Exploration of Energy Saving in HVDC Transmission

HVDC transmission is a high-power long-distance direct current transmission that utilizes the advantages of stable direct current with no inductive reactance, capacitive reactance does not work, and no synchronization problems. The power transmission process is direct current. It is often used in submarine cable power transmission, communication between asynchronously operating AC systems, etc.


High-voltage Direct Current


1. Introduction to HVDC transmission


HVDC transmission technology is used for long-distance transmission of electric energy through overhead lines and submarine cables; at the same time, it is also used for the connection between independent power systems in some occasions where traditional AC connection is not suitable. The world's first commercial HVDC transmission line was born in Sweden in 1954. It was used to connect the mainland of Sweden and Gotland, and was completed by ASEA.


2. Function of HVDC transmission


In a HVDC transmission system, electric energy is derived from one point of the three-phase AC grid, converted into DC at the converter station, and transmitted to the receiving point through overhead lines or cables; after the DC is converted into AC at the other side of the converter station, it is then into the receiver's AC grid. The rated power of DC transmission is usually greater than 100 MW, and many are between 1000-3000 MW.


HVDC transmission is used for long-distance or ultra-long-distance transmission because it is more economical than traditional AC transmission.


With the application of high-voltage direct current transmission system, the power level and direction can be quickly and accurately controlled. This performance can improve the performance and efficiency of the AC grid it is connected to. The direct current transmission system has been widely used.


High-voltage direct current transmission is a power transmission method in which three-phase alternating current is rectified into direct current through a converter station, and then sent to another converter station through a direct current transmission line for inversion into three-phase alternating current. It basically consists of two converter stations connected to the AC system at both ends and a DC transmission line.


The cost of DC transmission lines is lower than that of AC transmission lines, but the cost of converter stations is much higher than that of AC substations. It is generally believed that the overhead line exceeds 600-800km, and the cable line exceeds 40-60km. DC transmission is more economical than AC transmission. With the development of high-voltage and large-capacity thyristors and control and protection technologies, the cost of converter equipment has gradually decreased. DC transmission has developed rapidly in recent years. my country's Gezhouba-Shanghai 1100km, ±500kV DC power transmission project has been completed and put into operation. In addition, the Xiangjiaba-Shanghai DC transmission project with a total length of more than 2,000 kilometers has also been completed and was put into operation on July 8, 2010. This line is currently (as of early 2011) the longest HVDC transmission project in the world.


3. Main advantages of HVDC transmission


The advantage is that without increasing the short-circuit capacity of the system, it is convenient to realize the non-synchronous network operation of the two power systems and the network connection of power systems with different frequencies; the use of power modulation of the DC system can improve the damping of the power system, suppress low-frequency oscillations, and improve AC transmission in parallel operation. power transmission capacity of the line. Its main disadvantage is that it is difficult to lead out the branch lines of DC transmission lines, and most of them are only used for end-to-end power transmission. Canada originally planned to develop and build a five-terminal direct current transmission system and now a three-terminal direct current transmission system has been built. The main technical difficulty in realizing the multi-terminal DC transmission system is the line power control problem under various operating modes. At present, it is generally believed that it is technically difficult to realize the economic rationality of the DC transmission system with more than three terminals to be studied.


4. Main equipment for HVDC transmission


Including converters, converter transformers, smoothing reactors, AC filters, DC arresters and control and protection equipment.


The converter, also known as the converter valve, is the key equipment of the converter station, and its function is to realize rectification and inverter. At present, most converters use thyristors and silicon-controlled rectifiers) to form a three-phase bridge rectifier as the basic unit, which is called a converter bridge. Generally, a converter system is composed of two or more converter bridges to realize the function of converting AC to DC and DC to AC.


The converter will generate multiple harmonics such as 5, 7, 11, 13, 17, 19 and so on during the rectification and inversion process. In order to reduce the harmonics entering the AC system, a filter must be installed on the AC bus of the converter station. It is composed of three devices in series: a reactance coil, a capacitor and a small resistor. Multiple harmonics can be filtered out through tuning parameters. Generally, the 5th, 7th, 11th, and 13th harmonic filter banks are installed on the AC side bus of the converter station.


