RU2381613C1 - Method to dry solid insulation of electrical machine windings - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: invention relates to electrical engineering, particularly to production of electrical machines and can be used in production of transformers, as well as in production and repair of motors. In compliance with proposed method, insulation in transformer tank is heated by air heat carrier and low-frequency currents on feeding 1.0 Hz-voltage to high-frequency winding, low frequency winding being short-circuited. Cyclic evacuation of said tank is effected by single- or multi-stage high-rate evacuation with the help of receiver and fast-operation electric valve for 10 seconds with subsequent holding at vacuum for up to 10.0 min, or by single- or multi-stage vacuum relief to atmospheric pressure in filling transformer tank with dehumidified air. Cycles are repeated unless required amount of humid is accumulated in solid insulation. Multi-stage evacuation prevents mechanical deformation of transformer tank and rules out origination of whatever defects in solid insulation made from multi-layer cardboards and wood board. To accelerate drying, heated vacuum cap can be used. Conditions of high-rate evacuation, receiver volume and preset pressure come-up time are calculated by enclosed formulae. Solid insulation drying can be effected on both transformer production and its operation during oil replacement.

EFFECT: intensified drying.

3 cl, 3 dwg, 1 tbl, 2 ex

Description

The invention relates to the field of electrical engineering, namely to the technology for the production of electrical machines, mainly to transformer construction.

A known method of drying solid insulation, according to which when wetting the solid insulation, dried during the manufacture of transformers, drying is carried out in oil (RTM 16. 687.000.-73).

However, this method is ineffective, since the presence of oil limits the creation of high temperatures, the moisture capacity of the oil prevents the free removal of moisture from the solid insulation, draining and refilling of a large volume of oil is required.

The method is energy-intensive and time-consuming, requires a large amount of preparatory work, is used with insignificant insulation surfaces.

A known method of drying transformers by spraying a hot dielectric fluid (US Patent No. 2718709, CL. 34-91).

The transformer tank is filled with dielectric fluid to the level of the beginning of equalization isolation. Equipment for heating and cleaning the dielectric fluid is connected to the tank and the liquid is circulated in the tank from the bottom up with spraying it in the upper part of the tank. The liquid is heated to 80-110 ° C. A vacuum of not more than 125 mm Hg is maintained in the tank. residual pressure. The dried dielectric liquid, getting on the insulation, heats and removes the moisture released from it (drying in solvent vapor), which is removed from the liquid in the treatment plant. Sparseness in the tank helps to remove moisture.

Moisture is removed slowly due to the short contact time of the dielectric liquid with the solid insulation and its small (liquid) moisture capacity, a high-performance installation for continuous drying of the liquid dielectric is required; dielectric liquid is used to dry it when it is heated to 80-110 ° C for a long time ( 10-15 days) should not emit aging products that pollute solid insulation.

The closest in technical essence and the achieved effect is the invention (SU A.S. 671664, Н02К 15/12) "Method for drying the solid insulation of electrical machines and apparatus", taken as a prototype. The method includes heating solid insulation by spraying heated oil with simultaneous evacuation. Drying is carried out in cycles, each of which includes heating the insulation to a temperature not exceeding the aging temperature of the oil, turning off the heating and spraying the oil. Continue the evacuation process until the insulation temperature reaches 40-50 ° C, and the residual pressure is maintained at a level not exceeding 5 mm Hg. throughout the cycle. The cycles are repeated until the desired moisture content in the insulation is reached.

The disadvantage of this method is the length of the process due to the fact that during the drying of the insulation the temperature gradient prevents the movement of moisture to the surface of the insulation, since the surface temperature is greater than in the center of the body, and also in vacuum the process of thermal diffusion of moisture proceeds in the form of steam rather than in the form of a liquid.

The aim of the present invention is to create conditions that accelerate dehumidification from the solid insulation of transformers, to reduce the drying time while maintaining the quality of solid insulation - the absence of cracks and splitting.

