Section4&5-Doble Testing Basic Theory Part1&2

Doble Training

Knowledge Is Power

SM

Apparatus Maintenance and Power Management for Energy Delivery

Doble Testing Basic Theory

Why Do We Test?

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2-1

Doble Training

Purpose Of Tests: To Detect… •
Overall Power Factor: Detect Moisture and contamination in oil and deeper in cellulose •
Bushing C1: Contamination in the main body •
Bushing C2: Contamination in the oil and tap area •
Exciting Current: Core defects, bad connections, shorted turns; especially useful for LTC’s •
Turns Ratio: Shorted turns •
Leakage Reactance: Winding movement 3

How To Pinpoint A Problem

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2-2

Doble Training

Core Form Transformer Winding Details Some currents we want to measure… …and some we do not. Unmeasured current returns to Guard without being measured.

m A

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What We’re Measuring From the components of our test current, we measure current (IT), capacitance (IC), and watts (IR).

IT

IC

IC
IT HV

O IR

CH

E

CHL

CL

LV

m A 6

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Doble Training

Relationship Between mA, pF, Watts, And Percent Power Factor IC IT We apply a voltage across our insulation, and measure the current that flows (IT) in mA. We measure the capacitive component (IC in pF) and the resistive component (IR in Watts)

See the relationship clearly (Fig 1) but more realistically (Fig 2) IT

IC

Cos
= PF = IR/IT


IR Fig 1

E

IR Fig 2 7

Relationship Between Current and Capacitance

This relationship applies when the resistive component is very small. 8

2-4

Doble Training

Why We Measure Capacitance

A


C= 4d C A  d

d

A

= Capacitance = Area (size of capacitor) = dielectric constant = Distance between plates

•
All of these variables are Physical Parameters

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The Transformer As A Capacitor A Representation of Transformer CH Insulation: A Capacitor

Upper Plate: (High Voltage Winding) AT

A

d

CH CHL CL

Dielectric: (oil, porcelain, paper products)

Lower Plate: (the grounded transformer tank)


BT

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2-5

Doble Training

Original Winding Capacitances Based on Normal Distances

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Distances Changed Due To Winding Distortions, Changing Capacitances

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2-6

Doble Training

Why We Measure Watts (Loss)

The measured Watts represent the energydissipating tendency of the insulating material (i.e., the Dielectric Loss portion of the insulating material). This component of the measured current is created by foreign materials not part of the insulation system. Examples: •
Moisture •
Arcing Byproducts •
Metallic Particles from forced oil motors •
Airborne Particles

Resistive component of current in Watts 13

What The Watts Measurement Means

IR

IC

Surface leakage on a bushing with contaminated porcelain can be highly resistive because of moisture and particles of contamination. Since the inside of the bushing is clean and free of moisture, the C1 current measured on the inside should be highly capacitive. 14

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Doble Training

A Word About Negative Watts... The test set sees VT as the voltage, but only I2 as the current. I2 referenced to V2 would give us a positive watts reading, but I2 referenced to VT does not. The current IG and Resistance RG determine the degree to which the watts go negative. Note that the current is negative only when referenced to VT; when referenced to V2, it is positive. So there really is no such thing as negative Watts.

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Watts As It Relates To VT Or V2 The angular difference
between VT and V2 is caused by the element

Without RG With RG

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Doble Training

How To Calculate Power Factor

To express power factor in percent (% PF), multiply by 100:

Power Factor Is:

10 kV equivalent values

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Know These Relationships

•
% Power Factor = (Watts X 10)/mA •
Capacitance (pF) = Current (mA) X 265

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2-9

Doble Training

You Can Check Your Data EX: Line 2 1. Watts X 10 =1.91

2. Divided by 9.077mA=.21%

3. %PF Measured

4. Capacitance= 9.077 mA X 265=2405 pF 19

Power Factor Vs. Test Voltage Tip-Up = Power Factor at Line-to-ground voltage Power Factor at 25% Line-to-ground voltage As test voltage is increased, the power factor will increase depending on the void density. %PF

