Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 146

generators connected to the distribution grid were tripped (by protection),
soon followed by the larger-scale generation. Despite implementing automatic
protection measures in the form of pumped storage shedding and load shedding
(described later), the frequency had fallen below 47.5 Hz by 3.28 am, and a major
blackout of Italy was inevitable. Over 50 million people were affected, with some
consumers off supply for up to 18 hours.
There are many reasons why a generator may be tripped from the system,
and it is sufficiently common that protection plans should be designed to mini-
mise the effect of such a disturbance (Armor, 2003). When units are being
committed to a power system consideration must, therefore, be given not only to
the predicted demand pattern, but also to the impact of a generator trip. Loss of
the largest source of supply (infeed) to the system will generally, but not always,
be of greatest concern. At best, it may take several minutes before fast-start units,
typically OCGTs, can be brought online to make up the shortfall in generation.
Since frequency (and hence system) collapse may occur in seconds, there must
be the ability within the existing unit commitment for a number of units to
increase their outputs rapidly to curtail the initial fall in frequency. The so-called
spinning reserve requirement is expensive, since it implies that extra units are
committed to the system, and it results in a non-economical sharing of system
load. Intuitively, the spinning reserve available for a particular commitment of
generators is the difference between the sum of the power ratings of all operating
units and their actual loadings. So, if a unit is operating x MW below its upper
limit, then it should be able to provide x MW of spinning reserve. As Figure 5.6
illustrates, the actual emergency reserve available from a thermal generating unit
is less than this.
Due to time delays present in the power station boiler, mainly the reheating of
steam between the high and intermediate pressure stages of turbine steam expansion,
the immediate reserve available will be reduced. It is worth noting, however, that the
addition of the reheater improves the thermal efficiency of the plant significantly.
Assuming that a generating unit has a governor droop of 4 per cent, then a 1 per cent
fall in frequency would require a (1/4) 100 ¼ 25 per cent short-term increase in

Practical

Ideal

Boiler limit

45°

P min P max

Electrical power (MW)

Figure 5.6 Thermal unit spinning reserve characteristic