The second restoration method (case 2) is used where the DG
roughly matches the local load. In these cases, the
interconnection protection trips the main incoming breaker
(breaker A) as illustrated in Fig. 8. In many cases, the distributed
generation facility may have internal underfrequency load
shedding as is the practice at petro-chemical and pulp and paper
facilities to match the local load to available distributed generation
after the utility separation. To re-synchronize the distributed
generation facility to the utility system, a more sophisticated
synchrocheck relay is required which not only measures phase
angle but also slip frequency and voltage difference between the
utility and distributed generation systems. Typically, such relays
supervise automatic, manual and supervisory reclosing.
Typically, a microprocessor or switching scheme is used to
switch the source of line side voltage to the automatic
synchronizing equipment to VT’s on the utility supply breaker.
This breaker (A) is then used to re-synchronize to the utility
system.
Fig. 8.