Capacitance and Tan Delta Test of Transformer Windings: Step by Step Procedure

The capacitance and tan delta test stands as one of the most critical and reliable non-destructive diagnostic methods for assessing the insulation quality of transformer windings. In the field of electrical engineering, particularly during transformer testing and commissioning, this test provides invaluable insights into the condition of winding insulation before the equipment is put into operation. The test measures two essential parameters: capacitance \((C)\) and dissipation factor \((tan δ)\), which together reveal the health of the transformer’s insulation system.

For electrical engineers, technicians, and commissioning professionals, understanding the principles, procedures, and interpretation of capacitance and tan delta testing is fundamental to ensuring the safe and reliable operation of power transformers. In this blog post we will explore the test methodology, test modes, measurement procedures, and acceptable limits for transformer windings.

What is Capacitance in Transformer Windings?

Capacitance is a fundamental electrical property that represents the ability of a system to store electrical charge at a given voltage. In transformer windings, capacitance exists between:

1. High Voltage (HV) winding and Low Voltage (LV) winding – denoted as CHL
2. HV winding and ground (tank) – denoted as CH or CHG
3. LV winding and ground (tank) – denoted as CL or CLG

The capacitance value in transformer windings depends on three primary factors:

• Area of the winding conductors (proportional to capacitance)
• Dielectric constant of insulating material (oil-paper insulation system)
• Distance between conductors and ground (inversely proportional to capacitance)

Mathematically, capacitance is expressed as:

\(C = ε₀ × εᵣ × A / d\)

Where:

• \(ε₀\) = Permittivity of free space (8.854 × 10⁻¹² F/m)
• \(εᵣ\) = Relative permittivity of insulating material
• \(A\)= Area of the conductor
• \(d\) = Distance between conductors

Why is Capacitance Important?

Changes in capacitance values can indicate several abnormal conditions in transformer windings:

• Mechanical displacement of windings due to short-circuit forces or transportation
• Partial breakdown of insulation resulting in reduced distance between conductors
• Presence of moisture in the oil-paper insulation system, which increases the dielectric constant
• Change in insulation material properties due to aging and thermal stress
• Oil degradation affecting the overall dielectric constant of the insulation system

Tan Delta (Dissipation Factor) of Transformer Windings

Tan Delta \((tan\, δ)\), also known as dissipation factor or loss angle, measures the power dissipated through the insulation material when an AC voltage is applied. It represents the quality and condition of the insulation by quantifying the resistive losses within the dielectric material.

The Physics Behind Tan Delta

In a perfect capacitor (ideal insulation), the current leads the voltage by exactly 90 degrees, and all current flowing through the insulation is purely capacitive \((I_C)\). However, in real transformer windings, the insulation has some resistive component due to impurities, moisture, contamination, and aging. This results in:

• Capacitive current \((I_C)\): The ideal current through a perfect capacitor
• Resistive current \((I_R)\): Current through the resistance of the imperfect insulation

The loss angle \((δ)\) is defined as the angle by which the phase shift falls short of 90 degrees. Therefore:

\(tan\, \delta = \frac{I_R}{I_C} = \frac{1}{(2πfCR)}\)

Where:

• \(I_R\) = Resistive component of current
• \(I_C\) = Capacitive component of current
• \(f\) = Frequency of applied voltage
• \(C\) = Capacitance
• \(R\) = Resistance of insulation

Interpretation of Tan Delta Values

• Low tan δ values (< 0.3%): Indicates good insulation condition with minimal losses
• Moderate tan δ values (0.3% – 0.5%): Normal condition for aged equipment
• High tan δ values (> 0.5%): Indicates insulation deterioration due to moisture, contamination, or aging
• Very high tan δ values (> 1.0%): Suggests serious insulation failure risk and immediate action required

An important characteristic of tan delta testing is observing how the dissipation factor changes with applied voltage. In good insulation, tan delta remains relatively constant as voltage increases. However, in contaminated or degraded insulation, tan delta increases significantly at higher voltages (known as “tip-up”), indicating the presence of impurities or defects.

