Unlike motors and generators, electrical transformers do not undergo mechanical wear and tear. However, they are not exempt from mishaps and deterioration over time.
Exposure to large short-circuit currents can have considerable effects on transformer windings, resulting in displacements and excessive heating. Additionally, electromagnetic vibrations end up causing wear on insulations and clearances of the windings. Finally, absorbed external moisture is another threat that accelerates the premature aging of insulations and can lead to internal discharges.
It is in this context that the periodic performance of tests becomes fundamental to ensure the integrity and reliability of transformers. Among these tests are those for measuring the resistance of windings, which we will see below:
Tests for measuring winding resistance aim at the precise detection of short circuits in the coils and an estimate of the aging of the coils.
For this purpose, the Kelvin bridge method is used, using direct current.
This method is based on the comparison between an unknown resistance (which corresponds to the resistance of the windings to be measured) and a known resistance.
Kelvin Bridge
A precision measurement instrument used to determine very low electrical resistances, usually below 1 ohm.
It is based on the Wheatstone Bridge but has a modification that eliminates the resistance of the measuring cables, making it ideal for measuring low impedance resistances, such as electrical contacts, motor and transformer coils, wires and cables, and welded joints.
Determining the Test Frequency
The frequency at which the winding resistance measurement test is performed depends on several factors that affect the integrity and reliability of transformers.
Type of Transformer | Operating Conditions | Maintenance History
Power Transformers: Every 12 months.
Distribution Transformers: Every 24 months.
Transformers under severe conditions: Every 12 months, due to the challenge of adverse conditions.
Transformers with a history of problems: Every 12 months, to ensure closer monitoring.
Determining the ideal frequency of tests should be based on a comprehensive assessment of the operating conditions and maintenance history of each transformer, aiming to preserve its integrity and reliability.
Equipment and Procedures
To accurately conduct the winding resistance measurement test on transformers, it is essential to have the appropriate equipment and follow the correct procedures. The following elements are necessary to carry out the test efficiently and safely:
1. Necessary Equipment:
a) Microohmmeter: The microohmmeter is a high-precision instrument that generates a low-voltage current, allowing precise measurement of the low-value resistance of windings.
b) Test Terminals: Used to establish the connection between the microohmmeter and the terminals of the transformer windings.
c) Personal Protective Equipment: A crucial step is the de-energization and isolation of the transformer from the electrical system before the test. To ensure safety, the use of personal protective equipment such as safety glasses, specific boots for electrical work, and gloves is essential.
2. Procedures:
To conduct the winding resistance measurement test, follow these steps:
a) De-energization and isolation: De-energize the transformer and isolate it completely from the electrical system.
b) Connection of test terminals: Connect the test terminals of the microohmmeter to the terminals of the transformer windings according to the necessary specifications.
c) Measurement execution: Turn on the microohmmeter and take the resistance reading.
HVEX Solutions for Transformer Tests
HVEX offers a set of advanced technological solutions to simplify and enhance tests on transformers, such as:
TTR Test Module:
HVEX's TTR test module, present in the OHMVEX ohmmeter, is a portable and robust tool designed to assess not only the transformation ratio but also the angular displacement in transformers.
This system offers high accuracy, incorporates fuses and varistors for electrical protection, and features automatic control to avoid connection errors. Additionally, it provides connection diagrams in the control software to facilitate TTR tests.
For resistance measurement, the equipment offers controlled ramp operation, automatic scaling, and a fast response time for result stabilization. These features make HVEX solutions ideal for precise and efficient tests on transformers, contributing to preventive maintenance and the reliability of these critical equipment in the electrical sector.
This solution is a highly accurate microohmmeter that plays an essential role in measuring the resistance of transformer windings. The OHMVEX provides low-voltage and high-precision current, allowing precise measurements in windings of reduced value.
Why acquire the OHMVEX?
Portability and robustness
Precise resistance measurement
Advanced TTR tests
Efficient communication
Integrated database