High voltage direct current (HVDC) power systems, which transmit enormous amounts of power over long distances, employ direct current (DC). When it comes to long-distance power transmission, high-voltage direct current (HVDC) cables are less expensive and have lower losses than alternating current (AC). It connects networks with a variety of frequencies and characteristics together.
A constant voltage and current wave flow through the wire, changing direction every millisecond, resulting in heat losses when using an alternating current transmission system. In contrast to alternating current lines, voltage and current waves in direct current do not change direction. Because of the rapid transfer of electricity, high-voltage direct current cables improve the efficiency of transmission lines.
The generated alternating current voltage is transformed to direct current at the transmitting end of a combined alternating current and direct current system. The direct current voltage is changed to alternating current voltage for distribution at the receiving end. There must be conversion and reversing equipment installed at both ends of the line.
Only long-distance transmission lines of more than 600 kilometres in length and subterranean cables of more than 50 kilometres in length are cost-effective when using high-voltage direct current transmission.
When the substation is constructed, alternating current electricity is generated, which can subsequently be converted to direct current electricity with the help of a rectifier. Reciprocating rectifiers and inverters are located at both ends of a transmission line at the high-voltage direct current (HVDC) or converter substation. The inverter terminal converts direct current to alternating current, whereas the rectifier terminal converts alternating current to direct current.
On the user's side, DC is joined with overhead lines and changed to alternating current by inverters in the converter substation. The power remains constant at both the transmit and receive ends of the link during the entire process. In long-distance transmission, direct current (DC) is employed because it reduces losses while increasing efficiency.
Two terminal DC systems, also known as point-to-point systems, are defined as systems that have more than two converter stations and one transmission line connected to each other. When it comes to multi-terminal DC substations, the phrase refers to a system that contains more than two converter stations that are interconnected by DC terminal lines.
Despite the fact that DC lines are less expensive than AC lines, DC terminal equipment is relatively expensive when compared to the wiring used for AC terminals (shown in the graph below). Consequently, the starting cost of the high-voltage direct current transmission system is higher than that of the alternating current transmission system.
The point at which the two curves overlap is known as the breakeven distance. The cost of the HVDC system decreases when the distance between the breakeven point and the system is increased. The breakeven distance for overhead transmission lines ranges between 500 and 900 km, depending on the type of line.
Given all of the advantages of DC, high-voltage direct current (HVDC) lines appear to be more efficient than alternating current lines. A substantial initial investment is required for the HVDC substation, and the technology used in the substation is extremely sophisticated. As a result, for long-distance transmission, it is preferable to generate power in alternating current, convert it to direct current for transmission, and then convert it back to alternating current for end use. This system is both cost-effective and efficient, increasing the overall efficiency of the system.