SUBSTATION SURVEILLANCE USING RADIO FREQUENCY INTERFERENCE ...iiee.org.ph/wp-content/uploads/2014/12/Partial-Discharge-Detection... · substation surveillance using radio frequency

SUBSTATION SURVEILLANCE

USING RADIO FREQUENCY

INTERFERENCE (RFI)

MEASUREMENTS Eduardo Pabuna

WESTCO ELECTRICAL

[email protected]

SUBSTATION SURVEILLANCE USING RADIO FREQUENCY INTERFERENCE (RFI) MEASUREMENTS

• Introduction.

• Partial Discharge – a precursor to failure.

• Radio Frequency Interference (RFI)

detection - understanding what we are

measuring.

• Complimentary EMI detection techniques

using different sensors

• Case studies

• Conclusions

Agenda


A successful substation asset management strategy depends

heavily on predictive maintenance assessments conducted

periodically:

Surveillance based techniques to identify potential sites of

insulation degradation

Followed by more investigative and invasive quantitative

techniques

Adds value to the maintenance work, selectively identifying

and replacing those items of plant that have deteriorated

significantly.

Identifies HV apparatus that presents a potentially safety

hazard to staff and public.

Cost effective and non expert use.

Key Drivers


Surface discharges caused by

pollution on the insulator surfaces.

Leakage currents on insulator

surfaces.

Insulator damage.

Corona.

Loose connections

External discharges

Voids in the insulation.

Poor conductor-insulation

interfaces.

Metal irregularities/contaminants

Internal discharges

PDS100 – PD Surveyor

PD Detection by RFI/EMI


Partial Discharge (1)

• PD occurs when insulation defects exist which produce

distorted and enhanced electric field stress.

• Causes of PD in insulation system:

Voids in epoxy resins, polymers, paper

Bubbles in liquids/oils

Metal depositions/irregularities/contaminants

Electrodes and insulation surfaces

Poor terminations/loose joints

• Environmental stresses can act to accelerate the

deterioration of the insulation system in HV apparatus.



Can also arise through:

Poor design and manufacture

Damage of equipment

Poor installation processes/workmanship

General “ageing” or deterioration of materials

Lightening strikes, transients, overloading



• PD is a symptom of

degradation

• Once present – it

dominates as it’s own

“inherent” stress

degradation mechanism

• Precursor to complete

insulation failure and

breakdown

poor preparation

Cable Termination Area

poor preparation

poor

preparation

erosion traces


Detection of partial discharges

Detection is based on the energy exchanges that take place during the discharge

Heat Light

Chemical changes,

gases

Sound/

noise

Electromagnetic

Radiation

Impulse current

pulses

Dielectric

losses

http://www.ldic.de/en/Products/Partial_Discharge/PD_Smart.html


RFI Emissions

IEC60270 measurements

– Quantify “apparent charge” involved in PD event

– Integration of current pulse

– Requires physical connection to measure current

pulse

RFI measurements

– Radiated signals depends on the dynamics of

charge motion

– Signals generated when charge is accelerated, e.g.

• Charge oscillation

• Charge movement

– PD current pulse comprises moving charges thus

radiation will ensue


RFI Case in UK, 1984:

Experience of a transformer fire that followed a bushing failure:

During 6 months before this failure neighbors were complaining

about Radio & TV interference, foretelling what was coming.

Since then, RFI surveys are routine.


Partial discharge pulse RFI emission frequency

spectrum PD current pulse comprises moving charges thus radiation

will ensue

It emits an impulsive electromagnetic wave that travels in

the surrounding media.

Understanding What We Are Measuring


Understanding What We Are Measuring

Detecting and measuring a partial discharge event is

analogous to listening for the strike of the hammer on

the bell – at distance what we hear are the resonant

harmonics.


Discharge pulse propagation

discharge

source

RFI propagation

Amplitude of

higher

frequency

emissions drop

of quicker with

distance

Source of

discharge may

be located to an

area or specific

item of

equipment

Radio frequency, or RF signals, weaken as they travel

through free space because of divergence or dispersal,

much the same way light travels away from a light bulb.


How?

• RFI from PD source

radiates intermittently

• PDS100 scans

VHF/UHF in steps of 6

MHz for 1-1000 ms

(set by user)

• Captures RF energy

by amplitude


Scanning for RFI emissions


How?

