High Voltage Systems

ELEC9712

AppendixPartial Discharge

The University of New South Wales

School of Electrical Engineeringand Telecommunications

Content

Introduction

Quantities measured

Test circuits and measuring systems

Calibration

Practical considerations

1

Introduction

What is Partial Discharge?

Localized electrical discharge that only partially bridgesinsulation between conductors

Breakdown is due to presence of small defects in insulation, which creates local electric field enhancement

Different types of PDs

Internal dischargesOccurs in cavities within solid or liquid dielectricCavities surrounded by dielectric orBounded on one side by a conductor

Surface dischargesOccurs at interface between 2 different dielectrics

Corona dischargesOccurs in presence of strong inhomogeneous fields built up around sharp points or edges of HV electrodesSharp protrusions from conductor can also cause corona discharge

Damaging effects of PDCause gradual damages to insulation

Shorten lifespan of equipment

Needs to measure PDWhen designing HV equipment, strict manufacturing process needed to ensure quality of insulation

Non-destructive test techniques required to detect and identify defects present in insulation

AS 60270 – 2001 Standard

High-voltage test techniques – Partial discharge measurements

ScopeQuantities measuredTest and measuring circuitsAnalog and digital measuring methodsCalibrationTest proceduresInterference

Ca

Cb

Cc

abc model

Apparent charge: detectable PD quantity at terminals of test object. It is a representative measure of the PD, but it is NOT equal to the true amount of charge involved at the source of the PD.

Example:

Breakdown strength of air 3kV/mm (uniform field), say 2kV/mm (field enhancement).Cylindrical void (0.5mm depth, 1mm diameter) Cc=0.0139pFSay Cb=Cc/100=1.39x10-4pF (much larger depth)Terminal voltage (100kV) gives 1kV across void and cause breakdownTrue charge= 1kV x 0.0139pF = 13.9pCSay test object is 100pF so Ca=100pFVoltage dip at terminal = 1.4 mVApparent charge = 1.4mV x 100pF = 0.14pC

Paschen’s curve for air at 20oC

can only measure apparent charge at terminals

usually much smaller than actual charge at discharge site

more charge larger current more damage to insulation

2

Measured quantities

Measured Quantities

Basic quantities

Derived (integrated) quantities

Others

Measured quantities

Basic quantities related to each individual PD pulseapparent charge q (coulombs) and polarityphase angle (degrees) wrt 50Hz applied voltage

Measured Quantities

Measured quantities

Quantities derived over a time interval T (many AC cycles) to characterise overall trend of PD activity.

1. Discharge current I (A or C/s)

2. Discharge power P (W)

3. Quadratic rate D (C2/s)

4. Repetition rate n (pps)

Measured quantities

Voltage

Applied test voltage UPD inception voltage Ui

PD extinction voltage Ue

3

Test circuits andmeasuring systems

CkCa

MICDCC

Z

Ca: test object capacitanceCk: blocking capacitorZmi: input impedance of measuring systemCD: coupling deviceCC: connecting cableMI: measuring instrument

Zmi

Circuit for PD measurement

PD measuring system = CD + CC + MI

Measurement system characteristics

Transfer impedance

Lower and upper limit frequencies

Mid-band frequency and bandwidth

Superposition error

Pulse resolution time

Integration error

-6 dB

f1 f2 Frequency

PD pulse

Calibration pulse

Pass band

Measurement system

Magnitude

Lower limitfrequency

Upper limitfrequency

Frequency spectrum

Measuring system

Wide-band

Narrow-band

Measuring system

Wide-band PD instrumentsFrequency band (recommended)

Response to PD current pulse is a well-damped oscillationFrom this response, can determine apparent charge and polarity of PD current pulsePulse resolution time 5μs to 20μs

10μs/div

1V/dive.g

1

2

30 kHz 100 kHz 500 kHz

100 kHz 400 kHz

ff

f

≤ ≤≤

≤ Δ ≤

Measuring system

Narrow-band PD instrumentsFrequency band (recommended)

Response to PD current pulse is a transient oscillation+ve/-ve peak values of envelope proportional to apparent charge, independent of polarity of PD current pulseLarge pulse resolution time, typically > 80μs

50 kHz 1 MHz9 kHz 30 kHz

mff

≤ ≤≤ Δ ≤

Measuring system

Digital PD instrumentsMinimum requirement

Display value of largest repeatedly occurring PD magnitude.Update display at least every second

Additional requirementsMeasure qi, ui, ϕi of each individual PD pulse.Derived quantities I, P, D, nRadio disturbance voltage URDV

Measuring system

Largest repeatedly occurring PD magnitude

apparent charge displayed by a peak reading meter with a charge/discharge time constant < 0.44sEquivalent pulse train response:

A digital PD measuring instrument (Omicron MPD540)

Couplingcapacitor

Coupling device(quadrupole)

