Power transformer

Introduction

A transformer is a kind of static electrical equipment used to convert AC voltage and current to transmit AC power. It is based on the principle of electromagnetic induction to achieve electrical energy transfer. Transformers can be divided into power transformers, test transformers, instrument transformers and special-purpose transformers for their purposes: power transformers are necessary equipment for power transmission and distribution, power users to distribute power; test transformers are subjected to withstand voltage (boost) for electrical equipment. Test equipment; instrument transformers are used for electrical measurement and relay protection of power distribution systems (PT, CT); special-purpose transformers include electric furnace transformers for electric smelting, electric welding transformers, rectifying transformers for electrolysis, and small voltage regulating transformers.

A power transformer is a static electrical device that is used to convert a certain value of an alternating voltage (current) into another voltage or current of a different value (current). When the primary winding is connected to alternating current, an alternating magnetic flux is generated, and the alternating magnetic flux acts to induce an alternating electromotive force in the secondary winding through the core magnetic conduction. The level of the secondary induced electromotive force is related to the number of turns of the secondary winding, that is, the magnitude of the voltage is proportional to the number of turns. The main function is to transmit electrical energy, so the rated capacity is its main parameter. The rated capacity is a conventional value for expressing power. It is the size of the transmitted electrical energy. It is expressed in kVA or MVA. When the rated voltage is applied to the transformer, it is used to determine the rated current that does not exceed the temperature rise limit under specified conditions. The more energy-efficient power transformer is an amorphous alloy core distribution transformer, and its biggest advantage is that the no-load loss value is extremely low. The ultimate in ensuring no-load loss values ​​is a core issue to be considered throughout the design process. When the product structure is arranged, in addition to the effect that the amorphous alloy core itself is not affected by the external force, the characteristic parameters of the amorphous alloy must be accurately and reasonably selected in the calculation.

effect

Power transformers are one of the main equipment for power plants and substations. The role of the transformer is not only to increase the voltage to send electricity to the electricity area, but also to reduce the voltage to the voltage used at each level to meet the needs of electricity. In short, both boost and buck must be done by the transformer. In the process of transmitting electric energy in the power system, voltage and power loss are inevitably generated. When the same power is delivered, the voltage loss is inversely proportional to the voltage, and the power loss is inversely proportional to the square of the voltage. Using a transformer to increase the voltage reduces the loss of power transmission.

The transformer consists of two or more coil windings wound on the same core. The windings are connected by an alternating magnetic field and operate according to the principle of electromagnetic induction. The installation location of the transformer should be considered for easy operation, maintenance and transportation, and a safe and reliable place should be chosen. The rated capacity of the transformer must be reasonably selected when using the transformer. When the transformer is running at no load, a large amount of reactive power is required. These reactive powers are supplied by the power supply system. If the capacity of the transformer is too large, it will not only increase the initial investment, but also make the transformer run at no load or light load for a long time, so that the proportion of no-load loss increases, the power factor decreases, and the network loss increases. This operation is neither economical nor reasonable. If the transformer capacity is too small, it will overload the transformer for a long time and damage the equipment. Therefore, the rated capacity of the transformer should be selected according to the needs of the electrical load, and should not be too large or too small.

Inspection

When the transformer and power distribution owner is on duty, each class will be inspected once and unattended once a week. After heavy load changes, abnormal weather, new installations and overhaul of transformers, special inspections should be added, and the cycle is uncertain.

A. Whether the load current is within the rated range, whether there is a drastic change, and whether the operating voltage is normal.

B. Whether the oil level, oil color and oil temperature exceed the allowable value, and whether there is leakage oil phenomenon.

C. Whether the porcelain casing is clean, cracks, damage and stains, discharge phenomenon, whether the contact terminals are discolored or overheated.

D. Whether the degree of discoloration of the silica gel in the moisture absorber is saturated, and whether the transformer running sound is normal.

E. Is there any air in the gas relay, whether it is full of oil, whether the oil level gauge glass is broken, and whether the diaphragm of the explosion-proof tube is intact.

