Transformers Testing

Transformer testing instruments

Types of tests

DC tests

AC tests

Winding resistance measurement

Why is it important?

How it is performed

Kelvin's method (4-wire)

How is it done?

Loading process

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Loading time

Stabilization process

How to saturate the core

Too high currents heat up the winding (< 10% of In)

50 A - Test current Cooper heating

Measuring the test current

Test current

Measuring on the LV side can be time consuming

Use the high-voltage winding as an additional aid for faster saturation

LV winding measurement with HV winding support

Test current

How is the test current determined?

Discharge

Safety

IEEE guide for field diagnostic testing of power transformers, regulators, and fluid-filled reactors, IEEE std C57152TM

What is a "good" result?

Tips from transformer manufacturers

VH deviation between phases - less than 03 - 05%

LV deviation less than 3-5%: more tolerance to error due to other circuit elements and smaller resistance values

The LV deviation between phases - less than 3-5%: more tolerance to error due to other circuit elements and smaller resistance values

The final decision is made based on the technical expertise of the test supervisors and their evaluation of the conditions

The final decision is made based on the technical expertise of the test supervisors and their evaluation of the conditions

Transformer demagnetization

Why is it important?

The remanent magnetism can cause a variety of problems:

Causes of transformer magnetization:

Comparison of maximum inrush current with and without residual flux

How?

  1. Demagnetization should be performed before AC tests, especially before FRA and excitation current testing, before AVGD, etc
  2. If the test object is a three-phase (auto)transformer, demagnetization of all 3 phases should be performed
  3. The initial demagnetization current should be as high as possible If the demagnetization is performed after a DC test on a transformer, such as winding resistance, the initial demagnetization current should have the same value as the current used for the DC test
  4. Demagnetization should be performed from the side of the transformer that has the neutral point accessible If both the high voltage (HV) and low voltage (LV) sides have/do not have accessible neutral point, demagnetization should be performed from the HV side

When to demagnetize?

AC test devices

Turn-ratio measurement

Excitation current measurement

Why is it important

?

How is it performed

?

Vector group detection

Why is it important

How is it done?

Phase angle measurement

Why is it important?

How it is performed

SFRA Measurement