Practical cases of fault prevented using Partial Discharge

measures

Andrej Sepcic RAM, Altanova, Italy

• Localized electrical discharge that only partially bridges the insulation between conductors

• Symptom of insulation deterioration

• A micro spark

• PD normally develop in air gaps or on insulation surfaces, due to defects in the insulation system

WHAT IS PD ?

THREE BASIC TYPES OF PD

Internal PD can occur inside the insulating system defect, such as voids or contaminants, and can eventually trigger electrical treeing.

Surface PD can occur in surfaces of insulating materials if the electric field is high, and can provoke insulation tracking.

Corona PD originated in correspondence of sharp tips or protrusions or in the HV connections, normally is not harmful.

HV

HV

HV

MAIN DEFECTS IN CABLE SYSTEMS

Main defects type:

Internal PD in the cable and its accessories

Surface PD on cable terminations

Corroded shield

Broken Jacket

MAIN DEFECTS IN CABLE SYSTEMS

HA

RM

FU

LN

ESS

MAIN INSULATION

JOINT

AND TERMINATION

SURFACE PD

CORRODED SHIELD

BROKEN JACKET

Internal PD in the cable and its accessories

Surface PD on cable terminations

WHY WE DETECT PD ?

Effective assessment with enhanced Technology able to

- find very small PD source, even below noise level – avoiding false

negatives

- separate noise and corona and identify true PD – avoiding false

positive

For a 230kV termination or joint, cost of false positive can exceed 200.000$,

considering:

- 3-5 stand-by days to repair/replace

- New joint or termination plus installation

- Extra testing day

HOW WE DETECT PD ON CABLES ?

passive inductive sensors for PD

installed around the ground connection of the accessories;

on/off line tests and permanent monitoring

Preferred sensor due to its high sensitivity

Close HFCT

Clamp HFCT

PD coupler installation along the bars

HOW WE DETECT PD ON GENERATORS ?

Capacitive Couplers PD coupler installation in the terminal box

Example of permanent sensor installation

for on-line test

HOW WE DETECT PD ON GENERATORS ?

WHAT DOES A TYPICAL RESULT LOOK LIKE

Black Phenomenon:

Background Noise

By applying real time filters, it is possible to see

the PD activities below the noise level

Pattern Recognition

Blue Phenomenon:

Surface PD

Red Phenomenon:

Internal PD

PRESENTATION GUIDELINES

Black Phenomenon

Disturbances/Noise

MULTIPLE PD

PHENOMENA

FOUND BELOW

NOISE LEVEL!

Red Phenomena:

Slot discharges

Sensitivity increase (test on 15kV Generator)

Blue Phenomena:

Embedded delamination Green Phenomena:

Stress grading

CASE STUDY 1

LOCATION EUROPE

EUT GAS TURBINE 120 MW

GENERATOR

RATED VOLTAGE

15 kV

INSULATION

VINTAGE

TYPE OF TEST

ON-LINE

PD SENSOR VARIOUS

. Boroscopic inspection confirms diagnosis.

CASE STUDY

CASE STUDY 1 – PD MEAS. SETUP

PD

Coupler

CASE STUDY 1 – PD MEAS. SETUP

PD

Couplers

Bus Bar

RESULTS U - PHASE

Classification map

Entire Pattern acquisition

Exciter noise

External discharge to

Phase U. Cross talk of distributed micro voids from Phase W

Distributed

microvoids in phase U

Discharges due to bar

to bar or bar to ground activity in overhang

Sub patterns III Level identification

RESULTS U - PHASE

Classification map Entire Pattern acquisition

Visual Inspection

RESULTS U - PHASE

RESULTS U - PHASE

PD traces found in 12 of the 60 slots

(7 white traces and 8 suspecious traces)

SLOT DISCHARGES

Results of endoscopic inspection on generator

On-line PD

measurements

Gather data to assess

the whole machine

conditions

In particular

Machine affected by:

ם Distributed micro-voids (all three phases)

ם Bar to bar (ground) discharges (phase U)

ם Slot Discharges

Unavoidable process in modern thermosetting insulation systems

The high frequency currents may degrade, in the course of time, both the stress grading system and the slot coating protection in the region where the bars leave the iron core.

Final considerations

Dangerous phenomenon: inspection and maintenance required

RESULTS U - PHASE

CASE STUDY 2

LOCATION Middle East

EUT HV Cables

RATED VOLTAGE

400kV

INSULATION XLPE

LENGTH 12km

VINTAGE New

TYPE OF TEST

Off-line

PD SENSOR High Frequency Current Transformer

Off-line PD detected inside one joint of an HV Cable

CASE STUDY 2 – PD MEAS. SETUP

GIS Termination

High Frequency PD pulses were

achieved by means of Clamp HFCT

39mm placed around the ground

connection of the GIS Termination.

Thanks to the Clamp version of the

HFCT it is possible to perform on-line

PD measurements without ground lead

disconnection or out of service of the

EUT.

HFCT Clamp

CASE STUDY 2 – PD MEAS. SETUP

HFCT Clamp

LFCT Accessible Joint

High Frequency PD pulses were achieved by means of Clamp

HFCT 39mm placed around the ground connection of the Joint.

Thanks to the Clamp version of the HFCT it is possible to perform

on-line PD measurements without ground lead disconnection or out

of service of the EUT.

RESULTS RED - PHASE

Equivalent Frequency analysis for the Internal PD activity detected

on the TF Classification Map (Red Cluster) allows to conclude:

PD activity inside one Joint of the phase under test

IDENTIFICATION

RED PHASE

R

RESULTS - LOCALISATION

Instruments made it possible to localize the Joint where PD derived from through Amplitude-Frequency analysis (First Level of PD Location).

4,5 MHz

7 MHz

RESULTS ARRIVAL – TIME ANALISYS

Second Level of PD Location consist of the Arrival Time Analysis with GPS absolute time synchronization. This analysis is 100% effective and conclusive to locate PD sources.

Termination A

GIS SYSTEM

J.B. 4

Acq. Unit #1

with on-board GPS

GPS Satellite J.B. 3 J.B. 1

Acq. Unit #2

with on-board GPS

Acq. Unit #3

with on-board GPS

RESULTS ARRIVAL – TIME ANALISYS

J.B. 4

J.B. 3

Termination A

This method allows to localize the PD source thanks to the arrival time differences of the pulses between the three different locations.

The PD was localized in J.B.

4

During the inspection, Techimp diagnosis was confirmed: Burning traces were founded in the

Joint Bay 4

SUGGESTED ACTIONS

Considering amplitude and repetition rate of detected PD it was suggested to:

1 - Replace the Joint affected from Internal PD activity in order to avoid unexpected failures

and consequent explosion during service.

2 – Regular basis PD Measurements in order to do a periodically check of the cable.

SOLUTION PD measurement on/off-line

SUGGESTED ACTIONS

Thank you for your attention !