Partial discharge in isolation: the process of partial discharge

Partial discharge is an electrical discharge that occurs in a small area of insulation, where the strength of the electric field exceeds the breakdown strength of the material. It can occur in voids within the solid insulation, along the surface of the insulating material, inside gas bubbles in liquid insulation.

The causes of partial discharges

By definition, adopted by international standards, a partial discharge is called an electric discharge, locally shunting the insulation on a separate section of the structure.

This process arises from the ionization of a gas or a liquid dielectric and can occur at the interface between two media and inside the insulation. The origin and development depends on the type of dielectric and the structural features of the insulation of the object. Partial discharges in insulation are a consequence of the presence of inhomogeneities in the structure of the dielectric and the characteristics of the voltage acting on it. Such heterogeneities can be various extraneous impurities and contaminants, gas cavities, humidification zones. Such defects are formed in the insulation structure, as a rule, as a result of disruption in the process of its manufacture and in the operation of equipment (under the influence of mechanical influences, deformation, vibration).

What are triands and their formation in the structure of the insulating material

In the insulating material , a tree-like structure is formed from the cavity present in it, a tri-ing. Partial discharges develop in the branches of the triands. Under the influence of the electric field and discharges, the triands increase in size and quantity, thereby increasing the degree of degradation of the polymer material. Dendrites have increased conductivity and lead to progressive destruction of the dielectric.

Since the discharge of a partial discharge in the gaseous medium requires a voltage lower than that for the same effect in the liquid or in a solid foreign inclusion, the presence of such defects in the insulation can become the most probable cause of the beginning of the destruction of this material. This occurs as a result of the fact that in a cavity filled with gas the electric field strength is higher than in a solid or liquid region and the electrical strength of the gaseous medium has a lower value than other fractions of insulation.

Types of triings

Electric-induced triands are formed by the action of alternating and pulsed voltages, as well as at very high values. During operation of the equipment, these values do not cause immediate breakdown of insulation, but can provoke ionization of the gas in inhomogeneities. If the structure of the material does not have sufficiently large cavities, dendrites can develop for a relatively long time.

The presence of bubbles of increased size leads to the occurrence of partial discharges when the cable operates at the rated voltage.

Water trings are formed when moisture enters the interior of the insulation as a result of diffusion or through microcracks in the material.

When condensation of moisture in the inclusions, dendrites are formed, after which their intensive formation and growth begins due to the appearance of additional voids. This leads to a decrease in dielectric strength and to breakdown of the cable.

The main reasons for the degradation of insulation include both electrical aging due to partial discharges occurring in inclusions during overvoltage and in the rated operating mode, as well as thermal aging of the material.

Under the influence of partial discharges, the process of destruction of insulation starts, the size of the affected area increases.

The conditions for the appearance of partial discharges depend on the shape of the electromagnetic field of the insulating structure and the electrical properties of a particular zone of material.

Partial discharges usually do not lead to a through-breakdown of insulation, but are the cause of changes in the structure of the dielectric, and if the system is operated for a fairly long time, it can cause a breakdown of the insulation layer. Their occurrence always indicates a local inhomogeneity of the dielectric. Characteristics of partial discharges are sufficiently good to judge the degree of defectiveness of the insulating structure.

The greatest danger they represent when the equipment operates at alternating and pulsed voltage.

Physical phenomena accompanying partial discharges in isolation

Overheating of insulation leads to an acceleration of the process of its destruction due to an increase in the number of points in which new defects appear, leading to an increase in the amount and volume of dendrites. This leads to an increase in the intensity of the field in this area.

A partial electrical discharge exerts a thermal effect on the insulation, and also destroys it by charged particles and reactive products formed as a result of the discharge.

In addition, partial discharges cause the generation of impulse currents in the channels created by them. In the event of breakdown, all this is accompanied by electromagnetic radiation, shock waves, light flashes and the decay of the isolation at the molecular level.

Partial discharges are among the main causes of damage to high voltage equipment. This is explained by the fact that the appearance of partial discharges is the initial stage of development of most defects in high-voltage insulation.

As a result of these processes, conditions are created for the occurrence of insulation breakdown.

The stages of discharges

If a certain voltage threshold set for a particular insulating material is exceeded, partial discharges can be initiated in it, which do not lead to an immediate burnout of the insulation, and therefore can be completely tolerated. They got the name - initial.