Acquisitionunit+battery

ComputerOpticalfibre

USBcontroller

http://mtronix.de/

A digital PD measuring instrument (Lemke LDS-6)Ref.: http://www.ldic.de

Source: http://www.pd-systems.com/

4

Calibration

Calibration

To check that specified PD magnitude measured correctly

Determine correct scale factor k for measurement of apparent charge q

Calibration is required with each new test object

Calibration: procedure Use a calibrator, test object de-energised

Inject current pulses of a known charge magnitude qo

across terminals of test object

In the range 50% to 200% of specified PD magnitude

Calibrator capacitor Co < 0.1 Ca

CkCa

MICDCC

Ca: test object capacitanceCk: blocking capacitorZmi: input impedance of measuring systemCD: coupling deviceCC: connecting cableMI: measuring instrument

Calibrator

Uo

Co

Indirect calibration

Direct calibration

Typical capacitance of HV components / equipment:

Insulators (post, suspension) some 10 pFBushings (simple, graded) ~ 100 – 1000 pFInstrument transformers ~ 200 – 500 pFPower transformers

< 1 MVA ~ 1000 pF> 1 MVA ~ 1000 – 10000 pF

HV power cablesOil-paper impregnated ~ 250 – 300 pF/mGaseous insulated ~ 60 pF/m

5

Practical issues

Permissible PD in equipment

Permissible level varies with type of equipment.

Refer to relevant standards, e.g. AS 60044.1-2007Instrument transformers - Current transformers

Need to specify if Um not less than 7.2 kV

Permissible PD in equipment

AS 60076.11-2006 Power transformers - Dry-type transformers

Need to specify if windings have Um not less than 3.6 kV

Voltage application for routine PD test

Basic PD measuring circuitfor a 3-phase transformer

Disturbances Sources of disturbances

Detecting disturbances

Disturbance levels

Reduction of disturbancesScreening and filteringBalanced circuitsElectronic processing and recovering of signalsTime-window method (phase position gating, noise sensor gating)Polarity discrimination methodsPulse averagingFrequency selectionDigital signal processing

Balanced circuit arrangement

Polarity discrimination circuit arrangementSource: AS60270-2001

Red

Yellow

Blue

50Hz

PD signal coupling between phases.

Y-phase50Hz

interference

gating pulseactive

PD pulse

gating pulsenot active

at 3.2kV, only noise, ~2500pC

Hoxton Park (14/9/01) A2 cable

at 6.4kV, detect PD of ~400pC

Australian RF communications

AM radio: 526 - 1605 kHz

CB radio: 26 - 28 MHz

FM radio: 88 - 108 MHz

Marine radio: 156 - 162 MHz

VHF TV channels 0 - 11: 46 - 216 MHz

UHF TV channels 28 - 43: 527 - 632 MHz

6

PD sensors

Capacitive: pick up PD signal through electric field

Inductive: pick up PD signal through magnetic field

Galvanic: sense PD conductive current

Radiative: antenna to sense electromagnetic waves

Pre-mouldedcable

accessories

CIGRE WG21-16

Capacitive sensors

Example of utilising coupling circuit inherent in the test object to measure PD.

http://www.pd-systems.com/

Split 1 Split 2

PDA

+ -

Ring Bus

Coaxial Signal Cable

Coaxial Signal Cable

Permanent Coupler C

Permanent Coupler C1 2

External Noise

LINE

NEUTRAL

PD

PDA capacitive HV coupler using a 28kV XLPE cable with coaxial connection to an external terminal box for on-line PD monitoring.

HF-CT clamp around conductor that connects the cable screen to Earth.

can be installed while cable is energised.

Inductive sensors

Metallic ring clamped around the plastic outer sheath of a cable with helically wound wires of the earth screen.

132kV Endurance CT Sydney West 330kV/132kV substation.

High-frequency CTwrapped around DDF link.

Rogowski coil

HF-CTs may or may not use magnetic cores, e.g. ferrite.

Rogowski coils have lower transfer impedances (hence poor sensitivity), and limited bandwith (not suitable for high frequency application).

Sensor (made by a resistor or inductor) placed across metallic screen interruption of joints that is sensitive to the current.

CIGRE WG21-16

Galvanic sensors

6

Non-standardPD measurements

(electrical)

PD measurements - electrical

Conventional (IEC 60270)Up to 1 MHz

Non-conventional (beyond 1MHz)Part of MF band (1-3 MHz)HF band (3-30 MHz)VHF band (30-300 MHz)UHF band (300-3000 MHz)

UHF PD measurements

Electromagnetic waves (TEM, TE, TM modes)

SpecFrequency range: 300 MHz to 3GHzNarrow-band bandwidth: ~5 MHzor wide-band: 300MHz to 1.5 GHz

Sensors (antennas to detect EM wave radiation)disc sensor, cone sensormicro-strip

UHF sensors for GIS (ABB)http://www.pd-systems.com/

Conclusions

AS 60270 Standard

the single most important quantity to measure is the largest repeatedly occurring magnitude of the apparent charge.

(If you can only remember 2 things from this lecture …)