per pole). With the DC power changes from 0.1 pu to 1.2 pu, the firing angle/extinction angle, the DC voltage, the reactive power consumption and the tap position are shown in Figure 7.  
Figure 7: Cont. Based on the above calculation results, the following conclusions can be drawn:
* Due to the existence of the DC resistance, the DC voltage of the rectifier is always greater than that of the inverter. The voltage difference becomes larger as the DC power increases. * An appropriate increase in the number of the on-load tap changers will be beneficial to the decrease in the firing angle at the rectifier and the extinction angle at the inverter, thus lowering the consumption of the reactive powers. * At the low DC power level, due to the restriction of the minimum filter control mode at the rectifier side, the LCC operates under a larger firing angle and the CCC operates under a larger apparent extinction angle. Thus, the reactive power consumptions of both sides will increase. Meanwhile, the DC voltage is below the rated value and the tap changers of the converter transformer are at the higher position. The model of a \(\pm\)500 kV/3000 MW LCC-CCC HVDC transmission system (by modifying the Guizhou-Guangdong II \(\pm\)500 kV/3000 MW LCC-HVDC) is established in PSCAD/EMTDC when the DC power is selected as 1.0 pu. To better illustrate the features of the CCC, the simulation and calculation results of the CCC at the inverter side are shown in Table 8. The simulation results of the conventional LCC at the inverter side are also presented. The results obtained show that the relative errors between the calculation and the simulation results are within 5%. The CCC consumes less reactive power than the conventional LCC under the same conditions. To further display the characteristics of the thyristor valves, waveforms of the valve voltages and the valve currents are shown in Figures 8 and 9, respectively. The results show that the peak voltage across the valves of the CCC is about 1.1 times that of the LCC, which can also be seen in Table 8. Due to the extra commutation voltage provided by the commutation capacitors, the voltage stress across the valves of the CCC lags that of the LCC, which will reduce the apparent extinction angle of the CCC and diminish the reactive power consumption. Moreover, the thyristor valve currents of the CCC exhibit larger fluctuations than those of the LCC during the commutation process. Figure 7: Relationships between some main circuit parameters and the DC power: (**a**) relationship between the angles and the DC power, (**b**) relationship between the DC voltage and the DC power, (**c**) relationship between the reactive power consumption and the DC power, (**d**) relationship between the tap position and the DC power.