PD-INSTRUMENT TRAFO-Doble.pdf

7/28/2019 PD-INSTRUMENT TRAFO-Doble.pdf

http://slidepdf.com/reader/full/pd-instrument-trafo-doblepdf 1/25

PD IN INSTRUMENT

TRANSFORMERS



INTRODUCTIONINTRODUCTION

All electrical appliances are insulated with

liquid, solid & gaseous materials. Insulation is subjected to varying voltages,

temperature, & frequency.

The dielectric losses must be low & insulationresistance should be high in order to preventthermal breakdown of these materials.

The void formation within the insulatingmaterials must be avoided as thesedeteriorate the dielectric materials.



Partial DischargePartial Discharge-- DefinitionDefinition

Partial Discharge can be defined as localizedPartial Discharge can be defined as localized

electrical discharges in the insulating media,electrical discharges in the insulating media,

which only partially bridges the insulationwhich only partially bridges the insulationbetween the conductors.between the conductors.

It can occur in Solids, Liquids, and Gases inways which will not affect the breakdownvoltage during a “high voltage” proof test.

Yet Partial Discharge will eventually causefailure in most systems, possibly years afterthe system is placed in service.



INTERNAL DISCHARGES (VOID) :This type of discharges occurs in the cavities or inclusions inside the insulating material.

This type of discharge is seen in solids.

SURFACE DISCHARGE (FLOATINGCOMPONENTS / CORONA) :

These discharges are occurring at the surface of the

dielectric material. Such type of the dischargesoccurs in the strongly inhomogeneous field around a sharp point at high voltage.

This type of discharge is seen in Liquids / gases.

SOURCES OF DISCHARGES:



PARTIAL DISCHARGESPARTIAL DISCHARGES Examples

Corona or Gas discharge due to non-uniform fieldon sharp edges of the conductor.

Surface discharges & discharges in laminatedmaterial on the interfaces of different dielectricmaterial (gas/solid)

Cavity discharges in solid or liquid insulating

material Treeing Channels – [continuous PD] High

intensity fields are produced in an insulatingmaterial at its sharp edges and this deteriorates theinsulating material.



PARTIAL DISCHARGE PHENOMENONPARTIAL DISCHARGE PHENOMENON

Gas Discharge

Surface

Discharge

CavityDischarges

TreeingChannels



PD MEASURING TECHNIQUESPD MEASURING TECHNIQUES

Electrical method – standard & accuratestandard & accurate

Optical method –used for transparentused for transparent

materialsmaterials

Acoustic method - advancedadvanced

Noise – old, high Tanδ

is a sign of PD.

The measuring principles are based on the energyenergy

conversion processconversion process associated with electricalelectricaldischargesdischarges such as emission ofelectromagnetic light, noise or formation of

chemical compounds.



PD MEASURING TECHNIQUESPD MEASURING TECHNIQUES

Corona Discharges can be detectedACOUSTICALLY or OPTICALLY, which hasthe benefit of both detecting & locating the

source.

But a void buried in dielectric system cannot

be seen. Hence ELECTRICAL DETECTIONtechniques have been developed to detectsuch defects.



SIGNIFICANCE:

Partial discharge test helps to identify the flawsin the insulation. It is an important tool to detect:

Air gaps/voids in solid insulation. Gas bubbles in liquid/composite insulation.

Presence of unwanted / foreign particles like

metallic or magnetic materials. Loose connection on the high/low voltage

terminals.

Sharp points in the insulation. Overstress on insulation.

Improper processing or drying of insulation.



VOID DISCHARGESVOID DISCHARGES

CAUSES OF CAVITIES

Gas filled cavities in solid can result from many

causes. In case of epoxy castings, gas filled cavitiescan be caused by,

Air leaking into the mold during curing.

By insufficient pressure on the liquid epoxy duringcuring so that vapour pressure of epoxy componentcauses a gaseous cavity to develop.

THE GASES & PARTIAL PRESSURES WITHIN

THE CAVITY WILL HAVE MAJOR IMPACT ON

DISCHARGE CHARACTERISTICS.



PARTIAL DISCHARGE - THE CONCEPT

Once a discharge has been established, it can bemaintained with a lower voltage than the minimumvoltage required producing the discharge initially.

The minimum voltage to establish a discharge

initially is known as the discharge inceptiondischarge inceptionvoltage (Vi)voltage (Vi) and the minimum voltage to maintain adischarge once it has been established is known as

the discharge extinction or exception voltagedischarge extinction or exception voltage. Voltage (Ve): It is important that the exception

voltage Ve should be always greater that the ratedvoltage of the HV equipment, because, when anover voltage occurs, the PD may get established butwhen the voltage reduces to the working level, thedischarges should vanish.