Three Modes of Capacitance and Tan Delta Testing

The measurement of capacitance and tan delta in transformer windings is performed using three distinct test modes. The choice of mode depends on which specific capacitance value you wish to measure. Each mode differs in which current pathways are measured by the test set.

1. UST Mode: Ungrounded Specimen Test

Purpose: To measure capacitance between two ungrounded terminals (windings) only, isolating the measurement to a specific insulation path.

Connection Method:

• High voltage test lead connects to one winding (e.g., HV winding)
• Low voltage test lead connects to the other winding (e.g., LV winding)
• Ground lead connects to tank/earth
• Current through ground is automatically bypassed and not measured

What is Measured:

• Only the capacitance between the two connected windings: CHL (HV to LV capacitance)
• Current flows only through the capacitive path between the two windings
• Ground leakage current is eliminated from measurement

When to Use UST:

• Measuring capacitance between HV and LV windings
• Measuring capacitance between HV and tertiary windings
• Isolating individual sections of insulation for precise assessment

2. GST Mode: Grounded Specimen Test

Purpose: To measure all current paths to ground, providing a comprehensive assessment of the winding’s total capacitance to ground.

Connection Method:

• High voltage test lead connects to one winding (e.g., HV winding)
• Low voltage test lead connects to ground/tank
• Ground lead also connects to ground/tank
• Both ground paths and winding-to-winding paths are measured

What is Measured:

• Total capacitance including all parallel paths: CH + CHL (when HV is energized)
• Sum of capacitance to ground and inter-winding capacitance
• Both capacitive and resistive currents flowing to ground are included

When to Use GST:

• Measuring total capacitance of a winding to ground
• Initial screening for insulation degradation
• When complete assessment of all loss paths is required

3. GSTg Mode: Grounded Specimen Test with Guard

Purpose: To measure capacitance to ground only by eliminating or “guarding” the current flowing through other winding paths.

Connection Method:

• High voltage test lead connects to one winding (e.g., HV winding)
• Low voltage test lead connects to ground/tank
• Guard lead connects to the other winding (e.g., LV winding)
• Current through LV winding is shunted to ground and not measured

What is Measured:

• Only the capacitance from the measured winding to ground: CH (isolated from inter-winding capacitance)
• Eliminates the influence of inter-winding capacitance
• Provides the true winding-to-ground capacitance

When to Use GSTg:

• Measuring winding-to-ground capacitance in isolation
• Assessing individual winding insulation condition
• Obtaining accurate baseline measurements for trending

Verification Check

An important property of these three modes is that measurements can be verified using the following relationship:

\(GST = UST + GSTg\)

Or mathematically:

\(CH + CHL = CHL + CH\)

Test Procedure for Two-Winding Transformers

Preparation Before Testing

1. Isolation: Ensure the transformer is completely isolated from the power system and all external equipment. Remove all jumpers from bushing terminals.
2. Earthing: Open all earthing connections from neutral terminals (for Y-connected windings) to ensure clean measurement paths.
3. Terminal Preparation:
   • Short all three phases of HV winding together
   • Short all three phases of LV winding together
   • For single-phase transformers, short applicable terminals
4. Cleanliness: Ensure all bushing terminals are clean and dry to prevent surface leakage.
5. Temperature Recording: Record the ambient temperature and transformer oil temperature.
6. Test Equipment: Use a 10 kV or 12 kV fully automatic capacitance and tan delta test kit for accurate measurement.

Standard Test Combinations for Two-Winding Transformer

For a two-winding transformer, measure capacitance and tan delta in the following combinations:

UST Mode Connection Diagram

Measurement Verification

After completing all measurements, perform inter-check calculations to verify accuracy:

Check 1: \(CH = (CH + CHL) – CHL\)
Check 2: \(CL = (CL + CHL) – CHL\)
Check 3: \(CHL\) measured in UST should approximately equal CHL derived from GST combinations

If variations exceed 5%, repeat the measurements or investigate potential equipment issues.

