Design And Construction Of A Microcontroller Based Automatic 3-Phase Selector

ABSTRACT

This project work is on the design and construction of automatic phase selector for 3-phase power supply. It provides a means of switching from one phase of AC mains to another in the case of failure in the existing phase; This project has been improved on the existing types of electromechanical device that has being in use over the years.

Hence this has been achieved by the use of microcontroller and high current relay switches and it is powered by 12V dc power supply. The aim of this work is to design and construct an Auto Phase Selector introduces an automatic solution to overcome power fluctuation/phase interruption by selecting next most healthy available phase to feed the equipment without any notice of power outage.

 CHAPTER ONE

•      INTRODUCTION

In Nigeria today the problems of power outage across phases is so rampant thus leading to some sensitive equipment and appliances being redundant, this could sometimes be the cause of one phase going out with respect to the other.

It may surprise one to note that in an attempt to solve this problems, so many unskilled electricity consumers has  in recent past resorted to some crude means of swapping between phases to obtain power. So many souls have been lost in this act. This is an undesirable condition to consumers and the need now arises to design a device that can automatically select among the phases and make power supply available at the consumers terminal.

However, this can only be possible if one have a three phase met or and there is supply from any of the service line or the entire line, this project known as a three phase automatic phase selector has been constructed with a view of solving the problems stated above. The design and construction was based on the principle of Electro magneto-Dynamism and alignment as demonstrated in the contactor arrangement.

Among other components are the timers miniature-circuit breakers the contactors and indicator bulbs, the ease with which the device is operated is well elaborated to ensure simplicity effort were made to present a stop by step operation of the project. As much as there is no project without limitation and applications, we have as well dedicated a section of this paper to look into it.

In order to ease the effort of technicians in restoring the devices should there be any malfunction associated.

Faults and how they are cleared is presented, however, there are no user serviceable parts in the device therefore all maintenance should be referred to a qualified electrical personnel.

In this system, If any one or two phases in a 3-phase supply interrupted/goes low and you want equipment to work on normal voltages then this circuit will give solution. This system is designed to monitor the presence of supply to the three phases and to display condition of each phase on an LCD.

1.2                                          OBJECTIVE OF THE PROJECT

The objective of this work is to provide uninterruptable power supply for single phase loads even in the failure of one or two phases in a three phase system.

1.3                                      SIGNIFICANCE OF THE PROJECT

Industries require three phases power to run their machinery. Some of them require continuous \ uninterrupted power to maintain their data. Auto phase selector unit for those equipment whose supply is single phase. The single phase supply is selected automatically from three phases supply.

Auto Phase Selector introduces an automatic solution to overcome power fluctuation \ phase interruption by selecting next most healthy available phase to feed the equipment.

1.4                                     STATEMENT OF THE PROBLEMS

In every home, office or industries, automatic phase selector plays a vital role, that is, It provides a means of switching from one phase of AC mains to another in the case of failure in the existing phase; This project has been improved on the existing types of electromechanical device that has being in use over the years.

In the course of designing this project, different kinds of problem was notice such as:

• Difficulty in troubleshooting with circuit without the circuit diagram
• Difficulty in connecting the output without the three phases short-circuiting, until a multiplexing circuit was gotten.
• Difficult in wiring because of the strong wiring the project required..

1.5                              SCOPE AND LIMITATION OF THE STUDY

This work covers only a three phase automatic phase selector which can only be used for providing a means of switching from one phase of AC mains to another in the case of failure in the existing phase.

1.6                                          APPLICATION OF THE PROJECT

Applications of this project are as below:

• Small and Medium Scale Industries
• Residential Apartments.

 CHAPTER TWO

2.0                                                LITERATURE REVIEW

2.1                                     REVIEW OF ELECTRICAL PHASES

There is a major characteristic of an AC electricity supply that requires explanation -phases. A DC circuit has two wires through which the current in the circuit flows from a source of electricity through a load and back to the source. A single-phase AC circuit also has two wires connected to the source of electricity. However, unlike the DC circuit in which the direction of the electric current does not change, the direction of the current changes many times per second in the AC circuit. The 220 volt electricity supplied to our homes is single phase AC electricity and has two wires – an “active” and a “neutral”.

