Condenser Bushings: Partial Discharge (PD) Evaluation & Its Importance from the Point of View of Long-Term Reliability.

Partial discharge (PD) is a localized dielectric discharge that occurs in insulation systems under high voltage stress that only partially bridges the insulation between conductors and which may or may not occur adjacent to conductors. Partial discharges are in general a consequence of local electrical stress concentrations in the insulation or on the surface of the insulation. Generally, such discharges appear as pulses of duration of much less than 1 µs.

PD does not immediately result in a breakdown but can deteriorate insulation properties over time, leading to catastrophic bushing failure. PD activity is a key indicator of insulation degradation. Undetected or unmanaged PD occurrences can compromise the bushing’s performance and may lead to the transformer failure or unplanned outages.

PD in Insulation takes place due to inclusion of impurities in the insulation system, or entrapment of Gas Bubbles etc.

Different types of PD may be observed in bushings include:

1. Internal discharges in voids within OIP/RIP insulation.

2. Surface discharges along the contaminated surfaces.

3. Corona discharges in air gaps or at sharp edges of the conducting parts (e.g. terminal connections) where electric fields are concentrated.

Figure 1. Various partial discharges in simple insulation configurations.

Consequences of PD on Bushing Integrity

1. The presence of PD leads to gradual degradation of the bushing insulation system by:

2. Thermal damage: Localized heating caused by discharges erodes internal/external insulating materials.

3. Chemical degradation: Discharges generate ozone (O₃) gas and other reactive byproducts.

4. Carbonization and tracking: Creates permanent conducting channels/paths resulting in increased stress.

5. Mechanical weakening: PD activities may initiate cracks/flaws in composite insulation.

IEC 60137: 2017 defines maximum values of PD quantity for transformer bushings as per Table 1. These PD Values are to be assessed while lowering the voltage after application of one minute withstand voltage.

Type of bushing insulation	Maximum discharge quantity pC measured at
	Uₘ	1.5 Uₘ/√3	1.05 Uₘ/√3
Oil-impregnated paper	10	10	5
Resin-impregnated paper	10	10	5

In the long run, excessive PD activities can result in dielectric breakdown, oil contamination (in OIP bushings), and flashover.

How is PD measured at the Factory?

• The PD measurement in the factory is done in an electromagnetically shielded EHV laboratory that ensures ambient discharge values of < 2 pC. A typical PD measurement setup is shown in Figure 2.

Figure 2. PD Measurement in Transformer Bushing.

• The test circuit must be configured to ensure that background noise is lower than 2 pC. This guarantees precise and reliable measurement and monitoring of partial discharges.

• During testing, the voltage is gradually increased while monitoring for the inception of partial discharges. The PD measurements are recorded at specified test voltages as per above given table, after the one minute withstand voltage is applied and while lowering the voltage.

• The measurement shall be recorded after the dry power-frequency withstand voltage test at the values as per the insulation levels for the highest system voltage for equipment (Uₘ) during the decrease of the voltage from the dry power-frequency withstand test level. The table mentions the maximum permissible values of PD quantity.

The research (Figure 3) reveals that the PD inception voltage above the 1-minute AC withstand voltage ensures a robust insulation system with long-term PD-free operation. For ensuring insulation remains PD Free even after 30 years of service, it is necessary to ensure the PD inception is above one minute withstand voltage, ensuring insulation remains PD-free even after 30 years of service. This validates the design for lifetime reliability under electrical stress and aging conditions.

Figure 3. PD Inception Voltage- Time Characteristics

Following techniques are being experienced and attempted to detect and monitor PD Activities in the bushings:

1. Acoustic Emission method: The electrical energy of the partial discharges transforms into mechanical energy, an ultrasonic acoustic wave that spreads through the insulation systems (OIP/RIP) to the wall of external insulators (porcelain/composite/epoxy). These waves are detected by piezoelectric sensors.

2. Ultra-high Frequency (UHF) sensors: The partial discharges emit electromagnetic waves. UHF sensors capture these electromagnetic waves.

Difficulties in assessment of PD during service

• Electrical Noise and Interference: Ambient noise from switchgear, corona, switching transients, and mechanical equipment can mask actual PD signals.

• Intricacy in Signal Discrimination: It is challenging to differentiate between real PD events and external noise without advanced signal processing techniques.

• Requirement for Specialized Sophisticated Equipment: Advanced techniques (UHF sensors, acoustic sensors) and expertise are often needed for reliable diagnosis.

• Effect of Environmental Conditions: PD activities and measurement repeatability can be significantly affected by environmental factors such as temperature, humidity, and pressure.

• Risk of Misinterpretation: Misinterpretation of PRPD (phase-resolved partial discharge) patterns may either overestimate or underestimate insulation conditions, leading to poor maintenance decisions.

PD Evaluation: Key Terminologies

Parameter	Description
PD Inception Voltage (kV)	Voltage at which PD incepts/commences
PD Extinction Voltage (kV)	Voltage at which PD extincts/reduces
PD Magnitude (pC)	Largest PD pulse magnitude
PRPD Pattern	Pulse phase-resolved analysis to identify probable PD location

Preventive Measures to Mitigate PD in Bushings

• Avoid contamination: Ensure insulating materials remain free from conductive deposits, metal swarf, entrapped air, and moisture during production and assembly.

• Optimum design: Optimize the round sharp edges to enhance electric field uniformity and prevent dangerous concentrations that can cause PD and subsequently failure.

• Drying and impregnation: Effective drying of insulation to remove moisture and then impregnated with Insulating Oil.

By implementing effective monitoring practices, the risks of PD-related failures of bushings can be significantly reduced, ensuring higher long-term operational reliability and safety.

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