Test Procedures for Three-Winding Transformers

Three-winding transformers (with HV, Intermediate Voltage-IV, and LV windings) require additional test combinations:

Three-Winding Transformer Combinations

The total capacitance in GST mode should equal:

\(GST (HV) = CHG + CHLV1 + CHLV2\)

Test Procedures for Reactors

For shunt reactors and current-limiting reactors:

1. Terminal Preparation: Short all three-phase bushings at the reactor terminals
2. Connection:
   • HV lead of test kit → Connected to shorted bushings
   • LV lead of test kit → Connected to earth connection
3. Test Mode: Use GST (Grounded Specimen Test) mode
4. Pre-test: Ensure neutral connection with earth or NGR (Neutral Ground Resistor) is isolated before testing
5. Measurement: Record capacitance and tan delta values

Acceptable Limits and Standards

According to pre-commissioning standards and guidelines:

Acceptable Limit: 5% variation in all combinations

This means that when measurements are taken from different terminal combinations (reverse connections), the variation should not exceed 5%. For example:

If CHL measured as: HV(+) to LV(-) = 5820 pF

Then CHL measured as: LV(+) to HV(-) should be within: 5778 – 5862 pF

Typical Capacitance Values and Tan Delta Ranges

For Power Transformers (as typical reference):

Temperature Correction

Since capacitance and tan delta are temperature-dependent, results should be corrected to a standard reference temperature (typically 20°C or 25°C) for comparison with previous measurements:

Temperature Correction Formula:

\(\tan\,\delta (corrected) = \tan\,\delta (measured) \times exponential factor\)

The exact correction factor depends on the insulation material and oil type. Most test equipment performs automatic temperature correction based on entered ambient temperature.

How to Identify Insulation Problems Using C and Tan Delta

Capacitance and tan delta testing can reveal several specific insulation problems:

1. Moisture in Windings

Indicators:

• Increase in capacitance value (dielectric constant increases with moisture)
• Significant increase in tan delta (moisture increases conductivity)
• Non-linear voltage response: tan delta increases sharply at higher voltages

2. Winding Displacement or Mechanical Damage

Indicators:

• Decrease in inter-winding capacitance (CHL)
• Increase in winding-to-ground capacitance (CH and CL)
• Asymmetry in three-phase measurements (variation > 5%)

3. Oil Contamination

Indicators:

• Increase in tan delta without proportional increase in capacitance
• Poor voltage dependency (non-linear response)
• Variation in tan delta with frequency changes

4. Insulation Aging and Degradation

Indicators:

• Gradual increase in tan delta over time
• Increase in capacitance (due to insulation shrinkage and material degradation)
• Higher dissipation factor values compared to baseline

5. Partial Short Circuit or Breakdown

Indicators:

• Very high tan delta values (> 1%)
• Significant change in capacitance values
• Three-phase imbalance
• Measurement instability or inability to measure

Comparison with IR (Insulation Resistance) Testing

While both capacitance/tan delta testing and insulation resistance (IR) testing assess winding insulation, they measure different aspects:

Advantages of Capacitance and Tan Delta Testing

1. Early Detection of Insulation Problems: Detects moisture and degradation long before complete failure
2. Voltage Dependency Assessment: “Tip-up” characteristics reveal the nature of contamination
3. Non-Destructive: Equipment remains undamaged; can be performed multiple times
4. Standardized Procedure: Well-established standards and acceptance criteria
5. Trending Capability: Historical data enables predictive maintenance
6. Multi-component Assessment: Can test windings, bushings, and complete insulation systems
7. Cost-Effective: Prevents catastrophic failures and unplanned outages

Tan Delta and Capacitance Calculator

Conclusion

Capacitance and tan delta testing of transformer windings represents a critical component of pre-commissioning testing and ongoing condition monitoring for power transformers. By understanding the principles behind these measurements, mastering the three test modes (UST, GST, GSTg), and following established procedures, electrical engineers can effectively assess winding insulation condition and prevent catastrophic failures.

The test’s ability to detect moisture, contamination, mechanical damage, and insulation degradation makes it invaluable during commissioning phases. When combined with other diagnostic tests like insulation resistance and dielectric response analysis, capacitance and tan delta testing provides comprehensive insight into transformer health.

For commissioning engineers and maintenance professionals, regular trending of these measurements over the transformer’s lifetime enables data-driven decisions about maintenance, reconditioning, or replacement—ultimately improving system reliability, reducing outages, and extending equipment life.