understanding online partial discharge testing

SEPTEMBER 4 - 6, 2019

Understanding Online Partial Discharge Testing

Sanket BolarMegger USA


Need for condition based maintenance

• Time based maintenance – Tests done at prescheduled times to assess the condition of the asset

• Example, discharge test on a battery

• Condition based maintenance – Certain parameters associated with the condition of the asset monitored continuously

• Example, online DGA on a transformer


What is partial discharge?

• Localized electrical discharge that only partially bridges the insulation


Conventional method of PD measurement

• Mainly employed in factory testing• Measured quantity is called apparent charge • Apparent charge (pC) is the pulse charge

measured at the test terminal because of the charge transfer inside the test object due to partial discharge

• Measuring impedance calibrated by injecting a controlled pulse train

• Asset energized by a test voltage source and subjected to overvoltage conditions for a specified duration

• The apparent charge measured during the course of the test needs to be less than the limit specified in the standards


Unconventional methods of PD measurement

• Using conventional method not practical onsite because of the presence of noise

• Unconventional methods used for detecting partial discharge onsite

• PD detection based on physical properties • No correlation with apparent charge• The following technologies used –

• RF detection• Acoustic methods• Optical methods• Chemical methods

PD

RFI

Sound

Gases

Light


Sensor technologies

UHF measurement• PD pulse emits electromagnetic waves• UHF range: 300MHz to 3GHz• Most of the background noise is present in

lower frequencies• Some of the sensors used: Field grading

sensors, window sensors, disc sensors, etc.


Sensor technologies

Acoustic methods• PD emits sound waves in the ultrasonic range• Measurement range: 10kHz to 300kHz• Sensors: Piezo-electric transducers,

accelerometers, condenser microphones, etc.• PD localization using multiple sensors possible• Difficulties in PD localization: Velocity of

propagation differs with media, reflections and refractions can occur


Sensor technologies

HF measurements• HFCT uses inductive coupling• PD results in short duration current pulses

which are measured by HFCT connected on the ground of the equipment

• Difficulty: HFCT can also detect current pulses from other sources on the grounding grid

• Transient Earth Voltage (TEV) sensors use capacitive coupling

• Transient voltages develop on the metal enclosure because of internal PD

• TEV sensor can be placed on enclosure to detect PD

• Used mostly on switchgear• Difficulty: External sources can also induce TEV


PD data for corona discharge


PD data for void discharge

At 5 kV At 9 kV


PD data for void discharge


PD data for surface discharge

At 3.5 kV At 6 kV



At 160 MHz



At 1.5 kV



At 9 kV


Case study I - Bushing on an oil circuit breaker


Case study II – Surface discharge on cables


Questions?

• Hope you slept comfortably

understanding online partial discharge testing

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