This goal is achieved by the fact that in the drying method, carried out in cycles, each of which includes heating the insulation while vacuuming the transformer tank, the insulation is preheated at atmospheric pressure with an air coolant and by applying a low frequency voltage to the high-voltage winding to 1.0 Hz when short-circuited low-voltage winding to a temperature not exceeding the aging temperature of the insulation, evacuation of the transformer tank is carried out by single or multi-stage high-speed evacuation m using a receiver and a fast-acting solenoid valve for up to 10 seconds, followed by exposure to a vacuum of up to 10.0 min and a single or multi-stage vacuum discharge to atmospheric pressure by letting dried air into the transformer tank, and the cycles are repeated until the required amount of moisture in the solid insulation is obtained .

Insulation heating by applying a low frequency voltage not exceeding the breakdown voltage to the high-voltage winding when the low-voltage winding is short-circuited allows directing thermal diffusion of moisture from the inner surface of the insulator to the outer one, evaporation of moisture occurs in the entire insulation volume, and in the inner layers there is more than surface, as a result of which a pressure gradient arises, which is the main driving force of vapor transfer inside the insulator. High-speed evacuation provides the process of thermal diffusion of moisture in the form of a liquid, due to the pressure drop obtained in the capillaries, the intercapillary space and on the surface of the insulator, which is the driving force for the transfer of liquid from the depth of isolation. Periodic purging of the tank with heated air coolant intensifies the process of moisture removal, especially at the initial stage of drying the insulation.

Vacuum drying of the insulation by high-speed vacuum is carried out in cycles and, depending on the strength properties of the transformer tank and insulation material (deformation of the transformer tank and / or cracking, delamination of dielectric materials is possible), each cycle is divided into several stages. The number of steps is selected as the largest of the obtained ratio of the pressure difference in the tank of the transformer and receiver to the maximum allowable pressure drop in the transformer ΔP and the ratio of the difference between the initial moisture content and the final to the maximum allowable moisture difference ΔW to ensure insulation quality.

Transformers with a permissible pressure drop for a tank of less than one atmosphere are placed for drying solid insulation under a heated hood with the possibility of creating a vacuum in it with one-stage high-speed vacuum with a pressure drop in relation to the receiver of 1 atm and one-stage discharge to atmospheric pressure.

The free volume of the receiver and the set-up time of the set pressure in the transformer tank, characterizing high-speed evacuation, are calculated according to the following relationships:

V _p = V _c.c. × (R _{f.s.o -R n.p.} ) / (R _n.p. -R _ro );

t = [(P _NR / R _k.s.o) ^{- (k-1) / 2k} -1] / V;

Where:

V _p is the free volume of the receiver;

V _c.c. - free volume of the transformer tank;

R _k.s.o - the initial gas pressure in the transformer;

R _n.p. - saturation pressure of moisture vapor depending on the temperature in the transformer tank;

P _ro - the initial pressure in the receiver;

t is the set pressure in the transformer tank;

K is the indicator of the polytropic gas in the transformer tank;

B = (k-1) / 2 × (Go / ωo) is an indicator of the intensity of the outflow of gases;

Go - initial gas consumption;

g is the acceleration of gravity;

Fcr - the minimum cross-sectional area of a high-speed valve on a transformer;

R is the gas constant of the gas;

T _k.s.o - initial gas temperature in the transformer tank;

ωо - initial mass of gas in the transformer tank before turning on the valve.

To calculate the parameters according to the above mathematical expressions when drying the insulation of transformers using “caps”, instead of the value of the free volume of the transformer tank, the total value of the free volumes of the transformer tank and “cap” is taken.

Depending on the strength characteristics of the transformer tank, there are three possible schemes for implementing the proposed method of insulation drying, which are shown in figures 1-3.

Figure 1 shows a diagram of the installation of drying the solid insulation of the transformer using heating with an air coolant and low-frequency currents with high-speed evacuation of the transformer tank inside the receiver cabinet.

Figure 2 shows the installation diagram for drying the solid insulation of a transformer with a tank that can withstand a pressure drop of (at least one) atmosphere, heated by an air coolant and low-frequency currents with high-speed vacuum.

