Transformer Winding Resistance Test by DC Current

How Do You Do Transformer Winding Resistance Test by DC Current?

Considering the basic principle of operation is to inject a DC current through the transformer winding to be measured, and then read the voltage drop across that winding. 

We may refer to the formula for DC voltage across the transformer as, V = I * R  

where, 

V= DC voltage across transformer winding;

I = DC current through transformer winding; &

R = desired calculated resistance of the transformer winding.

How to Prepare for Transformer Winding Testing?

1. Selecting the Proper Current Range, transformer manufacturers typically recommend that the current output selected should not exceed about 10% of the rated winding current;
2. Cause erroneous readings due to heating of the winding; 
3. Always choose the highest current output possible for the expected resistance value; 
4. Typical ranges are 0.1-10 % of rated winding current;
5. The temperature of the winding shall be assumed to be the same as the temperature of the insulating liquid;
6. It should not be assumed that the winding is at the same temperature as the surrounding air;
7. For star-connected winding, the resistance shall be measured between the line and the neutral terminal;
8. For delta-connected winding, winding resistance measurement shall be done between pairs of line terminals.

As in delta connection, the resistance of individual winding can not be measured separately, the resistance per winding shall be calculated with the following formula: 

Resistance per winding = 1.5 x Measured value 

The resistance is measured at ambient temperature and then converted to resistance at 75˚C or 85˚C  for all practical purposes of comparison with specified design values, previous results, and diagnostics.

Winding Resistance at reference temperature of 75° C or 85° C 

R = Rm (234.5 + Ref. Temp)/(234.5 + Meas. Temp) 

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How to Measure Winding Resistance at Delta Side?

Average of the (3) Phase measurements multiplied by 9/2 or 4.5. This is the 3-phase sum resistance at the temperature at which the resistance was measured.  

Resistance measured: H1-H3 = 0.665, H2-H1 = 0.664, H3-H2 = 0.666 at an oil

temperature of 26°C.  

The average resistance of the (3) phases is 0.665. This (0.665) multiplied by 4.5 equals 2.993. The unit is a 55/65 rise transformer, therefore to correct the resistance to the reference temperature of 75°C, use the correction formula: 

 (234.5 + 75) / (234.5 + 26) = 1.1881

So the corrected 3-phase sum resistance is, 2.993 * 1.1881 = 3.554 ohms;

Working backward from the test report: HV position 3 resistance reported on the test report is 3.554. Determine the oil temperature of the transformer at the time of resistance measurement (say it is 26°C again).   3.554 / 1.1881 = 2.991  

2.991 / 4.5 = 0.665  

The measured resistance between two bushings (one phase) should be approx. 0.665 ohms.  

How to Measure Winding Resistance at Star or Y-Side?

 If measuring between L-N, then add each of the L-N measurements together. This is the 3-phase sum resistance at the measured temperature. The resistance needs to be corrected the same as for the Delta connection above.

If measuring a Wye connection L-L, then add each of the L-L measurements and divide by 2. This is the 3-phase sum resistance at the measured temperature. Correct resistance to reference temperature as above. 

How Measure Line to Neutral Resistance?

Resistance measured between X1-XO = 0.0394, X2-XO = 0.0393, X3-XO = 

0.0397 at an oil temperature of 26°C. The 3-phase sum is:

0.0394 + 0.0393 + 0.0397  = 0.1184;

Corrected to 75°C: 

0.1184 * 1.1881 = 0.1407 ohms. 

How Measure Line-to-Line Resistance?

Resistance measured between X1-X2 = 0.0782, X2-X3 = 0.0783, X3-X1 = 0.0785

at an oil temperature of 26°C. The 3-phase sum is: 

(0.0782 + 0.0783 + 0.0785) / 2 = 0.1175;

Corrected to 75°C”

0.1175 * 1.1881 = 0.1396 ohms .

The 3-phase sum of the resistance between the L-N and L-L measurements are 

slightly different because the L-N measurement has the XO lead cable in the circuit. 

How Confirm the Measurement is Correct?

Comparing the factory value or factory standard and the previously measured value can help to confirm whether the measured value is satisfactory or not.

So, note the below points and compare:

1. Compared to the original factory measurements
2. Compared to previous field measurements
3. Comparing one phase to another 
4. Industry-standard (factory) permits a maximum difference of  0.5% from an average of three-phase wingdings.  
5. Field readings may vary slightly more than this due to the many variables. If all readings are within 1% of each other, they are acceptable. 

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When Do Need to carry the Transformer Winding Resistance Test?

Test In the factory: 

1. To know whether the manufacturing is correct or not;
2. To calculate conductor losses; 
3. Calculations of DC component of conductor losses;
4. Calculation of winding temperature at the end of a temperature test cycle. 
5. Measuring the resistance of the winding assures that the connections are correct and the resistance measurements indicate that there are no severe mismatches or opens. 

Test on Site:

To evaluate possible winding damage, such as short circuits between winding or between turns; open circuits; contact problems; condition of the tap changer. 

Test at Installation:

1. The risk of damage is significant whenever a transformer is moved damage will often involve a current-carrying component such as the LTC,  DETC or a connector,
2. Serves as a verification of the manufacturers work,
3. Installation measurements should be filed for future reference 

Transformer Routine Maintenance:

Transformer routine maintenance is required to verify operating integrity and to assure reliability. 

1. To detect damage to the transformer;
2. To determine if it is safe to re-energize;
3. To determine if corrective action is necessary;
4. To establish priority of corrective action. 

At Internal Transformer Inspections: 

Internal inspections are expensive due primarily to the cost of oil processing. When such opportunities do present themselves the inspection should be  planned and thorough.