Current control of three-phase PWM Inverter for flywheel energy storage system
The thesis is concerned with the use of flywheel energy storage system (FESS) in utility load levelling application. The work presented consists of two parts, first, an evaluation of utility load levelling schemes with FESS as the energy storage medium, and second, the development of power electronic interface of FESS to the utility. The thesis presents a study to evaluate FESS load levelling schemes in a UK electricity supply and distribution company. It identifies and quantifies the costs and benefits of the schemes, and carries out a financial appraisal based on Net Present Value (NPV) and Internal Rate of Return (IRR) methods. The results indicate that the DSM schemes utilising FESS can be financially viable for a UK electricity supply and distribution business in a mass-produced (low-cost) FESS scenario, and provide FESS manufacturers and developers with cost goals for such applications. The conclusions drawn provide the motivation for further technical research undertaken within the programme of work. The main work presented is in the area of power electronic interface between FESS, as well as other energy storage devices or energy sources, and the utility for embedded generation. The thesis particularly focuses on the design of current controllers for an interface in the form of three-phase voltage-source pulsewidth modulated (VS-PWM) inverter connected to the utility via LCL filter. Two different current controller structures based on suboscillation current control method have been analysed and designed, aided by computer simulation studies carried out using a general purpose dynamic system simulation software, Matlab Simulink. General properties of a three-phase VS-PWM inverter have been investigated to establish a basic understanding of its operation. The phenomenon of phase interaction in a system with no neutral connection has been examined and the effect of practical inverter nonlinearities caused by interlock time delay (dead time) and on-state voltage drops of the semiconductor devices has been discussed. Various PWM current control techniques have been investigated, including the three-independent hysteresis current control, advanced hysteresis current control, suboscillation current control and space vector current control. The suboscillation current control method has been chosen as produces a well defined harmonic spectrum in the output current without the need for complicated computations and extensive hardware, and can be easily implemented in analogue to avoid problems •sampling and computation time delay generally associated with digital controllers. Fundamental appreciation of the suboscillation PWM technique has been established from analytical synthesis of the modulation process, providing a rational basis for the current controller and computer simulation model validation. An inherent disadvantage of the suboscillation control method has been found to be its limited controller gain, causing a steady-state error to and the effect of inverter nonlinearities to be quite significant. In the first current controller structure, a simple compensation has been utilised to enable the gain to be increased beyond the conventional limit. Simulation results show that the steady-state error the current waveform has been improved and the effect of inverter nonlinearities has been It also makes the current controller less susceptible to the inherently noisy environment, current controller has been experimentally built and tested to validate the simulation results and to validate the practical aspects of its implementation. In the second current controller structure, a cyclic feedback system based on Iterative Control (ILC) has been utilised to eliminate the periodic error in the current waveform. The structure is more complicated and the cyclic feedback system requires digital implementation. Simulation results indicate that the cyclic feedback system is effective in eliminating periodic error in the current waveform. Due to time constraints and hardware limitations, ital implementation of the system has not been possible but is recommended for future research.