SCED Limit Calculation Matthew Young/Jian Chen 9/15/2017 Problem
SCED Limit Calculation Matthew Young/Jian Chen 9/15/2017 Problem Overview When the MW limit for a transmission constraint is calculated, there is a possibility for abnormally low limit results if the flow over the constrained equipment is very small. Resulting in the constraint binding at an unrealistic MW limit. PUBLIC 2 Current SCED Limit Calculation (Constant Power Factor) The SCED MW limit of the active constraint is calculated by EMS. The real and reactive flows are used to calculate the current power factor.
The power factor is presumed to remain constant from one SCED run to the next. The power factor is then applied to the MVA limit to get the MW limit. PUBLIC 3 Existing problem When calculating the PF, if P << Q, the resulting MW limit can be very low in relation to the actual equipment rating. PUBLIC 4 Impact to SCED As a result, a very low MW limit could be sent to SCED,
causing a high shadow price in SCED dispatch. SZEPCMN8: HLD_FMR1 on 5/17/2017 PUBLIC 5 Review of similar events Since Jan.1 2012, there have been 11 similar events, when MW limits dropped below 20% of MVA ratings and caused high shadow prices (>$1000) in SCED. Date 6/14/2012 8/23/2012 12/9/2012 12/28/2012 2/21/2013 5/19/2015
5/3/2017 5/14/2017 5/17/2017 5/28/2017 7/21/2017 PUBLIC Contingency XLOB58 XLOB58 XVE2R89 BASE CASE SAJORIO5 SCRDLOF9 SKLELOY8 SSCLWF28 SZEPCMN8 XFTS89 XFTS89
Binding with Low Limit 2 1 1 1 8 1 2 1 1 1 1 6 Alternative Method One alternative method to calculate the SCED MW limit is based on the assumption that the change in reactive power through the
constrained equipment will be small and can be ignored. As shown in the above equation, this method would run into problems when reactive flow is higher than equipment rating. To prevent this from happening, a hybrid-method is suggested: PUBLIC 7 Method Comparisons PUBLIC 8 Method Comparisons
PUBLIC 9 Method Comparisons Test with all active constraints from July, 2017. Total of 16,718 SCED constraint intervals are tested. MW limit/MVA rating 80%~100% PUBLIC Constant Power Factor Hybrid PF/MVAR 16623
16718 60~80% 49 0 40~60% 23 0 20~40% 12 0
0~20% 11 0 10 Method Comparisons PUBLIC 11 Conclusion Constant Power Factor Method Unrealistic MW limits could be sent to SCED when the power factor of a constraint is
very low. Based off of historical data, there were few of these events since Jan.1, 2012, with minimal impact to the market. Hybrid PF/MVAR Method Could present the opportunity for some MVA limit overshoot. While a possible solution to unrealistic MW limits, would affect all constraints going forward. PUBLIC 12
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