The distribution line supplying your home may be single phase and have only two wires strung between the poles (we will use the overhead power lines as examples because they can be easily seen). However, the distribution line may be made up of 4 lines. What are the others? The other lines carry the currents from two other electrical circuits, making a total of three circuits or phases. The reason why there are only 4 lines is because the 3 phases have a common neutral line (i.e. 3 active lines and 1 common neutral line).

Because the magnitude and direction of the electricity flowing in each of the phases is slightly displaced in time from the electricity flowing in the other phases, the current flowing in the common neutral will be the sum of the neutral currents from the 3 phases. The resultant current in the common neutral is smaller in a 3 phase system than in systems with other numbers of phases. This ability to use a common neutral of relatively small capacity has large economic advantages and is the main reason why 3 phases are used.

3 phase electricity has another advantage. We mentioned above that, in Canada, the voltage between the active and neutral in the single phase, low voltage supply to our homes is 220 volts and that this phase is only one of the phases in the 3 phase system. The voltage between the phases of a 120/208V 3 phase system is 208 volts (in Canada). A 208 volt, 3 phase supply is able to deliver more energy than a 120 volt, single phase supply. 3 phase supplies are normally restricted to large electrical loads, such as large electric motors.  Commercial buildings are often wired for three phase power.  Air conditioners for instance are run on the three phase power while single phase power is typically used for most electrical, electronic and lighting equipment.

A single phase supply must have a neutral, whereas a 3 phase supply does not require a neutral. More complicated reasons deal with fixing the voltage of the single phase supply relative to the ground (because domestic appliances have their metal enclosures connected to ground) and for fault protection purposes. 3 phase, medium voltage, distribution systems and high voltage transmission systems therefore use one wire for each phase and no neutral.

The above discussions focused on active and neutral conductors (wires) as being the means to convey the electricity. One type of system uses the ground as the return path, with only the active being conveyed by a wire conductor. This type of single-phase supply system is called the Single Wire Ground Return system and is use to supply small loads which are located far from the main distribution networks.

2.2                          REVIEW OF THREE-PHASE ELECTRIC POWER

Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of poly phase system and is the most common method used by electrical grids worldwide to transfer power. It is also used to power large motors and other heavy loads. A three-phase system is usually more economical than an equivalent single-phase or two-phase system at the same line to ground voltage because it uses less conductor material to transmit electrical power. The three-phase system was independently invented by Galileo Ferraris, Mikhail Dolivo-Dobrovolsky and Nikola Tesla in the late 1880s.

In a balanced three-phase power supply system (by far, the most common type), three conductors each carry an alternating current of the same frequency and voltage relative to a common reference (Typically such a reference is connected to ground and often to a current-carrying conductor called the neutral) but with a phase difference of one third the period; hence the voltage on any conductor reaches its peak at one third of a cycle after one of the other conductors and one third of a cycle before the third conductor. From any of the three conductors, the peak voltage on the other two conductors is delayed by one third and two thirds of one cycle respectively. This phase delay gives constant power transfer over each cycle. It also makes it possible to produce a rotating magnetic field in an electric motor and generate other phase arrangements using transformers (For instance, a two phase system using a Scott-T transformer).

With a perfectly balanced three phase supply the instantaneous voltage of any phase is exactly equal in magnitude but opposite to the sum of the other two phases. This means that if the load on the three phases is balanced as well, the return path for the current in any phase conductor is the other two phase conductors.

Hence, the sum of the currents in the three conductors is always zero and the current in each conductor is equal to and in the opposite direction as the sum of the currents in the other two. Thus, each conductor acts as the return path for the currents from the other two.

While a single phase AC power supply requires two conductors (Go and Return), a three phase supply can transmit three times the power by using only one extra conductor. This means that a 50% increase in transmission cost yields a 200% increase in the power transmitted.

Three-phase systems may also utilize a fourth wire, particularly in low-voltage distribution. This is the neutral wire. The neutral allows three separate single-phase supplies to be provided at a constant voltage and is commonly used for supplying groups of domestic properties which are each single-phase loads. The connections are arranged so that, as far as possible in each group, equal power is drawn from each phase. Further up the supply chain in high-voltage distribution the currents are usually well balanced and it is therefore normal to omit the neutral conductor.

