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Basic knowledge of transformers and how to configure transformer capacity

(Summary description)In the process of power transmission, transformation and distribution, the transformer is an important electrical equipment. Transformers can be seen everywhere in industrial enterprises, shopping malls, office buildings, and residential quarters. The role of transformers in the power industry is extremely important. As a power practitioner, you must understand the knowledge of transformers. The 11 common senses about transformers are summarized. After reading, you will have a new understanding of transformers.

Basic knowledge of transformers and how to configure transformer capacity

(Summary description)In the process of power transmission, transformation and distribution, the transformer is an important electrical equipment. Transformers can be seen everywhere in industrial enterprises, shopping malls, office buildings, and residential quarters. The role of transformers in the power industry is extremely important. As a power practitioner, you must understand the knowledge of transformers. The 11 common senses about transformers are summarized. After reading, you will have a new understanding of transformers.

Information

In the process of power transmission, transformation and distribution, the transformer is an important electrical equipment. Transformers can be seen everywhere in industrial enterprises, shopping malls, office buildings, and residential quarters. The role of transformers in the power industry is extremely important. As a power practitioner, you must understand the knowledge of transformers. The 11 common senses about transformers are summarized. After reading, you will have a new understanding of transformers.

1. The principle of transformer

A transformer is a device that uses the principle of electromagnetic induction to change AC voltage and current. The main components are the primary coil, the secondary coil and the iron core (magnetic core). The figure below shows several different shapes of transformers.

2. The role of the transformer

Transforming voltage: increasing or decreasing the voltage, the generator will go through the process of boosting, transmitting, and reducing the voltage before it is discovered, and transform the voltage into a voltage level for residential or industrial use.

Changing current: changing the voltage while changing the magnitude of the current

Isolation: The user system and the power supply system are isolated to reduce the scope of the accident.

3. What types of transformers are there?

A. Oil-immersed transformer: a transformer that uses transformer oil as the insulating medium and cooling medium, and the iron core and coil are all immersed in the insulating oil.

B. Dry-type transformer: The iron core and coil are cast with epoxy resin material to play an insulating role.

C. Amorphous alloy transformer: A new type of transformer developed in recent years, using amorphous alloy material as magnetic circuit material, with good temperature resistance.

4. What are the components of the transformer?

A. Body main body: including iron core, winding, insulating support

B. Voltage regulating device: namely tap changer, (on-load voltage regulating device and non-excitation voltage regulating)

C. Fuel tank and fin-shaped radiator (wings), casing

D. Safety devices: moisture absorbers, gas relays, oil filters and temperature measuring instruments, etc.

5. What is the use of transformer oil

(1) Insulation effect: The specific and strong insulation performance of the transformer oil produces sufficient insulation strength between the winding and the casing. Common types are DB25 and DB45.

(2) Heat dissipation: The transformer oil circulates naturally in the body and between the chip radiator according to the principle of heating up, and the heat is dissipated through the radiator to keep the temperature of the coil from being too high.

(3) Arc suppression function: When the on-load voltage regulating device is operated, arc light will be generated when the tap changer is opened, and the transformer oil can extinguish the arc light and play a protective role.

6. The role of the oil conservator (oil pillow)

The oil pillow (oil conservator) is installed diagonally above the oil tank, and there is an oil pipe connected with the oil tank. When the volume of transformer oil expands and shrinks with the change of oil temperature, the oil pillow plays the role of oil storage and oil replenishment. Prevent oil and air from being oxidized and damped. The oil pillow is equipped with an oil gauge tube, and some are also equipped with a liquid level indicator to observe the change of the oil level.

7. What is on-load voltage regulation?

On-load voltage regulation is a voltage regulation method in which the transformer can change the voltage by changing the tap position when the transformer is running with load.

8. What are the losses of the transformer during operation

Transformer losses include: no-load loss (iron loss): loss caused by iron core eddy current,

Load Loss (Copper Loss): Losses due to resistance heating of the windings.

9. Representation method of transformer model

Base Model: S

Design serial number or performance level code: 11

Rated capacity: 500KVA

High voltage side voltage: 10KV

10. What are the main technical data on the nameplate

Transformer primary and secondary rated voltage, rated capacity, wiring group, primary and secondary rated current, variable impedance voltage, tap changer adjustment voltage range, operating environment conditions, transformer oil type and weight, etc.

11. What is the function of a hygroscopic device (respirator)?

A. Since the temperature of the transformer will change at any time during operation, the oil level inside the transformer will rise or fall, and the gas must be discharged, otherwise the pressure will rise; it can be understood as the process of breathing, which prevents oil and The effect of direct air contact deterioration.

