CC BY
6УДК 621.75 Mufidzade N.A., Agayev I.E.
Mufidzade N.A.
Azerbaijan State Oil and Industry University (Baku, Azerbaijan)
Agayev I.E.
Azerbaijan State Oil and Industry University (Baku, Azerbaijan)
INVESTIGATION OF THE MAIN CHARACTERISTICS AND ELECTRICAL STRENGTH OF TRANSFORMER CIRCUIT INSULATION
Abstract: assessment of the wear condition of transformer insulation is one of the main conditions for their normal and long-term operation. Depreciation directly depends on the operating temperature of the equipment, external influences on them and the duration of operation. During operation, the main disadvantage is that the insulation loses its quality due to wear and tear. This increases the moisture in the Transformer during the aging process, further accelerating the aging process. Knowing the moisture content of solid insulation, getting an idea about the degree of wear and tear and its further operation can be decided. Various methods offrequency spectroscopy (FDS) are known. The service life of the power transformer is its structural materials It also depends on the temperature. Temperature processes in the transformer Due to their significant impact, comprehensive information about these processes is of great interest.
Keywords: insulation, transformer winding, electrical strength, insulation resistance.
Insulating materials applied to the duct are classified based on how long they can withstand heat. As we know, every time the load increases, the temperature of the circuit increases due to the flowing current. For every 10 degrees increase in temperature, the life of the insulation decreases by 50%. Based on that temperature factor, the insulation composition is divided into classes - fig. 1.
m
JW к •
Q \ с p "iTj lOdarsQC andiQtta yani 165 de rats С olduqda stator dolaginn i j la m a qabiliyvali SOii azalir
n \ 1 i \
г \
HI \ 1 A -1
-A-CLASS (105 *C) -8-C LASS (1 WC| —f-Class dsss:] — H-CLASS (UO*C| \ \ \
HJ 1 \ \ \
JO \ \
¿u \ \
0 s ч
0 SO 100 ISO Jtt 2S0 J00
tempratur
Fig. 1. Dependence of insulation tolerance on temperature. Table 1 lists insulation classes and their tolerance temperatures.
Table 1. Insulating compositions of coils according to different classes.
Class of Insulation Insulation Composition Insulation Withstanding Temperature
Y Cotton, Silk, Paper 90°C - 194F
A Paper, Silk, Cotton, Polyamide and Rubber 105°C - 22IF
E Synthetic rubber, enamelled wire, cellulose, Polyvinyl rubber, resin. Plastic Powder 120°C - 248F
В Mica, Fiber, Glass, Asbestos 130°C - 266F
F Mica, Tar 155°C -31 IF
H Mica, Fiberglass and Asbestos 180°C - 356F
С Mica, Ceramic, Glass, Teflon and Quartz >180°C - >356F
In general, the temperature of the windings of electric machines during normal operation should not exceed the insulation class specified in their passports. Otherwise, for every 10% increase in temperature, the operating life of the electric machine is
halved. For example: If the temperature of the circuit of the electric machine has risen to 1800C, then:
Will only work for 300 hours if it is Class A insulation.
Will work for 1800 hours if it is Class B insulation.
Will last 8500 hours if it is Class F insulation.
Will work for 10000 hours if it is class H insulation.
The most common accident is that the winding insulation is damaged and a single phase is shorted to earth. Deterioration of insulation is one of the main causes of ground fault. Insulation resistance is measured with a megometer with a voltage of 1000 ... 2500V. The measurement is carried out when the temperature of the top layer of oil is not lower than +100C. They measure the insulation resistance between the cores in each turn, as well as between the turns. During the measurement, the reading of the megometer is taken 15 and 60 seconds after the voltage. At this time, the absorption coefficient is determined as Kab. The value of absorption coefficient in moistened insulation should not be less than 1.3. It is recommended to evaluate the condition of the insulation by comparing the results of the measurements with the preliminary data known in advance at the same temperature. In the absence of initial data, the approximate average operating data of the minimum allowable value of the insulation resistance can be used. According to DUIST 11677-75, the test of electrical strength of transformer insulation is carried out by two methods. They test the head insulation (insulation between windings) of the transformer with normal frequency elevated voltage. At this time, the test voltage is tested, with a short-circuited circuit, all other circuits of the transformer and the magnetic conductor are closed, and is applied between the earthed tank (Fig. 2). The test is carried out after 10 ... 20 hours have passed since the oil was poured into the transformer and when the temperature of the upper layers of the oil is around +200C. First, they try the AG loop and then the YG loop. They increase the voltage intermittently - smoothly, from 0 to the full test voltage. They keep the test voltage until one minute has passed from the moment it is established, and then slowly reduce it. If during the supply of high voltage, there is no
perforation of the insulation, no vapor separation, or no decrease in the test voltage, then the transformer is considered to have passed the test (Fig. 2).
TV3
Fig. 2 Test scheme of the electrical strength of the main (main) insulation of the transformer: TV1 - autotransformer, TV2 - transformer, R - resistor,
TV3 - the transformer under test.
The test of the longitudinal insulation of the transformer (between the windings, layers and sections) is carried out with a high voltage induced in the transformer itself. This test is carried out in the no-load mode by applying a voltage equal to 1.3 nominal for one minute to the outputs of one of the circuits (usually AG). Each transformer coming out of repair must pass a verification test. Before testing, the transformer is externally inspected and the windings are checked for proper connection. During the external inspection, the insulation quality of the windings of the transformer is carefully checked, the connection of the output ends to the clamp plate and marking are reviewed. In the external inspection process, the general condition of the transformer core, the flat stacking of the sheets, the insulation quality of the winding studs and the quality of the steel sheet retention are evaluated. According to the current standards, small power transformers must undergo the following inspection tests after repair:
1) checking the transformation coefficient on all outputs.
2) measuring the insulation resistance of the circuit.
3) measuring the resistance of the wires of the windings with a constant current.
4) testing of electrical strength of circuit insulation.
5) measurement of no-load losses and current of the transformer.
6) measuring the short-circuit voltage and loss of the transformer.
The insulation resistance of the transformer windings to the body should be checked with a megohmmeter with a voltage of up to 500V. The measured value is compared with the previous results. The minimum value of insulation resistance should not be less than 5MOm. In the period of operation, the value of the insulation of the loop measured in the cold state should not be lower than 1MOm. The test of the electrical strength of the winding insulation is carried out according to the following scheme in order to check the insulation between the body and the winding by applying a test voltage in single-phase small power transformers (Fig. 3).
The trial period is considered to be 1 minute. The test voltage is found from the following expression.
where Un is the nominal voltage of the tested circuit of the transformer. The test result is considered positive if the indication of the devices does not change sharply during the test process, and short-circuiting does not occur.
Conclusion. In the article, according to the insulation classes of transformer windings, their insulation strength was investigated by several methods. Several factors affecting transformer windings during operation have been studied. The advantages and disadvantages of various methods for measuring the insulation quality of the circuit have been investigated.
Fig. 3. Electrical scheme of the test stand.
Usin = 2Un + 1000
REFERENCES:
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3. Greenwood, Electrical Transients in Power Systems, John Wiley & Sons, Inc., 1991;
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5. Martinez-Velasco, "Chapter 1 Parameter Determination for Electromagnetic Transient Analysis in Power Systems," in Power System Transients: Parameter Determination, Boca Raton, CRC Press, 2009, pp. 1-16;
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