June 2, 2017

Transformer Turns ratio test

As mentioned before, the types of transformer testing… We will show in this article a very important test of the transformer testing. Transformer Turns ratio test

Purpose of this test:

We perform Transformer turns ratio test to:

• Determine the turn’s ratio between the primary and secondary windings.
• Calculate the error ratio between the design value and the actual value, this value shall not exceed limit given in relevant standards, normally 0.5% at no load.

• Detect any short circuit between winding in a single side.
• Verify that transformer winding are connected for the correct vector group.

Precautions before turns ratio test:

• Disconnect the transformer electrically.
• Clean the terminals of transformer.
• Disconnect the voltage source before changing the voltage tap changer.

Steps for transformer ratio testing:

We have two cases to do this transformer testing, Firstly using measurement device as shown in figure:

• On the device, adjust the test device by specifying the vector group of the transformer, for example DY 11.
• Also determine the number of voltage tap changer points, for example 7 points.
•  the amount of voltage step 2.5.
• And determine the normal number of voltage tap changer for example point 4.
•  determine the value of the voltage on the primary winding is 11 kV and value Voltage on the secondary winding is 415 volts corresponding to the point of the normal voltage tap changer which is No. 4
• Then, through the test device, which has two voltage values ​​of 40 volts or 80 volts AC, select one of them.
• Then starts injecting that voltage onto one of the high voltage windings of the transformer.
• Measure the voltage on the two ends of the secondary windings, the ratio of which is the transformer ratio.
• Disconnect the voltage source before changing the voltage tap changer.
• Change The voltage tap changer mode and the test frequency.
• Repeat the test at another tap changer point and compare the output with the existing data of transformer.

Test result:

Measure the voltage ratio for each tapping connection of the transformer. In the test report the specified tapping voltage ratios are stated, as well as the measured ratios. If the error rate exceeds 0.5%, the transformer has failed the test and is rejected.

We can also do this test with another method:

transformer turns ratio test can also done by using an external voltage source 380 volts we carry

• The voltage tap changer is set to position 1
• and connect a three phase voltage source 380 volts AC on the High voltage side.
• Then measure the line voltage on the low voltage side as well as the phase voltage.
• Divide the line voltage of the high voltage side on the line voltage of the low voltage side
• And compare the result of the transformer ratio of point “1” to the voltage tap changer in the transformer nameplate with a measurement result where the error ratio should not exceed ±5.0 %.
• Then we disconnect the voltage source from the transformer.
• With the change of the voltage tap changer mode to mode number “2” and
Repeat the previous steps and disconnect the transformer and change the voltage tap changer mode to “3” and repeat previous steps.
• Then disconnect the adapter and change the voltage switch mode to “4” and repeat previous steps.

The previous steps should be equal to the measured transformer ratio with the existing nominal transformer ratio on the transformer’s nameplate where the error ratio should not exceed ± 5.0% of the nominal transformer ratio.