Page 76 - Mathematical Models and Algorithms for Power System Optimization

66 Chapter 3

P N P N
In Fig. 3.8, T i¼1 P Gi and i¼1 P EL (t) are two fixed curves in the whole period T. The area
enclosed by the two curves has constituted the total maintenance reserve area of the system,
which is a constant. If a GMS is feasible, then the maintenance area of a scheduled unit must be
less than this area. Otherwise, the system may not meet the load demand, and such a GMS is
hazardous to the system, that is, the system has no ability to fulfill the GMS. The introduction of
an operation index can quantify the system’s GMS ability, so that different GMS control
strategies can be selected in a targeted manner.

(1) Operation index OPindex>1.0. This indicates that the capacity of units scheduled for
maintenance is greater than allowable in the studied time interval. Under such a condition,
not all units scheduled for maintenance can be arranged in the studied time interval.
Possible control process would be:
Step 1: Where possible (judged by the experts), move some to-be-maintained units out of
the studied period, then recalculate the operation index.
Step 2: If the feasible movement in Step 1 still fails to make the operation index less than
1.0, then there is only one way to achieve this, which is to decrease the partial forecast
loads of the system.
When OPindex>1.0, obviously, the generation output cannot meet the load demand, thus,
it is necessary to delay maintenance (if possible) or curtail the loads.
(2) Operation index OPindex<1.0. This indicates that the system allowable maintenance
margin is greater than the scheduled one. This is the essential prerequisite condition to
make the GMS calculation. Under such a condition, the GMS is feasible if other system
constraints can be met at the same time period. There are many constraints for the system
to complete GMS. Based upon the expert’s experience, the main constraints are:
1. Maintenance window constraint of units.
2. Maintenance time nonoverlapping constraint of units.
The two constraints play a significant role in completing the GMS, which raises requests
for the system to make the GMS respectively from two aspects:
1. Maintenance window constraint reflects the running state whether generating units
shall be maintained, thus, this constraint can be satisfied as long as there is adequate
maintenance reserve capacity in the system.
2. Maintenance time nonoverlapping constraint reflects whether generating units are
technically qualified for maintenance (such as site, manpower, equipment etc.),
therefore, this constraint may not be satisfied even if there is adequate maintenance
reserve capacity. The following are two unsatisfied situations:
a. For all generating units that participated in the inspection of concurrent
maintenance time nonoverlapping constraints, if the sums of their maintenance
time and minimum intervals between maintenance times is greater than the total