What Is a Power Transformer?

A power transformer is a kind of stationary electrical equipment. It is a device used to change an AC voltage (current) of a certain value into another voltage (current) of the same frequency or different values.

Power transformer

Power transformer is a static

Transformer is a kind of static electrical equipment used to transform AC voltage and current and transmit AC power. It is based on the principle of electromagnetic induction to achieve power transfer. Transformers can be divided into power transformers, test transformers, instrument transformers and special-purpose transformers according to their uses: power transformers are necessary equipment for power transmission and distribution and power user distribution; test transformers withstand voltage (boost) of electrical equipment Test equipment; instrument transformers are used for electrical measurement and relay protection of power distribution systems (PT, CT); special-purpose transformers include smelting furnace transformers, welding transformers, electrolytic rectifier transformers, and small voltage transformers. ^[1]

Outline drawing of three-phase oil-immersed power transformer

1-nameplate; 2-signal

Power transformer is one of the main equipments of power plants and substations. The role of the transformer is not only to increase the voltage to send electrical energy to the area where it is used, but also to reduce the voltage to the voltage used at all levels to meet the demand for electricity. In short, both step-up and step-down must be done by a transformer. In the process of transmitting electrical energy in the power system, there are bound to be losses in voltage and power. When transmitting the same power, voltage loss is inversely proportional to voltage, and power loss is inversely proportional to the square of the voltage. use

In the development of the past ten years, China's electric power construction has developed rapidly with remarkable results. Among them, the installed capacity of power generation has grown rapidly, the construction speed of the power grid has been advancing by leaps and bounds, the adjustment of the power supply structure has been continuously optimized, the level of technical equipment has been greatly improved, the energy saving, emission reduction and consumption reduction effects have been significant, and the power construction has achieved leapfrog development. This has provided a strong impetus for the steady and rapid development of China's economy and society, and played an important support and guarantee role in improving people's lives.

According to the National Bureau of Statistics, from 2007 to 2011,

1.IF

Power transformers are classified by purpose: boost (power station 6.3kV / 10.5kV or 10.5kV / 110kV, etc.), liaison (220kV / 110kV or 110kV / 10.5kV between substations), step-down (35kV / 0.4kV for distribution or 10.5kV / 0.4kV).

Power transformers are classified by number of phases: single-phase, three-phase.

Power transformers are classified by windings: double windings (each phase is installed on the same core, the primary and secondary windings are separately wound and insulated from each other), three windings (three windings per phase, the primary and secondary windings are separately wound and insulated from each other), Autotransformer (a set of winding taps as primary or secondary output). The three-winding transformer requires the capacity of the primary winding to be greater than or equal to the capacity of the secondary or tertiary winding. The three winding capacity percentages are in the order of high voltage, medium voltage, and low voltage: 100/100/100, 100/50/100, 100/100/50, and the second and third windings cannot be operated at full load. Generally, the voltage of the tertiary winding is relatively low, and it is mostly used for near-field power supply or compensation equipment, and is used to connect three voltage levels. Autotransformer: There are two types: step-up or step-down. Because of its low loss, light weight, and economical use, it is widely used in ultra-high-voltage power grids. 400V / 36V (24V) is commonly used for small autotransformers, which are used to supply power for safety lighting and other equipment.

Power transformers are classified by insulation medium: oil-immersed transformers (flame-retardant, non-flame-retardant), dry-type transformers, 110kVSF6 gas-insulated transformers.

The core of the power transformer is a core structure.

The three-phase power transformers configured in general communication engineering are double-winding transformers.

Three-phase transformers and three-phase transformer groups can be connected into star, triangle, and zigzag shapes. They are represented by Y, D, and Z symbols on the high-voltage side, and by y, d, and z symbols on the low-voltage side. The high pressure is represented by YN and ZN symbols, and the low pressure is represented by yn and zn symbols. According to different wiring combinations of three-phase windings, there are 12 types of wiring groups. However, for the convenience of manufacture and use, China originally specified 5 types of wiring groups: Y, Yn0 (Y / Y0-12); Y, Yn (Y / Y-12); YN, Yn (Y0 / Y-12) ; Y, d11 (Y / -11); YN, d11 (Y0 / -11).

Y, Yn0 (Y / Y0-12): used for distribution transformers. The primary and secondary windings are all star-connected, and the secondary windings are neutral grounded.