B. Install a silica gel tank on the exhalation channel of the oil. The transformer breathes through the silica gel, and the discharged moisture is absorbed. When the moisture reaches a certain level, the color of the silica gel will change from blue to red. At this time, the silica gel needs to be replaced. shown in the picture.

12. What is a Buchholz relay? What is the role?

Buchholz relay (also known as gas relay) is a protection device used in transformers. It is installed between the oil conservator of the transformer and the pipeline between the oil tank. When the internal fault of the transformer is used to decompose the oil to generate gas or cause the oil flow to surge , make the contacts of the gas relay act, and send out a "heavy gas" trip signal and a "light gas" alarm signal to the duty personnel through the internal electrical common contacts to remind the transformer that the transformer is abnormal or faulty.

In AC circuits, there are three concepts of electrical power: active power, reactive power, and apparent power.

kVA stands for apparent power, which includes reactive power and active power, kW stands for active power, and kVar stands for reactive power. There is also a concept of the relationship and conversion between kW and kVar - power factor cosФ, active power kW=UIcosФ, reactive power kVar=UIsinФ, and apparent power kVA=UI (U is voltage, I is current).

When applying for electricity to the power supply bureau, you should fill in the active power you need to use, that is, how many kW you need. If the total power consumption of the building is calculated to be 4500kW, such a large power consumption, the power supply bureau must require the installation of reactive power compensation devices. Assuming that your compensated power factor can reach above 0.9, calculate with 0.9. Then the apparent power needs to be 4500÷0.9=5000kVA, and then considering a certain load margin and the specification and model of the transformer, I am afraid that the minimum transformer must be 5600kVA. Of course, this is just a calculation. In actual application, it depends on factors such as the simultaneous rate of your load.

Regarding the selection of transformer capacity, there are the following rules for reference!

1. Knowing the transformer capacity, find the rated current on the voltage level side:

Suitable for any voltage class.

Formula: The capacity is divided by the voltage value, and its quotient is multiplied by six and divided by ten.

Case: Apparent current I=apparent power S/1.732*10KV=1000KVA/1.732*10KV=57.736A Estimated I=1000KVA/10KV*6/10=60A

2. Knowing the transformer capacity, quickly calculate the current value of the primary and secondary protection fuse-links to match the transformer high-voltage fuse-links, and compare the capacity and voltage.

For low-voltage fuse-links, the capacity is multiplied by 9 and divided by 5

3. Measure the current on the secondary side of the power transformer and calculate the load capacity it carries

Knowing the secondary voltage of the distribution transformer, the measured current is obtained in kilowatts.

The voltage level is 400 volts, one amp and 0.6 kilowatts.

The voltage level is 3,000 volts, and one amp is 4.5 kilowatts.

The voltage level is six thousand volts, and one amp is an integer of nine kilowatts.

The voltage level is ten thousand volts, and one amp is fifteen kilowatts.

The voltage level is 35,000, and one amp is 55 kilowatts.

4. Knowing the capacity of the transformer, find the common voltage coefficient of the rated current on the voltage level side, and the capacity multiplication coefficient to obtain the current,

The rated voltage is 400 volts, the coefficient is 1.445,

The rated voltage is 6,000 volts, the coefficient is 0.096,

The rated voltage is 10,000 volts, and the coefficient is exactly 0.06.

Note: The rated current of the corresponding voltage level can be obtained by directly multiplying the transformer capacity by the corresponding coefficient.

5. Select the melt current value of the primary and secondary fuses according to the rated capacity and rated voltage of the transformer

The melt flow on both sides of the distribution transformer is simply calculated according to the capacity.

The unit of capacity is kVA, and the unit of voltage is kV.

The high voltage capacity is divided by the voltage, and the low voltage is multiplied by 1.8,

The unit of current is obtained, and then the level is subtracted or added.

Case: The rated capacity of the three-phase power transformer is 315KVA, the rated voltage of the high-voltage end is 6KV, and the rated voltage of the low-voltage end is 400V;

The rated current of the high-voltage side melt is (315÷6)A=52.5A; the rated current of the low-voltage side melt is (315×1.8)A=567A

Note: The specification of the fuse should be selected according to the difference between the calculated value and the melt current gauge.

6. According to the rated current of the transformer, select the melt current value of the primary and secondary fuses

The melt flow on both sides of the distribution transformer is several times the rated current.

The value of the high pressure side is larger, and the number of different capacities is different.

