Direct Current Transmission - Composition of Converter Station, DC Line, AC Side and DC Side

Direct current transmission is a power transmission method that converts the alternating current generated by the power plant into direct current through the rectifier and transmits it to the receiving end, and then uses the inverter to convert the direct current into alternating current and sends it to the receiving end AC grid.

It is mainly composed of converter stations (rectifier stations and inverter stations), DC lines, power filters on the AC side and DC side, reactive power compensation devices, converter transformers, DC reactors, protection and control devices, etc. (see Figure DC Basic composition of the transmission system). Among them, the converter station is the core of the direct current transmission system, which completes the conversion between AC and DC.

1. Introduction to direct current transmission

The AC power generated by the power plant is converted into DC power by the rectifier and sent to the power receiving end, and then the DC power is converted into AC power by the inverter and sent to the AC grid of the receiving end. It is mainly used in long-distance high-power transmission and networking of asynchronous AC systems. It has the advantages of less line investment, no system stability problems, fast adjustment, and reliable operation.

2. Advantages of direct current transmission

Compared with AC transmission, direct current transmission has the following advantages:

2.1 When the same power is transmitted, the cost of the DC line is low, the tower structure of the overhead line is relatively simple, the line corridor is narrow, and the cables with the same insulation level can run at a higher voltage;

2.2 The power and energy loss of direct current transmission is small;

2.3 Little interference to communication;

2.4 There is no capacitive current and no reactance voltage drop when the line is in steady state operation, the voltage distribution along the line is relatively stable, and the line itself does not need reactive power compensation;

2.5 The AC systems at both ends of the direct current transmission line do not need to run synchronously, so it can be used to realize the asynchronous connection between AC systems of different frequencies or the same frequency;

2.6 The direct current transmission line itself does not have the inherent stability problems of AC transmission, and the transmission distance and power are not limited by the stability of the synchronous operation of the power system;

2.7 The respective short-circuit capacity of the AC systems interconnected by direct current transmission lines will not increase significantly due to the interconnection;

2.8 The adjustment and control of the power and current of the direct current transmission line is relatively easy and fast, and various adjustments and controls can be realized. If the AC and DC run in parallel, it will help to improve the stability of the AC system and improve the operating characteristics of the entire system.

3. Disadvantages of direct current transmission

The development of direct current transmission is also limited by some factors. First of all, the converter station of direct current transmission is more complex than the substation of AC system, with higher cost and higher requirements for operation and management; 40% to 60% of the direct current transmission power; the converter device will generate harmonics on both the AC side and the DC side during operation, and a filter must be installed; when the direct current transmission uses the earth or sea water as a circuit, it will cause corrosion of metal components along the way , protective measures are required. To develop multi-terminal direct current transmission, it is necessary to develop high-voltage DC circuit breakers.

4. History of direct current transmission

Transmission of electrical energy with direct current. People's application and understanding of electric energy first started from DC. In 1882, the French physicist and electrician M. Deppler sent the electric energy generated by the 3-horsepower DC generator installed in the Miesbach coal mine to Munich, 57 kilometers away, with a DC voltage of 1500-2000 volts. At the International Exposition, the first power transmission test was completed. Since then, in the early 20th century, the voltage, power and distance of experimental direct current transmission reached 125 kV, 20 MW and 225 kilometers respectively. However, since the DC generators are used in series to obtain high-voltage DC power supply, the motor at the receiving end is also operated in series, which not only limits the commutation of high-voltage and large-capacity DC motors, but also complicates the way of series operation and has poor reliability. It has not been further developed for nearly half a century. In the 1950s, the high-voltage and large-capacity controllable mercury arc rectifier was successfully developed, which created conditions for the development of high-voltage direct current transmission; at the same time, the expansion of the scale of the power system made the limitations of the stability of AC transmission more obvious. direct current transmission technology has been paid attention to again. In 1954, a 96-kilometer submarine cable direct current transmission line was built between Sweden and Gotland, with a DC voltage of ±100 kV and a transmission power of 20 MW. It was the world's first industrial high-voltage direct current transmission line. Wire. The emergence of silicon controlled rectifier components in the late 1950s opened up a new way for the manufacture of converter equipment. Over the past 30 years, with the advancement of power electronics technology, direct current transmission has had new developments. By the 1980s, nearly 30 direct current transmission projects had been put into operation in the world, with a total transmission capacity of about 20,000 MW, and the longest transmission distance exceeded 1,000 kilometers. And there are many larger-scale projects that are being planned, designed and constructed.

5. The development history of direct current transmission

From the 1930s to the 1950s, people explored the use of various devices to form converters as DC high-voltage power supplies to replace DC generators, and thus developed a controllable mercury arc valve converter, which opened up a new path for the development of high-voltage and high-power direct current transmission. Since the world's first commercial direct current transmission project - Gotland direct current transmission project was completed in 1954, direct current transmission has been re-emphasized by people and has risen rapidly. In the 1970s, with the thyristor With the rapid development of technology, the technical advantages of HVdirect current transmission are becoming more and more obvious. Therefore, the successful commercial application of Gotland direct current transmission project marks the rise of direct current transmission.

