Energy-saving exploration of high-voltage direct current transmission technology

With the coming of summer, the power grid will enter the peak load period again. In order to cope with the peak of power consumption in summer this year, power supply companies at all levels have taken different measures to ensure power supply. In the field of power transmission and transformation, the application of energy-saving technology and energy-saving equipment is an eternal theme in the power grid's energy-saving measures, and it is also an effective measure to solve the problem of power shortage for large power customers.

Since the 1980s, the pace of development of power transmission technology has accelerated significantly, and methods to improve transmission capacity have continued to emerge, including high-tech technologies such as DC transmission technology, flexible AC transmission technology, and frequency division transmission technology, as well as capacity-increasing and transformation technologies for existing high-voltage AC transmission lines, such as step-up transformation, multi-conductor capacity-increasing transformation, and conversion of AC transmission lines to DC transmission technology. DC transmission has very important practical significance for improving the transmission capacity of existing transmission systems and tapping the potential of existing equipment, and its implementation can achieve twice the result with half the effort.

Three economical characteristics highlight the energy-saving effect

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

First, the line cost is low, saving cable costs. DC transmission only requires two conductors, and using the earth or seawater as a loop only requires one conductor, 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 significant. There are fewer DC transmission wires, the resistance heating loss is small, there is no reactive power loss of inductive reactance and capacitive reactance, and the increase in transmission power reduces the unit loss, which greatly improves the energy saving effect in power transmission.

Finally, the line corridor is narrow, saving land acquisition costs. Taking the same level of 500 kV voltage as an example, the DC line corridor is only 40 meters wide. For transmission lines of hundreds or thousands of kilometers, the amount of land saved is considerable.

In addition to economical efficiency, the technicality of DC transmission is also remarkable. DC transmission has fast regulation speed and reliable operation. It can ensure stable output under normal circumstances and provide emergency support in the event of an accident, because DC transmission can quickly adjust power and achieve power flow reversal through thyristor converters. In addition, there is no capacitor charging current in the DC transmission line, the voltage distribution is stable, and the load size does not cause voltage abnormalities and does not require parallel reactors.

Improving space-based high-power power electronics will improve DC transmission performance

The core technology of DC transmission is concentrated in the converter station equipment. The converter station realizes the mutual energy conversion between DC and AC in DC transmission projects. In addition to the same equipment as AC substation in the AC field, there are also the following unique equipment: converter valves, control and protection systems, converter transformers, AC filters and reactive power compensation equipment, DC filters, smoothing reactors and DC field equipment. The converter valve is the core equipment in the converter station. Its main function is to perform AC-DC conversion. It has developed from the initial mercury arc valve to the current electrically controlled and light-controlled thyristor valves.

Thyristors have been used in high-voltage direct current transmission for a long time. 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 performance of the new generation of direct current transmission, greatly simplify the equipment, reduce the land occupation of converter stations, and reduce the cost.

【Viewpoint】

Long-distance power transmission has obvious advantages

The AC power generated by the power plant is converted into DC power through the converter valve, and then sent to the receiving end through the DC transmission line, and then converted into AC power and injected into the receiving end AC power grid. Industry experts agree that high-voltage DC 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 in the event of a fault. It is particularly suitable for long-distance point-to-point high-power transmission.

Among them, the light DC transmission system uses turn-off devices such as turn-off thyristors and insulated gate bipolar transistors to form converters, making medium-sized DC transmission projects competitive even in shorter transmission distances.

In addition, the inverter composed of the shut-off devices 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 access distributed power sources such as fuel cells and photovoltaic power generation. The light DC transmission system is more helpful in solving the problem of clean energy grid stability.

【Engineering Application】

1. The ±660 kV Ningdong-Shandong DC transmission project was put into operation on February 28, 2011. Shandong's capacity to receive external power increased from 3.5 million kilowatts to 7.5 million kilowatts. According to statistics, Shandong can save 11.2 million tons of raw coal every year. As a result, the province reduced sulfur dioxide emissions by 57,000 tons and sulfur dioxide emissions by 1.1 percentage points, greatly promoting the construction of a resource-saving and environmentally friendly society.

In the first quarter of 2011 alone, Shandong Power Grid received 9.13 billion kWh of electricity from outside the province, a year-on-year increase of 176%.

2. Jinping-Sunan ±800 kV UHV DC transmission project uses 900 square millimeter conductors, which have obvious energy-saving and environmental protection effects, strong resistance to natural disasters, and can further promote power technology innovation and industry technology upgrades. Compared with the traditional 630 square millimeter cross-section conductors, the Jinsu UHV DC line uses 900 square millimeter cross-section conductors. Based on an annual operation of 3,000 hours, it can save 43,200 kWh of electricity per kilometer of line per year, and the entire line will create a direct benefit of more than 40 million yuan a year.

Based on the power supply coal consumption of 360 grams of standard coal per kilowatt-hour, the entire line will reduce standard coal consumption by 77,350 tons per year and reduce carbon dioxide emissions by about 201,200 tons. In terms of resisting natural disasters, the horizontal load of large-section conductors in strong winds is reduced by about 10%, and the vertical load of 15 mm ice is reduced by about 7%.

3. The Three Gorges-Shanghai ±500 kV DC transmission project has a total length of 1,048.6 km and a transmission capacity of 3 million kW. If medium-strength all-aluminum alloy conductors are used to replace ordinary conductors, at normal power, if the transmission hours per year are 4,000 hours, 79,800 kWh/km of electricity can be saved, and the entire line can save 83.72 million kWh of electricity per year.