Increasing amounts of renewable generation have changed the nature of the electrical grid. A “green” grid will by nature be subject to greater unexpected changes in generation and an important number of those will be significant enough to undermine the reliability of the grid. For this reason, grid operators have become interested in “flexible resources” which can either add or subtract energy quickly when actual generation is more or less than that which was scheduled.
A flexible resource is defined by its desired operating characteristics. The California ISO identifies these characteristics:
• Sustain upward or downward ramp;
• Respond for a defined period of time;
• Change ramp directions quickly;
• Store energy or modify use;
• React quickly and meet expected operating levels;
• Start with short notice from a zero or low-electricity operating level;
• Start and stop multiple times per day; and
• Accurately forecast operating capability.
The problem is that the resources currently available have only one or two of these operating characteristic. A Combined Cycle (CC) plant, for example, has good ramp rates, that is, it can provide large amounts of generation up or down. However, a CC plant cannot feasibly start and stop several times a day. A Simple Cycle (SC) peaking plant does have that ability but does not provide the same ramping capability. Neither plant can react quickly to unexpected changes, that is, within the time frame of importance in the current operating environment.
Although a single resource cannot provide the desired flexibility, a mix of resources working together under the management control of UniGen can.
Consider a mix of resources that includes a wind farm, a SC natural gas plant, and an energy storage system. Individually, they are not truly flexible as defined by the California ISO. But working together under the management of UniGen, the composite plant exhibits every one of the characteristics in the California ISO's definition.
“However, for reliable operation, adequate amounts of system flexibility are required to accommodate large amounts of variable generation. Without this flexibility, the penetration of variable generation may be limited in order to ensure the reliability of the bulk power system. Therefore, planning and design processes will need to change, depending on basic system characteristics, to provide the flexibility needed to meet targeted levels of variable generation.” *