Transformer Power Factor Test

Tests Performed in De-Energized State of Transformer : • Capacitance and Power Factor –Windings to ground and to each other –Oil –Bushings –Arresters

Transformer Testing

Transformer Construction

Transformer Insulation Model

CHL

CH

CL

Low

Voltage

Windings

High

Voltage

Windings

• For Overall tests, transformer insulation is sectioned into three groups:

– CH

– CL

– CHL

• Reasons – Better assessment of the

individual components.

– Reduces chance of a problem being masked by “averaging”

High

Low

CH

CL

CHL

Tank and

Core

• CH - Insulation between High-Voltage conductors and grounded Tank & Core (H Bushings-Winding Insulation-Structural Insulating Members-Oil)

• CL - Insulation between Low-Voltage conductors and grounded Tank & Core (X Bushings - Winding Insulation-Structural Insulating Members-Oil)

• CHL - Insulation between High- and Low-Voltage Windings (Winding Insulation-Barriers-Oil)

Two-Winding Transformer

• CH - Insulation between the High-Voltage Winding Conductors and the Grounded Tank & Core – High Voltage Bushings –Winding Insulation – Structural Insulating Members – DETC, LTC and Series-Parallel Switch Insulation – Insulating Fluid

Two-Winding Transformer Dielectric

Circuit

• CL - Insulation Between the Low-Voltage Winding Conductors and Grounded Tank & Core – Low Voltage Bushings –Winding Insulation – Structural Insulating Members – DETC, LTC and Series-Parallel Switch Insulation – Insulating Fluid

Two-Winding Transformer Dielectric

Circuit

• CHL - Insulation between the HV and LV Windings –Winding Insulation Barriers – Insulating Fluid

• Possibility of a Static Shield

Two-Winding Transformer Dielectric Circuit

• Acceptance – Establish a baseline – Make sure you get what you paid for

• Routine – Establish Trend – Preventative Maintenance

• Troubleshoot – Disturbance on system, suspected fault – Isolate problem

• Assess Condition after relocation or repair

When to Do Test

High Voltage Winding Setup

Guard

Low Voltage Lead

I & W Meter

Tank & Core

Ground

Lead

HV Cable

Test Set

Low High

Measuring CH + CHL ICHL

Guard

Low Voltage Lead

I & W Meter

Tank & Core

Ground

Lead

HV Cable

Test Set

Low High

GST-Ground

ICH + ICHL

ICHL

ICH

ICH

Measuring CH Guarding CHL

Guard

Low Voltage Lead

I & W Meter

Tank & Core

Ground

Lead

HV Cable

Test Set

Low High

GST-Guard

ICH

ICHL

ICH

ICH

ICHL

Measuring CHL Guarding CH ICHL

Guard

Low Voltage Lead

I & W Meter

Tank & Core

Ground

Lead

HV Cable

Test Set

Low High

UST

ICHL

ICHL

ICH

ICH

ICH

Verification of the HV Results

Line #1 Measures CH+CHL , Tank and LV Winding Grounded

Line #2 Measures CH only to Ground, LV Winding Guarded

Line #3 Measures CHL only using UST Lead

Line #4 Calculated from Lines 1 minus 2. Compare to Line #3

Low Voltage Winding Setup

Low Voltage

Lead

I & W Meter

Tank & Core

Ground Lead

HV Cable

Test Set

Low High

Guard

Measuring CL + CHL

Low Voltage

Lead

I & W Meter

Tank & Core

Ground Lead

HV Cable

Test Set

Low High

Guard

GST-Ground

ICL + ICHL

ICL

ICHL

Measuring CL Guarding CHL

Low Voltage

Lead

I & W Meter

Tank & Core

Ground Lead

HV Cable

Test Set

Low High

Guard

GST-Guard

ICL

ICL

ICHL

Measuring CHL Guarding CL

Low Voltage

Lead

Tank & Core

Ground Lead

Test Set

Low High

Guard

ICL

ICHL

ICHL

UST

HV Cable

Verification of the LV Results

Line #5 Measures CL+CHL , Tank and HV Winding Grounded

Line #6 Measures CL only to Ground, HV Winding Guarded

Line #7 Measures CHL only using UST Lead

Line #8 Calculated from Lines 5 minus 6. Compare to Line #7

Analysis of Test Result

• < 0.5% -- GOOD

• >0.5% but < 0.7% -- DETERIORATED

• >0.5% but <1.0% & increasing -- INVESTIGATE

• > 1.0% -- BAD

GENERAL GUIDELINES

Data Interpretation of Modern Oil-Filled Power Transformers (Temperature Corrected)

Analysis of Test Result

• Change in Capacitance –Physical change has occurred

• 0-3% Good

• 3-5% Deteriorated

• 5-10% Investigate

• >10% Bad

Capacitance change detects movement and deformation of transformer windings.

C = Ae

d

Significance of Measured Capacitance

• Evaluating Transformer Condition – Involves Many Methods – Some more sensitive to certain problems than others – Baseline Results Beneficial

• Power Factor – Sensitive to Physical Characteristics & Quality of Insulation – Segment Insulation into Small Pieces

Summary