High-power Wind Energy Conversion Research Articles

Design of a Medium Voltage Generator with DC-Cascade for High Power Wind Energy Conversion Systems

This paper shows a new concept to generate medium voltage (MV) in wind power application to avoid an additional transformer. Therefore, the generator must be redesigned with additional constraints and a new topology for the power rectifier system by using multiple low voltage (LV) power rectifiers connected in series and parallel to increase the DC output voltage. The combination of parallel and series connection of rectifiers is further introduced as DC-cascade. With the resulting DC-cascade, medium output voltage is achieved with low voltage rectifiers and without a bulky transformer. This approach to form a DC-cascade reduces the effort required to achieve medium DC voltage with a simple rectifier system. In this context, a suitable DC-cascade control was presented and verified with a laboratory test setup. A gearless synchronous generator, which is highly segmented so that each segment can be connected to its own power rectifier, is investigated. Due to the mixed AC and DC voltage given by the DC-cascade structure, it becomes more demanding to the design of the generator insulation, which influences the copper fill factor and the design of the cooling system. A design strategy for the overall generator design is carried out considering the new boundary conditions.

Modeling of an efficient high power wind energy conversion system using self-excited multi-phase machines

Self-Excited Multi-phase Induction Generators (SEMPIG) producing power from wind turbines infar-flung fields are receiving the interest of recent researchers. Small and medium energy wind generators involve 3-phase induction generators. SEMPIG with significant reliability, better fault tolerance, reduced harmonics, and high power-handling capability using reduced current per phase and maintaining the same per phase voltage makes it the promising contender in high-power WECS. This investigation analyzes the proficiency of energy efficient stand-alone self-excited seven-phase induction generator (SE7PIG) in wind energy schemes. This paper develops a cogent model from the two-phase equivalent circuit of SE7PIG using Matlab/Simulink. The study examines the modeling aspects of the SE7PIG with incorporated significant 7-rectifier and 7-phase inverter. The machine operational parameters speed, torque, voltage and current are presented. The 2MW WECS with 7-phase induction generator is analyzed with and without DC link converter. The results are compared with the 3-phase counterpart to illustrate the efficacy of SE7PIG for megawatt wind-power generation. The paper confirms the competence of SE7PIG with better fault tolerance and significant energy output with a decrease in phase current.

Current source back‐to‐back converter for wind energy conversion systems

This paper proposes a new back-to-back current source converter (BTB-CSC) suitable for mediumvoltage high power wind energy conversion systems (WECS). It employs a dual three-phase permanent magnet synchronous generator and two current source inverters with a phase-shift transformer at the grid side. The proposed BTB-CSC has the following advantages: reduced power circuit and control complexity; low switching losses (zero switching losses at the inverter side); and independent control of active and reactive power. PSCAD/EMTDC simulations are used as to assess the steady-state and dynamic behaviour of the proposed system under different operating conditions. It is shown that the proposed WECS can ride-through ac faults. Experimental results from scaled prototype of the proposed WECS are used to validate the simulations.

Application of modular multilevel converter in medium voltage high power permanent magnet synchronous generator wind energy conversion systems

Traditional low voltage high power wind energy conversion system (WECS) requires heavy cables and large step-up grid interfacing transformers. In recent years, power rating of offshore wind turbines already exceeds 10 MW, and medium voltage (MV) converters have become strongly demanded. Modular multilevel converter (MMC) is suitable for MV high power applications. However, there are two technical difficulties for MMC when applied in MV permanent magnet synchronous generator (PMSG) WECS. The first one is the large sub module (SM) voltage fluctuation caused by low frequency and high amplitude PMSG phase current. The other one is the low voltage ride through (LVRT) problem. Overall control strategy based on carrier-phase-shifted -sinusoidal pulse width modulation technique of the MMC WECS is proposed. For the first difficulty mentioned, a second-order circulating current injection method based on closed-loop control is proposed. For the LVRT problem, a distributed braking chopper solution is provided. The effectiveness of the injection method and the distributed braking chopper solution are validated by simulation of a 5 MW/3.3 kV MMC WECS. Influences of the current injection amplitude on SM voltages and arm currents are discussed and a trade-off method is given to optimise system design, which is also verified by simulation.

MPPT Control for Current Source Converter Based PMSG Wind Energy Conversion System

Current source converter (CSC) configuration tailored to high-power grid-connected wind energy conversion system (WECS) has been an attractive solution for direct drive of permanent magnet synchronous generators (PMSGs). A maximum power point tracking (MPPT) scheme for a PMSG-based WECS is presented in this paper. On the generator side, a fully controlled current source converter is inserted as a circuit interface to handle a wide range of the variable wind speeds. Rectifier side controller extracts maximum power through closed-loop regulation of generator speed. The available maximum power fed to the grid is adjusted by regulating dc-link output current of CSC according to the variable wind speed. Simulation results obtained from a megawatt PSIM model are provided to verify the proposed concepts.