Design And Construction Of 25amps Single Phase Automatic Change Over

Project and Seminar Material For Electrical Electronics Engineering EEE

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 the Study

As the quest for change over on public power supply and stand by generation increase. Engineering have researches deep into the different methods of achieving to both manual and automatic types. It may not be taken seriously in some places where power supply is up to 90 constant or where intermittent outage causes no great damage. However, place where they have business centers, industrial centers and even cold rooms need to almost 100% supply. Theraja, B.L and Therajah, A.K(2000). So, in this case there is need to provide a standby power supply in the event of power outage. The most important task is just to provide a standby power supply by great attention is paid to the reliability of the system. That is immediately and change over without fluctuations.

The first type of change over switch were purely manual operated. They usually have three switch able position, main, off and generator. An arm is provided from where the operator can have access to the switching. Here, the effectiveness of the switching (i.e. how fast it is affected each time power needs to be changed) largely depends on the operator. The change over system will be ineffective when the operator is not around.

The second generation of change over switches were made semi-automatic in which the mains power supply directly powers the contactors. This arrangement is wired is such a way that if the mains power supply fails, the contractors will all drop leaving the supply to the generator position. The effectiveness of the system will now depend on the operator top start up the generator on time.

The main problem suffered by the above mentioned two versions of the change over switch is that the operator must have to be continuously on duty for it to be very effective. They lent use full closed loop system to operate.

The present-day technology has made it possible for the change over switch to come in a number of designs switch using the closed loop system at feed back control. Some of them use comparators, logic circuits and electronic timers to achieve the task. Some even used simple microprocessor chips to function. The design of this project is such that a combination of comparators digitals switches (logic IC), electronics timers and electromagnetic switches are used to achieve a very fast and effective change over in this case. [Mbaocha C. (2012)].

The automatic change over switch makes use of protective relays and contractors among other things which constantly monitor the power system to ensure maximum continuity of the electrical service with minimum damage to life and property. They are found mostly in domestic installations, hospital theaters, conference halls, computer centers, data and research centers, laboratories and industries that require three phase constant power supply. This equipment however is used to protect some house holds appliances from electrical faults which may arise due to voltage drop. It was designed and constructed purposely to alleviate the suffering encountered by the consumers following supply authority’s consistent failure to meet its obligations of constant and uninterrupted power supply. A circuit breaker is in corporate as a back up protection to improve the reliability of the system and to meet it’s specification, although more back-up protection could have been incorporated monitoring the supply but due to financial constraints, it has to be limited to this operation. The schematic diagram of the automatic change over switch was clearly explained as far as possible to be apprehended by any person within or outside the engineering field. It is my intention however to present the work to Institute of Management and Technology (IMT) students in Engineering and to the general public for consumption.

1.1.1 General Description and Application

Brief Description: The changeover switch is said to be automatic because it employs full closed loop system. It has comparators that test both the input and output conditions and then uses the information to switch the relays to the appropriate positions.

There are three important switching position which are:

Switching of the contactors (i.e. C/O)
Switching of the generator started if power fails
Switching off the generator as soon as power is restored

The system uses 3 comparators, to test the voltage conditions to a certain full voltage, low voltage or even power failure. When the phase voltage is between 190v and 230v not neutral, the voltage from the converters to the input of comparators is higher than voltage reference at the non-inverting input. Hence, the output of the comparators will be below (zero volts).

If there is low voltage (below 190v) in one or more of the phase voltages, the voltage at the inverting input will be less than reference voltage at the non-inverting input this will produce a high output (+9v) from the comparator. One or more of this art the input of the OR gate will cause the output of the OR gate to be high and after a delay time of about 30 seconds this voltage appears at the inputs of the AND gate.

Now the inputs of the AND gate will depend on the position of the enable switch. If it is switched off, the information from the comparators will not pass the AND gate. But if it is switched on (i.e. both inputs will now be high) the information will be reflected at the output of the AND gate and it will be used to achieve the 3 switches generator on/off and then change over switch.

Also, if there is complete power failure in one or more phase, the operation of the system will be the same as described above for low voltage. For the switching actions, the information at the output of the AND gate operates the 3 switches at the same time. It supplies voltage to the coil of contactor “A” leaving the contacts on the normally close (NC) position ready for generator supply. Also, the generator starter and turn off receive information, the turn off will switch on d.c supply to the high voltage coil while the starter supplies starting pulses (through the multi vibrator) to the kick starter. When the generator starts running, a signal is sent to the starter to disable it.

Also, the generator sends signals to the delay circuit and after some delay time. It will switch on contactor “B” and the generator will start supplying. But if the power is restored and the comparators okay the supply (i.e if it is above 190v) the OR gate output goes low and time delay, the C/O will be affected again and the generator will be turned off by cutting off d.c supply to the high voltage coil. The aim of the delay is to stoop any fluctuation in power which is less than 15 seconds from reaching the control relays the power supply uses a 9v backup battery with a 9v regulator. It is interesting to note that when supply is restored, the system automatically returns the consumers to his original power supply as the case may be.

1.2 Statement of the Problem

From various surveys in the industrial sector of Nigeria, it was generally noticed that industries are vulnerable to long and short interruptions (that are considered “reliability issues”, in the power system analysis). The single-phased automatic changeover switch to design is a complete system with various sub-systems and components arranged and linked to function primarily as a means of manipulating the supply of electrical power to any desired load.