Three-phase supplies have properties that make them very desirable in electric power distribution systems:

• The phase currents tend to cancel out one another, summing to zero in the case of a linear balanced load. This makes it possible to reduce the size of the neutral conductor because it carries little to no current; all the phase conductors carry the same current and so can be the same size, for a balanced load.
• Power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations.
• Three-phase systems can produce a rotating magnetic field with a specified direction and constant magnitude, which simplifies the design of electric motors.

Most household loads are single-phase. In North American residences, three-phase power might feed a multiple-unit apartment block, but the household loads are connected only as single phase. In lower-density areas, only a single phase might be used for distribution. Some large European appliances may be powered by three-phase power, such as electric stoves and clothes dryers.

Wiring for the three phases is typically identified by color codes which vary by country. Connection of the phases in the right order is required to ensure the intended direction of rotation of three-phase motors. For example, pumps and fans may not work in reverse. Maintaining the identity of phases is required if there is any possibility two sources can be connected at the same time; a direct interconnection between two different phases is a short-circuit.

2.3 REVIEW OF THREE PHASE GENERATION AND DISTRIBUTION

At the power station, an electrical generator converts mechanical power into a set of three AC electric currents, one from each coil (or winding) of the generator. The windings are arranged such that the currents vary sinusoidally at the same frequency but with the peaks and troughs of their wave forms offset to provide three complementary currents with a phase separation of one-third cycle (120° or ^2π⁄₃ radians). The generator frequency is typically 50 or 60 Hz, varying by country.

At the power station, transformers change the voltage from generators to a level suitable for transmission minimizing losses.

After further voltage conversions in the transmission network, the voltage is finally transformed to the standard utilization before power is supplied to customers.

Most automotive alternators generate three phase AC and rectify it to DC with a diode bridge.

2.4                  REVIEW OF THREE-WIRE AND FOUR-WIRE CIRCUITS

A transformer for a “high-leg delta” system; (Assuming a 200 V, 3-Phase supply) 200 V 3-phase motors would be connected to L1, L2 and L3. 200 V Single-phase load would be connected between L1 and L2. Single phase 100 V supplies (180 degrees “out of phase”) would be obtained between either L1 or L2 and the neutral (N). L3 (wild or high leg) will be 173.2 V with respect to the neutral.

There are two basic three-phase configurations: delta and wye (star). As shown on the left, a delta configuration requires only 3 wires for transmission but a wye (star) configuration may tilize a fourth wire. The fourth wire, if present, is provided as a Neutral and is normally Grounded. The ‘3-wire’ and ‘4-wire’ designations do not count the ground wire used above many transmission lines which is solely for fault protection and does not carry current under non-fault conditions.

A four-wire system with symmetrical voltages between phase and neutral is obtained when the neutral is connected to the “common star point” of all supply windings. In such a system, all three phases will have the same magnitude of voltage relative to the Neutral. Other non-symmetrical systems have been used.

The four-wire wye system is used when ground referenced voltages or the flexibility of more voltage selections are required. Faults on one phase to ground will cause a protection event (fuse or breaker open) locally and not involve other phases or other connected equipment. An example of application is a local distribution in Europe (and elsewhere), where each customer is fed from one phase and the neutral (which is common to the three phases). When a group of customers sharing the neutral draw unequal phase currents, the common neutral wire carries the currents resulting from these imbalances. Electrical engineers try to design the system so the loads are balanced as much as possible within premises where 3-phase power is utilized. These same principles apply to the wide scale distribution of power to individual premises. Hence, every effort is made by supply authorities to distribute all three phases over a large number of premises so that, on average, as nearly as possible a balanced load is seen at the point of supply.

In North America, a high-leg delta supply is sometimes used, where one winding of a delta connected transformer feeding the load is center-tapped and that center tap is grounded and connected as a Neutral, as shown on the right. This setup produces three different voltages. If the voltage between the center tap (neutral) and each of the two adjacent phases is 120 V (100%), the voltage across any two phases is 240 V (200%) and the Neutral to “high leg” voltage is ≈ 208 V (173%).^[7]

The reason for providing the delta connected supply is usually to power large motors requiring a rotating field. However, the premises concerned will also require the “normal” North American 120 V supplies, two of which are derived (180 degrees “out of phase”) between the “Neutral” and either of the center tapped phase points.

