This is the most detailed article I have ever read about the basic knowledge of transformer boxes!
Suitable locations for box type substations
1. Used as a road light box transformer: Both European and American box transformers have advantages over traditional civil electrical rooms in low load power systems. Box type substations have much lower construction costs than ordinary electrical rooms and have a beautiful appearance. European style box type substations can also use landscape type enclosures, making them a beautiful scenery.
2. Power consumption in parks and residential areas: Box type substations are often used for power supply in parks, residential areas, and other places. Parks and residential areas often pursue aesthetics and space occupation, and box type substations effectively solve these two problems, with beautiful appearance and small footprint.
3. Industrial and mining electricity: Factories, mines and other areas have high requirements for power supply equipment due to complex terrain and expensive time costs. Box type substations are easy to install, require less on-site construction, and have a protection level of IP33, which can greatly reduce construction volume and duration. They are installed in areas with complex terrain.
4. Temporary power supply: The construction unit often changes the location of power supply with the relocation of the construction site, so a set of movable distribution equipment is needed. The box type substation has good mobility and sufficient load capacity, which can fully meet the temporary power supply needs of the construction site.
Classification and advantages and disadvantages of box type substations
How to configure a box type substation
1、 High voltage side configuration of transformer box
(1) Box type transformers are divided into terminal type and ring network type according to the primary wiring method on the high-voltage side. The terminal type box transformer does not need to consider additional wiring: if the high-voltage side has one in multiple out or multiple in one out (mostly one in two out or three out, or two in one out or two out), it can be considered as a ring network box transformer.
(2)The high voltage side of the European box transformer is equipped with Circuit Breakers, as well as load switch fuse combination protection. Considering the cost of the transformer, for transformers with a capacity not exceeding 800 kV · A and low requirements for high-voltage side protection configuration, the latter is often used.
(3) The high-voltage load switches installed in European style transformer boxes are mostly FLN-12 sulfur hexafluoride load switches.
(4) The rated current of the protective high-voltage fuse melt is selected according to the following formula:
Irr=KIgmax
In the formula, Irr represents the rated current of the melt, A；
K - coefficient, generally taken as 1.3:
Igmax - rated current on the high voltage side of the transformer.
(5) Considering the safety of operation and maintenance, the configuration of the high voltage side of the European box transformer should meet the following requirements.
① When the load switch is in the disconnected state, there is a grounding device to ensure maintenance safety.
② The load switch and isolation switch are mutually locked to ensure safe operation.
(6) The American box type transformer is assembled in the same oil immersed box due to the combination of a high-voltage load switch fuse and a transformer, including a position load switch, backup fuse, plug-in fuse, and transformer. The ring network type is achieved by converting the internal connection terminals of the position load switch.
2、 Configuration of Low Voltage Main Distribution Cabinet for Box Transformer
The low-voltage main distribution cabinet of the box transformer generally needs to be equipped with incoming low-voltage isolation switches, low-voltage main circuit breakers, and low-voltage main metering devices. Low voltage isolation switches must be installed to meet the needs of daily maintenance and repair. The low-voltage main circuit breaker is selected based on the transformer capacity and the short-circuit current at the installation location of the circuit breaker.
The low-voltage main circuit breaker of the transformer is generally selected as a frame type intelligent circuit breaker, and its selection and setting requirements are as follows.
(1) The rated current of the circuit breaker. Generally, 2.0-2.5 times the capacity of the transformer is sufficient. The corresponding rated breaking current is greater than the short-circuit current value on the low voltage side of the transformer, and rarely needs to be verified.
(2) Long delay (i.e. inverse time limit, overload) overcurrent release setting current Lset, requiring In ≤ Iset1 ≤ Iz. Among them, In is the rated current on the low-voltage side of the transformer, and Iz is the current carrying capacity of the low-voltage bus. In order to fully utilize the capacity of the transformer without affecting its service life, Iset1 should be equal to or close to In.