A changeover switch can be generally described as a device that allows the conversion from one power source to another (for example, supply from PHCN to supply from a standby generator). The switching obtainable from the changeover switch is usually manual, that is the user has to move a lever to change from one source to another. This is usually associated with time wasting as well as some health hazards like electric shock and trauma. In order to eliminate this human intervention as well as introduce speed and precision in the changeover operation there is need for an automatic changeover switch. This is typically a switching system whose function is based on the predetermined configuration unit. It selects the available power source without intervention of the user to ensure the availability of supply at the desired times provided from one source to another can only be achieved by device or a system that determines when the change should actually take place and which source to be given preferences to supply the load. This brings about the need for a unique control system.

The basic problem to be addressed here is how to connect two different sources of electrical power simultaneously to a single unit (the automatic changeover switch) that can serve as a link between these power sources and the load or network. Moreover, preference is given to the power source such that one source supplies the load at a time and when the first (mains) source fails, the link immediately connects the second (generator) source to the load.

1.3 Aims and 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 suppress power failure and ensure regular power supply for the industry, hospitals, schools and homes. The problem of power failure can be checkmated with the use of stand-by generating set.

An Engineering Author, “Tony Rudkin” said in his book titled “Upgraded Signal Source with Improved Performance and Reliability” that the cost and depredation 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 firm 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 hospital, undergoing a surgical operation and power supply suddenly go off, the patient might lose his or her life due to the power outage.
Furthermore, if the president of the country is making nationwide broadcast and all of a sudden power went off in the transmitting station, it would be viewed as an attempt to sabotage the government ruling and some people must pay for it. In his book, Tony Rudkin also said that the depredation caused by such reduce efficiency of the organization and leads to a great deal of frustration. Sequel to the rate at which more sophisticated electrical/electronics gadgets are being procured and installed in our homes, hospitals 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 25amp automatic change over switch with a phase failure detecting circuit also known as “Automatic Mains Failure” which switches on the head from power Holding Company (PHC) to a generator when power fails and from generator to PHC when power comes back.

1.4 Significance of the Work

This work serves the purpose of saving the electrical appliances in a household and offices from power fluctuation-related damages which could be occasioned by overloading of unprotected changeover switches. Such a device protects electrical appliances from possible harmful effects of voltage sag. It provides an average user the comfort of enjoying the use of electrical appliances at home and offices without the interruption of work and switching over between the public power source and alternative power source. Convenience of not having to walk all the way to the alternative power supply source to turn it off or on is also provided.

It can also create entrepreneurship opportunity for our teeming unemployed youths of the country owing to the large number of people that use alternative power supply that seek automatic changeover from the public power supply to the alternative power supply.

1.5 Scope and Limitation

The automatic change over switch will be able to transmit 25amp of current, the switch aimed at achieving the following automatic actions;

To change power over to generator
To change back to PHC
To change the generator.

The automatic change over unit is operated in single phase system. The automatic change over switch has the following advantages;

It minimizes damages to lives/equipment since it has its own monitoring system and its switching requires no human contact with the switch, thus eliminating human error.
It reduces its change over timing to the minimum due to its fast response to power outage.
It maintains high quality of service through its fast and prompt response.
Moreover, the size and captivity 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.

1.7 Definition of Terms

Microcontroller

Microcontroller is a small computer on a single integrated circuit containing processor core, memory and programmable input/output peripherals, also designed for embedded applications in contrast to the microprocessor used in personal computers or other general-purpose applications

Electric Power

This is the rate at which electric energy is transferred by an electric circuit. Electric power can be produces by electric generators; can also be produced by other sources such as electric batteries. The Standard International Unit (S.I. Unit) of power is the Watt which is one joule per second.

Mains Power Supply

Also known as the public power supply is the general-purpose alternating current (A.C.) electric power supply. In Nigeria, it is the PHCN (Power Holding Company of Nigeria) that generates and dissipates the public power supply.

Standby Generator

This is a back-up electric system that operates manually or automatically. Within seconds of utility outage an automatic transfer switch senses the power loss and commands the generator to start, and then transfer the electrical loads to the generator.

Automatic Power Change-Over

This is an electrical mechanism that changes from one power source to another, for instance from public power supply to a standby generator and vice versa. It switches off the generator set after alerting you and switches over to the public power supply when there is light.

Transfer Switch

This is electrical switch that switches a load between two sources either from mains power supply to a standby generator or vice versa. Some switches are manual, while others are automatic and switch power when they sense that one source has lost or gained power.

Chapter Five

Conclusion and Recommendation

5.0 Conclusion

After a research work and findings on the existing changeover devices through the use of cut-out fuses, a normal switching system and even automatic coil-energized changeovers, a step-further was taken to put in place active component monitors to provide as input data to a microcontroller software programmed chip in controlling the time and method of switching over to a standby generator once there is low or no supply from public power supply. It makes the design highly sensitive to avoid damaging effect of low voltage in particular or instant changeover to the alternative source for a continuous supply to the load uninterrupted.

5.1 Problems Encountered

There was problem of inconsistent power supply during the design stages. This caused mostly the delays as well as unstable data values, more so when the device was power input based.
Cost financing of the project seriously affected the progress of the work.
Sourcing for components was also a constraining factor as it led to going to distant cities such as Lagos and Port Harcourt in order to purchase some major components of the project.

5.2 Recommendations

The level of this project achieved is only at a prototype level and is therefore unsafe for installation as a household device except the various power ratings of the components are upgraded.
System design of the project can be enhanced for a multi-phase-based input instead of the single-phased input.
All connections at various stages of the circuit design should be carefully re-examined before the introduction of the high input voltages. This can be disastrous if ignored

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