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Unintended Consequences

Does anyone care about rising redispatch costs?

Fortnightly Magazine - February 2007

would cause the system to become unstable, causing cascading blackouts (within less than a minute of the outage), something must be done in advance of the outage to ensure the system would not become unstable should the outage occur. For this example, we will assume that such an unstable condition would not result from any outage contingency.

Even without a system-stability concern, system operators still consider whether the loss of a transmission line might cause a problem ( e.g., a thermal overload of remaining transmission lines). As can be seen from this example, with both generators 1 and 2 operating at full capacity to serve the load, there appears to be adequate transmission to move the 500 MW of power from these two units to the load, since 800 MW of thermal capacity is on the cut path between the two busses. Of course, the system operator will want to run a load flow to assure that no single line gets overloaded. Assume we have run that load flow and verified that none of the 5 transmission lines overload.

Next, assume we want to check the loading on the lines if the 280-MW line 1 trips off. As can be seen, even without line 1, there still is 520 MW of thermal capacity on the cutplane between the two busses. However, if we run the load flow, we might discover that the loading on the lines does not distribute according to the capacity of the lines. We might find that the 20-MW line 5 in fact loads to 25 MW. This exceeds its thermal limit.

Is this a problem that would prohibit us from performing the economic dispatch of running generators 1 through 4 to meet the load on this hour? Recall that while we are studying the possibility that the 280-MW line 1 trips off, we have no indication that such a trip will occur. In fact, historic data suggests that in all probability, the 280-MW line 1will not trip.

The choices available to the grid operator are the following:

1) Because of the possible tripping of the 280-MW line 1 and resulting overload on line 5, redispatch the system to back down generator 1 or 2 and run the more expensive generator unit 5;

2) Allow the economic dispatch of running generators 1 through 4, but open line 5 in the middle so that it becomes two radial lines. This way, if the 280-MW line 1 trips, then line 5 will not overload (assuming we have tested that lines 2, 3, and 4 can handle the 500 MW of transfer if line 1 trips);

3) Allow the economic dispatch of generators 1 through 4, but “arm” generator dropping on generator 2 so that if line 1 trips, relays will disconnect generator 2. We will need to test to be assured that the 250-MW spinning reserve unit will pick up to cover the loss of generator 2 without any resulting instability. Then the more expensive generator 5 could be called into operation;

4) Allow the economic dispatch of generators 1-4,