Transformers:
Transformer forms the heart of any electrical system. This is 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 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 i.e., 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 loss 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 minimize 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.
This write up doesn't intend to dwell at length with the design or constructional aspects of a transformer. Instead, I shall describe in brief, the different tests carried out on transformers.
Tests on a transformer can be carried out in three categories:
1) Type tests or special tests
2) Routine tests
3) Factory acceptance tests
Type tests:Most common type tests that are carried out on a transformer are
a) Lightning Impulse test
b) Dynamic short circuit tes
c) Temperature rise test
In addition to this, there a number of other special tests like Tan delta measurement, Zero phase sequence measurement, Harmonics measurement, Sweep Frequency Response Analysis. These tests are categorized as optional and are carried out on transformers based on the customer requirements
Routine tests and Factory acceptance tests:
Normally these tests are same. Routine tests are carried out by the manufacturer's quality control department on 100% quantity and acceptance tests are carried out in the presence of the buyer's representative. Buyer witnesses the routine tests on randomly selected samples according to customer's discretion or as per an agree Quality assurance Plan (QAP)
Tests on transformers are carried out as mentioned in the Indian standard specification IS 2026 or International Electro-technical commission (IEC - 76).
Routine tests that are to be carried out on a transformer are
1) Visual examination
2) Dimensional verification
3) Turns ratio measurement
4) Vector group verification
5) Magnetic balance check
6) Insulation value measurement
7) Power frequency High voltage test
8) Induced over voltage test
9) No Load Loss and No load current measurement
10) Load Loss and Impedance voltage measurement
11) Winding resistance measurement
12) Tests on accessories (if any) like Bucholz relay, Winding temperature and oil temperature indicators, Magnetic Oil gauge, Pressure release valves.
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