(1) There is no problem with the stability of synchronous operation between the AC systems at both ends of the DC transmission, and the transmission energy and distance are not limited by the stability of synchronous operation.
(2) The DC power line network is used to facilitate regional power distribution management, facilitate timely emergency assistance between AC systems in the event of a failure, and limit the spread of accidents.
(3) The DC control system has the advantages of fast response, precise adjustment, convenient operation, and multi-purpose control.
(4) The voltage distribution along the DC transmission line is stable, there is no capacitive current, and no parallel compensation is required.
(5) DC transmission at both ends is convenient for layered structure, layered structure, and capacity expansion, and the maximum benefit can be obtained as soon as possible.
Comparison of HVDC and HVAC
(1) DC transmission line investment is small. DC transmission usually takes over the operation of the bipolar neutral ground. A DC line requires only two wires and a three-phase AC line requires three, but both transmit about the same amount of power. Therefore, the load on the tower can be reduced, and the width and footprint of the pipe gallery can be reduced. Under the condition of carrying the same power and distance, the investment of DC overhead lines is generally two-thirds of the investment of AC overhead lines.
(2) Less investment in DC cable lines. If the same cable insulation is used for DC, the allowable working voltage is twice that of AC. For the same voltage, the cost of DC cables is much lower than that of AC cables.
(3) The substation system is larger than the substation system. Installing a substation is more complicated than installing an AC substation. In addition to converter transformers, there are also converter auxiliary equipment such as expensive thyristor converters. Therefore, the substation system is higher than the AC substation system of the same capacity and corresponding voltage.
(4) Under similar conditions, if the transmission line is larger than the equivalent line, the construction cost of DC transmission will be lower than that of AC transmission.
(5) Low operating cost. According to operating experience, the annual depreciation and maintenance costs of plant and station equipment are not only AC and DC, but also account for about the same share in construction costs. However, under the condition that the effective transmission power of the same conductor line part is the same, the power failure of DC transmission is two-thirds of that of AC transmission.
