Power network

Structure:

A large power network (combined power network) is always developed and interconnected by many sub-power networks, so the hierarchical structure is a major feature of the power network. The general power network can be divided into transmission network, secondary transmission network, high voltage distribution network and low voltage distribution network.

The transmission grid is generally composed of backbone power lines with a voltage of more than 220kV, which connects large power plants, large capacity users and adjacent sub-power networks. The voltage of the secondary transmission grid is generally 110 ~ 220kV, which is connected to the transmission grid and the high voltage distribution network, which is a regional network, connecting regional power plants and large users. Distribution network is a network that supplies power to medium users and small users, 10 ~ 35kV is called high-voltage distribution network, and 1kV is called low-voltage distribution network.

Wiring mode:

The wiring mode of the power network can be roughly divided into two categories: without backup and with backup.

No-spare connections include single-loop radial, trunk, and chain networks. There are alternate connections including double-circuit radial, trunk, chain, ring and two end power supply networks. No spare wiring is simple, economical and easy to operate, but the power supply reliability is poor. The automatic reclosing device of overhead line can make up for the above shortcomings to some extent.

On the contrary, the backup junction has a high reliability of power supply, and the fault or maintenance of one line generally does not affect the power supply to the user, but the investment is large and the operation is complicated. Among them, the ring power supply and the two ends of the power supply are more commonly used.

Type:

1, according to the range of power supply, transmission power and voltage levels, the power network can be divided into local network, regional network and long-distance network three categories.

The voltage is 110kV and 110kV below the power network, the voltage is low, the transmission power is small, the line distance is short, the main power supply to the local substation, known as the local network;

Voltage above 110kV, 330kV below the power network, its transmission distance and transmission power are relatively large, generally supplied to large regional substation, known as regional network;

The power supply distance is more than 300km, and the voltage is more than 330kV and 330kV, which is called the long-distance network.

If it is divided only from the level of voltage, the power network can be divided into low-voltage power grids (below 1kV), medium voltage power grids (1 ~ 20kV), high-voltage power grids (35~220kV), ultra-high-voltage power grids (330kV, 330kV or more), and the newly developed UHV (AC 1000kV, DC ±800kV) power grids. In addition, power grids can be divided into DC power grids and AC power grids according to different types and characteristics. China's power grid according to the regional division, can be divided into northeast power grid, North China power grid, Northwest power grid, East China power grid and central China power grid five trans-regional power grids and southern power grid.

2. Classification by voltage level

According to the voltage level, there are low-voltage networks, high-voltage networks, ultra-high-voltage networks, usually the power grid below 1kV is called low-voltage networks; 1 ~ 330kV high voltage network; Power grids of 500kV and above are called ultra-high voltage networks. According to the provisions of the power network planning regulations, the lines above 110kV are usually called transmission lines, and below 110kV, including 110kV lines are called distribution lines, 110, 63kV power grids are called high-voltage distribution lines, and 20kV, 10kV lines are called medium-voltage distribution lines, and 400V lines are called low-voltage distribution lines.

3. Classification by grid structure

(1) Open power grid: where the user can only get electricity from a single direction of the power grid, called open power grid.

(2) Closed power grid: where the user can get electric energy from more than two directions of the power grid, called closed power grid.

Voltage level:

The voltage hierarchy of the power network is divided into five levels:

① Low voltage: below 1 kV;

② Medium voltage: 1 ~ 10 kV;

③ High voltage: 10 ~ 330 kV;

④ Ultra-high voltage: 330 ~ 1000 kV;

⑤ UHV: more than 1000 kV.

China's commonly used long-distance transmission voltage is 110 kV, 220 kV, 330 kV, the transmission trunk generally adopts 500 kV ultra-high voltage, the new Northwest power grid transmission trunk adopts 750 kV ultra-high voltage. The higher the voltage, the further the transmission distance.

Design content:

1. Determine the load of the power network

The load data investigated, understood and collected from the relevant departments can not be directly used for design and application. For example, its construction area, the equipment capacity of the production workshop, the annual output of industrial enterprises, and the proportion of various enterprises and other users in the region. Only after analysis and sorting, can we determine the load data for the design of the power network, the calculation load of the substation and the number and capacity of the transformer from these indirect load data.

2. Select the rated voltage of the network

It is one of the important issues in the design of power network to determine the power supply range and voltage level of power network and which voltage level is more suitable for power users who require power supply.

3. Select the cross-sectional area of the power line

According to the technical and economic comparison, the wire type and the cross-sectional area of the power network are determined.

4. Determine the wiring scheme of the power network

According to the important degree of power users and the reliability requirements of power supply, a reasonable power supply scheme is put forward, and the main wiring mode of substation and power network is determined. The tasks in the design of power network are interrelated, and only by considering all the factors mentioned above comprehensively can we get a reasonable design scheme.

To meet the requirements of users, a variety of power supply schemes can be used. The design is based on the specific requirements of users, as well as the previous design and operation experience to propose several more feasible schemes, after comparison, choose the more reasonable scheme.