Knowledge | What is a box-type transformer? Classification, Structure, Maintenance, and Common Fault Troubleshooting of Box-Type Transformers
Structure of Box Transformer
The overall structure of a transformer substation is mainly divided into three parts: High-Voltage Switchgear, transformers, and low-voltage distribution equipment.
According to system requirements, high voltage switches can be equipped with sulfur hexafluoride or Vacuum Circuit Breakers, ring network switches, load switches with fuses.
A measuring device can also be installed on the high-voltage side. The low-voltage side is generally equipped with a main switch and branch feeder switch, while some only install feeder switches to directly feed low-voltage end users. Compensation capacitors, measuring devices, etc. can also be installed. Distribution Transformers are generally selected as oil immersed or dry-type transformers.
The high voltage chamber of a European box transformer is generally composed of high voltage load switches, high voltage fuses, lightning arresters, etc. It can perform power outage and transmission operations and has overload and short circuit protection. The low-voltage chamber is composed of low-voltage air switches, current transformers, ammeters, voltmeters, etc. Transformers are generally made of S9 or dry-type.
There are two ways to construct the box, namely the "eye" shaped layout and the "cross" shaped layout. The high and low voltage chambers arranged in a "mu" shape are relatively wide, which facilitates the implementation of ring network or dual power supply wiring for ring network power supply solutions.
The American box type combination transformer is divided into two parts: front and rear. The front part is the wiring cabinet, which includes high and low voltage terminals, high voltage load switches, plug-in fuses, high voltage tap changer operating handles, oil level gauges, oil temperature gauges, etc; The rear is the fuel tank body and heat sink, with transformer windings, iron cores, high-voltage load switches, and plug-in fuses all located inside the fuel tank. The box adopts a fully sealed structure.
The integrated box has recently been developed by domestic manufacturers and is not widely used. It has a double-layer structure, with high and low voltage chambers placed above the transformer chamber
European style, American style, and integrated box type transformers each have their own advantages and disadvantages. European style box type transformers have a larger volume, with high and low voltage switches and transformers located in the same large housing, which has poor heat dissipation conditions and requires the installation of mechanical exhaust devices. American style box transformers have relatively good heat dissipation conditions due to the direct external heat dissipation of transformer cooling fins, but their design is worse than European style transformers, and their appearance is difficult to match with green environments such as residential areas. The integrated transformer takes up less space and has similar advantages and disadvantages to the American transformer. In addition, American style and integrated transformer substations can only manufacture those with a capacity of 630kVA or less domestically, while European style transformer substations can reach 1250kVA.
The models of ordinary box type substations are divided into three categories
(1) High voltage switchgear model;
(2) Dry type transformer cabinet model;
(3) Low voltage switchgear model.
The meaning of the first three letter symbols is:
Z-combination formula; B - Substation; N (W) - Indoor (outdoor, optional); X-box type; Y-Mobile.
Operation and Maintenance of Box Transformer
（1） Basic Requirements for Box Transformer Operation
1. The floor for placing box type equipment should be chosen at a higher height and not in low-lying areas to prevent rainwater from entering the box and affecting operation. When pouring concrete platforms, a gap should be left to facilitate the laying of cables in and out.
2. There must be two reliable connections between the box and the grounding grid. The grounding and neutral of the box transformer can share one grounding grid. The grounding grid is generally connected by grounding piles at the four corners of the foundation.
3. No illegal stacking is allowed around the box type equipment to ensure ventilation and operation inspection of electrical equipment. The box type transformer is mainly cooled by natural air circulation, and the transformer room door should not be blocked.
4. Loop switches, transformers, lightning arresters and other equipment in high-voltage distribution equipment should be regularly inspected and maintained. Defects should be promptly repaired, and insulation preventive tests should be conducted regularly. During operation, it is necessary to correctly release the mechanical interlock and use an insulating rod for operation.
（2） The inspection and maintenance of the transformer box should be carried out regularly according to the inspection and maintenance cycle (no less than once a month), testing the temperature at the cable terminal connection, checking the operation of the equipment, and conducting face testing if necessary.
The general inspection items are as follows.
1. Whether the foundation is firm, whether the holes are blocked, and whether there is moisture in the cabinet.
2. Whether the grounding device is complete, the connection is good, and the grounding resistance meets the requirements.
3. Is there any change in the outdoor environment that affects traffic and pedestrian safety.
4. Check the load condition of each feeder line, whether the three-phase load is balanced or overloaded, whether the switch on/off position and instrument indication are correct, and whether the control device is working properly.
5. The dust removal inside the transformer box should be carried out once every year. The surfaces of the high-voltage and low-voltage cabinets and the gas box can be wiped with a damp cloth. The transformer inside the transformer room should be cleaned with air blowing or a vacuum cleaner.
6. Daily maintenance and inspection of the fan. If the fan is not running, use a temperature and humidity controller, adjust it to below the current temperature, and start the fan for inspection.
7. Maintenance and Overhaul of High Voltage Switches and Low Voltage Switch Electric Operating Mechanisms
(1) Check the pointer of the pressure gauge to see if it is in the green area. If it enters the red area, perform opening and closing operations. Notify the manufacturer immediately for handling.