Unipolar is divided into one line and one ground and unipolar two lines. DC transmission generally adopts bipolar lines. When one pole of the converter is out of operation, the DC system can operate as unipolar and two-wire, but the transmission power must be reduced by half.


5. Exploration of energy saving in HVDC transmission


Since the 1980s, the pace of development of power transmission technology has been significantly accelerated, and methods to improve transmission capacity have emerged. There are high-tech technologies such as DC transmission technology, flexible AC transmission technology, and frequency division transmission technology, as well as existing high-voltage AC transmission technologies. The capacity-increasing transformation technology of transmission lines, such as step-up transformation, reconduction capacity-increasing transformation, AC transmission line to DC transmission technology, etc. DC transmission has very important practical significance for improving the transmission capacity of the existing transmission system and tapping the potential of the existing equipment, and it can achieve twice the result with half the effort when implemented.


5.1 The three major characteristics of economy highlight the energy-saving effect

From an economic point of view, DC transmission has the following three main advantages:

First of all, the line cost is low, saving cable costs. DC transmission only needs two wires, and only one wire is used for the circuit using the earth or sea water, which can save a lot of line investment, so the cable cost is much lower.


Secondly, the operating power loss is small, and the transmission energy saving effect is remarkable. The number of DC transmission wires is small, the resistance heating loss is small, there is no reactive power loss of inductive reactance and capacitive reactance, and the increase of transmission power reduces the unit loss, which greatly improves the energy saving effect in power transmission.


Finally, the line corridor is narrow and the land acquisition fee is low. Taking the same level of 500 kV voltage as an example, the DC line corridor is only 40 meters wide, which saves a considerable amount of land for transmission lines of hundreds or thousands of kilometers.


In addition to the economy, the technology of direct current transmission is also remarkable. The direct current transmission regulation speed is fast, the operation is reliable. Stable output can be guaranteed under normal conditions, and emergency support can be realized in case of accidents, because the DC power transmission can quickly adjust the power through the thyristor converter and realize the reversal of the power flow. In addition, there is no capacitive charging current in the DC transmission line, the voltage distribution is stable, and the load does not have abnormal voltage and does not require parallel reactance.


5.2 Space for improvement High-power power electronic devices will improve DC transmission performance

The core technology of DC power transmission is concentrated in the converter station equipment. The converter station realizes the mutual energy conversion between DC and AC in the DC power transmission project. In addition to the same equipment as the AC substation in the AC field, it also has the following unique equipment: converter valve , control and protection system, converter transformer, AC filter and reactive power compensation equipment, DC filter, smoothing reactor and DC field equipment, while the converter valve is the core equipment in the converter station, and its main function is to perform AC DC conversion, from the initial mercury arc valve to the current electric control and light control thyristor valve.


Thyristors have a long history of use in HVDC transmission. In the past 10 years, the breaking capacity of high-power electronic devices such as turn-off thyristors and insulated gate bipolar transistors has been continuously improved. The research, development and application of new high-power power electronic devices will further improve the new generation of DC Power transmission performance, greatly simplifies equipment, reduces the area occupied by the converter station, and reduces the cost.


5.3 Obvious advantages of long-distance power transmission

The alternating current generated by the power plant is converted into direct current through the converter valve, and then sent to the receiving end through the direct current transmission line and then converted into alternating current, which is injected into the receiving end AC grid. Industry experts agree. High-voltage direct current transmission has the advantages of strong line transmission capacity, low loss, no need for synchronous operation of the AC systems on both sides, and small loss to the power grid when a fault occurs. It is especially suitable for long-distance point-to-point high-power transmission.


Among them, the light-duty DC transmission system uses turn-off thyristors, insulated gate bipolar transistors and other devices that can be turned off to form converters, so that medium-sized DC transmission projects are also competitive in short transmission distances.


