In electrical power transmission, high voltage DC is advantageous in many ways over high voltage AC systems. Today we will see the advantages and disadvantages of AC transmission, and the advantages and disadvantages of DC transmission. That is, will compare AC and DC economical transmission systems. Here is a comparative discussion of both systems of transmission:
Advantage of DC Transmission Systems
Below are 10 advantages of DC transmission systems.
(1) In electrical power transmission AC system requires three wires, and DC requires two wires. Power transmission is also possible with only one conductor using the earth as the return path (neutral). As a result, copper is saved in DC transmission.
(2) DC transmission has no problem with inductance, capacitance, power factor, and surge.
(3) There is no skin effect in DC, so the entire cross-section of the line conductor is used.
(4) Electrical stress on insulation in DC system at same working voltage is 1/√2 times that of AC. As a result, less insulation is required in DC at the same operating voltage.
(5) Charging current, which causes dissipation in the line even at no load condition is absent in DC.
(6) Corona loss in the DC line is less.
(7) DC transmission system causes less interference in adjacent communication lines than AC.
(8) Underground cables can be used for high voltage transmission as the electrical stress on the insulation in DC is low and the dielectric loss is negligible.
(9) Voltage regulation of DC transmission line is better than that of AC at equivalent load and same voltage at sending end.
(10) No stabilizer is required even for long transmission in DC.
- Also read: Limitations of AC Transmission
Disadvantages of DC Transmission Systems
Below are 4 disadvantages of DC transmission systems.
(1) High voltage cannot be produced in DC due to commutation problems and
(2) Voltage cannot be stepped-up directly to DC for transmission.
(3) DC circuit breakers and switches have limitations.
(4) DC transmission is relatively complex and expensive.
Advantages of AC Transmission Systems
Below are 3 advantages of AC transmission.
(1) The biggest advantage of the AC system is that in AC, the voltage can be easily stepped up to a higher value as per the required efficiency by using step-up transformers at the generation end or transmission end and step-down voltage can be done by using step down transformers at the distribution end.
(2) The Second advantage of the AC system is that electric power can be easily generated at high voltage.
(3) Maintenance of electrical substations used in AC systems is very easy and cheap.
Disadvantages of AC Transmission Systems
Below are 6 disadvantages of the AC transmission system.
(1) Distance between conductors should be kept sufficient to keep insulation between conductors adequate to reduce corona loss of overhead transmission lines.
(2) AC transmission system requires more copper than DC.
(3) Longer the length of the transmission line, the greater the capacitance. Even when the line is open, there is a constant dissipation of the charging current.
(4) Construction of transmission lines in AC systems is not as easy as DC.
(5) Parallel operation of two alternators requires synchronization.
(6) Alternator speed control should be limited to a very low range.
The distribution system of electricity is better in AC than in DC. Because AC voltage can be controlled very easily by using a transformer.
The best approach is to use AC systems for generation and distribution and DC systems for transmission. Currently, DC electrical power transmission is possible using mercury arc rectifiers and thyristors. DC to AC and AC to DC can now be converted very efficiently and cheaply using mercury arc rectifiers and thyristors. The capacity of such devices is up to 33 MW power at 400kV.
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