(3) Short delay (i.e. fixed time limit, overcurrent) overcurrent and the set current Iset2 of the release must meet the requirement of 1.2Ifj ≤ Iset2 ≤ Id/1.3. Among them, Ifj is the short-term load peak current of the transformer, and Id is the single-phase grounding fault current of the transformer. This requirement is easy to meet for transformers with D and yn11 connections, but it is not easy to meet for transformers with Y and yn0 connections. This is also the reason why many box transformers use D and yn11 transformers. The setting time of the short delay overcurrent release of the transformer low-voltage main circuit breaker is generally 0.6 seconds, which can ensure that the time difference with the next level circuit breaker is not less than 0.2 seconds.
(4) Instantaneous overcurrent release set current Iset3. Due to the short delay overcurrent protection, in order to ensure better selectivity, the Iset3 value can be set larger, generally taking Iset1, which is 12-15 times the value.
3、 Reactive power automatic compensation device for box type transformer
According to the current national standards, the reactive power compensation capacity is generally required to be configured at 30% of the transformer capacity, which of course meets the requirements for usage conditions with high natural power factor. However, with the rapid popularization of inductive household appliances such as air conditioners, washing machines, refrigerators, and energy-saving lamps, configuring at 30% of the transformer capacity sometimes cannot achieve the expected effect. It is advisable to configure at 40% to 60% of the transformer capacity and automatically compensate in 10 control circuits. Although the cost has increased, it is negligible compared to the cost of the entire box transformer, and the compensation effect is good.
4、 Configuration of low-voltage outgoing cabinet
The number of circuits in the low-voltage outgoing cabinet of a box transformer is generally determined based on the load distribution in its power supply area and the appropriate reservation of 1-3 circuits. The specifications and models of circuit breakers selected for each circuit are determined based on their corresponding calculated loads. The selection and setting requirements for low-voltage circuit breakers in each feeder are similar to those for transformer main circuit breakers. But the prerequisite is to know the calculated load current of each feeder. Only then can it be reasonably configured.
Precautions for outdoor installation of box type substation
1. The grounding conductor of the box type substation must be a copper conductor, and the cross-sectional area must not be less than 30mm2. For some equipment that intentionally reduces the size of the grounding conductor, it is easy to be damaged by lightning strikes.
2. The grounding network of the box type substation also needs to ensure sufficient coverage area and depth. The grounding network of the box type substation should be buried below 1 meter underground, and some vertical grounding electrodes should be added to reduce resistance.
3. Box type substations should be installed in areas without strong vibrations and impacts, and there should be no significant electromagnetic induction nearby. Cable entry and exit lines should meet the construction safety distance.
4. Box type substations should be installed in dry and ventilated areas, horizontally, with an inclination of no more than 5 degrees.
5. The installation area of the box type substation shall not contain hazardous substances such as explosions, as well as corrosive gases or liquids.
6. The outdoor use of the box type substation does not require a shift, so it occupies a small area and has low operating costs.
Common electrical fault handling measures for box type substations
1. Overheating measures for the transformer box. Overheating of electrical equipment in box type substations can easily lead to insulation aging over a long period of time, resulting in loose joints. Internal short circuits in box type substations can also cause temperature rise and even lead to fires. The transformer winding is also prone to heating during operation, and continuous heating can easily damage the transformer. To address the issue of overheating in box type substations, it is necessary to promptly clean up residual substances such as dust inside the substations. Secondly, it is necessary to strengthen the temperature monitoring of various equipment inside the transformer box and issue high temperature warning signals in a timely manner.
2. The on load switch of the transformer box is faulty. Load switches in box type substations rarely fail except for their own faults. The main reasons for on load switch faults are as follows:
- Firstly, the switch has been used for too long, causing the switch frame to deform, resulting in the switch becoming loose or falling off;
- The second component has insufficient strength, resulting in damage to the tap changer during switching operations;
- The third issue is that the switch quality does not meet the standard, and after using it for a period of time, there may be situations such as spindle breakage.