CAUSES OF PD IN INSTRUMENT TRANSFORMERSCAUSES OF PD IN INSTRUMENT TRANSFORMERS ::

Presence of air bubbles and voids in regions of highelectrical stresses in the insulation.

In PTs with cast resin insulation, if the coil impregnation is

not sufficient or if the layer insulation itself has materialdefects or is damaged during winding, PD may occur.

Defective electrodes and conductive grading layers thatare arranged around the potential lead in case of PTs andthe secondary leads in case of CTs.

Sharp edges or not carefully rounded ends of suchconductive layers are the cause for the low PD inception

voltage. PDs are even initiated by impurities as well as locally

enhanced humidity values or a very high humidity contentof the insulating material itself.



PD TEST SETPD TEST SET--UPUP





REFERENCE STANDARD --- IS : 11322 - 1985

PERMISSIBLE VALUES OF PARTIAL DISCHARGE LEVELS

TYPE OFINSTRUMENT

TRANSFORMER

PRSTRESSVOLTAGE>10Sec

MEASURINGVOLTAGE>1 Min

TYPE OFINSULATION

PERMISSIBLEDISCHARGELEVEL

Current Transformer 0.8 x 1.3 Um 1.1 Umv3

SOLID 50 pC

Phase to Earth

Voltage Transformer

0.8 x 1.3 Um 1.1Um

v3

SOLID 50 pC



CT PD TEST SUMMARYCT PD TEST SUMMARY

CT Failure rate reduced from 8.1% to 2.1% for 2003 to 2010CT Failure rate reduced from 8.1% to 2.1% for 2003 to 2010



PT PD TEST SUMMARYPT PD TEST SUMMARY

PT Failure rate reduced from 20% to 0% for 2008 to 2010PT Failure rate reduced from 20% to 0% for 2008 to 2010



SNAPS OF INSTRUMENT TRANSFORMER FAILURESNAPS OF INSTRUMENT TRANSFORMER FAILURE



Mating surfaceMating surface

roughnessroughness

reduced fromreduced from

0.4micron to0.4micron to0.1micron !!!!!!!!0.1micron !!!!!!!!

SILVER PLATING FOR COPPER BUSBARSSILVER PLATING FOR COPPER BUSBARS



BEFORE AFTER

SILVER PLATING FOR CUPPER BUSBARSSILVER PLATING FOR CUPPER BUSBARS

IMPROVEMENTSIMPROVEMENTS



35.0°C

68.9°C

40

50

60

SP01

35.0°C

68.9°C

40

50

60

SP01

CT TERMINATION @ 88*C CT TERMINATION @ 48*C

COMPARISONCOMPARISON

GTGT--1 33KV CABLE CHAMBER1 33KV CABLE CHAMBER –– ‘‘ Y Y’’PHASEPHASE



CABLE TERMINATION @ 62*C CABLE TERMINATION @ 40 *C

40.3°C

45.7°C

41

42

43

44

45

SP01

39.4°C

45.5°C

40

42

44

SP01

COMPARISONCOMPARISON

14ST114ST1 33KV CABLE CHAMBER33KV CABLE CHAMBER –– ‘‘ Y Y’’PHASEPHASE



PD TESTING AT HAZIRA - BACKGROUND

z On 31st Dec’2000, at 10:22 PM, RIL Hazira witnessed

total power outage & all running plants came to a

grinding halt.z Basic cause was flashover in cable chamber of 33kv

isolator panel for GRTF-2 at Switchyard.

z Field observations revealed that fault was initiated from

33kv CT of ‘Y’ phase.

z All the three phase CTs were sent to ERDA for detailed

checking.

z 2 nos of ‘R’ & ‘B’ phases CTs failed in PD testing. ‘Y’phase CT could not be tested as it was in damaged

condition.

z Gamma Radiography was done to verify the structural

arrangement .



TEST RESULT OF GAMMA RADIOGRAPHY

FOR SWITCHYARD CTs

CT OBSERVATIONS DETAILS

‘R’ ph More gap seen in secondary winding.

Primary coil not aligned properly.

‘Y’ ph Winding not proper in secondary side.

‘B’ ph No significant defect.



THANK YOU !

Documents

PD-INSTRUMENT TRAFO-Doble.pdf