[b]1 Characteristics of Prefabricated Substations[/b] (1) Equipment tends to be oil-free: In prefabricated substations, the main transformer, direct distribution transformer or station service transformer, TV, and TA are generally dry-type, and the circuit breaker is a vacuum circuit breaker. The use of oil-free equipment will effectively avoid the leakage problem that is difficult to eradicate in conventional substation equipment. The substation can implement condition-based maintenance, reduce maintenance workload and maintenance costs, and has a good operating environment. It is especially suitable for heavily polluted areas, with low operating costs and considerable overall benefits. (2) Small footprint: Since my country has very little arable land, the construction of prefabricated substations reduces the footprint and the equipment layout is concentrated. Generally, the footprint of a prefabricated substation can be reduced by more than 50% compared to a conventional substation. (3) Simplified layout: The oil-filled equipment in the substation is replaced with oil-free equipment. The equipment area is small and the layout is concentrated, which is becoming increasingly simplified and miniaturized. The equipment protection does not need local display function. All electrical signals and data acquisition are transmitted to the main station for display, which makes it easy to achieve unmanned operation. (4) Secondary microcomputerization: The secondary side of the box-type substation adopts microcomputer-based automated protection devices, which have remote control, remote signaling, and remote measurement functions. According to the main transformer capacity, it can also realize four remote functions. This makes the substation operation level reach a high level of intelligence, and the operation is simple and highly applicable. [b]2 Overview of the application of box-type substations in our province[/b] In the rural power grid renovation project, seven 35kV power supply points in our province have adopted box-type substations. Currently, two are in operation and five are under construction. In terms of equipment configuration, we try to use oil-free equipment. TV, TA, compensation devices, switching equipment, etc. are selected as oil-free equipment. In order to save investment, oil-immersed transformers are still used for the main transformer. This can save investment and increase the load capacity of the main transformer. The box-type substations in our province all adopt this mode of external main transformer. The protection configuration and communication method are the same as those of conventional substations. [b]3 Case Study of Datong Queershan Box-Type Substation[/b] (1) Determination of Queershan Station Scheme: The original 35kV Queershan Substation was located in the northeast of Zuoyun County, Datong City, and was responsible for supplying electricity to Queershan Town, Guanjiabao Township and Zhangjiachang Township. It mainly supplied electricity to coal mines, and the nature of the load was very important. Over the years, due to the blind mining of small coal mines, many mining subsidence areas have been formed in the area. In May 1999, our substation operators suddenly discovered that there was obvious subsidence in the geological conditions of the station area and the surrounding area, and cracks appeared in many places in the station buildings, which had endangered the safety of personnel and equipment. After the substation foundation subsidence occurred, we conducted two months of monitoring and soil surveying and found that the changes were getting bigger and bigger. It was necessary to take temporary remedial measures immediately to reinforce the substation buildings to ensure the safety of the operators. In order to find out the cause of the substation foundation subsidence, our company successively hired relevant geological departments of the province and the region to conduct on-site surveys. It was found that there were five coal mining areas below the station, which were serious mining subsidence areas, and it was necessary to relocate and rebuild. The entire 10km radius around the Queershan substation is a coal mining subsidence area. All buildings within this area have experienced varying degrees of subsidence, posing significant challenges to the selection of a site for the relocation of the substation. Based on local load conditions, if the substation were located in a non-mining area more than 10km away, it would be deviated from the load center, increasing the power supply radius and hindering economic operation. After comprehensive comparison, the only viable site for the new substation is within the mining subsidence area. Foundation treatment for building a substation within a mining subsidence area is extremely difficult. First, the large area of ​​the subsidence area makes foundation treatment too costly; second, the geology of the subsidence area is constantly changing, making long-term treatment impossible, requiring only a transitional approach; and third, the ground load within the substation must be minimized. Due to these three factors, we must consider them when determining the relocation plan and be prepared for another relocation. Only a prefabricated substation can meet the requirements for relocation. While prefabricated substations offer advantages such as convenient operation and easy relocation compared to conventional substations, their overall cost is higher. Therefore, the prefabricated substation needs to be modified and optimized to reduce project costs. We considered three aspects in the modification of the box-type substation: First, the main transformer should be reused and placed outside the box, instead of using a dry transformer, which can save money; second, the 10kV and 35kV equipment should be placed in two boxes according to voltage level, which is convenient for installation and management; third, it should meet the requirements of unmanned operation and realize dispatch automation. (2) Design concept of Queershan box-type substation: While considering the overall cost and ease of use, the design of Queershan box-type substation was optimized. The scale of the reconstruction should be the same as the original station to meet the local power supply needs. In terms of equipment selection, we should try to reuse the old equipment and modify the equipment to reduce the cost. ① Wiring method: There is no busbar structure in the station. The 35kV incoming line enters the box through the isolating switch, and the 10kV cable goes out. The 35kV single bus connection and the 10kV single bus branch connection are all located inside the box. ② The secondary of the substation adopts integrated automation, which has four remote functions and meets the requirements of unmanned operation. Secondary panel configurations should be minimized. The box should contain 3 protection panels, 2 DC panels, 1 AC panel, 1 energy meter panel, and 1 communication cabinet. ③ The secondary equipment inside the box should be reused as much as possible. The switch cabinet should have five protection functions. The main transformer should be oil-immersed 2×3150kVA. The carrier equipment should be