Partial discharges are small, localized electrical discharges within an insulation system that do not yet lead to complete voltage breakdown. They are an important indicator of weak points in the insulation – detecting them early prevents costly consequential damage. In modern transformer maintenance, partial discharge measurement is therefore considered one of the most informative methods for reliably assessing the condition of the insulation system throughout its service life. Partial discharges are often the first warning sign of serious insulation problems – detecting them early reliably prevents costly consequential damage.
How do partial discharges occur?
Partial discharges occur at locations with locally elevated electric field strength – for example, in tiny air pockets in the cast resin, at sharp edges, or due to contamination in the insulation system. They usually take place in nanoseconds and are neither visible nor audible during operation. Over time, however, partial discharges gradually degrade the insulation material. If left untreated, they will sooner or later lead to a complete insulation breakdown.
Key Characteristics at a Glance
- An indicator of weak points in the insulation system
- Measured in picocoulombs (pC) as a measure of discharge magnitude
- Factory routine test according to IEC 60270
- Online monitoring available for strategically important installations
- Early detection prevents costly consequential damage
- Quantitative assessment of insulation condition complements classic withstand voltage tests
- Acoustic and electromagnetic sensors complement electrical measurement
- Phase-resolved measurement allows fault types to be identified
Frequently Asked Questions
How are partial discharges measured?
In electrical partial discharge measurement, the high-frequency signal of each individual discharge pulse is captured and expressed as charge in picocoulombs. Modern methods also use acoustic sensors, UHF antennas, or fiber optics. Measurement is carried out at the factory as a routine test or on site during commissioning and maintenance.
What values are considered critical?
For new high-voltage transformers, values below 100 pC at rated voltage are usually required. Values above 500 pC indicate weak points and should be investigated more closely. For cast-resin transformers, the typical limit values are lower, as the cast material reacts particularly sensitively.
Why is partial discharge measurement so informative?
Unlike classic high-voltage tests, which only provide a pass/fail result, partial discharge measurement provides quantitative information about the condition of the insulation system. It is therefore an important tool for both quality assurance and predictive maintenance.
What typical fault types can be identified from partial discharges?
Phase-resolved measurement methods can distinguish between different fault types: air pockets in the insulating material, surface discharges, corona discharges at sharp electrodes, or discharges in oil gaps. This information is crucial for targeted repair and maintenance measures.
What measurement systems are standard in modern installations?
Common systems include electrical measurements according to IEC 60270, UHF sensors for contactless online monitoring, and acoustic sensors for localization. Multi-channel systems that simultaneously evaluate several sensor types allow a very reliable assessment of insulation condition.
Our specialists are available for partial discharge measurements, diagnosis of insulation problems, and preventive maintenance of your transformer fleet – get in touch with us.