Date of Award

May 2018

Degree Type

Thesis

Degree Name

Master of Science

Department

Engineering

First Advisor

Lingfeng Wang

Committee Members

Yi Hu, Guangwu Xu

Keywords

availability, microgrid, operational conditions, power electronics, realibity

Abstract

Microgrid, a distributed energy system consisting of distributed energy and loads, aims to ensure reliable and affordable energy security in urban and rural communities. With the growing global energy need and the emerging threat of climate change, green renewable energy is becoming a new favorite in the field of power generation. Microgrids have received wide spread attention and application for their minimizing carbon dioxide and greenhouse gas emissions. Microgrids do so by maximizing clean local energy generation as well

as reducing the stress of the transmission and distribution system.

With the use of renewable energy, the reliability performance of microgrids becomes an issue because many renewable energy sources are intermittent. As power electronics increasingly serve as interfaces for renewable energy integration in microgrids, the reliable performance of power electronics plays an important role in microgrid reliability. In recent years, although the reliability of microgrid and power electronics has been studied, most of the research was limited to the reliability evaluation on the long-term planning timescale. However, the operational reliability evaluation considering power electronic influence was rarely studied.

Power electronic devices such as converters are important parts of the microgrid system. The constant failure rate of converters has been widely used in power system planning. It has proved to be useful for calculating long-term or medium-term average reliability indices. However, under different operating conditions, the average failure rate cannot fully represent the failure rate of the component.

In this thesis, a converter real-time failure model in different micro-sources was built and tested. These models were applied to an 11-node microgrid test system to calculate the operating reliability indices under different situations. Then, the sensitivity analysis was carried out, and the influence of various factors was evaluated.

The converter real-time failure model is built based on the power losses of power electronics caused by the variation of the weather data (wind speed, ambient temperature, and illumination). To calculate the availability for test systems, systems were simplified to several sub-systems according to the power flow. By using the reliability block diagram (RBD) method and combining with the real-time availability of the converter, each sub-system’s hourly availability was calculated. In the simulation, the system availability considering the influence of power electronics was calculated and a comparison was made with the system availability without considering the influence of power electronics. To calculate operational reliability indices, a short-term outage model was applied and varied cases were used to test the influencing factor. The sensitivity analyses demonstrated the influence of seasons, wind turbine parameters and meteorological conditions.

According to the simulation results, the reliability performance of the system can be more accurately reflected by these condition-dependent models. The ambient temperature is the main affecting factor for wind turbines and the illumination is the most important factor for photovoltaic arrays. The availability of subsystems varies significantly due to the different operating environments. The studies also indicated that the number and type of micro-sources in microgrid have great influence on the reliability indices of the overall system. The results of one year's simulation illustrate that the reliability of the system has a certain seasonality. And it also shows the dependence of operational reliability on factors such as wind turbine parameters, system topology as well as local meteorological conditions.

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