pd testing intro08

7/24/2019 PD Testing Intro08

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Introduction to PD Testing

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Mob: 0417 17 8026


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What is a partial discharge?

Partial discharges (PD) are localized electrical dischargeswithin an dielectric insulation system, restricted to only a

part of the dielectric material, thus only partially bridging theelectrodes. So the breakdowns stay local

The insulation may consist of solid, liquid or gaseous

materials, or any combination. The term partial discharge includes a wide group of

electrical discharge phenomena.

Materials are differently effected by PD but discharge activityhas in general detrimental effects on the insulation material

Partial Discharge Measurement is sensitive to overall andlocalised defects


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PD wide group of discharge phenomena

Internal discharges:

In voids or cavities within solid or liquidmaterials (incl. at boundaries of differentinsulation materials)

Continuous impact of discharges in soliddielectrics forms discharge channels (treeing)

External discharges:

Surface discharges: At boundaries of differentinsulation materials tracking

Corona discharges: Discharges in gaseousdielectrics when strong, inhomogeneous fieldsare present

Discharges due to electrically floating potentials

(related to gaseous dielectrics)

HV

HV

HV


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Reason for PD Ignition

Aging processes, due to

Electrical overstress

Mechanical overstress

Thermal overstress

Incorrect assembly, manufacturing

defects


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When should HV plant be tested for PD

PD measurements should be performed at the factory toensure correct design and manufacturing quality.

The On-site Commissioning tests and periodic PD tests toensure that no transport damage has occurred and that thewas been assembled correctly.

The On line PD monitoring provides a continuoussurveillance of discharge activity for risk assessment ofthe asset (trend analysis, warnings, nursing of suspectequipment etc.


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Almost all HV insulation systems can be adversely affected by PD:- Rotating Machines

- Transformers

- Cables

-Switchgear Components


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Typical Locations of PD Ignition

Cavities, interfacesof different dielectricproperties and atsharp electrodeedges and

protrusions


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In this Perspex block acarbonised breakdown

channel is developing fromone electrode.

The material ahead is still

insulating and highimpedance. This preventsany significant current flowand the tree extends onlyslowly, branch by branch.But eventually it will breakthrough.


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Close up of 11kv CW Pumpstator winding

Before and after lab ageing


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Surface Effects

Close-up of damage in previous slide. Here the PD have eroded awaythe resin leaving dry glass cloth and sheets of mica-paper


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Why do PD occur?

Practical dielectric insulation often contains voids or cavities

The voids/cavities are usually filled with a medium (i.e. gas),often with lower breakdown strength than the main

dielectric (the breakdown strength of air is ~100 times less

than many solids)

The electric field in a composite dielectric is distributedaccording to capacitances, and the dielectric permittivity ofthe medium (gas) is usually lower than the solid. Thus, anair-filled void will have a field stress enhancement related

to the dielectric/solid permittivity r


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Example void in solid

Cc: Capacitance of void

Cb: Capacitance of solid in seriesw/void

Ca: Capacitance of the rest of the

solid

Va: Applied voltage of solid

Vc: Voltage across void

V+/V-: Inception volt. for PD in void

Groups of discharges originate froma single void and give rise to

current pulses (pos. and neg.)


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Why do PD occur?

If the voltage across the void is high enough (>theinception voltage), the field stress in the void will exceed itsdielectric strength and the voltage across it collapses/breaksdown, i.e. a partial breakdown/discharge has occurred

The solid dielectric in series with the void will withstand theexternally applied voltage and choke off the PD

The PD will re-ignite if the voltage builds up to the inceptionvoltage again or on voltage reversal

The PD breaks chemical bindings in the dielectric, the soliderodes and gets thinner and eventually fails


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Detection of partial discharges

Detection is based on the energy exchanges that take placeduring the discharge

Heat Light

Chemical changes,

gases

Sound/

noise

Electromagnetic

radiation

Impulse

current pulses

Dielectric

losses


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Optical

Mechanical

Chemical

Acoustic

Opto-acoustic

HV

Electrical IEC 60270

HF/VHF/UHF

Optical Effects

(Light)

Pressure Wave

(Sound)

Discharge Effects

Dielectric Losses

High Frequency

Waves

Chemical Effects

Heat

Macroscopic-Physical

EffectsDetection Methods


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PD Detection Methods

1. Electrical transients: The flow of charge at the defect will causean equivalent charge transfer within the apparatus also reflectedin the external circuit allows effective and calibratabledetection (IEC 60270)

2. Electromagnetic radiation: PD generates high frequencyelectromagn. radiation up to 1 GHz)

VHF/UHF sensors (inductive/capacitive sensors)

Spectrum analyzers Skin effect currents leaving dielectric through gaskets

TEV

3. Chemical changes: Measurements of by-products/gases DGA in dielectric liquids SF6-gas Ozone etc.


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PD Detection Methods

4. Sound/noise/vibration: Tens of kHz regime

Contact probes/transducers to detect/locate

internal discharges (GIS, cable accessories,transformers, switchgear)

Airborne/remote detection of corona and surfacedischarges: Windings, cable terminations etc.

