Feeder Booster for Electrical Transmission Lines and Its Functions

A feeder booster is an important piece of equipment for electrical energy transmission lines, especially for long feeders of transmission lines where the voltage dropped a the endpoint due to some technical limitations of transmission lines that are not easy to eliminate as required.

Basically, a booster is a generator whose function is to add to or inject into a circuit, a certain voltage that is sufficient to compensate for the IR drop in the feeders etc. IR drop means the voltage drop due to the resistance (R) of the transmission conductor when flowing current (I) through the conductors.

In an electrical transmission network system, it is not possible to keep the feeder length equal or uniform load distribution for all; in that caseload and voltage becomes unbalanced in the system network. In a d.c. the system, it may sometimes happen that a certain feeder is much longer as compared to others and the power supplied by it is also larger. In that case, the voltage drop in this particular feeder will exceed the allowable drop of 6% from the declared voltage. This can be remedied in two ways:

1. by increasing the cross-section of the feeder conductor, so that its resistance and hence IR drop is decreased;
2. or by increasing the voltage of the station bus bars.

The difficulty for both cases is that increasing the conductor means the cost of transmission lines will increase a few times and it is not economical for the utility companies without increasing the per unit electricity cost; on the other hand the bus-bar voltage increasing is not easy technically for a certain feeder.

The second method is not practicable because it will disturb the voltage of other feeders, whereas the first method will involve a large initial investment towards the cost of increased conductor material.

To avoid all these difficulties, the usual practice is to install a booster in series with this longer feeder as shown in the Figure above. Since it is used for compensating drops in a feeder, it is known as a feeder booster. It is a (series) generator connected in series with the feeder and driven at a constant speed by a shunt-motor working from the bus bars. 

The drop in a feeder is proportional to the load current, hence the voltage injected into the feeder by the booster must also be proportional to the load current, if exact compensation is required. In other words, the booster must work on the straight or linear portion of its voltage characteristic.

Example: 

A 2-wire system has the voltage at the supply end maintained at 500. The line is 3 km long. If the full-load current is 120 A, what must be the booster voltage and output in order that the far end voltage may also be 500 V.

Take the resistance of the cable at the working temperature as 0.5 ohm/kilometre.

Solution: 

Total resistance of the line is = 0.5 × 3 = 1.5 Ω

Full-load drop in the line is = 1.5 × 120 = 180 V

Hence, the terminal potential difference of the booster is 180 V (i.e. 180/120 = 1.5 volt per ampere of line current).

Booster-output = 120 × 180/1000 = 21.6 kW

 A Feeder Booster adds voltage to the feeder and compensates for the voltage drop. Hence it increases the efficiency and reliability of the system. That is why it is one of the important devices in power systems.