Partial discharge is an electrical discharge that occurs in a small area of insulation where the electric field strength exceeds the breakdown strength of the material. It can occur in voids within solid insulation, along the surface of insulating material, inside gas bubbles in liquid insulation.
Causes of partial discharges
According to the definition adopted by international standards, a partial discharge is an electrical discharge that locally shunts the insulation in a separate section of the structure.
This process occurs due to the ionization of a gas or liquid dielectric and can occur at the interface between two media and inside the insulation. The emergence and development depends on the type of dielectric and the design 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 inhomogeneities can be various impurities and impurities, gas cavities, humidification zones. Such defects are formed in the insulation structure, as a rule, inas a result of a violation of the process of its manufacture and during the operation of equipment (under the influence of mechanical influences, deformation, vibration).
What are trees and their formation in the structure of an insulating material
In the insulating material, from the cavity present in it, a tree-like structure is formed - treeing. Partial discharges develop in branches of treeings. Under the influence of an electric field and discharges, treeings 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 a partial discharge in a gas medium requires a lower voltage than for any effect in a liquid or in a solid foreign inclusion, the presence of such defects in the insulation can be the most likely cause of the onset of destruction of this material. This is due to the fact that in a cavity filled with gas, the electric field strength is higher than in a solid or liquid area and the electrical strength of the gaseous medium has a lower value than other insulation fractions.
Types of treeings
Trings of electrical origin are formed when exposed to alternating and impulse voltage, as well as at very high values. During operation of the equipment, these values do not cause an immediate breakdown of the insulation, but can provoke gas ionization ininhomogeneities. If there are no sufficiently large cavities in the structure of the material, dendrites can develop for a relatively long time.
The presence of oversized bubbles leads to partial discharges when the cable is operated at rated voltage.
Water trees are formed when moisture gets inside the insulation as a result of diffusion or through microcracks in the material.
When moisture condenses in inclusions, dendrites are formed here, after which their intensive formation and growth begins due to the appearance of additional voids. This leads to a decrease in the electrical strength of the dielectric and to the breakdown of the cable.
The main causes of insulation degradation include both electrical aging due to partial discharges occurring in inclusions at overvoltage and in the rated operating mode, and thermal aging of the material.
Under the influence of partial discharges, the process of insulation destruction starts, the size of the affected area increases.
The conditions for the occurrence of partial discharges depend on the shape of the electromagnetic field of the insulating structure and the electrical properties of a particular zone of the material.
Partial discharges usually do not lead to a through breakdown of the insulation, however, they cause changes in the structure of the dielectric, and with a sufficiently long operation of the system, they can cause a through breakdown of the insulating layer. Their occurrence always indicates local heterogeneity.dielectric. The characteristics of partial discharges make it possible to judge the degree of defectiveness of the insulating structure quite well.
They pose the greatest danger when equipment is operated on alternating and impulse voltage.
Physical phenomena accompanying partial discharges in insulation
Insulation overheating accelerates the process of its destruction by increasing the number of points at which new defects appear, leading to an increase in the number and volume of dendrites. This leads to increased tension in the fields of the area.
Partial electrical discharge has a thermal effect on the insulation, and also destroys it with charged particles and reactive products resulting from the discharge.
In addition, partial discharges cause the appearance of pulsed currents in the channels they create. During a breakdown, all this is accompanied by electromagnetic radiation, shock waves, light flashes and the breakdown of insulation 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 in the development of most defects in high-voltage insulation.
As a result of these processes, conditions are created for the occurrence of insulation breakdown.
Discharge stages
When a certain voltage threshold is exceeded, set for a specificinsulating material, partial discharges can be initiated in it, which do not lead to immediate burnout of the insulation, therefore, they can be quite acceptable. They got the name - initial.
Further increase in voltage, an increase in the size and number of inclusions, the number of trees in the process of continuous operation of the 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.
