Transient Control of Synchronous Machine Active and Reactive Power in Micro-Grid Power Systems

Abstract

There are two main topics associated with this dissertation. The first is to investigate phase–to–neutral fault current magnitude occurring in generators with multiple zero–sequence current sources. The second is to design, model, and tune a linear control system for oper- ating a micro–grid in the event of a separation from the electric power system. In the former case, detailed generator, AC8B excitation system, and four–wire electric power system models are constructed. Where available, manufacturers data is used to validate the generator and exciter models. A gain–delay with frequency droop control is used to model an internal combustion engine and governor. The four wire system is connected through a transformer impedance to an infinite bus. Phase–to–neutral faults are imposed on the system, and fault magnitudes analyzed against three–phase faults to gauge their severity. In the latter case, a balanced three–phase system is assumed. The model structure from the former case – but using data for a different generator – is incorporated with a model for an energy storage device and a net load model to form a micro–grid. The primary control model for the energy storage device has a high level of detail, as does the energy storage device plant model in describing the LC filter and transformer. A gain–delay battery and inverter model is used at the front end. The net load model is intended to be the difference between renewable energy sources and load within a micro–grid system that has separated from the grid. Given the variability of iiboth renewable generation and load, frequency and voltage stability are not guaranteed. This work is an attempt to model components of a proposed micro–grid system at the University of Wisconsin Milwaukee, and design, model, and tune a linear control system for operation in the event of a separation from the electric power system. The control module is responsible for management of frequency and active power, and voltage and reactive power. The scope of this work is to ❼ develop a mathematical model for a salient pole, 2 damper winding synchronous generator with d axis saturation suitable for transient analysis, ❼ develop a mathematical model for a voltage regulator and excitation system using the IEEE AC8B voltage regulator and excitation system template, ❼ develop mathematical models for an energy storage primary control system, LC filter and transformer suitable for transient analysis, ❼ combine the generator and energy storage models in a micro–grid context, ❼ develop mathematical models for electric system components in the stationary abc frame and rotating dq reference frame, ❼ develop a secondary control network for dispatch of micro–grid assets, ❼ establish micro–grid limits of stable operation for step changes in load and power commands based on simulations of model data assuming net load on the micro–grid, and ❼ use generator and electric system models to assess the generator current magnitude during phase–to–ground faults.