Chapter One1.0 Introduction

Most electronic appliance users know from experience that electrical power supply from the mains is not reliable. All over the world there is significant electrical power supply interruptions which are common after hurricane or severe storm. This power outage means no lights, television, and refrigerator e.t.c. Also there is increase in occurrence of power supply disturbance, which can be viewed as a form of power pollution. High voltage spikes and momentary voltage drops are therefore common. These power supply disturbances may affect the performance of sensitive equipment in private and corporate organisations causing critical loss of data and even damage to equipment.

The supplies that should be available for use as recommended by the Institute of Electrical Engineers (IEE) should be continuous, uninterrupted, with constant frequency and within the load demand in terms of voltage and current. These requirements have become even more relevant in view of the high sensitivity and sophistication of modern technological equipment in use today.

In Nigeria power today outages and poor quality power supply are the order of the day. A wide range of factors have been identified for this problem. They include natural disasters, vandalism, maintainability and sustainability inadequacies, lack of local content, and absence of integrated and collaborated capacity building programme. The problem of poor quality power supply can not be properly addressed without a review of other factors such as lack of political will to invest adequately in power sector, absence of replacement policy resulting in obsolete equipment, unsustainable human capacity building, and inadequate reward and remuneration system to motivate human resources team to perform well.

The progress made in developing alternative sources of energy over the last decades has shown that independent power system (those using sources other than fossil fuels ) are not only possible but are also very practical. In fact a wide variety of generating equipment is now available to allow individuals take advantage of just any renewable sources of energy. For a number of reasons however most of these systems produce only direct current (DC), and often do so only at low voltages. Nonetheless it is generally agreed that the greatest potential use for alternative energy in the future will be to serve alternating current (AC) loads, since these exist in the vast majority of homes.

The two types of electricity; direct current and alternating current, each has its advantages and disadvantages, and most types of devices will only run on one or the other. Therefore, it is useful to be able to change electricity from one form to the other. The process of changing AC into DC is called conversion. Conversion also refers to changing one DC voltage to another and other things as well. Devices that perform this process are called converters, but are also sometimes called adapters, and if being used for charging batteries, they are often just called chargers. Changing DC into AC is the opposite process and is called inversion. A device that does this is called an inverter.

Most people use converters on a daily basis even if they don't realize it, while inverters are used only for special applications. This is because most people have AC power in their houses and therefore have little need for a device that creates AC from a DC source. However, inverters are useful for a wide range of applications, including letting you run small 220 VAC household appliances from car battery or electrical system, which is DC. In the PC world, inverters are a major component in uninterruptible power supplies, changing stored battery energy into a form your AC-powered PC power supply can use.

1.1 Background and Aim of the research

An inverter is a device that takes a DC input and produces a sinusoidal AC output. A commercial 2.0 KVA inverter should be able to power many appliances in the home or a small business office with one photocopier (1.0 KW), three computer systems (300Watts), three lighting points (180Watts), two ceiling fans (100 Watts) and a small printer (200 Watts) from a battery bank - charged from mains supply in UPS fashion. The inverter is to be designed to handle the energy requirements of a household while yet remaining efficient during periods of low demand. Inverters can be designed in a number of topologies depending on the situation and its requirements. The efficiency of the inverter is highly dependent on the switching device, topology and switching frequency of the inverter.

The aim of this thesis is to produce an efficient DC to Single Phase 220 Volt AC inverter with sine wave output.

1.2 Scope

In the scope of this thesis, the DC - AC inversion stage will be most critical. A good battery bank is essential to proper operation of inverters. Inverters should have a stable DC supply to work with, but it will have to cope with other issues such as reactive power correction and maintain a good level of voltage regulation in the most efficient manner possible. For this reason a microcontroller has been incorporated into the system. In the same manner, the choice of topology depends entirely on the operating condition and tradeoffs need to be made with regards to limitations on power, efficiency and cost. The most important part of the DC-AC inversion process is in the generation of the sinusoidal input signals to the gates of the MOSFETs. This will be achieved using the same microcontroller loaded with a suitable assembly language program to obtain PWM output to drive the switching MOSFETs.

1.3 Research problem and hypothesis

The research problem can be split up into design and construction of two major components a DC - AC inverter with control circuit and a step-up center tapped transformer for output. The overall system schematic can be seen in Figure 1.

UNRELIABLE/ INACCESSIBLE

MAINSSUPPLY

MICRO-CONTROLLERCIRCUIT

OUTPUT TRANSFORMER

DC –ACINVERTER

SWITCHINGCIRCUIT

RELAY1CIRCUIT

BATTERYCHARGERCIRCUIT

BATTERYBANK

24 Volts DC

AC OUTLET220Volts

PRINTERFANCOMPUTERLIGHTINGPHOTOCOPIER

RELAY2 CIRCUIT

Figure 1: System Schematic

The inverter converts the 24V DC to a 50Hz modified sine wave which is presented at the input of the center tapped output transformer, which will then step up the 24V AC to a usable 220V 50Hz AC supply. The switching device is a high power MOSFET that was particularly attractive due to its high switching speeds and high power handling capabilities at low voltages. The control for this stage was chosen to be a single IC based analogue solution to allow the focus of this thesis to remain on the inverter stage. By defining the output as usable implies that it must be within accepted tolerance limits for total harmonic distortion and voltage regulation under a wide range of predicted loads.

The central research problem is the design and construction of this inverter with a micro-controller based control system with some effort at achieving cost reduction and high system efficiency.

1.4 Significance/ Benefit of Research to Humanity

First, there are still a significant number of locations in Nigeria that are not connected to a supply grid. Often, the cost of connecting these locations to the grid far outweighs the initial expenditure required to set up some form of renewable energy source on location. Also electrical power supply from the mains is not reliable and interruptions are common even in locations where the supply grid is present. This proposition is made more attractive with the introduction of high efficiency inverters. This is due to the fact that an inefficient inverter leads to higher rating requirements on all preceding devices - often expensive solar panels and battery banks. The cost of higher ratings on these preceding devices far outweighs the cost of researching and designing an efficient power inverter. Recent advances in micro0controller technology has led to ‘single-chip’ solutions for the control of inverters which makes complicated control systems cheaper and efficient due to reduced number of components used. This thesis hopes to utilise these advances to create a high efficiency inverter capable of handling varying supply and load conditions.

1.5 Arrangement of the Report

Chapter one presents an introduction to the project work as has been done above. Chapter two will begin with a literature review on relevant articles written on the subject of both power inverters and their control. Chapter three will present the design of a 2.0 KVA Commercial inverter. Chapter four will explain the construction of the inverter with the control system and the output transformer it will then present the results of the open circuit tests and short circuit tests performed. In addition the bill of quantities will be presented in chapter four. Chapter five will include an analysis of data and attempt to draw appropriate conclusions based on experiments done. It will also look into future implications of this research.