Figure 3 shows the layout of the "cap" for drying the solid insulation of the transformer with a tank that can withstand an external pressure drop of less than one atmosphere, heated by an air coolant and low frequency currents with high-speed vacuum.

Figure 1 shows a diagram of the installation of drying the insulation of transformers in a volumetric vacuum chamber. With a permissible differential pressure (external) for the tank, ΔР is less than one atmosphere (usually ΔР≈0.3 atm), the transformer (one or several depending on their size) 1 is placed in a vacuum chamber 2, for example, a volumetric cabinet of a vacuum filling plant. In this case, the volume cabinet is a drying apparatus and a vacuum receiver, and the oil-free volume of the transformer tank is a vacuum drying chamber. On the transformer cover, a high-speed solenoid valve 3 with an open pipe is installed in the free volume of the cabinet.

Insulation temperature and pressure sensors are installed in the tank and in the cabinet.

To heat the insulation, a low-frequency voltage source 4 and an air coolant supply system consisting of an electrovalve 5, a heater (thermocompressor) 6, a dehumidifier 1, an electrovalve 8, a condenser 9, an electrovalve 10 are connected. When a solenoid valve 8 is closed and an solenoid valve 11 is opened, a closed movement cycle is organized air coolant with dehumidifier disconnected.

To carry out high-speed evacuation (when the high-speed electro-valve 3 is closed and the electro-valves 5, 10, 16 are closed), air is pumped out of the free volume of the cabinet through the electro-valve 12, the condenser 9 by the vacuum post 13 with the electric valve 14 or 15 open, respectively.

The signals from the temperature and pressure sensors are fed to the pressure and temperature recorder 17 and to the control unit 18, which allows you to turn off the heating, evacuation and gives commands to all solenoid valves and valves.

At the first stage of high-speed evacuation, the air is pumped out of the receiver cabinet to a pressure of P ₁ ≥ (P _atm -ΔP) and the fast-acting electrovalve 3 is opened. After equalizing the pressure in the receiver and the transformer tank, the electrovalve 3 is closed and the second stage of high-speed evacuation is carried out: the receiver is pumped out to pressure P ₂ ≥ (P ₁ -ΔP), open the electrovalve 3, after equalizing the pressure and closing the valve 3, carry out the next stage of high-speed evacuation, etc. After all stages of evacuation have been completed, exposure is carried out under vacuum for up to 10 minutes, while additional pumping of air from the receiver and the transformer tank is possible. At the end of the cycle, heated, dried air is let in through the electrovalve 5 into the transformer tank. Ambient atmospheric air is supplied to the cabinet through solenoid valves 12 and 16.

If the drying regime of solid insulation must be performed under more stringent restrictions on gradient changes in vacuum, for example, to prevent cracking of wood-laminated plastic (chipboard), the number of stages of high-speed evacuation increases.

After performing the required number of cycles, ensuring the achievement of permissible norms of moisture content of solid insulation, drying is stopped.

Figure 2 shows a diagram of the installation of drying the solid insulation of the transformer 1 with a tank that can withstand a pressure drop of (external) at least one atmosphere, heated by an air coolant and low-frequency currents with high-speed vacuum.

The connection of the low-frequency voltage source 4 and the air coolant for heating the insulation of the windings, as well as the installation of sensors for measuring temperature and pressure, are carried out as described above. The sealed tank of transformer 1 is connected by a pipeline through a high-speed electrovalve 3 with a capacitor 9 and through an electrovalve 12 with a separate (standard) vacuum receiver 2. The receiver is pumped out by a vacuum post 13 when opening the necessary vacuum electrostatic shutters 14 or 15. A feed system is connected to another branch pipe on the transformer tank air coolant, consisting of an electrovalve 5, a heater (thermocompressor) 6, a dehumidifier 7 and an electrovalve 8 connected to a condenser 9. P and closed electrovalve 8, 16 and open electric valve 11 the heated air will circulate without passing through the desiccant, thereby increasing its operating life.

