Abstract: Environmental requirements and design experience, as well as intelligent features of oil-immersed underground combined transformers of 10kV and below.
Keywords: underground combined transformer, environment, design, intelligent.
1 Introduction:
Before the 1990s, overhead lines, like spider webs, seriously affected the beautification and environment of modern cities. With the planning and development of the city and the continuous improvement of urban power grids, the overhead lines have been replaced by cables into the underground. Various high and low voltage power distribution equipments have been placed underground, which has gradually become a new direction for urban power distribution transformation, which can make urban streetscapes The city appearance has improved. Among them, underground transformers are an important part of buried products. The design and manufacture of underground combined transformers is the key to the quality and safety of the system.
Underground transformers are generally installed in the access holes of the trench and in small pits. For its special operating environment, its sealing waterproof, insulation, anti-corrosion and anti-rust and temperature rise control issues are a key part of product design. The performance of the designed products is generally designed according to the requirements of the most advanced level of domestic distribution transformers, designed as S15 type amorphous alloy core transformer, S11, S13 type energy-saving environment-friendly transformer. In the case of large load changes, the on-load capacity-regulating transformer can be used underground to make it more energy efficient.
With the coordinated development of power grids, the improvement of technology and information, automation and interaction, the realization of grid intelligence is a key step in the development of power grids. It is imperative to realize intelligent control of substation equipment.
The author of this article is based on years of experience in designing its products for your reference.
2. Technical requirements and parameters of underground transformers
(1) Ambient temperature in the pit
Maximum ambient temperature +55 ° C
Minimum ambient temperature -10 ° C
Maximum monthly average temperature +45 ° C
(2) Installation environment
No explosive or corrosive gas, no violent shock at the installation site. The transformer can be used in pits or soil, and the tank and exposed parts can be immersed in water for long-term operation.
(3) Performance parameters
Product technical indicators: based on national energy-saving product indicators.
Voltage level: high voltage 6kV, 10kV
Low voltage: 0.4kV
Rated frequency: 50Hz
Rated capacity: 30kVA~1600kVA
(4) In addition to the provisions of GB1094.1~3, GB1094.5, GB/T6451 and JB/T10217, underground transformers should also meet the following requirements:
1) Good sealing to meet the requirements of product operation. Product water immersion test is required. The water surface is at least 200mm above the highest point of the product, and the duration is not less than 20h. After that, the insulation resistance measurement is carried out under water, and the external pressure and induction withstand voltage test are applied. The test is to comply with the provisions of JB/T501.
2) The ambient temperature of the underground transformer operation is generally allowed to be 10 °C higher than the normal ambient temperature, so the temperature rise limit of each part of the underground transformer is generally required to be 10K lower than the corresponding national standard value of the conventional product, that is, the oil top temperature rise control At 50K, the winding temperature rise is controlled at 55K.
3) The underground transformer tank or the outside of the cabinet and all exposed metal parts are exposed to harsh environment for a long time. Therefore, the external parts of these components should be coated with anti-corrosion materials or made of anti-corrosion materials, and the surface should be smooth and smooth.
4) There should be mechanical strength requirements for the underground type transformer tank: no damage should be caused under the positive pressure of 50 kPa and no permanent deformation should be allowed; no cracking should occur under the positive pressure of 100 kPa.
5) The pit of the underground transformer can be installed according to the rated capacity. There is proper space around it, and the drainage facilities are installed. The pit cover is provided with ventilation and manhole to facilitate heat dissipation and observation of operation.
6) Underground combined transformer primary system diagram: Figure 1
3. Underground transformer design
The underground transformer body is equipped with special low-loss, high-efficiency S11, S13, S15 transformers.
3.1 Iron core
In order to meet the needs of national energy conservation and emission reduction, reduce the no-load loss of the transformer, and save a lot of electric energy. For this reason, the core adopts new materials and new processes, such as amorphous alloy core (the no-load loss is 70%-80% lower than that of S11, However, the price is higher, the noise is slightly larger) and the oriented crystalline cold-rolled silicon steel sheet (27QGH100/23ZDKH90/23ZDKH85 low-loss silicon steel sheet) is wound core structure, and the laminated core adopts multi-level joint structure to reduce the core volume and reduce the weight. The no-load loss is reduced, the no-load current is reduced by 80%, the noise is reduced by more than 10dB, the winding wire weight is reduced, and the transformer volume is reduced. When designing, the height of the transformer is reduced as much as possible, and the volume of the pit is correspondingly reduced.
3.2 Winding
When the capacity is < 315KVA, the high and low voltage windings are made of long coils wound with copper wire; when the capacity is 315KVA-1600KVA, the long and round coils with low voltage is copper foil and high voltage is copper wire; The thermal rating is Class B with a large temperature rise margin. The underground transformer fully meets the requirements stipulated by national standards and professional standards of the machinery industry.
The copper wire and copper foil are designed with oxygen-free copper material, and the load loss is greatly reduced. Therefore, the heat generation is low and the overload capability is strong. The overload is allowed to double 2 hours and 2 hours, and the overload is 1.6 times 7 hours instead of Will affect the working life of the transformer.
3.3 High side protection
It can be seen from a system diagram that the high-voltage side protection adopts the protection method of the American box.
