# A PMSG Wind Energy System Featuring Low-Voltage Ride-through via Mode-Shift Control
## 7 Discussion  
At present, DFIG and PMSG are two mainstreams of wind turbine generators that dominate the energy market. However, the PMSG option facilitates the elimination of the gearbox, which is easily broken. Besides featuring high energy conversion efficiency compared to other types at low maintenance costs, PMSG WECS offer high-power density and complete active and reactive power control. However, these features are met at the cost of larger and heavier generators. For grid integration, PMSG are employed with full-scale power converters that feature isolation from grid disturbances, thus adding robustness to their fault ride through capabilities. Hence, the PMSG-based WECS is considered in this paper. In [49], a review on commercialized drivetrain system technologies of wind turbines is performed. It was found that direct driven PMSG is the dominant technology of Siemens Games and GE for fixed/floating offshore platforms. PMSG power converters must be sized at the full-scale rating of the generator capacity, thus adding cost and size to the overall system. To manage the trade-off between the PMSG fault tolerance capabilities and its high converter cost, the PMSG WECS considered in this paper applies an economic converter topology rather than the traditional back-to-back one. The considered converter topology is a passive front-end converter (diode rectifier) assisted with a DC/DC boost converter for the MSC stage and a three-phase VSI as the GSC stage. The considered MSC topology features seven passive switches with only one active switch and one gate-driver, compared to the classical active front-end topology featuring six active switches and six gate-drive circuits. Thus, the considered MSC converter size and cost are reduced as well as its implementation and control complexity. Besides, less switching losses are exhibited, especially at low and average turbine speeds, making it a predominant solution for low and medium power ranges. However, this is achieved at the cost of higher generator torque ripples and generator current harmonics, as this converter experiences limited generator-side control. To achieve a fault-tolerant PMSG-based WECS featuring efficient LVRT, a swap control feature is proposed to modify the classical converters' control scheme during voltage dips. During LVRT mode, the proposed modification shifts the role of DC-link voltage regulation to be overtaken by the MSC independently of the grid current control and VAR support achieved by the GSC. This feature has the following benefits. First, the MPPT is seized, and only adequate wind power is extracted by the MSC sufficient to maintain DC-link nominal voltage level and prevent any rises. Second, the VSI prime focus is VAR injection instead of performing two control functions. VAR injection is achieved according to dip depth  
Figure 15: FFT analysis of experimental grid current during 70% LVRT for (**a**) non-swap control and (**b**) swap control. at fast dynamics, which is totally independent from the slow dynamic \(V_{dc}\) control. Third, computing the grid current's \(d\)-component from the \(I_{d}-I_{q}\) relation, rather than from the affected DC-link voltage loop, improves grid current THD as well as fastens grid current control dynamics. Fourth, the proposed swap enhancement is quite flexible since it can be implemented on the existing systems without any hardware modifications. It is applicable to any existing WECS involving full-rated converters such as synchronous generator-based WECS, which are the considered systems in this research. Finally, any rises in DC-link voltage or grid current during dips, due to energy mismatch, are mitigated by storing any surplus energy in rotor inertia without needing an extra hardware device, which adds to system cost-effectiveness. At voltage dips, using the proposed swap control, VAR injection for grid support reduces the amount of transferable active grid power, following Equations (36)-(38).