**4. Simulation and Implementation**

ment with the simulation results.

ler.

**4. Simulation and Implementation**  In this study, the POWERSIM simulation software is used in both simulation and hardware implementation, and a personal computer equipped with an i7-9700 CPU, @ 3.00 GHz is used as a control desk. The sampling frequency used in the hardware design of the proposed LVRTC is 50 kHz. It is the same as that of the switching frequency used. Figure 9a shows the schematic architecture of the simulated LVRTC system connected to the power grid. For the simulation scenario, the grid voltage variation sequence is planned as follows: 0.5 pu, 0.6 pu, 0.7 pu, 0.8 pu, 0.9 pu, 1.0 pu, 1.2 pu, and then 1.0 pu. The corresponding reactive power commands for the LVRTC are generated according to the grid code. The simulation results are shown in Figure 10. Figure 11 shows the detailed steadystate waveforms of Figure 10. It can be observed that the LVRTC successfully feeds appropriate reactive current to the grid during faults. Figure 12 shows the results of the experimental tests using the constructed 2 kVA SiC-based three-phase inverter as shown in Figure 9b, and a programmable AC power supply emulating the grid. In this study, the TI's DSP (TMS320F28335) is used as the control core for the proposed LVRTC hardware, in which six SiC switching devices using ON Semiconductor's NTHL060N090SC1 with the driver integrated circuits of Si8271 and the sensor devices for sensing currents and In this study, the POWERSIM simulation software is used in both simulation and hardware implementation, and a personal computer equipped with an i7-9700 CPU, @ 3.00 GHz is used as a control desk. The sampling frequency used in the hardware design of the proposed LVRTC is 50 kHz. It is the same as that of the switching frequency used. Figure 9a shows the schematic architecture of the simulated LVRTC system connected to the power grid. For the simulation scenario, the grid voltage variation sequence is planned as follows: 0.5 pu, 0.6 pu, 0.7 pu, 0.8 pu, 0.9 pu, 1.0 pu, 1.2 pu, and then 1.0 pu. The corresponding reactive power commands for the LVRTC are generated according to the grid code. The simulation results are shown in Figure 10. Figure 11 shows the detailed steady-state waveforms of Figure 10. It can be observed that the LVRTC successfully feeds appropriate reactive current to the grid during faults. Figure 12 shows the results of the experimental tests using the constructed 2 kVA SiC-based three-phase inverter as shown in Figure 9b, and a programmable AC power supply emulating the grid. In this study, the TI's DSP (TMS320F28335) is used as the control core for the proposed LVRTC hardware, in which six SiC switching devices using ON Semiconductor's NTHL060N090SC1 with the driver

voltages are used to facilitate a flexible experimental system. Regarding the task of DSP's programming, the SimCoder tool of POWERSIM software (2021) is used to convert the

implementation stage, the control desk (PC) is used to communicate with the DSP, and the controller parameters can be adjusted online to achieve the best control performance and accuracy. Figure 9c shows the DSP programming steps in hardware implementation of LVRTC. In this study, the identical system condition used in the simulation case is implemented here in the hardware test. Figure 13 shows the detailed steady state waveforms of Figure 12. It can be clearly observed that the implementation results are in good agree-

integrated circuits of Si8271 and the sensor devices for sensing currents and voltages are used to facilitate a flexible experimental system. Regarding the task of DSP's programming, the SimCoder tool of POWERSIM software (2021) is used to convert the AD modules, various functional blocks, and controllers into the required DSP control program. Then, the DSP control program can be burned into the DSP chip. In the hardware implementation stage, the control desk (PC) is used to communicate with the DSP, and the controller parameters can be adjusted online to achieve the best control performance and accuracy. Figure 9c shows the DSP programming steps in hardware implementation of LVRTC. In this study, the identical system condition used in the simulation case is implemented here in the hardware test. Figure 13 shows the detailed steady state waveforms of Figure 12. It can be clearly observed that the implementation results are in good agreement with the simulation results. *Micromachines* **2022**, *13*, x FOR PEER REVIEW 11 of 21

(**c**)

**Figure 9.** (**a**) Schematic of simulated grid-connected LVRTC; (**b**) the photograph of the LVRTC SiCbased 3P inverter circuit prototype; (**c**) the DSP programming steps in hardware implementation of LVRTC. **Figure 9.** (**a**) Schematic of simulated grid-connected LVRTC; (**b**) the photograph of the LVRTC SiC-based 3P inverter circuit prototype; (**c**) the DSP programming steps in hardware implementation of LVRTC.

**Figure 10.** Simulation results of LVRT capability: (**a**) DC voltage and its command signal/grid phase A voltage and current (20 times); (**b**) inverter d-axis current and its command/q-axis current and its command. **Figure 10.** Simulation results of LVRT capability: (**a**) DC voltage and its command signal/grid phase A voltage and current (20 times); (**b**) inverter d-axis current and its command/q-axis current and its command.

