Nonlinear Source Emulator

Abstract

The world is rapidly changing from using the fossil based energy, which is facing exhaustion and having a lot of environmental issues, to the use of renewable energy sources such as sun energy and wind energy. Energy from the sun has become an important source for terrestrial applications and remains the prime source of energy in non-terrestrial applications such as those in sky-explorers. However, a renewable energy source is expensive, bulky, and its performance is weather dependent, which make testing of downstream converters very difficult. As a result, a nonlinear source emulator (NSE) is a good solution to solve the problems associated with the use of real nonlinear sources in testing phases. However, a recent technical survey conducted during this work shows that most existing NSEs have only been concerned with simulating nonlinear systems in terrestrial applications. Furthermore, their dynamic performance were not fast enough in order to imitate how a real nonlinear energy source would react under extreme conditions and operation modes. Particularly, a system in the sky can experience a step change of sunlight irradiation. Moreover, operation modes may include load step between nominal and open circuit, and load step between nominal and short circuit. Under these conditions, a practical nonlinear source system will react almost instantly, whereas the fastest among existing NSEs had a transient of about 3 milliseconds. It is the highlight of this thesis, to demonstrate the development of a proposed NSE system with high dynamic performance. The goal of the work is to achieve a state-of-the art transient time of 10 µs. In order to produce the arbitrary nonlinear curve, the exponential function of a typical diode is used, but the diode can be replaced by other nonlinear curve reference generator unit. Because nonlinear energy sources come in different sizes and power rating, a single NSE may not be sufficient to simulate a wide selection of nonlinear sources. For this reason, the proposed NSE system is realized as modules. Stacking or connecting multiple modules in parallel will allow simulation of nonlinear source systems with higher output power. In this work, a module will consist of two fundamental units: an isolated power supply and an NSE. The isolated power supply has to possess a very low circuit input-to-output capacitance (very low Cio) in order to reduce the effect of conductive common-mode current produced by the high rate of change of voltage over time (high dv/dt) at the NSE output. v/xvii The contributions of the thesis are based on the development of both units: the low Cio isolated power supply and the high dynamic performance NSE. Both units are investigated theoretically and experimentally. For the very low Cio power supply, we propose a new topology and control, together with a novel transformer structure, in which, its two windings are separated by a significant distance, in order to attain a low interwinding capacitance. A mathematical model is proposed to accurately model the interwinding capacitance of the proposed transformer. The result achieved is a total converter Cio of 10 pF in a 300-W prototype, which is 30 times lower than that of existing approaches. For the NSE, we propose a new circuit consists of an ultrafast tracking converter and a novel nonlinear curve reference generator based on diode curve. Even though the nonlinear curve is based on diode p-n junction, the proposed NSE can simulate other arbitrary nonlinear sources if the diode is replaced by other appropriate nonlinear curve reference generator units. The prototype is 200-W rating. The experimental results show that the proposed NSE can react to a fast change in input source (such as an abrupt change of wind speed for wind turbine emulator), as well as to a load step from nominal to open circuit and vice versa, all within 10 µs. The proposed NSE, therefore, offers the state-of-the-art dynamic performance among devices of the same kind. It also offers a complete solution for simulation of nonlinear source systems of different sizes, both in terrestrial and non-terrestrial applications. Key words: Current transformers, dc-dc power converters, hysteresis, parasitic capacitance, system, stacking, switching converters.