YN, d11 (Y0 / -11) is used for high-voltage transmission lines, so that the high-voltage side of the power system may be grounded.

Y, zn11: The primary winding is a star connection, and the secondary winding is a zigzag connection with a neutral point ground (star connection).

But the above types of wiring do not include D, yn11. In the communication industry, urban power grids, industrial and mining enterprises and civil buildings of 10 / 0.4 / 0.23kV power distribution systems, the three-phase transformers of the national shaped products Y and Yn0 have been used for many years, following the standards adopted by the former Soviet Union. However, there are two wiring methods (D, Yn0; D, yn11) for distribution transformers produced by imported technology from abroad. The transformers selected by domestic and foreign-funded enterprises and the distribution transformers in most countries use D, Yn11 wiring.

China's national standard JGJ16-2008 "Code for Electrical Design of Civil Buildings" has the following requirements for the selection of transformer wiring groups:

For one of the following situations, the wiring group is D, Yn11 type transformer:

² The three-phase unbalanced load is more than 15% of the rated power of each phase of the transformer.

² Those who need to increase the single-phase short-circuit current value to ensure the sensitivity of the low-voltage single-phase grounding protection device.

² Those who need to limit the third harmonic content.

10KV high voltage

Primary and secondary sides of ordinary transformer

A. Except for the factory test, the new transformer should be inspected on-site before the installation is completed and put into operation; it is the same after overhaul. (Short-distance transportation may not be carried out without bumps, but tests such as pressure resistance should be performed.)

When the substation owner is on duty, each shift is inspected, and the unattended one can be on a weekly basis.

Transformer operation and maintenance mainly includes four aspects: basic requirements, equipment switching operation, inspection and accident handling.

(1) Basic requirements

1) High-voltage maintenance personnel must be certified to work, and those without a certificate are not authorized to operate.

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

3) Outdoor oil-immersed transformers should be tested for insulation oil once a year, and indoor oil-immersed transformers should be tested for insulation oil every two years.

(2) Transformer switching operation sequence: stop the load side first and then stop the power supply side when the power is cut off; reverse the above operation sequence when power is on.

1) Power is transmitted step by step from the power supply side to the load side. If there is a failure, it is easy to determine the scope of the failure, and timely judgement and treatment are performed to prevent the failure from expanding.

2) In the case of multiple power sources, stopping the load side first can prevent reverse charging of the transformer. If the power supply is stopped first, the protection device may malfunction due to failure.

(3) Inspection and inspection: According to the requirements of the safe operation regulations of the transformer substation, in addition to the inspections required by the shift staff, the inspection of the transformer substation shall be conducted five times per shift. The inspection items are as follows.

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

2) Keep the porcelain bottles, sleeves, and magnetic surfaces clean, and observe whether there are cracks, breaks, or discharges. The oil level of the oil-immersed transformer should be in compliance with the standard, the color should be normal, and there should be no oil leakage.

3) The fan running sound of the dry-type transformer and the indication of the thermostat are normal.

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

(4) Accident handling of transformers

1) Any one of the following cases should be stopped immediately

a. The internal noise is loud and crackling.

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

c. Oil pillows and explosion-proof barrels are sprayed with oil and smoke (oil-immersed transformers).

d. Severe oil leak, oil level is no longer visible (oil-immersed transformer).

e. The transformer smoked and caught fire.

f. The casing is severely cracked and discharged.

j. The terminal is blown and phase-open operation occurs.

2) Processing steps

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

b. When the transformer is on fire, open the bottom throttle so that it is lower than the fire.

3) Accidents that are allowed to be handled after contacting the competent authority and their processing steps

a. When the transformer load exceeds the requirements of the operation regulations, it shall be reported to the superior responsible person in time, and the load and temperature shall be monitored.

b. If the sound is abnormal, the terminal is overheated or the redness is melting, it should be reported to the superior in charge in time to take measures in time.

c. The transformer temperature exceeds the allowable temperature rise, and the cause should be identified as soon as possible. Check whether the three-phase load is balanced (whether there is a short circuit between turns), whether the cooling device of the transformer is normal, whether the on-load tap-changer tap-changer is not in good contact, and whether there is a short circuit between the transformer core silicon steel sheets.

