DC-link Voltage Control of a Full Power Converter for Wind Generator Operating in Weak-Grid Systems

When the wind power accounts for a large portion of the grid power, it may need to help the grid voltage and frequency regulation. This paper investigates a permanent-magnet wind generator with a full power voltage-source converter in weak-grid mode, where the dc-link voltage needs to be controlled from the generator side instead of the grid side. The energy relationship of the wind generator, dc-link energy storage, and load is established. An intrinsic right-half-plane zero, together with the wind power characteristics, the mechanical system inertia, and the dc-link energy storage, is identified as the physical limitations for the control. With the understanding of the system energy relationship and limitations, a hybrid adaptive control algorithm is proposed that searches for the optimal generator acceleration to achieve the maximum wind generator power change rate to match the load power variation. The proposed control scheme is verified through simulation of a 1.5-MW wind system as well as through the experiment of a scaled 1-kW, DSP-/field-programmable-gate-array-controlled, permanent-magnet-generator-based test bed. The results show that it is feasible to regulate dc link by the generator-side converter through the generator speed control. Some important applications issues are also investigated, including the dc-link energy storage requirement, wind speed change impact, and control transition between the weak-grid and strong-grid modes.

I. Introduction

Today, while most wind power is transferred to the grid via variable-speed doubly fed induction generators in large wind turbines, the permanent-magnet generator (PMG) interfaced to the grid through a full power converter is being increasingly adopted due to its higher power density and better controllability, especially so during grid faults [1]–[5]. Full power conversion schemes can use various popular ac–ac converter topologies [6]–[8]. Fig. 1 shows a typical full power wind converter based on the back-to-back two-level voltage-source converter (VSC) topology. In normal grid-connected operation, when wind power is relatively a small portion of a strong grid, the grid-side converter is used to regulate the dc-link voltage while the generator-side converter regulates the PMG speed to achieve the desired power transfer under a given wind condition [9]–[16]. This is normally achieved following the maximum power point tracking (MPPT) scheme [17]–[20]. Fig. 1.

PMG-based wind system with full power VSC.

Fig. 2.

Basic dc-link voltage control scheme.