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.

Solid core insulators : Testing

The standard specifications followed are IS 2544 and IEC 60168

Tests are categorised in three parts

• Routine test
• Acceptance test
• Type tests
• Routine test
• Visual examination

Bending load test at 50% of the load for 1 minute in all 4 directions

• Acceptance test
• Visual examination
• Dimensional verification
• Temperature cycle test
• Mechanical Failing load test
• Porosity test
• Galvanization test

Like in disc and Pin insulators, here too acceptance tests are carried out on sampled pieces. IS/IEC standards specify the sampling plan. The sample as per the standard for a lot of below 500 Nos of insulators is not specifically mentioned. Instead this is to be decided between the buyer and the supplier. Normally 1% of the total number of offered quantity is selected for tests

Tests for Disc and Pin insulators

Porcelain Insulators - String and Rigid type: testing

Before explaining about testing or inspection of insulators, we have to know about the classification of insulators based on its electrical strength. There are two types of insulators that has been classified for the purpose of identifying the tests to be carried out. They are type A and Type B. All tests need not be done on all the insulators

Type A insulators are those insulators in which the length of the shortest puncture path thro the solid insulating material is equal to the half the length of the shortest flash over path outside the insulator ..

Type B insulators are those insulators in which the length of the shortest puncture path thro the solid insulating material is equal to less than the half the length of the shortest flash over path outside the insulator ..

Tests are classified based on the types. As per the standards, Class A insulators are subjected to only mechanical tests during the routine or acceptance tests and all the Electrical tests are prescribed as type tests on them

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.

Power Frequency High Voltage Test on PT

This is the test that determines the healthiness of insulation A high voltage at the power frequency is applied on one of the windings with the other being shorted together and earthed along with the tank. HV test voltage varies for VTs used in Earthed and Un-earthed systems. In earthed system, one end of the primary of the PT is connected to the earth through an earth link. whereas in the unearthed system, primary is not earthed. In case of earthed system, the high voltage applied on the primary side will be only 3KV irrespective of its rated voltage. In case of Un-earthed system, the High voltage is applied on the primary side with secondary and tank are earthed together.

For eg: If the test is done on the 33KV side of a PT, 70KV is applied on the winding with the secondary shorted and connected to earth. This voltage is maintained at the terminals for one minutes, during which there shall not be any flash over or collapse of voltage.

INSTRUMENT TRANSFORMERS

Instrument transformers , like conventional transformers, work on the principle of mutual induction. Instrument transformers, unlike conventional transformers: are used for measuring high voltage and current parameters in electrical circuits, are also used for obtaining electrical feed backs to protection devices in the electrical circuits.

There are two types of instrument Transformers.

1.Current transformers ( IS 2705 , IEC 60044-1)

2.voltage transformers (IS 3156, IEC 60044-2)

CURRENT TRANSFORMERS

Current transformers are used to step down high current to very low current that can be handled comfortably.

WHY CURRENT TRANSFORMERS?

• High currents can not be measured directly using ammeters due to limitations of current carrying capacity of the measuring instruments.
• Also, high currents can not be used in protective relays. To allow high current to pass, larger size of conductor is required .
• This leads to practical difficulties in handling the devices
• The practical solution to this difficulty is to step down the current to a very low value that can be as low as 1 amp or 5 amp.
• Unlike a regular transformer, a current transformer steps down only current.
• Voltage developed in the secondary will be negligibly less
• (ie, current in the secondary x resistance of the winding)
• The basic principle of CT functioning is ampere-turn matching
• AT primary = AT secondary