Power Delivery Laws

The complex interaction of a power delivery system is governed by a number of physical laws relating to the natural phenomena that have been harnessed to produce and move electric power. These interactions have created a number of “truths” that dominate the design of delivery systems:

• It is more economical to move power at high voltage. The higher the voltage, the lower the cost per kilowatt, to move power any distance.

• The higher the voltage, the greater the capacity and the greater the cost of otherwise similar equipment. Thus, high voltage lines, while potentially economical, costs a great deal more than low voltage lines, but have a much greater capacity. They are only economical in practice if they can be used to move a lot of power in one block.

• Utilization voltage is useless for transmission of power. Transmitting power at utilization voltage level results in unacceptably high electrical losses, severe voltage drops, and astronomical equipment cost.

• It is costly to change voltage level. Voltage transformation is a major expense, which does nothing to move the power any distance in and itself.

• Power is more economical to produce at very large amounts. Large generators produce power more economically than small ones., thus it is most efficient to produce power at a few locations utilizing large generators.

• Power must be delivered in relatively small quantities at low voltage level.

Source: Power Distribution Planning Reference Book, H. Lee Willis

The Delivery System's Mission

A delivery system’s primary mission is to deliver electric power to the consumers at their place of consumption and in read-to-use form. The system must deliver power to the customers, which means it must be dispersed throughout the utility franchise in uneven proportions.

There are a bulk of customers in central downtown area of a city but are scattered as it goes to the outskirts. The primary requirement for the power delivery system is to reach every customer through an electrical path, sufficient enough to satisfy the customer’s demand.

The electrical path reaching the customers is not enough; it must be a reliable one. A reliable path provides an uninterrupted flow of power to the customers. A reliable delivery system delivers all of the power demanded, not just a part of what is needed, in a sustained manner. A 99.9% reliability of service means nearly 9 hours of electric service interruption each year, in percentage it’s impressive but the truth it is not. And it is unacceptable to nearly all first world countries.

Aside from providing the customers with highly reliable power, the delivery system must also make sure that the delivered power is of acceptable quality. Mostly, the quality of the power delivered can be measured through the state of the voltage of the electricity being delivered. Delivery voltage range is about ten percent of the utilization voltage. Voltage fluctuations have different and mostly damaging effect on the appliances thus a ten percent voltage fluctuation from the utilization voltage is unacceptable. If the fluctuation is too great or happens too often, then it is a sign of poor quality.