(2) The lubrication of mechanical components can be achieved by using universal lithium based grease (butter) for lubrication and conducting opening and closing operation tests.
(3) According to the requirements of routine testing, insulation and leakage current tests are conducted on cables and lightning arresters for routine testing.
8. Routine testing of auxiliary temperature and humidity controllers; Routine testing of smoke alarm devices; Tightening and Inspection of Terminal Blocks: Tightening and Inspection of Standard Components.
9. Terminal block maintenance: Due to thermal expansion and contraction, the terminal block may become loose. Annual inspections should be conducted to re tighten all terminals in the terminal room. Attention: Before re tightening, please confirm that both the primary AC circuit and the secondary control circuit are powered off to avoid electric shock!
10. Precautions for maintaining the transformer box
(1) The box transformer door is equipped with a windproof mechanism, which is a mechanism to ensure that it remains fully open. When closing the box door, be sure to lift the root of the windproof mechanism upwards, and then close the door without pulling or forcefully, to prevent the mechanism or door from deforming and affecting the normal use of the box transformer!
(2) After the local manual operation of the high-voltage load switch is completed, the load switch operating handle should be returned to the handle bracket on the inside of the outer door to avoid loss.
(3) When the backup circuit of the high-voltage ring main unit is temporarily not connected to the cable, the backup circuit should be locked before the ring main unit is powered on, or the cable socket should be sealed with a matching insulation cap to avoid accidents!
(4) The dust cap equipped on the ring main unit when the box transformer leaves the factory cannot replace the insulation cap!
(5) It is not allowed to insert any short-circuit plug into the test hole during operation, otherwise it will cause damage to the voltage sensor.
(6) The low-voltage isolation switch can only be operated when in the unlocked position. Do not forcefully pull or tug!
Correct operation of the transformer box
1. Closing operation
Close the cable room door - disconnect the grounding switch - close the load switch.
2. Opening operation
Separate load switch - close grounding switch - open cable compartment door.
(1) Precautions during operation
(1) When performing the opening and closing operation of the load switch, the switch must be pushed to the final position of opening or closing. Do not release or pull out the operating handle before the switch completes the action, otherwise the spring rebound will harm the operator.
(2) When performing the opening and closing operation of the load switch, the operating handle should be turned outward to prevent injury to the operator during operation.
(3) Before performing the opening and closing operation of the load switch, the electric manual switch knob on the upper left of the corresponding unit panel must be pulled out and rotated 90 ° to fix it in the manual position before manual operation of the load switch can be carried out, otherwise the mechanism may be damaged.
Troubleshooting methods for the causes of fault phenomena
(1) The frame circuit breaker cannot be closed
- 1. Control circuit malfunction.
- 2. After the intelligent release is activated, the red button on the panel does not reset.
- 3. Energy storage institutions do not store energy
exclusion method
- Use a multimeter to check the open circuit point.
- Identify the cause of the trip, eliminate the fault, and press the reset button.
- Manual or electric energy storage.
(2) Plastic case circuit breaker cannot be closed
After the mechanism was tripped, it did not reset.
2. The circuit breaker has an undervoltage coil and no power supply at the incoming end.
exclusion method
- Identify the cause of the trip and troubleshoot before resetting.
- Make the incoming end live, reset the handle, and then close the switch.
(3) The circuit breaker trips when it is closed
1. There is a short circuit phenomenon in the outgoing circuit
exclusion method
1. Do not repeatedly close the switch. The fault must be identified and resolved before closing.
(4) The capacitor cabinet cannot automatically compensate
- The power supply of the control circuit has disappeared.
- The current signal line is not connected correctly
1. Check the control circuit and restore the power supply.
How to install box type transformers in the construction of box type transformers?
Construction of grounding device for transformer box
1. A grounding grid is installed around the box type substation, which is connected to the grounding grid of surrounding buildings.
2. The burial depth and welding requirements of the grounding device shall comply with the design requirements and relevant specifications.
3. After the installation of the grounding device, the grounding resistance test can only be carried out on clear weather when the ground humidity reaches the specified standard. If there is a grounding device with a grounding resistance value that does not meet the requirements, corresponding grounding electrodes and grounding busbars should be added according to the regulations until the grounding resistance value meets the design requirements.
4. The connection between the grounding device and the equipment should be reliable and aesthetically pleasing.
Installation and construction of outdoor box type substation
1. Approval: According to the requirements and usage requirements of the box transformer design drawings provided by the box transformer manufacturer, order the corresponding materials and equipment. Before ordering the transformer inside the box transformer, provide the equipment model to the power supply department for approval.
2. Wiring: After on-site investigation, determine the reasonable entry point for the 10kV power supply and prepare the corresponding power supply construction plan
3. Pre embedding: carry out the foundation construction of the box type substation, embed corresponding components and cable protection steel pipes.
4. Installation: After the foundation reaches more than 70% of the design strength, the box type substation device should be inspected on site first. The equipment can only be installed after the accessories are complete, the equipment is intact, and there is no rust or mechanical damage. During this process, attention should still be paid to the maintenance of the foundation.
5. Inspection: After the construction of the box type substation is completed, the construction unit shall first conduct equipment self adjustment and self inspection to ensure that it is qualified. The construction unit shall then entrust a qualified testing department to conduct on-site testing of the box type substation.