%PF @ L-G %PF @ 25% L-G

E 25% L-G L-G Tip-up occurs mostly in dry-type insulation specimens such as Dry Type Transformer, rotating machinery, and cables, but can occur in oil filled transformers when the insulation is contaminated. 20

2-10

Doble Training

Power Factor Tip-UP

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Tipup Tests: Which Contaminants? These will cause tipup: –
Metallic particles from pump bearings in the insulation –
Copper Sulfide –
Carbon in the insulation

These will not cause tipup: –
Water –
Normal ageing byproducts –
Corrosive Sulfur in the oil –
Carbon or metallic particles in the oil 22

2-11

Doble Training

Transformers And The Tipup Test EXAMPLES OF WHEN TO USE A TIPUP TEST 1.
There is a bushing failure and you do a clean up. Check the main insulation power factor including tip-up to see how much conductive material might be trapped in the insulation. 2.
The oil shows a high particle metal content and we know the bearing went on one of the pumps. Check for tip up to see if the metal particles have gotten into the main insulation 3.
When a high power factor is measured, perform a tipup test to see if the source of high power factor is water and/ or normal aging (no tip-up), or some other contaminant. 23

Summary

Current

(mA)

Capacitance (pF)

A-C dielectric Loss (Watts)

Percent Power Factor

Power Factor Tip-Up

Power Factor 24

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Doble Training

Basic Insulation & Power Factor Theory Test Current I How does test current return to its source?

Possible Return Leads:

Red Blue Ground

Guard

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Basic Insulation & Power Factor Theory Test Current I Whick test currents get measured? Current returned directly to guard (IA) does not get measured. IA IB Guard Current returned through measuring circuit (IB) does. THUS WE CAN CONTROL WHICH CURRENTS TO MEASURE. 26

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Doble Training

Test Circuits •
GST means Grounded Specimen Test. At least part of the test current is measured through the ground lead; the rest is measured through the LV leads, if used and not guarded. •
UST means Ungrounded Specimen Test. The ground lead is not used for measurement. Only the selected LV leads are measured. No currents to ground can be measured. 27

Circuits Used To Measure CH, CHL, and CL Insulation

GST Test Circuit: Currents to Ground are measured

Guard Test Set Ground Lead: (Set automatically)

UST Test Circuit: Currents to Ground are not measured; Ground is Guard! Guard

Measured Not Measured 28

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Doble Training

Location Of Ground Relative To Guard: What Is Measured Test Current I

Test Current I

GST-Ground UST

GST-Guard

Guard Guard Guarded and Ungrounded cables not measured

Guarded and Grounded cables not measured 29

Grounded Specimen Test GST-Ground Energize a bushing 60 Hz Electrostatic Interference IE (returns to ground)

m A

57/63 Hz Doble Test Current (returns to Guard)

Current to ground is measured using ground lead 30

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Doble Training

GST GROUNDED SPECIMENT TEST

UST UNGROUNDED SPECIMENT TEST

GND - RB Measure Red, Blue, & Ground GAR - RB Measure Only Ground GAR - R

Measure Only Blue & Ground

GAR - B

Measure Only Red & Ground

UST - RB Measure Only Red and Blue UST - R

Measure Only Red

UST - B

Measure Only Blue 31

Controlling Measured Current On Ch Measurement Without being guarded, the LV winding influences the HV winding measurement to ground; get a mixed measurement.

LV

HV

CH CHL m A

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Doble Training

Controlling Measured Current On Ch Measurement With the LV lead connecting the LV winding to guard, the influence of the LV winding is eliminated. Get a pure measurement.

LV

HV CHL

CH m A

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Controlling Measured Current Section A will influence the section B measurement if not guarded. Get a mixed measurement. A

B

m A

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Doble Training

Controlling Measured Current With Section A guarded, the influence of A is redirected via the LV lead to Guard, and doesn’t get measured. Get a pure measurement.

A

B

m A

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The End

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