Single-phase loads

Single-phase loads may be connected across any two phases, or a load can be connected from phase to neutral. Distributing single-phase loads among the phases of a three-phase system balances the load and makes most economical use of conductors and transformers.

In a symmetrical three-phase four-wire, wye system, the three phase conductors have the same voltage to the system neutral. The voltage between line conductors is √3 times the phase conductor to neutral voltage.

V_L-L = √3 V_L-N

The currents returning from the customers’ premises to the supply transformer all share the neutral wire. If the loads are evenly distributed on all three phases, the sum of the returning currents in the neutral wire is approximately zero. Any unbalanced phase loading on the secondary side of the transformer will use the transformer capacity inefficiently.

If the supply neutral is broken, phase-to-neutral voltage is no longer maintained. Phases with higher relative loading will experience reduced voltage and phases with lower relative loading will experience elevated voltage, up to the phase-to-phase voltage.

A high-leg delta provides phase-to-neutral relationship of V_L-L = 2 V_L-N , however, L-N load is imposed on one phase. A transformer manufacturer’s page suggests that L-N loading to not exceed 5% of transformer capacity.

√3 is ≈ 1.73, so if V_L-N was defined as 100%, V_L-L would be ≈ 100% × 1.73 = 173% If V_L-L was set as 100%, then V_L-N ≈ 57.7%

Unbalanced loads

When the currents on the three live wires of a three-phase system are not equal or are not at an exact 120° phase angle, the power-loss is greater than for a perfectly balanced system. The method of symmetrical components is used to analyze unbalanced systems.

Non-linear loads

With linear loads, the neutral only carries the current due to imbalance between the phases. Devices that utilize rectifier-capacitor front-end such as switch-mode power supplies, computers, office equipment and such produce third order harmonics that are in-phase on all the supply phases. Consequently, such harmonic currents add in the neutral which can cause the neutral current to exceed the phase current.

2.5                                    REVIEW OF THREE-PHASE LOADS

An important class of three-phase load is the electric motor. A three-phase induction motor has a simple design, inherently high starting torque and high efficiency. Such motors are applied in industry for many applications. A three-phase motor is more compact and less costly than a single-phase motor of the same voltage class and rating and single-phase AC motors above 10 HP (7.5 kW) are uncommon. Three-phase motors also vibrate less and hence last longer than single-phase motors of the same power used under the same conditions.

Line frequency flicker in light can be reduced by evenly spreading three phases across line frequency operated light sources so that illuminated area is provided light from all three phases. The effect of line frequency flicker is detrimental to super slow motion cameras used in sports event broadcasting. Three phase lighting has been applied successfully at the 2008 Beijing Olympics to provide consistent light level for each frame for SSM cameras.  Resistance heating loads such as electric boilers or space heating may be connected to three-phase systems. Electric lighting may also be similarly connected.

Rectifiers may use a three-phase source to produce a six-pulse DC output. The output of such rectifiers is much smoother than rectified single phase and, unlike single-phase, does not drop to zero between pulses. Such rectifiers may be used for battery charging, electrolysis processes such as aluminium production or for operation of DC motors. “Zig-zag” transformers may make the equivalent of six-phase full-wave rectification, twelve pulses per cycle, and this method is occasionally employed to reduce the cost of the filtering components, while improving the quality of the resulting DC.

One example of a three-phase load is the electric arc furnace used in steelmaking and in refining of ores.

In Germany, a 1965 publication shows some “full size” stoves are designed for a three-phase feed. However, the individual heating units may be connected between phase and neutral to allow for connection by three individual circuits on the same single-phase supply.

Phase converters

Phase converters are used when three-phase equipment needs to be operated on a single-phase power source. They are used when three-phase power is not available or cost is not justifiable. Such converters may also allow the frequency to be varied (resynthesis) allowing speed control. Some railway locomotives use a single-phase source to drive three-phase motors fed through an electronic drive.