Jim Bumby

A Multilevel Modular Converter for a Large, Light Weight Wind Turbine Generator

In an onshore horizontal axis wind turbine, generator and converter are usually in the nacelle on the top of the tower, while the grid step-up transformer is placed at the bottom. Electric power is transmitted down through flexible cables of high current rating which are expensive and can suffer from large I2 R loss. An offshore wind turbine usually has to include the step-up transformer in the nacelle. This adds significantly to the mechanical loading of the tower even new designs aim to reduce the transformer size and weight. In either case, a transformer-less, high voltage, high reliability generating unit for nacelle installation would be an attractive technology for large wind turbines. This study presents a power electronic solution based on a permanent magnet generator design. A multilevel cascaded voltage source converter is developed to synthesize a high sinusoidal output voltage. The dc link voltages of inverter modules are balanced by rectifiers fed from isolated generator coils while the inverter switching strategy equalizes the power sharing between the modules. The switching strategy also reduces the low order harmonics to constrain the sizing of the dc link capacitors. The modulating effect between the ac and dc sides of the inverter is taken into account. This paper describes the generator-converter arrangement, analyzes the inverter switching effects and derives the switching strategy which is verified by simulation and laboratory experiment.

Coordinated control of an HVDC link and doubly fed induction generators in a large offshore wind farm

Doubly fed induction generators (DFIGs) are an economic variable-speed solution for large wind turbines while high-voltage dc (HVdc) transmission is being considered for the grid connection of some offshore wind farms. This paper analyzes the need for coordinating the control of the DFIGs and the HVdc link so that the two topologies can work together, giving system designers and operators a choice that may be useful in some applications. It is desired that individual generators be controlled for power tracking in a way similar to that used when they are connected directly to an ac grid, although a grid voltage reference for the DFIG control is no longer available as an independent source in this case. The study shows that machine control should explicitly maintain the flux level, which then allows the HVdc link to regulate the local system frequency and, indirectly, voltage amplitude. Interactions between DFIGs and the HVdc link are investigated and simulations performed to verify the proposed control strategy.

Unbalanced Grid Fault Ride-Through Control for a Wind Turbine Inverter

Large variable speed wind turbines with fully rated, direct-in-line converters are increasingly used as silicon cost continues to decline. While grid codes require wind turbines to ride-through grid faults, including unbalanced faults, voltage source inverters interfaced to the grid are inherently sensitive to any unbalance in the a.c. voltage. This paper presents an unbalanced grid fault ride-through control method for such a wind turbine configuration. The method can be easily integrated into the power tracking control needed during normal operation. It is shown that the major constraint is the 2nd order harmonic to be absorbed by the d.c. link capacitor and that the control effectiveness depends on the voltage and current capabilities of the semiconductor devices in the inverter. Simulation and experimental results are used to validate the proposed control method and to identify the power limit that is necessary during ride-through.

Use of turbine inertia for power smoothing of wind turbines with a DFIG

Fluctuating power is of serious concern in grid connected wind systems and energy storage systems are being developed to help alleviate this. This paper describes how additional energy storage can be provided within the existing wind turbine system by allowing the turbine speed to vary over a wider range. It also addresses the stability issue due to the modified control requirements. A control algorithm is proposed for a typical doubly fed induction generator (DFIG) arrangement and a simulation model is used to assess the ability of the method to smooth the output power. The disadvantage of the method is that there is a reduction in energy capture relative to a maximum power tracking algorithm. This aspect is evaluated using a typical turbine characteristic and wind profile and is shown to decrease by less than 1%. In contrast power fluctuations at intermediate frequency are reduced by typically 90%.

New Constant Electrical Power Soft-Stalling Control for Small-Scale VAWTs

This paper presents a constant power soft-stall control scheme for a vertical-axis wind turbine (VAWT) fitted with a permanent magnet generator. Small-scale VAWTs are attractive due to their ability to capture wind from different directions without using yaw; this simplifies the design and gives the turbine tolerance to turbulence in urban areas. One difficulty with VAWTs is to prevent overspeeding and component overloading at excessive wind velocities. This paper suggests that necessary control can be achieved by controlling the electric power in order to operate the turbine in the stall region. This control strategy attenuates the stress in the system, while smoothing the power generated. It is shown that the controller can stabilize the nonlinear system using an adaptive current feedback loop. The controller does not need wind velocity or turbine speed, but is based on the current and voltage measured on the dc side of the rectifier connected to the permanent magnet synchronous generator. Maximum power point tracking (MPPT) control is provided in normal operation. Simulation and experimental results are presented.

2-D lumped-parameter thermal modelling of axial flux permanent magnet generators

Results from lumped parameter thermal modeling of an axial flux permanent magnet generator based on the application of the 2-D equivalent thermal circuit are presented. The components of the generator and the internal air-flow domain are split into a system of connected and interacting control volumes. Energy and mass conservation equations are then solved for each of volume to determine its thermal state. This method takes into account heat transfer due to both conduction and convection. Two case studies have been performed to validate the accuracy of the 2-D equivalent thermal circuit model by comparison with CFD results.

Computations on heat transfer in axial flux permanent magnet machines

Effective cooling is of paramount importance for axial flux permanent magnet (AFPM) machines due to their high power density. This paper presents a computational investigation on the effect of variation in some geometric parameters, (running clearance, rotor groove depth and rotational speed), on the cooling effectiveness of an AFPM machine. The numerical model used has been validated by comparison with a small test rig and the basic flow pattern inside the generator has been described.

Perturbation parameters design for hill climbing MPPT techniques

This paper presents a design strategy for photovoltaic (PV) power tracking control, based on the hill-climbing method. The study establishes guidelines to determine the variables of the hill-climbing algorithms, the time interval between and the magnitudes of the disturbances. This is effective to improve the efficiency of MPPT in rapid radiation changes. The controller design performance is demonstrated by simulation.

CFD analysis of the thermal state of an overhead line conductor

At present commercial CFD packages such as Fluent, ANSYS CFX, and Star-CD are widely used for investigation of heat and mass transfer processes in various fields of engineering. These codes can also be successfully applied to estimate the thermal state of major components of electrical distribution networks, such as overhead lines, underground cables and transformers. This paper presents some results obtained from numerical modelling of the temperature field in the Lynx overhead conductor in both cross and parallel wind conditions using 2-D and 3-D CFD models. The CFD results obtained demonstrate that for an applied load of 433 A and considering the summer rating (Lynx conductors ER P27 [1]) the maximum temperature in the conductor is considerably lower (16 degrees) than the prescribed design conductor temperature. This indicates that there is headroom for increasing the ampacity of the conductor.