July 6, 2015

What’s behind transformer field testing?

Abstract To prevent unexpected outages it is necessary to implement a field test programme. It has to be decided what tests should be performed and...

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Abstract

To prevent unexpected outages it is necessary to implement a field test programme. It has to be decided what tests should be performed and what information is expected to be obtained from these tests. This paper describes a common set of field tests on the basis of prescribed standards and years of field experience. Off-line and on-line tests are described as well as their use in diagnosis and trending. The paper also highlights the importance of training in this process to make conclusions and recommendations reliable.

Keywords: field testing, life extension, predictive maintenance, off-line test, on-line test

Introduction

Power transformers are key assets in electric energy generation, transmission and distribution systems, and also an important element in the industry. In order to get maximum service reliability in this critical component, it is necessary to perform a scheduled set of suitable diagnosis techniques on each transformer.

We can classify test failure modes in power transformers according to their different components. From a practical and didactical point of view, it is helpful to divide the transformer into the following components and associated potential faults:

• Electric circuit. Copper paths: contact problems at bushings, joints, windings, welded and soldered connections, tap changers 
• Magnetic circuit. Steel, yokes, loosening 
• Geometry of windings. Symmetry, change of geometry between windings and windings to core, distances 
• Insulation system. Oil, paper, contamination, water ingress, distances, ageing. This component usually defines the life of the power transformer

Field testing

The electric utility and electric power industries need to define their strategy and maintenance policy for their costly power transformer assets. Today, there are some on-line tests that will help to achieve this, but most of the checks rely on regular off-line measurements.

Off-line testing

Table 1 lists the standard off-line tests available for power transformers. Today there are instruments to conduct all the tests listed in the table. Sometimes a single instrument will include most of tests with the associated software to simplify its use for diagnosis and trending.

Table 1. Off-line tests for power transformer field testing

DC	Insulation, OLTC, Bushings & Windings						AC
Basic insulation:				Advanced insulation:
Insulation resistance, polarization index, Cg, core to ground				FDS (Dielectric Spectroscopy). Tan δ to windings (CH, CL, CHL), HV excitation current. Tan δ to bushings (C1/C2) & hot   collar
Advanced insulation:				No load test:
PDC / recovery voltage				Turns ratio / LV excitation current
Winding resistance:				FRA: Frequency Response Analysis
Static/dynamic (OLTC)				Short circuit impedance/voltage

 Basic insulation testing (DC)

In traditional insulation resistance testing, the Insulation Resistance (IR₁) between windings and between each winding and the ground is measured. Test instruments use a DC voltage (typ. = 5 kV dc). The insulation value is recorded over time, indicating the 1 minute and 10 minute (IR₁₀) values. From these, the Polarization Index (PI) is calculated as PI=IR₁₀/IR₁. Whilst the IR values have a strong dependency on temperature, the polarization index reduces this dependency. This measurement can detect serious problems, such as moisture, contamination or breakdowns. Testers usually plot the IR vs. time and also take a reading of the capacitance value Cg.

Advanced insulation testing (DC)

The two techniques below were developed to get an insight into oil-paper insulation. Even with good IR and PI results, power transformers can have a degraded insulation with moisture or contamination that will affect their lifetime. In the transformer, it is paper lifetime that usually limits transformer life, and these dc tests are tools that can help in assessing its condition.

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