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Unintended Consequences

Does anyone care about rising redispatch costs?

Fortnightly Magazine - February 2007

but have operators take action to reduce generation on generator 2 if line 1 trips. We know that the loss of line 1 results in thermal overloads on one or more remaining lines. We also know that thermal overloads on the line mean that the line tends to warm up and “sag” as a result of thermal overloads. We also know that this phenomenon of warming up and “sagging” takes some time ( e.g., 15 minutes before the line sags below acceptable clearance levels). Therefore, if line 1 trips, operators have a little time before the loss of line 1 manifests itself into a problem on the grid. Therefore, well-trained system operators have time (once they get the alarm that line 1 has tripped), to take action to avoid the problem on the grid. For example, the system operator could manually cause generator 2 to reduce its generation, with the make-up power coming from the spinning-reserve unit and then call on the more expensive generator 5 to allow recovery of the spinning reserve;

5) Allow the economic dispatch of generators 1 through 4, but have operators take action to open line 5 in the middle if line 1 trips. As indicated above, if line 1 trips, operators have a little time before the loss of line 1 manifests as a problem on the grid. Therefore, well-trained system operators have time (once they get the alarm that Line 1 has tripped) to take action to avoid the problem. For example, the system operator could open line 5 in the middle; or

6) Allow the economic dispatch of generators 1 through 4, but have operators take action to: (a) reduce generation on Generator 5; and (b) simultaneously drop load to offset the loss of generation. Then immediately call on the more expensive generator 5 to start up to pick up the load that was dropped.

Among these 6 choices, only choice number 1 actually requires us to move to a less than optimal generation dispatch in anticipation of a possible (albeit very low probability) loss of a transmission line.

Are we redispatching unnecessarily in the name of security constrained economic dispatch?

Prior to the formation of ISOs or RTOs, owners of transmission lines generally would not cause uneconomic dispatch prior to the low-probability loss of a line. Instead, these owners reasonably would be expected to choose to use one or more of steps 2 to 6 above as the plan to deal with a potentially problematic, but low-probability, line loss.

With the establishment of ISOs or RTOs, we are finding that redispatch is being done more often to deal with a potentially problematic, but low-probability, line loss. This redispatch is being done, apparently, to demonstrate that the ISO is performing security constrained economic dispatch. This is the transmission system equivalent of “cover your rear,” but it has the unintended consequence of imposing more cost on the transmission system than the simple, common-sense solution illustrated above.

The redispatch often taking place is manifesting itself as “congestion charges.” As a result, the existence of high