Environmental Engineering Reference
In-Depth Information
If islanding is to be permitted, then the system must be designed to be capable of operating
both grid-connected and standalone. Generally speaking, the standalone mode is the more
diffi cult to engineer since it must include generation/load matching in order to ensure that
voltage and frequency are kept between acceptable limits. Generators that are designed to
operate both grid-connected and standalone are often called
standby
generators and are
installed primarily to provide security of supply to particularly important loads, such as hos-
pitals and computer centres. Standby generators are usually powered from diesel or gas.
Systems employing renewables are rarely designed to operate both grid-connected and
standalone. The extra cost of properly engineering a standalone mode is unlikely to be justi-
fi ed unless grid power cuts are very frequent.
If a generator is not designed to operate in a standalone mode, it must be capable of shut-
ting down immediately when the grid connection is lost, otherwise the system may become
dangerous for the following reasons:
The island may not be properly earthed and may present an electric shock hazard to
personnel.
A system that appears to be disconnected from the mains but is in fact still live also presents
a shock hazard.
The voltage and/or frequency may deviate from the normal range and this may damage
other equipment connected within the island.
The island's frequency will quickly deviate from that of the main network and if the two
networks are reconnected under these circumstances, equipment (particularly synchronous
machines) will probably be damaged.
The fault level within the island will be reduced and, if a fault were to develop, the result-
ing current might be insuffi cient to operate protective devices such as circuit breakers and
fuses decisively.
6.5.2 Loss - of - mains Protection for Rotating Machines
Satisfactory
loss - of - mains
detection is surprisingly diffi cult to achieve with rotating machines.
The detection must be reliable, fast and stable and not prone to wrong tripping. The require-
ment that it must be fast stems from the widespread use of
auto - reclose circuit breakers
in
distribution systems and from the fact that these are not normally fi tted with synchronization
checking. Auto-reclosers are designed to restore a supply very quickly after a transient fault.
Most faults in distribution systems (upwards of 80%) are transient and thus reclosers play a
very important role in reducing customer-minutes lost. Reclose times in distribution systems
can be as short as 1 second, which is good for consumers but also implies that loss-of-mains
protection must be designed to operate well within 1 second.
For small induction generators, under/overvoltage and under/overfrequency protection is
considered adequate protection against loss-of-mains (in the UK). For synchronous generators
and large induction generators the preferred loss-of-mains detection techniques are:
rate of change of frequency (ROCOF) and
vector shift.