In addition, the converter composed of devices that can be turned off can also be used to supply power to isolated small systems such as offshore oil platforms and islands. In the future, it can also be used in urban power distribution systems to connect to distributed power sources such as fuel cells and photovoltaic power generation. The light DC transmission system is more helpful to solve the problem of grid stability of clean energy.


6. Application Status of HVDC Transmission


6.1 Application of HVDC Power Supply Technology

The application of high-voltage DC power supply technology in my country is mainly reflected in the fact that China Telecom Corporation is using and promoting high-voltage DC power supply technology, and telecom companies and power system developers are constantly researching high-voltage DC power supply. wide range of applications. Although a variety of voltages can be selected for high-voltage DC power supplies, there is still no strong support from back-end equipment manufacturers. When selecting the power supply voltage, it is necessary to ensure that the entire power supply system can operate normally. If the problems in the high-voltage DC power supply technology are continuously solved, the high-voltage DC power supply technology will develop rapidly.


6.2 Factors Affecting the Development of HVDC Power Supply Technology

With the continuous development of the communication industry, there are more and more requirements for power supply. The application of high-voltage DC power supply is relatively extensive, but the development of high-voltage DC power supply still has many restrictive factors:

6.2.1 Impact of back-end equipment on high-voltage DC power supply technology

Although the high-voltage DC power supply can meet the basic needs of the back-end equipment power supply in many industries, the standard of the high-voltage DC power supply is not the standard power supply required by the back-end equipment, so that there will be certain risks during the operation of the entire system. The main performance In the following respects. Technical risk

Although there are many back-end devices using high-voltage DC power supply, according to the analysis of the pilot operation status of high-voltage DC power supply, some devices may not support high-voltage DC power supply, and the detection of whether the equipment supports high-voltage DC power supply can only be determined through operation. It can be detected, but the detection takes a certain amount of time, so there will be many risks before the detection results come out. Legal risk

When the high-voltage DC power supply is used, the back-end equipment fails, which is not good for the operator. While facing a great risk test, the use of the high-voltage DC power supply is likely to cause both parties to the contract to fall into legal disputes.


6.2.2 The finalization of the power system and the restriction of the quantity on the high-voltage DC power supply technology

Because there is no relevant technical standard system for high-voltage DC power supply technology, although it has been widely used in many departments, there is still a lack of technical guidance and experience in the use of high-voltage DC power supplies, so there is a situation where high-voltage DC power supply products have not been finalized , while the number of products powered by HVDC cannot be determined.


6.2.3 Restrictions of relevant supporting devices on the development of high-voltage DC power supply technology

In the high-voltage DC power supply system, although many supporting devices are very common, there will also be some relatively rare devices, such as power distribution components such as fuses and circuit breakers. High-voltage DC power supply has high voltage requirements, so the requirements for these devices are also very high. These devices are not often seen in the market, which has brought obstacles to the development of high-voltage DC power supply technology.


6.2.4 Restriction of the monitoring system on the development of HVDC power supply technology

If the high-voltage DC power supply technology is to be applied on a large scale in the power environment monitoring system, the technical requirements will be very high. There is no difficulty in switching the power supply, but it is difficult to realize the supporting battery pack. Because so far, there is no supplier that can provide a dedicated battery monitoring system.


6.3 Development prospect of HVDC power supply technology

Many Chinese telecommunications companies are gradually developing servers with 240V DC voltage compatible with AC power. According to the concept of power supply safety first, telecom companies are gradually realizing the development goals of energy saving and compatibility of electrical products. During this process, China Telecom chose high-voltage DC power supply as the power supply for equipment. Relevant reports show that in the data power supply market of telecom companies, the number of high-voltage DC power supplies has completely exceeded that of traditional uninterruptible power supplies, and it is decided to continue to expand the application range of high-voltage DC power supplies in future development. At the same time, different communication companies are also working hard to promote the development of high-voltage DC power. These companies directly introduce high-voltage DC power into customized servers, so that high-voltage DC power will promote the development of high-voltage DC power. Therefore , it can be said that the high-voltage DC power supply has a strong development prospect, and the high-voltage DC power supply is gradually replacing the traditional uninterruptible power supply.

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