After heating the insulation in the transformer tank to a predetermined temperature (not exceeding the aging temperature of the insulation), the heating systems are turned off and high-speed vacuum cycles are performed. The magnitude of the permissible pressure changes is limited only by the characteristics of solid insulation, that is, a single-stage high-speed evacuation cycle is allowed, followed by exposure to vacuum for up to 10 minutes. In the holding mode under vacuum, only the transformer tank can be pumped out of air - by shutting off the solenoid valve 12, the electric shutters 14, 15 and opening the solenoid valve 10.

To carry out the next evacuation cycle, drained heated air coolant is supplied to the transformer tank. After drying of the solid insulation of the windings is completed, the transformer tank is filled with oil.

This system for drying the solid insulation of windings can be implemented as a mobile unit for drying the insulation of transformers in operation when changing the oil in them.

Figure 3 shows the most effective (in terms of time parameters) scheme for drying solid insulation in transformers with tanks that have limitations on the difference in external pressure, which can be arranged by placing the transformer under a small-sized (comparable in volume to a transformer) heated cap with the possibility of creating vacuum with a single-stage high-speed evacuation with a pressure drop in relation to the receiver of 1 atm and a single-stage discharge to atmospheric pressure. Heating the hood eliminates the possibility of moisture condensation, especially at the initial stage of drying.

It is possible to implement pulsed evacuation using 2 receivers to more quickly achieve operating pressure under the hood.

Heating of the solid insulation of the windings of the transformer 1 with an unsealed tank located on the bed 19 and covered with a cap 20 is carried out by low-frequency currents 4 and air coolant from the heater (thermocompressor) 6 through the solenoid valve 5 to the transformer tank. The movement of air coolant can be carried out in two circuits. 1st — from the tank into the cavity under the cap and then through the high-speed solenoid valve 3 to the condenser 9, the solenoid valve 8, the dryer 7 to the thermocompressor 6 and through the open solenoid valve 5 to the tank cavity. 2nd — with a closed electrovalve 8 and an open electrovalve 11, air will not pass through a dehumidifier 7. This circuit can be used when heating solid insulation at the initial stage of its drying, at high values of moisture removal from the insulation and thereby save the life of the desiccant.

To perform high-speed evacuation, the cap cavity is connected by a high-speed solenoid valve 3 through a capacitor 9, the solenoid valve 13 with a separate (standard) receiver 2, which is pumped out by a vacuum post 13 with open gate valves 14 or 15.

The high-speed vacuum cycle is single-stage (if there are no restrictions on the drying regimes of solid insulation such as chipboard) and this reduces the drying time of the insulation. When holding under vacuum, it is possible to pump out air only from under the cap and tank of the transformer - by closing the solenoid valve 12, solenoid valves 14, 15 and opening the solenoid valve 10, thereby reducing the time to reach the ultimate vacuum: 0.5-1.0 mm Hg

The creation of two parallel production lines based on dryer hoods will increase the drying performance of transformer insulation up to 4 times.

Example 1

The table shows the data on the drying of the windings of an oil-filled transformer in a vacuum cabinet with multi-cycle 3-stage high-speed vacuumization using satellite samples from 2 mm electrical cardboard with a size of 100 × 100 mm and large insulation segments from chipboard with a size of 49 × 48 × 100 mm. Heating was carried out to 105 ± 2 ° C, then a multi-cycle (after 120 min) high-speed 3-stage evacuation was performed. High-speed evacuation was performed for a time from 3.5 to 9.8 s. The exposure time under vacuum ranged from 3 to 10 minutes

Table No. step
cycle Pressure, mmHg Pressure equalization time, s Humidity% Receiver Transformer tank Alignment (exposure time under vacuum, min) Cardboard Chipboard 1/1 500 760 513 3.6 - - 2/1 250 513 263 5,6 - - 3/1 0.5 265 fifteen 9.8 - - one 8 8 8 (3) one 500 760 - - 4.3 3.8 1/59 500 760 513 3,5 - - 2/59 250 513 263 5,4 - - 3/59 0.5 265 fourteen 9.3 - - 59 1.3 1.3 1.3 (10) 59 760 760 - - 0.09 0.12

After 118 hours of drying, moisture removal from the chipboard 4.74% was achieved in the absence of cracks and splitting.

