Voltage source converters (VSCs) are widely used in renewable energy conversion systems, such as wind generation and photovoltaic systems. Rich wind and solar resources are often located in remote areas and far from the load centres in many parts of the world. For integrating bulky renewable power into grids, hundreds of wind turbines (WTs) or solar generation units are grouped and tied to AC systems via long transmission lines, which results in a relatively low SCR (Short Circuit Ratio).

The direct drive permanent magnet synchronous generator (PMSG) Type-IV WT is increasingly used due to its high efficiency and low maintenance. To output active power captured from the turbine and exchange reactive power with the AC system, the gridconnected converter of the WT must be synchronized with the grid. The synchronization can be achieved through two different types of schemes: Grid-Following (GFL) and Grid-Forming (GFM). However, the present GFL control schemes are inappropriate for weak grid conditions. The analysis shows that the GFL control scheme is a conditionally stable system and cannot drive the converter output current toward the synchronous state after the asynchronous state transition. Although the GFM converter technology achieves better stability in a weak grid than the GFL control, there are still more realistic problems and challenges when applied to Type-IV wind turbines.

Unlike previous work, this thesis introduces a novel synchronized approach of the gridside converter with an AC system based on the newly defined concept of instantaneous magnitude and phase. This method only uses local measurement variables (DC terminal voltage, AC terminal three-phase voltage and current) and local reference commands (DC voltage, AC voltage magnitude and active-reactive power). The synchronization can be achieved by controlling the converter terminal active-reactive power and instantaneous voltage magnitude to their reference value.

The system model is developed in the dq-frame with a fixed angular speed and is valid for synchronous and asynchronous states. Two nonlinear power control algorithms based on the instantaneous magnitude and phase of three-phase signals are presented to regulate the power outputs to track their references. One method achieves power regulation through an inner current control loop and another directly by a power control loop, namely direct current control (DCC) and direct power control (DPC) based on instantaneous magnitude and phase, respectively. The stability of these two control schemes in the whole system are proven via the Lyapunov theory.

In the proposed DCC control scheme, the control of the GSC’s instantaneous active and reactive power is realized indirectly through the closed loop of the current. The output current of the GSC can be controlled quickly and accurately in a wide range of grid conditions. This control scheme has a simple control structure as it does not require complex mathematical models and calculations. Under unbalanced grid conditions, the novel current control scheme is proposed and includes two current controllers, a central controller, and an additional compensation controller. The central controller does not involve positive and negative sequence separation. The compensation controller is specially designed for controlling the negative-sequence current and is only activated when the voltage unbalances are detected.

To obtain a fast and direct power transition trajectory control, DPC based on the instantaneous magnitude and phase control algorithm is proposed. Unlike the proposed DCC control method, DPC does not need to convert power variables into current variables but directly adopts instantaneous active and reactive power as dynamic control variables. Compared to the DCC scheme, the proposed DPC control method has the advantages of good robustness and fast power response. A limitation of the conventional DPC control approach, based on real and reactive variables, is that it does not provide inherent overcurrent limiting capability. A novel current limiting strategy for the proposed DPC control to overcome this drawback is presented.

Simulation results using PSCAD/EMTDC demonstrate that the grid-side converter using the proposed control schemes are superior to the traditional vector current control and the state-of-the-art DPC control in the literature under various grid condition, i.e., a wide range of SCR (SCR≥1), temporary grid faults, large grid impedance varying and grid frequency drift. Additionally, the proposed control schemes can easily be retrofitted into the existing GFL VSCs without needing any extra hardware.