DESIGN AND CONSTRUCTION OF AN AUTOMATIC SINGLE PHASE CHANGE OVER SWITCH

DESIGN AND CONSTRUCTION OF AN AUTOMATIC SINGLE PHASE CHANGE OVER SWITCH

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Abstract

Considering the epileptic power supply in Nigeria and other developing countries today, a high demand of an alternative service is required. A single phase automatic change over switch from the public mains supply to the auxiliary supplies (single phase ac generator) and vice-versa has been developed. The design was realized using major components like a step down transformer (220V-12V dc), atmega8 microcontroller, rectifiers, voltage regulators, 555 timers, relays,  circuit breaker and others like resistors, diodes, and capacitors. The device automatically switches from public mains to an auxiliary whenever there is an outage in the public mains. This device also detects the public main power supply when available and switches from auxiliary power supply to the mains with a delay period of 4 seconds in starting and switching off the generator. The device has been constructed successfully, and it demonstrated a tendency to automatically switch from mains to the auxiliary source and vice-versa without human interception.


CHAPTER ONE

INTRODUCTION

1.1     Background of study

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 equipment, microprocessor controlled industrial machines etc.

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

Essential Loads- these also 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.

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.

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.

Power outage in general does not promote development to public and private sector. The investors do not feel secure to come into a country with constant or frequent power outage. 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, bank transactions and lots more. An automatic change over switch can be entertained to allow smooth and fairly uninterrupted supply of electricity for operation and services that require no discontinuity in electricity supply. This is achieved by the use of integrated circuits that have timing abilities and relays to effect switching.

Power supply instability in developing countries creates a need for automation of electrical power generation or alternative sources to back up the utility supply. This automation becomes predominantly high. Most industries and commercial processes are partly dependent on generators and public power supply. The public mains supply remains epileptic especially in tropical African countries where Nigeria forms a part. Therefore, if the processes of power changeover between these two power supplying sources are manual, human error during change over connections may occur; leading to machine damage, electric shock electrocution as well as increased down time consequently introducing massive losses.

1.2     Statement of Problem

Electricity plays a major role in economic development of a nation, and this forms the basis of this study, with interests in human, infrastructural and economic development.  In most developing and undeveloped parts of the world, the supply of electricity for industrial, commercial and domestic use is highly unstable.  This gives rise to the frequent use of alternative of power supply to meet up with the energy demands.

Of all the existing types of changeover switch, the most populous changeover is the manual change over which consists of a manual change over switch box, switch gear and cut-out fuse or the connector fuse. Ahmed et. al., 2006 stipulates that, this change over switch box separates the source between the generator and public supply, as the contact is manually switched from mains terminal to the generator terminal and vice versa. Many challenges are experienced in this manual changeover as it requires lots of human activities in its operation. Electric shock can occur during the switching due to electric wire contact with the device metal casing as a result of crack in the ceramic insulator in the switch. This has led to several deaths and damage to electrical appliances. Nwaforet. al., 2012 also stipulates one of the disadvantages as wear and tear of some mechanical parts due to frequent movement from one source to the other.

Electrical changeover switch is another type of changeover switch, which involves the use of electrochemical type relays, contactors, voltage monitoring relays and delay timer relays as main components of the device. The device is cheap and the construction is easy, but associated with noise in switching of relays, leading to wear and tear, loss of reliability, and thereby making the device to be prone to failures.  

The electronic change over switch has proven to be a better device compared to both the manual and electrical changeover switch as it gives no noise, eliminates wear and tear, reduces damages to lives, and maintains high quality of service as it provide quick response for the interchanging of power to the load automatically from the main supply to the generator and vice versa. The electronic changeover device employed in this research work changes over immediately when the public mains is out. However, the generator starter/off has a periodic delay to start/off the generator. This requires the generator to have a kick starter and off facility, and a battery in good condition. This paper presents the construction of a single phase power changeover switch that switches power supply from public mains supply to an auxiliary source (ac generator) and vice versa.

1.3     Aims

The main aim of this study is to 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.

1.4     Objectives

Due to inconsistent supply of power, there is a growing need for an alternative source of power supply. This has led to heavy capital investment in a bid to curb power failure and ensure regular power supply for our industries, hospitals and homes. The problem of power failure can be checkmated with the use of standby generating set.

An Engineering author Tony Rudk Inn said in his book titled "Upgraded signal source with improved performance and reliability" that the cost and depreciation associated with breakdown vary from one application to the other, and in some cases, the user has little choice but to ensure that a stand-by unit is available to take over on event of failure of primary system.

