Dielectric Frequency Response Analysis (DFR / FRA) Testing of Power Transformers

What is DFR analysis in Dielectric Frequency Response?

Dielectric Frequency Response Analysis (DFR) which is also known as Frequency Domain Spectroscopy (FDS) (or) Dielectric Response Analysis (DRA) is an advanced diagnostic test used to assess the insulation condition of oil filled power transformers. 

This test is especially effective in determining the moisture content in cellulose insulation (paper insulation) and evaluating the overall health of the transformers insulation system.

The insulation system of a transformer mainly consists of:

1). Cellulose insulation (paper & pressboard)

2). Transformer insulating oil

These 2 materials together form the dielectric insulation between windings and between windings and ground. 

Over a period of time, moisture, aging and contamination degrade the insulation performance. 

Dielectric Response Analysis helps to detect these issues accurately without dismantling the transformer.

This test is widely used for:

• Condition assessment of power transformers.

• Remaining life estimation.

• Maintenance planning.

• Preventive maintenance programs.

Effects of Moisture on Transformer Insulation

Moisture can causes serious deterioration in the insulation performance.

1). Decrease in Dielectric Strength

Moisture reduces the insulation withstand capability which is increasing the risk of dielectric failure.

2). Accelerated Aging of Insulation

Moisture accelerates the cellulose decomposition reducing transformer life.

3). Bubble Formation at High Temperature

At high temperatures moisture vaporizes & used to form bubbles inside the insulation. 

These bubbles can cause internal flashover and catastrophic failure.

4). Increased Dielectric Losses

Moisture increases power factor (tan δ) resulting in higher dielectric losses.

5). Increased Risk of Internal Faults

Moisture weakens the insulation and increases the risk of 

• Winding-to-winding faults and 

• Winding-to-ground faults.

Purpose of Dielectric Response Analysis

The primary purpose of Dielectric Frequency Response Analysis (DFRA) is to determine the moisture content in cellulose insulation and determine the insulation condition of the transformer.

Objective

1). Moisture Assessment in Cellulose Insulation

Cellulose insulation is highly hygroscopic and absorbs the moisture over time. 

Even a small amounts of moisture significantly affect the insulation strength. 

DFR provides the accurate moisture percentage (%) in paper insulation.

2). Evaluation of Insulation Condition

DFR helps to identify:

• Insulation aging,

• Oil contamination,

• Insulation degradation &

• Conductivity changes in insulation.

3). Maintenance and Asset Management

The test helps utilities and maintenance engineers to decide:

• Whether drying of insulation is required.

• Transformer condition (Good / Moderate / Poor).

• Remaining service life.

Importance of Moisture Detection in Transformers

Moisture is one of the most dangerous contaminants in transformer insulation. 

It may enter into the transformer due to:

• Sources of Moisture,

• Aging of cellulose insulation,

• Leaky seals & gaskets,

• Breather malfunction,

• Improper maintenance,

• Oil contamination &

• Atmospheric moisture ingress.

Principle of Dielectric Frequency Response Analysis

Dielectric Response Analysis used to measures insulation response over a wide frequency range to evaluate the insulation properties.

The test measures:

• Capacitance,

• Power Factor (Tan Delta),

• Dissipation Factor &

• Polarization and Depolarization currents.

These values vary with the frequency and provide information about:

• Moisture content in the cellulose insulation,

• Oil conductivity &

• Insulation aging condition.

Frequency Range used in DFR Test

Typical frequency range is:

0.0001 Hz to 1000 Hz

Different frequency ranges provide different information as

Frequency Range	Information Obtained
Very Low Frequency	Moisture in solid insulation
Mid Frequency	Oil conductivity
High Frequency	Oil and geometric capacitance

Test Equipment 

• OMICRON DIRANA – Dielectric Response Analyzer

The OMICRON DIRANA is one of the most widely used instruments for the Dielectric Response Analysis.

Features of DIRANA

• Combines Frequency Domain Spectroscopy (FDS),

• Combines Polarization Depolarization Current (PDC),

• Automatic moisture calculation,

• Automatic temperature correction,

• Model-based insulation analysis &

• Accurate moisture determination without oil sampling.

