HVDC transmission is a strong alternative to EHV-AC transmission in some of the applications such as long distance high power transmission by overhead lines, under water transmission, transmission by underground cables, system interconnection by means of overhead lines or underground cables or back-to-back HVDC coupling stations, frequency conversion (50/60 Hz) and hence preferred.
On the basis of applications, HVDC transmission systems can be classified as follows:
1. Long Distance High Power Transmission System:
DC overhead lines are cheaper than three-phase transmission lines of the given transmission capacity. With large distances, the saving in cost becomes greater than the additional expenditure on the terminal equipment. HVDC becomes favourable above 800 km, 1,000 MW when the cost of EHV-AC line/substation exceeds that of equivalent HVDC line/substation.
A two-terminal dc link employing overhead lines is used for transmission of large blocks of relatively cheap power from remote sources (say hydroelectric) to load centres situated far away from the generating stations.
2. Underwater Transmission System:
ADVERTISEMENTS:
In case of 3-ɸ transmission, the maximum length of the underwater cable is 25-40 km. This is due to the problem of charging current, and in case of oil-filled cables it is difficult to ensure oil flow over long distances. For longer distances HVDC is the only solution. In the absence of dielectric losses, the dc cables can be of the impregnated type for voltages of at least up to 400 kV, and practically no limit is set to its length.
3. Underground Transmission System:
Sometimes, the power transmission in congested areas has to be done through underground cables. Wherever the distances involved exceed 40 km, dc transmission is the only feasible solution. DC transmission in such cases offers definite advantages, since the equipment for compensating charging kVA is not required and also there is no reduction in the transmission capacity of cables.
Moreover, there is no dielectric heating of the insulation or inductive heating of armouring. Hence the heat dissipation problem in dc cables is far less as compared to the 3-ɸ ac cables. The extra cost of the converters is compensated by saving in cable costs at distances from 50-100 km depending on the conditions in each individual case.
4. DC Link in Parallel with AC Link Transmission System:
For interconnection between two ac systems having their own load frequency control, HVDC links have several advantages over ac links. HVDC links form an asynchronous-tie i.e., the two ac systems interconnected by HVDC tie- line need not in synchronism with each other.
ADVERTISEMENTS:
HVDC interconnection is superior to EHV-AC interconnection in many respects and is preferred due to its technical superiority. With HVDC interconnection, power flow can be controlled, the frequency disturbances are not transferred, short-circuit levels remain unaffected at both ends, transient stability of an existing ac link can be considerably improved.
A dc link has an extremely rapid speed of response (of the order of 50 ms) and, therefore, it can change its transmitted power much more rapidly than, say a turbine generator can adjust its output. Thus the system stability can be significantly improved.
HVDC interconnection can provide a weak tie (of lesser capacity) between strong and a weak ac network. This is difficult with ac interconnection.
HVDC system control can be modified to dampen oscillations in load or power angle δ and thereby improve the stability of an existing ac link.
ADVERTISEMENTS:
Most important task of interconnector is transfer of required amount of power in required direction and to assist the interconnected ac network to maintain transient stability. AC interconnectors have severe limitations whereas dc interconnectors do not.
5. HVDC Back-to-Back Transmission System:
Such a system provides an asynchronous interconnection between two adjacent ac systems through converter station and zero (or negligible) length dc line. Such schemes are employed for coupling ac systems of different frequencies (e.g., 50/60 Hz) or of different system controls. The back-to-back coupling stations can be located at any suitable location, where two networks meet geographically and exchange of required amount of electrical power is desired.
Vindhyachal HVDC back-to-back link is for exchange of power between Northern and Western regions. Each block of 250 MW is capable of operating independently in either direction and can transfer power in the range of 25 MW to 250 MW depending on the system conditions.
6. Multi-Terminal HVDC Interconnection or Links Transmission System:
This is the new HVDC possibility. Three or more ac networks can be interconnected asynchronously by means of a multi-terminal HVDC link. Power flow from each connected ac network can be controlled suitably. Large power can be transferred.
ADVERTISEMENTS:
Almost all dc links set up so far are two-terminal links except Kingsnorth-London scheme in which each half of the sending-end power is independently controlled and delivered to two different receiving-end stations. One of the essential requirements in a multi-channel dc scheme is the use of large dc circuit breakers.