# Overload Mitigation of Inertial Grid-Forming Inverters Under Frequency Excursions
## 5 Experimental Validation  
The experimental tests aim to replicate the simulation scenarios under controlled laboratory conditions, verifying that the proposed mitigation strategies behave as expected in hardware. The overload mitigation schemes are validated using the experimental test bench illustrated in Figure 15. The setup consists of two power converters connected in back-to-back topology. The first converter operates as a single-phase rectifier, generating a regulated 320 V DC bus from the 110 V AC utility grid. The second stage is a three-phase inverter, used to test the GFM control algorithms. Both converters are built using INF-50 hardware from Dutt Electronics. The GFM inverter is interfaced with a 200 V three-phase AC grid, which is emulated using a bidirectional 320-AMX bidirectional power supply from Pacific Power. An LC filer is used to connect the inverter to the PCC, with damping resistors placed in series with the filter capacitors to damp resonance effects. Additional inductors are included to emulate grid impedance. All passive components and sensing devices are off-the-shelf components. The DC bus voltage control of the rectifier and the GFM strategy of the inverter are executed on a OP4512 real-time simulator from OPAL-RT. This platform also regulates the Pacific AMX-320 voltage via analog outputs and features a built-in 10 kHz data acquisition task for capturing sensor measurements and control signals. The experimental system replicates the parameters used in the simulation (Tables 1 and 2) to ensure consistency between simulations and experiments. Figure 14: Power response of GFM inverter under a frequency oscillation using different overloading protection schemes. (**a**) Grid frequency profile. (**b**) Active power response. The frequency excursion test described in Section 4.2 is repeated in the experimental setup for the VP strategy and the proposed overload mitigation strategies, confirming the theoretical findings previously presented. The experimental results are displayed in Figures 16-19. These figures include measured active power waveforms and key internal control signals that contribute to validate the overload mitigation strategies. These control signals are \(P_{v}\) in VP strategy, \(\omega_{v}\) in parallel PI strategy, \(\delta^{\prime}\) in AL strategy, and \(P_{EFS}\) in EFS strategy. For comparison, experimental and simulation data are plotted together. Figure 16 confirms that the VP strategy effectively removes the frequency regulation without altering the inertial power, resulting in active power exceeding the imposed limits by up to 0.25 pu. During the first frequency excursion (t = 1 s), the \(P_{v}\) signal adjusts the power setpoint of the PSL to account for the portion of frequency regulation power that cannot be delivered. As the grid frequency deviates a maximum value of 0.01 pu from allowable range, \(P_{v}\) reaches 0.2 pu in accordance with (3). In Figure 17, the parallel PI strategy limits the overload around the theoretical 0.05 pu already estimated in the previous section. The \(\omega_{v}\) signal confirms the activation of the parallel PI during the first frequency excursion, adding a negative frequency component to the output of the PSL. The evolution of \(\omega_{v}\) during the second frequency excursion (t = 3 s) also illustrates the slow recovery process of this strategy. The active power dynamics of the GFM inverter are determined by the parallel PI until \(\omega_{v}\), which is the output of the PI, settles to zero. This signal requires around 250 ms to reach this value and recover normal operation. The experimental validation of AL strategy reveals practical limitations inherent to its design. Since this method relies on an open-loop active power control mechanism based on phase shift limitation, precise tuning is critical to mitigate the effects of unmodelled delays and parameter uncertainties, ensuring effective power limitation. In this context, the phase delay and the filter inductance value need to be accurately identified.