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Comparative Assessment of Look-Ahead Economic Dispatch and Ramp Products for Grid Flexibility

Qian Zhang, Le Xie, Long Zhao, Congcong Wang

TL;DR

This work addresses how look-ahead economic dispatch (LAED) and ramp products (RP) perform under high renewable penetration and rolling-window dispatch. It develops linear programming models for multi-interval LAED and RP co-optimization, and proves that an enhanced RP formulation can match LAED's feasible region at a single time step when intertemporal deliverability constraints are enforced. However, equivalence generally breaks under rolling-window operation because the two approaches optimize different intertemporal objectives and lead to different end-of-window states. Case studies on 2- and 10-generator systems show LAED typically achieves similar or lower load shedding than RP for the same look-ahead horizon, highlighting limitations of current RP designs and suggesting enhancements such as mid-duration RPs to improve performance.

Abstract

High renewable penetration increases the frequency and magnitude of net-load ramps, stressing real-time flexibility. Two commonly deployed remedies are look-ahead economic dispatch (LAED) and ramp products (RPs), yet their operational equivalence under the industry-standard rolling-window dispatch implementation is not well understood. This paper develops linear optimization models for multi-interval LAED and RP-based co-optimization, and proves that an enhanced RP formulation can match LAED's dispatch feasible region at a single time step when additional intertemporal deliverability constraints are enforced. We then show that this equivalence does not generally persist under rolling-window operation because LAED and RP formulations optimize different intertemporal objectives, leading to divergent end-of-window states. Using different test systems under stressed ramping conditions and multiple load levels, we show LAED achieves similar or lower load shedding than RP implementations with the same look-ahead horizon, with the most pronounced differences under high-load, ramp-limited conditions. The study highlights the limitations of current ramp product implementations and suggests enhancements, such as introducing more mid-duration RPs.

Comparative Assessment of Look-Ahead Economic Dispatch and Ramp Products for Grid Flexibility

TL;DR

This work addresses how look-ahead economic dispatch (LAED) and ramp products (RP) perform under high renewable penetration and rolling-window dispatch. It develops linear programming models for multi-interval LAED and RP co-optimization, and proves that an enhanced RP formulation can match LAED's feasible region at a single time step when intertemporal deliverability constraints are enforced. However, equivalence generally breaks under rolling-window operation because the two approaches optimize different intertemporal objectives and lead to different end-of-window states. Case studies on 2- and 10-generator systems show LAED typically achieves similar or lower load shedding than RP for the same look-ahead horizon, highlighting limitations of current RP designs and suggesting enhancements such as mid-duration RPs to improve performance.

Abstract

High renewable penetration increases the frequency and magnitude of net-load ramps, stressing real-time flexibility. Two commonly deployed remedies are look-ahead economic dispatch (LAED) and ramp products (RPs), yet their operational equivalence under the industry-standard rolling-window dispatch implementation is not well understood. This paper develops linear optimization models for multi-interval LAED and RP-based co-optimization, and proves that an enhanced RP formulation can match LAED's dispatch feasible region at a single time step when additional intertemporal deliverability constraints are enforced. We then show that this equivalence does not generally persist under rolling-window operation because LAED and RP formulations optimize different intertemporal objectives, leading to divergent end-of-window states. Using different test systems under stressed ramping conditions and multiple load levels, we show LAED achieves similar or lower load shedding than RP implementations with the same look-ahead horizon, with the most pronounced differences under high-load, ramp-limited conditions. The study highlights the limitations of current ramp product implementations and suggests enhancements, such as introducing more mid-duration RPs.
Paper Structure (18 sections, 2 theorems, 10 equations, 7 figures, 3 tables)

This paper contains 18 sections, 2 theorems, 10 equations, 7 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Modeling of LAED and Ramp Product
  3. System Description
  4. Operational Security Loss
  5. Look-Ahead Economic Dispatch
  6. Ramp Product for Variation
  7. Missing Capabilities in Ramp Products
  8. The Limits of Incomplete Ramp Products
  9. Lack of Ramp Increment Control
  10. Lack of Rolling Difference Enforcement
  11. Enhanced Ramp Products
  12. Rolling Window Dispatch
  13. Feasibility at a Single Time Step
  14. Feasibility in Rolling Windows
  15. Case Study
  16. ...and 3 more sections

Key Result

Proposition 1

[Equivalence of Feasible Regions at a Single Time Step] The feasible region of the enhanced ramp products rp3 is equivalent to that of the LAED laed with the same look-ahead horizon, provided that the initial generation dispatch values satisfy $g_i(t-1)$ being identical in both formulations for all

Figures (7)

  • Figure 1: Illustration of missing ramp increment control in ramp products
  • Figure 2: Illustration of missing rolling difference enforcement in ramp products
  • Figure 3: The rolling-window dispatch with window size $W=2$
  • Figure 4: The comparison between the system with only a 10-min ramp product and the system with the 10-min ahead LAED (Scenario 1)
  • Figure 5: The comparison between the system with only 10-min ramp product and the system with 10-min ahead LAED (Scenario 2)
  • ...and 2 more figures

Theorems & Definitions (6)

  • Definition 1
  • Definition 2
  • Proposition 1
  • proof
  • Proposition 2
  • proof