Transformer Excitation Current Testing

Summary

In this webinar, Tom Sandry, the director of Workforce Development at Vector Power, dives into the intricacies of transformer excitation current testing. He refreshes participants on magnetism, magnetic induction, and Faraday's and Lenz's laws, setting the stage for a deeper understanding of the test's purpose. Sandry explains how excitation current tests can detect winding and core issues in transformers, even when other tests show normal results. He discusses the physics behind the test, the significance of phase patterns, and how to perform the test with necessary safety precautions. The webinar concludes with a quiz to reinforce the key learnings and a Q&A session, providing a comprehensive guide for professionals in the field.

Takeaways

• 🧲 The webinar by Tom Sandry from Vector Power focused on Transformer excitation current testing, which is crucial for diagnosing transformer health.
• 🔍 Magnetism is foundational to understanding excitation current testing, with magnetic fields, flux, and lines of force being key concepts.
• 👐 The left-hand rule helps determine the relationship between current flow and the direction of magnetic lines of force around a conductor.
• 📚 Magnetic permeability is analogous to electrical conductivity, with high permeability materials concentrating magnetic flux lines effectively.
• 🔗 The relationship between magnetism and electrical current is demonstrated through Ohm's law in the magnetic world, relating MMF, reluctance, and flux.
• 🌐 Electromagnetic induction, discovered by Michael Faraday, is the process where a voltage is induced in a conductor that cuts through magnetic field lines.
• 👣 Lenz's Law explains the polarity of induced voltage, which opposes the change causing the induction, setting up a magnetic field around the conductor.
• 🚀 The excitation current test is valuable for detecting winding and core problems in transformers, even when other tests seem normal.
• 🔍 The test can identify issues like abnormal core grounds, winding faults, low tap changer problems, and manufacturing defects.
• 📈 Phase patterns in three-phase transformers can indicate potential problems; normal patterns are high-low-high or low-high-low, with deviations suggesting issues.
• 🛠️ The procedure for performing excitation current tests involves careful setup, safety considerations, and comparison of results to previous tests or benchmarks.

Q & A

What is the main focus of the webinar presented by Tom Sandry from Vector Power?

The webinar focuses on Transformer excitation current testing, covering topics such as magnetism, magnetic induction, the physics behind the test, and how to identify faults in transformers using this method.

Why is a refresher on magnetism important for understanding excitation current testing?

A refresher on magnetism is important because the theory behind excitation current testing is deeply rooted in the understanding of magnetic fields, magnetic flux, and the principles of electromagnetism.

What is magnetic flux and how is it represented?

Magnetic flux is the group or number of magnetic field lines emitted outward from the North Pole of a magnet. It is represented by the symbol 'Φ' (phi), and its SI unit is the Weber.

Can you explain the left-hand rule for current-carrying conductors?

The left-hand rule helps determine the relationship between the current flow through a conductor and the direction of the magnetic lines of force around it. If you point your thumb in the direction of the positive potential and wrap your fingers around the conductor, your fingers indicate the direction of the magnetic field.

How does adding an iron core to a coil affect the magnetic field?

Adding an iron core inside a coil increases the flux density. The iron core becomes magnetized, enhancing the magnetic field around the conductor.

What is magnetic permeability and how is it related to relative permeability?

Magnetic permeability (mu) refers to the ability of a material to concentrate magnetic lines of flux. Relative permeability (Mu_r) is the ratio of the permeability of a material to the permeability of a vacuum (mu_0). It indicates how easily a material can be magnetized compared to a vacuum.

What is the significance of the excitation current test in transformer maintenance?

The excitation current test is significant for detecting possible winding or core problems in transformers, even when other tests like turn ratio and winding resistance tests appear normal. It can identify issues such as abnormal core grounds, winding faults, and manufacturing defects.

Why can't we measure the magnetic flux directly in a transformer?

Direct measurement of magnetic flux is not possible because it would require being inside the transformer while it is energized. Instead, magnetizing current, which is easier to measure and record, is used as an indicator.

What are the typical phase patterns observed during excitation current testing on three-phase transformers?

The typical phase patterns observed are the high-low-high pattern, the low-high-low pattern, and the pattern where all phases provide similar measurements. These patterns are tied to the core configuration and the interaction of the magnetic field with the core.

How should the excitation current test be performed on a three-phase transformer with a Y connection?

For a three-phase Y connection, the test should be performed by measuring each phase (H1 to Ho, H2 to Ho, H3 to Ho) in the ungrounded specimen test (UST) mode. Safety should be observed for other windings, and terminals that are normally grounded should be grounded during the test.

What does an unusual excitation current test result suggest?

Unusual results from the excitation current test may suggest potential problems with the transformer, such as residual magnetism, winding faults, or core issues. It may be necessary to demagnetize the transformer and repeat the test for accurate results.