DISTRIBUTION LINE LOSS

DISTRIBUTION LINE LOSS

There are two types of power distribution that electric utilities used are known, overhead power distribution and underground power distribution which utilizes power cables and cabinet substations. These two types of distribution methods possess similar and unique characteristics with respect to each other. Overhead lines uses bare wires while underground lines are securely insulated.

In evaluating losses on distribution primary lines, a complex and detailed approach is usually necessary since in a power distribution system a great variety of possible line configurations and load conditions exist. We have the familiar distribution set up like a radial, looped and interconnected configuration.

It is important for a utility that they are able to evaluate their line losses since through this they can be able to base their decisions regarding loss reduction that can produce significant savings when considered along with other system improvement and plans and new construction.

Like any other line loss calculations, distribution line loss follows the I squared R law, so when a utility decided to consider reducing their line losses all they have to do is first reduce the flow of current in the conductors. So, in a constant power distribution, when we refer to the power formula = Voltage x Current, in order for electric utilities to reduce the amount of current they can operate their lines at the highest voltage practical under the circumstances.

“As the line voltage level in a power distribution INCREASES, the amount of current and lines losses DECREASES”

To illustrate the effect of voltage conversion to power distribution line losses refer to the sample problem from TVPPA Distribution System Line Reduction Manual below.

A single-phase 7.2kV distribution line having a total circuit resistance of 1 ohm carries a load of 720kVA. The distributor re-insulates this line without replacing conductors and connects it to a 14.4kV source. What are the line losses before and after the conversion?

SOLUTION 1

Before conversion, the line current is:
(720kVA) / (7.2kV) = 100 amperes
Losses = I squared R = (100 amperes)^2  x 1 ohm = 10,000 watts = 10kW

SOLUTION 2

After conversion, the line current is:
(720kVA) / (14.4kV) = 50 amperes
Losses = I squared R = (50 amperes)^2 x 1 ohm = 2,500 watts = 2.5kW

The voltage conversion would result in a 75% loss reduction. This example is representative of the level of loss reduction that can be obtained with voltage conversion.

The analysis of distribution line losses is not far from the transmission line approach except corona and skin effect which is said to be negligible in voltage levels that of with distribution system. A distribution system is however more complicated since the network in a distribution system is more complex because of the complexity of the loads connected to it and the possible configurations that can be done.