Effect of discharges in the structure on equipment
One of the main design elements of transformers and electrical machines is winding insulation. It is continuously exposed to such destructive factors as: thermal effects due to the long flow of currents; vibration loads due to the operation of the magnetic circuit (for transformers) and the drive mechanism (for electrical machines); consequences of inrush currents and short-circuit currents.
All of these factors lead to insulation damage and partial discharges. For electrical machines, this is the most common cause of failure, and for transformers, failure due to damage to the winding insulation is in second place after damage to the bushings.
Why you need to measure discharges
Measuring the processes that occur when partial discharges occur is necessary to be able to prevent insulation breakdown and minimize themintensity in insulating materials.
In connection with the use of XLPE insulation in the construction of power cables, power equipment, high-voltage transformers, overhead power lines, it is necessary to constantly monitor partial discharges that affect the safety of their operation.
Insulation breakdown prevention and test methods
It is necessary to carry out checks on the condition of the insulating material during operation in order to detect developing damage and prevent accidental failures due to partial discharges on equipment.
To control the degree of defectiveness of high-voltage equipment insulation, there are:
- Tests with increased voltage, equivalent in magnitude to its possible increase during operation. This is necessary to establish the values of the dielectric strength of the insulation during short-term voltage increases.
- Non-destructive test methods to determine the life of its operation.
This makes it possible to conduct reliable diagnostics on operating equipment, without decommissioning equipment, and, therefore, the elimination of economic losses.
Existing methods for diagnosing partial discharges make it possible to detect a defect at an early stage of its development and, thereby, prevent costly repairs or replacement of failed equipment.
Some methods allow you to localize the defect area, and only damaged areas will be subject to repairinsulation.
When testing equipment with high voltage, insulation quality deteriorates as a result of exposure to voltages several times higher than the working values.
Diagnostic methods for detecting partial discharge allow the most accurate assessment of the degree of residual performance of the equipment without having a destructive effect on its insulation. Diagnostics of partial discharges during operation is hampered by the fact that usually there is other equipment around the object being checked, which is a source of interference. These signals may not differ in parameters from the signals of the desired object, as they may also be partial discharges.
Therefore, in order to separate the interference signals and the measured partial discharge, you must first measure the interference signals with the voltage turned off on the object under test, and then measure it in the operating mode.
In this case, the sum of the partial discharge signals and background will be recorded.
The difference between these measurements will show the value of the PD signal.
The obtained characteristics allow us to evaluate the nature of defects and the discharge itself.
The partial discharge method does not harm the insulation and is widely used because the test process does not use high voltage to adversely affect the insulation.
Electrical discharge method
Method requires measuring instrument contact with insulation.
It allows you to define a large number of partial discharge characteristics.
This is the most accurate of allpartial discharge measurement methods.
Acoustic registration method
This method is based on the use of microphones that pick up sound signals from live equipment.
Sensors are installed in complex switchgears and other electrical power equipment and operate remotely.
Disadvantage: partial discharges of small magnitude are not recorded.
Electromagnetic or remote method
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 magnitude of the discharges.
Specific discharges in transformers
Powerful power transformers are part of the power systems, and high-voltage equipment is installed near them, in which partial discharges can exist. The signals from them are sent to the controlled transformer in various ways.
If the transformer is connected to overhead power lines that are subject to lightning, the signals from them will be recorded when measuring the characteristics of partial discharge in the transformer insulation.
When a transformer is located in an open substation, corona discharges periodically occur on its external current-carrying parts, depending on temperature, humidity and other factors.
Changing the load and the presence of devices in transformers that regulate their parameters during operation, for example, devices thatregulating operation under load, leads to a change in the characteristics of partial discharges, which may decrease or increase.
All of these factors lead to the fact that many measurements on transformers can show a distorted picture of the state of the insulation.
The readings taken from the transformer under test will be superimposed by noise pulses from nearby equipment.
In such cases, it is necessary to use a properly selected measurement technique in order to exclude the influence of interference on the received data on partial discharges in transformers.
