## 3 Control Method and Control Characteristics of the Switching Process of the DFVSPS Unit
### Generating Mode
#### 3.1.3 On-Grid Operation Stage  
After grid connection, the DFVSPS unit is in the grid-connected operation state. There are two control methods when the DFVSPS unit in the grid-connected operation stage, which are power priority control strategy. According to the research of related literatures, the power priority control strategy has better control characteristics when realizing power control of unit features, while the speed priority control strategy has an advantage in speed regulation. In the research of this paper, the power of the unit should be controlled whether it is in the load ramping stage, load rejection stage or the stable operation stage. Therefore, the power priority control strategy is adopted. The overall control strategy diagram is shown in Figure 10.  
The grid-side converter implements dc voltage control, the rotor-side converter realizes the control of the power, and the guide vane opening control system of pump-turbine realizes the speed control of the unit. Since these control strategies are common, this article will not repeat them. The control process for each stage is shown as follows. Load ramping stage and load rejection stage: According to the full characteristic curve of the pump turbine, the speed of the unit in the load ramping stage and load rejection stage are maintained near the synchronous speed, and the power of the unit is constantly adjusted. According to the control strategy diagram and the DFIM model, the power and electromagnetic torque of the unit are controlled by the \(d\)-axis current of the rotor. The constant adjustment of \(d\)-axis current will lead to constant changes of unit speed. Since the purpose of the guide vane control system at this time is to maintain the unit speed at the synchronous speed, the guide vane opening is continuously adjusted, changing the flow and thus changing the output power of the pump turbine. In order to prevent a large change  
Figure 9: Comparison of three-phase voltage before and after grid connection. in speed when the power changes, a stepwise increase or decrease of the load is adopted, that is, the load is increased or decreased by 30 MW every 5 s until the power reaches the target value. Stable operation stage: According to the full characteristic curve, the unit is running near the rated operating point at this time. The output power can be adjusted according to the grid demand. The speed can be adjusted continuously according to the change of the operating environment of the unit to make the unit operate at the optimal efficiency. Combined with the overall control strategy of the power priority control method and the mathematical model of each part of the DFVSPS unit, a simulation model is established in MATLAB/Simulink to simulate the control characteristics of the on-grid operation in the generating mode. The parameters of the proportion integration (PI) regulator are obtained by trial and error, and the waveform is shown in Figure 11 (Since this paper focuses on the tracking performance of low frequency band of the control system, the PI controller is used). Figure 11: Simulation waveforms of unit in generating mode: (**a**) active power in load ramping stage; (**b**) active power in stable operation stage; (**c**) active power in load rejection stage; (**d**) speed in stable operation stage; (**e**) guide vane opening in stable operation stage; (**f**) actual dynamic responses for a DFVSPS unit in stable operation. Figure 10: Overall control diagram of power priority control strategy. Figure 11 shows the simulation waveforms of the unit in the on-grid operation stage under generating mode. The red line in Figure 11a-c is the reference value of unit active power and the black line is the unit active power simulation waveform obtained by the electromagnetic transient simulation model. Figure 11a shows the output power uploaded from the 30 MW to 240 MW.