High Voltage Direct Current (HVDC) power transmission systems (also known as power highways or electric highways) use direct current (DC) to transmit power versus the more common alternating current (AC) systems. Most HVDC power transmission lines use voltages between 100 kV and 800 kV.
In comparison with AC transmission systems, high-voltage direct current (HVDC) transmission has many advantages due to its long transmission distance, large transmission capacity, and easy power regulation. And is widely used in long-distance power transmission, asynchronous networking, island power supply, and other fields. At present, the main protection schemes for DC transmission lines include traveling wave protection, differential Under voltage protection, DC differential protection, etc.
HVDC allows power transmission between asynchronous AC transmission systems. As the power flow through the HVDC link can be controlled independently of the phase angle between source and load. HVDC also allows the transmission of power between grid systems operating at different frequencies (e.g., 50 Hz and 60 Hz). Allowing power to be exchanged between incompatible networks, improves the stability and economy of each grid.
HVDC and HVAC are the two most important power transmission methods today, and the differences are analyzed in terms of technical limitations, technical comparisons, and economic benefits.
For AC transmission, the biggest advantage of overhead lines is the ability to bundle multiple conductors. If the overhead lines in the table are 2 conductors per phase, the rated power per system can also reach the power per system of 500 kV for underground cables.
For DC cable transmission, ±500kV transmission can be achieved if Mass impregnated cable (viscous impregnated cable) is used. Overhead line transmission can reach a power rating of 7200 MW (8 conductors per pole) when using ±800 kV transmission. But with the development of technology voltage levels have been able to reach ± 1100kV (LCC-HVDC), China is now under construction with a total length of 3324 km of Changji – Guzhuan ultra-high-voltage DC transmission is the use of ± 1100kV voltage level. But for VSC-HVDC because it is limited by the IGBT voltage level and output power, the voltage level and power compared to LCC-HVDC will be much lower.
Here to point out is for AC submarine transmission line, the current technology can not transmit voltage higher than 275kV, the power per phase higher than 400MVA power (there may be progress). So for offshore power transmission HVDC transmission is the only choice (overhead lines can not be used in offshore situations).
HVDC transmission has the following main advantages over high-voltage AC transmission
(1) When the power delivered is the same, DC lines are cheaper
Three-phase AC power requires three wires to transmit, each wire to ground insulation in accordance with its peak ([formula] times the effective value) design. DC power requires only two wires (only one if it is unipolar) and the peak value of DC power is equal to the RMS value. Therefore, DC transmission can save a lot of transmission materials, and also reduce a lot of transportation and installation costs. Secondly, DC transmission has a simpler structure of overhead line towers, narrow line corridors, and cables with the same insulation level that can run at higher voltages.
(2) DC transmission of power consumption is small
Because DC overhead lines use only one or two wires, so the active loss is small and has a “space charge” effect, its corona loss and radio interference are smaller than the AC overhead lines.
(3) suitable for submarine power transmission
General AC power transmission is mostly overhead, the line and the formation of a capacitance of the earth, but because the composition of the air capacitance is very small, so the impact on the circuit transmission is very small. But for underground cables and submarine cables, because the cable and the land between the sea will form a larger capacitance, so the capacitive resistance is smaller than the high-altitude erection, equivalent to the formation of additional branches, which in turn causes additional line loss.
(4) The problem of reactive angle stability
In the AC transmission system, all synchronous generators connected to the power system must be kept in synchronous operation. If a DC line is used to connect two AC systems, the above stability problem does not exist because there is no reactance in the DC line. That means DC transmission is not limited by the transmission distance.
(5) Can limit the short-circuit current of the system.
When connecting two AC systems with AC transmission lines, the increase in system capacity will increase the short-circuit current, which may exceed the original circuit breaker blocking capacity, which requires the replacement of a large amount of equipment, increasing the amount of investment. When the DC transmission, there is no such problem.
(6) Fast regulation and reliable operation
DC transmission through the thyristor converter can easily and quickly adjust the active power and realize the tide flip. If a bipolar line is used, when one pole fails, the other pole can still be earth or water as a circuit to continue to transmit half of the power. This also increases operational reliability.
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