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Transformer is a device that uses the principle of electromagnetic induction to change the AC voltage. The main components are the primary coil, the secondary coil and the core (core). The main functions are: voltage conversion, current conversion, impedance transformation, isolation, voltage regulation (magnetic saturation transformer) and so on.
I. Current status of transformers in China
There are about 1,500 transformer manufacturing enterprises in China. There are nearly 30 production enterprises of transformers above 500kV; about 50 enterprises of 220kV and above; about 130 production enterprises with a certain scale within the statistical scope of production enterprises of 110kV and below. In the past three years, the annual demand for transformers in China has been around 1.5 billion kVA. Industrial Insight Research "Transformer Market Analysis Report" shows that China's transformer industry has a production capacity of about 3 billion kVA, of which Shen, Xi and Baosan Group have a capacity of about 500 million kVA. In the past few years, China's power construction has developed rapidly, and the demand for transformers has increased rapidly. As a result, a considerable number of domestic transformer companies have rapidly expanded their production capacity, and even individual enterprises have doubled their production capacity. Since 2009, the national power construction speed has slowed down, and domestic transformer demand has decreased. In particular, State Grid Corporation and Nanwang Company used centralized bidding to purchase transformers, and the low price bid led to unprecedented competition among enterprises. In addition, due to the fierce competition in the domestic market, the bid price is lower than the cost price, which led some companies to start overseas routes, export products, and build factories abroad. In addition, many companies have begun to develop new, energy-saving, environmentally friendly, and intelligent products. However, the general economic benefits of domestic transformer companies have dropped significantly, and many companies are on the verge of loss or loss. A large number of distribution transformer companies have even seen a situation of malicious competition at the expense of product quality. In addition, the quality of transformer products has decreased and the accident rate has increased. The pass rate of the distribution transformer is only 70%. The accident rate of running transformers on the Internet is on the rise, and there are many accidental transformers in the past five years, and the safety of the power grid is threatened.
The rapid development of China's power construction has driven the development of China's transformer manufacturing industry. In 2011, the national transformer output reached 1.43 billion kVA, an increase of 6.86%. In 2011, there were 1,461 enterprises in China's transformer manufacturing industry (main business income of more than 20 million yuan); sales reached 290.14 billion yuan, total profit reached 16.608 billion yuan, asset scale was 268.840 billion yuan, and product sales profit was 339.72. 100 million yuan.
The competition in China's transformer industry is fierce. Foreign multinational companies have seized a large market share, and the number of domestic transformer manufacturing companies is also growing rapidly. For example, Shandong Yongcheng Transformer has fierce competition in the middle and low-end transformer market. There are more than 20 enterprises with 220KV transformer production capacity, and more than 100 enterprises with 110KV transformer production capacity. The transformer enterprises with a production level of 500KV or above have built high barriers to entry through technology and capacity, and the market structure has stabilized.
In recent years, China's power demand has grown rapidly, and the high-speed construction of the power grid has driven the market demand for power transmission and transformation equipment. As a key electrical equipment for power transmission, the market demand has also maintained steady growth. According to the “China's Transformer Manufacturing Industry Market Demand Forecast and Investment Strategic Planning Analysis Report” issued by Forward Industry Research Institute, the sales revenue of China's transformer industry in 2013 was 368.034 billion yuan, an increase of 16.07% year-on-year. The compound growth rate of industry in 2005-2013 was 20.76. %.
The number of enterprises in the transformer industry in China is very large. In 2013, there were 1,650 companies in the whole industry, and there were 1,303 small and medium-sized enterprises, accounting for nearly 80%. Most small enterprises have weak technical strength and can only produce low-end products below 110, resulting in a serious excess of production capacity in the low-end transformer market. According to statistics, the current capacity utilization rate of domestic transformers is only 50%.
According to the plan, during the “Twelfth Five-Year Plan” period, the State Grid will invest about 2.55 trillion yuan for power grid construction. Compared with the 1.5 trillion yuan during the “11th Five-Year Plan” period, the “12th Five-Year” grid investment volume has increased by 68% year-on-year. In terms of subdivision, there will be 500 million yuan for 2.55 trillion yuan for UHV grid investment, 500 billion for distribution network investment, and about 1.55 trillion for other voltage grade grid line investments.
Second, the working principle
A transformer is a device that converts an alternating voltage, an alternating current, and an impedance. When an alternating current is passed through the primary coil, an alternating magnetic flux is generated in the iron core (or the magnetic core) to induce a voltage (or current) in the secondary coil. .
The transformer consists of a core (or core) and a coil. The coil has two or more windings. The winding connected to the power supply is called the primary coil, and the other winding is called the secondary coil.
Third, the main parameters
The main technical data is generally marked on the nameplate of the transformer.
