Advantages of DC transmission technology:
(1) There is no system stability problem, which can realize the non-synchronous interconnection of the power grid while all the synchronous generators in the AC power system keep running synchronously. Thus, under a certain transmission voltage, the transmission power and distance allowed by AC transmission are limited by the structure and parameters of the network. Measures for improving stability also need to be adopted to increase the cost. The DC transmission system to connect the two AC systems, the DC line is not reactance, there is no such stability problems. Therefore, the transmission capacity and distance of DC transmission are not limited by the stability of synchronous operation, but also two systems with different frequencies can be connected to realize non-synchronous networking and improve the stability of the system.
(2) Limit the short circuit current. Such as the AC power transmission line connecting the two AC systems, short circuit capacity increases, and even need to replace the circuit breaker or additional current limiting device. However, connecting two AC systems with DC transmission lines, 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 regulation, reliable operation. DC power through the silicon controlled rectifier can quickly adjust the active power, to achieve "trend flip" (power flow direction change), in the normal to ensure stable output, in case of accidents, to achieve sound system emergency system failure Support, but also to achieve oscillation damping and subsynchronous oscillation suppression. In parallel operation of AC and DC lines, if the AC line short circuit, you can briefly increase the DC transmission power to reduce the generator rotor acceleration and improve system reliability.
(4) There is no capacitive charge current. There is no capacitive current in the steady state of the DC line, and the voltage distribution along the line is stable and free from voltage. When the AC line is lightly loaded, the voltage of the terminal and the middle part of the AC line increases abnormally, and no parallel reactance compensation is required.
(5) Save the corridor. Considering the same voltage of 500 kV, the corridor of a DC transmission line ~ 40 m, an AC corridor ~ 50 m, while the former transmission capacity of about twice the latter, that is, DC transmission efficiency of about 2 times the exchange.
The lack of DC transmission technology:
(1) The commutation device is more expensive. This is the main reason that limits the application of DC transmission. When transporting the same capacity, the cost per unit length of the DC link is lower than that of the AC; however, the cost of the commutation equipment at both ends of the DC transmission is much more expensive than that of the AC substation. This raises the so-called "equivalent distance" issue.
(2) consume more reactive power. In general, the reactive power consumed by converter stations at each terminal is about 40% ~ 60% of the transmission power, and reactive power compensation is required.
(3) produce harmonic effects. The converter produces both harmonic voltage and harmonic currents on the AC and DC sides, overheating capacitors and generators, and instability of the converter control, interfering with the communication system.
The main advantages of UHV AC transmission:
(1) increase transmission capacity and transmission distance. With the expansion of the power grid area, the transmission capacity and transmission distance of electric energy are also increasing. The higher the required grid voltage level, the better the compact transmission.
(2) Improve the economy of electric energy transmission The higher the transmission voltage, the lower the price of the unit capacity of transportation.
(3) to save the line corridor and substation area. In general, a back to 1150 kV transmission lines can replace 6 back to 500 kV lines. The use of UHV transmission corridor utilization.
(4) Reduce the power loss of the line. For China, every 1% increase in voltage equates to an additional 5 million kW of power each year. The 500 kV line loss is more than 5 times larger than the 1200 kV line loss.
(5) is conducive to networking, simplifying the network structure and reducing the failure rate.
The main drawback of UHV transmission:
The main disadvantage of UHV transmission is that the stability and reliability of the system are not easily solved. Since 1965-1984, a total of six AC power grid disruptions occurred in the world, four of them in the United States and two in Europe. These serious disruptions caused by large power grids show that it is difficult to solve such problems as security and stability, accident chain reaction and large-scale blackouts in large power grids with interconnection. Especially in the early emergence of UHV lines, can not form the main grid, the line load capacity is low, the centralized power sent to bring greater stability issues. The subordinate power grid can not operate in solution, which can not effectively reduce the short-circuit current of the receiving terminal grid, which threaten the safe operation of the power grid. In addition, UHV AC transmission has a great impact on the environment.