Lord Consulting

An Overview of the Application of Frequency Response Analysis (FRA) Technology to the Condition Assessment of Transformers

This paper was presented at the 2006 AVO/LORD International Technical Conference

Abstract

This paper provides an overview of the application of Frequency Response Analysis (FRA) Technology to the conditional assessment of transformers. FRA is an exciting test technology which has been extensively used around the world over the past 10 years for detecting the integrity of winding structures of power transformers. This technology allows for the detection of small physical distortions to the transformer’s winding structure without visual inspection.

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TABLE OF CONTENTS

1 INTRODUCTION    3
2 CAUSES OF TRANSFORMER FAILURE    3
3 basics of frequency response analysis (fra)    6
Why is the FRA Test Performed    6
How is the FRA Test Performed    6
4 TRANSFORMER MODELING    7
Transformer Equivalent Circuit    7
Resonance (RLC Circuits)    9
4.1.1 Series Resonance Circuit    9
4.1.2 Parallel Resonance Circuit    10
Simple Transformer Modeling    11
5 performinG a FRA test measurement    13
FRAMIT Equipment Setup    13
5.1.1 Transformer Preparation    13
5.1.2 Test Leads & Connection Protocol    13
5.1.3 Frequency Range:    13
5.1.4 Measurement Result    13
5.1.5 FRAMIT Database    13
5.1.6 Transformer Records    13
5.1.7 Test Records    14
5.1.8 Test Reports    14
What FRA Tests Are Performed    15
5.1.9 FRA Open Circuit    16
5.1.10 FRA Short Circuit    17
6 diagnostics of FRA Test Results    19
What does the FRA Open Circuit Test Results show    19
Open Circuit Test: Low Frequency Range (0 – 50 kHz)    21
6.1.1 Core Problems:    21
6.1.2 Shorted Turns:    22
Open Circuit Test: Medium Frequency Range (0 – 500 kHz)    23
6.1.3 Axial Collapse    23
6.1.4 Hoop Buckling Failure (Radial Deformation)    24
Open Circuit Test: High Frequency Range (0 – 2000 kHz)    25
6.1.5 Movement of, or damage to, main and tap winding leads    25
6.1.6 The effect of testing the transformer WITH and WITHOUT oil    26
6.1.7 The effect of testing the transformer on DIFFERENT tap settings    26
Short Circuit Test: Very Low Frequency Range (0 – 3 kHz)    27
7 Implementation of FRA test technology    28
8 Conclusion    29