1、 Composition of power system

（1） Power system

According to the specified technical and economic requirements

A unified system is called power system. The power system also includes relays to ensure its safe and reliable operation

Protection and safety automatic device, dispatching automation, communication and other auxiliary systems (also called secondary systems). Fundamentals of Power System

The task is to provide users with sufficient, reliable, qualified and inexpensive power.

（2） Electric power network

The part of the power system that transmits, transforms and distributes electric energy is called the power grid. Power grid includes transmission grid and distribution

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The main function of the transmission network is to increase the voltage from the power plant far away from the load center through the transformer and pass

The high-voltage transmission line is transmitted to the hub substation adjacent to the load center. At the same time, the transmission line also connects adjacent power systems and

It is the function of adjacent hub substations.

Distribution network is a power network that directly distributes power from hub substation to users. Generally, the distribution network is divided into high

Voltage, medium voltage and low voltage distribution network. In China, the voltage of high-voltage distribution network is generally and that of medium voltage distribution network

The voltage of low-voltage distribution network is generally three-phase four wire system

2、 Characteristics of power system operation

First, the economic aggregate is large. At present, the asset scale of China’s power industry has exceeded more than 200 billion, accounting for one fourth of the total state-owned assets. Power production directly affects the healthy development of the national economy.

The second is simultaneity, whether the electric energy can be stored in large quantities, the unity of each link is indivisible, and the transition process is very fast. The power produced in an instant must be equal to the power used in an instant, so each link of power generation, transmission, and distribution to users is very important.

Third, the power production is highly centralized and unified. No matter how many power plants and power supply companies, the power grid must be unified in dispatching, management standards and management methods; There are strict requirements for safe production, organizational discipline and professional ethics.

Fourth, applicability. The service objects of the power industry are comprehensive, involving all people in the whole society. The power quality and price level are closely related to the interests of the majority of power users.

Fifthly, the national economy must take the lead in developing electric power.

3、 Rated voltage of power system

The voltage of the power grid is graded. The rated voltage grade of the power grid is formulated and promulgated by the state after comprehensive analysis and demonstration according to the needs of national economic development, the rationality of technology and economy, the manufacturing level of electrical equipment and other factors.

At present, the highest AC voltage level in China is 1000KV, which was put into operation from Changzhi to Jingmen on December 30, 2008; The highest DC voltage level in China is plus or minus 500KV, including Gezhouba – Shanghai Nanqiao Line, Tianshengqiao – Guangzhou Line, Guizhou – Guangdong Line, and Three Gorges – Guangdong Line.

At present, the commonly used voltages in China include 220V, 380V, 6kV, 10kV, 35kV, 110kV, 220kV, 330kV, 500kV and 1000KV. Among them, 10KV and below voltage lines are called power distribution lines, and 35KV and above voltage lines are called power transmission lines. It also stipulates that the voltage above 1KV is “high voltage”, the voltage below 1KV is “low voltage”, and the safety voltage is 36V and below.

The rated voltage of electric equipment is consistent with that of the power grid. In fact, due to the voltage loss in the power grid, the actual voltage at each point deviates from the rated value. In order to ensure the good operation of electrical equipment, it is obvious that electrical equipment should have a wider range of normal working voltage than the allowable deviation of grid voltage. The rated voltage of the generator is generally 5% higher than the rated voltage of the same level grid to compensate for the voltage loss on the grid.

The rated voltage of transformer is divided into primary and secondary windings. For the primary winding, when the transformer is connected to the end of the grid, it is equivalent in nature to a load on the grid (such as the factory step-down transformer), so its rated voltage is the same as that of the grid. When the transformer is connected to the generator outlet (such as the power plant step-up transformer), its rated voltage should be the same as that of the generator. For the secondary winding, the rated voltage of the secondary winding of the transformer shall be 5% higher than the rated voltage of the grid in consideration of its own voltage loss (calculated by 5%) when the transformer is loaded. When the transmission distance at the secondary side is long, the line voltage loss (calculated by 5%) shall also be considered. At this time, the rated voltage of the secondary winding shall be 10% higher than the rated voltage of the grid