4) Light-gas operation of oil-immersed transformers, warning signals and signal relays

a. First, find out the reasons: whether the oil level reduces the oil level, the transformer fails to generate a small amount of gas, the internal transformer short circuit causes the oil temperature to rise, whether there is gas in the gas relay, the secondary circuit and the gas protection device.

b. Treatment method: immediately turn off the alarm signal, restore the signal board and turn the switch handle to the open position, and disconnect the trip pressure plate for heavy gas protection. If the internal fault of the gas relay, it should be replaced in time. 10

Oil sample testPerformance indicators such as withstand voltage and impurities are carried out every three years. Long-term full-load or overloaded transformers can shorten the cycle.

The insulation resistance of high and low voltage is not less than 70% (10M) of the original factory value.

1. The shell of the transformer should be reliably grounded. The working neutral line and the neutral grounding line should be laid separately. The working zero line cannot be buried underground.

2. The neutral grounding loop of the transformer should be made into a detachable connection bolt near the transformer.

3, with

1. Oil leakage at the welding place

Mainly the poor welding quality, the existence of virtual welding, desoldering, the presence of pinholes in the weld,

, transformer primary current = S / (1.732 * 10), secondary current = S / (1.732 * 0.4).

, transformer once

The following conditions should be met at the same time: __ The connection groups should be the same, the voltage ratio should be the same (± 0.5% error is allowed),

The high voltage windings used in domestic transformers are generally connected into a Y connection method, and the connection method between the medium voltage winding and the low voltage winding depends on the system conditions. The so-called system situation refers to

GB1094.1-2008 "Power Transformer" Part I General Rules Transformer "Part V withstand short-circuit capability GN / T6451-1999" Technical parameters and requirements for three-phase oil-immersed power transformers "
GB / T16274-1996 "Technical parameters and requirements for oil-immersed power transformers 500kV level"
GB1207-1997 "Voltage Transformer"
GB1208-1997 "current transformer"
GB6450-1986 "dry-type power transformer"
GB10228-1988 "Technical conditions for three-phase air self-cooling dry-type power transformers"
GB10228-1997 "Technical parameters and requirements for dry-type power transformers"
GB10229-1988 "Reactor"
GB10230-1988 "On-load tap-changer"
GB4109-1997 "Technical conditions for high pressure casing"
GB156-2003 "Standard Voltage"
GB / T12325-2003 "Allowable Deviation of Power Quality Supply Voltage"
GB19212.1-2003 "Safety of Power Transformers, Power Supply Units and Similar Products" Part I General Requirements and Tests ZBK41005-1989 "Sound Levels of 6 to 220kV Transformers"

1. Epoxy insulation

Power transformers are the main equipment for power transmission and distribution in power systems. The power transformer mainly reduces the high-voltage power of the power grid to a voltage of 6000 volts (V) or 380 volts (V) that can be used directly to supply power to electrical equipment.

If the transformer is overloaded or short-circuited, the insulating material or insulating oil will decompose due to high temperature or electric spark, expand and even vaporize, causing the internal pressure of the transformer to increase sharply, which may cause the transformer casing to explode, a large amount of insulating oil spraying out and burning. The flow will further increase the risk of fire.

Pay attention to fire explosion during operation:

(1) Do not overload operation: Long-term overload operation will cause the coil to heat up, gradually insulate the insulation, and cause a short circuit.

(2) Regular inspection of insulation oil quality: The oil quality should be tested regularly, and the unqualified oil should be replaced in time, or other measures should be taken.

(3) Prevent the insulation deterioration of the transformer core, and the insulation aging caused by the long-term heating of the core.

(4) Prevent the insulation from being damaged accidentally due to overhaul. If abrasion damage is found, deal with it in time.

(5) Ensure that the wires are in good contact, and that poor contact causes local overheating.

(6) To prevent lightning strike, the transformer will burn out due to breakdown of insulation.

(7) Short-circuit protection: If a short circuit occurs in the transformer coil or load, if the protection system fails or the protection setting is too large, the transformer may be burned. For this purpose, reliable short-circuit protection must be installed.

(8) Protective grounding.

(9) Ventilation and cooling: If the transformer coil wire is Class A insulation, the insulator is mainly paper and cotton yarn. Each time the temperature rises by 8 ° C, its insulation life will be reduced by about half; the normal temperature of the transformer is below 90 ° C, and the life is about 20 years; if the temperature rises to 105 ° C, the life is 7 years. Transformers should be operated with good ventilation and cooling.

Widely used in lighting, machine tools, electrical and electronic equipment, medical equipment, rectifiers, military, aerospace, etc. Product performance can meet various special requirements of users.