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

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Richard Breytenbach

BSc Electrical Engineer
Starlogic Instrument Development
South Africa

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

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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.

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