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PURPOSE. Prevention of Apparatus Failure and Power System Interruptions due to Insulation Failure Results in : Enhance System Reliability Minimize Damage to Apparatus Enhances Safety to Personnel Minimize Loss of Revenue. Benefit. Extension of Apparatus Life

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PURPOSE

Prevention of Apparatus Failure and

Power System Interruptions due to Insulation Failure

Results in :

  • Enhance System Reliability

  • Minimize Damage to Apparatus

  • Enhances Safety to Personnel

  • Minimize Loss of Revenue


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Benefit

  • Extension of Apparatus Life

    • Degradation of Insulation, if detected before failure, can generally be restored to its original condition

    • Defer replacement costs

  • Better Utilization of Resources

    • Inspection interval may be safely extended or scheduled to utilize resources efficiently and effectively

    • Verification of new apparatus

    • Verify that new apparatus meets purchased specification and agrees with factory test reports

    • Assures proper field Assembly


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Definition

  • What is a Power Factor/ Dissipation Factor / Tangent Delta Test?…

    • The underlying principle of this test is to measure the fundamental AC electrical characteristics of insulation.


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Clarification

  • Insulation vs. Dielectric

    • Insulation relates to a medium’s ability to prevent the flow of current, I.e. poor conductivity.

    • Dielectric implies that the medium or material has specific measurable properties such as: Dielectric Strength, Dielectric Constant, Dielectric Loss and Power Factor.


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Dielectric Constant

  • In 1836, Michael Faraday (the father of the Capacitance) discovered that when the plates between a capacitor were filled with another insulating material, the capacitance would change.

  • This factor is the dielectric constant e

  • By definition the dielectric constant of a Vacuum is 1.0.

Oil e=2.2

Vacuum

Cvacuum=10 pF

Coil = ex Vacuum = 22 pF


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Dielectric: An Atomic View

-

-

-

-

+

+

+

+

-

-

+

+

-

-

-

-

+

+

+

+

A Dielectric at Rest with no applied

voltage. The molecules have no polar

orientation.

molecules

The molecules acquire a dipole moment resulting in a Negative charge building up on the Positive plate and a positive at the negative plate. This polarization creates an opposing electric field resulting in a diminished applied voltage.

+

E

-


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Perfect Insulator

  • The Capacitor


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Ideal Insulation System

  • Evaluating Insulation System


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Resistive Component

  • The Resistor

IR = IT

IR = E/R

W=EIR

q=0o

E

IR


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Basic Insulation Circuit

  • Basic Power/Dissipation Factor Circuit


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Definition

  • The Term Power/Dissipation Factor Describes

    • The phase angle relationship between the applied voltage across and the total current through a specimen.

    • The ratio of the real power to the apparent power.

    • The relationship between the total and resistive current


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Basic Insulation & Power Factor Theory

IC

IT

d

Q

IR

E

  • Power Factor Vs. Dissipation Factor Vs. Tangent Delta


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Insulation System

C1

C2

C3

Insulation Systems can be modeled as a Series of dielectrics...

C1C2C3C4

Or as parallel dielectrics...

or a combination of the two.


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DC Testing on Series Insulation

BAD

DC Test Voltage

Short

Stop

BAD

DC Test Voltage

Test Results: Good

Insulation System: Bad

Stop

  • If the first dielectric is good. The DC Test will indicate good; any

  • remaining dielectrics will not be tested.

Test Results: Good

Insulation System: Bad

  • For a DC test to be good, only one dielectric needs to be in good condition

During an AC Test the power factor will change as each dielectric fails.


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DC Testing on Series Insulation Cont.

DC Test Voltage

Short

Short

Test Results: Bad

Insulation System: Bad

For a DC Test to indicate an unsatisfactory result, all dielectrics must

be in poor condition.


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DC Testing on Parallel Insulation

DC Test Voltage

  • If one dielectric fails in a parallel dielectric, the test will fail.

  • There is no way to tell if the other dielectrics are good or bad.

During an AC Test the power factor will change as each dielectric fails.


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Advantage of AC vs. DC Tests

  • The AC test has a common factor in the form of a ratio (% Power Factor), which is independent of the amount of insulation.

  • The AC test is not hindered by a layer of “good” insulation in series with a “bad” insulation, since it merely requires a capacitance coupling.

  • The AC test provides a direct measure of dielectric loss and capacitance, both of which are useful in the diagnosis of the deterioration of many forms of insulation.

  • The DC test measurement depends on the length of time the voltage is applied. (PI)


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Limitation of the AC Dielectric-Loss

  • The ability to detect localized defects decreases as the inherently normal dielectric-loss and capacitance of the insulation systems increases.

