High Voltage Systems ELEC9712 Appendix Partial Discharge The University of New South Wales School of Electrical Engineering and Telecommunications Content Introduction Quantities measured Test circuits and measuring systems Calibration Practical considerations
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ELEC9712 - Appendix Partial Discharge · High-voltage test techniques – Partial discharge measurements Scope Quantities measured Test and measuring circuits Analog and digital measuring
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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
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