# A PMSG Wind Energy System Featuring Low-Voltage Ride-through via Mode-Shift Control
## 6 Experimental Verification  
Practical validation of the proposed approach has been applied to a low-power prototype of schematic diagram presented in Figure 11 and the rig photograph shown in Figure 12. The experimental rig parameters are listed in Table 4. Two mechanically coupled machines are employed (a DC-motor and a PMSG) to emulate the PMSG-wind turbine system. The experimental results are obtained when the PMSG is driven at a constant speed \(n\) of 900 rpm and produces a balanced 3-phase output voltage \(v_{gen}\) at a frequency \(f_{gen}\) of 30 Hz. The difference between the prototype PMSG and the simulated system frequency is the reduced number of poles available in the experimental prototype. Assuming that the MPPT algorithm for the wind turbine is at a steady state (fixed wind speed), the system is tested at a constant driven rotor speed instead of using a mechanical wind turbine. Figure 10: Simulation results during 30% symmetrical grid voltage with the proposed control scheme: (a) abc grid voltages \(v_{grid}\) and d– axis grid voltage \(V_{d}\) (p.u. ); (b) abc grid currents \(i_{grid}\) and d– axis grid current \(I_{d}\) (p.u. ); (c) DC– link voltage \(V_{dc}\) and reference \(V_{dc}^{*}\) (p.u); (d) grid active power \(P_{grid}\) and reactive power \(Q_{grid}\) (p.u). Figure 9: Simulation results during 30% symmetrical grid voltage dip without the proposed control scheme: (a) abc grid voltages \(v_{grid}\) and d– axis grid voltage \(V_{d}\) (p.u. ); (b) abc grid currents \(i_{grid}\) and d– axis grid current \(I_{d}\) (p.u. ); (c) DC– link voltage \(V_{dc}\) and reference \(V_{dc}^{*}\) (p.u); (d) grid active power \(P_{grid}\) and reactive power \(Q_{grid}\) (p.u). Correspondingly, the boost converter current \(I_{b}\) is set to 2A and the generator extracted power \(P_{gen}\) is 100 W. To ensure a safe test rig operation, a 100 V \(V_{dc}\) has been selected since DC-link voltage might increase above this threshold during voltage dip, reaching over 200 V. The system is connected to a 50 Hz, 40 V line-line grid voltage \(v_{grid}\). Grid active power \(P_{grid}\) reaches 95 W, while the grid injected current \(i_{grid}\) corresponds to 1.4A (2A peak). The control system and the switching strategy for GSC and the MSC have been implemented using a 32-bit TriCore microcontroller TC1796 from Infineon. Voltage dips were emulated using a coupling, multi-turn step-up transformer connecting the GSC to the grid through the primary side. Due to the limited power level of the experimental test rig, the moment of inertia of the experimental prototype is lower than that of the simulated system. As a result, the rotor speed might cross the overspeed limit during the sag. Therefore, the fault duration is adjusted to 60 msec to avoid PMSG speed increase while keeping same depth of the voltage dip (70%) and thus validating the proposed technique concept. Experimental results for the proposed technique have been compared to the results obtained with the conventional non-swap control action as shown in Figures 13 and 14. Table 5 summarizes experimental performance results during the 70% grid voltage dip. Figure 11: Block diagram of experimental set-up. Figure 12: Photograph of the experimental prototype: (**A**) coupling transformer; (**B**) power converters; (**C**) prime mover and PMSG; (**D**) grid side and filter; (**E**) voltage and current transducer boards; (**F**) DSP Tri-core 197. Using the proposed control technique, the DC-link voltage \(V_{dc}\) is preserved at a constant value to 100 V, while phase a-grid current \(i_{a-grid}\) at 1.4A (2A peak), during the dip, compared to \(V_{dc}=160\) V and \(i_{a-grid}=2.8\) A (4A peak) without the suggested control as shown in Figures 13b and 14b, respectively. This DC-link rise is successfully limited for converters' safety; in addition, grid currents are maintained within their nominal value during normal and LVRT operation.