A partial discharge is an electric 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, on the surface of an insulating material, inside gas bubbles in liquid insulation.
Causes of partial discharges
By definition adopted by international standards, a partial discharge is an electric discharge that locally shunts insulation in a separate section of the structure.
This process occurs due to the ionization of a gas or liquid dielectric and can occur on 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 the result of heterogeneities in the structure of the dielectric and the characteristics of the voltage acting on it. Such heterogeneities can be various impurities and contaminants, gas cavities, and humidification zones. Such defects are formed in the insulation structure, as a rule, as a result of a violation of its manufacturing process and during equipment operation (under the influence of mechanical stresses, deformation, vibration).
What are triings and their formation in the structure of an insulating material
In the insulating material , a tree-like structure is formed from the cavity present in it - a triing. In the branches of the triings, partial discharges develop. Under the influence of an electric field and discharges, triings 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 appearance of a partial discharge in a gaseous medium requires a lower voltage than for what effect in a liquid or in a solid foreign inclusion, the presence of such defects in the insulation may be the most likely cause of the 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 section and the electric strength of the gas medium is lower than other insulation fractions.
Types of Triings
Triings of electrical origin are formed when exposed to alternating and pulse voltage, as well as at very high values. During operation of the equipment, these values do not cause an immediate breakdown of insulation, but can provoke ionization of 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 oversized bubbles leads to partial discharges during cable operation at rated voltage.
Water triings are formed when moisture enters the insulation as a result of diffusion or through microcracks in the material.
During moisture condensation in inclusions, dendrites form here, after which their intensive formation and growth begins due to the appearance of additional voids. This leads to a decrease in the dielectric strength and to cable breakdown.
The main causes of insulation degradation include both electrical aging due to partial discharges occurring in inclusions during overvoltage and in the nominal operating mode, as well as thermal aging of the material.
Under the influence of partial discharges, the process of destruction of the insulation 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 material zone.
Partial discharges usually do not lead to 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 through breakdown of the insulating layer. Their appearance always indicates local heterogeneity of the dielectric. The characteristics of partial discharges make it quite good to judge the degree of defectiveness of the insulation structure.
They pose the greatest danger when the equipment is operated on alternating and pulse voltage.
Physical phenomena accompanying partial discharges in isolation
Overheating of the insulation 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 an increase in tension in the fields in this area.
Partial electric discharge has a thermal effect on the insulation, and also destroys it with charged particles and chemically active products resulting from the discharge.
In addition, partial discharges give rise to pulsed currents in the channels they create. During the breakdown, all this is accompanied by electromagnetic radiation, shock waves, light flashes and insulation decay 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 breakdown of insulation.
Discharge stages
If a certain threshold voltage value set for a specific insulating material is exceeded, partial discharges can be initiated in it, which do not lead to an immediate burning out of the insulation, and therefore can be quite acceptable. They got the name - primary.
A further increase in voltage, an increase in the size and number of inclusions, the number of triings 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.
The effect of discharges in the structure on equipment
One of the main structural elements of transformers and electrical machines is the insulation of windings. It is continuously exposed to such destructive factors as: thermal effects due to the prolonged flow of currents; vibration loads due to the operation of the magnetic circuit (for transformers) and the drive mechanism (for electric machines); consequences of inrush currents and short circuit currents.
All these factors lead to insulation damage and partial discharges. For electric 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 is discharge measurement necessary?
Measurement of processes that occur when partial discharges occur is necessary in order to be able to prevent breakdown of insulation and minimize their intensity in insulating materials.
In connection with the use of cross-linked polyethylene insulation in the construction of power cables, electric power equipment, high-voltage transformers, overhead power lines, it is necessary to constantly monitor partial discharges that affect the safety of their operation.
Breakdown Prevention and Test Methods
It is necessary to carry out checks of the condition of the insulating material during operation in order to identify developing damage and prevent accidental failures due to partial discharges on the equipment.
To control the degree of defective insulation of high-voltage equipment, there are:
- Tests with increased voltage, equivalent in value to its possible increase during operation. This is necessary to establish the values of the electrical strength of the insulation with short-term voltage increases.
- Non-destructive test methods for determining the lifetime of its operation.
This makes it possible to conduct reliable diagnostics on operating equipment, without decommissioning the equipment, and, therefore, eliminating economic losses.
Existing methods for the diagnosis of partial discharges make it possible to detect a defect in the early stages of its development and, thereby, prevent expensive repairs or replacement of failed equipment.
Some methods allow you to localize the area of the defect, 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 exposure to voltages several times higher than the operating values.
Diagnostic methods for detecting a partial discharge allow the most accurate assessment of the degree of residual performance of the equipment without exerting a destructive effect on its isolation. The diagnosis of partial discharges during operation is complicated by the fact that usually around the object under test there 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, it is first necessary to measure the interference signals with the voltage off at the test object, and then measure on 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 obtained characteristics make it possible to evaluate the nature of the defects and the discharge itself.
The method of partial discharges does not harm the insulation and is widely used, since the increased voltage, which negatively affects the insulation, is not used in the verification process.
Electrical discharge method
The method requires the contact of measuring instruments with insulation.
It allows you to determine a large number of partial discharge characteristics.
This is the most accurate of all partial discharge measurement methods .
Acoustic Registration Method
This method is based on the use of microphones that receive sound signals from operating equipment.
Sensors are installed in complex switchgears and other electric power equipment and operate remotely.
Disadvantage: partial discharges of small size are not fixed.
Electromagnetic or remote method
Partial discharge detection using the microwave method is a simple and efficient process. For this, a directional antenna device is used.
The disadvantage of this method is the inability to measure the magnitude of the discharges.
The specifics of discharges in transformers
Power transformers are parts of power systems, and high-voltage equipment is installed near them, in which partial discharges can exist. The signals from them in various ways arrive at a controlled transformer.
If overhead power lines undergoing lightning discharges are connected to the transformer, then the signals from them will be recorded when measuring the characteristics of the partial discharge in the transformer insulation.
When the transformer is located in an open substation, on its external current-carrying parts periodically, depending on temperature, humidity and other factors, corona discharges occur.
Changing the load and the presence in the transformers of devices that regulate their parameters during operation, for example, devices that regulate operation under load, leads to a change in the characteristics of partial discharges, which can decrease or increase.
All these factors lead to the fact that many measurements on transformers can show a distorted picture of the insulation state.
The readings taken from the transformer under test will be superimposed by interference pulses from the operation of nearby equipment.
In such cases, it is necessary to use a correctly selected measurement procedure in order to exclude the influence of interference on the received data on partial discharges in transformers.