Microgrid Optimal Scheduling Amin Khodaei, Ph.D. University of - - PowerPoint PPT Presentation

Microgrid Optimal Scheduling

Amin Khodaei, Ph.D. University of Denver

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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 An islanding at hour 2 causes more than 4 MW load curtailment!

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5 10 15 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Power (MW) Time (h) DG DES Utility grid

20 40 60 80 100 120 140 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Price ($/MWh) Time (h) Real-time price

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 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 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 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 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 Hours (1-24)

2 4 6 8 10 12 14 16 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Load (MW) Time (h) Load

Microgrid Optimal Scheduling

• Grid-connected objective: minimize operation cost + energy purchase cost

+ customer inconvenience cost

• Islanded operation objective: minimize load curtailment when islanded
• Components:

– DERs (DG, DES) – Loads (fixed, adjustable)

Grid-connected Operation Determine the optimal schedule of DERs, adjustable loads, and the main grid power Islanded Operation Determine dispatch during islanded hours. Find load curtailments. Generate cuts Cut Schedule

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

• T-3 islanding:

1: grid-connected 0: islanded

• 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

• 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

• 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

• 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

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

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Example

• Applying the T-τ islanding criterion to the same microgrid:

1: additionally committed

• 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

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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

Hours (1-24)

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.

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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.

Islanded operation

Microgrid

Normal operation Uncertain time

• f incident

Microgrid Microgrid

Resynchronized Uncertain time

• f restoration

Normal operation Resilient operation Repaired and restored PM PM PM=0

Resilience-Oriented Optimal Scheduling

• Resilience-oriented microgrid optimal scheduling
• Robust optimization is

used for solving the problem

• min operation cost
• max-min load curtailment

Grid-connected operation Determine the optimal schedule of DERs, adjustable loads, and the main grid power Resilient Operation Calculate the worst case mismatch when islanded. Generate resiliency cut. Curtail loads to remove mismatch, if required Schedule Resilience cut Uncertainty information: Load, generation, price and islanding uncertainty intervals

to be further explored…

• Community Microgrids
• Integrated Microgrids
• Market-based Scheduling

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Community microgrids

• Privacy issues are important (not all resources can be directly controlled)

Master Controller Net load Communal DERs and loads Price signal Buildings (Individual DERs and loads) Control signal

Integrated microgrids

• Data transfer
• Power transfer
• Primary/secondary controller
• Coupled/Provisional microgrid

Microgrid 2 Microgrid 1 Main Grid

Market-based scheduling

• DMO as an intermediate

entity between the microgrids and the ISO to enable microgrids market participation

• Microgrid scheduling will

be based on pre-assigned (awarded) amounts of power transfer from the main grid

Bid Award Bid Bid

Distribution Market Operator (DMO) Load serving Entity (LSE) Microgrids

Award

Independent System Operator (ISO) GENCOs TRANSCOs

Transmission info Award

ISO Level Utility Level Customer Level

Power flow Power flow Load forecast Power flow

Nonresponsive Loads

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