  • Defects which are voltage dependent may not be detected if the initiation voltage of the defect is greater than the test voltage.


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Voltage sensitive characteristics

  • When we closely examine insulation, very small gaps or “voids” could exist. These voids develop an electrostatic potential on their surfaces. These small gaps become ionized: Partial Discharge/Corona.

Voids


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Results Interpretation

Increase in Tan Delta indicates:

  • Chemical deterioration due to ageing and temperature or local overheating

  • Contamination of water, carbon deposits, bad oil, dirt etc.

  • Severe leakage through cracks and surfaces

  • ionization


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FACTORS AFFECTING TAN DELTA

  • TEMPERATURE

  • HUMIDITY

  • SURFACE LEAKAGE

  • ELECTROSTATIC INTERFERENCE

  • SYSTEM FREQUENCY


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FACTORS AFFECTING TAN DELTATEMPERATURE

  • AT LOW TEMP. DUE TO HIGH VISCOSITY, MOTIONAL RESPONSE OF IONS AND POLAR COMPOUNDS SMALL AND HENCE LOW DIELECTRIC LOSS.

  • AS TEMPERATURE INCREASES, THE VISCOSITY DECREASES AND HENCE AMPLITUDE OF MOTION INCREASES TO INCREASE THE DILECTRIC FRICTIONAL LOSSES.


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FACTORS AFFECTING TAN DELTAHUMIDITY AND SURFACE LEAKAGES

  • IN HUMID ATMOSPHERE TEST OBJECT ACQUIRE DEPOSIT OF SURFACE MOISTURE WHICH RESULTS IN SURFACE LEAKAGE ERRORS

  • DIRT ON SURFACE OF TEST OBJECT INCREASES LEAKAGE CURRENT


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FACTORS AFFECTING TAN DELTAELECTROSTATIC INTERFERENCE

  • IN ENERGISED SUBSTATIONS CAPACITIVE COUPLING BETWEEN SPECIMEN AND CHARGED SYSTEM IS FORMED

  • INTERFERENCE SUPPRESSORS ARE USED IN TESTING KITS

  • MEASUREMENT IN NORMAL & REVERSE POLARITY APPLIED


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MODES OF MEASUREMENT

  • UST

  • GST

  • GSTg


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UST

  • USED WHEN TESTING IS DONE ON UNGROUNDED (ISOLATED FROM EARTH) OBJECT LIKE BUSHINGS, CTs WITH TEST TAP, CVTS AND GRADING CAPACITORS OF CBs


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GST

  • USED WHEN OBJECT IS GROUNDED AND WHEN TWO SPECIFIC POINTS (ISOLATED FROM GROUND) ARE NOT AVAILABLE FOR TAN DELTA MEASUREMENT. EXAMPLE: TRANSFORMER / REACTOR WINDING , CTs WITHOUT TEST TAP


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GSTg

  • USED TO SEPARATE THE TOTAL VALUES OF GST TEST FOR BETTER ANALYSIS

  • FOR EXAMPLE IN TRANSFORMER

    HV-IV /LV+G-------GST------CHL + CHG

    HV-IV / LV WITH Guard-----GSTg---CHG


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C – TAN DELTA MEASUREMENT

  • TRANSFORMER (BUSHING & WINDING)

  • REACTOR (BUSHING & WINDING)

  • CVT

  • CT

  • CB (GRADING CAPACITOR)

  • LA (NEWLY INTRODUCED)


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TEST FREQUENCY

  • Bushing----2Y

  • Winding----4Y

  • Grading Capacitor----4Y

  • Current Transformer---Y

  • CVT---4Y


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Transformer Testing


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Transformer Types

  • Power and distribution transformers may be either single-phase or three-phase

    • Three-Winding

    • Two-Winding

    • Autotransformer (with or without a tertiary winding)

  • They may be liquid-insulated, gas-insulated, or dry-type. For test purposes, the procedure used depends on the number of accessible, separatewindings.


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Transformer Insulation Systems

High

Voltage

Windings

CHG

CHL

CLG

Low

Voltage

Windings

For simplicity, transformer

insulation is grouped into three

representative insulation systems

CHG

CLG

CHL


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WINDING COMBINATION FOR C & TAN DELTA MEASUREMENT IN AUTO TRANSFORMERS


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Dielectric Circuit: Two Winding Transformer

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)

High

CH

Tank and Core

CHL

CL

Low


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Physical Representation of Three-Phase Two-Winding Transformer

High-Voltage Windings

Low-Voltage Windings

One of Three Phases Shown

CH

CHL

CL

Core

Leg

Transformer Tank


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The Overall Test

CH

CHL

CL

  • Before you start the overall test:

  • Short Circuit High Voltage Windings

  • Short Circuit Low Voltage Windings

  • Disconnect the neutral bushing from

  • ground

This is one of the most

common sources of error

in test results!

