1 Microgrid Optimal Scheduling Amin Khodaei, Ph.D. University of Denver
Microgrid Definition • By DOE: A group of interconnected loads and Distributed Energy Resources (DERs) with clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid. • Characteristics: • The electrical boundaries must be clearly defined • There must be control systems in place to dispatch DERs in a coordinated fashion and maintain voltage and frequency within acceptable limits • The aggregated installed capacity of DERs and controllable loads must be adequate to reliably supply the critical demand (islanding)
Value Streams • Improved reliability (by islanding and introducing self-healing at the local distribution network) • Higher power quality (by managing local loads) • Reduction in carbon emission (by the diversification of energy sources) • Economic operation (by reducing T&D costs, sellback to utility) • Offering energy efficiency (by responding to real-time market prices, reducing congestion levels ) • Improved resilience (by enabling lasting islanding) • BUT: Most of these benefits cannot be achieved unless the microgrid is intelligently operated
Importance of Islanding Price-based scheduling (4 DGs and 1 DES) – grid-connected only 18 20 16 Load DG DES 14 15 Utility grid 12 Load (MW) 10 Power (MW) 10 8 6 4 5 2 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Time (h) -5 140 Time (h) Real-time price 120 100 Price ($/MWh) Hours (1-24) 80 G1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 60 G2 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 40 G3 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 0 0 20 G4 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 0 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 ESS 0 -1 -1 -1 -1 -1 0 0 0 0 0 0 0 0 0 1 1 1 1 1 0 0 0 0 Time (h) An islanding at hour 2 causes more than 4 MW load curtailment!
Microgrid Optimal Scheduling • Grid-connected objective: minimize operation cost + energy purchase cost + customer inconvenience cost • Islanded operation objective: minimize load curtailment when islanded Grid-connected Operation • Components: Determine the optimal schedule of DERs, – DERs (DG, DES) adjustable loads, and the main grid power – Loads (fixed, adjustable) Schedule Cut Islanded Operation Determine dispatch during islanded hours. Find load curtailments. Generate cuts
Multi-period Islanding • New concept of T- τ islanding: – T is the scheduling horizon (typically 24 hours) – τ is the number of consecutive islanding hours • Microgrid should be able to supply loads for τ consecutive hours in a T- hour scheduling horizon. • Islanding scenarios will be generated, each with τ consecutive hours of islanding Sce. 1 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Sce. 2 1 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Sce. 3 1 1 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 • T-3 islanding: Sce. 4 1 1 1 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1: grid-connected Sce. 5 1 1 1 1 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Sce. 6 1 1 1 1 1 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0: islanded Sce. 7 1 1 1 1 1 1 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
7 Coordinating Grid-connected and Islanded Islanding Cut: • Generated when the islanding is not feasible, i.e., microgrid does not have sufficient online capacity to supply local loads • Indicates that the islanding mismatches can be mitigated by readjusting the microgrid schedule in the grid-connected operation. • This cut results in a change in the schedule of 1) DERs, and 2) Adjustable loads, based on islanding considerations. • The cut changes adjustable loads operating time interval while adding a customer inconvenience cost to the objective • If not feasible after certain number of iterations, load curtailment will be applied as a last resort
8 Example • Applying the T- τ islanding criterion to the same microgrid: Hours (1-24) 1 : additionally committed G1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 G2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 G3 1 1 1 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 G4 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 ESS -1 -1 -1 -1 -1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 0 0 0 0 • The operation cost is increased by about 4.4% (~$500). • The cost increase can be considered as the cost of islanding • This small cost increase provides a huge benefit as the microgrid islanding without load interruption is ensured. It can be extended to consider resilience; Uncertainty considerations would be critical
9 Uncertainty Considerations • Uncertainty: some factors, having a major influence on scheduling decisions, are not under control of the microgrid controller and/or cannot be predicted with certainty. – Forecast errors: microgrid load, non-dispatchable unit generation, and market price . (depend on a variety of factors which are out of control of the microgrid master controller, such as weather and site conditions, decisions of market players, transmission network congestion, etc.) – Main grid supply interruption: the time of incidents is unknown. Furthermore, depending on the range and severity of outages in the main grid, the required time to repair the power system and restore the power supply would vary.
Resilient Operation • For ensuring resilience, the microgrid controller must plan ahead for main grid supply interruptions while taking forecast uncertainties into account, and accordingly perform a seamless islanding when required. Resilient operation Normal operation Repaired and restored P M P M =0 P M Uncertain time Uncertain time of restoration of incident Microgrid Microgrid Microgrid Normal operation Islanded operation Resynchronized
Resilience-Oriented Optimal Scheduling • Resilience-oriented microgrid optimal scheduling Grid-connected operation • Robust optimization is Determine the optimal schedule of DERs, adjustable loads, and the main grid power used for solving the problem Resilience cut Schedule • min operation cost Resilient Operation • max-min load curtailment Calculate the worst case mismatch when islanded. Generate resiliency cut. Curtail loads to remove mismatch, if required Uncertainty information: Load, generation, price and islanding uncertainty intervals
12 to be further explored… • Community Microgrids • Integrated Microgrids • Market-based Scheduling
Community microgrids • Privacy issues are important (not all resources can be directly controlled) Master Controller Communal DERs and loads Buildings (Individual DERs and loads) Net load Price signal Control signal
Integrated microgrids • Data transfer • Power transfer • Primary/secondary controller • Coupled/Provisional microgrid Main Grid Microgrid 1 Microgrid 2
Market-based scheduling GENCOs TRANSCOs • DMO as an intermediate entity between the Transmission info Bid Award microgrids and the ISO to ISO Level Power Independent System Operator enable microgrids market flow (ISO) participation Bid Award Load forecast • Utility Level Microgrid scheduling will Load serving Entity (LSE) Distribution Market be based on pre-assigned Operator (DMO) (awarded) amounts of Power Power power transfer from the flow flow main grid Bid Award Customer Level Nonresponsive Loads Microgrids
Microgrid review paper • S. Parhizi , H. Lotfi, A. Khodaei, and S. Bahramirad, “State of the Art in Research on Microgrids: A Review,” IEEE Access, vol. 3, pp. 890 -925, July 2015. • Open access – can be freely downloaded • ~400 papers are reviewed • Presents a review of issues concerning microgrids and provides an account of research in areas related to microgrids, including distributed generation, microgrid value propositions, applications of power electronics, economic issues, microgrid operation and control, microgrid clusters, and protection and communications issues
Thank you amin.khodaei@du.edu
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