Functions of DIRANA

• Measures dielectric response curve,

• Calculates moisture percentage in paper insulation,

• Evaluates oil conductivity,

• Determines insulation aging condition &

• Transformer Insulation Structure Relevant to DFR.

The major cellulose insulation is located between 

• High Voltage (HV) winding and Low Voltage (LV) winding.

• Windings and transformer tank

• Inter-turn insulation

DFR test primarily measures the insulation between HV and LV windings.

Applicable Standards

• IEC 60076-3 – Insulation Levels and Dielectric Tests

• IEC 60076-18 – Measurement of Frequency Response

• NEMA TR 1 – Transformers, Regulators, and Reactors

• NEMA TR 2 – Standard Test Procedure for Transformers

• NEC Article 250 – Grounding and Bonding

Test Connections

Correct connection is more important for accurate measurement.

Basic Connection Method

• Output terminal → High Voltage winding (HV)

• Input terminal → Low Voltage winding (LV)

• Guard terminal → Transformer tank (ground)

Purpose of Guard Connection

Guard connection eliminates unwanted currents such as:

• Surface leakage currents,

• Tank currents &

• External interference currents. 

This ensures an accurate insulation measurement.

Step-by-Step Test Procedure

Step-1: Transformer Preparation

Ensure that transformer is get isolated.

Disconnect from the power supply.

Ensure that transformer is properly grounded.

Clean the bushings and all the connecting terminals.

Step-2: Connect Measurement Equipment

Connections include:

• HV output → HV winding

• Measurement input → LV winding

• Guard → Transformer tank

Ensure:

Metallic connections are clean and no rust (or) paint at any connection points and also check proper grounding.

Step-3: Guard Cable Connection

Guard cable should be connected to:

• Transformer tank and

• Ground point

This is used to prevent unwanted capacitive and resistive currents.

Step-4: Perform Frequency Sweep

The analyzer applies test voltage over a wide frequency range and measures:

• Capacitance,

• Power factor and

• Dissipation factor.

Test voltage is typically low and non-destructive.

Step-5: Record Dielectric Response Curve

The instrument plots:

Power Factor vs Frequency curve

This curve provides the information about insulation condition.

Step-6: Temperature Measurement

Transformer insulation temperature should be recorded because the dielectric response depends on the temperature.

Temperature correction is applied automatically by DIRANA.

Test Overview

DFR combines 2 test methods:

1). Frequency Domain Spectroscopy (FDS)

FDS measures insulation response over frequency range.

FDS provides accurate moisture measurement.

2). Polarization Depolarization Current (PDC)

PDC measures polarization and depolarization currents.

PDC provides information about the insulation aging and moisture.

Interpretation of Dielectric Response Curve

The dielectric response curve contains valuable information about transformer insulation.

Low Frequency Region

Low frequency region provides information about:

• Moisture in cellulose insulation.

• Aging condition.

• Higher response at low frequency indicates higher moisture.

Mid Frequency Region

Mid frequency region provides information about:

• Oil conductivity.

• Oil contamination.

• Shift in slope indicates oil degradation.

High Frequency Region

High frequency region provides information about:

• Oil insulation condition.

• Geometric capacitance.

Moisture Calculation

The instrument compares the measured curve with internal model curves.

Based on comparison it calculates:

• Moisture percentage in paper insulation (%),

• Oil conductivity &

• Insulation aging condition.

Typical moisture levels:

Moisture %	Condition
< 1%	Excellent
1 – 2%	Good
2 – 3%	Moderate
> 3%	Poor
> 4%	Critical

Reporting

The test report includes:

General Information

• Transformer rating

• Voltage level

• Serial number

• Location

Test Data

• Frequency response curve

• Moisture percentage

• Oil conductivity

• Capacitance values

• Power factor values

Temperature Data

• Oil temperature

• Ambient temperature

Template

Summary

Dielectric Frequency Response Analysis (DFR) is one of the most advanced and reliable methods for assessing transformer insulation condition. 

It provides accurate moisture measurement in cellulose insulation, evaluates oil conductivity and helps to determine the insulation aging condition.