Insulation of electrical installations

Insulation of electrical installations is divided into external and internal.

To external insulation, high-voltage installations include insulating gaps between the electrodes (wires power lines (power lines), timing tires (RU), external live parts electrical appliances etc.), in which the role of the main dielectric performs atmospheric air. Isolated electrodes are located at certain distances from each other and from the ground (or grounded parts of electrical installations) and are fixed in a certain position with the help of insulators.

To internal insulation includes insulation of windings of transformers and electrical machines, insulation of cables, capacitors, compacted insulation of bushings, insulation between the contacts of the switch in the off state, i.e. insulation, hermetically sealed from the environment by a casing, casing, tank, etc. The internal insulation is usually a combination of different dielectrics (liquid and solid, gaseous and solid).

An important characteristic of external insulation is its ability to restore its electrical strength after removal of the cause of the damage. However, the dielectric strength of the outer insulation depends on the atmospheric conditions: pressure, temperature and humidity. The dielectric strength of external insulators is also affected by surface contamination and precipitation.

The peculiarity of the internal insulation of electrical equipment is aging, i.e. deterioration of electrical characteristics during operation. Dielectric losses heat up the insulation. Excessive heating of the insulation may occur, leading to thermal breakdown. Under the influence of partial discharges occurring in gas inclusions, the insulation is destroyed and contaminated with decomposition products.

Breakdown of solid and composite insulation — an irreversible phenomenon leading to damage to electrical equipment. Liquid and internal gas insulation is self-healing, but its characteristics deteriorate. It is necessary to constantly monitor the condition of the internal insulation during its operation in order to identify the defects that develop in it and to prevent the emergency damage of the electrical equipment.

External insulation of electrical installations

Under normal atmospheric conditions, the dielectric strength of air gaps is relatively low (in a uniform field with interelectrode distances of about 1 cm ≤ 30 kV / cm). In most insulation constructions, when high voltage is applied, highly inhomogeneous electric field… The electric strength in such fields at a distance between the electrodes of 1–2 m is approximately 5 kV / cm, and at distances of 10–20 m it decreases to 2.5–1.5 kV / cm.In this regard, the sizes of overhead transmission lines and switchgear rapidly increase as the rated voltage increases.

The expediency of using the dielectric properties of air in power plants with different voltage classes is explained by the lower cost and relative simplicity of creating insulation, as well as the ability of air insulation to fully restore the dielectric strength after removing the cause of the discharge gap failure.

External insulation is characterized by the dependence of the dielectric strength on weather conditions (pressure p, temperature T, absolute humidity H of the air, type and intensity of precipitation), as well as on the condition of the surfaces of the insulators, i.e. amount and properties of impurities on them. In this regard, the air gaps are selected to have the required dielectric strength under unfavorable combinations of pressure, temperature and humidity.

The electrical strength on the insulators of the outdoor installation is measured under conditions corresponding to different mechanisms of the discharge processes, namely, when the surfaces insulators clean and dry, clean and wet with rain, dirty and damp. Discharge voltages measured under the specified conditions are called dry discharge, wet discharge and dirt, or moisture discharge voltages, respectively.

The main dielectric of the external insulation is atmospheric air — it is not subject to aging, i.e. regardless of the voltages acting on the insulation and the operating modes of the equipment, its average characteristics remain unchanged over time.

Regulation of electric fields in external insulation

With highly inhomogeneous fields in the external insulation, corona discharge is possible at electrodes with a small radius of curvature. The appearance of the corona causes additional energy losses and intense radio interference. In this regard, measures to reduce the degree of inhomogeneity of electric fields are of great importance, which make it possible to limit the possibility of corona formation, as well as to slightly increase the discharge voltages of the external insulation.

Regulation of the electric fields in the outer insulation is carried out with the help of screens on the reinforcement of the insulators, which increase the radius of curvature of the electrodes, which increases the discharge voltages of the air gaps. Split conductors are used on overhead transmission lines of high voltage classes.

Internal insulation of electrical installations

Internal insulation refers to parts of an insulating structure in which the insulating medium is a liquid, solid or gaseous dielectric, or combinations thereof, which do not have direct contact with atmospheric air.

The desirability or necessity of using internal insulation rather than the air around us is due to a number of reasons. First, the internal insulation materials have a significantly higher electrical strength (5-10 times or more), which can sharply reduce the insulation distances between the wires and reduce the size of the equipment. This is important from an economic point of view. Secondly, the individual elements of the internal insulation perform the function of mechanical fastening of wires; liquid dielectrics in some cases significantly improve the cooling conditions of the entire structure.

Internal insulating elements in high-voltage structures are exposed to strong electrical, thermal and mechanical loads during operation. Under the influence of these influences, the dielectric properties of the insulation deteriorate, the insulation "ages" and loses its dielectric strength.

Mechanical loads are dangerous for the internal insulation, because microcracks can appear in the solid dielectrics that make it up, where then, under the influence of a strong electric field, partial discharges will occur and the aging of the insulation will accelerate.

A special form of external influence on the internal insulation is caused by the contacts with the environment and the possibility of contamination and moisture of the insulation in case of breaking the hermeticity of the installation. Wetting the insulation leads to a sharp decrease in leakage resistance and an increase in dielectric losses.

The inner insulation must have a higher dielectric strength than the outer insulation, i.e. a level at which breakdown is completely excluded throughout the service life.

The irreversibility of internal insulation damage greatly complicates the accumulation of experimental data for new types of internal insulation and for newly developed large insulation structures of high and ultra-high voltage equipment. After all, each piece of large, expensive insulation can only be tested for failure once.

Dielectric materials must also:

• have good technological properties, i.e. must be suitable for high-throughput internal isolation processes;

• meet environmental requirements, i.e.they must not contain or form toxic products during operation, and after the entire resource has been used up, they must undergo processing or destruction without polluting the environment;

• not to be scarce and to have such a price that the isolation structure is economically viable.

In some cases, other requirements may be added to the above requirements due to the specifics of a particular type of equipment. For example, materials for power capacitors must have an increased dielectric constant, materials for switching chambers — high resistance to thermal shocks and electric arcs.

Many years of practice in the creation and operation of various high voltage equipment shows that in many cases the whole set of requirements is best satisfied when a combination of several materials is used in the composition of the internal insulation, complementing each other and performing slightly different functions.

Thus, only solid dielectric materials provide the mechanical strength of the insulating structure. They usually have the highest dielectric strength. Parts made of a solid dielectric with high mechanical strength can act as a mechanical anchor for wires.

Usage liquid dielectrics allows in some cases to significantly improve the cooling conditions due to the natural or forced circulation of the insulating liquid.