Environmental Engineering Reference
based upon different protection principles may be used on the super-grid network.
The costs and complication of protection rise accordingly and this reflects either
the strategic worth of rapid fault clearance to maintain generator stability or the
importance of the integrity of a particular part of the system. Many transmission
and distribution systems are re-closed automatically after a fault to minimise cus-
tomer outage. The complications of this are dealt with below.
4.9.2 Transmission connected wind farms
A combination of protection complexity and system security is likely to feature in
the debate over how wind farms may be connected. A plain, a switched or a fused
'T' connection may be acceptable at distribution level, but is unlikely to be
acceptable at transmission level. Where distance protection (Weedy et al. , 2012) is
one of the systems in use, it is likely that the protection may mal-operate because,
by 'seeing' to the end of the zone 1 range necessary to protect the backbone power
circuit, the scheme will 'over-reach' into and perhaps beyond the transformer
supplying the wind farm. Thus the backbone system may trip for disturbances
within the wind farm. The following diagrams explain the problem.
Figure 4.13 shows the normal arrangement for a plain feeder protection
scheme. The relays are effectively measuring impedance by relating voltage and
fault current. For faults within a small impedance distance (say 80% of the distance
to substation 2 - to prevent over-reaching into transformers at substation 2) the
fault is cleared quickly by zone 1 protection which has no added time delay. Zone 2
detects faults beyond that range and introduces a time delay. Often faults in the last
20% of the distance leading to substation 2 are detected as a reverse flow at 2 and
an acceleration signal is sent to 1 to allow a zone 1 trip time to occur.
Distance protection scheme