Environmental Engineering Reference
In-Depth Information
The grid
Fault current
from grid
Circuit
breaker
Additional fault current
from generator
Generator
Fault
Load
Figure 6.6
Increased fault current through a circuit breaker
In some cases, a proposed generator will make a relatively small contribution to the fault
level but this could be enough to increase the existing fault level above the maximum allow-
able for a particular switchgear component. This component may be serving many other
customers and be large relative to the generator. Upgrading this component could easily cost
far in excess of the generator itself. A fault level increase is often the limiting factor for
installation of embedded generation in urban areas where the lines are short and impedances
low.
The good news is that the large majority of distributed renewable energy sources are not
interfaced to the network through a directly connected synchronous generator but through a
power electronic converter of the PWM type. Such converters consist of very fast switching
elements and generally contribute negligible overcurrents during faults. In most cases there
will be no need to upgrade the local circuit breakers.
Conversely, care must be taken to ensure that the inability of power electronic converters
to provide fault current does not confl ict with a protection system design that might rely for
example on high currents to blow fuses.
6.5 Islanding
6.5.1 Introduction
A generator is said to be
islanded
if it continues to supply a local load after being discon-
nected from the main network. On a larger scale, an island can include many generators and
many loads.
