Transformers : Testing

 

Transformer forms the heart of any electrical system. This is an equipment that helps electrical engineers tremendously to deal with the transmission loss and the heavy drop of voltage at the customer end.

A transformer is essentially an equipment that steps up or steps down alternating (current)  voltages without changing the frequency. Any transformer can be used as a step up or a step down transformer depending upon the requirement. The higher voltage side carries less current and the lower voltage side carries higher current ie, N1/N2 = VI/V2 = I2/I1 whereas N1 and N2 are number of turns on primary and secondary sides, V1 and V2 are voltages on Primary and secondary and I1 and I2 are current in primary and secondary. The major parts of a transformer are two windings – primary and secondary and a core that carries the flux. The power supply on one of the windings induces a magnetic field and due to this, a voltage is induced in the other winding. This is the simple statement of faraday's law of induction. To carry the magnetic field that causes induction in the other field, a highly permeable magnetic material is used, so that there is no loss of flux between the windings. Any ferrous material can do this job, but to reduce loss of flux and the resultant loss in energy. In addition, the losses in the core material causes excessive heating of core, which is not desirable. So, it is important to have the best quality core in the transformer that can carry maximum flux without incurring too much loss. In the modern days, there are a number of types of core materials that really minimise the core to very great extend. The most commonly used and easy to handle among the core material is Cold Rolled Grain Oriented (CRGO) steel.

Transformer Breathing

Oil filled transformers 'breath'. As the temperature increases, volume of oil increases and when temerature goes down, its volume decreases. So, during expansion, volume of oil increases. As a result, air, present inside the transformer tank is displaced by oil. Now, this air has to be evacuated from the tank. Otherwise, the air will get compressed which will result in increased pressure inside the tank. To make way for the air to the atmosphere, a pipe is provided from the top side of the conservator. Displaced air passes to atmosphere through this pipe. While volume of oil decreases, air enteres from atmosphere through this pipe.

A "breather" is provided at the end of the pipe from where the air enters/exit into/from the transformer tank. This breather is a chamber filled with silica gel which can absorb water ( moisture), that may be present in the air. Air must be free from moisture while it enters the transformer. Presence of moisture may contaminate transformer oil. Contaminated oil may, in turn, damage the insulation - by bringing down the value of insulation resistance

There is a cup at the bottom of the breather. This cup is filled to a certain level with transformer oil. Air exits/enters through this oil. Oil is provided as an extra filter.

The bubble that we saw in the vedio clipping of the Breathing action is the exit of air from the tank through the silica gel and then through the transformer oil in the cup.

Please note that the pipe which is called the breather pipe will not carry any oil. This is meant only to carry air.

There are certain small capacity transformers that are completely sealed. There will be no conservator in these transformers. These transformers are filled with transformer oil to a certain level and the remaining part of the tank will be filled with nitrogen. While transformer oil expands, pressure increases. Due to pressure, nitrogen dissolves in the oil. This neutralises the pressure. While oil cools down, nitrogen will be leberated from the oil and thus complimenting the decreased pressure.

This article is contributed by Sri. Natesan P. P.

If Bucholz Relay acts?

If a transformer breaker trips with bucholz relay action, the transformer should not be charged before conducting detailed investigation. The high voltage and low voltage sides of the transformer should be isolated. DC supply to and from Bucholz relay is also to be isolated. The gas collected in the Bucholz relay has to be tested as detailed below.

1. Examine Bucholz relay.
2. Note and record the quality and colour of gas collected. A colour less accumulation usually indicates the presence of air.
3. Release a small quantity of gas and smell. Peculiar smell other than oil smell is not desirable.
4. Prepare silver nitrate solution of any concentration in a oil bottle (the solution must be clear).
5. Allow some quantity of gas to bubble through this clear solution of silver nitrate.
6. If there is no change in the colour of the solution, it generally indicates that the gas is merely air. Whitish precipitation-decomposition of paper. Yellow precipitation-decomposition of wood. deep grey-gas of over heated oil due to burning of iron. Black-gas of decomposed oil due to electric arc.
7. If a precipitate is obtained, thorough examination of the transformer should be made at the earliest opportunity.

Source: Engineers Diary 2003