measurements can also be used for real-time stability monitoring and for the adaptive control of IBRs during changing grid and operation conditions. Measurement Using Multimegawatt Grid Simulators Multimegawatt grid simulator facilities are used in several parts of the world to conduct grid integration tests-such as low- and zero-voltage ride-through tests, frequency and phase jump tests, steady-state volt-var characterization, and operation with different grid strengths-on wind turbines and inverters. Grid simulator facilities can also be configured to conduct impedance measurements of IBRs. Grid simulators can be programmed to inject small-signal voltage perturbations at different frequencies superimposed on the fundamental voltages for impedance measurements. Voltage perturbations can be designed in the dq, sequence, and phasor domains to obtain impedance responses in different domains. Figure 11 displays the 7-MW grid simulator, the controllable grid interface (CGI), at the Flatirons Campus of ac-Side Analysis ac-Side Currents 50 3 40 Magnitude (dB) 2 Zp(s) 1 Zacn(s) 30 0 -1 20 -2 10 -3 9.5 97 Hz 0 20 60 100 (Hz) 400 dc-Side Impedance 50 37 Hz Magnitude (dB) 40 200 Zacn(s) 0 193° -200 -400 20 9.7 400 Zp(s) Phase (°) 9.6 t (s) 60 100 (Hz) 30 10 0 -10 0.1 400 Zdc (s) 20 1 10 100 1 104 (Hz) Magnitude (Ω) Figure 9. The reflection of resonance at 97 Hz on the ac side of an HVdc converter station in its dc-side input impedance at 37 Hz. 104 103 Zinv(s) X: 554 Y: 409.6 Zgrid(s) 102 101 101 102 Phase (°) 200 103 X: 554 Y: 122.4 100 0 -100 -200 101 Resonance Frequency: 554 Hz X: 554 Y: -62.15 102 103 Grid-Side Transformer Phase Margin: -45° Figure 10. An impedance analysis of interaction between a 430-kW PV plant and the grid. O t t Output Transformer ARU and Four NPC-VSC in Parallel Figure 11. A 7-MW grid simulator. ARU: active rectifier unit; NPC-VSC: neutral-point clamped voltage source converter. (Source: NREL.) IEEE Electrific ation Magazine / MARCH 2 0 2 1 61