Partial discharge detection is a proven method for detecting the early sign of electrical insulation degradation in medium voltage and high voltage equipment. It is supported by several documents and committees such as the International Electrotechnical Commission (IEC) and the Institute of Electrical and Electronics Engineers (IEEE).
What are partial discharges ?
Partial discharges are defined in IEC 60270 as a localized electrical discharge that partially bridge the insulation between conductors and which can or cannot occur adjacent to a conductor. Partial discharges occur in almost all types of insulation (gaseous, liquid and solid). Take the example of a void in an ipoxy insulation. The air permittivity is a lot smaller than the permittivity of ipoxy resulting in a very high electrical field across the void. If the voltage across the void becomes higher than the dielectric strength of air, a discharge occurs. The following image shows this phenomenon.
A partial discharge can also occur around sharp edges because of the surrounding high concentration of electrical field.
Effects on electrical equipment
Electrical insulation is never perfect. Imperfections and irregularities can occur at many stages including manufacturing, transportation, installation and maintenance testing if not performed properly.
Throughout the life of the equipment, the electrical insulation is subject to thermal, electrical and mechanical stresses. Other factors, such as normal aging and the environmental condition in which the equipment is located, should also be taken in consideration.
For example, the next image shows different imperfections that can be found in a single-phase power cable.
Over time, those imperfections or small defects can become worse and ultimately, create the complete breakdown of the insulation.
The following pictures show actual cases of problems created by partial discharges.
Partial discharge measurement
Partial discharge measurement can be performed as a routine test, during commissioning and periodically as preventive maintenance.
Measurements performed as routine tests are usually done in a specialized high voltage laboratory with a controlled environment and a very low ambient noise. They are done according established standards.
Partial discharge measurements for commissioning and preventive maintenance can be more difficult to perform because of the environment of the field testing. Several mitigation methods are used to distinguish the external noises and the actual partial discharges.
Our team can perform partial discharge measurement in our high voltage laboratory and in the field. We can do those measurements on motors, generators, transformers, medium voltage cables and on many other electrical equipment. Usually, it is also possible to find the location of the partial discharges occurring in a cable.
Furthermore, we can provide and install permanent inductive or capacitive sensors to perform on-line partial discharge measurements, therefore eliminating the need of unnecessary outages.
Why perform partial discharge measurements
Unlike dielectric withstand tests like Hipot and VLF, partial discharge measurements are non-destructive tests. This means that it is more unlikely to damage equipment while performing partial discharge measurements. Indeed, when performing online measurements, the test voltage is the operation voltage. The device under test is then not stress with a higher voltage. When performing off-line measurement, the partial discharge magnitudes and repetition rates are constantly monitored. This allows the operator to stop the test if needed before damaging the test object.
Partial discharge measurement is an excellent method to assess the condition of motors, generators and power transformers. The test objects can be tested at fixed intervals and a trend can be analyzed.
Also, testing medium voltage cables for partial discharges and measuring the dissipation factor is excellent methods to assess the condition of a cable. Like power transformers, motors and generators, a trend can be established and analyzed over time.
For more information or for any inquiries, do not hesitate to communicate with us.