Jianyun Chai

A transformerless modular permanent magnet wind generator system with minimum generator coils

This paper has presented a modular converter for multi-coil direct-drive permanent magnet wind generator system. The converter modules are cascaded to achieve medium voltage output (6kV~33kV), thus eliminating the grid-side step-up transformer, which is desirable for both on-shore and offshore wind turbines. Each converter module is composed of a rectifier, dc-link and an inverter. The generator coils with 90 degree phase shift are rectified through the power factor correction (PFC) circuit and connected in series to get unity power factor, stable dc-link power and higher dc-link voltage. The generator armature inductance is used as the AC-side PFC boost inductor, thus reducing the system size and cost. The inverter adopts a neutral point clamped (NPC) five-level converter to match the dc-link voltage level and are cascaded to achieve multilevel medium voltage output. The vector control scheme is used to regulate the converter active and reactive power transferred to the grid. Simulation results with a 1.5MW wind generator and converter system and experimental results with a scaled 3kW system validate the proposed topology and control method.

Control of variable pitch, variable speed wind turbine in weak grid systems

In the case of operating in a weak grid system, when wind power becomes a significant portion of the power system or even the sole energy source, the wind power generator and its converter are expected to help maintain the grid voltage amplitude and frequency, operating like the conventional large synchronous generator (SG). The grid-side converter needs to work as a voltage source to regulate the terminal (grid) voltage amplitude and frequency, which can be achieved by controlling the reactive power and active power flow, respectively. For the direct drive permanent magnet synchronous generator with full power converter, the reactive power can be supplied from the grid-side converter and the active power should be provided by the captured wind power. The power balance between the source (wind) and the grid (load) can be achieved by actively controlling the generator speed and the pitch angle, thus adjusting the wind power input. In the paper, the coordinated control of generator speed and pitch angle are proposed, enabling the generator output power to track the load power. The proposed scheme fully utilizes the system rotational energy by the speed control and the pitch control is used for limiting the speed range and the wind power capture as well. The droop method is used for power sharing among multiple paralleled grid-side wind power converters and the converter dynamics is also considered. The simulation is carried out with different wind speed and load conditions and the results show the proper operation of the proposed control scheme.

A doubly fed induction machine based solution to medium frequency power supply

The 400Hz medium frequency power supply is a key component feeding the critical loads on ships and airplanes. In order to solve some problems met in the existed system, this paper developed a complete mathematical model and operation strategy for the doubly fed induction machine (DFIM) based motor-generator frequency conversion system, including the process of starting, grid synchronization and vector-controlled speed regulation of the system. A unique feature of the proposed system is that the two synchronous speeds of the DFIM and the generator are different, therefore, only a standard one quadrant AC-DC-AC converter with about 1/6 power rating of the DFIM is needed, which benefits from low cost and simple structure. Furthermore, the rotor inverter is also used as the starter to limit the starting current in the DFIM. A new grid synchronization scheme is also proposed to achieve the soft connection by actively adjusting the DFIM stator voltage angle and amplitude. Experimental results with a scaled 7.5kW system are presented to show the validity of the proposed method.

Virtual synchronous control of grid-connected DFIG-based wind turbines

This paper proposes a virtual synchronous control strategy for Doubly-Fed Induction Generator (DFIG)-based wind turbines. On the basis of the proposed excitation control model orientated by rotor flux, and the P-f, Q-V droop control models which emulate the governor and exciter of synchronous machine, this paper established the basic virtual synchronous control structure for DFIG. This control strategy can provide DFIGbased wind turbines with intrinsic regulation ability of frequency and voltage without using the phase-locked loop (PLL) of the grid. Considering the power limitation under certain wind speed, the proposed adaptive droop control method realized the maximum power point tracking (MPPT) of wind turbines in steady state. Simulation results demonstrate the feasibility and effectiveness of the presented virtual synchronous control for grid-connected DFIG. This control strategy can not only provide the possibility of unifying the grid-connected wind turbines as the standard synchronous power, but also decrease the negative effect on the grid frequency and voltage with high penetration of wind energy.

A converter based adjustable speed drive for doubly fed induction machine with centrifugal loads

Adjustable speed drive is quite attractive for pump, compressor and other centrifugal load applications in sense of its flexibility and high operating efficiency, compared with the conventional constant speed drive system. In this paper, a slip power recovery scheme combined with the constant volts/hertz control for doubly fed induction machine (DFIM) is proposed and investigated. The machine is started and accelerated to the setting speed region by constant volts/hertz control without the help of starting resistors or auto-transformer. Two starting alternatives can be used according to different turns ratios between the stator and rotor windings. After grid synchronization, the vector control scheme takes over and regulates the machine speed in the setting speed region based on the machine mathematical model. Some important application issues such as “soft grid connection” and rotor position estimation are also discussed. The proposed control scheme has been experimentally verified through the DSP-controlled 30kW system by starting from the rotor side. The drive system can successfully run in the full speed range with the proposed control method and control sequences.

Virtual synchronous control strategy for doubly-fed induction generator under asymmetrical grid faults

Much attention has been paid to Virtual Synchronous Control (VSynC) strategy for Doubly-Fed Induction Generator (DFIG) due to its advantage in providing dynamic supports of frequency and voltage. However, current researches of VSynC strategy focus on analysis of its electromechanical behaviors, and neglect its electromagnetic transient responses and fault characteristics, hindering its application in grid-fault situation. This paper studies the electromagnetic transient behaviors of VSynC-based DFIG and proposes a voltage compensation VSynC strategy to implement fault ride through during asymmetrical grid faults. The proposed strategy improves the inherent defect of existing VSynC strategy, capable of limiting the overcurrent in DFIG rotor circuits and suppressing the oscillations of electromagnetic torque. Simulation results validate the effectiveness of the proposed strategy.