Ensure that the Neutral Bushing is also shorted and

disconnected from Ground


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Hookups for the Overall Tests

High

CH

GST-Ground

Test #1 : CH+CHL

CHL

CL

Low

High

GST-Guard

CH

Test #2: CH

CHL

CL

Low

High

CH

UST

Test #3: CHL

CHL

CL

Low


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Dielectric Circuit for Three-Winding Transformers

High

Tank and Core

CH

CHL

Low

CHT

CL

CLT

Tert

CT


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Bushing Types

  • Condenser type:

    • Oil-Impregnated paper insulation

    • Resin bounded paper insulation

  • Non-condenser type:

    • Solid

    • Alternate layers of solid and liquid insulation

    • Gas-filled


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Bushing Tests

  • Main insulation - C1

    • Center Conductor to Tapped Layer

  • Tap insulation - C2

    • Tapped Layer to Ground Flange

  • Overall insulation

    • Center Conductor to Flange, only applicable when not installed on an apparatus

    • Hot Collar Test


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Center Conductor

Sight-Glass

Liquid or Compound Filler

Insulating Weather shed

Main Insulating Core

Tap Insulation

Tap Electrode

Mounting Flange

Ground Sleeve

Tapped Capacitance-Graded Layer

Lower Insulator

Components of a Typical Oil-Impregnated

Capacitance-Graded Bushing


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Liquid/Compound Filler

Tapped Capacitance-Graded Core Layer

Main insulating Core

Insulating Weather shed

Capacitance Graded

Core Layers

Tap Cover

Filler Plug

Connection to Tapped Core Layer

Tap Electrode

Tap Insulation

Mounting Flange

Permanently Grounded Core Layer

Ground Sleeve

Typical Bushing Potential Tap Construction


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Condenser Bushing Construction

Main Insulation C1

Grounded Layer/Flange

Center Conductor

CK

CA = CB = CC = CD = CE = CF= CG = CH = CI = CJ

V1 = V2 = V3 = V4 = V5 = V6 = V7 = V8 = V9= V10

Tap

Electrode

Line-to-Ground System Voltage

The Condenser Bushing allows an Energized Conductor to Penetrate

a Ground Plane.

Voltage is stressed equally across each layer of the condenser bushing


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High-Voltage Cable

  • Test Includes

  • Main C1 Core Insulation

Main-Insulation/C1 Test

Standard Method

Test Mode: UST

LVL

Test Set Ground Lead

Bushing and

Apparatus Ground


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C1

Center Conductor

C2

Dielectric Circuit: Main-Insulation/C1 Test

High Voltage Lead

Low-Voltage

Lead

Test Tap

C1

Guard

Test Ground

Current & Loss

Meter

Test-Set

Ground Lead


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  • Test Includes

  • Tap Insulator

  • Core Insulation between tapped Layer and Bushing Ground Sleeve

  • Portion of Liquid or Compound Filler

  • Portion of Weathershed near Flange

LVL

Test Mode: GST-Guard

HVL

Tap-Insulation/C2 Test

Standard Method

C2

Guard


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C1

Center Conductor

C2

LVL

HVL

Tap-Insulation/C2 Test

Standard Method

C2

Guard

Test Ground

Test Mode: GST-Guard


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High-Voltage Cable

  • Test Includes

  • Main C1 Core Insulation

  • Insulating Weathershed

  • Sight-Glass

  • Lower Insulator

  • Portion of Liquid or Compound Filler

Test Mode: GST-Ground

Guard

Test Set

Ground Lead

Overall Test

Bushing and

Apparatus Ground


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LVL

Single Hot Collar Test

UST Mode

Test Set Ground Lead

Test Mode: UST

Guard

  • Test Includes

  • Portion of Insulating Weathershed

  • Sight-Glass

  • Core Insulation in Upper Area

  • Liquid or Compound Filler in the Upper Area

Bushing and

Apparatus Ground


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CVT Testing


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CT Testing


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GRADING CAPACITOR

  • The grading capacitors are having certain capacitance value .This value is need to be measured to ensure the healthiness of grading capacitor for proper voltage gradation. This value may change due to short circuiting of some capacitor stacks, which may lead to the failure of grading capacitor and circuit breaker.


