Environmental Engineering Reference
In-Depth Information
So, if there is an initial power imbalance, D P , then the accelerating torque, D T ,
is obtained as the total system inertia multiplied by the angular acceleration, a .
Consequently, at rotational speed w , an expression for a can be obtained as
d w
dt ¼ D T
D P =w
2 E system =w 0
rad = s 2
a ¼
I system ¼
where E system encompasses the total stored energy of all generators and loads
connected to the system at the nominal rotational speed w 0 .If f is defined as the
system frequency, and it is assumed that ww 0 , then the initial ROCOF can be
readily calculated as
df
dt ¼ D P f 0
Hz = s
ð 5 : 1 Þ
2 E system
Likewise, the inertial response, D P gen , of an individual unit can be determined
in relation to its stored energy, E gen ,as
D P gen ¼ 2 E gen
df
dt ¼ 2 H gen S max
df
dt
f 0
MW
ð 5 : 2 Þ
f 0
where S max represents the generator rating.
Clearly, the larger the power system, the greater will be the system stored energy
relative to the credible loss of generation, D P . Hence, the fall in frequency will be
slower, giving generators more time to respond. It follows that the loss of a generator
in a small, synchronously isolated power system, e.g. the islands of Ireland or Crete,
will be more significant than for a large interconnected power system, e.g. mainland
Europe. For small power systems, the initial ROCOF will tend to be higher, requiring
highly responsive generating units, perhaps less than 5 seconds. This burden can be
particularly onerous during periods of low demand when the system inertia is less.
The spinning reserve duty is then spread across fewer units, and individual generators
are likely to provide a greater proportion of the total system load. In contrast, for a
relatively large power system, such as the Great Britain system, the most significant
scenario normally considered was not the loss of one generator, but two (or the
connecting transmission line), namely the 1,320 MW (2 660 MW) nuclear power
station of Sizewell B. In April 2014 this infrequent infeed loss risk was increased to
1,800 MW, while within the European continental system, 3,000 MW of regulating
reserve is required within 30 seconds following major contingencies.
5.2.2.2 System restoration
Following a loss of generation event, or other major power imbalance, the system
inertial response will determine the initial rate of fall of frequency and the max-
imum response time required of operational generating units. The subsequent
actions that a particular power system operator undertakes will depend on the
resources and facilities available, but are likely to include the following options:
Currently operational thermal units will increase their steady-state output.
Quick-start units, e.g. OCGTs, will be brought online.
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