Further increase of voltage, increase in the size and number of inclusions, number of triings in the process of continuous operation of equipment leads to a sharp increase in the intensity of partial discharges. Their occurrence sharply reduces the shelf life of the insulation and can lead to its breakdown. Such discharges are called critical.

Influence of discharges in the structure on equipment

One of the main elements of the design of transformers and electrical machines is the insulation of the windings. It is continuously subject to such destructive factors as: thermal effects due to prolonged flow of currents; Vibration loads due to the magnetic circuit (for transformers) and the drive mechanism (for electric machines); Consequences of leakage of starting currents and short-circuit currents.

All these factors lead to damage to insulation and the occurrence of partial discharges. For electric machines, this is the most common cause of failure, and for transformers, failure as a result of damage to the insulation of the windings is second only to damage to the inputs.

Why you need to measure discharges

The measurement of the processes occurring when partial discharges occur is necessary to obtain the possibility of preventing the breakdown of insulation and the maximum decrease in their intensity in insulating materials.

In connection with the use of XLPE insulation in the construction of power cables, electrical equipment, high-voltage transformers, overhead transmission lines, it is necessary to constantly monitor the partial discharges affecting the safety of their operation.

Prevention of insulation breakdowns and test methods

It is necessary to check the condition of the insulating material during operation in order to detect the developing fractures and to prevent emergency failures due to partial discharges on the equipment.

To control the degree of defectiveness of insulation of high-voltage equipment, there are:

• Tests with increased voltage, equivalent in magnitude to its possible increase during operation. This is necessary to establish the values of the electrical strength of insulation during short-term voltage rises.
• Nondestructive test methods for determining the time resource of its operation.

This makes it possible to conduct reliable diagnostics on operating equipment, without removing equipment from operation, and, therefore, excluding economic losses.

Existing methods for diagnosing partial discharges can detect a defect in the early stages of its development and, thereby, prevent expensive repairs or replacement of failed equipment.

Some methods allow localizing the defect area, and only damaged areas of insulation will be subject to repair.

When testing equipment with high voltage, the insulation quality deteriorates as a result of stresses several times higher than the operating values.

Diagnostic methods for detecting partial discharges allow the most accurate estimate of the degree of residual performance of the equipment without rendering a destructive effect on its isolation. The diagnostics of partial discharges during operation is complicated by the fact that usually around the object being inspected is other equipment that is a source of interference. These signals may not differ in parameters from the signals of the desired object, since they can also be partial discharges.

Therefore, to separate the interference signals and the measured partial discharge, you must first measure the interference signals with the voltage disconnected at the tested object, and then measure it in the operating mode.

In this case, the sum of the partial discharge and background signals will be recorded.

The difference in the results of these measurements will show the value of the partial discharge signal.

The characteristics obtained make it possible to estimate the character of the defects and the discharge itself.

The partial discharge method is not harmful to insulation and is widely used, since the test does not use increased voltage, which negatively affects the insulation.

Electric discharge method

The method requires the presence of a contact of measuring instruments with insulation.

It allows to determine a large number of characteristics of a partial discharge.

This is the most accurate of all methods of measuring partial discharges.

Acoustic recording method

This method is based on the use of microphones that receive audio signals from operating equipment.

Sensors are installed in complex switchgears and other electrical equipment and operate remotely.

Disadvantage: partial discharges of small magnitude are not fixed.

Electromagnetic or remote method

The detection of partial discharges using the microwave method is a simple and effective process. For this, a directional antenna device is used.

The disadvantage of this method is the impossibility of measuring the magnitudes of the discharges.

Specificity of discharges in transformers

Powerful power transformers are parts of power systems, and high voltage equipment is installed near them, in which partial discharges can exist. Signals from them come in various ways to the monitored transformer.

If overhead power lines subjected to lightning discharges are connected to the transformer, signals from them will be recorded when measuring the partial discharge characteristics in the transformer insulation.

When the transformer is located in an open substation, corona discharges occur periodically on its outer live parts, depending on temperature, humidity and other factors.

The change in load and the presence in the transformers of devices that regulate their parameters during operation, for example, devices that regulate operation under load, lead to a change in the characteristics of partial discharges, which can be reduced or increased.

All these factors lead to the fact that many measurements on transformers can show a distorted picture of the state of insulation.

The readings taken from the transformer under test will be impulses of interference from the work of the nearby equipment.

In such cases, it is necessary to use a correctly selected measurement technique to exclude the influence of interference on the obtained data on partial discharges in transformers.