Transformer

Why Low Voltage (LV) winding is placed near to the transformer core?

Anwer: For a given conducting material insulation required depends on the voltage. Hence if High Voltage (HV) winding is placed near to the transformer core, more insulation is required to insulate between the transformer core and the High Voltage (HV) winding. This results in increase in the cost of the insulation material and also size of the transformer increases significantly. Thus Low Voltage (LV) winding is placed near the core which requires less insulation between the core and LV winding.

What are the different insulation materials used in power transformers?

Answer: In power transformers the primary insulation medium and cooling medium is transformer oil. It serves the purpose of both insulation and cooling. Apart from transformer oil different insulation materials employed are oil impregnated paper, press board, wood, mica, and asbestos.

What is Transformer Breathing?

Answer: When transformer under full load, power transformer oil present in the transformer heats up and gets expand. During this process gas at the top of the oil gets expel out in to the conservator present at the top of the power transformer along with hot oil and cool oil from conservator comes down. This process is called breathing out of the transformer.

When the load on the power transformer is removed or during no load condition, transformer oil cools and air is drawn in to the transformer. This is called breathing in of the transformer. The incoming air may consists of moisture which should be removed else results in the deterioration of the dielectric strength of the transformer oil. Hence the air entering the power transformer is made to pass through the breather where moisture is removed from air through silica gel

Transformer Testing methods

Transformer Tests:

Tests on the transformers are carried for verifying the capabilities of transformer to withstand Thermal stresses, Dielectric stresses, short circuit electro-dynamic stresses and environmental stresses.

Routine tests:

These are carried out on every transformers before dispatch to ensure that it is in accordance with the specifications.Some of the tests come under routine test include:

·                     Measurement of the winding resistance

·                     Verifying the polarity of the windings

·                     Measurement of load losses and the impedance voltages

·                     Measurement of no load losses and no load no load current.

·                     Electrical tests at the power frequency

Type Tests:

Type tests are performed on the first transformer of one type and are intended to check the design characteristics. It is presumed that every transformer would also comply with the type test, since its design is identical.

·                     Temperature rise test

·                     Tests of ability to withstand full wave impulse

·                     Tests for switching impulse withstand

·                     Noise level test

Special Test:

Special tests are conducted in the presence of the purchaser or his representatives as specified in the tender

·                     Partial discharge test

·                     Checking the level of Radio interference voltage

·                     Vibration test

·                     Test on ability to withstand Short -circuit current

·                     Measurement of Noise level

Voltage Regulation of Transformer

Voltage Regulation:

When the secondary of the transformer is loaded (from no load to full load), the secondary voltage changes even though primary supply voltage is held constant at rated value.

% Voltage Regulation = (V_{2(no load)} - V_{2 (full load)})*100 / (V_{2(no load)})

With the rated voltage applied to the primary winding, the secondary  terminal voltage varies with the load current and power factor even the primary voltage is kept constant. This variation is called voltage regulation.

Voltage regulation depends on the voltage drop in the impedance of the transformer, load current and load power factor.

Leading and Lagging power factor regulation:

·                     For lagging power factor load the secondary voltage decreases with increase in the load current (when the transformer is loaded). Thus for lagging power factor loads, the regulation is positive ( voltage drop observed as the load current increases). 

·                     For leading power factor load, the secondary voltage increases slightly with increase in the load current. This type regulation. Thus for leading power factor loads, the regulation is negative (raise in voltage as load current increases) 

For better performance of the system voltage regulation should be as minimum as  possible i.e,  Change in secondary voltage with load should be as minimum as possible when primary voltage is kept constant.