Page 23 - Mathematical Models and Algorithms for Power System Optimization

Daily Economic Dispatch Optimization With Pumped Storage Plant 13

difference is 2650MW and the average load is 5244MW. To ensure the security and
overall economic efficiency of the power system operation, some units have to be started
up or shut down. However, even a 20MW conventional coal-fired thermal power unit requires
about 10h to recover from shutdown to full loading operation. Therefore, if this thermal
power unit is used to regulate peak load, the system capability of tracking the load variations
would be very poor.
To adapt to the large load variations from 2000 to 4000MW within the normal upper and lower
output limits of all generation units, some more units shall be started up during the peak load
period yet some units will have to be shut down during valley load period. However, the
economic efficiency of units is often poor because of the startup and shutdown operations.
Moreover, there may be many startup and shutdown combinations with units in different time
periods, making it impossible to guarantee the overall economic efficiency of the units’
operation. Therefore, the peak regulating capacity of the pumped storage plant must be fully
utilized to shift the peak load and reduce the number of startups and shutdowns of thermal
power plants, so as to improve the overall economic efficiency, security, and rationality of the
power system operation.

The load variations of the power system can be represented by the minimum load rate P min /P max
(where P min represents the minimum load and P max the maximum load) and average load rate
P ave /P max (where P ave represents the average load). Generally, the minimum load rate is
expected to be larger than 0.7–0.75, and the average load rate larger than 0.85–0.9. If the peak
load is only regulated by the thermal units, the installed capacity should be more than the
difference between P max and P min . If the peak load is regulated by the pumped storage plant, the
installed capacity just has to meet the difference between P avep and P max (where P avep
represents the average load after putting pumped storage plant into operation), of which P avep
may be slightly larger than P ave . Generally, the installed capacity of the pumped storage plant is
required to be 3%–6% of the total installed capacity in the system, or about one-third of the
peak-valley load difference, then the remaining load variation is easily regulated by
thermal units.
Therefore, when analyzing the economic efficiency of a pumped storage plant, the combination
of the thermal power plants used for base load and the pumped storage plant used for regulation
should be considered, rather than simply using the same peak load regulating capacity to
compare the economic efficiency of the pumped storage plant and thermal power plant.
Generally, the comparison can be made based on the equivalent peak load regulating capacity
by the two methods outlined as follows:

(1) Comparing the generating costs of the equivalent peak load: according to the calculation
of the unit cost of power generation from the pumped storage units and thermal power
peak load regulating units with peak load of 1500h per year in China, the ratio (ratio¼cost
of pumped storage/cost of thermal generation) is 63% and 70%, respectively, in
Guangdong.