12 Operational flexibility
12.1 Typical operational regimes
Power station output, and therefore generation, needs to be regulated constantly to match consumer demands. Economics demand that the base load should be provided by the most efficient plant, which mainly includes nuclear and large-capacity coalfield-sited coal-fired stations.
More expensive, or less efficient, generating plant (small-capacity coal and oil-burning stations) are therefore needed to vary output to match demand. The plant that is required to run intermittently to match demand operates what is known as a two-shift regime, so called because of the requirement to run during the two daytime shifts but to shut down during the night shift. Such plant may also be called upon to shut down over the weekend. It is now common for 500 MW and 660 MW machines to carry out two-shifting operations
in addition to load-following cycles. This requirement for improved flexibility has resulted in all 500 MW and 660 MW turbine-generator units on the UK system being designed to be capable of operating a two-shifting regime, in addition to the requirement for high efficiency base-load operation during early life. Typical operational requirements for modern high merit turbine plant are shown in Table 2.2.
Table 2.2 Typical operational requirements
Hot starts (Two-shifting)
Approx 6 hours shutdown with run-up to full-load in 30 minutes
24-72 hours shutdown
Starts following >120 hours prolonged shutdown
Changes of load in 50- 100% load range. Assuming minimum of 3 hours between cycles.