When the voltage has increased the efficiency of the transmission line increases and the cost of conductor material in feeder cables gets reduced but in the case of the dc system it is not possible to change the voltage between feeders and distributors without using rotating machinery which is undesirable, so the feeders and distributors are operated at same voltage i.e consumer voltage is the same as the voltage at feeding points.
The above difficulty is largely overcome by employing a 3-wire system which consists of two outers and an earthed middle wire known as a neutral wire. In such a system consumer apparatus or lamps are connected between the neutral and one of the outers. The voltage between the outers being twice that of the consumer’s terminals increases the transmission efficiency and reduces the cost of conductor material.
If the load on both sides is equal, no current flows through the neutral, and such a load is known as a balanced load. But if the load on two sides is different then the difference of load current on two sides, known as out-of-balance current, flows in the neutral. The neutral wire is generally taken off half cross-section of either outer and only out-of-balance current flows through the neutral wire.
If currents flowing through +ve and -ve outers are I1 and I2 respectively then the current in the middle wire will be a difference between currents I1 and I2 flowing in the outers.
When the load on the +ve side is greater than on the -ve side, current equal to the difference of outer currents will flow in the neutral wire from the load end to the supply end as shown in the figure below,
But when the load on the -ve side is greater than on the +ve side, current equal to the difference of outer currents will flow in the neutral wire from the supply end to the load end as shown in the figure below,
Typical layout of the 3-wire dc distribution system
Advantages of 3-wire DC Distribution
- Mainly two voltages are available i.e. high voltage for power loads and low voltage for domestic and commercial loads.
- 3-wire dc distribution is able to deliver more power i.e. twice that with two-wire distribution without affecting the efficiency with the same volume of conductor material.
- If both of the 3-wire and 2-wire feeders transmit the same amount of power over the same distance with the same efficiency, the three-wire feeder, if the load is balanced and the cross-section of the middle wire is half of the cross-section of either outer, requires only 0.3125 times conductor material as required by two wire feeder.
Let power delivered to consumers be P watts, the voltage at the power station and feeding point be V1 and V2 volts respectively.
In a 3-wire system the voltage is double i.e. 2V2 and therefore, each outer will carry a current of P/2V2 so long as the load is balanced.
Since for a given length of a conductor, the area of cross section and therefore volume is inversely proportional to resistance, and if the volume of conductor material required in a two-wire system is v units,v/2 in each wire, the volume of each outer in the 3-wire system=((v/2)/4)=v/8 and volume of middle wire=v/16.
Disadvantages of 3-wire DC Distribution
- The requirement of additional wire partially reduces the safety and necessity of a balancer set causing an increase in cost.
- In an overhead system, cross-arms of greater length are required.