5. Light: Ultra violet cameras to locate PD (e.g. DayCorCorona Camera)

6. Heat: Infrared cameras/scanning to detect more intensePD


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Off-Line, On-site, Out of service

VLF: Very Low Frequency

0.1Hz OWTS: Oscillating Wave Test System

Resonant Test (10 - 400Hz)

Power Frequency 50/60Hz

On-Line, In-service

Normal working voltage

Spot Test or Continuous Monitoring

Wideband (100kHz to 400MHz)

PD Test Methods


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PD Off-Line Detection Circuit


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Measurements options according IEC 60270

Bridge Circuit

Measurement Impedance in

Series mit Coupling Capacitor

Measurement Impedance in

Series mit Test Object

Coupling via Bushing Tap

II. TE-Messpr inzipien Elektr ische TE-Messung


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II. TE-Messpr inzipien Elektr ische TE-Messung

Calibration Circuit

Calibration of test Circuit


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Narrow Band PD-Measurement

- Bandwidth 9 kHz and 30 kHz

- Centre Frequency between 50 kHz und 10 MHz

Wideband PD-Measurement

- Bandwidth typically between 40 kHz und 400 kHz (IEC60270=


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PD in the Frequency Domain

typical noise spectrum

on-site

frequency spectrum of

PD pulses

frequency

characteristics of PD

measuring systems


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Interference sources & suppression techniques

Filtering- frequency selective

filtering of sinusoidal

noise

Windowing

- Software or hardware

windowing of pulse

shaped periodical noise

Gating / Masking

subtraction of pulseshaped stochastical noise

Synchronous

measurements

Bridge measurementSeparation of pulses

based on individual

pulse characteristics


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Digital PD detectors

Record discharge parameters associated with every

individual PD pulse. Data is analysed instantly to

produce results.


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7x Types of Partial Discharges

Void in

Insulation

Sharp, Irregular

surface onconductor

Floating metalwork

near conductors

Tree Growth

in insulation

Corona from sharp

objects at high voltage

Surface

Discharges

Discharges from field

induced situations

Internal PD always dangerous

External PD dangerous depending on plant


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Corona Discharges (point at HV)


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Surface Discharges


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Phase Resolved Partial Discharge (PRPD) Pattern

AnalysisDelamination - thermal ageing often caused by thermal

stress

Pattern and distribution of pulses determines thelocation of the voids e.g. inside the material or

delamination from the conductor


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More Surface Discharges increase in test voltage, notelow magnitude, analysis by magnitude only can

lead to false conclusions


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Phase Resolved Partial Discharge

(PRPD) Pattern Analysis

Slot Discharges

Conductor

Wedge Stator Core

Slot

Strands

Ground

Insulation

Void


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Computer Assisted PD Recognition

Data basestructure isopen foradaptation

by the user


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On-Line PD Testing Now a Field RealityTypical waveforms for PD Types

Segment Waveforms

Time (uSec)

80757065605550454035

nt

0.005

0.004

0.003

0.002

0.001

0

-0.001

-0.002

-0.003

-0.004

Main Waveform

Time (uSec)

0.450.40.350.30.250.20.150.10.05

Chan1

0.006

0.004

0.002

0

-0.002

-0.004

-0.006

Example of Typical High Frequency,

Oscillatory Switchgear PD Pulse(Frequency Band: 4MHz to over 100MHz)

Segment Waveforms

Time (uSec)

14131211109876543210

Mainsegment

0.03

0.02

0.01

0

-0.01

-0.02

-0.03

Example of Typical Monopolar Cable

PD Pulse on PILC 33kV Cable (-ve pulse)(Frequency Band: 200kHz to 4MHz)

Examples of Typical Noise Pulses(Switching noise at 40KHz, RF noise at 600KHz)

Ch 1

Ch 2

Ch 3

Segment Waveform

Time uSec

14131211109876543210

Volts(mV)

2,000

1,500

1,000

500

0

-500

-1,000

-1,500

-2,000


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On-Line PD Testing Now a Field RealityCable PD Cursors from PDGold data

Risetime Pulse Width

Segment view

Time uSec

65432

Volts(mV)

25

2015

10

5

0

-5

-10

-15

-20

-25

Falltime

The PD magnitude

in picoCoulombs

(pCs) is the areaunder the PD pulse.


