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Comparison of high voltage DC and AC cable advantages

Information Source:Henan Huadong Cable Co., Ltd  Release date:2014-02-09

The advantages of DC transmission technology:

Economic aspects:

(1) the cost of circuit low. For overhead transmission lines, three wire AC and DC, generally two, by land or sea water loop for as long as a root, can save a lot of line construction cost. For cable, because DC strength insulating medium far higher than AC strength, such as the oil paper cable, DC allowed working voltage is about 3 times of AC, DC cable investment less.

(2) annual energy loss. DC overhead transmission line with only two, conductor resistance loss than the AC small; no inductance and capacitance reactive loss resistance; no skin effect, make full use of wire section. In addition, DC overhead line "space charge effect" that the corona loss and radio interference are less than the AC line. Therefore, DC overhead transmission line investment in transmission line construction and operating costs than the exchange of economic.

The technical aspects:

(1) there is no systematic stability, can realize the non synchronous interconnected power grid, and AC power system of the synchronous generator all maintain synchronous operation. Thus, in the transmission voltage, AC input capacitance Xu transmission power and distance by the network structure and parameter limit, must take the measures to improve the stability, increase the cost of. While connecting the two AC system with DC power transmission system, because of the DC line no reactance, the stability problem does not exist. Therefore, the transmission capacity HVDC transmission and distance is not affected by the synchronous operation stability limit. Can also be connected to two different frequencies of the system, the non synchronous network, and improve the system stability.

(2) to limit the short circuit current. Such as the use of AC transmission line connecting the two communication system, short circuit capacity increase, or even need to replace the circuit breaker or a current limiting device. However, connecting the two AC system with DC transmission line, DC system "constant current control ', will quickly put the short-circuit current limitation in the vicinity of rated power, not because the Internet increases short-circuit capacity.

(3) Regulation of fast, reliable operation. DC transmission through the thyristor inverter can quickly adjust the active power, the realization of "tide turn" (power flow direction changes), can guarantee the stable output in the normal, in case of an accident, can realize the emergency support sound system on fault system, also can realize the oscillation damping and sub synchronous oscillation suppression. In the AC and DC transmission lines in parallel operation, if the AC line of short circuit, can temporarily increase the DC transmission power to reduce the generator rotor speed, improve the reliability of the system.

(4) no capacitor charging current. The DC line steady state without capacitance current, the voltage distribution along the smooth, non empty, AC line by the end of the middle voltage and the abnormal increase of the phenomenon of light load, also need not parallel reactance compensation.

(5) save the transmission line corridor. Consider the same voltage of 500 kV, a DC transmission line corridor to 40 m, an AC line corridor to 50 m, while the former transport capacity is about 2 times as fast, i.e. DC transmission efficiency is about 2 times in exchange.

Deficiency of HVDC transmission technology:

(1) converter is more expensive. This is the main reason to limit the application of DC transmission. In the delivery of the same capacity, the DC line length unit cost than AC and DC transmission is low; both ends of the commutation equipment cost a lot more than your AC substation. This leads to a so-called "equivalent distance".

(2) the reactive power consumption without much. Generally each end station consume no power is about 40% ~ 60% of the transmission power, without the need for reactive power compensation.

(3) harmonic effect. The converter produces harmonic voltage and harmonic current in AC and DC side, control the capacitors and power engine overheating, the converter is not stable, the interference to the communication system.

(4) technology and equipment, DC waveform without zero, arc difficult. Realizes the function of the switch control pulse signal current lack of DC switch but by blocking converter. If multiple DC line pool of a region, a failure may also be caused by a plurality of inverter station closed, and in multi power supply can not individually cut accident line and cut off the line, which will have a significant impact on system.

(5) from the operation and maintenance of DC line pollution, fast speed, the pollution flashover voltage is low, pollution problem is more serious than the AC line. Compared with the western developed countries, the atmospheric environment in our country is relatively poor, which makes the DC line cleaning and anti pollution flashover is more difficult. DC flashover rate was significantly higher than that of the AC line equipment failure and pollution serious causes.

(6) do not use transformers to change voltage grade. HVDC transmission is mainly used for long-distance transmission, AC asynchronous interconnection and submarine cable transmission etc.. Compared with the direct current transmission, the existing 500kV AC transmission (economic transportation capacity is 1000 kW, the transportation distance is 300~500 km) has been unable to meet the needs, only by improving the voltage level, the use of special high-voltage transmission mode, in order to obtain higher economic benefits.

The main advantages of UHV AC power transmission:

(1) improve the transmission capacity and transmission distance. With the expansion of the regional power grid, power transmission capacity and transmission distance can also increase. The required voltage grade is higher and higher, the better the effect of compact type transmission.

(2) improve the economic efficiency of power transmission. The transmission voltage higher transport capacity of lower prices.