Mainly include: rated capacity, rated voltage and its tapping, rated frequency, winding junction group and rated performance data (impedance voltage, no-load current, no-load loss and total load loss).
A. Rated capacity (kVA): rated voltage. The capacity that can be delivered during continuous operation at rated current.
B. Rated voltage (kV): The working voltage that the transformer can withstand when operating for a long time. To meet the needs of the grid voltage change, the high-voltage side of the transformer has tap taps, and the output voltage of the low-voltage side is adjusted by adjusting the number of turns of the high-voltage winding.
C. Rated current (A): The current allowed by the transformer under the rated capacity for long-term passage.
D. No-load loss (kW): When the rated voltage of the rated frequency is applied to the terminals of one winding, the active power drawn by the remaining windings is open. It is related to the performance and manufacturing process of the core silicon steel sheet and the applied voltage.
E, no-load current (%): When the transformer is under no load on the secondary side under rated voltage, the current passing through the primary winding is generally expressed as a percentage of the rated current.
F, load loss (kW): short circuit the secondary winding of the transformer, the rated current is applied to the rated winding position of the primary winding, and the power consumed by the transformer at this time.
G, impedance voltage (%): short circuit the secondary winding of the transformer, slowly increase the voltage in the primary winding, when the short-circuit current of the secondary winding is equal to the rated value, the voltage applied to the primary side at this time is generally rated voltage The percentage representation.
H, phase number and frequency: the beginning of the three phase is denoted by S, and the beginning of the single phase is denoted by D. China's national standard frequency f is 50Hz. There are 60Hz countries abroad (such as the United States).
I. Temperature rise and cooling: The difference between the temperature of the transformer winding or the upper oil temperature and the temperature around the transformer is called the temperature rise of the winding or the upper oil surface. The temperature rise limit of the oil-immersed transformer winding is 65K, and the oil surface temperature rise is 55K. There are also a variety of cooling methods: oil immersion from cold, forced air cooling, water cooling, tubular, and sheet.
J. Insulation level: There are insulation grade standards. The example of the insulation level is as follows: the high voltage rated voltage is 35kV, and the low voltage rated voltage is 10kV. The transformer insulation level is expressed as LI200AC85/LI75AC35, where LI200 means the transformer high voltage lightning impulse withstand voltage is 200kV, power frequency withstand voltage For 85kV, the low-voltage lightning impulse withstand voltage is 75kV, and the power frequency withstand voltage is 35kV.
 
Fourth, common faults
(1) When the transformer is powered or tested after being stopped, it is often found that the voltage is not normal, such as two phases high and one phase is low or the indication is zero; some new commissioning transformers have high three-phase voltages, making the part The electrical equipment is burned due to excessive voltage;
(2) The high voltage fuse is blown and not powered;
(3) The transformer cannot be powered after the thunderstorm;
(4) The transformer sound is not normal, such as making a "beep" or "beep" sound; in the operation, it sounds like a frog "wow wow wow";
(5) The high voltage terminal is burned out, and the high voltage casing has serious damage marks;
(6) Under normal cooling conditions, the transformer temperature is abnormal and continues to rise;
(7) The oil color changes too much, and carbon is present in the oil;
(8) The transformer emits a squeaking sound, which injects oil from the safety air passage and the oil storage tank, deforms the oil tank and the heat pipe, leaks oil, and seeps oil.
Fifth, the solution
(1) When constructing a new substation, the high and low voltage fuses should be installed in time according to the specifications. During the operation of the transformer, it is found that the fuse should be replaced in time after being burned or stolen;
(2) Reasonable configuration of high and low voltage fuses:
1 The transformer with a capacity of 100kVA or more shall be equipped with a fuse of 2.0~3.0 times of rated current;
2 Transformers with a capacity below 100kVA should be equipped with fuses with 1.5~2.0 times rated current;
3 The low-voltage side fuses should be selected slightly according to the rated current.
(3) Strengthen the actual measurement of the electric load. During the peak period, use the clamp-type ammeter to measure each of the distribution transformers, adjust the load reasonably, and avoid the three-phase unbalanced operation of the distribution transformer;
(4) For the 10kV distribution transformer low-voltage side voltage is within the range of +7%~-10%, it is generally not allowed to adjust the tap-changer. When adjusting the tap-changer, it shall be tested and adjusted by the test technician;
(5) Regularly check whether the three-phase current is balanced or exceeds the rated value. If the three load currents are seriously unbalanced, measures should be taken to adjust them in time;
(6) Before the annual thunderstorm season arrives; the lightning arresters on all distribution transformers should be sent to the repair department for testing, and the test should be installed in time after passing the test;
(7) The following inspections should be done before commissioning:
1 with load division and closing switch three times, no misoperation;
2 Test three times with the test button, it should operate correctly;
3 Test the grounding test three times with the test resistor and operate correctly.
(8) Regularly clean the dirt on the surface of the distribution transformer casing, check whether the casing has flash marks, whether the grounding is good, whether the lead wire used for grounding has broken strands, desoldering or breaking. Check the grounding resistance with a megger. >4Ω.
