Three Phase Transformer

       Transformer is a static device which transfers the electrical energy from one circuit to another circuit through inductively coupled conductors with change in electrical parameters (like current and voltage) and with same frequency. Main parts of a three phase transformer are as follows-

1-Core

2-Windings

3-Conservator

4-Buchholz relay

5-Breather

6-Tape changer

7-Tank

8-Lightening arrester (Rod arrester)

Transformer-Core-Coil-Assembly-

 Core consists of “Cold Rolled Grain Oriented Steel”. Steel is used in the form of small thin sheets and these thin sheets are assembled to form the complete core as shown in figure.CRGO is used in construction of core due to its magnetic properties are better than the steel in simple form. Losses in transformer are also be reduced by using of CRGO laminations.
 

Complete transformer assembely with windings-

completely assembled core

There is two types of windings in transformer,they can used in both sides primary and secondary.

1-High tension winding (H.T winding)

2-Low tension winding (L.T winding)

In case of Distribution or Step down Transformer  H.T is used in primary side and L.T is used in secondary side. In Step up transformer ,the connection is vice-versa. H.T always carries high value of voltage but High Current flows in L.T side.so the wire used in High Voltage or H.T winding always have less gauge diameter then  the wire used in L.T winding. Therefore we have required large no. of turns in H.T winding side.

Winding Connections

Conservator -

Conservator is the transformer oil storage tank,If the main tank oil reduce the conservator will feed the oil to main tank. and if the transformer oil gets expanded than that expanded oil will go to conservator.

Conservator

Buchholz relay and its working-

The Buchholz relay has two oil-filled chambers with floats and relays arranged vertically one over the other. If high eddy currents, local overheating, or partial discharges occur within the tank, bubbles of resultant gas rise to the top of the tank. These rise through the pipe between the tank and the conservator. As gas bubbles migrate along the pipe, they enter the Buchholz relay and rise into the top chamber.

As gas builds up inside the chamber, it displaces the oil, decreasing the level. The top float descends with oil level until it passes a magnetic switch which activates an alarm. The bottom float and relay cannot be activated by additional gas buildup. The float is located slightly below the top of the pipe so that once the top chamber is filled, additional gas goes into the pipe and on up to the conservator. Typically, inspection windows are provided so that the amount of gas and relay operation may be viewed during testing.
If the oil level falls low enough (conservator empty), switch contacts in the bottom chamber are activated by the bottom float.

Inside View of Buchholz relay

Buchholz Relay

These contacts are typically connected to cause the transformer to trip. This relay also serves a third function, similar to the sudden pressure relay.

A magnetically held paddle attached to the bottom float is positioned in the oil-flow stream between the conservator and transformer tank. Normal flows resulting from temperature changes are small and bypass below the paddle. If a fault occurs in the transformer, a pressure wave (surge) is created in the oil. This surge travels through the pipe and displaces the paddle. The paddle activates the same magnetic switch as the bottom float mentioned above, tripping the transformer.

Breather-

A transformer breather is an accessory of an oil filled type transformer which is attached into the oil conservator tank; this serves as the breathing point of the unit, that when the insulating oil of the transformer gets heated up, it expands and goes back to the conservator tank and subsequently pushes the dry air out of the conservator tank through the breather which is filled with silica gel, when the oil cools down, it retracts and sucks fresh air from the atmosphere through the breather and from this point, the silica gel dries up the air that goes back in to the conservator tank.

Tap changer and its application-

A transformer tap is a connection point along a transformer winding that allows a certain number of turns to be selected. This means, a transformer with a variable turns ratio is produced, enabling voltage regulation of the output. The tap selection is made via a tap changer mechanism.

Rod gap arrester-

It is a very simple type of diverter and consists of two 1.5 cm rods, which are bent at right angles with a gap in between as shown in Fig. One rod is connected to the line circuit and the other rod is connected to earth. The distance between gap and insulator (i.e. distance P) must not be less than one third of the gap length so that the arc may not reach the insulator and damage it.

Generally, the gap length is so adjusted that breakdown should occur at 80% of spark-voltage in order to avoid cascading of very steep wave fronts across the insulators.

Typical rod gap arrester

The string of insulators for an overhead line on the bushing of transformer has frequently a rod gap across it. Fig 8 shows the rod gap across the bushing of a transformer. Under normal operating conditions, the gap remains non-conducting. On the occurrence of a high voltage surge on the line, the gap sparks over and the surge current is conducted to earth. In this way excess charge on the line due to the surge is harmlessly conducted to earth.