# A PMSG Wind Energy System Featuring Low-Voltage Ride-through via Mode-Shift Control
###### Abstract
An LVRT requirement implies that wind power plants must remain connected even in the presence of transient grid voltage dips. This maintains the network voltage and frequency stability by injecting active and reactive power into the grid under a predetermined profile related to the grid dip severity. Thus, LVRT is quite a challenging requirement, affecting the design and control of WECS power converters, for grid stability support and system reliability enhancement. Several studies have been conducted to investigate the performance of various LVRT techniques on various WECS during grid faults. Commonly, these techniques can be divided into hardware solutions or software ones (modified control-based methods), which will be presented in the following section [11, 12, 13, 14]. In this paper, a PMSG-based MW-level WECS is considered, applying passive-end MSC to achieve an economic fault-tolerant system able to compromise between generator robustness against grid faults and converter cost-effectiveness. For LVRT, a swap feature modification is proposed for the classical control scheme, enabling it to successfully work during normal conditions or utility voltage sags. Normally at no fault, the MSC achieves MPPT while the GSC regulates the DC-link voltage by controlling the grid current to transfer the extracted active power to grid [15]. In this case, no reactive power (VAR) control is required. However, during grid faults, if the classical non-swap control is still applied, the GSC action will be affected as it is directly coupled to the grid. The GSC ability to control the DC voltage becomes limited, since it can no longer deliver the wind power extracted by the MSC to the grid. Consequently, the energy surplus must be dissipated into the system, causing DC-link over voltages and grid current increase [15]. Meanwhile, the GSC should satisfy grid requirements and inject VAR into the grid, as per the standards [16]. Thus, the GSC is required to achieve both functions of VAR control and DC-link voltage regulation simultaneously. Since both control loops have different dynamics and different time constants, this will affect the VAR control response since DC-link voltage dynamics are slower due to the large DC-link capacitor size. On the contrary, applying the proposed swap control action, system operation will be shifted from MPPT mode to LVRT mode. In this mode, the responsibility of DC-link voltage control is overtaken by the MSC rather than the GCS, which independently controls grid current to inject the suitable active and reactive power into the grid based on the sag depth. This increases the system capability of decoupling the grid current controller from the DC-link voltage controller during LVRT conditions since both controllers feature different time constants and dynamics. Moreover, this prevents any noise or power oscillation propagation from one controller to another. Any surplus energy is stored in the rotor inertia with no additional circuit required [13]. Finally, rises in inverter DC-link voltage and grid currents are eliminated, thus maintaining power converter safety and meeting system interconnection requirements. Compared to other alternative topologies in the literature [15; 17; 18; 19; 20; 21; 22; 23], the proposed approach features these merits, albeit with a converter topology containing the minimum number of active switches. This in turn reduces converter switching losses and cost, simplifies its implementation and minimizes its control complexity. A simulation set-up is implemented along with an experimental correlated prototype to verify the effectiveness of the proposed swap technique. This paper is divided into nine sections. Section 2 describes the LVRT capability and recent modified control-based mitigation studies. Section 3 describes the considered PMSG WECS configuration and model. In Section 4, the authors explain the proposed model-shift LVRT control for grid faults.