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April 2006 1

PD-Diagnosis Physical

basic practical

experience with OWTS

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April 2006 2

What are Partial Discharges?

Simulation

Electrical description

PD in PILC and XLPE

Solutions

Practical Experiences

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April 2006 3

Joint with Locations of possible PD Stress

1 Gap build up

2 Cavities / Voids

3 Cavities with conductive peaks

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April 2006 4

Source: TU Delft

Paper insulated three phase cable joint with PD source

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Optical and accoustical Partial Discharges (PD)

Model to simulate partial discharges

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Model of a PD fault

C

a)

Dielectric material and capacitive equivalent circuit to model an inner fault

a) High voltage insulation

b) Equivalent circuit

b)

1i(t)

u(t)

2

Test object

C

C

C

u

u F

R

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Current and voltage simulated for a PD fault

Voltage and current for the capacitive equivalent circuit

a) Graphical simulation of the voltage on C

b) Current and voltage shape measurable on the test lead

t mt

u(t)

tt

t

i(t)

b)

t mt

u (t)

u(t)

t t

t

a)u (t)10

+u z

-u z

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PD faults in paper insulated cables based on sharp edges on the conductor.

The reason can be for example poor workmanship.

Source: TU Delft

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Impurities at the inner

semicon lead to a

growth of vented

trees which often

change into an

electrical tree

Example of a cable insulation short time before a breakdown

occurs

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April 2006 10

PD Faults in XLPE power cables

PD in a PE/XLPE cable insu lation :

Impurities or changes in the outer

semiconducting layer

change of water tree in electr. tree

old cables of the first generation

e.g. initiated with test voltages

Electrical Trees lead to a breakdown of the insulation under service

conditions in a time range of weeks/months

Except treeing based on migration and diffusion processes of additives;

no measurable partial discharges !

Mechanical damages of the outer semicon

Water Trees do not create PD at all !!Due to external influences in polymeric

cables sheet faults can be determined

and localized easily

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April 2006 11

PD Faults in XLPE power cables

PD in joints and terminat ion s:

Mounting faults

Material faults

After thermal stress

PD effects in terminations start later normally under

service conditions

Silikon is able to fill microvoids and holes for some

time.

Using shrink techniques PD are directly measurable,there are no HV PD-tests for all splices

Reasons for an increase of faults in joints are mainly

poor workmanship or additional high thermal stress.

The complexivity of mounting joints lead to a high risk of subjective failures

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April 2006 12

PD Faults in PILC- cable segments

PD in joints and terminat ion s:

Mounting failures, especially in transition joints Absence of mass in cable accessories

Leakages in joints

increase of the electrical field stress in bad conductor connections

Impuritiy based increase of the electrical field stress at the outer semicon

Characteristical effects :Self healing due to mass wandering under high load

PD-intensity up to 1000 pC has long term stability

Basic rules:

PD-levels at Uo in a range of several 100 pC can be accepted

PD-levels in joints of 1000 pC at Uo repeat measurements later

to check the trend!! Mapping recommended, replace at high levels

Good cable segments are free of PD under service voltage Uo

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April 2006 13

Overview PD inception voltage and PD faults in paper insulated cables

51%

29%

20%

16%

33%

51%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

PDIV PD Localisation

Cable joints

Cable termination

Cable insulation

PD activity above 1,3Uo

PD activity upto 1,3Uo

PD activity upto Uo

Source : Frank Wester ; Nuon Alkmaar

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April 2006 14

Examples of values

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April 2006 15

Why OWTS ?

The measuring system works with the resonant method and uses a

sinussoidal oscillating wave test voltage (compact dimensions and lowweight)

The oscillating frequency depends on the cable capacitance and is fixed in

a range of 50 Hz up to several 100 Hz

The voltage stress of the test object is similar to the nominal service

conditions The duration of the voltage stress is only several 100 ms and therefore

nearly all long-term influences can be neglected.

Analysing the decrease of the test voltage, the PD extinction voltage and

the tan delta value can be determined easily.

PD level measurement according to IEC 270 at a bandwidth 150 ..650 kHz

PD fault locating and mapping feature with a bandwidth up to 3 MHz with

semi- and fully automatic analysing Software

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April 2006 16

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April 2006 17

Compact-PD Cable test van with rear and interior view

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April 2006 18

OWTS portable standalone unit with resonant coil and control unit

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April 2006 19

OWTS - Function

OWTS block diagram

Filtert

t

PD

V

PD Coupling

Unit

150 M

15 k

1 k

1 nF

10 F

Semi-Conductor

Switch IGBT

Inductance 0.7 H

Voltage Devider

Cable under

Test

DC Source

0 ... 36 kV

PC with Display and A/D

Converter

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April 2006 20

PD-Diagnosis at 50 Hz AC and OWTS Method

Ac 50 Hz Energizing OWTS

PD registration

20 ms

20 ms

1...20ms

1...20ms

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April 2006 21

PD Calibrator

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April 2006 22

Localization of the PD source with the pulse

reflection method

xlttt

xlt

Q

xt

Q

2:differencetime

2:

2impulsreflected

:2

impuls

12

2

1

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April 2006 23

OWTS - Calibration (1)

A / D Converter for PD Mapping

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April 2006 24

OWTS - Calibration (2)

A / D Converter for IEC Standard

measurements

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April 2006 25

Use of the calibration data after saving under e.g. L1_1000pC

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April 2006 26

OWTS Measuring window

Test voltage

Partial discharges

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April 2006 27

PD- Inception- and extinction voltage and service voltage

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April 2006 28

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April 2006 29

Dissipation factor measurement

Determination of the tan d

value which results from

the decrease of the test

voltage

Sensitivity: 1 E-3

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April 2006 30

Step by step evaluation of measured PD data

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April 2006 31

Zooming in on original pulse and first reflection

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April 2006 32

1100m cable, type NKBY 3x120 6/10 kV nominal voltage 6/10 kV, manual evaluation

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April 2006 33

PD threshold level for evaluation

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April 2006 34

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April 2006 35

Automatical evaluation with TDR Software-Tool

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April 2006 36

XLPE cable segment with joint problems

at 110 m L2 and L3, and L1 at 450 m

Cable length: 940 m

Installation: 1982

Service voltage: 15 kV

Test frequency: 423 HzCapacitance: 0,22 F

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April 2006 37

New 120m long 15 kV paper cable with mass problems on phase L3

from 2 m to 30 m distance from the test setup.

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April 2006 38

Mixed cable segment, 22kV, with increased PD intensity on phase L1 and customer recom-

mondation to check the transition joints at 360, 1234, 1850 m distance from the test setup

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April 2006 39

PD pattern at 8 kV in a 20 kV cable

Mapping histogram at 8 kV

TEAG PD measurement, 8 kV on L2

Example for a surface

discharges in a splice

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April 2006 40

LEW PD fault location, L2 , 20 kV XLPE cable

Two detected faults, termination and splice

Number of PD events versus location

Discharges in

a joint

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April 2006 41

PD measurement 20 kV mixed section

Needle-Plane PD in transition joint

Splice at 570 m

Phase L1, 17 kV

PD level 1200pC

20 kV on L1

PD level 6500pC