±1100 UHV: The world's "four most" direct current transmission projects are electrified. The Zhundong-South Anhui ±1100 kV UHV direct current transmission project represents the highest achievement of my country's transmission projects. The transmission distance, transmission capacity, voltage level, and technical level are among the best in the world.

6. Application of direct current transmission

direct current transmission is mainly used in 5 aspects:

6.1 Long-distance high-power transmission;

6.2 Linking AC systems operating on different frequencies or on the same frequency but not synchronously;

6.3 As the connection line between the network interconnection and the regional system (easy to control without increasing the short-circuit capacity);

6.4 Use submarine cables to transmit electricity across the strait or use underground cables to supply power to large cities with high electricity density;

6.5 In the power system, the parallel operation of AC and direct current transmission lines is adopted, and the rapid adjustment of direct current transmission lines is used to control and improve the operation performance of the power system.

With the development of power electronics technology, the advancement of high-power thyristor manufacturing technology, price reduction, and improvement of reliability, the increase in the availability of converter stations, and the increasingly mature direct current transmission technology, direct current transmission will inevitably get more in the power system. Applications. At present, the development of high-voltage DC circuit breakers, the study of the operating characteristics and control of multi-terminal DC systems, the development of multi-terminal DC systems, and the study of the operating mechanism and control of AC-DC parallel systems have received extensive attention.

The new development of many scientific and technological disciplines has opened up broad prospects for the application of direct current transmission technology. A variety of new power generation methods - magnetic fluid power generation, electric gas power generation, fuel cells and solar cells, etc. all produce direct current. The electric energy should be transmitted in the form of direct current, and converted into the alternating current power system by an inverter; extremely low temperature cables and superconducting cables are also more suitable for direct current transmission, and so on. The future power system will be a mixed AC and DC system.

7. direct current transmission equipment

7.1 DC screen

The general name of DC screen is intelligent maintenance-free DC power supply screen, referred to as DC screen, and the general model is GZDW. Simply put, a DC screen is a device that provides a stable DC power supply. (When the input has 380V power supply, it is directly converted into 220V, and when there is no input (mains power and backup power), it is directly converted into battery power supply - DC 220V: in fact, it can also be said to be an industrial special emergency power supply) . The power operating power supplies in power plants and substations are all DC power supplies today, which provide power for control loads, power loads, and DC accident lighting loads, and are the basis for control and protection of contemporary power systems. The DC screen is composed of a power distribution unit, a charging module unit, a step-down silicon chain unit, a DC feed unit, a power distribution monitoring unit, a monitoring module unit and an insulation monitoring unit. It is mainly used in small and medium-sized power plants, hydropower stations, various substations, and other users using DC equipment (such as petrochemical, mining, railway, etc.) in the power system. Electrical protection and fault lighting and other occasions.

The DC screen is a new type of DC system with digital control, protection, management and measurement. The monitoring host part is highly integrated and adopts a single-board structure (All in one), which includes functions such as insulation monitoring, battery inspection, grounding line selection, battery activation, silicon chain voltage stabilization, and microcomputer central signal. The host is equipped with a large LCD touch screen, and various operating states and parameters are displayed in Chinese characters. The overall design is convenient and concise, and the man-machine interface is friendly, which is in line with user habits. The DC screen system provides powerful functions for remote detection and control, and has remote control, remote adjustment, telemetry, remote signaling functions and remote communication interfaces. Through the remote communication interface, the operating parameters of the DC power system can be obtained remotely, and the operating status and fixed values can also be set and modified through this interface to meet the requirements of power automation and power system unattended substations; equipped with standard RS232/485 serial Line interface and Ethernet interface, which can be easily incorporated into the power station automation system.

7.2 DC power supply

DC power supply (DC power) has two electrodes, positive and negative. The potential of the positive electrode is high, and the potential of the negative electrode is low. Form a current from the positive pole to the negative pole. A constant water flow cannot be maintained solely by the difference in water level, but a constant water level difference can be maintained by means of a water pump to continuously send water from a low place to a high place to form a steady water flow. Similar to this, the electrostatic field generated by the charge alone cannot maintain a constant current, but with the help of a DC power supply, the non-electrostatic effect (referred to as "non-electrostatic force") can be used to move the positive charge from the negative electrode with a lower potential. Return to the positive electrode with higher potential through the power supply to maintain the potential difference between the two electrodes, thus forming a stable current. Therefore, a DC power supply is an energy conversion device that converts other forms of energy into electrical energy supply circuits to maintain a steady flow of current.

The non-electrostatic force in a DC power supply is directed from the negative pole to the positive pole. When the DC power supply is connected to the external circuit, a current from the positive pole to the negative pole is formed outside the power supply (external circuit) due to the promotion of the electric field force. And inside the power supply (internal circuit), the action of non-electrostatic force makes the current flow from the negative pole to the positive pole, so that the flow of charge forms a closed cycle.