See Also
A Time Step Simulation can be used to obtain optimal power flow and security-constrained optimal power flow solutions on a time point by time point basis for users that have the OPF and SCOPF add-ons. On the Summary page, the following Solution Types can be specified:
- Power Flow
- Unconstrained Optimal Power Flow, which is equivalent to Economic Dispatch
- Optimal Power Flow (OPF)
- Security-Constrained Optimal Power Flow.
Note that different time points can be solved by any of the previously listed solution methods in the same time step simulation. However, the solution settings of the previous time points are used as initial conditions for the solution of the next time point.
Simulator OPF and SCOPF tools are among the most advanced optimization packages for power systems. They have been extended in the latest versions of Simulator with many features, and have become complex analysis system. For users that are unfamiliar with the OPF/SCOPF solutions, read the sections on Optimal Power Flow and Security-Constrained Optimal Power Flow before setting up OPF/SCOPF time step simulations.
Power Flow Time Step Simulation
The power flow simulation allows the user to obtain AC or DC power flow solutions for a set of time points. During the Time Step Simulation all the power flow options defined in the Simulator Options page as well as in other dialogs are used for the solution. A key concept of the Time Step Simulation is that if you select Solve Time Point from the Summary page or from the grids of the Input page, you would obtain the same solution that if you would hit the Solve Power Flow Button. This is true, when the input data does not contain schedule data but only time point input data. If your Simulation contains schedule data, there may have been scheduled actions that were applied in previous time points that are not being applied when you select the Solve Time Point option.
The power flow solution will observe all the power balance constraints, control limits and area interchange constraint defined in the power flow settings.
Unconstrained OPF Time Step Simulation
In the unconstrained OPF solution, the Time Step Simulation removes all the constraints that would normally act in the OPF and optimizes the system to find the minimum operating cost settings. In doing so, Simulator will change the set points of the specified controls (generators and phase shifters) to minimize the cost of all Areas and Superareas set to OPF AGC control.
Besides the power flow solution options, the Unconstrained OPF simulation will take all the options that have been defined for a regular OPF solution. Most of these options are defined in the OPF Options Dialog, which is accessed using the OPF Options and Results button on the Add Ons ribbon tab. All the settings such as objective function, cost of unenforceable constraints, control available, prices for controls, etc. are defined in this dialog. Other options are defined for each particular object such as generators, loads, areas, lines, interfaces, etc.
OPF Time Step Simulation
When using the OPF solution type, the Time Step Simulation applies the time point and schedule input data and optimizes the control areas set to OPF to minimize cost while enforcing normal operation constraints: transmission line thermal limits, interface limits, generator control limits, and load control limits. In doing so, the OPF algorithm detects the controls that need to be moved, the constraints that are binding at the solution point, and the unenforceable constraints, i.e., constraints that cannot be enforced with the available controls.
Some of the quantities that are of interest in the solution of the OPF algorithm are:
- Locational Marginal Prices: These need to be selected for display on the Buses Results page. Average LMP prices and other LMP metrics can also be displayed in the Results page for Areas, Injection Groups, Super Areas, and Zones.
- Binding Constraints as well as Marginal Cost of Limit Enforcement for lines and interfaces. These fields can be seen in the Results: Constraints page and can be added to the Results page for Lines, Transformers, and Interfaces.
- Unconstrained, Final and Congestion Cost, displayed in the Results Summary Page for the entire system. These costs can also be displayed for Areas, Owners, Superareas and Zones in the Results page.
- Profit $/hr for Generators, Owners, Superareas, Areas, and Zones can be displayed in the corresponding grids of the Results page.
SCOPF Time Step Simulation
The SCOPF combines the power of Simulator’s OPF with the Contingency Analysis Tool to optimize a system for minimum cost while enforcing both normal operation and contingency constraints. At each time point, the SCOPF solution provides the optimal operation of the system so that if contingencies occur they would not create security violations. The locational marginal prices created in this manner are security-constrained signals to the market.
The solution of SCOPF Time Step Simulation depends on the options that have been set up for the following tools:
- Power Flow
- Optimal Power Flow
- Contingency Analysis
- Security Constrained Optimal Power Flow
- Time Domain OPF Options
The SCOPF Time Step Simulation does the following for each time point:
- Applies the input data to the power system
- Applies scheduled actions determined by the schedule input data
- Solves a power flow
- If specified, solves an unconstrained optimal power flow (economic dispatch)
- Initializes the base case of the security constrained OPF by solving a power flow or an OPF
- For the initial system conditions, solves the list of contingencies
- Solves the SCOPF optimization problem: minimizes operating cost while enforcing normal and contingent constraints
- Displays the results in all the result grids
The SCOPF is on its own a complex computation that often requires significant computer resources. This is due mostly to the solution of the list of contingencies and the calculation of their sensitivities. The size of the problem can be dimensioned by:
- Size of the system, given by the number of buses and the areas to be optimized. This is difficult to reduce since the optimization problem is normally defined for a certain region.
- Number of contingencies, which can be reduced by developing a contingency screening using peak loading conditions.
- Number of constraints (monitored elements), which can be reduced by selecting critical element, e.g. interfaces and higher voltage transmission lines.
- Number of time points in the list.
Another mechanism to speed up the computation of the PF/OPF/SCOPF Time Step Simulation is to use DC solutions in some of the internal routines:
- AC or DC power flow
- AC or DC contingency analysis. This one will produce the larger time savings.
- AC or DC SCOPF