(3) save line corridors and substation area. In general, a 1150 kV transmission line can be instead of 6 to 500 kV line. The use of special high-voltage transmission to improve the utilization rate of the corridor.

(4) no capacitor charging current. The DC line steady state without capacitance current, the voltage distribution along the smooth, non empty, AC line by the end of the middle voltage and the abnormal increase of the phenomenon of light load, also need not parallel reactance compensation.
(5) save the transmission line corridor. Consider the same voltage of 500 kV, a DC transmission line corridor to 40 m, an AC line corridor to 50 m, while the former transport capacity is about 2 times as fast, i.e. DC transmission efficiency is about 2 times in exchange.

Deficiency of HVDC transmission technology:

(1) converter is more expensive. This is the main reason to limit the application of DC transmission. In the delivery of the same capacity, the DC line length unit cost than AC and DC transmission is low; both ends of the commutation equipment cost a lot more than your AC substation. This leads to a so-called "equivalent distance".

(2) the reactive power consumption without much. Generally each end station consume no power is about 40% ~ 60% of the transmission power, without the need for reactive power compensation.

(3) harmonic effect. The converter produces harmonic voltage and harmonic current in AC and DC side, control the capacitors and power engine overheating, the converter is not stable, the interference to the communication system.

(4) technology and equipment, DC waveform without zero, arc difficult. Realizes the function of the switch control pulse signal current lack of DC switch but by blocking converter. If multiple DC line pool of a region, a failure may also be caused by a plurality of inverter station closed, and in multi power supply can not individually cut accident line and cut off the line, which will have a significant impact on system.

(5) from the operation and maintenance of DC line pollution, fast speed, the pollution flashover voltage is low, pollution problem is more serious than the AC line. Compared with the western developed countries, the atmospheric environment in our country is relatively poor, which makes the DC line cleaning and anti pollution flashover is more difficult. DC flashover rate was significantly higher than that of the AC line equipment failure and pollution serious causes.

(6) do not use transformers to change voltage grade. HVDC transmission is mainly used for long-distance transmission, AC asynchronous interconnection and submarine cable transmission etc.. Compared with the direct current transmission, the existing 500kV AC transmission (economic transportation capacity is 1000 kW, the transportation distance is 300~500 km) has been unable to meet the needs, only by improving the voltage level, the use of special high-voltage transmission mode, in order to obtain higher economic benefits.

The main advantages of UHV AC power transmission:

(1) improve the transmission capacity and transmission distance. With the expansion of the regional power grid, power transmission capacity and transmission distance can also increase. The required voltage grade is higher and higher, the better the effect of compact type transmission.

(2) improve the economic efficiency of power transmission. The transmission voltage higher transport capacity of lower prices.

(3) save line corridors and substation area. In general, a 1150 kV transmission line can be instead of 6 to 500 kV line. The use of special high-voltage transmission to improve the utilization rate of the corridor.

(4) to reduce the loss of power line voltage, in our country, every 1% increase every year, the equivalent of the new increase of 5000000 kW power, 500 kV transmission is 1200 more than 5 times as large as kV loss.

(5) for networking, simplify the network structure, reduce the failure rate.

The main disadvantage of UHV transmission:

The main disadvantage of UHV power transmission is the problem of the stability and reliability of the system is not easy to solve. Since the 1965-1984 world has taken place 6 times large AC grid collapse accidents, of which 4 occurred in the America, 2 times in europe. These serious accidents grid tile solution that uses AC interconnected power grid security and stability, there is a chain reaction accident and large area blackout, difficult problems. Especially in the early stage occurred in UHV transmission line, cannot form the main frame, line load capacity is low, centralized supply power brings stability problems of larger. Lower level network cannot solve the loop operation, so it can not effectively reduce the receiving end grid short-circuit flow, which threaten the safe operation of power grid. In addition, UHV AC transmission has a great influence on the environment.

Summary:

The UHV AC and HVDC each have advantages and disadvantages, can be used for long distance and large capacity power grid interconnection circuit transmission lines and large area, both have advantages and disadvantages. Construction of power transmission line is the main consideration of economic, and interconnect lines to the stability of the system in the first place. With the development of technology, the advantages and disadvantages of both sides may also transform into each other. Two kinds of transmission technology will be in very long period of time the coexistence and competition. In the ultra high voltage AC transmission, if the use of 750kV in the 500kV voltage level (maximum operating voltage of 800kV), likely due to two level voltage far too close, will cause the electromagnetic loop network, flow control difficult, power loss and other issues, and, even if in the future by using flexible AC transmission technology or compact transmission technology limited increase transmission capacity, is still difficult to meet the need of long-term development of the power system. In summary, compared with the 750kV UHV AC transmission, has certain advantages in the large capacity long-distance transmission and building strong power grid across the country, all in the technology and equipment and technical problems more, in the construction investment and operation is more economic.

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