(9) Analysis of the causes of transformer leakage
1. Rubber seal failure and weld cracking
The transformer has many solder joints and long weld seams, while the oil-immersed transformer is a collection of various welded and connected joints based on steel welded shells. A 31500kVA transformer has more than 70 total solder joints, and the total length of the weld is nearly 20m. Therefore, there may be more leakage paths. The cause of direct leakage is rubber seal failure and weld cracking, pores, slag inclusions, etc.
2, sealant parts aging, cracking, deformation
Transformer leakage occurs mostly at the joints, while more than 95% is mainly caused by sealant parts. The quality of the sealant depends mainly on its oil resistance. The oil resistance is poor, and the aging speed is faster. Especially at high temperatures, the aging speed is faster, which can easily cause the seal to age and crack. Deterioration, deformation, and failure, causing the transformer to leak oil.
3. Manufacturing quality of transformers
During the manufacturing process of the transformer, there are many solder joints, long welds, difficult welding, welding materials, welding specifications, processes, and techniques, etc., which will affect the welding quality, causing pores, blisters, virtual welding, and desoldering. .
4, the quality of the plate butterfly valve is not good
Another part of the transformer that often leaks is in the plate butterfly valve. The transformer produced earlier is a rougher, thinner, single-layer seal that is used in the conventional plate-type butterfly valve. It is a phase-out product and can easily cause transformer leakage.
5, improper installation method
The flange connection is not flat. When installing, the gasket can not be evenly stressed. The bolts around the gasket are not evenly stressed. The flange joint is deformed and misaligned, so that the side of the gasket is too strong and the force on one side is too small. The seal on the side with a small force is likely to cause leakage due to insufficient compression. This phenomenon occurs mostly at the connection of the gas relay and the connection between the radiator and the body; the other point is that when the gasket is installed, the compression amount is insufficient or too large. When the compression amount is insufficient, the operating temperature of the transformer increases and the oil becomes thin, resulting in a transformer. The oil seepage, the compression amount is too large, the gasket is seriously deformed, and the aging accelerated service life is shortened.
6, improper shipping
During the consignment, construction and transportation, parts collide and improperly hoisting and transporting, causing parts to be deformed, welds to be welded, cracks, etc., causing leakage.
Six, the main classification
The classification of commonly used transformers can be summarized as follows:
1. According to the number of phases:
1) Single-phase transformer: for single-phase load and three-phase transformer group.
2) Three-phase transformer: used for the rise and fall voltage of three-phase systems.
2, according to the cooling method:
1) Dry-type transformer: relying on air convection for natural cooling or increasing fan cooling, it is mostly used for small-capacity transformers such as high-rise buildings, high-speed toll stations, local lighting, and electronic circuits.
2) Oil-immersed transformer: rely on oil as cooling medium, such as oil-immersed self-cooling, oil-infiltrated air-cooled, oil-immersed water-cooled, forced oil circulation, etc.
3. According to the purpose:
1) Power transformer: used for the rise and fall voltage of transmission and distribution systems.
2) Instrument transformers: such as voltage transformers, current transformers, measuring instruments and relay protection devices.
3) Test transformer: It can generate high voltage and carry out high voltage test on electrical equipment.
4) Special transformers: such as electric furnace transformers, rectifier transformers, regulating transformers, capacitive transformers, phase shifting transformers, etc.
4, according to the form of winding:
1) Two-winding transformer: used to connect two voltage levels in the power system.
2) Three-winding transformer: Generally used in power system regional substation, connecting three voltage levels.
3) Autotransformer: A power system used to connect different voltages. It can also be used as a normal boost or post-down transformer.
5, according to the iron core form:
1) Core transformer: Power transformer for high voltage.
2) Amorphous alloy transformer: Amorphous alloy core transformer is a new type of magnetic conductive material, the no-load current is reduced by about 80%. It is an ideal distribution transformer with energy saving effect, especially suitable for loads in rural power grids and developing regions. The rate is lower.
3) Shell-type transformer: special transformer for high current, such as electric furnace transformer, electric welding transformer; or power transformer for electronic equipment and television, radio, etc.
Eight, the basic composition
The components of the transformer include the body (core, winding, insulation, lead), transformer oil,
(Fig. 2) Fuel tank and cooling device, pressure regulating device, protective device (hygroscopic device, safety air passage, gas relay, oil storage cabinet and temperature measuring device, etc.) and outlet bushing.