Capacity of 100 and below, two to three times the amount of flow,

If it is more than 100, it should be reduced by a multiple of 2 to 1.5.

There are regulations for the minimum high pressure, which cannot be less than three amps.

The low voltage is equal to the rated value regardless of the capacity value.

7. Installation requirements for distribution transformers

The distance from the ground is at least 2 meters and 5 meters, and barriers are installed on the ground.

The obstacle height is at least 1.8 meters, leaving the quarter-finals of the distribution change point,

If the economy allows, it is more appropriate to use a box type.

Unless there is a temporary use, it should not be placed on the open ground.

The indoor installation should be ventilated, and the surrounding channels should be appropriate.

8. Regulations on the quality of power supply voltage of distribution transformers

The power supply voltage is guaranteed, and the equipment operates normally.

There are regulations for the level deviation, and the voltage levels are different.

The voltage between the lines is positive and negative seven, and the negative ten positive seven voltage is the phase,

If the requirements are more special, the supply and demand sides will negotiate.

Analysis: In my country's low-voltage power supply system, the line voltage is 380V, and the allowable deviation is ±7%, that is, 353.4 to 406.6V; the phase voltage is 220V, and the allowable deviation is -10% to +7%, that is, 198 to 235.4V.

9. Detection of insulating windings of transformers

Change distribution operation to ensure safety, and measure insulation to check for hidden dangers.

Use a megohmmeter for measurement, and select the meter according to the voltage.

More than three, five, two thousand and five thousand, and less than ten thousand, use one thousand.

The E terminal of the instrument should be grounded, and the G terminal should be added if the pollution is serious.

Untested windings and components, reliable grounding to ensure safety.

The rotation speed of the hand crank is one hundred and two, and after the test, the discharge is discharged and then the stitches are removed.

Note: For transformers of 35KV and above, a 2500V megohmmeter should be used; for transformers of 10KV and below, a 1000V megohmmeter should be used. The L terminal is connected to the winding of the transformer, and the E terminal is grounded.

10. Parallel operation of two transformers

Two transformers in parallel, four conditions must be prepared;

The wiring group should be the same, and the transformer ratio should be the same;

The impedance voltage should be consistent and connected to each other in the same phase sequence;

The capacity difference should not be much, preferably not more than three to one.

11. Reasons for blown fuses of distribution transformers

If the high-voltage fuse is blown, there are six reasons to judge.

The fuse specification is selected to be small; it is difficult to bear the damage of poor quality;

There is a short circuit in the high voltage lead; the internal insulation is broken down;

Lightning strike damaged; casing rupture or breakdown.

If the low-voltage fuse blows, there are five reasons to judge.

The fuse specification is selected to be small; it is difficult to bear the damage of poor quality;

Excessive load for a long time; winding insulation is broken down;

Transmission line failure, short-circuit to ground or phase-to-phase.

12. Measure the no-load current of a 380V single-phase welding transformer without a nameplate, and calculate its rated capacity

Formula: Three hundred and eight welder capacity, no-load current multiplied by five.

Analysis: The single-phase AC welding transformer is actually a special-purpose step-down transformer. Compared with ordinary transformers, its basic working principle is roughly the same. In order to meet the requirements of the welding process, the welding transformer works in a short-circuit state and requires a certain arc ignition voltage during welding. When the welding current increases, the output voltage drops sharply. According to P=UI (the power is constant, the voltage is inversely proportional to the current).

When the voltage drops to zero (ie, the secondary side is short-circuited), the secondary side current will not be too large, etc., that is, the welding transformer has the external characteristic of a steep drop, and the external characteristic of the welding transformer is the voltage drop generated by the reactive coil. obtained. At no-load, since there is no welding current passing through, the reactance coil does not produce a voltage drop. At this time, the no-load voltage is equal to the secondary voltage, that is to say, the no-load welding transformer is the same as the ordinary transformer no-load. The no-load current of the transformer is generally about 6%~8% of the rated current (the country stipulates that the no-load current should not be greater than 10% of the rated current)

13. Knowing the capacity of the three-phase motor, find the formula for its rated current: divide the capacity by the number of kilovolts, and multiply the quotient by the coefficient of 76.

Case: Known three-phase two-hundred-two motor, kW 3.5 amps. 1KW÷0.22KV*0.76≈1A is a known high-voltage three-kilovolt motor, four kilowatts and one amp. 4KW÷3KV*0.76≈1A Note: The formula is applicable to the calculation of the rated current of three-phase motors of any voltage level. When the formula is used, the unit of capacity is kW, the unit of voltage is kV, and the unit of current is A.

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