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DESIGN AND CONSTRUCTION OF AN AUTOMATIC SINGLE PHASE CHANGEOVER SWITCH

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ABSTRACT

The main aim of any electric power supply in the world is to provide uninterrupted power supply at all times to all its consumers. Although, in developing countries, the electric power generated to meet the demands of the growing consumers of electricity is insufficient, hence power instability and outage.

Power instability or outage in general does not promote development in the public and private sector. The investors do not feel secure to come into a country with constant or frequent power failure. These limit the development of industries. In addition, there are processes that cannot be interrupted because of their importance for instance, surgery operation in hospitals, transfer of money between banks and lots more. Power instability and outage in developing countries (Nigeria) creates a need for alternative sources of power to backup the mains supply.

Automatic changeover switches find a wide application scope wherever the reliability of electrical supply from the utilities is low and it is used in lighting/motor circuits wherever continuity of supply is necessary, for switching to an alternative source from main supply and vice versa.

This project is a design of an automatic changeover switch. This means that when there is any mains failure, the automatic changeover switch will change to an alternative power supply (Generator), and back to the main supply when it is restored. The purpose of this project is to maintain constant supply to the main circuit that is being supplied by making-up for the time lapse or delay that usually accompanies the manual switching from one source to another.

The design comprises of the power connection circuit and control connection circuit. The main components to be used include; contactors, relays and timer.

CHAPTER ONE

1.0 INTRODUCTION

The need for continuous power supply and its reliability has increased rapidly over the years, especially in all those areas where uninterrupted power supply is a must. Modern systems are power dependent. Their complexity has increased as continuous information and communications are needed to control automated process, be in industries, commercial complexes, hospitals, hotels or even modern residences. The need, as such, for independent standby power system has therefore increased manifold. The power distribution, control, monitoring and protection of standby power systems need to be integrated. Standby generator systems, for example are required to serve:-

Sensitive Loads- are supplied by UPS systems. The period of non-availability of power, before the standby supply takes over, is bridged by battery banks. Typical loads are computers, hospital equipments, micro processor controlled industrial machines etc.

Critical Loads- thesemostly involve standby generator systems which supply power to lighting systems, air conditioning, elevators etc in Airports, Hotels and commercial complexes.

Essential Loads- thesealso use standby generator systems mostly in process industries as they relate to high restarting times or high down times. Automatic transfer from mains supply to standby supply is vital for all the above kinds of loads.(11)

In the event of power failure, the standby power is usually expected to take over automatically. Electrical starting equipment, battery bank and diesel generator are required for the automatic operation. The automatic transfer is achieved mostly by automatic mains failure systems. The process of on-load transfer has to be monitored and controlled for a smooth Changeover and within safety limits of all elements of the system. This is achieved by an Automatic Changeover Switch.

Changeover switches find a wide application wherever the reliability of electrical supply from the utilities is low and they are used in lighting/motor circuits wherever continuity of supply is necessary, for switching to an alternative source from mains supply and vice versa. They are switch disconnectors with independent manual operation capable of making, carrying and breaking currents under normal circuit conditions which may include operating overload conditions and also carrying currents under specified abnormal circuit conditions such as those of short circuit for a specified time.(1)

Automatic changeover switch (also known as automatic transfer switch ATS) is an integral part of a power generation process, allowing smooth and immediate transfer of electric current between multiple sources and load. When the generator is in operation, the transfer switch prevents any feedback current to the load. It also ensures that the different power sources are synchronized before the load is transferred to them. The transfer switch senses when there is interruption if the mains supply remains absent. Fluctuations and voltage drop below a particular level within a specified time in the mains supply will also cause the automatic transfer switch to transfer the load to the generator. The starting of the generator is done by a relay which switches the battery voltage to the ignition coil of the generator. In about a few seconds when the generator is producing full power, the transfer switch disconnects the load from the mains supply and connects it to the generator supply, restoring electricity to the load. The transfer switch continues to monitor the mains supply and when it is restored, it switches the load from the generator back to the mains supply. Once the generator is disconnected, it goes through a cool down routine and shuts down automatically.(2)

1.1 PROJECT AIMS AND OBJECTIVES

The aims and objectives of this project is to

1.     Design and construction of an automatic changeover switch that will switch load from the mains power supply (PHCN supply) to a back-up power supply (generator) with simplicity.


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