The Function and Structure of Box Substation

Box Substation Function Structure

A. The Function and Characteristics of Box Substation

Box type substation refers to an outdoor distribution device that installs high and low voltage switchgear and transformers together in a closed box. The main function is to provide the required electrical energy for high-voltage or low-voltage users. It has the following characteristics:

1. Small footprint. Installing a box type substation can save more than two-thirds of the land area compared to building a substation of the same size.

2. Flexible combination methods. The structure of box type substations is relatively compact, with each box forming an independent system, which makes the combination method flexible and variable. The user can freely combine some modes according to the actual situation to meet the needs of different places.

3. Beautiful appearance, easy to coordinate with the environment. The box casing can be selected in different colors according to different installation locations, making it easy to coordinate with the surrounding environment. It is particularly suitable for densely populated areas such as urban residential communities, stations, ports, airports, parks, green belts, etc. It can be used as a fixed substation or a mobile substation.

4. Low investment and short construction cycle. Box type substations reduce investment by 40% to 50% compared to conventional substations of the same scale, and the construction and installation cycle can be significantly shortened. At the same time, the daily maintenance workload of the box type substation is also relatively small.

B. Types of box type substations

1. Assembly type

Install the complete set of high and low voltage devices and transformers into metal boxes, and leave operating corridors in the high and low voltage distribution devices. This type of box transformer has a large volume and is rarely used nowadays.

2. Combination device type

Instead of using the existing complete set of devices, the high and low voltage control and protection electrical equipment are directly packed into the box, forming a whole. The design is considered maintenance free, with no operating corridors and a small box, also known as European style box transformers or ordinary box transformers.

3. One body type

Simplify the high-voltage control and protection devices, and put the high-voltage and low-voltage distribution devices and the main body of the transformer into the transformer oil tank together to form a whole, smaller, close to the oil immersed transformer of the same capacity, which is 1/3 of the European box type transformer. Also known as American style box transformer or compact box transformer.

C. Structure of Box Substation

1. Structure of European style box transformer

(1) In China, European style box type substations are commonly used, with three parts each consisting of one room, forming a "mu" or "pin" structure. Commonly used models include ZBW and XWB. The cross-sectional and wiring diagrams are shown in Figure GYPD00102003-1. The high voltage chamber of the European style box transformer is a ring network, double line or terminal unit load switch, with a small volume and compatibility with the ring network cabinet. Adopting a safe and reliable compact design, with comprehensive anti misoperation interlocking function, high reliability, and convenient operation and maintenance. High voltage equipment generally adopts a combination of load switch and fuse. The fuse is equipped with an impactor. After the fuse is blown, the striker will push the release mechanism, and the load switch three-phase will jump open at the same time to avoid phase loss operation. At the same time, if an electric operating mechanism is used, distribution automation function can be implemented. The 10kV cable outlet of the ring network structure is equipped with a ground fault indicator and zero sequence sensor. Transformers can be oil immersed or dry type, with D, yn11 wiring. Considering unfavorable heat dissipation, natural ventilation or top forced ventilation can be used. The low-voltage room is equipped with metering and reactive power compensation, and the secondary circuit and number of outgoing lines can be designed according to user needs to meet different needs. The outer shell is made of steel plate or alloy plate, with a double-layer top cover and good insulation. The appearance and color can be coordinated with the environment. The high-voltage chamber, transformer chamber, and low-voltage chamber are separated into independent compartments by partitions. For monitoring and maintenance, lighting devices are installed in each compartment, and the lighting switch is controlled by the door.

2. Structure of American style box transformer

(1) The American style box transformer shares a high voltage switch and transformer in the same oil tank. Common American style box transformer models include ZB1336, GE, and COOPER. The structure and wiring are shown in figures GYPD00102003-2 and GYPD00102003-3.

The American style box transformer adopts a fully sealed and fully insulated structure with high safety, which can be used for ring networks and terminals. The high voltage is controlled by a T-type or V-type three-phase linkage four position oil load switch, as shown in Figure GYPD00202002-04 and GYPD00202002-05

2) The transformer adopts a backup fuse connected in series with a plug-in fuse to provide protection. The backup fuse is installed inside the box and only acts when there is an internal phase fault in the box transformer, used to protect the high-voltage line. The plug-in fuse fuses when there is a short circuit or overload on the secondary side.

The high-voltage cable is connected to a sleeve with good insulation performance, which is connected to the elbow cable plug to seal the live part. The elbow cable plug is a fully shielded insulated plug that can be fully loaded and unplugged, equivalent to a load switch. The structure is shown in Figure GYPD00102003-6, but if it is connected to a short circuit point, the elbow plug and sleeve should be replaced.

D.Operation of Box Substation

1. The box type substation should be placed at a relatively high place, not in low-lying areas, to prevent rainwater from entering the box and affecting equipment operation. When pouring concrete platforms, gaps should be left on the high and low voltage sides to facilitate the laying of cables in and out. When excavating the foundation, if encountering the ground formed by the accumulation of garbage or corrosive soil, it must be excavated to solid soil, then backfilled with good soil quality and compacted, and then filled with three layers of soil or gravel to ensure the stability of the foundation.

2. The grounding and neutral connection of the box type substation share a common grounding network. The grounding grid generally adopts the method of driving grounding piles at the four corners of the foundation, and then connecting them as a whole. There must be two reliable connections between the box type substation and the grounding grid. After operation, the grounding connection should be checked regularly to ensure it is not loose or corroded. Regularly measure the grounding resistance, not exceeding 4 Ω.

3. The box type substation mainly adopts natural air circulation cooling. Therefore, debris should not be piled up around it, especially the transformer room door. Louver ventilation holes should also be regularly removed to ensure that the equipment does not exceed the maximum allowable temperature.

After the low-voltage circuit breaker trips, the cause should be identified before power can be transmitted to prevent the accident from expanding.

5. The high-voltage chamber of the box type substation should be equipped with zinc oxide lightning arresters, and the installation method should be convenient for testing and replacement.

6. Equipment such as ring network switches, transformers, lightning arresters, etc. in the high-voltage room should be regularly inspected and maintained, defects should be discovered and dealt with in a timely manner, and insulation preventive tests should be conducted regularly. If it is discontinued for more than 3 months and is put into operation again, a full project preventive test should be conducted.

7. To replace a high-voltage fuse without breaking capacity, the transformer must be powered off, and the mechanical interlock must be properly released during operation, and an insulated operating rod must be used.

8. All incoming and outgoing cable holes of the box type substation should be sealed to prevent small animals from entering and causing accidents.

9. Must have a warning sign of "high voltage danger" and a nameplate number for electrical equipment

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