See Also
The following options are all contained on the DC Options tab after choosing the Power Flow Solution page of the Simulator Options.
Use DC Approximation in Power Flow
When this box is checked, Simulator will solve the load flow using the DC approximation method. When not checked, Simulator performs the Full Newton AC load flow algorithm. Once a case, especially a large one, has been solved using a DC approximation, it tends to be quite difficult to revert back to the Full Newton AC load flow from a solved DC approximation.
DC Power Flow Model
These choices also affect the calculation of sensitivities on the PTDF, LODF, and TLR dialogs and ATC Tool calculations.
Generally, the only inputs that affect the DC power flow equations are line impedances and line status (open/closed). However there are three possible modeling choices (for which there is not general agreement in the industry) that will affect the DC equations in small ways. These choices are made here on the DC Power Flow Model options:
Ignore Series Resistance (r) or Ignore Series Conductance (g)
The series term of a transmission line consists of an r and x value which represent a complex number impedance. In the DC power flow approximation, the value that used is the imaginary part of the inverse of impedance (called admittance)
g + jb = 1 / (r + jx) = r / (r^2 + x^2) – j x / (r^2 + x^2)
Thus
g = r / (r^2 + x^2)
b = – x / (r^2 + x^2) *** This term is used in the DC power flow equations
The only term that is used in the DC power flow equations is the b term. Some say that a DC power flow means that r = 0 which means that b = -1/x and g = 0. Others say that a DC power flow only means assuming that g = 0, which means that b = - x / (r^2 + x^2). There is not a good consensus in the industry as to what is correct regarding this, so Simulator offers an option as to whether you ignore r or ignore g. The default option in PowerWorld is to ignore the resistance.
Ignore Transformer Impedance Correction Tables and Ignore Phase Shift Angle Effects
These are both ignored by default.
When a transformer impedance correction table is specified for a transformer, the series impedance of the transformer will vary by a multiplier with the tap or phase of the transformer. The tap ratio is not generally relevant to a DC power flow, however the phase-shifter angle could vary when using the DC approximation. This option specifies whether to ignore the impact of this impedance correction. For phase shifters the multiplier is normally between 1.00 and 1.50 and thus tends to increase the impedances as you move away from zero degrees.
As a branches phase shift varies, the DC power flow equations could be modified to take this into account. This effect results in the DC power flow equations for which the "b" term above is multiplied by the cosine of the phase angle across the line. This effect tends to decrease the impedance as you move away from zero degrees.
Because the impedance correction tends to increase impedance and the phase effect tends to decrease impedance it is reasonable to ignore both of these, and this is done by default. The other issue with not ignoring these is that both create a situation where the equations for the DC approximation are dependent on the system state. In other words, the matrix equations become a function of the phase-shift angle. This results in a situation in which the matrices must be recalculated and re-inverted each time a phase angle is moved. This removes some of the advantages of the DC approximation. This can be especially problematic when using the OPF or SCOPF.
Compensate for Losses by Adjusting the Load
Traditionally a DC load flow is treated as lossless. However, you can approximate the loss in the load flow by artificially adjusting the load in the case to include estimated losses. To do so, click on the DC Loss Setup button to open the DC Power Flow Loss Setup dialog for setting the DC Loss Multipliers.
Compensate for Dispatch Sensitivities with User-Specified Values
This option allows for the bus MW loss sensitivities to be used in the OPF and ED dispatch algorithms, if the type of loss sensitivity on the General tab is set to User-Specified.