Test on Transformer

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Test on transformerTesting of Transformers The structure of the circuit

equivalent of a practical transformer is developed earlier.

The performance parameters of interest can be obtained by

solving that circuit for any load conditions. The equivalent

circuit parameters are available to the designer of the

transformers from the various expressions that he uses for

designing the transformers. But for a user these are notavailable most of the times. Also when a transformer is

rewound with different primary and secondary windings the

equivalent circuit also changes. In order to get the

equivalent circuit parameters test methods are heavily

depended upon. From the analysis of the equivalent circuit

one can determine the electrical parameters. But if thetemperature rise of the transformer is required, then test

method is the most dependable one.

There are several tests that can be done on the

transformer :-

1.Winding resistance test2. Polarity Test

3 .Open Circuit Test

4 .Short Circuit Test

5. Dvdf Test

6. Hv Test

7. T.T.R Test


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1. Winding resistance test:-

Winding resistance measurements in transformers are

of fundamental importance for the following purposes:- Calculations of the I2R component of conductor

losses.- Calculation of winding temperature at the end of a

temperature test cycle.- As a base for assessing possible damage in the field.

Transformers are subject to vibration. Problems orfaults occur due to poor design, assembly, handing,poor environments, overloading or poor maintenance.Measuring the resistance of the windings assures that

the connections are correct and the resistance

measurements indicates that there are no severemismatches or opens. Many transformers have tapsbuilt into them. These taps allow ratio to be increased

or decreased by fractions of a percent. Any of theratio changes involve a mechanical movement of a

contact from one position to another. These tapchanges will also be checked during a winding

resistance test.Regardless of the configuration either wye or delta,

the measurements are made phase to phase andcomparisons are made to determine if the readings

are comparable. If all readings are within one percentof each other, then they are acceptable. Keep in mindthat the purpose of the test is to check for gross

differences between the windings and for opens in theconnections. The tests are not made to duplicate the


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readings of the manufactured device which was testedin the factory under controlled conditions and perhapsat other temperatures

2. Polarity Test:-

transformer polarity test are used two know about the

poalrity of a transformers .for these purpose to

transformer are used and his low voltage winding and

high voltage winding are coonected in parallel to each

other the secondary windings of a transformer which

has commonly high voltage winding fuse are

connected to his secondary side of a transformer a manpoint to be noted here that a wire used for fuse are so

small rating similarly when the polarity of

autotransformer same no current will be flow as a

reason the dont blow. otherwise when the polarity are

different the current will be flow and fuse will be blow

out


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3. Open Circuit Test

Method :-

The secondary of the transformer is left open-circuited. A

wattmeter is connected to the primary. An ammeter is

connected in series with the primary winding. A voltmeter

is optional since the applied voltage is same as the

voltmeter reading. Rated voltage is applied at primary.

If the applied voltage is normal voltage then normal fluxwill be set up. As the Iron loss is a function of applied

voltage, normal iron loss will occur. Hence the iron loss is

maximum at rated voltage. This maximum iron loss is

measured using the wattmeter. Since the impedance of the

series winding of the transformer is very small compared to

that of the excitation branch, all of the input voltage isdropped across the excitation branch. Thus the wattmeter

measures only the iron loss. It should be noted that the iron

losses consist of the hysteresis loss and the eddy current

loss. This test only measures the combined loss. Although

the hysteresis loss is less than the eddy current loss, it is not

negligible. The two losses can be separated by driving the

transformer from a variable frequency source since thehysteresis loss varies linearly with supply frequency and the

eddy current loss varies with the square.

Since the secondary of the transformer is open, the

primary draws only no load current which will have

some copper loss. This no load current is very small

and because the copper loss in the primary is


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proportional to the square of this current, it is

negligible. There is no copper loss in the secondary

because there is no secondary current.

Current, voltage and power are measured at the

primary winding to ascertain the admittance and powe

factor angle.

Another method of determining the series impedance

of a real transformer is the short circuit test.

Calculation:-

The current is very small.

If is the wattmeter reading then,

The above equation can be rewritten as,
http://en.wikipedia.org/wiki/File:Open_circuit_test.png


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Thus,

Impedance:-

By using the above equations, and can be

calculated as,

Thus,

or

Admittance:-

The admittance is the inverse of impedance. Therefore,

The conductance can be calculated as,


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Hence the suspectance,

or

Here,

is the wattmeter reading

is the applied rated voltage

is the no load current

is the magnetizing component of no load current

is the core loss component of no load current

is the exciting impedance

is the exciting admittance

4.Short Circuit Test:-


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The test is conducted on the high voltage (HV) side of

the transformer where the low voltage (LV) side or thesecondary is short circuited. The supply voltage

required to circulate rated current through the

transformer is usually very small and is of the order of

a few percent of the nominal voltage and this voltage

is applied across primary. The core losses are very

small because applied voltage is only a few percentage

of the nominal voltage and hence can be neglected.Thus the wattmeter reading measures only the full load

copper loss.

Procedure:-
http://en.wikipedia.org/wiki/File:Short_Circuit_test.jpg


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For carrying short circuit test on power transformer:-

1. Isolate the power transformer from service.

2. Remove HV/LV jumps and disconnect neutralfrom earth/ground.

3. Short LV phases and connect these short circuited

terminals to neutral

4. Energise HV side by LV supply.

5. Measure current in neutral, LV line voltages, HV

voltage and HV line currents

Calculations:-

is the Full load copper loss

is the applied voltage

is the rated current

is the resistance as viewed from the primary

is the total impedance as viewed from the primaryis the reactance as viewed from the primary


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Analysis:-

If neutral current is near to zero transformer windings

are operational.

If neutral current is higher or equal to line current

between LV phase one of the winding is open.

5. Dvdf Test:-


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The DVDF (double voltage double frequency) test is

an overvoltage withstand test. It is performed byapplying a higher than nominal voltage (2 times) to the

transformer for a relatively short period of time

(usually 1 minute). Since a prolonged (significant)

overvoltage will cause the transformer to overexcite,

the volts per hertz are kept constant by applying a

higher than nominal frequency (2 times in this case).


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6. Hv Test:-

Seperate source voltage withstand test (High Voltagetests on HV & LV). This test checks the insulation

property between Primary to earth, Secondary to earth

and between Primary & Secondary.

HV high voltage test:-LV winding connected together

and earthed. HV winding connected together and given28 KV ( for 11KV transformer) for 1 minute.

LV high Voltage test :- HV winding connected

together and earthed. LV winding connected together

and given 3 KV for 1 minute.

Equipment used:-High Voltage tester ( 100 KV &3KV).


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Test on Transformer