In the technical design, the protection component selects the oil-immersed load switch and the series two-stage fuse with reliable performance and advanced structure as the high-voltage side protection of the underground transformer. The two-stage fuses are the plug-in fuse and the backup fuse respectively, which are reduced. Operating costs and protection against failures. The plug-in fuse is a dual-sensitive (temperature, current) fuse that is blown when a short-circuit fault occurs on the low-voltage side of the transformer, and the oil temperature is too high. The backup protection fuse is used to disconnect the internal short-circuit current of the transformer, mainly to protect the transformer when a short-circuit fault occurs in the high-voltage side winding.
According to the needs of users, it can also be designed as a underground transformer without auxiliary protection equipment, which has a simple structure and low price.
3.4 High and low voltage outlet
The high and low voltage of underground transformers are generally cable entry and exit lines. The waterproof, fully sealed, fully insulated and fully shielded wiring is used to ensure that the unpowered part of the whole transformer is bare and achieve zero insulation distance. The high-pressure uses the integrated bushing seat and the plug-in cable connector to enter the line. The cable connection and the cable are sealed and sealed with heat-shrinkable tubes. The low-voltage outlet terminal uses a low-voltage cable bushing (low-pressure sealed lead device). When the capacity is ≥ 630KVA, the base is used for the raised seat, and the cable is sealed with the marine cable seal module. The above devices are easy to install, corrosion-resistant, aging-resistant, maintenance-free, and the equipment can be normally supplied with water for 24 hours in water, and the operation is safe and reliable.
After the cover of the other electrical components is fixed, a metal sealing box is attached to the top of the cover to enclose them.
3.5 Design of insulation structure
The insulation is made of Class H insulation material. The main insulation is made of H and F insulation materials, which enhances the insulation strength. The insulation oil uses high-burning point β oil, which is safe and fireproof and enhances the overload capacity. However, this design is expensive. According to the needs of users, A grade or B grade hybrid heat-resistant insulation materials can be selected to reduce the cost of the transformer.
3.6 cabinet
The protection level of the cabinet generally meets the requirements of IP68 according to GB4208, that is, the product is installed below the ground and runs normally in the immersion water for a period of time. A fully sealed fuel tank is used, and the box edge is sealed with oil-resistant cork rubber rubber between the cover and the cover. Since all operations of the underground transformer are performed at the top, the operation panel is placed at the top of the tank, and the layout of the components on the tank cover is shown in Fig. 2. The oil tank is welded with high mechanical strength steel plate. After the welding is completed, the shot peening is carried out. The overall sealing test of the fuel tank is carried out according to the product standard. The radiator adopts the thick steel plate welding structure, which greatly improves the leakage prevention capacity of the box. It is treated with anti-corrosion materials such as spray painting and spray coating, and has good anti-corrosion, waterproof and moisture-proof properties.
Figure 2 Schematic diagram of the cover installation
3.7 Temperature rise and heat dissipation
The underground transformer is buried deep in the ground or placed in the pit. During the operation, the loss-heat-heat dissipation process is inevitable. The flat oil tank and the flat radiator are used, and the surface is dark-painted to enhance the radiation, which is beneficial to the hot oil circulation. The signal is transmitted through a temperature measuring device on the transformer and a thermocouple in the pit. The fan in the light box housing can automatically exhaust and supply air into the foundation of the pit to reduce the temperature of the entire system. The temperature rise limit of each part is 10K lower than the corresponding national standard value of the conventional product, so that it has no temperature rise failure during the service life cycle.
3.8 LED display and advertising light box type outdoor switchgear LED display and advertising light box is a creative product that meets the needs of various markets in China. It not only beautifies the surrounding environment, but also can be built with low-voltage power distribution cabinet and miniaturized high-voltage switchgear. The electric solution is flexible and diversified, and the underground transformer capacity is large. The advertising light box is directly installed above the underground transformer to form an integrated underground combination transformer. The structural features are as follows:
(1) The protective casing of the switchgear adopts the advertising light box structure. The two doors of the light box casing can be opened at full angle and have spring support positioning, which is convenient for operation and maintenance of the switchgear.
(2) The two sides of the light box casing are advertising boards, which adopt LED light-emitting elements with small volume and uniform light source. The LED lights make the advertising board and the surrounding environment harmonious and unified, adding light and color, reducing the noise emission of the transformer and the radiation discharge of electromagnetic waves. Energy conservation and environmental protection requirements. Achieve a bumper harvest of economic and social benefits.
(3) The door lock of the advertising light box and the switchgear is set independently, and the maintenance of the light box does not affect the operation of the switchgear.
(4) The light box housing is equipped with an audible and visual alarm device, which can be damaged by the outside world and can send out an alarm signal.
(5) The switchgear adopts double-layer door structure, the protection level reaches IP34, and the second layer is the instrument door. After ensuring that the light box-type casing is damaged, the low-voltage cabinet still reaches the protection level of a complete independent outdoor low-voltage product. Display and remote meter reading.
4. Oil-immersed underground combined transformer structure
5. The composition and characteristics of intelligent underground transformer
The product will be built-in or externally placed on the sensor body of the transformer body and the intelligent components of the transformer, combined with modern mobile communication technology and computer technology, forming a mechatronics intelligent product - intelligent underground combination transformer. Its working principle is shown in Figure 4.
The 10kV distribution substation is the node of the power network, which changes the voltage from 10kV to 0.4kV, collects current and distributes electrical energy. Integrating modern electronics into high-precision A/D sampling devices, high-performance 32-bit or 64-bit DSP processors, the system has the following features:
(1) Online detection function
The underground combination box has built-in GPRS communication module and communication interface RS232, RS485 and USB interface, which can set parameters and read data remotely or locally. It can receive the timing command issued by the main station and has the function of remotely upgrading the terminal application.
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