If some of these big firms do not make provisions for stand-by power source, frustration could set in which may lead to the closure of business and thus throwing workers into unemployment. Also in the case of hospitals undergoing a surgical operation and the power supply suddenly goes off, the patient might lose his life due to power outage.

Furthermore, if the President of the country is making a nationwide broadcast and all of a sudden there is power failure in the transmitting station, it would be viewed as an attempt to sabotage the government ruling and some people must go for it.

Sequel to the rate at which more sophisticated electrical/electronic gadgets are being procured an installed in our homes, hospital and business premises, there is a justifiable need for a faster and more reliable change over system in an event of power outage.

In view of these considerations, this project is aimed at designing and constructing a workable automatic change-over switch with phase selector circuit which switches on the load from National Electric Power Authority (NEPA) to a generator when power comes back. Also when there is a phase failure in any of the NEPA phase carrying the load, the change-over switch will be changed to alternative phase.

1.5     SCOPE AND LIMITATION OF THE STUDY

The automatic change over switch is aimed to achieve the following automatic actions.

To start the generator

To change power to generator                                           

To change back to PHCN

To change the generator

To change to alternative phase in case of phase failure.

The automatic change over unit can be operated in single or three phase-system. The automatic change over switch has the following advantages.

i.        It minimizes damage to lives/equipment since it has its own monitoring system and its switching requires no human contact with the switch, thus eliminating human error.

ii.       It reduces its change-over timing to the minimum due to its fast response to power outage.

iii.      It maintains high quality of service through its fast and prompt response

iv.      It avoids and reduces fuel consumption since the generator is switch off immediately PHCN supply is restored.

Moreover the size and capacity of the Unit will depend upon the load for which it will be used. The Unit is also portable, easy, convenient and safe to install.

1.6     DEFINITION OF TERMS

SWITCHES

In electrical engineering, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another. In the design, I used the switches S1 S2, S3. Switch S1 is to prevent the generator from starting in case PHCN restores power, Switch S2 is to prevent the auxiliary contact from energizing the relay that connects the starter; Switch S3 is to start the generator manually.

CABLES

A cable is defined as an insulated conductor, single or stranded which makes an overall mechanical protection. Cables have two major parts, the conductor and the insulator. The conductor is that part of the cable which allows current to flow through it, while the insulator is the outer covering which serves as a protection to the conductor and does not allow current to flow through it. Cables are channels through which current are being transferred in various instrument/equipment.

RESISTORS

Resistors are devices that can resist the flow of electrical current. They are bi-directional components. Resistors are made of metal, carbon or ceramic. They are of two types namely fixed type and variable types.

In resistors, colour coding is used to identify their values which are measured in a unit Ohms with the symbol (Ω). The schematic diagram of resistors is shown below.

CAPACITORS  

Capacitors are electronics components that store electrical changes/energy by electrostatic stress in a dialectric. Capacitors basically consist of two conducting surface separated by a layer of an insulating medium lied dielectric. The dielectric may be air, vacuum, per, mica, ceramic plastic electrolyte. Each of these materials has the values called dielectric constant.  

DIODES

Diodes are two terminal semi-conductors that allow current to flow in one direction. The terminals are anode and cathode. When connecting positive terminal of the source (EMF) to the positive terminal of the diode and negative of the source to the negative of the diode, it is said to be in forward biased mode and it will conduct. But when connecting positive terminal of the source to the negative terminal of the diode and negative of the source to the positive terminal of the diode, it is said to be in reverse biased mode and it will not conduct.        

TRANSFORMER

This is a static electrical machine through which power from one part of the circuit (primary side) is transferred to the other part (secondary side) of the circuit by means of electromagnetic induction. It input and output current/ voltage differs depending on the windings.

TRANSISTORS  

Transistors are made up of P-type and N-type semi-conductor materials. It is being formed into two junctions. PN junction and NP junction separated by either N or P semi—conductor in between the junctions. This results in a three terminal electrode of emitter, collector and base.   

TRIAC 

This is a five layer bi-directional device which can be triggered into conduction by both positive and negative triggering pulse at it gate. It is used to control AC power to a load by switching ON and OFF during positive and negative half cycles of the input AC power.   

1.7. PROJECT REPORT ORGANIZATION

This project report is presented in five chapters to appropriately illustrate the steps involved in its implementation.

Chapter One introduces the project overview, objectives and its significance. Chapter Two is basically the literature review and some theories relevant to the design.

Chapter Three x–rays the methodology and analysis while

Chapter Four elaborates on the implementation, testing and result.

Finally, Chapter Five is about the summary of the entire theories and recommendation.



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