11.docx

1 General requirements for tests -temperature between 10 °C and 40 °C

1.2-Tests shall be made at the manufacturer's works.

- The test basis for all characteristics other than insulation is the rated condition. -Tapped windings shall be connected on their principal tapping. -All measuring systems used for the tests shall have certified according to the rules of 4.11 of ISO 9001. - All external components and fittings that are likely to affect the performance of the transformer during the test shall be in place.

2 Measurement of winding resistance 2.1 General The winding resistance and the terminal between the temp. which measured and which should be recorded and We use the current in the measure .We also should care that the self induction now it’s minimized .

2.2 Dry-type transformers first we let the transformer in a constant ambient temperature for at least 3 h Winding resistance and winding temperature shall be measured at the same time. The winding temperature shall be measured by sensors placed in a duct between the high-voltage and low voltage windings.

2.3 Oil-immersed type transformers first we let the transformer under oil without excitation for at least 3 h then measured the oil temperature and the temperature of the winding shall be the same as the average oil temperature the difference in temperature between the top and bottom oil should be small. To obtain this result more rapidly, the oil may be circulated by a pump.

3 Measurement of voltage ratio The voltage ratio shall be measured on each tapping. The polarity of single-phase transformers and the connection symbol of three-phase transformers shall be checked.

4 Measurement of the harmonics of the no-load current The harmonics of the no-load current in the three phases are measured and the magnitude of the harmonics is expressed as a percentage of the fundamental component.

5 Auxiliary circuits insulation test After the tap-changer is assembled on the transformer, a power frequency test shall be applied to the auxiliary circuits as specified in IEC 60076-3.

6 Transformer vector group test The vector group of transformer is an essential property for successful parallel operation of transformers. Hence every electrical power transformer must undergo through vector group test of transformer at factory site for ensuring the customer specified vector group of transformer.

6.1 Procedure of Vector Group Test of Transformer Let’s have a YNd11 transformer. Connect neutral point of star connected winding with earth. Join 1U of HV and 2W of LV together. Apply 415 V, three phase supply to HV terminals. Measure voltages between terminals 2U-1N, 2V-1N, 2W-1N, that means voltages between each LV terminal and HV neutral. Also measure voltages between terminals 2V-1V, 2W-1W and 2V-1W. For YNd11 transformer, we will find, 2U-1N > 2V-1N > 2W-1N 2V-1W > 2V-1V or 2W-1W.

7 Open and Short Circuit Test Open and short circuit tests are performed on a transformer to determine the: 1. Equivalent circuit of transformer. 2. Voltage regulation of transformer. 3. Efficiency of transformer.

7.1 Open Circuit Test open circuit test on transformer is used to determine core losses in transformer and parameters of the shunt branch of the equivalent circuit of the transformer.

The ammeter reading gives the no load current Ie. As no load current Ie is quite small compared to rated current of the transformer, the voltage drops due to this current that can be taken as negligible. let us consider wattmeter reading is Po.

Where, Rm is shunt branch resistance of transformer. If, Zm is shunt branch impedance of transformer.

Therefore, if shunt branch reactance of transformer is Xm,

These values are referred to the LV side of the transformer due to the tests being conducted on the LV side of transformer. These values could easily be referred to HV side by multiplying these values with square of transformation ratio.

7.2 Short Circuit Test

short-circuit test of a transformer is used to determine copper losses in the transformer at full load. It is also used to obtain the parameters to approximate the equivalent circuit of a transformer.

Let’s say, voltmeter reading is Vsc. The watt-meter reading indicates the input power during the test. As we have short-circuited the transformer, there is no output; hence the input power here consists of copper losses in the transformer. Since the applied voltage Vsc is short circuit voltage in the transformer and hence it is quite small compared to the rated voltage, so, we can neglect the core loss due to the small applied voltage. Hence the wattmeter reading can be taken as equal to copper losses in the transformer. Let us consider wattmeter reading is Psc.

Where, Re is equivalent resistance of transformer. If, Ze is equivalent impedance of transformer.

Therefore, if equivalent reactance of transformer is Xe.

These values are referred to the HV side of the transformer as the test is conducted on the HV side of the transformer. These values could easily be converted to the LV side by dividing these values with the square of transformation ratio.

8 Insulation resistance test of transformer Insulation resistance test of transformer is one of the most important transformer tests. insulation resistance test of transformer is divided into three parts: Firstly… Measure insulation resistance between low voltage windings and the main body of the transformer. We use MEGGER test at low voltage side    

Adjust MEGGER tester at 1000 V DC. Record the value of insulation resistance measured by MEGGER tester after applying 1 min. Repeat this test between another phase and transformer body as example (a & TR body , b & TR body , c & TR body) Insulation resistance of new transformer should be greater than 1 Giga ohm and old transformer should greater than 300 Mega ohm.

Secondly … Measure insulation resistance between low voltage windings and high voltage windings. We also use MEGGER tester.    

Adjust MEGGER tester at 2500 V DC. Record the value of insulation resistance measured by MEGGER tester after applying 1 min. Repeat this test between other phases as example (A & a , A & b , A & c , A & n , B & a , B & b , B & c , B & n , C & a , C & b , C & c , C & n ). Insulation resistance at all mentioned phases should greater than 1 GIGA ohm.

Thirdly … Measure insulation resistance between high voltage windings and the main body of the transformer. We use MEGGER test at low voltage side as shown in fig.    

Adjust MEGGER tester at 5000 V DC. Record the value of insulation resistance measured by MEGGER tester after applying 1 min. Repeat this test between another phase and transformer body as example (A & TR body , B & TR body , C & TR body) Insulation resistance of new transformer should be greater than 1 Giga ohm and old transformer should greater than 300 Mega ohm.

9 Insulation Dielectric Test The dielectric test of transformer is generally performed in two different steps, likewise, separate source voltage withstand test and induced voltage withstand test of transformer, which we have discussed one by one below.

9.1 Separate Source Voltage Withstand Test of Transformer This dielectric test is intended to check the the ability of main insulation to earth and between winding.

Procedure 1. All three line terminals of the winding to be tested are connected together. 2. Other winding terminals which are not under test and also tank of the transformer should be connected to earth. 3. Then a single-phase power frequency voltage of shape approximately sinusoidal is applied for 60 seconds to the terminals of the winding under test. 4. The test shall be performed on all the windings one by one. 5. The test is successful if no break down in the dielectric of the insulation occurs during test.

9.2 Induced Voltage Test of Transformer

The induced voltage test of transformer is intended to check the inter turn and line end insulation as well as main insulation to earth and between windings1. Keep the primary winding of transformer open circuited. 2. Apply three phase voltage to the secondary winding. The applied voltage should be twice of rated voltage of secondary winding in magnitude and frequency. 3. The duration of the test shall be 60 second. 4. The test shall start with a voltage lower than 1/3 the full test voltage, and it shall be quickly increased up to desired value. 5. The test is successful if no break down occurs at full test voltage during test.