Electric Power Industry – Structure and Rated Voltage of Power System

Electric Power Industry - Structure and Rated Voltage of Power System
  1. The position of the power industry in the national economy

The production and life of human need to consume energy, and the source of these energy is called energy. According to the law of conservation of energy, energy can only be changed from one form to another, which is called energy conversion, but there is no “production” and “consumption” in the true sense. According to the source of energy, energy is divided into primary energy and secondary energy.

Primary energy refers to the energy directly provided by nature to humans. Examples include chemical energy from fossil fuels such as wind, solar, coal, and petroleum.
Secondary energy refers to energy generated by converting primary energy. For example, electricity “produced” by a power plant. The advantages of using electrical energy are:
(1) Easy to transmit Electric energy is transmitted with wires, so it can be easily transmitted whether in the house or outside, in the sky or underground.
(2) Easy to convert Electric energy can be converted into mechanical energy through a motor, into thermal energy through an electric furnace, and into light energy through lighting fixtures.
(3) Easy to control The electrical equipment can be started and stopped by switching electrical appliances, and the size of the converted power can be controlled by the controller. Therefore, electric energy is widely used in production and life. The electric power industry is the leading industry in the development of the national economy. Electric energy is inseparable from life. It is closely related to the material civilization and spiritual civilization of society.

Power outages will reduce production and cause great inconvenience to people’s lives, and even lead to personal injury and equipment damage. Large-scale power outages are disasters, as evidenced by the huge losses caused by power system blackouts that have occurred successively around the world in recent years. For example, the North American blackout accident occurred on August 14, 2003 Eastern Time, the first large-scale power outage occurred in Manhattan, New York City, and then affected parts of the eastern United States and Canada. , causing huge economic losses.

  1. Structure and rated voltage of power system

2.1 Structure of the power system
FIG. 1 shows the basic structure of a power system. Power plants and users are the two main prime movers for “production” and “consumption” of electrical energy, converting other forms of energy into mechanical energy.

Figure 1 - AC Power System Structure Diagram
Figure 1 – AC Power System Structure Diagram

A generator converts mechanical energy into electrical energy. Compared with DC generators, AC generators are simpler in structure, more reliable in operation, and have a larger manufacturing capacity. Therefore, AC generators are currently used in power plants. The electrical equipment in the user converts electrical energy into energy forms such as mechanical energy or thermal energy required by the user. The network that connects power plants and users is called transmission network (or transmission system). Due to the advantages and disadvantages of AC and DC transmission, the current transmission system uses both AC and DC to transmit power.

The system composed of power plant, transmission network and users is called power system. The electrical part of the power system (excluding the prime mover and its preceding power part in the power plant) is called the power system.
In the figure, T is the prime mover; G is the generator; T1 is the step-up transformer; T2 is the step-down transformer; LAC is the AC transmission line; LDC is the DC transmission line; M is the motor; P is the lighting equipment.

2.2 Rated voltage of power system
The so-called rated voltage refers to the standard voltage under normal operating conditions. It is safe and economical for electrical equipment to operate within the specified standard voltage and its vicinity. The rated voltage of power systems and three-phase electrical equipment is expressed as line voltage.

2.2.1 Rated voltage of power grid
The rated voltage of the AC power grid refers to the standard operating value of the line voltage. The rated voltage of the three-phase AC power network formulated by the Chinese national standard is 3kV, 6kV, 10kV, 20kV, 35kV, 110kV, 220kV, 330kV, 500kV, 750kV and 1000kV. Among them, 35~220kV is called high voltage (HV – High Voltage), 220kV and above and below 1000kV are called extra high voltage (EHV – Extra High Voltage), 1000kV and above are called ultra high voltage (UHV – UItra High Voltage). At present, the rated voltage of the AC power network that has been built is mostly 10kV, 110kV, 220kV, 500kV, and the northwest region is 10kV, 110kV, 330kV, 750kV. 3kV and 6kV are mainly used for the internal power supply network of the factory. Suzhou Zhongxin Industrial Park replaced 10kV with 20kV, the purpose is to properly increase the capacity of the low-voltage distribution network, and at the same time not make the short-circuit current too large. With the gradual formation of the national network, China has begun to plan and construct a 1000kV UHV power grid.

