Type-4 wind power plants is also presented in Figure 13 for different ratios between the GFM and GFL wind power plants up to an 81% level of penetration. Inertial, frequency droop, and voltage droop controls were enabled in the Type-4 GFL wind power plant models. Frequency-megawatt and voltage-Mvar droops were enabled in the Type-4 GFM wind power plants. The highest share of the GFM wind provides a superior improvement on the frequency response, as shown in Figure 13. This is because GFM wind turbines can automatically and rapidly increase their power in response to frequency deviations. The same is true for other inverter-based GFM resources, such as solar photovoltaics (PVs) and battery energy storage systems. More detailed visualizations of the response of the system at different penetration scenarios are shown in Figures 14 and 15 for GFL- and GFM-dominated systems, respectively. The outputs of the selected individual GFL and GFM wind power plants and one of the remaining Type-4 wind power plants operate with a 10% curtailment to have active power reserve for the provision of frequency-response services. synchronous generators are illustrated in per units normalized to the MVA rating of each individual plant [Figures 14(b) and 15(b)]. Figures 14(b) and 15(b) show the frequency of the system during the event measured using a PLL (orange lines) and the speed of the synchronous generator in per units (blue lines). The frequency measured by a PLL depends on its components characteristics, and under dynamic conditions differs from generator speed. The presence of 16% GFM wind (Figure 14) provides significant frequency-response improvements compared to the synchronous generator and GFL-only cases, shown previously in Figure 13 (blue and orange lines). In the case of 66% GFM wind (Figure 15), there is a small " dent " in the system frequency after the loss of 10% of synchronous generation. This demonstrated excellent frequency-stabilizing characteristics of GFM wind, although the same is true for GFM solar PV generation (if operating with sufficient headroom) and battery energy storage systems with GFM inverters. 1.0005 1.001 0.999 0.9995 1 Frequency Measured by PLL Generator Speed 0 246 8101214 Time (s) (a) 0.4 0.5 0.6 0.7 0.8 0.9 1 GFM Wind GFL Wind Synchronous Generators 0 2468 10 Time (s) (b) Figure 15. The system response to a loss of generation (16% GFL and 66% GFM wind). p.u.: per unit. (a) Frequency. (b) Power. 62 IEEE Electrification Magazine / MARCH 2022 12 14 Power (p.u.) Frequency (p.u.)