Environmental Engineering Reference
In-Depth Information
Circuit
breaker
opens
rapidly
Circuit
breaker
opens
later
Figure 4.17
Reverse flow protection
4.9.3 Distribution connected wind farms
Many distribution systems are operated as closed loop or ring networks. Such
systems are protected using directional over-current and earth fault schemes with
plain over-current and earth-fault backup. The relays are time graded from the
source substation, and any attempt to introduce a significant alternative source
around the ring would result in inappropriate tripping for a fault. A simple example
demonstrates the principle.
As can be seen in Figure 4.18, both the over-current and directional over-cur-
rent protection is time graded from the source. Suppose a fault occurs between
substations 3 and 4. Energy flows from the source in both clockwise and anti-
clockwise paths. The first relay to activate for anti-clockwise current is the 0.5 s
relay at substation 3. The first relay to operate for clockwise current is the 1.3 s relay
at substation 4. Hence the line from 3 to 4 is removed from service and substation
3 remains supplied from 2 while substation 4 is supplied only from the source.
All customers connected remain supplied.
If a new source is introduced, say at 3, then energy flowing clockwise from this
source may cause operation of the 0.1 s relay at substation 1, hence removing
customers at substations 1, 2 and 3 from service. There seems little alternative with
present commercially available protective equipment but to replace the scheme
with either a distance or unit protection scheme.
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