An alternate means of transmitting electricity is to use high-voltage direct current (HVDC) technology. As the name implies, HVDC uses direct current to transmit power. Direct current facilities are connected to HVAC systems by means of rectifiers, which convert alternating current to direct current, and inverters, which convert direct current to alternating current. Early applications used mercury arc valves for the rectifiers and inverters but, starting in the 1970s, thyristors became the valve type of choice.
Thyristors are controllable semiconductors that can carry very high currents and can block very high voltages. They are connected is series to form a thyristor valve, which allows electricity to flow during the positive half of the alter-nating current voltage cycle but not during the negative half.
Since all three phases of the HVAC system are connected to the valves, the resultant voltage is unidirectional but with some residual oscillation. Smoothing reactors are provided to dampen this oscillation.
HVDC submarine cables are either of the solid type with oil-impregnated paper insulation or of the self-contained oil-filled type. New applications also use cables with extruded insulation, cross-linked polyethylene.
Although synchronous HVAC transmission is normally preferred because of its flexibility, historically there have been a number of applications where HVDC technology has advantages:
1 The need to transmit large amounts of power (>500 mW) over very long distances ( >500 km), where the large electrical angle across long HVAC transmission lines (due to their impedances) would result in an unstable system.
2 The need to transmit power across long distances of water, where there is no method of providing the intermediate voltage compensation that HVAC requires. An example is the 64 km Moyle interconnector, from Northern Ireland to Scotland.
3 When HVAC interties would not have enough capacity to withstand the electrical swings that would occur between two systems. An example is the ties from Hydro Quebec to the United States.
4 The need to connect two existing systems in an asynchronous manner to prevent losses of a block of generation in one system from causing transmission overloads in the other system if connected with HVAC. An example is the HVDC ties between Texas and the other regional systems.
5 Connection of electrical systems that operate at different frequencies. These applications are referred to as back-to-back ties. An example is HVDC ties between England and France.
6 Provision of isolation from short-circuit contributors from adjacent systems since dc does not transmit short-circuit currents from one system to another.
With the deregulation of the wholesale power market in the United States, there is increasing interest in the use of HVDC technology to facilitate the new markets.
HVDC provides direct control of the power flow and is there-fore a better way for providing contractual transmission services. Some have suggested that dividing the large synchronous areas in the United States into smaller areas interconnected by HVDC will eliminate coordination problems between regions, will provide better local control, and will reduce short-circuit duties, significantly reducing costs.
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Advantages of HVDC
As the technology has developed, the breakeven distance for HVDC versus HVAC transmission lines has decreased. Some studies indicate a breakeven distance of 60 km using modern HVDC technology.
Some of the advantages identified are:
- No technical limits in transmitted distance; increasing losses provide an economic limit;
- Very fast control of power flow, which allows improvements in system stability;
- The direction of power flow can be changed very quickly (bi-directionality);
- An HVDC link does not increase the short-circuit currents at the connecting points. This means that it will not be necessary to change the circuit breakers in the existing network;
- HVDC can carry more power than HVAC for a given size of conductor;
- The need for ROW is much smaller for HVDC than for HVAC, for the same transmitted power.
Disadvantages of HVDC
The primary disadvantages of HVDC are its higher costs and that it remains a technology that can only be applied in point-to-point applications because of the lack of an economic and reliable HVDC circuit breaker.
The lack of an HVDC circuit breaker reflects the technological problem that a direct current system does not have a point where its voltage is zero as in an alternating current system. An HVAC circuit breaker utilizes this characteristic when it opens an HVAC circuit.