Transformer insulation classification

   Each power transformer, when assessing its dielectric strength, can be represented as consisting of three systems - a system of parts that are energized in a powered transformer, a system of grounded parts and an insulation system that separates both the first two systems and separate parts that are energized.
   The system of live parts includes all metal parts and parts that are used to conduct the operating current (windings, contacts of voltage step switches, taps, feed-through buses and bushing pins, etc.), as well as all parts galvanically connected to them (protective screens, capacitor rings, metal caps of bushings, etc.). The system of earthed parts should include: a magnetic system with all metal parts that serve for its fastening; tank and cooling system, also with all parts and metal fittings in oil-immersed transformers or a protective casing in dry-type transformers.
   Insulation separating live parts from each other and separating them from grounded parts in power transformers is made in the form of structures and parts made of solid dielectrics - electrical insulating cardboard, cable paper, varnished cloth, wood, textolite, paper-bakelite products, porcelain, etc. other materials. The parts of the insulating gaps not filled with a solid dielectric are filled with a liquid or gaseous dielectric - transformer oil in oil transformers, atmospheric air in dry transformers. Other liquids and gases are sometimes used as such a dielectric, and the whole transformer is filled with a compound or filled with quartz sand. 
   Insulation of windings can be subdivided into
    the main insulation, that is, the insulation of each of the windings from the grounded parts and from other windings, and the longitudinal insulation - between different points of the given winding, that is, between the turns, layers and elements of capacitive protection. The isolation of taps and switches can also be subdivided in the same way. The division of insulation into main and longitudinal can be attributed to oil and dry transformers. The voltage class of the winding is the continuous operating voltage. The voltage class of the transformer winding coincides with the rated voltage of the electrical network into which the winding is connected. The voltage class of the transformer is the voltage class of the high voltage winding. Each voltage class of the transformer is assigned a rated operating voltage and certain test alternating voltages at 50 Hz and impulse. So, for a voltage class of 35 kV, the nominal voltages according to GOST are 35; 36.75 and 38.5 kV; the highest operating voltage is 40.5 kV; test alternating voltage 50 Hz is 85 kV. and the pulse for the full wave is 200 kV.   

Requirements for transformer insulation

    The insulation of the transformer must withstand without damage the electrical, thermal, mechanical and physicochemical effects to which it is exposed during the operation of the transformer.
The cost of insulation is a significant proportion of the cost of a transformer. For transformers of voltage classes 220-500 kV, the cost of insulation, including oil, reaches 15-20% of the cost of the entire transformer.
   The main tasks in the design of transformer insulation are: determination of those influences, primarily electrical, which the insulation is subjected to during operation; selection of the basic design of insulation and the shapes of insulating parts; selection of insulating materials filling the gaps and the dimensions of the gaps.
    In operation, the power transformer is constantly on, and its insulation is under prolonged exposure to the operating voltage, which it should
withstand without any damage indefinitely. Permissible continuous overvoltage must be specified in the standards for specific types and groups of transformers. According to the GOST requirement, power transformers must also be designed to operate under certain conditions with short-term voltages exceeding the rated voltage up to 15 and 30%. In the electrical system in which the transformer operates, due to normal switching processes (switching on and off high powers, etc.) or emergency processes (short-circuit, line breaks, etc.), short-term overvoltages occur, reaching in some rare cases values close to four times the phase voltage. The duration of these overvoltages is measured in hundredths of a second and, as a rule, does not exceed 0.1 s. Normal operating voltage and switching overvoltage mainly affect the main winding insulation. Overvoltage impulse waves caused by lightning atmospheric discharges can also occur in the overhead network. When they reach the transformer, they act on its insulation. Atmospheric overvoltages in some unfavorable cases reach 10 times the phase voltage with a duration measured in microseconds. The impact of atmospheric lightning surges affects mainly the longitudinal insulation of the transformer windings, in particular, the insulation between turns, between layers of turns and between individual winding coils.
     In the event of overvoltages of one type or another, in the case of insufficient dielectric strength of the insulation, an electric discharge or even a breakdown may occur, i.e., local destruction of the insulation. To simplify the calculation and standardization of the requirements for the dielectric strength of the insulation of the finished transformer, the electrical calculation of the insulation is made so that it can withstand the acceptance and acceptance tests provided for by the relevant standards. The test standards are drawn up taking into account the values, duration and nature of electrical influences that are possible in practice, contain the necessary safety margins and are enshrined in GOST. The standards are periodically revised in accordance with the clarification of the technical requirements for transformers, the development of their production and the improvement of operating conditions. These standards are strictly mandatory for all enterprises producing transformers (see table). 

 

Voltage class, kV

 

      Test voltages

applied effective Utest kV

pulse amplitude (kV) at wave

complete

cut

3

18

44

50

6

25

60

70

10

35

80

90

15

45

108

120

20

55

130

150

35

85

200

225

110

200

480

550

150

230

550

600

220

325

750

835

330

460

1050

1150

500

630

1550

1650

 The electrical strength of the transformer insulation is ensured, first of all, by the correct consideration of those electrical influences that this insulation experiences in operation, and the correct choice of standards, i.e., test voltages and methods of influencing the insulation during acceptance and acceptance tests of transformers. It is the conditions of dielectric strength that determine the choice of the fundamental design of the insulation and the shapes of its parts. The windings and all current-carrying parts of the transformer are heated during its operation. Both long-term and short-term (emergency) exposure to high temperatures on winding insulation causes aging of the insulation, which gradually loses its elasticity, becomes brittle, its electrical strength decreases, and it collapses. In a properly sized and properly operated transformer, the winding insulation should last 25 years or more.
    The necessary heat resistance of the insulation, which guarantees long-term trouble-free operation of the transformer, is achieved by limiting the permissible temperature of its windings and oil, the use of insulation materials of the appropriate class that can withstand prolonged exposure to the permissible temperature, and a rational design of the windings and insulating parts, ensuring their normal cooling. When electric current passes through the windings and other live parts, mechanical forces arise between them. In an emergency case of a short-circuit of a transformer, the mechanical forces, reaching values that are higher, the greater the power of the transformer, can cause destructive stresses in the intercoil or support insulation of the windings.
     The choice of insulating materials is made taking into account the insulating properties, mechanical strength and chemical resistance in relation to transformer oil in the case of an oil transformer. The material should not enter into chemical reactions with oil at temperatures up to 110 ° and should not contribute to chemical and physical changes in the oil as a catalyst. In transformer construction, sufficient experience has been accumulated to select insulating materials for oil and dry transformers that have the necessary insulating properties, are chemically resistant and have sufficient mechanical strength to withstand mechanical stress during emergency processes in the transformer. The materials used in oil transformers, such as insulating cardboard, paper of various grades, porcelain, cotton tape, do not chemically interact with oil, do not degrade themselves and do not contribute to chemical decomposition and oil contamination. Insulating materials that have resins, varnishes and enamels in one form or another, for example, enamel wire insulation, paper bakelite products, varnished cloth, textolite should contain resins, varnishes and enamels that are insoluble in transformer oil. In commonly used transformer designs, insulation is exposed, as a rule, only to compressive forces, and the most common insulating materials, for example, electrical insulating cardboard, cable paper, paper-bakelite products, textolite, allow compressive stresses up to 20-40 MPa, which is practically sufficient so that the insulation does not break down.
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