Environmental Engineering Reference
In-Depth Information
52.0
Generators start tripping
50.5
Upper statutory limit
50.2
Upper operational limit
50.0
49.8
Lower operational limit
49.5
Lower statutory limit
48.8
Load shedding starts
47.0
Load shedding
complete
Figure 3.9
Operational and statutory limits of system frequency for the UK power network. (Source:
National Grid plc)
In conclusion, even if all generation were 100% reliable, a substantial reserve would still
be required because it is simply not possible to predict the demand on a power system
exactly.
3.4.3 Frequency Limits
Although perfect balance between supply and demand is not achievable, the balance has to
be good enough so that under normal operating conditions the frequency does not drift outside
prescribed operational limits.
Figure 3.9 shows that for the UK, statutory limits on permissible frequency variation are
±
1% (95% of week) with absolute limits in emergencies of +4% and
−
6%.
0.4% which are tighter than
the legal requirements. Under unusual circumstances e.g. substantial loss of demand, the
frequency may rise uncontrollably and generating sets are fi tted with overfrequency protec-
tion that will trip them off the system if the frequency reaches 52 Hz. At the other extreme,
due to loss of generation or less frequently, unexpected demand increase, the frequency may
fall below its statutory limit.
Large groups of noncritical loads (i.e. excluding hospitals, emergency services etc.) are
fi tted with staggered under-frequency protection so that switching off such loads, i.e. load
shedding begins at about 48.8 Hz and is completed by 47 Hz. Further drops in frequency could
trigger cascade tripping of power stations. Such cascade tripping incidents are not unknown,
several having taken place in developed countries during the last decade. Complete shutdown
of the electricity supply or blackout is an occasional, but traumatic event for millions of
affected citizens trapped in lifts, underground trains, etc. Full restoration of supply may take
as long as several days.
However, the National Grid's usual operational limits are
±
