Mini Project on Transformers

INDEX I. Introduction II. Construction and Parts of Power transformers

(a)Construction of Power Transformer (b).Parts of power transformer 2.l. Transformer tank 2.2. Core Assembly 2.3. Winding Construction 2.4. Bushings 2.5. Transformer Oil 2.6. Radiators and Cooler Fans 2.7. Conservator Tank. 2.8. Silica gel Breather. 2.9. On Load Tap Changer (OLTC)

III. Protection of a transformer

(a)Gas operated relays 3. l . Buchholz relays 3.2. Oil surge relays / OLTC Buchholz relay 3.3. Pressure Relief Valve (b)Oil| & Winding Temperature Indicator (c)Differentia| protection (against internal faults) (d) Over current and earth fault protection (against through faults)

IV. Various tests on a power transformer

4.1. Insulation Resistance test 4.2. Transformer Ratio test 4.3 Magnetizing current test 4.4 .Magnetic balance test 4.5 .Short circuit test 4.6. Vector Group test 4.7. Winding Resistance test 4.8. Tan delta test

V. Causes for transformer failures and its reduction techniques

5.1 Reasons for Failure 5.2 Failure Reduction Technique

CHAPTER I INTRODUCTION

Transformers transfers electrical energy between circuits completely insulated from each other. During this transfer of energy voltage level may be stepped up/down (depending on the tums) with consequent decrease/increase currents from primary to secondary windings, thus maintaining the power transferred constant. Therefore when voltage is stepped up the current is stepped down. This makes it possible to use very high voltages for transmission lines resulting in a lower current. Higher voltage and lower current reduce the conductor size, transmission line losses and improves the voltage regulation as well. Transformers have made possible economic delivery of electric power over long distances.

In our country the electrical energy is usually generated at 11 or 15.75 or 2l kv, stepped up to l32;220;400 or 765 kV with the help of step up transformers for transmission and then stepped down the voltage to ll kV for feeding distributing transformers stepping down the voltage further to 400/230 volts for the consumer uses.

PRINCIPLE OF TRANSFORMER:

The physical basis of the transformer involves the Faradays laws of electro magnetism in which an alternating flux induces voltage in the coil and Lenz Law which states that the effect produced will oppose the cause. It consists of two inductive coils which are electrically separated but magnetically linked through a path of low reluctance by mutual induction. If one coil is connected to the source of alternating voltage an alternating flux is set up in the laminated core most of which is linked with the other coil produces mutually induced emf. If the second coil circuit is closed, a current flows in it and so electric energy is transferred from the primary to secondary coil.

CHAPTER ll CONSTRUCTION and PARTS OF TRANSFORMER

2(a) Transformer construction: The transformer is simple in construction and consists of magnetic circuit linking with two windings known as primary and secondary windings. Besides magnetic circuit and windings it consists of a suitable container for the assembled core and windings such as a tank, a suitable medium for insulating the core and the windings from its container such as transformer oil, suitable bushings (either of porcelain , oil filled or condenser type) for insulating and bringing out terminals of the windings from the transformer tank, temperature gauge for measurement of temperature of hot oil or hottest spot temperature and oil gauge to indicate the oil level inside the tank. Transformers are also provided with the conservator tank in order to slow down deterioration of oil and keep the main tank full of oil.

Fig.2.1: Cut View of a Power Transformer

2(b) Parts of a transformer: 2.1. Transformer tank: Small capacity tanks are fabricated from welded sheet steel, while larger ones are assembled from plain boiler plates or cast aluminum parts, usually mounted on a shallow fabricated steel base. The lids of these transformer tanks can be of cast iron, a water proof gasket being used at the joints. For cooling purpose, cooling tubes are welded with the tank, but in case of radiators. Separate radiators are individually welded and then bolted-on to the transformer. A tank must withstand the stresses developed by jacking and lifting and shall be no longer than necessary to accommodate the core, windings and internal connections with appropriate electrical clearance.

2.2. Core Assembly: Core Assembly forms the magnetic circuit linking the two windings of the transformer. Those parts of the, magnetic circuit which carry the transformer windings, are called the limbs or legs, and those parts which connect the legs and serve for closing the magnetic circuit are termed yokes. The core material used in its construction should satisfy:  

Maximum flux is created with minimum magnetizing current Minimum core loss.

The use of steel in magnetic circuit introduces iron or core loss but ensures a high permeability of the magnetic circuit. Because of the high permeability the magnitude of exciting current necessary to create the required flux in the core is small. The presence of steel core causes 100% of the magnetic flux created by the primary o be linked with secondary. The magnetic frame of the transformer is built up of laminated hot rolled or cold rolled oriented steel consisting of 3.5% silicon. The higher content of silicon increases the resistivity of the core, thereby reducing the eddy current core loss.

As the flux in the core is pulsating one ,it becomes necessary that the transformer cores are laminated and the laminations should be insulated and made as thin as possible in order to minimize the eddy current loss. 2.3. Winding Assembly: The most important features that the windings of a transformer should possess are:

a) The winding should be economical both as regards initial cost, with a view to the market availability of copper b) The efficiency of the transformer in Service c) The heating conditions of the windings should meet standard requirements d) The winding should be mechanically stable in respect to the forces appearing when sudden short circuit of the transformer occurs e) The winding should have the necessary electrical strength in respect to the over voltages.

Transformer windings are made of solid or stranded copper or aluminum strip conductors. Heavy current capacity needs conductors of large cross section to reduce eddy current losses in the conductors, several small wires or parallel straps are preferred to one large strap. This gives rise to unequal reactance components the conductor which can be eliminated by transposition of conductors.

2.4 Bushings: Bushings are incorporated to bring the Extra High voltage winding terminations through the cover of the transformer tank. Variety of bushings used for various voltage classes were mentioned below: Voltage

Type of bushing

Class