Electrical Distribution Systems Explained
The very existence of an AC power system is dependent upon the availability of transformers. Without them it could not operate. In its broadest sense distribution system refers to the manner in which electrical energy is transmitted from the generators to the many points of use. In its more specialized sense it refers to the lines and circuits over which electric energy passes through city streets or rural roads to the ultimate consumer (Figure 1).
The electrical distribution system consists of three major components: geñeration, a high-voltage transmission grid, and a distribution system. The "handoff" from electric transmission to electric distribution usually occurs at the substation. Distribution substations are commonly equipped with remote control and monitoring facilities so that operation of switchgear and auxiliary equipment can be activated remotely. Much control, protection, transformation, and regulation take place in a power distribution system. Figure 2 illustrates the typical stages through which the distribution system must go in delivering power to industrial users.
Single or one-line diagrams are generally used to show the main power distribution for large commercial and industrial installations. This type of diagram is known as a single-line diagram because one single line is used to represent the three-phase conductors, neutral conductor, or grounding conductor. Figure 26-66 shows how Wye and Delta transformer connections are represented on a single-line diagram. Note that each transformer shows whether the primary and secondary are connected as Wye or Delta.
Fig. 1. Typical electrical distribution system.
Energy transmitted at the voltage at which it is used is known as secondary distribution. The system of lines and circuits supplying energy to the secondary distribution system at a higher voltage is the primary distribution system. The connecting link between the primary and secondary distribution system is the "distribution transformer." Distribution voltages are those voltages connected to primaries of these transformers. Utilization voltages are those voltages that are connected to the loads.
Fig. 2. Stages in the delivery of power to an industrial electrical distribution user
A distribution system must be able to meet the needs and requirements of all users from the smallest to the largest safely and economically. To do this, the growing requirements of electricity users must be anticipated well in advance and provisions made to meet these upon their earliest need. Factors which are considered in the design of a distribution system include the following:
- Type of structure: The type of distribution structure used will depend on the type of utilization equipment and usages required by the customer. In most installations power is supplied to the building at utilization voltage and a simple radial system is employed for the distribution of the power.
- Present utilization and future needs: This will involve some degree of load forecasting. The cost of carrying excessive capacity as idle investment is compared with the cost of replacing smaller equipment with larger and other methods of increasing capacity when required.
- Projected life of the structure
- Flexibility of the structure
- Load requirements: These may include maximum demand and time intervals for maximum demand.
- Location of service entrance and load equipment • Switchgear, distribution equipment and panels
- Type of installation methods used
