Real-Time Active-Reactive Optimal Power Flow with Flexible Operation of Battery Storage Systems
Abstract
:1. Introduction
- (1)
- A multi-time-scale dynamic (i.e., with difference equations rather than only algebraic equations) AR-OPF framework is developed to optimally react to the spontaneous changes in wind power and ensure the feasibility of operations in real time when BSSs exist in DNs.
- (2)
- The framework offers the possibility of simultaneous optimization of all of the following mixed-integer variables in a prediction horizon:
- Wind power curtailment of each WF (continuous);
- Active power charge/discharge of each BSS (continuous);
- Reactive power dispatch of each WF and BSS (continuous);
- Length of charge and discharge periods of each BSS (discrete);
- Length of charge-discharge cycles of each BSS (discrete);
- Number of charge-discharge cycles of each BSS in the prediction horizon (discrete);
- Status of charge/discharge of each BSS (binary);
- Slack bus voltage (discrete); and
- Active-reactive reverse power flow to an upstream network (continuous).
- (3)
- Fully flexible optimal operation strategies for BSSs are determined for the dynamic AR-OPF while minimizing the expended life costs of the BSSs as a function of DoD and the number of charge-discharge cycles.
2. Problem Formulation
3. Real-Time Dynamic AR-OPF Framework
- (1)
- Provide hourly forecasted wind power, demand, and price profiles in advance of each prediction horizon .
- (2)
- Solve the corresponding dynamic MINLP AR-OPF problem. In this step, optimal flexible operation strategies for BSSs are computed for the upcoming (e.g., 24 h, with hourly discretization). The detailed problem formulation is described in Section 4.
- (3)
- The variables of BSSs computed in phase 1 will be used as fixed input parameters for the second phase. Note that other decision variables will be recomputed in phase 2.
- (4)
- Provide forecasted values of wind power, demand, and price ahead of each prediction horizon (e.g., 2 min). Note that the length of the prediction horizon should depend on the availability of the forecasted data as well as the computation time in step (7).
- (5)
- To describe uncertain wind power, generate wind power scenarios for each WF using a continuous bounded stochastic distribution with an identical probability between two adjacent scenarios. For this purpose, intervals are defined for the wind power , , such that:
- (6)
- Send the generated wind power scenario combinations (obtained in step 5) to the MINLP AR-OPF.
- (7)
- Solve the MINLP AR-OPF problems corresponding to each scenario combination for the upcoming . Note that the optimization problems at this step are not dynamic as the optimal operation strategies of BSSs are already given as input parameters. Since reactive power flow has influence on nodal voltages [40,41], the reactive power dispatch of the WFs can lead to voltage violations, in particular when the wind power fluctuates. For this reason, we use a back-off strategy [14] to satisfy voltage constraints in the RT-OPF. Since the optimization problems in this step are independent, they are solved using parallel computation in order to ensure that the solutions for all the scenario combinations are available within the prediction horizon .
- (8)
- Send the solutions of the MINLP AR-OPF problems (obtained in step 7) as a lookup table to a reconciliation algorithm.
- (9)
- Using the reconciliation algorithm, reconcile the lookup table by substituting the un-converged problems with solutions by which the safety of the operations is ensured while minimizing the degree of conservatism.
- (10)
- Send the reconciled lookup table for the to a selection algorithm.
- (11)
- Provide the values of wind power measured at each sampling interval (e.g., 20 s) to the selection and power factor modification algorithms.
- (12)
- The selection algorithm selects a solution strategy based on the measured values of wind power for each sampling interval . The selected scenario ensures the safety of the operation with the minimum of the objective function.
- (13)
- Send the selected scenario to the power factor modification algorithm.
- (14)
- Modify the power factor of WFs before realizing the solution using the power factor modification algorithm. Due to the possible difference between the measured wind power and the selected scenario, realizing the reactive power dispatch can lead to violations of power factor limits. Therefore, the power factor modification algorithm ensures satisfaction of the power factor constraints.
- (15)
- Send the decision variables to the network at each sampling interval .
