1. Application background

The box-type substation is different from the conventional civil construction power distribution room, and its main features are:

(1) The substation is designed, manufactured and installed in the manufacturing plant, and the internal electrical wiring is completed.

(2) The substation has passed the required type test assessment.

(3) The substation has been verified by the factory test.

Box substation is a kind of factory prefabricated compact power distribution equipment that combines high-voltage switchgear, distribution transformer and low-voltage power distribution device according to a certain wiring scheme. It has strong complete set, small size, small area, and power transmission. A series of advantages such as short cycle, flexible site selection, strong environmental adaptability, convenient installation and use, reliable operation, low investment and quick results.

Box transformers are used in urban areas and can be installed in roadside green areas, road intersections, living quarters, production plants, high-rise buildings, etc. The floor space of the box substation with the same capacity is only 1/5～1/10 of the area occupied by the power distribution room, and at the same time, the construction amount of the project is greatly reduced, and the investment is low and the effect is quick. It has been widely used in our Taixing city.

2. The overall structure of the box station

In the combined box substation, the room where the high voltage switchgear is located is generally called the high voltage room; the room where the transformer is located is generally called the transformer room; the room where the low voltage power distribution device is located is called the low voltage room. These three rooms have two layouts in the box-type station, namely "目" type layout and "品" type layout. It is also the arrangement of what we usually call the European change and the American change.

3. Medium voltage switchgear in the box change

In the box-type station, if it is terminal wiring, use (load switch + fuse) combined electrical cabinet; if it is ring network wiring, use ring network power supply unit. The ring network power supply unit is generally equipped with a load switch, which is composed of two load switch cabinets as incoming and outgoing lines and a transformer protection cabinet (load switch + fuse) combined into an electrical cabinet. As my country is implementing ring network power supply in the construction and transformation of urban networks to reduce power supply interruptions, it is expected that ring network power supply units will have a great development.

The SF6 ring network power supply unit is characterized by greatly improved personal safety, at the same time, it is easy to install, less maintenance, save space, and has significant economic benefits. The ring network power supply unit is composed of intervals, generally at least three intervals, that is, 2 cable in and out intervals and 1 transformer loop interval.

The urban network generally uses ring cables. Where overhead lines are used, the overhead lines can be led to the ring network power supply unit, and then the cables are introduced and led out. Generally speaking, the bus bar and switch are enclosed in an SF6-filled shell. Because the shell is closed, it is not affected by the external environment, which improves the reliability of the equipment, and because it is insulated with SF6, it can be small in size. Also, due to the low inflation pressure, problems such as sealing are also easy to solve.

4. Transformer matching and heat dissipation treatment

The transformer for box change is a step-down transformer, which generally reduces 10kV to 380V/220V. The transformer capacity is generally 50～1600kVA, and the commonly used capacity for box change is 80～630kVA. The transformer selects S11 or higher grade fully sealed, maintenance-free, low-noise, cost-effective oil-immersed transformer (noise ≤50dB) or new dry-type transformer SCB10, etc. When a dry-type transformer is used, the transformer room must be equipped with a heat dissipation system. At present, the province requires the use of S11 series distribution transformers, and some also use amorphous alloy transformers. Its advantage is that the no-load loss is very small, only 1/4 to 1/3, but its price is 1.3 to 1.6 times higher. But with the improvement of manufacturing technology, once the price drops, amorphous alloy transformers will occupy the core position of the market. According to the requirements of Jiangsu Electric Power Company, the wiring group must be connected by Dyn11.

There are two ways to install the transformer: one is to expose the transformer to the air, but it is still in the shell, which is easy to replace (European style box layout); the other is to install the transformer in a closed compartment (American style box layout). The latter transformer room is designed to prevent the room temperature from rising due to sunlight radiation. The surrounding walls are equipped with heat insulation materials and a double-layer plywood structure. The top cover is designed as a gas building structure with air cushion or heat insulation material. Automatic forced exhaust ventilation device (axial fan or frame fan). Usually, the opening and stopping of the device is automatically controlled by the temperature monitoring device in the transformer room, and the temperature setting value is set at 80-90% of the allowable temperature; under normal indoor temperature, natural ventilation is used to dissipate heat.

5. Features of low-voltage side

On the low voltage (0.4kV) side, the outgoing line protection can be equipped with multi-channel air switch or knife melt switch; automatic cycle switching reactive power compensation device, the compensation capacity is generally 30% of the transformer capacity; it has an independent electric energy metering box.

The low-voltage switch equipped with an intelligent controller can realize the early warning function, and can provide a signal unit and operation control, as well as a meter function.

6. Shell manufacturing process

The poor adhesion and short life of the surface protection layer restrict the use of ordinary steel plates in outdoor box-type substations. Some shells are made of stainless steel plates, aluminum alloy plates or polystyrene composite panels, instead of metal shells (such as glass fiber reinforced cement boards, glass fiber reinforced plastic boards, colored boards, etc.) is a popular trend. When using glass fiber reinforced cement board and glass fiber reinforced plastic board, in order to light and prevent rainwater from entering, the top cover can be made into a smooth water-proof drip edge.