For comparison, for the same period of time at a heating temperature of 105 ± 2 ° С, the moisture removal from the particleboard in the drying cabinet was 3.76%. It took 176 hours to reach a moisture removal level of ~ 5% in an oven - almost 1.5 times more, and when drying using heating to 105 ± 2 ° С and slow (constant) evacuation, the time spent is more than 30%. In these drying options, cracking was noted in the particleboard samples.

Example 2

Drying of the elements of solid insulation of transformer windings (electrical paper, multilayer cardboard, chipboard) according to the method using the “cap” was carried out on an experimental setup consisting of a glass cap (diameter 193 mm, height 485 mm, volume 13.4 dm), metal frame, a receiver with a volume of 100 dm ³ , a closed metal vessel (with a diameter of 160 mm, a height of 120 mm) - a simulator of a transformer tank. There were two holes with a diameter of 15 mm in the wall of the metal vessel - one for supplying air through the tube through the bed, the other for connecting to the cap cavity. A metal vessel with an electric heater was installed on the bed and elements of solid insulation were placed in it. The insulation was heated through the vessel wall by an electric heater and by blowing air heated by a technical hairdryer (through a tube through the bed). Heated air escaped from the metal vessel into the cavity of the cap, partially heating its walls, and then through the main in the bed to the outside.

Vacuum and air lines were blocked by ball valves Du20.

From the receiver and from the hood, the air was pumped out by a 2FY-3B vacuum pump.

After heating the samples of solid insulation to a temperature of 105 ° C, a high-speed single-stage evacuation was carried out for 7 seconds by opening a high-speed electrovalve (KVM-25NZh, Du 25 grade) on the line connecting the receiver with the cap cavity (through the bed). Then, exposure was carried out under vacuum for up to 10 min with pumping air from the cavity of the cap (and the metal vessel at the same time) to a pressure of 0.5 mm Hg.

Depending on the type of solid insulation after 3 to 15 cycles of drying by high-speed evacuation, the moisture content in the samples of solid insulation decreased to 0.1 ... 0.03%.

Claims (3)

1. The method of drying the solid insulation of electrical machines and apparatuses, mainly transformers, in cycles, each of which includes heating it with simultaneous evacuation of the transformer tank, characterized in that the insulation is preheated at atmospheric pressure with an air coolant and by applying a low frequency voltage to the high voltage winding to 1.0 Hz with a short-circuited low-voltage winding up to a temperature not exceeding the aging temperature of the insulation, the transformer tank is evacuated with one- or multi-stage high-speed evacuation using a receiver and a fast-acting electrovalve for up to 10 s followed by holding under vacuum for up to 10.0 min and with one or multi-stage vacuum discharge to atmospheric pressure by letting dried air into the transformer tank, and the cycles are repeated until the required amount of moisture is reached in solid insulation. 2. The method according to claim 1, characterized in that transformers with a permissible pressure drop for a tank of less than one atmosphere are placed to dry solid insulation under a heated hood with the possibility of creating a vacuum in it with one-stage high-speed vacuum with a pressure drop in relation to the receiver of 1 atm. and one-stage discharge to atmospheric pressure. 3. The method according to claim 1 or 2, characterized in that the free volume of the receiver and the set time of the set pressure in the transformer are calculated according to the following relationships:
V _p = V _f.s. (P _f.s.o -P _n.p. ) / (P _{n.s.- p ro} );
t = [(P _n.p. / P _K.co ) ^{- (k-1) / 2k} -1] / V,
where V _p is the free volume of the receiver;
V _K.s - the free volume of the transformer tank;
R _k.s.o - initial pressure in the transformer tank;
P _np - saturation pressure of moisture vapor depending on the temperature in the transformer tank;
P _ro - the initial pressure in the receiver;
t is the set pressure in the transformer tank;
K is the indicator of the polytropic gas in the transformer tank;
B = (k-1) / 2 × (G ₀ / ω ₀ ) is an indicator of the intensity of the outflow;
G ₀ is the initial gas flow rate;
ω ₀ - the initial mass of gas in the tank of the transformer before turning on the valve.

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