4. Dynamic MINLP AR-OPF
4.1. Operation Modes of BSSs
4.2. Detailed Problem Formulation
4.3. Equations of BSSs
5. Case Study
6. Conclusions
Author Contributions
Funding
Conflicts of Interest
Abbreviations
Constant parameters of a BSS | |
Active/reactive energy price | |
Average depth of discharge of a BSS | |
Energy level in a BSS | |
Lower/upper limit of energy level in a BSS | |
Expended life cost of a BSS | |
Objective function | |
Total cost of active/reactive energy imported from an upstream network | |
Total expended life costs of BSSs | |
Network active/reactive power function | |
Dynamic model equations | |
Indices for buses | |
m | ndex for sampling interval Ts, i.e., m = 1, …, M |
n | Index for prediction horizon Tp2, i.e., n = 1, …, N |
nc | Index for wind power scenario combinations |
Index for charge-discharge cycles of a BSS, i.e., ncyc = 1, …, Ncyc.max | |
Number/maximum number of charge-discharge cycles of a BSS in each prediction horizon Tp1 | |
Battery total number of cycles | |
Index for wind power scenarios i.e., | |
nw | Index for wind farms (WFs), i.e., |
Active power charge/discharge of a BSS | |
Active/reactive power demand | |
Power factor of a WF | |
Upper limit of a WF power factor | |
Lower limit of a WF power factor | |
Active/reactive power at the slack bus | |
Wind power of a WF | |
Reactive power dispatch of a BSS | |
Apparent power in a feeder | |
Set of buses/BSS buses/WF buses | |
Upper limit of apparent power in a feeder | |
Apparent power of a power conditioning system (PCS) in a BSS | |
Maximum capability of a PCS in a BSS | |
Upper limit of apparent power at slack bus | |
Total operation cost of storage units | |
Per unit cost of storage units | |
Initial/final time | |
Length of charge-discharge cycles of a BSS | |
Charge/discharge periods of a BSS | |
Duration of time steps for BSSs | |
Computation time for dynamic OPF in Phase 1 | |
Prediction horizon in phase 1/phase 2 | |
Sampling interval | |
Vector of continuous/discrete/binary decision variables | |
Lower/upper limits on continuous decision variables | |
Voltage at a PQ bus | |
Lower/upper limit of voltage at a PQ bus | |
Voltage at slack bus | |
Lower/upper limit of voltage at slack bus | |
Vector of state variables | |
Initial states | |
Lower/upper limits on state variables | |
Binary variable for charge/discharge of a BSS | |
Wind power curtailment of a WF | |
Step change of voltage at slack bus | |
Coefficient for active/reactive power limit at the slack bus | |
Battery charging efficiency | |
Battery discharging efficiency | |
Vector of random variables |
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TP1 = 24 h | PFw.min = 0.85 | SPCS.max = 6 MVA |
TP2 = 2 min | PFw.max = 1 | Eb.min = 5.4 MWh |
TS = 20 s | Ss.max = 20 MVA | Eb.max = 18 MWh |
Vmin = 0.94 pu | SUCu = 150 $/kVA | ηch = ηdis = 1 |
Vmax = 1.06 pu | a = −4775 | Ncyc.max = 4 |
γPs = γQs = −1 | b = 6542 | td = 1 h |
Operation Mode | (s) | (MWh) | (MWh) | ($) | ($) | ($) | ($) | ($) | ($) | ||
Mode 1 | 4 | 4 | 135.32 | 2.41 | 2.611 | 667.73 | 679.86 | –1166.41 | –3184.85 | 1347.59 | –3003.67 |
Mode 2 | 4 | 4 | 212.96 | 2.368 | 2.516 | 665.25 | 674.06 | –1210.2 | –3244.31 | 1339.31 | –3115.2 |
Mode 3 | 1 | 1 | 241.15 | 2.742 | 3.052 | 172 | 177.01 | –1230.4 | –3279.67 | 349.01 | –4158.06 |
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Mohagheghi, E.; Alramlawi, M.; Gabash, A.; Blaabjerg, F.; Li, P. Real-Time Active-Reactive Optimal Power Flow with Flexible Operation of Battery Storage Systems. Energies 2020, 13, 1697. https://doi.org/10.3390/en13071697
Mohagheghi E, Alramlawi M, Gabash A, Blaabjerg F, Li P. Real-Time Active-Reactive Optimal Power Flow with Flexible Operation of Battery Storage Systems. Energies. 2020; 13(7):1697. https://doi.org/10.3390/en13071697
Chicago/Turabian StyleMohagheghi, Erfan, Mansour Alramlawi, Aouss Gabash, Frede Blaabjerg, and Pu Li. 2020. "Real-Time Active-Reactive Optimal Power Flow with Flexible Operation of Battery Storage Systems" Energies 13, no. 7: 1697. https://doi.org/10.3390/en13071697
APA StyleMohagheghi, E., Alramlawi, M., Gabash, A., Blaabjerg, F., & Li, P. (2020). Real-Time Active-Reactive Optimal Power Flow with Flexible Operation of Battery Storage Systems. Energies, 13(7), 1697. https://doi.org/10.3390/en13071697