Page 64 - Mathematical Models and Algorithms for Power System Optimization

54 Chapter 3

3.2.2 Principles to Set Priority for Unit Maintenance

Determining the priority sequence of unit maintenance is the key to shorten the GMS
calculation time. Without a priority sequence, the arrangement order of GMS with N generator
units will have too many possible options (up to N!). For example, for a system with only a
dozen units, the number of options would be 12! It is a large astronomical number. It is
impossible to evaluate each option individually to choose the best scheme within an acceptable
computation time. Fortunately, for a practical system, most of the options (close to N!) are
unreasonable and unnecessary to be evaluated. Thus, before the GMS calculation, it is feasible
and very necessary to preset a priority sequence for the unit maintenance in accordance with the
system’s reliability, economy, and rationality.
The general principles for setting the priority of unit maintenance are:

(1) Principle of handling production expenses. Units of high production expenses should have
higher priorities of maintenance for the sake of economy.
(2) Principle of handling unit capacity. Units of larger capacity exert a greater impact on the
system’s reliability, so from the perspective of system reliability, a unit with a large
capacity should have a higher priority for maintenance, especially for units with the same
production expenses.
(3) Principle of handling the last maintenance completion time. Normally, the maintenance
sequence of units in the same power plant should not be easily rearranged from a
rationality point of view. Thus, in the same power plant, units whose last maintenance
completion time was earlier should have higher maintenance priorities than those whose
last maintenance completion time was later.

The maintenance priority of the unit can be determined based on the previously mentioned
comprehensive principles or manually specified by the grid dispatcher in reference to these
principles in terms of the difficulty of the unit’s maintenance schedule, that is, those with a long
maintenance time, large unit capacity, and difficult arrangements can be given priority. In
addition, under abnormal conditions, the unit maintenance priority should also be able to
change. For example, in the case of a serious fault, the broken-down units need higher priorities
for maintenance in the GMS.

3.2.3 Way of Processing the Objective Function

There are two processing methods for GMS to select an objective function:

(1) For systems with high load level and small system reserve, GMS can select the maximum
reserve margin of maintenance as the objective function.
(2) For systems with low load level and large system reserve, GMS can select the minimum
production and maintenance expenses as the objective function.