Electrical

What is a partial discharge?

Partial discharge is an electrical discharge and it is a localized electrical discharge that happens in an insulation system. The partial discharge is restricted to only a certain part of the dielectric. So basically partial discharge would affect the insulation life and it won’t create a complete breakdown. The partial discharge is the breakdown of the small portion of the solid or liquid electrical insulation system because of high stress. The partial discharge can happen in a solid, liquid, or gaseous insulating medium, the PD would occur across a portion of the insulation between two conducting electrodes without bridging the gap completely.

What is a dielectric strength?

The dielectric strength can be described as the ability of the insulating material to handle electrical stress. This dielectric strength of a material is dependent upon how the material is used like the temperature it is subjected to, voltage frequency, field distribution, and also the duration of the stress. If a cable is subjected to certain conditions that exceed its specified insulation range, then there will be discharge and it can be classified into two types partial discharge and complete break-down.

Where could partial discharge occur and how does it occur?

  • It can happen in voids in solid insulation like polymer, paper, etc
  • Gas bubbles in liquid insulation
  • Around an electrode in a gas
  • Partial discharge can happen in high voltage equipment where the insulation deteriorates with age or it could be because of thermal overstressing.

The partial discharge activity can be seen in all kinds of high voltage devices from the switchgear to the transformers and it can also be seen in overhead lines. The partial discharge is the cause of major failures, but it can indicate the condition of the insulation. The equipment which is subjected to partial discharge are cables, circuit breaker, insulators, generators, surge arrestors, bushings, motors, and capacitor. The partial discharge would take place if the electric field is higher than the breakdown field of an insulating medium. Field strength inside a void exceeds the stress of the surrounding dielectric and so due to this discharges occurs in the void above a definite voltage and this can be measured externally and can lead to the erosion of the surrounding material. The dielectric strength of the gases is usually low when compared to the dielectric strength of solid, liquid, or impregnated dielectrics.

What are the benefits of online partial discharge measurement?

  • It can indicate the insulation degradation in advance
  • This test is a non-intrusive test which means that it won’t interrupt the service which is performed under normal load and operating voltage
  • It won’t affect the equipment under test, and it does not test to failure
  • It won’t use any overvoltages and because of this it won’t expose the equipment which is being tested to higher voltage stresses than it experiences under the normal operating conditions
  • The measurement results will be useful for the future test
  • Mostly the partial discharge occurs within the test object and this will be helpful to repair the local problems
  • The cost to do this testing is very low when compared to the offline testing

What are the causes and effects of a partial discharge?

  • Improper installation could be a cause
  • It can happen because of aging
  • Manufacturing defects
  • Environmental problems
  • It can also happen while testing and monitoring high voltage cables
  • Weak insulation
  • Arcing because of poor contacts
  • Corona
  • Surface discharge
  • Overloading
  • Lightning strikes

If we apply high voltages to a device that produces partial discharge then we can notice that the effect would start at certain voltage level. If this is started then we must reduce the voltage to a lower voltage before it ceases. So these two voltages are called inception and extinction voltages. The partial discharge could occur during the AC voltage testing and this could occur at certain times during the positive and negative half cycle. If this takes place with proper magnitude according to a certain time then arching will damage the insulation. A tree-like pattern will be created in the dielectric and this would lead to failure and this is called erosion breakdown.

The major effect of a partial discharge is insulation breakdown, in medium and high voltage equipment the partial discharge is the indication of the insulation breakdown. The partial discharge would emit energy as electromagnetic emissions and it will be in the form of radio waves, heat, and light. It would emit ozone and nitrous oxide gases and it also does the acoustic emissions in the audible and ultrasonic range.

Effects of PD

  • Insulation degradation over time
  • Failure of the equipment
  • It will be harmful to people when they come in contact with the damaged insulation
  • Loss of money

What are the types of partial discharge ?

Cavity discharge

It occurs because of the presence of cavities in solid or liquid insulation and the discharges are formed when the gas within the cavity is over-stressed.

Surface discharge

This type of partial discharge is caused by the formation of conducting path on the dielectric surface because of the surface erosion under-voltage applications.

Corona discharge

This type of discharge happens because of the presence of inhomogeneous fields. During this process, there could be a hissing sound, violet glow, production of ozone, and radio interference.

Treeing channel

This type of discharge would happen if branching channels are created on the surface of the solid dielectric material because of cumulative particle discharges. Continuous discharge impact in solid dielectric would form discharge channels.

What is a PD test and what are the types of partial discharge testing?

By the PD testing, we are doing predictive maintenance and it would help to detect the partial discharge in cables and accessories, and thus we can replace it before a major failure occurs and if it is unchecked then the failure of equipment or the system which the cable is used could be affected and this can be avoided by doing a PD testing. The PD test would detect the insulation defects and also it can detect the insulation deterioration due to its operating conditions.

Problems that can be detect by PD tests are

  • Defective splices
  • Defective terminations
  • Contaminated or dirty terminations
  • Damaged cable insulation
  • Termination producing corona

Partial discharge testing by sensors

We can use sensors to detect the partial discharge, by using special sensors we can detect partial discharge at system voltage. Most sensors are used in areas where it is hard to access or in places where there is a safety concern. We can use permanently mounted sensors for periodic online PD detection. The sensors are attached to the equipment by the engineers and they will remain on the equipment for the online diagnostic information.  

These sensors would be connected to the splice or termination by using the frequency spectrum of the discharges. The PD signals from a cable system can be detected by using external inductive or external capacitive sensors. Built-in capacitive sensors are also used to do the PD measurement. Capacitive couplers are used in rotating machines in order to detect the PD.

Very low frequency testing

This is an AC high potential testing and this test is a replacement for the DC hi-pot testing because the DC testing is destructive and it is not capable to determine the insulation quality. The VLF testing is used to do the hi-pot or to do the withstand testing in cables this type of testing is also suitable for transformers, switchgear, rotating machines, etc. So in this test high voltage will be supplied to the insulation it is a withstand test, so high voltage will be supplied to the dielectric for fifteen minutes and the supply frequency will be 0.1HZ.

Tan delta testing

This is a type of offline testing, basically, it is a diagnostic tool that can detect the level of deterioration in the cable and also it can detect the life expectancy. This type of testing will be very useful to rate the cable and also this testing has the ability to detect the water tress. Tan delta or dissipation factor is the most powerful offline nondestructive diagnosis tool used to detect the cable insulation. This test can be performed during the manufacturing process.

Ultrasonic PD detection

When problems occur in medium and high voltage electrical equipment like the partial discharge and corona then it will create a sound wave and it can be detected by ultrasonic testing. This type of testing is done without disrupting the plant operation. This type of testing is a reliable method to check for partial discharge. In this type piezoelectric transducer is used to convert the acoustic energy into electric energy, accelerometers, and acoustic emission sensors are the sensors that are used for acoustic detection.

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