A Complete Guide To Transformer Winding Resistance Testing

Measurements of winding resistance are an essential diagnosing tool to evaluate probable transformer damages due to poor design, installation, processing, unfavourable climates, overload or lack of maintenance.

The main aim of this testing with a transformer winding resistance meter is to assess whether winding differences or whether connections are open. The measurement of transformer winding resistance ensures that each circuit is efficiently wired and therefore all connections are tight.

Transformer winding resistance changes due to shorted turns, loose connections or worsening transformer tap contacts. Whatever their configurations, they are usually phase-to-phase resistance observations and readings are evaluated to one another to determine whether or not they are reasonable. You can easily contact a transformer winding resistance meter supplier, and get the necessary equipment for yourself.

Precautions During Testing:

It is crucial to understand all safety alerts and take appropriate precautions before executing a transformer winding resistance test. Ensure that all devices to be tested have a correct base and that all high-voltage power systems are processed as powered until the appropriate lockout/drainage procedures have been proven otherwise.

Where the current still circulates through the transformer mostly during the test, it is essential not to consider removing current or power leads. This leads to the development of an extremely high voltage across the area where voltage is broken and a deadly voltage might also generate.

Connecting Test Set:

Together with the main and secondary terminals of the transformer, readings of all windings must be insulated from outer links at each phase. Test device connections must be formulated in the following order:

• Ground - Make sure that only the transformer has been explicitly grounded to the local station and afterwards connect the test soil.
• Test Leads - Attach the voltage output to the test set and confirm the strength of all connections with the test pipe detached from the system under test.
• Connect Transformer - Every configuration of transformers requires various test connections. Further caution should be given that leads cannot fall off during testing or that leads are connected to or too close to each other. In (between) the current leads and the transformer, the voltage lines should be lined up.
• Input Power - Please connect to the test set. Make sure that the available power ground has a low impedance to the ground of the local plant before trying to make this connection.

The following connections could be used for single-phase and simplistic Delta-WYE setups. Please note that every transformer configuration is distinct, and may not cover your particular configuration see the user guide for more information that has come with your test kit.

• Single Phase Transformer
• 3-phase Delta Winding
• 3-phase WYE Secondary Winding
• Dual Winding Test (Single Phase)
• Dual Winding Test (Three-Phase)

The jumper that connects the two windings must be linked to opposite polarities of the transformer to decrease the fundamental saturation period.

When the resistance between the two coils exceeds a factor of 10, measurements which are more accurate by testing each coil individually may be desirable.

Winding Resistance Measurements:

The reading must be monitored and recorded when trying to measure winding resistance after the resistance value is reestablished. In the beginning, resistance values would also "drift" due to transformer inductance, which is most common in large delta windings.

Drifting only should last for a few seconds on comparatively small transformers, the drift may take as little as a minute for single-phase high-voltage transformers; the drifting time needed for large transformers may take a few minutes or longer. Any current changes will keep changing the value of the resistance.

Tap Changer:

Many power transmission and distribution transformers have tap changing devices to increase or reduce the turn ratio contingent on the supply voltage. Due to the mechanical mobility of a change of relation from one situation to another, throughout a winding resistance test, every tap should be verified.

During regular maintenance, due to time limitations or other considerations, it will not always be possible to test every tap. In these cases, only the assigned tap position can measure the strength of each winding.

The transformer must be unloaded between tap changes for the "Off-load" taps. "On-load" tap changers and voltage regulators may well be controlled also with a test set remaining on it while shifting from tap to tap, this not only saves a lot of time but can also confirm the make before break component of the tap changer.

Test Results:

Usually, the analysis of the results of winding strength is compared on the basis of each strength with each adjoining winding with the same tap. If all the measurements are within 1%, the test is considered to have been passed.

Final Words:

Comparisons can also be conducted with original test data which are assessed on the production plant using temperature correction data, taking into account that resistance tests on the spot do not repeat the manufacturer's test track, which very often occurred at the time of production in a contained way.