Design And Construct A Three Phase Auto-Transformer Of Rating 2.5Kva, And Step Down Voltage Of 415 And Secondary Taps Of 0.,180V, 200V, 230V And 240V.

ABSTRACT

The project presents here is on design and construct a three phase auto-transformer of rating 5kva, and step down voltage of 415 and secondary taps of 0v, 180v, 200v, 230v and 240v.

A three phase autotransformer is a special type of electrical transformer in which a common winding is shared by both high voltage & low voltage sides. Here, three phases AC is provided along the primary side and the output is collected from the secondary side.

Actually, three phase auto transformers are used for such applications where small voltage corrections are required in the distribution system. Also, they are frequently used as an economical alternative of three phase distribution transformers to adjust the bus bar voltage with specific load requirements, when load isolation from the main line is not required.

CHAPTER ONE

1.0                                                      INTRODUCTION

A transformer which has only one winding per phase part of which is common to both primary and secondary circuits. A transformer is an electrical apparatus designed to convert alternating current from one voltage to another. It can be designed to “step-up” or “step-down” voltages and works on the magnetic induction principle. A transformer has no moving parts and is a completely static solid state device, which insures, under normal operating conditions, a long and trouble-free life. It consists, in its simplest form, of two or more coils of insulated wire wound on a laminated steel core. When voltage is introduced to one coil, called the primary, it magnetizes the iron core. A voltage is then induced in the other coil, called secondary or output coil. The change of voltage (or voltage ratio) between the primary and secondary depends on the turns ratio of the two coils.

Using an auto-transformer is an economical and compact means of connecting electrical equipment to a power supply or a different voltage. Part of the winding is common to both primary and secondary circuits so there is no isolation between the two. This may be acceptable on some power transformer. Typical applications include motor loads of industrial machinery, electric heating, air conditioners. Performs same function as an isolation transformer of same KVA and voltage rating without the isolation feature. May be used in either step-up or step-down configuration. All terminals are clearly identified and easy to connect. Transformer terminations rated below 330A are supplied with suitable hardware and lugs for cable connection. Terminations above 330A are supplied with termination pads.
Generally quieter than the equivalent isolation transformers. May be used in either step-up or step-down configuration. Low regulation (typically less than 3%).

1.1                                     PURPOSE OF THE PROJECT

The purpose of this study is to design and construct a three phase auto-transformer of rating 5kva, and step down voltage of 415 and secondary taps of 0v, 180v, 200v, 230v and 240v that can be used for such applications where small voltage corrections are required in the distribution system

1.2                            SIGNIFICANCE OF THE PROJECT

The purpose of an auto transformer will be one of two things. The auto transformer can increase the voltage a bit. The auto transformer can also decrease the voltage by a bit if that is what is needed.

Three phase auto-transformers are frequently used in power applications to interconnect systems operating at different voltage levels, such as to 66 kV to 138 kV transmission line.

1.3                                STATEMENT OF PROBLEM

An autotransformer does not provide electrical isolation between its windings as an ordinary transformer does; if the neutral side of the input is not at ground voltage, the neutral side of the output will not be either. A failure of the insulation of the windings of an autotransformer can result in full input voltage applied to the output. Also, a break in the part of the winding that is used as both primary and secondary will result in the transformer acting as an inductor in series with the load (which under light load conditions may result in near full input voltage being applied to the output). These are important safety considerations when deciding to use an autotransformer in a given application.

Because it requires both fewer windings and a smaller core, an autotransformer for power applications is typically lighter and less costly than a two-winding transformer, up to a voltage ratio of about 3:1; beyond that range, a two-winding transformer is usually more economical.

In three phase power transmission applications, autotransformers have the limitations of not suppressing harmonic currents and as acting as another source of ground fault currents. A large three-phase autotransformer may have a “buried” delta winding, not connected to the outside of the tank, to absorb some harmonic currents.

In practice, losses mean that both standard transformers and autotransformers are not perfectly reversible; one designed for stepping down a voltage will deliver slightly less voltage than required if it is used to step up. The difference is usually slight enough to allow reversal where the actual voltage level is not critical.

Like multiple-winding transformers, autotransformers use time-varying magnetic fields to transfer power. They require alternating currents to operate properly and will not function on direct current.

1.3                        APPLICATIONS OF PROJECT

• Used in both Synchronous motors and induction motors.
• Used in electrical apparatus testing labs since the voltage can be smoothly and continuously varied.
• They find application as boosters in AC feeders to increase the voltage levels.

Used in HV Substation due to following reasons:

1. If we use normal transformer the size of transformer will be very high which leads to heavy weight, more copper and high cost.
2. The tertiary winding used in Autotransformer balances single phase unbalanced loads connected to secondary and it does not pass on these unbalanced currents to Primary side. Hence Harmonics and voltage unbalance are eliminated.
3. Tertiary winding in the Autotransformer balances amp turns so that Autotransformer achieves magnetic separation like two winding transformers.

1.4                                         LIMITATION OF THE PROJECT

One of the limitations of the autotransformer connection is that not all types of three-phase connections are possible. For example, the ∆-Y and Y- ∆ connections are not possible using the autotransformer.

The Y-Y connection must share a common neutral between the high-voltage and low-voltage windings, so the neutrals of the circuits connected to these windings cannot be isolated.

A ∆ – ∆ autotransformer connection is theoretically possible; however, this will create a peculiar phase shift. The phase shift is a function of the ratio of the primary to secondary voltages and it can be calculated from the vector diagram.

This phase shift cannot be changed or eliminated and for this reason, autotransformers are very seldom connected as ∆ – ∆ transformers.

1.5             ADVANTAGES OF THE AUTOTRANSFORMER

1. There are considerable savings in size and weight.
2. There are decreased losses for a given KVA capacity.
3. Using an autotransformer connection provides an opportunity for achieving lower series impedances and better regulation. Its efficiency is more when compared with the conventional one.
4. Its size is relatively very smaller.
5. Voltage regulation of autotransformer is much better.
6. Lower cost
7. Low requirements of excitation current.
8. Less copper is used in its design and construction.
9. In conventional transformer the voltage step up or step down value is fixed while in autotransformer, we can vary the output voltage as per out requirements and can smoothly increase or decrease its value as per our requirement.

1.6       DISADVANTAGES OF THE AUTO TRANSFORMER

1. The autotransformer connection is not available with certain three-phase connections.
2. Higher (and possibly more damaging) short-circuit currents can result from a lower series impedance.
3. Short circuits can impress voltages significantly higher than operating voltages across the windings of an autotransformer.
4. For the same voltage surge at the line terminals, the impressed and induced voltages are greater for an autotransformer than for a two-winding transformer.
5. Autotransformer consists of a single winding around an iron core, which creates a change in voltage from one end to the other. In other words, the self-inductance of the winding around the core changes the voltage potential, but there is no isolation of the high and low voltage ends of the winding. So any noise or other voltage anomaly coming in on one side is passed through to the other. For that reason, Autotransformers are typically only used where there is already some sort of filtering or conditioning ahead of it, as in electronic applications, or the downstream device is unaffected by those anomalies, such as an AC motor during starting.