PD Patterns
Mohan Watwe
11-01-2017
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PD Patterns
• Different types of PD
• Different PD patterns
• ... How to distinguish...
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Corona Discharge (I)
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Corona Discharge (II)
Characteristics:
● First pulses at negative half-wave (for point at HV)
● For higher voltage also pulses at positive half-wave of higher
magnitude
● In region of voltage peak
● Repetition rate increases for voltage increase
● Transition to dc-like current for further voltage increase
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Corona Discharge (III) – Polartiy Effect
Corona discharges at sharp edges in air
(1) Needle at HV-Potential
Negative Polarity of the
Needle
(2) Needle at Ground-Potential
Positive Polarity of the
Needle
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Corona Discharge (IV) – Point at HV
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Corona (V) – Point at Ground
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Surface Discharges (I)
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Surface Discharges (II)
Characteristics:
● Discharges appear simultaneously in both half cycles
● Between zero and voltage peak
● Disappear immediately before voltage peak
● Longitudinal extension of discharge channel is possible
● Pulse magnitude is higher than in voids or perfect air
gaps
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Surface Discharges (III)
Locations:
● Bushings
● Cable terminations
● Generator windings
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PD along Dielectric Interfaces
Characteristics:
● Discharges appear simultaneously in both half cycles
(comparable to surface discharge in air)
● Pulse magnitude may become higher than in air
(due to much higher PD interception field strength)
Locations:
● Solid layers
(e.g. paper insulated cables)
● Combination of solid and liquid insulation
(e.g. winding insulation, power transformers)
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Void Discharges (I)
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Void Discharges (II)
Characteristics:
● Discharges appear simultaneously in both half cycles
(comparable to surface discharge in air)
● Between zero and voltage peak
● No increase of pulse magnitude with voltage increase
(discharge channel can not extend as it is limited by the
geometric dimensions of the void)
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Void Discharges (III)
Locations:
● Gas-filled cavities
Main reasons for the discharge ignition inside the inclusions:
● Lower electric strength of gaseous dielectrics
(in comparison to the solid or liquid insulation)
● Field enhancement inside the void
(due to the lower dielectric permittivity of gas in comparison to
the solid and liquid insulation)
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Void Discharges (IV)
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Void and Surface Discharges
Single-side contact at one Electrode (1) – (3) PD occurs simultaneously
in both half cycles between zero
(1) (2)
crossings and maxima
(1), (2)
Min. difference of approx. factor 3
in magnitude of both half cycles
Electrode at high potential Electrode at ground potential
No contact at the electrode – Discharges between the Electrodes
(3) (3)
Max. difference of approx. factor 3
in magnitude of both half cycles
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