(**c**)

**Figure 11.** *Cont.*

**Figure 11.** *Cont.*

**Figure 11.** Detailed view of inverter dq-axis currents and their commands/phase A voltage (V) and

*Micromachines* **2022**, *13*, x FOR PEER REVIEW 15 of 21

**Figure 11.** Detailed view of inverter dq-axis currents and their commands/phase A voltage (V) and three-phase currents (A) in Figure 10: (**a**) stage (1); (**b**) stage (2); (**c**) stage (3); (**d**) stage (4); (**e**) stage (5); (**f**) stage (6); (**g**) stage (7). **Figure 11.** Detailed view of inverter dq-axis currents and their commands/phase A voltage (V) and three-phase currents (A) in Figure 10: (**a**) stage (1); (**b**) stage (2); (**c**) stage (3); (**d**) stage (4); (**e**) stage (5); (**f**) stage (6); (**g**) stage (7). three-phase currents (A) in Figure 10: (**a**) stage (1); (**b**) stage (2); (**c**) stage (3); (**d**) stage (4); (**e**) stage (5); (**f**) stage (6); (**g**) stage (7).

**Figure 12.** Implementation results of LVRT capability: (**a**) DC voltage and its command signal (400 V/div)/grid phase A voltage (90 V/div) and current (5 A/div); (**b**) inverter d-axis current and its command (1.5 A/div)/q-axis current and its command (4.5 A/div). **Figure 12.** Implementation results of LVRT capability: (**a**) DC voltage and its command signal (400 V/div)/grid phase A voltage (90 V/div) and current (5 A/div); (**b**) inverter d-axis current and its command (1.5 A/div)/q-axis current and its command (4.5 A/div). **Figure 12.** Implementation results of LVRT capability: (**a**) DC voltage and its command signal (400 V/div)/grid phase A voltage (90 V/div) and current (5 A/div); (**b**) inverter d-axis current and its command (1.5 A/div)/q-axis current and its command (4.5 A/div).

**Figure 13.** *Cont.*

(**b**)

(**d**)

**Figure 13.** *Cont.*

(**f**)

**Figure 13.** *Cont.*

**Figure 13.** Detailed view of phase A voltage and three-phase currents/inverter dq-axis currents and their commands in Figure 12: (**a**) stage (1); (**b**) stage (2); (**c**) stage (3); (**d**) stage (4); (**e**) stage (5); (**f**) stage (6); (**g**) stage (7). **Figure 13.** Detailed view of phase A voltage and three-phase currents/inverter dq-axis currents and their commands in Figure 12: (**a**) stage (1); (**b**) stage (2); (**c**) stage (3); (**d**) stage (4); (**e**) stage (5); (**f**) stage (6); (**g**) stage (7).

### **5. Discussion 5. Discussion**

With the growth of wind power generation over the past decade, various LVRT technologies have been proposed to ensure the safe and reliable operation of WTGs and power systems. Some advanced LVRT control algorithms reviewed in this paper and reported in many other similar papers in the literature can be categorized into two groups, i.e., the protection strategies- and the control strategies-based methodologies. However, most of the reported control strategies were implemented by the WTG built-in converters with complex controllers. It should be noted that with a limited hardware capacity, during the period of fault, the real power output capability of the built-in converters of WTGs must be dynamically limited to meet real-time reactive power requirements, and comply with strict grid codes. Based on the LVRT grid code, when the voltage dipped over 0.5 pu, the total current capacity of inverter system of the WTGs should be used to output the reactive power. This can strongly limit the control flexibility of a real power restoration scheme in the post-fault period. In this paper, we have demonstrated a distributed LVRT compensator (LVRTC) control scheme that can, to some degree, solve the above problem, and increase the system control flexibility. In addition, with the proposed distributed LVRTC control scheme, the WTG in a wind farm has greater freedom in performing its protection schemes, or regulating its power flow to ensure system stability. With the growth of wind power generation over the past decade, various LVRT technologies have been proposed to ensure the safe and reliable operation of WTGs and power systems. Some advanced LVRT control algorithms reviewed in this paper and reported in many other similar papers in the literature can be categorized into two groups, i.e., the protection strategies- and the control strategies-based methodologies. However, most of the reported control strategies were implemented by the WTG built-in converters with complex controllers. It should be noted that with a limited hardware capacity, during the period of fault, the real power output capability of the built-in converters of WTGs must be dynamically limited to meet real-time reactive power requirements, and comply with strict grid codes. Based on the LVRT grid code, when the voltage dipped over 0.5 pu, the total current capacity of inverter system of the WTGs should be used to output the reactive power. This can strongly limit the control flexibility of a real power restoration scheme in the post-fault period. In this paper, we have demonstrated a distributed LVRT compensator (LVRTC) control scheme that can, to some degree, solve the above problem, and increase the system control flexibility. In addition, with the proposed distributed LVRTC control scheme, the WTG in a wind farm has greater freedom in performing its protection schemes, or regulating its power flow to ensure system stability.
