Home » POWER SYSTEM » A.C. AND D.C. TRANSMISSION BY OVERHEAD AND UNDERGROUND SYSTEM.

ELECTRICAL POWER TRANSMISSION:
Fundamentally electric power transmitted by overhead systems or underground system. In both cases, there are two types of process for electrical power transmission either by A.C or by D.C. The process of A.C and D.C transmission has its own advantage and disadvantages also over one another.
1) D.C. Transmission: This type of power transmission can be possible by several ways.
(i) D.C two wires, (ii) D.C two wires with midpoint earth and (iii) D.C three wires.

Advantages of D.C transmission:
► It requires only two conductors.
► There is no problem of inductance and capacitance.
► There is no surge problem.
► Voltage drop is less than A.C transmission, so better voltage regulation.
► There is no skin effect. So we can utilize the entire cross-section of the conductor.
► Less corona loss which reduces the interference with communication circuit.
► Less insulating material than A.C is required.

Disadvantages are:
► Due to commutation problem, electric power cannot be generated at D.C voltage.
► D.C voltage cannot be stepped up or down because transformer uses only A.C. voltage and current.
► D.C. switch, circuit breaker has its own limitations.

2) A.C. Transmission: The most common way of transmitting electrical power is A.C. transmission.
The various process of A.C. transmission are-
(a) Single phase A.C. system: These are also subdivided by
(i) Single phase two wire.
(ii) Single phase two wire with point earthed.
(iii) Single phase three wire.
(b) Two phase A.C system:
(i) Two phase four wires.
(ii) Two phase three wires.
(c) Three phase A.C. system:
(i) Three phase four wires.
(ii) Three phase three wires.

Advantages of A.C transmission:

► A.C voltage can easily be stepped up and stepped down at desired level by transformer.
► Maintenance of A.C. substation is easy and cheaper.
► In electric power system, from generation to distribution, throughout the network, a.c. voltage is handled. So no extra care is needed like D.C. voltage.

Disadvantages of A.C transmission:
► A.C. lines require more copper as it requires more conductor than D.C.
► A.C. transmission line construction is more complicated than D.C.
► As skin effect occurs only in A.C, effective resistance of the line increased.
► For line capacitance, continuous loss of power occur due to charging current.
► The voltage drop is greater than D.C.

COMPARISON OF CONDUCTOR MATERIALS:
Both A.C. and D.C. system of transmission, there are two ways.
i) Transmission by overhead system and
ii) Transmission by underground system.

In over head system,
Suppose P watt power has to be transmitted.
The length of transmission area is l meters.
Let the line loss is W watt and maximum voltage between any conductor and earth being V volt.

So, in two wire D.C. system with one conductor earthed, Load current, I=P/V.
If R is the resistance of each line conductor, R = ρl/a.
Where ρ = resistivity, a = cross-section of conductor.
So, line losses, W = 2I2R (as two conductor).
= 2(P/V)2ρl/a.
So, cross section of conductor, a =2P2ρl/WV2.
Volume of conductor material required, 2al = 4P2ρl2/ WV2.

In case of two wire D.C. system with midpoint earthed,
Volume of conductor material required, = P2ρl2/W V2, i.e. 1/4th or 0.25 times of two wire D.C. system with one conductor earthed.

In case of three wire D.C. system with one conductor earthed,
Volume of conductor material required, 2.5 P2ρl2/2WV2,
Or, 5/16th times (0.3125) of two wire D.C. system with one conductor earthed.

Now in underground system, let all the parameters are same, and maximum voltage between conductors are Vm in each case. So, in two wire D.C system,
Volume of conductor materials required,
Line losses, W = 2I2R (as two conductor).
= 2(P/V)2ρl/a.
So, cross section of conductor, a =2P2ρl/WV2.
Volume of conductor material required,= 4P2ρl2/ WV2.

In two wire D.C. system with midpoint earthed, volume of conductor materials required,
= 4P2ρl2/ WV2.
= same as two wire D.C. system.

In three wire D.C. system, volume of conductor materials required,
= 5P2ρl2/ WV2.
Or, 5/4 or 1.25 times that required for 2 wire D.C. system.

In single phase, 2 wire A.C. system,
Volume of conductor material required, = 8P2ρl 2/ WV2cos2θ. (Where cosθ= load power factor.)
Or, 2/ cos2θ times that required in a 2 wire D.C. system.

In single phase 2 wire system with midpoint earthed,
Volume of conductor material required, = 8P2ρl 2/ WV2cos2θ.
Or, 2/ cos2θ times that required in a 2 wire D.C. system and same as single phase 2 wire A.C. system.

In single phase 3 wire systems,
Volume of conductor material required, 10P2ρl 2/ WV2cos2θ.
Or, 2.5/cos2θ times that required 10P2ρl 2/ WV2cos2θ.in 2 wire D.C. system.

In 2 phase 4 wire systems,
Volume of conductor materials required =8P2ρl 2/ WV2cos2θ.
Or,2/cos2θ times that required in 2wire D.C. system.

In 2 phase 3 wire systems,
Volume of conductor materials required = 2.914P2ρl 2/ WV2cos2θ.
Or, 2.914/cos2θ times that required in 2 wire D.C. system.

In 3 phase 3 wire systems,
Volume of conductor materials required, = 6P2ρl 2/ WV2cos2θ.
Or, 1.5/cos2θ times that required in 2 wire D.C. system.

In 3 phase 4 wire systems,
Volume of conductor materials required, = 7P2ρl 2/ WV2cos2θ.
Or, 1.75/cos2θ times that required in 2 wire D.C. system.

From above discussion, we can conclude that, in both underground and overhead system D.C transmission has great saving in conductor material. But due to some technical unavailability and disadvantages, D.C system is not used for transmission. Considering the saving in conductor materials, the efficient system of transmission is now 3 phase A.C. system. However H.V.D.C is used for submarine cables, where transmission distance equal or more than 300km. on that case A.C. cannot be used because of excessive reactive power. Submarine H.V.D.C. system is used in country like Britain, Australia, Newzeland, Europe e.t.c.
From economical view 3 phase 3 wire A.C. system is the almost universal system of generation and transmission of electrical power whereas 3 phase 4 wire A.C. system is used in distribution area. The elements of a high voltage transmission lines in overhead systems are
(i) Conductors which usually 3 used for single circuit line and 6 for double circuit line. And the material is aluminium reinforced with steel.
(ii) Step up and step down transformers are used for step up the sending end voltage and step down the voltage at receiving end.
(iii) Line insulator used for mechanical support and isolation from ground.
(iv) Protective devices like ground wire, L.A, Circuit breaker, relay are used for protection.

Advantage of overhead line in transmission system:
As we have already known that electric power can be transmitted or distributed either by overhead lines or by underground cables. But in case of power transmission, mainly overhead lines are used because of power is generally transmitted over long distances between power station and load centers. So, costs of underground cable installation are much higher. Beside these power is transmitted in overhead line at high voltage level, where air acts as the insulation, which is not possible in underground system. Therefore power is transmitted by overhead lines and only in congested area, is distributed by underground system.

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