1. Establish baseline outside

substation or switch room

2. Walk from point to point inside

substation in service

3. Look for changes in amplitude of

the RF-activity

4. If indications of PD – approach

the apparatus and see if the

activity increases


Typical RFI spectrum without discharge source

(frequency sweep mode)


Compare Background signal with signal next to

source



Frequency Range Contributions

1) Corona in air around sharp metal will be dominant up to 10

MHz but will go as high as ~250 MHz

1) Surface discharge on dirty porcelains will dominate from

10 to 250 MHz. Heavily influenced by time of year, rain

washing etc.

3) PD will extend RFI up to 1GHz on the PDS100 (but actually

can go much higher).

The higher the frequency the greater the attenuation

Localization of PD requires the using the attenuation to your

advantage!

The closer you get to the source of PD the more sensitive

(elevated) the higher frequencies


Fault Characterization

Measured impulse events from an

arcing discharge source

(time resolved mode)


Case Study 1

Survey of Switchyard revealed high

RFI in vicinity of Isolator

Deviations from Background (Black) becoming larger when approaching

the isolator

The time resolved plot

shows a typical arcing

type discharge


• It was recommended to pay particular attention to the isolator and

to look for floating metal parts.

• Several Cu braids and springs were found to be burned off and

just lying in the centre pole enclosure of the isolator. In addition

the insulators on the load end (CSBP) were replaced due to

glazing damage of the surface due to a previous flashover.

• A repeat test after repair of the isolator showed no significant

deviation from background for all HV plant in this switchyard.

Case Study 1

Survey of Switchyard revealed

high RFI in vicinity of Isolator


Case Study 2

11KV switchboard

During a routine Survey some RFI activity

was found in front of only one 11kV

switchgear panel.


Case Study 4

400/275/22kV 800MVA Transformer

• RFI scans

performed

on places

indicated by

red dots

Radiator

400 kV

275 kV

Tapchanger

L7

L2

L1

L5

L4

L3

L8 L9

L6

L10

L11 L12 L13


Case Study 4


The PD scans at location L3 and the baseline is shown .

The PD measurements showed increases in amplitudes

when compared with the baseline measurement

indicating evidence of discharge activity.


Case Study 4



Case Study 4


• RFI scan at

L8 (TDM at

arrow

790MHz)

• Time domain mode at

780MHz – Pos. L8


Case Study 4


• Clear signs of severe PD

• The transformer failed 2 weeks after this

survey


• Strong RFI emissions were detected in the range of 750 to 900 MHz

indicating significant PD activity in the oil dielectric.

• Further interrogation of RF signals were done in time domain mode at

different PD frequencies.

Case Study 4

132kV CT


Case Study 4

132kV CT

Time Resolved Mode

showing

correlation with

system voltage

DGA results confirm

internal PD

C & TD Test data of CT’s insulation

measured Tan Delta at 3.0%.


Case 5 – 300 kV - Surge Arrester

RFI measurement was carried out

during routine substation survey.

• Very high amplitudes at the

highest frequencies indicated

serious PD.

• Other arresters in the same

switchyard followed the baseline.


EM Wave EM Wave

PD

HV Busbar

EM Wave

Joint

TEV Probe

PDS 100

Metal Cladding

Photo showing the

deployment of a TEV

probe

Graphic Showing Transient Earth Voltage

Detection of Partial Discharge Activity.

Far-Field Propagation

Other decoupling options TEV


Other Sensors: High Frequency CT’s


UHF Plate Sensors (Mounted in Hatch)

UHF Drain Valve Sensors

UHF Transformer Sensors


Use of Complementary EMI Couplers

Using a UHF coupler on a GIS

Substation. Time resolved trace @ 810 MHz

Frequency plot of injected pulse


Example of a Detected GIS Defect

Frequency trace for a

floating particle

Time resolved trace @ 871 MHz;

-repetition rate 1 s-1 UHF signal @ 2.5 GSs-1


• Impulsive broadband emissions are a

characteristic of insulation defects in HV

substations.

• The PDS100 instrument platform is

specifically designed for substation

surveillance and can greatly assist in early

stage recognition and reporting of PD.

• RFI offers a routine non-invasive and cost-

effective surveillance technique.

Summary


Thank You!

QUESTION?

Documents

SUBSTATION SURVEILLANCE USING RADIO FREQUENCY INTERFERENCE ...iiee.org.ph/wp-content/uploads/2014/12/Partial-Discharge-Detection... · substation surveillance using radio frequency