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Comparison Of Advantages Of High-voltage DC And AC Cables
May 14, 2018

The advantages of DC transmission technology:


   economic aspect:


(1) The line cost is low. For overhead transmission lines, AC uses three conductors, and DC generally uses two. If only one earth or seawater is used for the circuit, it can save a lot of line construction costs. For cables, since the DC strength of the insulating medium is much higher than the AC strength, such as the conventional oil-impregnated paper cable, the allowable working voltage of DC is about 3 times that of AC, and the DC cable investment is much less.


(2) Small annual energy loss. Only two DC overhead transmission lines are used. The resistance loss of the wire is smaller than that of the AC transmission. There is no inductive reactance and reactive reactive power loss. There is no skin effect, and the cross section of the wire is fully utilized. In addition, the "space charge effect" of DC overhead lines results in smaller corona losses and radio interference than AC lines. Therefore, DC overhead transmission lines are more economical than the exchange economy in the initial investment and annual operating costs of line construction.


   Technical aspects:


(1) There is no system stability problem, and the grid can be interconnected in the non-synchronous period, while all the synchronous generators in the AC power system keep running synchronously. It can be seen that under a certain transmission voltage, the transmission power and the distance allowable for AC transmission are limited by the network structure and parameters, and measures must be taken to increase the stability and increase the cost. The use of a direct current transmission system to connect two AC systems does not have the above-mentioned stability problem because the DC line has no reactance. Therefore, the transmission capacity and distance of HVDC transmission are not limited by the synchronous operation stability. It is also possible to connect two systems with different frequencies to achieve non-synchronous networking and improve the stability of the system.


(2) Limit the short-circuit current. If two AC systems are connected by AC transmission lines, the short-circuit capacity increases, and even a breaker or a current limiting device needs to be replaced. However, when two AC systems are connected by a DC transmission line, the “constant current control” of the DC system will quickly limit the short-circuit current to the rated power, and the short-circuit capacity will not increase due to the interconnection.


(3) Fast adjustment and reliable operation. The HVDC power transmission can quickly adjust the active power through the thyristor converter and realize “flippage of the power flow” (change of the power flow direction), which can ensure stable output under normal conditions. In the event of an accident, it can realize the emergency of a sound system to the faulty system. Support can also achieve oscillation damping and subsynchronous oscillation suppression. In the parallel operation of AC and DC lines, if there is a short circuit in the AC line, the DC transmission power can be briefly increased to reduce the acceleration of the generator rotor and improve the reliability of the system.


(4) No capacitor charging current. There is no capacitor current in the steady state of the DC line, and the voltage distribution along the line is stable. When there is no empty or light load, abnormal voltage rise occurs in the receiving end and the middle of the AC long line, and no shunt reactance compensation is needed.


(5) Save on line corridors. Considering the same voltage of 500 kV, the corridor of a DC transmission line is -40 m, and the length of an AC line corridor is -50 m, while the former transmission capacity is about twice that of the latter, that is, the DC transmission efficiency is about 2 times that of AC.


    The inadequacy of DC transmission technology:


(1) The converter device is expensive. This is the main reason for limiting the use of DC transmission. When delivering the same capacity, the cost per unit length of the DC line is lower than that of the AC; and the cost of the converter equipment at both ends of the DC transmission is much more expensive than that of the AC substation. This has caused the so-called "equivalent distance" problem.


(2) Consumption of reactive power is much. In general, the reactive power consumed by the converter station at each end is about 40% to 60% of the transmission power, which requires reactive power compensation.


(3) Generate harmonic effects. The converter generates harmonic voltages and harmonic currents on the AC and DC sides, causing overheating of capacitors and generators, unstable control of the inverter, and interference with the communication system.


(4) In terms of technology and equipment, there is no zero crossing in the DC waveform and arcing is difficult. In the absence of a DC switch, the switching function is achieved by blocking the control pulse signal of the converter. If multiple DC lines are brought together in one area, one failure may also result in blocking of multiple inverter stations. In the multi-terminal power supply mode, it is not possible to cut off the accident line alone and all lines must be disconnected, which will cause a major impact on the system.


(5) From the aspect of operation and maintenance, the DC line has a fast fouling rate and a low pollution flash voltage, and the pollution problem is more serious than the AC line. Compared with Western developed countries, China's atmospheric environment is relatively poor at present, which makes it more difficult to clean and prevent pollution flashover of DC lines. Equipment failures and serious contaminations cause the DC flash rate to be significantly higher than the AC line.


(6) The transformer cannot be used to change the voltage level. DC transmission is mainly used for long-distance large-capacity transmission, asynchronous interconnection between AC systems, and transmission of submarine cables. Compared with direct current transmission, the existing 500kV AC transmission (economical transmission capacity of 1 000 kW and transmission distance of 300-500 km) can no longer meet the needs. Only by increasing the voltage level, UHV transmission methods can be used to achieve a higher economy. benefit.


    The main advantages of UHV AC transmission:


(1) Improve transmission capacity and transmission distance. With the expansion of the power grid area, the transmission capacity and transmission distance of the electrical energy are also increasing. The higher the required grid voltage level, the better the effect of compact transmission.


(2) Improve the economical efficiency of power transmission. The higher the transmission voltage is, the lower the price of the transmission unit capacity is.


(3) Save line corridors and substations. In general, a 1150 kV transmission line can replace 6 500 kV lines. The use of UHV transmissions has improved corridor utilization.


(4) Reduce the power loss of the line. For China, every 1% increase in voltage is equivalent to a new increase of 5 million kW of electricity per year, and 500 kV of transmission is more than 5 times greater than the 1200 kV line loss.


(5) It is conducive to networking, simplifying the network structure and reducing the failure rate.


    The main disadvantages of UHV transmission:


    The main drawback of UHV transmission is that the stability and reliability of the system are not easy to solve. Since 1965-1984, there have been six exchanges of major power grid collapses in the world, four of which occurred in the United States and two in Europe. These serious large-scale power grid disintegration accidents indicate that large-scale power grids using AC interconnection have problems that are difficult to solve, such as safety and stability, chain reaction to accidents, and blackouts in large areas. Especially in the initial stage of the UHV line, the main grid cannot be formed, the line load capacity is low, and the centralized transmission of the power source brings about a large stability problem. The lower-level power grid cannot be operated in a loop, which can not effectively reduce the short-circuit current of the receiving power grid, which threatens the safe operation of the power grid. In addition, UHV AC transmission has a greater impact on the environment.


    to sum up:


    Due to the advantages and disadvantages of AC UHV and HVDC, they can be used for interconnection between long-distance large-capacity transmission lines and large-area power grids, both of which have their own advantages and disadvantages. The main consideration for the construction of transmission lines is economy, while the interconnection lines must put system stability first. With the development of technology, the advantages and disadvantages of both parties may also be transformed into one another. Both types of transmission technologies will coexist and have fierce competition for a long time. In the case of UHV AC transmission, if 750kV (maximum operating voltage 800kV) is used at a voltage level of 500kV, there may be problems such as multiple electromagnetic loops, difficult flow control, and large power grid losses due to the two voltages being too close to each other. Even if flexible AC transmission technologies or compact transmission technologies are adopted in the future, the limited increase in transmission capacity will still be difficult to meet the long-term development needs of the power system. In summary, compared with 750kV AC transmission, UHV has certain advantages in large-capacity long-distance transmission and construction of a strong national power grid. There are no insurmountable technical problems in technology and equipment, investment in construction and operation. It is also more economical.


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