Pd Testing Intro08

Introduction to PD Testing

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What is a partial discharge? • Partial discharges (PD) are localized electrical discharges within an dielectric insulation system, restricted to only a part of the dielectric material, thus only partially bridging the electrodes. 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 activity has in general detrimental effects on the insulation material • Partial Discharge Measurement is sensitive to overall and localised defects

HV

Internal discharges:

• In voids or cavities within solid or liquid

materials (incl. at boundaries of different insulation materials)

• Continuous impact of discharges in solid dielectrics forms discharge channels (treeing) External discharges: • Surface discharges: At boundaries of different insulation materials ⇒ tracking

HV

• Corona discharges: Discharges in gaseous dielectrics when strong, inhomogeneous fields are present Discharges due to electrically floating potentials (related to gaseous dielectrics)

PD – wide group of discharge phenomena

HV

Reason for PD Ignition • Aging processes, due to – Electrical overstress – Mechanical overstress – Thermal overstress

• Incorrect assembly, manufacturing defects

When should HV plant be tested for PD • PD measurements should be performed at the factory to ensure correct design and manufacturing quality. • The On-site Commissioning tests and periodic PD tests to ensure that no transport damage has occurred and that the was been assembled correctly. • The On line PD monitoring provides a continuous surveillance of discharge activity for risk assessment of the asset (trend analysis, warnings, “nursing” of suspect equipment etc.

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

- Transformers

- Cables

-Switchgear

Components

Typical Locations of PD Ignition Cavities, interfaces of different dielectric properties and at sharp electrode edges and protrusions

In this Perspex block a carbonised breakdown channel is developing from one electrode. The material ahead is still insulating and high impedance. This prevents any significant current flow and the tree extends only slowly, branch by branch. But eventually it will break through.

Close up of 11kv CW Pump stator winding

Before and after lab ageing

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

Surface Effects

PD damage due to generation of Ozone Ozone is a chemically reactive gas that combines with other gases in air to create Nitric Acid

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 distributed according to capacitances, and the dielectric permittivity of the medium (gas) is usually lower than the solid. Thus, an air-filled void will have a field stress enhancement related to the dielectric/solid permittivity εr

Example – void in solid Cc: Capacitance of void Cb: Capacitance of solid in series w/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 from a single void and give rise to current pulses (pos. and neg.)

Why do PD occur? • If the voltage across the void is high enough (>the inception voltage), the field stress in the void will exceed its dielectric strength and the voltage across it collapses/breaks down, i.e. a partial breakdown/discharge has occurred • The solid dielectric in series with the void will withstand the externally applied voltage and “choke off” the PD • The PD will re-ignite if the voltage builds up to the inception voltage again or on voltage reversal • The PD breaks chemical bindings in the dielectric, the solid erodes and gets ”thinner” and eventually fails

Detection of partial discharges • Detection is based on the energy exchanges that take place during the discharge Dielectric Heat

Light

losses

Electromagnetic

Chemical changes,

radiation

gases Impulse current pulses

Sound/ noise

Macroscopic-Physical Effects HV

Detection Methods

Optical Effects (Light)

Optical

Pressure Wave (Sound)

Mechanical

Opto-acoustic

Discharge Effects Dielectric Losses High Frequency Waves

Electrical

IEC 60270

Chemical Effects

Chemical

Heat

Acoustic

HF/VHF/UHF

1. Electrical transients: The flow of charge at the defect will cause an equivalent charge transfer within the apparatus also reflected in the external circuit ⇒ allows effective and calibratable detection (IEC 60270) 2. Electromagnetic radiation: PD generates high frequency electromagn. 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.

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 surface discharges: Windings, cable terminations etc.

