The most basic and universal means of detecting loss of parallel
operation with the utility is to establish an over/ underfrequency
(81O/U) and over/undervoltage (27/59) “window” within which
the DG is allowed to operate. The sensitive setting of both the
underfrequency and voltage windows prevent the DG from
supporting the power system during emergency conditions. When
the DG is islanded from the utility system, either due to a fault or
other abnormal condition, the frequency and voltage will quickly
move outside the operating window if there is a significant difference
between load and distributed generation levels. If the load and
generator are near a balance at the time of separation, voltage and
frequency may stay within the normal operating window and
under/overfrequency and over/undervoltage tripping may not take
place. If this is a possibility, then a transfer trip (TT) using a reliable
means of communication may be necessary. As discussed in the
previous section of this paper, when induction or synchronous DGs
are islanded with pole-top capacitors and the generator capacity is
near that of the islanded load, a resonant condition that produces a
non-sinusoidal overvoltage can occur. For these cases, an
instantaneous overvoltage relay (59I) that responds to peak
overvoltage needs to be used to detect this situation.
Interconnection protection requirements in the state of California
are defined in a filing to the state utility commission called Rule
21. A key provision of this rule is the unique application of a
directional power relaying (32) to detect loss of utility parallel
operation. This provision is only applicable to DG units that are
installed for peak shaving or load following and do not sell power
back to the utility.