Reliability Assessment Of Inverters

TABLE OF CONTENTS

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

TABLE OF CONTENT

CHAPTER ONE

1.0     INTRODUCTION

1.1     BACKGROUND OF THE PROJECT

1.2     AIM AND OBJECTIVE OF THE PROJECT

1.3     RESEARCH QUESTION

1.4     SCOPE OF THE STUDY

1.5     SIGNIFICANCE OF THE STUDY

1.6     RESEARCH ORGANISATION

CHAPTER TWO

2.0     LITERATURE REVIEW

2.1     REVIEW OF EARLY INVERTERS

2.2     REVIEW OF INVERTER CAPACITY

2.3     SAFETY OF INVERTER

2.4     REVIEW OF EXISTING LITERATURE

CHAPTER THREE

3.0              METHODOLOGY

3.1     STEPS INVOLVED IN ESTIMATING THE LIFETIME OF INVERTERS

3.2      INVERTER TESTING

3.3     INVERTER LOSSES AND THERMAL MODELING

3.4      CLASSIFICATION OF POWER LOSSES

3.4.1         Power MOSFET Losses

3.4.2           Switching Losses

3.4.3           Conduction Losses

3.4.5           Gate Driver Losses

2.4.6     Switch Output Capacitance Loss

3.4.7     Gate Charge Loss

3.4.8  Reverse Conduction Loss

3.4.9  Inductor Losses

CHAPTER FOUR

4.1     THERMAL MODELING

4.1.1     Implementation of Thermal Model

4.2       RESULTS OF THERMAL MODEL

4.3     Reduced-Order Models for Annual Temperature Estimation

4.4     Switching Model of the Inverter

4.5       Development of Average Model

4.6       Development of Averaged Loss Models

4.7       Rain Flow Counting

4.8     Estimation of Reliability and Useful Lifetime

4.9       Survey of Lifetime Models of Power Semiconductors

4.10    Cumulative Stress

4.11 Reliability Indices

4.12    Influence of Reactive Power

4.12   INVERTER SYSTEM RELIABILITY AND LIFETIME

CHAPTER FIVE

5.0     CONCLUSION

5.2     REFERENCES

CHAPTER ONE

1.0                                                INTRODUCTION

1.1                                  BACKGROUND OF THE STUDY

Most research, historically and present, has focused on the production costs and reliability of PV module technology, whereas the cost of the necessary power electronics components has been sparsely considered [Feldman et al, 2014]. As the price of PV modules decreases, the price of power electronics become more important because they now constitute 8%–12% of the total lifetime PV cost [Xue et al, 2011].

As efforts to reduce PV module costs yield diminishing returns, understanding and reducing inverter costs becomes increasingly critical and is a cost- effective investment toward achieving DOE Solar Energy Technologies Office goals.

A key price driver of power electronics is reliability [Ristow et al, 2008]. PV modules have long lifetimes with warranties up to 20 years [Jablonska et al, 2005]. In utility-scale fielded systems, the mean time between failure of inverters has been shown to be 300 to 500 times shorter than modules [Maish et al, 2013]. In one 27-month study, module failures accounted for only 5% of total energy losses, whereas inverter failures accounted for 36% of lost energy during the same period [Golnas et al, 2012].

Inverter reliability is a layered topic because of its complicated switching/monitoring system and multiple materials bonded together. In addition to providing output power meeting power quality standards, the inverter may be required to manage the power output of the PV module, connect/disconnect from the grid, read and report status, or monitor islanding. Meanwhile, trends in power electronics systems and devices during the past decade have placed increasing demands on the efficiencies of the thermal management and control systems used for metal-oxide- semiconductor field-effect transistor (MOSFET) and insulated-gate bipolar transistor (IGBT) modules. The pressure to decrease the size of power electronics systems and inverter subsystems has resulted in an overall reduction of 50% of the footprint area of many IGBTs during the past 10 years.

The aim of this work is to provide a detailed description of inverter reliability as it impacts inverter lifetime today and possible ways to predict inverter lifetime in the future. As a part of this work, we developed detailed inverter hardware and matching models that can potentially predict the lifetime of the inverter when used for different purposes and at different ambient temperatures.

1.2                            AIM AND OBJETIVES OF THE STUDY

The purpose of Reliability Testing of an inverter is to test the system thoroughly to ensure that all the defects and faults in the system are identified and rectified. The objectives of this work are:

1. To predict reliability, thermal cycling is considered as a prominent stressor in the inverter system.
2. To evaluate the impacts of thermal cycling, a detailed linearized model of the inverter is developed

• To provide a detailed description of inverter reliability as it impacts inverter lifetime today and possible ways to predict inverter lifetime in the future.

1.3                                           RESEARCH QUESTION

At the end of this work answers to the following questions shall be made known:

1. What is the main purpose of reliability assessment?
2. What is the purpose of testing reliability of a power electronics?

• What is reliable testing?

1.4                                           SCOPE OF THE STUDY

The scope of this work covers an operational reliability assessment approach of inverters considering a voltage/VAR control (VVC) function. The approach aims to quantify the reliability degradation and estimate the lifetime of inverters when they are utilized for the VVC function. This research also develops models and methods to compute the losses of the power electronics switches and other components in a inverter. The losses are then used to estimate the junction and heat sink temperatures of the power semiconductors in the inverter.

The model is verified by developing an in-house inverter. Additionally, to assess the scalability of the research, the hardware inverter is placed inside a thermal chamber to verify the losses for different ambient temperatures. After the verification of the model, a reduced-order model of the inverter is implemented to translate the profile of the ambient temperature and solar irradiance into the profile of the junction temperatures of the switches. The estimated junction temperature data are used to identify inverter reliability indices and predict the useful lifetime of the inverter system. After developing the models to predict the useful lifetime of the system, the impact of reactive power on the overall reliability of the system is studied.

1.5                                    SIGNIFICANCE OF THE STUDY

This work showcases and describes an approach to help assess and predict the   reliability of inverters. This report will also help the reader to understand why inverters fail.

1.6                                      RESEARCH ORGANISATION

Chapter 1 provides the detailed description of the state of knowledge on inverter reliability as well as researched topics relevant to this work. Chapter 2 describes the technical work on inverter losses and thermal modeling. Chapters 3 and 4 enlist model development for practical use and presents a deep dive into inverter lifetime models and their use  and chapter five concludes the whole work.