5. Light: Ultra violet cameras to locate PD (e.g. DayCor Corona Camera) 6. Heat: Infrared cameras/scanning to detect more intense PD

PD Detection Methods

PD Test Methods • 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 Off-Line Detection Circuit

II. TE-Messprinzipien – Elektrische TE-Messung

ƒ Measurement Impedance in Series mit Coupling Capacitor

ƒ Measurement Impedance in Series mit Test Object

ƒ Bridge Circuit

ƒ Coupling via Bushing Tap

Measurements options according IEC 60270

II. TE-Messprinzipien – Elektrische TE-Messung

ƒ Calibration Circuit

Calibration of test Circuit

II. TE-Messprinzipien – Elektrische TE-Messung

ƒ Narrow Band PD-Measurement - Bandwidth 9 kHz and 30 kHz - Centre Frequency between 50 kHz und 10 MHz

ƒ Wideband PD-Measurement - Bandwidth typically between 40 kHz und 400 kHz (IEC60270= <1MHz) - On-site sometimes higher measurement frequencies are used (typically up to 20 MHz)

PD in the Frequency Domain typical noise spectrum on-site

frequency spectrum of PD pulses frequency characteristics of PD measuring systems

– Filtering - frequency selective filtering of sinusoidal noise Windowing - Software or hardware windowing of pulse shaped periodical noise Gating / Masking – subtraction of pulse shaped stochastical noise Synchronous measurements Bridge measurement Separation of pulses based on individual pulse characteristics

Interference sources & suppression techniques

Record discharge parameters associated with every individual PD pulse. Data is analysed “instantly” to produce results.

Digital PD detectors

Internal PD – always dangerous

Void in Insulation

Tree Growth in insulation

Sharp, Irregular surface on conductor

‘Floating’ metalwork near conductors

External PD – dangerous depending on plant

Corona from sharp objects at high voltage

Surface Discharges

7x Types of Partial Discharges

Discharges from field induced situations

Corona Discharges (point at HV)

Surface Discharges

• Phase Resolved Partial Discharge (PRPD) Pattern Analysis Delamination - thermal ageing often caused by thermal stress

Pattern and distribution of pulses determines the location of the voids e.g. inside the material or delamination from the conductor

More Surface Discharges increase in test voltage, note low magnitude, analysis by magnitude only can lead to false conclusions

• Phase Resolved Partial Discharge (PRPD) Pattern Analysis Slot Discharges

Wedge

Stator Core

Conductor

Void Slot Ground Insulation

Strands

Computer Assisted PD Recognition Data base structure is open for adaptation by the user

Typical waveforms for PD Types On-Line PD Testing – Now a Field Reality Segment Waveforms 0.03 0.02 0.01

Main segment

Example of Typical Monopolar Cable PD Pulse on PILC 33kV Cable (-ve pulse)

0 -0.01 -0.02

(Frequency Band: 200kHz to 4MHz)

-0.03 0

1

2

3

4

5

6

7 8 9 Time (uSec)

10

11

12

13

14

Main Waveform 0.006

(Frequency Band: 4MHz to over 100MHz)

0.004 0.002 Chan 1

Example of Typical High Frequency, Oscillatory Switchgear PD Pulse

0 -0.002 -0.004 -0.006 0.05

0.1

0.15

Examples of Typical Noise Pulses

0.2 0.25 Time (uSec)

0.3

0.35

0.4

0.45

(Switching noise at 40KHz, RF noise at 600KHz) Segment Waveform

Segment Waveforms

Volts (mV)

0.005 0.004 0.003 0.002 0.001 0 -0.001 -0.002 -0.003 -0.004

0 35

ent

2,000 1,500 1,000 500 0 -500 -1,000 -1,500 -2,000

40

45

50

55 60 Time (uSec)

65

70

75

80

1

2

3

4

5

6 7 8 9 10 11 12 13 14 Time uSec

Ch 1 Ch 2 Ch 3

Cable PDGold© data On-LinePD PDCursors Testing –from Now a Field Reality

Segment view

Volts (mV)

25 20 15 10 5 0 -5 -10 -15 -20 -25

The PD magnitude in picoCoulombs (pCs) is the area under the PD pulse. 2

Risetime

3

4 Time uSec

Pulse Width

5

6

Falltime

A v a ila b le W a v e f o r m D is p la y 0 .0 3 5 0 .0 3

Available Wavef orm Display

0 .0 2 5 0 .0 2 0 .0 1 5 0 .0 1

0.012 0.008

Chan 1

0 .0 0 5 0 - 0 .0 0 5 - 0 .0 1

Chan 1

0.004

- 0 .0 1 5 - 0 .0 2 - 0 .0 2 5 - 0 .0 3

0 -0.004 -0.008

Noise

-0.012

- 0 .0 3 5 -0.016

2

4

6

8

10 T im e ( m S e c )

12

14

16

18 Time (mSec)

A vailable Wavef orm Display 0.012

A vailable Wavef orm Display 0.016

0.008

0.012 0.008 0.004

0 Chan 1

0

-0.004

-0.004 -0.008 -0.012

-0.008

-0.016

Cable PD

-0.012

-0.02

Noise

-0.024 -0.028

Time (mSec) A vailable Wavef orm Display

A vailable Wavef orm Display

Time (mSec)