F, transformer housing, lightning arrester, neutral grounding is good, transformer oil valve is normal.

G. Whether the doors and windows between the transformers, the louver rail fence and the fire-fighting equipment are intact, and whether the transformer foundation is deformed.

Operation and Maintenance

The operation and maintenance of the transformer mainly includes four aspects: basic requirements, equipment switching operations, inspection inspections and accident handling.

(1) Basic requirements

1) High-pressure maintenance personnel must be certified to work, and undocumented persons have no right to operate.

2) When power failure maintenance is required, it should be reported to the competent department for approval and notified to the user.

3) The outdoor oil-immersed transformer shall be tested for insulating oil once a year, and the indoor oil-immersed transformer shall be tested for insulating oil once every two years.

(2) Operation sequence of transformer switching: first stop the load side during power failure, then stop the power supply side; when transmitting power, it is opposite to the above operation sequence.

1) Power is sent from the power supply side to the load side step by step. If there is a fault, it is easy to determine the fault range, and make judgments and treatments in time to avoid the spread of faults.

2) In the case of multiple power supplies, first stop the load side to prevent reverse charging of the transformer. If the power supply side is stopped first, the protection device may malfunction due to a fault.

(3) Inspection inspection: According to the requirements of the safe operation rules of the substation, the on-duty personnel inspecting the shifts need to conduct inspection inspections, and each shift of the substation should be inspected 5 times. The inspection inspection items are as follows:

1) Whether the temperature and sound of the transformer are normal, whether there is odor, discoloration, overheating and smoke. The upper oil temperature of the oil-immersed transformer shall not exceed 95 °C according to the manufacturer's specifications (allowable temperature rise of 55 °C). In order to prevent the transformer oil from deteriorating too quickly, the upper oil temperature should not exceed 85 °C.

2) Keep the surface of the porcelain bottle, casing and magnetic material clean and observe whether there is crack damage or discharge. The oil level of the oil-immersed transformer should be in accordance with the standard, the color is normal, and there is no oil leakage.

3) Whether the fan running sound of the dry type transformer and the temperature controller indicate normal.

4) Whether the junction of the transformer high, low voltage and grounding is in good contact and there is no discoloration.

(4) Accident handling of transformers

1) One of the following conditions should be stopped immediately

a. The internal noise is very loud and there is a popping sound.

b. Under normal cooling conditions, the temperature rises sharply.

c. Oil pillows and explosion-proof barrels spray oil and smoke (oil-immersed transformers).

d. Serious oil leakage, no oil level (oil-immersed transformer).

e. The transformer smokes and catches fire.

f. The casing has severe cracking and discharge.

j. The terminal is blown and a phase failure occurs.

2) Processing steps

a. First, in accordance with the sequence of switching operations, disconnect the high and low voltage side switches of the transformer and take safety measures.

b. When the upper cover of the transformer is on fire, open the bottom throttle to make it lower than the fire.

3) Accidents allowed to be contacted by the competent authority and their handling steps

a. When the transformer load exceeds the provisions of the operating regulations, it shall promptly report to the person in charge of the superior and pay attention to monitoring the load and temperature.

b. If the sound is abnormal, the terminal is overheated or reddened, it should be reported to the person in charge at a higher level in time to take timely measures.

c. If the temperature of the transformer exceeds the allowable temperature rise, the cause should be ascertained as soon as possible. Check whether the three-phase load is balanced (whether there is a turn-to-turn short circuit phenomenon), whether the cooling device of the transformer is normal, whether the on-load tap changer is in poor contact, and whether the transformer core silicon steel sheet is short-circuited.

4) Oil immersed transformer light gas action, alarm signal and signal relay when the card is dealt

a. First, find out the reasons: whether the oil leakage causes the oil level to decrease, the transformer fails to generate a small amount of gas, the internal short-circuit fault of the transformer causes the oil temperature to rise, and the gas relay has gas, secondary circuit and gas protection device inside.