**Abbreviations**

The following abbreviations are used in this manuscript:


#### **Appendix A. Standard Grid Codes for LVRT and ZVRT**

**Figure A1.** Standard curves in grid codes for LVRT and ZVRT. The LVRT and ZVRT curves shown using the grid codes for China and South Australia [32,36].

#### **Appendix B. Regular Test Method of Emergent-Feathering**

It is stipulated in the WT design documents that the emergent-feathering performance must be tested regularly. The period is usually three months or half a year. The common method is to interrupt the safe loop intentionally to simulate an emergency and then to observe the pitch performance.

On 25 December 2018, a 3 MW WT operating in Hebei, China, could not feather during the test until the UPS in the WT controller was cut off in the emergency. After analysis, the accident might be correlated with the design flaws in Section 4.2. The accident process can be summarized as follows. First, two signal slides, the EFC signal in Figure 6 and 24 V UPS, were connected in the slip ring (the slip ring of a WT has limited space but a complicated wiring design and is a mechanical rotating device, and thus breakover errors are typically unavoidable). Then, when the safe loop was interrupted, the WT disconnected from the grid, which meant that the WT lost electromagnetic torque. Finally, driven by the mechanical torque, the aerodynamic system of the WT rotated with an overspeed, and the operating status is shown in Figure A2.

**Figure A2.** The operating status of the WT at the fault moment. The p.u. values of the electromagnetic torque, generator speed, rotor(hub) speed, and wind speed are 4300 N\*m, 1200 rpm, 12.9 rpm, and 10 m/s, respectively.

Figure A2 also shows that the rotor speed will double in approximately 70 s for the conditions of losing the electromagnetic torque and not-pitching, even if the wind speed is very low (nearly 4 m/s).

Overall, the improved test method is suggested as follows on the condition that the wind speed is less than 6 m/s and there is no rain.

(1) Stop the WT, lock the hub with bolts, check the pressure of the hydraulic pressure station, and pitch just one blade towards fine to 0◦ manually using the WT power.

(2) Press the EFC button in the WT controller cabinet, which also enables the backup power for the pitch system.

(3) Measure the capacitor voltages or battery impedances in the backup power.

(4) Repeat the above steps two times, and draw three feather curves in one plot for three blades (the *x*-axis represents the feather time, and the *y*-axis records the pitch angle, such as Figure 18).

The qualified performances can be defined as follows.

(1) Three pitch angles have finally reached 91◦.

(2) The entire feather time is less than 15 s. It will be also acceptable that the feather times in two tests are both less than 17 s, and all pitch angles reach 91◦. When the air temperature is below −20 ◦C, the limit of 17 s can be broadened to 21 s, but the capacitor voltages or battery impedances should meet the requirements.

(3) The feather curves should be smooth, and the following performance should satisfy the requirement that the differences among the pitch angles should be less than 10◦ at any moment.
