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I understand that we use SCUC (Security Constrained Unit Commitment) and SCED (Security Constrained Economic Dispatch) in day ahead electricity market – According to the literature, SCUC is used to take commitment decisions and SCED is used for dispatching (calculating MWs) energy.

I want to understand why do we need SCED when SCUC already provide commitment decisions and dispatch schedules? Why do we ignore dispatch schedules already calculated by SCUC and reculate them using SCED?

Edit 1: In PJM market, the day-ahead wholesale process consist of SCUC and SCED. Same offers/bids and fixed demand requirements are used in both SCUC and SCED (first execution of SCED, upcoming SCED may have different demand). As the demand remain the same, the dispatch from SCUC and first SCED should also be the same.

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  • $\begingroup$ One needs to compute both commitment decisions and prices, i.e., dual information. Unfortunately, the latter is not well-defined for mixed-integer problems like SCUC. Thus, one way to obtain prices is to solve the SCUC, fix the commitment decisions, and solve the resulting LP to get the power dispatch (primal variables) and prices (dual variables) --> that's essentially your day-ahead SCED. $\endgroup$
    – mtanneau
    Apr 28, 2021 at 13:18
  • $\begingroup$ Thank you. This is the 2nd important information I was looking for. I theoretically agree with your derivation of SCED from SCUC, but SCED still remain a proper LP problem, hence, dual information is well-defined. However, some markets in Europe use a fixed-MIP (created by fixing all binary variables of original MIP) to calculate prices -- do you see any limitations or corner cases with this approach? $\endgroup$
    – Amrit Gill
    Apr 28, 2021 at 21:45
  • $\begingroup$ What I meant is that SCED is essentially a SCUC where all the commitment variables have been fixed (what you referred to as "fixed-MIP"). From a theoretical/market perspective, there are pros and cons. See for instance Section 2.2 of this paper. $\endgroup$
    – mtanneau
    Apr 29, 2021 at 12:29

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With my best understanding (mostly from the practice of MISO), the goal of Day-ahead SCUC is to provide commitment decisions for the next day while the commitment decisions will also guarantee there are enough "average" power/energy with sufficient ramping for each of the considered discretized time-window. (See [1] for the manual reference)

In other words, we can also say SCUC is not perfect or ideal because:

  1. it's considering a moving "average" in general "sum" but not every couple of minutes at each generator & bus level -> you may miss the "peaks" (aka worse case situations like temporarily loss of winds for parts of your grid in 10 minutes, then back in another 10 minutes),
  2. checking all possible contingencies are difficult due to computational complexity -> in practice not all transmission constraints/limits & contingency limits are implemented -> in practice you can still miss some violations/contingencies,
  3. Same with 2), the transmission network may be omitted/aggregated/reduced to trade fidelity & accuracy for speed,
  4. there are usually no explicit models on reactive power there -> it's at most a DC solution but not an AC solution -> there is no guarantee on whether there will be voltage collapse plus unsure what are the reactive reserves needed (i.e. you may actually need to correct your decisions to start up more generators due to just the need to provide reactive reserves but not active reserves.)

To tackle at least some of the issues above, individual Day-Ahead SCED will be run separately, where these SCEDs will also use system state-estimation data to try to model extra detail with the transmission network(s). The idea is that hopefully these Day-Ahead SCEDs will capture some of these violations early on in the Day-Ahead market. In practice, we still have to run Future/Intra-Day/Look-Ahead Reliability Assessment Commitment Process (FRAC/Intra-Day RAC/LAC) to adjust many of the issues (e.g. a wrong prediction), plus the real-time market to correct the final bits.

Theoretically speaking I do agree we can always defined one super large Day-Ahead model merging the existing SCUC and all subsequent SCEDs together mathematically in full details (AC, all network constraints, all contingency constraints, etc), and there are research work putting AC equations on SCUC too. Computationally speaking, due to its mixed-integer complexities (and AC non linearity), decomposition methods are still widely used and the current industry seems fond of the current SCUC and SCED(s) decomposed architecture. Of course, it is forever evolving (see [2])

References:

[1] BPM 002. https://www.misoenergy.org/legal/business-practice-manuals/

[2] https://www.ferc.gov/sites/default/files/2020-06/T3-1_Pan_et_al_0.pdf

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  • $\begingroup$ Thank you very much. This is the information I was looking for. All is clear, nothing to add. $\endgroup$
    – Amrit Gill
    Apr 28, 2021 at 21:38
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This is because SCUC provided on/off + dispatch Sometimes when you are at time t the demand is not the same as the predicted value and you need to adjust the P values (not on/off since they can't be changed) then you will run a SCED to make sure everything is fine

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  • $\begingroup$ Thank you @Optimization team for your answer. I understand the use-case for SCED in case it is run in response to an update in demand. However, some RTOs run a SCED right after their SCUC in day-ahead market using the same demand. Could you please elaborate the use-case of SCED in this specific case? $\endgroup$
    – Amrit Gill
    Apr 28, 2021 at 9:20
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    $\begingroup$ In SCED you may also add DCOPF constraints (which is not easy to do in UC due to binary variables) So if you are sure about the on/off states but the actual dispatch is subject to uncertainty then SCED can be a quick and handy tool for making sure the system will operate securely $\endgroup$
    – Optimization team
    Apr 28, 2021 at 21:50

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