Discussion on the Characteristics of Ultra High Voltage AC and DC in the Comprehensive Launch of Ultra High Voltage Development

North Star Transmission and Distribution Network News: Ultra high voltage is currently the most advanced transmission technology in the world, with comprehensive advantages of long distance, large capacity, low loss, and less land occupation. The energy distribution in China is uneven, with a large amount of energy stored in the western part of China, while the eastern coastal areas are the load centers; At the same time, many cities in China are facing significant pressure from haze and energy conservation and emission reduction, so it is necessary to develop efficient ultra-high voltage power grids in China. Ultra high voltage specifically includes two forms: ultra-high voltage AC transmission technology and ultra-high voltage DC transmission technology. Below, we will explore the characteristics of these two forms of ultra-high voltage?
1、 The characteristics of ultra-high voltage direct current transmission (UHVDC) refer to direct current transmission and related technologies with voltage levels of ± 800kV (± 750kV) and above. The main characteristics of ultra-high voltage direct current transmission are large transmission capacity, long transmission distance, high voltage, and can be used for asynchronous interconnection of power systems.
1. Advantages of ultra-high voltage direct current transmission
(1) From an economic perspective: Given the advantages of low line cost and low annual energy loss of DC transmission, DC overhead transmission lines are more economical than AC systems in terms of initial investment and annual operating costs during line construction;
(2) There is no system stability issue: non synchronous interconnection of the power grid can be achieved, while all synchronous generators in the AC power system maintain synchronous operation;
(3) Limiting short-circuit current: If an AC transmission line is used to connect two AC systems, the short-circuit capacity will increase, and it may even be necessary to replace the circuit breaker or add a current limiting device. However, by connecting two AC systems with DC transmission lines, the "constant current control" of the DC system will quickly limit the short-circuit current to near the rated power, and the short-circuit capacity will not increase due to interconnection;
(4) Fast regulation and reliable operation: DC transmission can quickly adjust active power through controllable silicon converters, achieving "flow reversal" (change in power flow direction), ensuring stable output under normal conditions, providing emergency support for faulty systems in case of accidents, as well as suppressing oscillation damping and sub synchronous oscillation;
(5) No capacitor charging current: When the DC line is in steady state, there is no capacitor current, and the voltage distribution along the line is stable. When there is no empty or light load, there is an abnormal voltage increase at the receiving end and middle of the AC long line, and there is no need for parallel reactance compensation;
(6) Saving line corridors: Considering the same voltage of 500kV, the corridor of a DC transmission line is~40m, and the corridor of an AC transmission line is~50m. The former has a transmission capacity of about twice that of the latter, which means the DC transmission efficiency is about twice that of AC.
2. Shortcomings of ultra-high voltage direct current transmission technology
(1) The high cost of converter devices is the main reason limiting the application of direct current transmission. When conveying the same capacity, the cost per unit length of DC lines is lower than that of AC lines; The cost of the converter equipment at both ends of DC transmission is much higher than that of AC substations. This leads to the so-called "equivalent distance" problem;
(2) Excessive consumption of reactive power: Generally, each converter station consumes about 40% to 60% of the transmitted power, requiring reactive power compensation;
(3) Generating harmonic effects: The converter generates harmonic voltage and current on both the AC and DC sides, causing overheating of the capacitor and generator, unstable control of the converter, and interference with the communication system;
(4) In terms of technology and equipment, the DC waveform has no zero crossing and arc extinguishing is difficult: currently, there is a lack of DC switches, but the switching function is achieved through the control pulse signal of the blocking converter. If multiple DC lines converge in one area, a single fault may also cause multiple inverter stations to be locked, and in the multi terminal power supply mode, it is not possible to cut off the accident line alone and all lines need to be cut off, which will cause significant impact on the system;
(5) From the perspective of operation and maintenance: DC lines have a fast accumulation of pollution, low pollution flashover voltage, and more serious pollution problems than AC lines. Compared with developed Western countries, the current atmospheric environment in China is relatively poor, which makes it more difficult to clean and prevent pollution flashover of DC lines. Due to equipment malfunctions and severe pollution, the pollution flashover rate of DC lines is significantly higher than that of AC lines;
(6) Transformers cannot be used to change voltage levels: DC transmission is mainly used for long-distance high-capacity transmission, asynchronous interconnection between AC systems, and submarine cable power transmission. Compared with DC transmission, the existing AC 500kV transmission (with an economic transmission capacity of 1000kW and a transmission distance of 300-500km) can no longer meet the needs. Only by improving the voltage level and adopting ultra-high voltage transmission methods can higher economic benefits be achieved.
2、 The characteristics of ultra-high voltage AC transmission refer to AC transmission of 1000 kV and above, which has outstanding advantages such as large transmission capacity, long distance, low losses, and less land occupation.
1. Advantages of ultra-high voltage AC transmission
(1) Improving transmission capacity and distance: With the expansion of the power grid area, the transmission capacity and distance of electric energy continue to increase. The higher the required voltage level of the power grid, the better the effect of compact transmission;
(2) Improve the economy of electricity transmission: the higher the transmission voltage, the lower the price per unit capacity of transmission;
(3) Save line corridors and substation footprint: Generally speaking, one 1150kV transmission line can replace six 500kV lines. The use of ultra-high voltage transmission has improved the utilization rate of corridors;
(4) Reduce power loss of transmission lines: In China, for every 1% increase in voltage, it is equivalent to adding 5 million kW of electricity each year. 500kV transmission has more than 5 times greater line loss than 1200kV transmission;
(5) Beneficial for networking, simplifying network structure, and reducing failure rate: Utilizing ultra-high voltage transmission technology, especially DC transmission technology, to achieve long-distance and large-scale power transmission is beneficial for saving land resources and protecting the environment, and is an important way to optimize energy resource allocation.
The main drawback of ultra-high voltage AC transmission technology is that the stability and reliability issues of the system are not easily solved. Especially in the early stages of the emergence of ultra-high voltage lines, the main network structure cannot be formed, the line load capacity is low, and the centralized power supply brings significant stability problems. The inability of the lower level power grid to operate in a non loop manner results in the inability to effectively reduce the short-circuit current of the receiving end power grid, which poses a threat to the safe operation of the power grid. In addition, ultra-high voltage AC transmission has a significant impact on the environment. Both ultra-high voltage direct current transmission and ultra-high voltage alternating current transmission are aimed at improving transmission capacity, achieving high-power medium and long-distance transmission, as well as achieving long-distance power system interconnection, and building a joint power system. At the same time, the two can only complement each other and cannot replace each other. The mixed mode of "strong communication and strong straightness" will be the development direction of future ultra-high voltage construction.