## 8 Application of CGRR Index in Power System Operating Decisions
### System Operation Using CGRR Criterion  
It is recommended that an off-line CGRR evaluation of the past operating conditions of the system be first carried out for a specified period in order to comprehend the operating risks associated with past operator practice and the system operating performance. The off-line assessment can be used to determine the operating conditions with CGRR values significantly above and below the accepted criterion value. For these operating conditions, the load dispatch and/or the unit commitment should be rescheduled and CGRR evaluated until an acceptable value is obtained at the lowest possible cost. As an example, Figure 3 shows the CGRR of the IEEE-RTS obtained from an off-line evaluation of the different operating conditions as the load is varied from its minimum value to its peak. For example, the operating schedule to satisfy a load of 1995 MW using an N-1 unit commitment and economic load dispatch results in a CGRR of 0.002701, which exceeds the criterion value of 0.002698. It is necessary to re-dispatch the units in such a way that the total regulating margin will increase and reduce the CGRR to an acceptable level. It should be noted that the CGRR is also sensitive to the failure rates of the units carrying the regulating margin. The new load dispatch should be as economical as possible within the risk constraint when the committed units are re-dispatched to meet the CGRR criterion. The potential generating units for rescheduling are grouped into two classes, Class A and B. A Class A unit has a ramping capability greater than its current operating reserve, whereas a Class B unit has an operating reserve greater than its ramping capability. A part of the load from a Class A unit should be transferred to a Class B unit in order to increase the system regulating margin within the response time. This process is carried out in small load steps. The loading from the generator with the highest incremental operating cost in Class A is first transferred to the generator with the lowest incremental operating cost of Class B. After each incremental load transfer, the CGRR is calculated, and the load transfer process is repeated until the desired CGRR value is achieved. Table 7 illustrates the above described operating condition with 13 generating units committed to meet a load of 1995 MW in the IEEE-RTS. The table shows the economic load dispatch (ELD) and the re-dispatch to meet the CGRR criterion. The Class A and B units are also indicated along with the unit ID numbers in the first column. A part of the load from Class A units 7, 11, 12, and 13 were transferred to Class B units 8, 9, and 10 so that the CGRR criterion was satisfied. The resulting CGRR  
Figure 3: CGRR variation with different operating conditions. is 0.0026979 which is within the criterion value of 0.002698. The regulating margins associated with the two dispatches are also shown in the table. It can be seen that an increase in 71 MW of the total regulating margin was required to maintain the acceptable CGRR criterion. The off-line evaluation and results provide useful information to understand the required changes in unit commitment and load dispatch in order to maintain the CGRR at an acceptable level as operating conditions change over time. This creates a knowledge base to implement the CGRR approach in real time, applying predictive evaluation using the proposed techniques. The decision to make the transition to the new probabilistic approach in a continuously operating power system can begin at an operating condition, at which the CGRR value is within the acceptable level. The presented method can then be applied in real-time with repeated calculation of the CGRR by incorporating short-term forecast of system changes. This process is illustrated by the algorithm in Figure 4.  
The next operating decision in an operating condition is initiated by the short-term forecast of system changes within the lead time of the ready to start generating units available in the system.