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On Line Tan Delta Measurement

  • Measurement of Leakage Current from the Test Tap

  • Calculation of Capacitance and Tan Delta/ dielectric loss angle of HV & MV Bushings (ICT-6x2 Bushings and HVDC- 5x2 Bushings)

  • The Warning alarm in case of deviation/ drift to be initiated for maintenance staff

  • Second stage- Tripping of Transformers


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Sensor at the Bushing test tap


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Comparison


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Tan Delta with frequency


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Aged insulation


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CASE STUDY

  • OBJECTIVE

  • TO EVALUATE THE EFFECT OF TEMPERATURE ON TAN DELTA AND CAPACITANCE

  • TO SEE WHETHER TEMP. CORRECTION FACTORS AS GIVEN IN IEEE/C57 COULD BE APPLIED FOR POWER TRANSFORMERS OF ALL TYPES


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FIELD PROJECT

  • 4 NOS. POWER TRANSFORMERS SELECTED

  • CAPACITY 315 MVA, 400KV/220KV (2 NOS.)

    156MVA, 400KV/220KV (1 NO.)

    315 MVA, 400KV CONV. TRANS (1 NO.)

  • TEMP INCREASED TO 95-100C BY REGULATING LOAD

  • JUMPERS WERE REMOVED

  • TAN DELTA AND CAPACITANCE MEASURED WITH THREE TEST KITS

  • TEST REPEATED AFTER EVERY 5C


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TEST RESULTS FOR WINDINGS(MAKE-I)


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TEST RESULTS FOR WINDINGS(MAKE-II)


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TEST RESULTS FOR WINDINGS(MAKE-IV)


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TAN DELTA OF WINDINGS AT DIFFERENT TEMPERATURES


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DISCUSSIONS ON TEST RESULTS

1.TAN DELTA VALUES ARE DEFINITELY AFFECTED BY VARITAIONS IN THE TEMPERATURE

2. RATE OF INCREASE IS DIFFERENT FOR DIFFERENT MAKES

3.FOR ONE MAKE OF THE TRANSFORMER WINDING & BUSHINGS TAN DELTA DECREASES WITH TEMPERATURE

4.VALUE OF TAN DELTA UPTO 40°C IS ALMOST CONSTANT


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DISCUSSIONS ON TEST RESULTS

5. CAPACITANCE VALUES SLIGHTLY DECREASES WITH INCREASE IN TEMPERATURE

6. VALUES OF TAN DELTA AT HIGH TEMPERATURE WERE LESS THAN 0.005

7. TEMPERATURE COEFFICIENTS () WERE DIFFERENT FOR DIFFERENT MAKES OF TRANSFORMERS


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CONCLUSION OF CASE STUDY

  • TEMPERATURE CORRECTION FACTORS ARE DIFFERENT FOR DIFFERENT MAKE OF TRANSFORMERS

    2.HENCE CORRECTION FACTORS AS GIVEN IN IEEE/C57 CAN NOT BE APPLIED FOR ALL TRANSFORMERS

    3.SINCE DERIVATION OF TEMP. CORRECTION FACTOR IS DIFFICULT FOR ALL MAKES OF TRANFORMERS, IT IS RECOMMENDED TO MONITOR TAN DELTA AT LESS THAN 40°C


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FEW FAILURES---BINA

  • Tan delta of winding of 63 MVAr Line Reactor was found beyond permissible limit during commissioning. Tan delta could be improved after dry out of Reactor through hot air / heaters.


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FEW FAILURES---BINA

  • Tan Delta of Two CTs (One ABB make and one BHEL make) was found increased a lot in very short span of time. DGA was also found abnormal (H2 was heavily increased). CTs replaced


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FEW FAILURES---BINA

  • Tan delta of bushing of 63 MVAr Line Reactor was not getting measured. Tan delta point was found earthed. Bushing replaced.


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FEW FAILURES---BINA

  • PLCC was getting failed frequently. On detailed investigation Tan Delta point (HF point) of one of the CVT was found earthed. Insulation resistance of the point was measured. After confirmation CVT was removed from service.


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FAILURES

YOUR EXPERIENCE


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CONCLUSION

  • TAN DELTA AND CAPACITANCE TEST IS VERY IMPORTANT TEST TO MONITOR CONDITION OF INSULATION AND RESIDUAL LIFE ASSESSMENT

  • IT REQUIRES PRECAUTIONS TO BE TAKEN DURING TESTING

  • TEMP. CORRECTION FACTORS ARE TO BE APPLIED VERY CAUTIOUSLY


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