The transformers of a secondary network distribution system are connected in parallel through a special type of circuit breaker, called a network protector, to a secondary bus. Radial secondary feeders are tapped from the secondary bus to supply loads. A more complex network is a system in which the low- voltage circuits are interconnected in the form of a grid or mesh.
If a primary feeder fails or a fault occurs on a primary feeder or distribution transformer, the other transformers start to feed back through the network protector on the faulted circuit. This reverse power causes the network protector to open and disconnect the faulty supply circuit from the secondary bus. The network protector operates so fast that there is minimal exposure of secondary equipment to the associated voltage drop.
In some instances, a higher degree of reliability can be attained with a primary selective system. Protection against loss of a primary supply can be gained through the use of a primary selective system (fig. 4-4). Each unit substation is connected to two separate primary feeders through switching equipment to provide a normal and an alternate source. When the normal source feeder is out of service for maintenance or a fault, the distribution transformer is switched, either manually or automatically, to the alternate source. An interruption will occur until the load is transferred to the alternate source. Cost is somewhat higher than the radial system because primary cable and switchgear are duplicated.
In laying out a distribution system for a base, you should divide the base into a number of sections. These sections should be chosen so that the load in each section is close to one of the distribution centers. You take this action to keep the length of the mains as short as possible and to keep the voltage drop low between the distribution and the loads. The distribution or load centers should be located as near as possible to the electrical load center.
In the construction and maintenance of Navy power distributions systems, you should be aware of the overhead distribution pole locations and the types of overhead distribution equipment used. An excellent source of information on distribution systems is The Lineman's and Cableman's Handbook.
Your decision on the location of poles is limited because either you will be replacing existing poles or installing additional poles according to NAVFAC
Figure 4-4. - Primary selective distribution system.Continue Reading