The rated voltage of DC transmission refers to the standard operating value of the positive and negative poles to the grounding pole voltage. The rated voltage of DC transmission established by the Chinese national standard is ±500kV and ±800kV. The rated voltage of the existing DC transmission lines is ±500kV.

2.2.2 Rated voltage of electrical equipment
The rated voltage of electrical equipment is related to the rated voltage of the grid. One of the meanings of the rated voltage specified in the electrical equipment manufacturing specification is: the electrical equipment with a rated voltage of 220kV and above can operate for a long time within the range of 110% times the rated voltage without causing insulation breakdown; the rated voltage is 110kV and below. For electrical equipment, the maximum allowable voltage for long-term operation is 115% times the rated voltage.

  1. The rated voltage of the electrical equipment
    At present, the rated voltage of general high-voltage electrical equipment is 3kV, 6kV, 10kV, and the rated voltage of low-voltage electrical equipment is 380/220V, of which 220V is the phase voltage.
  2. The rated voltage of the generator
    For manufacturing economy, as the generator capacity increases, its rated voltage also increases accordingly. Currently, the rated voltages of generators in China are 0.4kV, 6.3kV, 10.5kV, 13.8kV, 18kV and 24kV. Among them, 0.4kV, 6.3kV, and 10.5kV correspond to the rated voltages of the power grid (ie, electrical equipment) of 0.38kV, 6kV, and 10kV. The generator capacity of this voltage is less than 100MW, which can directly supply power to the load near the power plant. Considering the loss of 5% of the transmission voltage, its rated voltage is 5% higher than the rated voltage of the grid.
  3. The rated voltage of the transformer
    According to the direction of power transmission, the power receiving end winding of the transformer is generally called the primary winding, and the power transmitting end winding is called the secondary winding. Therefore, the rated voltage of the primary winding of the transformer is the same as the rated voltage of the grid. Considering the voltage loss inside the transformer and the voltage loss from the transformer to the receiving terminal on the opposite side of the transmission grid, the rated voltage of the secondary winding of the transformer is generally 10% higher than the rated voltage of the grid.
    In order to facilitate the voltage regulation of the power grid, the high-voltage winding of the transformer has multiple taps (tap), and the middle one is called the main tap.

After each side is completely disconnected from the power grid, the transformer that can switch the taps in the state of no voltage and no current is called a non-excitation voltage regulating transformer; the transformer that can switch the taps without power failure during operation is called an excitation regulating transformer. pressure transformer. The excitation voltage regulating transformer has a special tap switching device to ensure that the power supply circuit is not disconnected during the tap switching process, and at the same time it can extinguish the arc generated when the tap is turned on and off. In addition to the main tap, the non-excitation voltage regulating transformer has 2~4 taps, and its voltage regulation range is ±5% or ±2×2.5%; the excitation voltage regulating transformer has another 16 taps, its voltage regulation The range is ±8×1.25%.

The primary winding of the step-up transformer dedicated to the generator outlet and the utility transformer is directly connected to the generator, so the rated voltage of this winding is the same as the rated voltage of the generator.
The transformer whose low-voltage winding is the receiving end is called a step-up transformer. Since the medium-voltage and high-voltage windings are the transmitting end, the main tap voltage is 10% higher than the rated voltage of the grid. The transformer whose high-voltage winding is the receiving end is called a step-down transformer. The voltage of its main tap is the same as the rated voltage of the grid, and the medium and low voltage windings are the transmitting ends, so the voltage of its main tap is 10% higher than the rated voltage of the grid.

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