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A v ai lab le W av e f o r m Dis pla y

T ime (mSec ) 1 8161 412108642

Chan1

0 .035

0 .03

0 .0250 .02

0 .015

0 .01

0 .005

0

-0 .005

-0 .01

-0 .015

-0 .02

-0 .025

-0 .03

-0 .035

Av ailable Wav ef orm Display

Time (mSec )

C

han1

0.012

0.008

0.004

0

-0.004

-0.008

-0.012

Av ailable Wavef orm Display

Time (mSec )

Chan1

0.032

0.028

0.024

0.02

0.016

0.012

0.008

0.004

0

-0.004

-0.008

-0.012

-0.016

-0.02

-0.024

-0.028


Time ( mSec)

Chan1

0.028

0.024

0.02

0.016

0.012

0.008

0.004

0

-0.004

-0.008

-0.012

-0.016

-0.02


Time ( mSec)

Chan1

0.016

0.012

0.008

0.004

0

-0.004

-0.008

-0.012

-0.016

-0.02

-0.024

-0.028

Ava ilable Wavef orm Display

Time (mSec)

Chan1

0.012

0.008

0.004

0

-0.004

-0.008

-0.012

-0.016

Cable PD

Cable PD

Noise

Noise

Noise

Varying waveforms from a single 50Hz power cycle period


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Example shows cable and switchgear PD events plus exciter noise on two channels.


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PD Severity

New equipment, 5pC typical in IEC standards

Apparent charge measured Off-line

On-Line results less clear No direct electrical connection of sensors

Wideband detection

Calibration difficult to achieve without an outage


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HV Apparatus

Test Standard Voltage PD level Publication Remarks

Bushings DIN VDE 0674 1.05 U / 3 300 pC (oil) IEC 137 (1984) (oil) Oil impregnated

Part 99/12.92 1.5 U / 3 10 pC (GH) (GH) Cast resin impregnated

10 pC (HP) (HP) Hard laminated paper

Capacitors DIN VDE 0360 (1.1 Um) (100 pC) IEC 358 (1990) Identical

Part 3 A 1/08.83 1.1 Um/ 3 10 pC

Cables DIN VDE 0472 IEC 885-2 (1987) Test procedure

Part 513/07.82 IEC 885-3 (1988) Test procedure

DIN VDE 0271 2 U0 20 pC (PVC) IEC 840 (1988) 1.5 U0: 10 pC (VPE)

/06.86

DIN VDE 0273 2 U0 5 pC (VPE) IEC 502 (1994) 1.5 U0: 20 pC (VPE)

/12.87 1.5 U0: 40 pC (PVC)

DIN VDE 0263 2 U0 5 pC (VPE)

/02.91

Cable Joints DIN VDE 0278 Test procedure

Part 1/02.91

DIN VDE 0278 2 U0 20 pC (VPE) Joints, Terminations

Part 2/02.91 40 pC (PVC)

DIN VDE 0278 2 U0 20 pC Pluggable and screwable

Part 6/02.91 encapsulated cable terminations

CT + PT DIN VDE 0414 (1.1 Um) 10 pC (liquid) IEC 44-4 (1980) Identical

Part 10/05.85 1.1 Um/ 3 50 pC (solid) (liquid) Liquid insulation

(solid) Solid insulation

Transformers and DIN VDE 0532 1.3 Um 300 pC IEC 76-3 (1980) Identical

Reactors Part 3/07.87 1.5 Um/ 3 500 pC

Dry Type DIN VDE 0532 1.1 Um/ 3 20 pC (GH) IEC 76-3 (1982) 1.1 Um/ 3

Transformers Part 6/01.94 (GH) Cast resin impregnated

Tap Switch for DIN VDE 0532 1.5 Um/ 3 50 pC IEC 214 (1989) Identical

Transformers Part 3/04.93

Insula ted Swi tchgears DIN VDE 0670 1.1 U 100 pC (HP) IEC 466 (1987) Identical

up to 38 kV Part 7/09.88 1.1 U / 3 10 pC (GH)

Gas-insula ted Switchgears DIN VDE 0670 1.1 U IEC 298 (1990) Harmonized

(GIS) Part 6/04.94 1.1 U / 3 HD 18755

DIN VDE 0670 1.1 U 10 pC (GH, solid) IEC 517 (1990) Identical

Part 8/02.94 1.1 U / 3

Low Voltage Components DIN VDE 0110 IEC 664-1 (1992) Test procedure technical identic

Part 20/08.90

OptocouplersDIN VDE 0884 1.6 U

IORM5 pC

/08.87

Table 1: Summary of national and international partial discharge test standards for HV apparatus, derived from IEC 270 respectively DIN VDE 0434

National German (DIN VDE) International (IEC)

Limi t values for

pd testing intro08

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