Robert Eric Betz

The Double Fed Induction Generator is commonly used in wind turbine systems. However, because one of its windings is directly connected to the grid, grid faults can result in significant over-currents and/or voltages in the rotor side converter (RSC). In addition large stator currents can increase the grid fault current. This paper explores several published control strategies designed to protect the RSC during faults. The basic ideas and details of how they are implemented are developed. Simulations will be presented to compare these strategies and evaluate their performance.

Grid-connected converters, when subject to grid fault conditions, should stay connected and support the network voltage via reactive power injection. The fulfilment of this requirement strongly depends on the dynamics and stability of the converter control system. It can be shown that local static instability, and the consequent local dynamic instability can be avoided if the grid impedance during the fault is known. This paper proposes a novel method to identify the grid impedance during the fault by using the grid fault disturbance as an identification pulse. The proposed identification method is implemented in a proof-of-concept detailed simulation model, and verified with experimental results.

Techniques for parameter determination of induction and doubly fed induction machines are well established. However, these techniques don't apply directly to the Brushless Doubly Fed Reluctance Machine (BDFRM). Even though the mathematical model for this machine is like that of a double-fed induction machine, the complexity of the machines magnetic fields due to differing winding pole numbers interacting with a reluctance rotor preclude standard testing. Very high leakage inductances affect the use of the normal locked rotor test, and non-sinusoidal air gap fluxes complicate modelingmodelling for Stand Still Frequency Domain and Time Domain testing. This paper examines the validity of a combination of testing and analysis techniques, including open circuit and pulse testing, which were eventually used to determine the BDFRM inductance parameters over a range of operating points. The accuracy of the results are validated by machine testing and comparison with the FEA machine design.

Power systems around the world continue to change with the increasing adoption of distributed generation in the form of renewable energy technologies. The increasing penetrations of intermittent renewable energy technologies necessitates improvements in power sharing control, with microgrids being a popular arrangement for large numbers of distributed energy sources. In recent years a new method for autonomously sharing power has been researched known as virtual oscillator control (VOC). Autonomous/Wireless power-sharing methods are however susceptible to steady state power sharing error due to heterogeneous network impedances. This paper presents a novel low-bandwidth secondary controller that adjusts the set-points of the virtual oscillator controller to mitigate power-sharing error caused by heterogeneous network impedances. The virtual oscillator controller is shown to exceed the performance of the droop controller during transient events, however a small inherent power coupling is demonstrated in the virtual oscillator system with heterogeneous network resistances.

Current clamp is the term used to describe the phenomena of inverter controlled current staying at zero when the current crosses from positive to negative. Current clamp is a consequence of the inverter dead-time. If the current is feeding a machine, then dead-time causes torque pulsations, possible load resonances and extra losses in the machine. This paper considers some alternative feedback strategies that attempt to minimise current clamp when traditional PI current controllers are used.

The Modular Multilevel Converter (MMC) is becoming a favoured topology for high voltage applications due to its performance, flexibility in control and its modular and scalable design which allow it to be used over a wide variety of system voltage levels and power ratings. The converter consists of a number of cascaded half bridge cells with isolated capacitors to make up an upper and lower arm in each phase. A low switching frequency Pulse Width Modulation(PWM) scheme utilising a heuristic model predictive controller is introduced and then a modified method presented. The modified method uses forward prediction of the converter arm current zero crossings to further reduce the device switching frequency. Computer simulation results are then presented to confirm the control scheme operation.

Current clamp is a term that describes the current staying at zero for extended periods of time upon current direction reversal in a inverter driven load. It is caused by the dead-time in the inverter that is required to prevent current shoot-through. Traditional PI controllers have difficultly eliminating this effect as very high gains are required which result in instability. This paper analyses an approach using augmented feedback in the dead-time region to minimise current clamp with PI controllers.

In this paper a prototype brushless doubly-fed reluctance machine is investigated which is aimed a meeting commercial machine performance figures. The machine has 4 and 8 pole 3-phase stator windings and a 6 pole radially-laminated ducted rotor. The machine is investigated using finite element analysis (FEA), static torque tests with DC current excitation, a MATLAB/Simulink model, and experimentally using direct torque control (DTC). The investigations show that the machine can perform and match performance found in commercial industrial drives and is controllable. This is a step forward from the work that has been published because the machine is shown to be of a design that is commercially viable, and controllable using known control techniques adapted for the machine. Most previous reports use proof-of-concept machines or machines that are not suitable for commercial operation. This machine is designed to be more commercial in its design.