To ensure the stable operation of the transformer, the production process requires strict selection of on load switches to ensure switch quality and installation technology.
3. The low-voltage switch of the transformer box trips. There are two main factors that cause the low-voltage switch of the transformer to trip. One is bus fault; The second is switch misoperation, etc. In the event of a low-voltage switch tripping fault, a careful inspection of the transformer box is required. Once overcurrent protection occurs on the low-voltage side of the main transformer, relevant equipment needs to be inspected and protected immediately. At the same time, eliminate switch misoperation faults, focusing on whether it is a bus fault or a line fault. Further inspect the relevant equipment to see if there is any leakage of the protective pressure plate used, and if necessary, conduct professional equipment inspection.
■ Maintenance of tripping fault in box type substation
During the operation phase of a box type substation, there are mainly two types of tripping faults: main transformer switch tripping and line switch tripping. Among them, main transformer switch tripping is further divided into three side tripping faults and low-voltage side tripping faults. The determination of the cause of the tripping of the transformer in the box type substation mainly relies on various monitoring data analysis.
When a tripping fault occurs in an outdoor box type substation, maintenance personnel should rush to the scene as soon as possible and restore power as soon as possible. The maintenance personnel first check if there is any serious damage to the equipment, and then inspect the operation of the trip switch and arc suppression coil. For spring energy storage switches, check whether the spring energy storage is faulty, and for electromagnetic switches, mainly check the switch power fuse.
When the low-voltage tripping fault occurs in the main transformer of the box type transformer, the main focus is on inspecting the line protection and main transformer protection in the substation equipment to find the cause of the accident. When overcurrent protection is the only cause of the fault on the low voltage side of the transformer, the line fault can be eliminated, and then the output terminal can be checked, followed by the inspection of the substation equipment. When there is both overcurrent protection on the low voltage side of the transformer and line protection, it can be directly determined as a line fault based on whether the line is disconnected. Maintenance personnel not only need to check the position of the line outlet, but also inspect the entire line to eliminate the line fault before proceeding with the switch trip fault handling.
Precautions for Fire and Explosion Prevention Operation of Box type Substation
1. Box type substations cannot operate under overload for a long time, especially after a few years of use, attention should be paid to issues such as coil aging and insulation aging to prevent short circuits.
If an Oil Immersed Transformer is installed in a box type substation, it is necessary to regularly check the insulation performance of the transformer oil and replace it in a timely manner if it is found to be unqualified.
3. During the operation of prefabricated substations, the condition of the transformer should be monitored at all times to prevent insulation aging caused by long-term heating of the transformer core. During maintenance, attention should also be paid not to damage the insulation layer.
4. During daily maintenance of box type substations, attention should be paid to wire contact issues and poor contact wires should be promptly dealt with.
5. Pre installed box type substation High-Voltage Switchgear, transformers, and low-voltage switchgear will be equipped with lightning arresters to prevent lightning damage to related equipment and protect the safe operation of the box type substation.
During the design phase of the box type substation, it is necessary to consider the ventilation and heat dissipation situation, and design sufficient ventilation openings and exhaust fans. The Ziguang box type substation is equipped with exhaust fans in both the transformer and low-voltage rooms, and the transformer room has an automatic temperature monitoring system that can effectively control the temperature inside the box type substation.
Conclusion
Box type substations are usually installed outdoors and are left unattended all year round. Only with regular inspections every year can a comprehensive maintenance of the box type substation be carried out. The maintenance of box type substations is a dangerous job, and if there is live maintenance, special attention should be paid to safety. The maintenance work of transformer substations is complex and tedious, with a large workload. If engaged in this work for a long time, it can easily cause psychological paralysis and lead to power accidents. Therefore, it is necessary to strengthen the security education work of the operation and maintenance team. The lifespan of a box type substation is usually 10-20 years, with the longest lifespan of a Ziguang box type substation reaching up to 20 years. Regular maintenance of the box type substation is essential for its normal operation.