Power Flow Solution: Common Options

 

The following options are all contained on the Common Options tab after choosing the Power Flow Solution page of the Simulator Options.

Power Flow (Inner) Loop Options (see Solving the Power Flow for information on solution loops)

MVA Convergence Tolerance

The MVA convergence tolerance serves as a measure for determining when the inner power flow loop of the Power Flow Solution process has reached an acceptable solution. The MVA mismatch is computed as the maximum real or reactive mismatch at any bus in the system. Usually, this value should be around 0.1 MVA. If you are having difficulty solving a particular case, it may be helpful to temporarily increase the MVA Convergence Tolerance to drive the solution closer to the actual solution, and then re-solve from this solution using the smaller MVA tolerance.

Maximum Number of Iterations

This option defines the maximum number of iterations Simulator will perform during the Power Flow Solution process in an effort to converge to a solution. If Simulator must exceed this number of iterations, it assumes that the power flow case is not converging and will terminate the solution process. If Simulator is configured to represent non-converging power flow cases as blackouts, the screen will turn gray and the blackout warning message will appear.

Do Only One Iteration

If checked, Simulator will only perform one iteration of the load flow solution process when Solve Power Flow is clicked, regardless of the Maximum Number of Iterations setting. This is useful to more closely examine a case with which you are experiencing difficulty.

 

MW Control (Outer) Loop Options (see Solving the Power Flow for information on solution loops)

Disable Automatic Generation Control (AGC)

If checked, the enforcement of the generation re-dispatch to account for MW interchange constraints for all areas is disabled. By default, this option is not checked.

Enforce Generator MW Limits

If checked, generator minimum and maximum MW limits are enforced for all generators whose Enforce MW Limits field is set to true. See Generator Information Dialog for more information. Otherwise, generator MW limits are not enforced. When using the economic dispatch or the Optimal Power Flow, the generator MW limits are always enforced regardless of user-settings.

 

Topology Processing Options (These options are only available with the Topology Processing add-on)

Use Topology Processing

This option is used for all Simulator applications: power flow, contingency analysis, sensitivity calculations, etc. This option should ALWAYS be checked for real-time cases, while it should be unchecked for planning cases. If a full-topology model was read and this option is not checked, the power flow will try to solve the model without removing the switching devices resulting in a very large ill-conditioned Jacobian. This option is also found on the Topology Processing Dialog.

Close Breakers to Energize Switched Shunts

If checked, an attempt will be made to close breakers in order to energize a Switched Shunt that is on either Discrete or Continuous control and is needed to meet its regulated value. This option is applied as part of the switched shunt control any time that the power flow is solved.

This option will be checked by default if loading an EMS case from Areva CSV or ABB Spider file types.

 

Controller (Middle) Loop Options (see Solving the Power Flow for information on solution loops)

Disable Checking Gen VAR Limits

If checked, the Mvar limits are ignored for all the generators in the case during a power flow solution. By default, this option is not selected.

Check Immediately

Before entering the inner power flow loop to solve the power flow equations, a decision is made about each generator in the case as to whether it will be treated as a PV or a PQ bus. It is assumed that a PV bus will maintain its setpoint voltage, while a PQ bus will maintain a constant reactive power output (for instance when the generator is at a Mvar min or max limit). Normally, the power flow equations are completely solved in an inner power flow loop using this assumption. After completing the solution, generator Mvar limits are checked to see if they have been reached (must change from PV to PQ), or if at a limit if the limit may be backed off of (must change from PQ to PV). If any generators change PQ/PV status, the inner power flow loop is resolved.

When the Check Immediately option is checked, the generator Mvar limits are checked after every inner power flow loop iteration instead to see if generator limits have been reached, or if a generator limit may be backed off. When this option is not checked then it will not check the generator limits inside the inner power flow loop. In some situations performing this check inside the inner power flow loop can help with convergence, however performing this check does slow down the solution process. By default, this option is not selected.

Check Back Off Immediately

Added in Version 19

This feature is a partial implementation of the Check Immediately option. It will only check whether generator limits should be backed off during the inner power flow loop iteration, but will not check if generators are outside their Mvar limits. When this is checked and Check Immediately is not chosen, generators being outside their Mvar limits will continue to occur in the controller control loop.

Added in Version 20 This option is checked by default.

Disable Switched Shunt (SS) Control

If checked, automatic control of switched shunts is disabled in all areas. By default, this option is not selected.

Disable SVC Control

If checked, SVC type switched shunts are disabled. By default, this option is not selected. Note: The area option for shunt control only applies to non-SVCs.

Disable LTC Transformer Control

If checked, automatic control of LTC transformers is disabled in all areas. By default, this option is not checked.

Disable Phase Shifter Transformer Control

If checked, automatic control of phase shifting transformers is disabled in all areas. By default, this option is not checked.

Disable DC Line Transformer Tap Control

If checked, automatic control of DC transmission line transformer tap control is disabled in the entire case. By default, this option is not checked.

Disable D-FACTS Control

If checked, automatic control of D-FACTS devices is disabled for all devices. By default, this option is checked.

Transformer Stepping Methodology

Choose either Coordinated Sensitivities or Self-Sensitivity Only. The default value is Coordinated Sensitivities.

When the regulated value of tap-changing or phase-shifting transformers move outside of their regulation range, then Simulator attempts to bring those transformers back inside their range. By default Simulator determines all the tap-changing and phase-shifter transformers which are out-of-range and coordinates the movement of these transformers in an attempt to bring all the regulated values back into range. This is what is called Coordinated Sensitivities. Generally this results in a better convergence, however it also can be slower when a large number of transformers are out-of-range together. Because it gets slower with a large number of transforms, if more than 50 transformers are involved in the switching, Simulator will always use the Self-Sensitivity Only option. As the solution process continues however, the number of transformers moving will reduce and eventually the Coordinated Sensitivities will be used again.

Choosing Self-Sensitivity Only will modify this methodology so that each transformer only looks at the sensitivity of its regulated value with respect to changing its own tap or phase. This calculation is faster, but may result in convergence problems due to transformers that interact with one another.

Prevent Controller Oscillations

Sometimes, a power flow will fail to converge because certain automatic controls such as Mvar limit enforcement at generators, transformer tap switching, and shunt switching oscillate between their control bounds. These oscillations very often are due to modeling inaccuracies. If this option is checked, Simulator will automatically detect such oscillating controls and fix them at their current value so that they no longer oscillate. You may find this option helpful if you feel that the modeling of automatic controls in your system is inaccurate.

Maximum Number Controller Loop Iterations

As part of the solution process, the outer loop of the solution algorithm is a check of any necessary controller changes due to changes in controlled values from the last iteration of the Newton-Raphson load flow solution. The maximum number of loops through the control change algorithm can be set here. This is not the same as the Maximum Number of Iterations, which applies to the actual Newton-Raphson inner loop algorithm and solves the actual power flow.