reused. The capacitors, TA, and TV should be dry-type and placed inside the box. The remaining equipment should be domestically produced equipment that meets the functional requirements. ④ The station adopts DC operation and uses fully enclosed lead-acid maintenance-free batteries. The control system adopts microcomputer technology and has a communication interface (RS232) with the integrated automation system to realize telemetry, remote signaling, and remote control functions. It collects relevant status quantities and analog quantities of the DC panel and sends them out in digital communication mode to provide reliable DC power for various relay protection, communication and various DC operation mechanisms in the station. (3) Principles for selecting equipment in the Queershan box-type station: ① The main equipment should not use oil-filled equipment as much as possible. However, due to the limited investment in the substation, the main transformer can still be reused without capacity expansion to meet the requirements. The circuit breaker, TV, TA and station transformer are oil-free. ② The secondary use of centralized integrated automation equipment meets the requirements of unmanned operation and automated dispatching. ③ The communication means in the station still adopts spread spectrum and carrier dual channels, and the original equipment is used to save investment and reduce costs. ④ Since the station site is still located in the mining subsidence area, the factors of relocation should be considered when selecting equipment. The station wall adopts hollow wall to reduce ground pressure. [b]4 Performance comparison between box-type substation and conventional station[/b] Taking the YB-40.5/12 box-type substation produced by Shantou Jinhuilong Switch Co., Ltd. selected for Datong Queershan Station as an example, the box-type substation has the following functions: (1) The switch cabinet in the box meets the four full functions, namely full insulation, full isolation, full enclosure, and full working conditions. The insulation partition in the cabinet uses SMC material, and the switch cabinet is equipped with a live display device. (2) The box comprehensively considers lighting, emergency lighting, ventilation, sand prevention, heat dissipation and heat preservation measures. Each unit cabinet is equipped with a heater, which is controlled by an automatic temperature and humidity controller. To meet normal operation and maintenance requirements, temperature, humidity, and measurement display meters are added, as well as condensers and smoke alarm devices. The humidity and smoke alarm probe signals are connected to the integrated automation system. The location of the carrier machine and spread spectrum equipment is also considered. (3) The secondary equipment of the substation is centrally placed in a temperature-controlled compartment, and a partition door with a heat insulation layer is used. The glass observation window is double-glazed to ensure that the temperature and humidity fully meet the operating conditions of the microcomputer protection device during operation, so as to ensure that the protection device can operate normally and reliably. (4) The box is double-sealed, and the interior is equipped with a heater and ventilation device, and is equipped with an automatic temperature and humidity controller. When the temperature is low and the humidity is high, the heater is turned on and the ventilation is started. When the temperature is low and the humidity is low, only the additional device is turned on, which can ensure that the internal temperature and humidity of the prefabricated substation are kept within a certain range, so as to ensure that the high and low voltage electrical equipment in the box will not produce condensation that affects the operation under any circumstances, thus meeting the requirements of the power sector. The top of the box is made into a ∧ shape for easy drainage. (5) The base frame of the enclosure is made of hot-rolled steel, and the cabinet frame is made of cold-formed steel. The two are assembled together and subjected to strict anti-corrosion treatment. The top plate and side walls are made of double-layer colored composite heat insulation board, and then decorated with aluminum alloy profiles, which not only ensures high strength and durability, but also increases aesthetics. (6) The enclosure is equipped with anti-theft, fire prevention and smoke alarm devices. When the enclosure door is pried open, the alarm bell will sound continuously and send a signal to the central control room at the same time. (7) The enclosure is equipped with a primary system simulation diagram, which can be made into a fully interconnected simulation system diagram as needed. In this way, when the enclosure door is opened, the equipment operation status can be seen at a glance. (8) The power supply reliability of the box-type substation is high. Since the box-type substation adopts indoor equipment and is equipped with microcomputer protection devices, it can realize unmanned operation and meet the requirements of county dispatch automation operation. (9) The box-type substation occupies a small area. Except for the two main transformers and the 35kV incoming line disconnectors, which are located outdoors, the other primary and secondary equipment of the 35kV Queershan substation are installed in two boxes. The substation has fewer foundations and structures and fewer buildings in the plant area. (10) Box-type substations are easy to operate and require less maintenance. Since the box-type substation switches are all indoor vacuum circuit breakers, the capacitors, TA, and TV are all dry-type products, and all insulation materials are flame-retardant materials, they meet the requirements of oil-free power supply equipment and control the fire risk to the lowest point. Moreover, the equipment operates stably and does not require daily maintenance. (11) Box-type substations are aesthetically pleasing and pollution-free. The box material can be guaranteed not to age, corrode, or leak for 20 years between -40℃ and +40℃. [b]5 Economic comparison between box-type substations and conventional substations[/b] (1) The construction cost of a box-type substation is about 300,000 yuan, while that of a conventional substation is about 700,000 yuan, which can save 400,000 yuan in investment. (2) Equipment purchase cost: The investment in containerized substations is much larger than that of conventional substations, which require approximately 1.5 million yuan, while that of conventional substations is 1 million yuan. Containerized substations require 500,000 yuan more than conventional substations. (3) Installation cost: Containerized substations require 300,000 yuan, while conventional substations require approximately 400,000 yuan due to the outdoor structure and other installation costs. (4) Land occupation: Containerized substations save more than 50% of land compared to conventional substations, saving 200,000 yuan. From the above four aspects, the overall cost of containerized substations is lower than that of conventional substations, saving 200,000 yuan in investment under the premise of the same capacity and equipment.