1, iron core
The iron core is the main magnetic circuit part of the transformer. It is usually made up of hot-rolled or cold-rolled silicon steel sheets with a high silicon content and a thickness of 0.35 mm\0.3 mm\0.27 mm, coated with an insulating varnish. The iron core is divided into two parts: a core column and a horizontal piece, and the iron core sleeve has a winding; the horizontal piece is used for closing the magnetic circuit. The basic form of the core structure is both a heart type and a shell type.
2, winding
The winding is the circuit part of the transformer, which is wound with a double wire wrapped insulated flat wire or an enamelled round wire.
Eight, safe operating procedures
1. Job safety duties
1.1 Responsible for the inspection and maintenance of the power transformer before installation, and make a record of inspection and maintenance.
1.2 Responsible for the integrity of the transformer during the installation process.
1.3 Strictly follow the safety technology and operation procedures.
2, job qualifications
2.1 Received specialized professional safety technology and skills training.
2.2 There is a uniform distribution of transformer and power distribution equipment to install the employment certificate, and the certificate is on duty.
3. Preparation for on-the-job work
3.1 Accept the safety technology, and clearly understand its contents, including: the installation height of the transformer, the primary high-voltage down-conductor, the secondary outlet, and the distribution box installation.
3.2 Before construction, check whether the power transformer specification model meets the design requirements.
3.3 Before construction, the person in charge of construction must personally check the layout of the site, and the operators should carefully check the site layout of their respective operating projects.
4. Safe operating procedures
4.1 Large oil-immersed transformers must be prepared according to the installation and operation instructions before installation.
4.2 The nitrogen-filled transformer is not fully exhausted (the oxygen content of the gas is >18%), and it is strictly forbidden for the staff to enter. When filling the nitrogen-filled transformer, no one should stay at the vent.
4.3 Large oil-immersed transformers During the oil draining and oil filtering process, the outer casing and each side winding must be reliably grounded.
4.4 When checking the core of the transformer, the core shall not be placed on the fuel tank and shall be placed on the clean support mat prepared in advance. Do not perform any work on the core before relaxing the hoisting rope.
4.5 When checking the transformer hoist, remove the cover and place it on the clean shovel before starting the core inspection. Special Guardian shall be provided around the hood, and it is strictly forbidden to touch any part of the core.
4.6 Transformer sling or sling must be stable when landing.
4.7 When conducting internal inspection of transformers, ventilation and lighting must be good, and special personnel should be inspected; workers should wear buttonless, non-pocket overalls and oil-resistant non-slip boots. The tools to be brought in must be reins, registered, invented, and tools are strictly guarded. The debris is left in the body.
4.8 Cover flange bolts must be symmetrical and evenly elastic.
4.9 When inspecting the large transformer core, scaffolding should be set up. It is strictly forbidden to climb the lead wooden frame up and down.
4.10 The oil storage and oil treatment site must be equipped with sufficient reliable fire-fighting equipment. A clear fire-responsibility system must be established. The site should be level and clean. There should be no fire and inflammable or explosive materials within 10m.
4.11 When the transformer accessory is defective, it is necessary to carry out the welding process, and the residual oil should be drained, and the surface oil is removed and transported to a safe place.
4.12 Defective welding of transformer leads When insulation repair is required on site, insulation and isolation measures shall be taken.
4.13 Allowing repair welding of tiny leaks in oil-filled transformers.
4.14 The top of the transformer should have open holes.
4.15 The welded part must be below the oil level.
4.16 Fire welding is strictly prohibited, and intermittent welding should be used.
4.17 The oil around the solder joint should be cleaned up.
4.18 There should be proper safety and fire prevention measures, and safety and technical disclosure should be given to the participants.
4.19 Safety technical measures and necessary management systems shall be established before the transformer is dried.
4.20 The power supply and wiring used in the drying transformer shall be calculated. There shall be an overload automatic shut-off device and an overheating alarm device in the circuit.
4.21 When drying the transformer, the thermometer should be installed on the iron core, the winding or the upper oil surface according to the drying method, but the mercury thermometer is strictly prohibited.
4.22 Drying transformers should be staffed on duty. On-duty personnel should frequently inspect the temperature of each part for overheating and abnormal conditions and make a record. On-duty personnel are not allowed to leave the dry site without authorization.
4.23 When short-circuit drying is used, the short-circuit line should be firmly connected. Insulated wire should be used when eddy current drying; low voltage power supply must be used when using bare wire, and reliable insulation measures should be taken.
4.24 When using an external power supply for drying, the transformer case should be grounded.
4.25 When using vacuum hot oil circulation for drying, the outer casing and each side winding must be reliably grounded.
4.26 Drying transformers shall not be placed with flammable materials and adequate fire fighting equipment shall be prepared.
5, other matters needing attention
5.1 In the process of installing the power transformer, the person in charge of the installation of the experienced equipment shall be on-site command.
5.2 Non-construction personnel shall not enter the work area.
5.3 When working at night, the work area should have good lighting.

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