3.2 Current Distribution Planning and Operational Procedures

3.2.2 Secondary Network Distribution System Design

In many cities, industrial parks, and other dense collections of customer loads, the distribution system can be networked with multiple sources of supply operating in parallel.  These networks are on the customer side of distribution transformers and are therefore often referred to as secondary networks. These secondary networks provide high reliability to customers since any fault with any one supply is automatically isolated by network protectors and power can still be supplied from the other sources.
In secondary networks, the network protector devices are generally operated to allow power to flow only one way, since their purpose is to detect reverse power flows that could be caused by faults. However, this poses a problem for installing DER systems at customer sites that are connected to secondary networks, since these DER systems could cause backflow through a network protector, causing it to trip since it thinks it is seeing a fault.  Some studies have been done to determine how much DER generation should be allowed for different load conditions of secondary networks, but there are no specific answers, other than that the more DER as a percent of load, the more likely that network protectors will trip off. FERC’s Small Generator Interconnect Process (SGIP) ( effective August 26, 2006) states that on a spot secondary network, generation shall not exceed the smaller of 5 % of a spot network's maximum load or 50 kW. However for grid secondary networks, jurisdictions have many different values or criteria for determining the permitted amount of DER generation.
Some newer network protectors are being designed to try to distinguish between DER generation and actual faults, but this is still a work in progress.