0.028

0.032 0.028 0.024 0.02

0.024 0.02 0.016

0.016 0.012 0.008 0.004 0

0.012 0.008 Chan 1

Chan 1

Chan 1

0.004

-0.004 -0.008 -0.012 -0.016 -0.02

0.004 0

-0.004 -0.008

Cable PD

-0.024 -0.028

-0.012

Noise

-0.016 -0.02

Time (mSec)

Time (mSec)

Varying waveforms from a single 50Hz power cycle period

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

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

Na tiona l Ge rma n (DIN VDE) Limit va lue s for Volta ge PD le ve l

HV Appa ra tus

Te st Sta nda rd Bushings

Ca pa citors

Ca ble s

Inte rna tiona l (IEC) Publica tion

Re ma rks

300 pC (oil) 10 pC (GH) 10 pC (HP)

IEC 137 (1984)

(oil) Oil impregnated (GH) Cast resin impregnated (HP) Hard laminated paper

IEC 358 (1990)

Identical

IEC 885-2 (1987) IEC 885-3 (1988)

Test procedure Test procedure

DIN VDE 0674 Part 99/12.92

1.05 U / √ 3 1.5 U / √ 3

DIN VDE 0360

(1.1 Um )

(100 pC)

Part 3 A 1/08.83

1.1 Um / √ 3

10 pC

DIN VDE 0472 Part 513/07.82 DIN VDE 0271 /06.86

2 U0

20 pC (PVC)

IEC 840 (1988)

1.5 U0: 10 pC (VPE)

DIN VDE 0273

2 U0

5 pC (VPE)

IEC 502 (1994)

1.5 U0: 20 pC (VPE)

2 U0

5 pC (VPE)

1.5 U0: 40 pC (PVC)

/12.87 DIN VDE 0263 /02.91

Ca ble Joints

CT + PT

DIN VDE 0278 Part 1/02.91

Test procedure

DIN VDE 0278 Part 2/02.91

2 U0

20 pC (VPE) 40 pC (PVC)

Joints, Terminations

DIN VDE 0278 Part 6/02.91

2 U0

20 pC

Pluggable and screwable encapsulated cable terminations

DIN VDE 0414

(1.1 Um )

10 pC (liquid)

Part 10/05.85

1.1 Um / √ 3

50 pC (solid)

Tra nsforme rs a nd

DIN VDE 0532

1.3 Um

300 pC

Re a ctors

Part 3/07.87

1.5 Um / √ 3

500 pC

Dry Type Tra nsforme rs

DIN VDE 0532 Part 6/01.94

1.1 Um / √ 3

Ta p Sw itch for Tra nsforme rs

DIN VDE 0532 Part 3/04.93

Insula te d Sw itchge a rs up to 38 kV Ga s-insula te d Sw itchge a rs (GIS)

IEC 44-4 (1980)

Identical (liquid) Liquid insulation (solid) Solid insulation

IEC 76-3 (1980)

Identical

20 pC (GH)

IEC 76-3 (1982)

1.1 Um / √ 3 (GH) Cast resin impregnated

1.5 Um / √ 3

50 pC

IEC 214 (1989)

Identical

DIN VDE 0670 Part 7/09.88

1.1 U 1.1 U / √ 3

100 pC (HP) 10 pC (GH)

IEC 466 (1987)

Identical

DIN VDE 0670 Part 6/04.94

1.1 U 1.1 U / √ 3

IEC 298 (1990)

Harmonized HD 18755

DIN VDE 0670 Part 8/02.94

1.1 U 1.1 U / √ 3

IEC 517 (1990)

Identical

IEC 664-1 (1992)

Test procedure technical identic

Low Volta ge Compone nts

DIN VDE 0110 Part 20/08.90

Optocouple rs

DIN VDE 0884 /08.87

1.6 UI OR M

10 pC (GH, solid)

5 pC

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