Transformer Diagnostic Testing

Performing Winding Resistance and Turns Ratio Tests

Vanguard Instruments Company, Inc.

1520 S. Hellman Ave. Ontario, California 91761, USA

TEL: (909) 923-9390 FAX: (909) 923-9391

June 2016 Revision 1

REV 1 TRANSFORMER DIAGNOSTIC TESTING

1

Power transformers play a vital role in power transmission. They are used to raise or lower the voltages in the power delivery system. High voltage (HV) power transformers are very expensive to manufacture and require long lead times since they are very labor intensive. A failed transformer is not only costly to replace but also causes loss of revenue to the utilities. Due to this reason, proper maintenance to ensure reliable performance and longevity is of utmost importance.

In operation, all transformers are subjected to internal as well as external fault current. Transformers will see more external than internal fault. Fault current imposes high mechanical stresses on the transformer's windings and its mechanical structure. These high stresses weaken and alter the mechanical structure of the transformer. Hoop buckling of windings and winding deformations are typical effects of high stresses caused by a short circuit in the system. Also the heat generated during normal operation and during a fault, oxidizes the transformer oil. Over time, the interaction of the insulation paper (used for conductor wrapping) with the hot transformer oil weakens the dielectric strength of the transformer.

In current operational environments, getting the required long outage to test transformers is not that common. So when the outage is planned, all the required tests need to be performed quickly. Although all tests need to be performed during the short outage time, personnel safety should never be overlooked or compromised. For transformer maintenance, personnel safety and speedy testing are the two most important factors. Several different tests can be performed on a transformer to analyze its condition. The transformer winding integrity can be verified by using the Sweep Frequency Response Analyzer test. The transformer bushing and overall insulation integrity can be analyzed using the Tan-delta (or Power Factor) test. Chemical analysis of the transformer oil should be conducted to detect moisture and determine the dielectric strength of the oil. Additionally, transformer turns ratio and winding resistance tests are used to detect shorted windings and possible connection issues. The focus of this application note is on winding resistance and turns ratio tests.

Winding resistance tests can verify the winding connections to the transformer bushings, or the winding contact resistance (as in the case of the load tap changer and voltage regulator). A properly-designed winding resistance meter can obtain fast and accurate resistance readings when performing an HV transformer winding resistance test.

A typical transformer winding resistance meter employs a high DC power supply (20Vdc to 60Vdc) to drive the winding into saturation. The use of a high DC voltage will quickly drive the winding into saturation and reduce the waiting time for stable readings. A high test current also delivers more accurate readings in noisy environments.

Since the transformer winding is highly inductive, it will store the energy injected into it during testing. This energy must be safely discharged in a controlled way to avoid the risk of injury to test technician as well as damage to the test instrument. For operator safety, this discharge circuit should be built into a transformer winding resistance meter. The test equipment should also provide clear audio and visual warnings to the user.

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The table below shows the Vanguard offerings for DC winding resistance measurement

Model DC Test Current # Windings Tested Demag Routine

TRM-20 Up to 20A Test two windings Yes

TRM-40 Up to 40A Test two windings Yes

TRM-203 Up to 20A Test three phases Yes

TRM-403 Up to 40A Test three phases Yes

WRM-10 Up to 10A Test two windings No

LTCA-10 Up to 10A Test three phases + LTC diagnostics No

LTCA-40 Up to 40A Test three phases + LTC diagnostics No

The table below shows the Vanguard offerings for turns ratio testing

Model Description Test Voltages Turns Ratio Range

ATRT-01 S3 Single phase 4, 40V 0.8 – 15,000 to 1

ATRT-01B S3 Single phase, Battery Powered 4, 40V 0.8 – 15,000 to 1

ATRT-03 S2 3-phase 8, 40, 100V 0.8 – 15,000 to 1

ATRT-03A S2 3-phase, Battery Power 8, 40, 100V 0.8 – 15,000 to 1

TRF-100 3-phase 4, 40, 100V 0.8 – 50,000 to 1

TRF-250 3-phase 4, 40, 100, 250V 0.8 – 50,000 to 1

Tri-Phase True 3-phase 8, 40, 100V 0.8 – 15,000 to 1

Vanguard Instruments Winding Resistance Meters and Transformer Turns Ratio Testers are housed in field-rugged enclosures and feature rugged membrane keypads for convenient data entry and control. Optional built-in thermal printers are available for field use. All devices can be used in stand-alone mode or can be computer-controlled with the included software.

For further information on Vanguard Instruments' offerings for current transformer testing, please contact our local channel partner or visit our web site at www.vanguard-instuments.com.

1520 S. Hellman Ave • Ontario, CA 91761 • USA

Phone: 909-923-9390 • Fax: 909-923-9391 www.vanguard-instruments.com

Copyright 2016 by Vanguard Instruments Company, Inc.

Transformer Diagnostic Testing • Revision A • June 23, 2016 • JG, HN, TA