# Application of MMC-RPC in High-Speed Railway Traction Power Supply System Based on Energy Storage
## 5 Simulation Analysis
### Verification of the MMC-RPC Based on Energy Storage under Improved VSG  
It is assumed that there is a 10 MW traction load on the left power supply arm of the traction network, which suddenly changes to 20 MW at 0.5 s. The adaptive VSG-DFC control and the traditional direct power control are respectively adopted on the inverter side of the MMC-RPC.  
As shown in Figure 20, when the traction load changes suddenly by 5 MW, the MMC-RPC system under the traditional DCL control lacks damping and inertia support. The output frequency will fluctuate to a certain extent. When the load changes suddenly at 0.6 s, the frequency will fluctuate up to 0.75 Hz, and the fluctuation time will last for 0.08 s.  
As shown in Figure 21, when the traction load changes by 15 MW, the frequency will fluctuate by up to 2.23 Hz, and the fluctuation time will last for 0.1 s. The simulation shows that the frequency instability is serious under the fluctuation of locomotive load, which affects the power supply reliability of the traction power supply system.  
Under the improved VSG control, the output frequency will not be affected by the sudden change of traction load. At the same time, when MMC-RPC is put into operation, the output frequency fluctuation under DCL control reaches 14 Hz, while the fluctuation under improved VSG control is smaller, and it can stabilize faster, the adjustment time is reduced by 20%.  
As shown in Figure 22, due to the sudden change of locomotive load, the output current amplitude increases from 270 A to 1030 A in 0.6 s. The current is stable at 0.61 s, and the harmonic content is low in the process of current fluctuation.  
Figure 21: Output frequencies of MMC-RPC under two control methods of 15 MW load change. Partial diagram of output frequencies by setting: (**a**) MMC-RPC is put into operation, (**b**) sudden load change.  
Figure 20: Output frequencies of MMC-RPC under two control methods of 5 MW load change. Partial diagram of output frequencies by setting: (**a**) MMC-RPC is put into operation, (**b**) sudden load change.  
As shown in Figure 23, the improved VSG control can maintain a stable basic voltage waveform while ensuring stable power transmission. At 0.6 s, the locomotive load changes abruptly, and the voltage ripple of the capacitor of the submodule rises from 30 V to 50 V. In the process of a sudden change of load, the capacitor voltage recovers more slowly than the output current in Figure 22. However, under the capacitor voltage balance control, the maximum voltage oscillation is lower than 70 V, and the capacitor voltage of the MMC-RPC sub module is stable at 5.5 kV at 0.75 s. Finally, the voltage fluctuation is maintained within 1.5% of the rated voltage with load change.  
As shown in Figure 24, after adding ES units, the capacitance-voltage fluctuation of the sub module can still be maintained within 1.5%. The stability of DC side of MMC-RPC based on ES is confirmed.  
Figure 24: Capacitor voltage comparison between no energy storage and energy storage.  
Figure 23: Capacitor voltage of sub-module in one bridge arm.  
As shown in Figure 25, after adding ES units, the capacitor voltage of the sub module can be stabilized near the expected value, and the voltage fluctuation is within 1.5%. The effect of voltage equalizing and voltage stabilizing control is good.  
As shown in Figure 26, in order to utilize regenerative braking energy, energy absorption is carried out through SCs. SCs under VDCM control reach the desired power in 0.06 s, which confirms the rapidity and stability of the control.  
According to Figures 20-26, ES-type MMC-RPC can maintain stable voltage, current and power during normal operation or sudden changes in locomotive loads. The simulation waveform verifies the stability of the topology and improves VSG control.