Like any other electrical equipment installed in a substation, power transformers not least need reliable electrical insulation. And if we talk specifically about transformers, then they can clearly separate the internal insulation and external insulation.
External insulation (air gaps)
The outer insulation of a transformer mainly includes air gaps. In practice, these are all possible gaps between the external conductive parts of the transformer, one way or another connected with the inputs:
- between winding inputs;
- along the porcelain covers of these inputs,
- as well as between the inputs and the grounded tank of the transformer.
Internal insulation (longitudinal and main)
The internal insulation of a transformer includes:
- insulation of the oil part of the leads;
- isolation between taps and auxiliary devices (such as switches),
- as well as winding insulation, which is divided into main and longitudinal.
Longitudinal winding insulation
The longitudinal insulation of a winding is the insulation between adjacent layers of its turns. It can have different dimensions and be made in the form of various structures, determined by lightning surges. In order for the voltage on the longitudinal insulation to be as low as possible during impulse actions, special capacitive screens are additionally installed and twisted windings are used with the turns connected in a certain sequence. To reduce impulse voltages, it is necessary to increase the operating voltage on the longitudinal insulation: a significant potential difference is obtained between adjacent turns. But the dimensions of the longitudinal insulation are still not too large.
Main winding insulation
The main winding insulation is:
- isolation between winding and magnetic core;
- between the windings belonging to the same core of the magnetic core;
- between the outer parts of the windings installed on adjacent rods of the magnetic core,
- as well as the insulation between the outer part of the winding and the wall of the transformer tank.
Until recently, the main insulation of the windings owed its dimensions primarily to lightning surges. Today, when the rated voltages in electrical networks are hundreds of kilovolts, the determining factor for building the main insulation of the transformer windings are internal overvoltages.
When internal overvoltages are effectively limited, the question arises of the dielectric strength of the main insulation over a long period of time, as well as the insulation distance.
One of the important factors influencing the design of insulation in a power transformer is the topic of heat removal. The bottom line is that the copper windings and the iron magnetic circuit during the operation of the transformer noticeably heat up. This means that the insulation must be such that it allows continuous cooling of the heating parts inside the transformer tank.
Oil barrier insulation
A modern way to implement the main insulation in a power transformer, taking into account the requirements regarding cooling, is the use of oil barrier insulation. As for the taps, they are isolated separately.
The longitudinal insulation of the windings may be oil-paper, or the coils and turns may be covered with their own insulating material.
The advantage of oil barrier insulation is that due to the circulation of oil in the tank, a sufficiently effective cooling of the transformer structure is automatically carried out. In addition, such insulation exhibits high short-term dielectric strength.
The barrier and its features
For the effective operation of the dielectric barrier, it must be located across the direction of the lines of electric field strength. For a bushing insulator, this condition is easily met by using a cylindrical barrier, since the electric field there is directed radially. But directly in the power transformer, the electric field is difficult to direct, so barriers of various shapes are combined here: a cylinder, a flat washer, an angled washer.
The higher the rated voltage, the more cardboard barriers are installed. And in order to even out the voltage distribution along the winding during lightning overvoltage, as well as to optimize the configuration of the electric field at the edge of the winding, a conductive ring with a cut is installed at its input zone, which increases the capacitance between the entry point to the winding and the rest of the coil. In addition, the longitudinal capacitance between the turns is increased by using a twisted winding. This is done to improve the distribution of surge voltage from the lightning surge phenomenon.
Oil-paper insulation has a higher electrical strength than oil-barrier insulation and makes it possible to reduce the dimensions of power transformers when used as the main winding insulation, which is extremely important for high-power transformers due to their ease of transportation. But paper-oil insulation has one serious drawback - it does not allow you to achieve the same effective cooling as with oil.
Managing Moisture in a Power Transformer: