PG&E generally has two types of electrical distribution systems: secondary network and radial. In PG&E's territory, only the downtown areas of San Francisco and Oakland are served by secondary network systems. Secondary networks are designed to meet the higher reliability needs and limited space commonly encountered in urban areas. The criteria that PG&E uses to install secondary networks are a function of the density of the load, economics, and a number of other related factors. (See Rule 2
for a complete description of PG&E’s distribution system; there is also a glossary (PDF, 119 KB)
of technical terms in PG&E’s Interconnection Handbook.)
In a secondary network, electricity is delivered through a complex and integrated system of multiple transformers and underground cables that are connected and operate in parallel. Power can flow in either direction on the lower voltage service delivery lines, commonly called secondary distribution lines. The loss of a single line or transformer in a secondary network does not cause an interruption of power, unlike radial systems where there is only one line and one path for power to flow, starting at the distribution substation and terminating at the customer’s service entrance or meter. If a radial line experiences an outage, service is interrupted to customers until repairs are completed; this is less likely to be the case in a secondary network distribution system.
In secondary networks, devices called “network protectors” are used to prevent power from “back-feeding” from one transformer through another. Network protectors are designed to open (that is, break the circuit) quickly when they detect back-feeding. Any power exported by a generator into this system will be detected as back-feeding by the network protectors. Also, as discussed in the “IEEE Guide for the Protection of Network Transformers
” (IEEE Standard C37.108-2002), most network protectors in service have not been designed or tested to operate as switching or isolation devices for operating electric generators. These concerns currently prevent PG&E from allowing installation of net energy generators within areas served by secondary networks.