Voltage and Reactive Equation Choice

 

There are many different methods of performing voltage control which impact the inner power flow equation solution in PowerWorld Simulator. The background on the theory is described elsewhere in the help documentation. The following is a precedence for how PowerWorld chooses the voltage control algorithm to use. This is then followed by the various fields of objects in Simulator which impact how this is done.

 

Precedence for Generator Voltage Control

1. A generator will remain at a fixed Mvar output and generally not participating in voltage control under any of the following situations
   1. AVR = NO
   2. WindControlMode = Follow Min Mvar Capability
   3. WindControlMode = Constant Power Factor
   4. MvarMax = MvarMin
   5. Generator belongs to a VoltageDroopControl that is either not enabled or configured in an invalid manner
2. Else If a generator is set to UseLineDrop = YES, then the generator will ignore all other settings and perform line drop compensation. When using line drop compensation, the only other input parameters that matter for the generator are VoltSet, Rcomp, and Xcomp.
3. Next, if the generator has been assigned to a VoltageDroopControl object then it will perform voltage droop control. When performing voltage droop control, Simulator will automatically group together all generators assigned to the same VoltageDroopControl that also regulate the same bus (or buses connected by very low impedance branches that are below the ZBR Threshold and discussed in the advanced power flow options).
4. If none of the other special options are chosen the generator will fall back to regulated the voltage at the specified RegBus including if it is a remotely regulated bus. The VoltSet and VoltSetTol are then used however as specified. Thus Voltage Setpoint Tolerance may be used.

 

Objects and Fields the are related to the choice of Voltage and Reactive Equation Choice

 

Gen Object

• VoltSet: This is the desired voltage or setpoint (in per unit) of the regulated bus
• VoltSetTol: This is the tolerance of the desired voltage. By default it is 0.0 meaning that the setpoint is enforced exactly. If non-zero then the Voltage Setpoint Tolerance equations are used.
• RegBus or RegBusNum : This field specifies the regulated bus that is being controlled. It does not need to be the terminal bus of the generator
• RegBusNumUsed: This field shows the number of what bus is actually regulated by this device. It may be different than RegBusNum due to zero-impedance- branch groupings that are internally determined. See description under Effect of the ZBR Threshold on Voltage Regulation for more information.
• AVR: Set this field to NO and the generator's Mvar output will remain fixed at the present Mvar output. Set this to YES so that the reactive power output can be varied by the various control features.
• MvarMax, MvarMin: This represent the maximum and minimum Mvar output of the generator. These can also be made a function of the real power MW output by defining reactive capability curves.
• UseCapCurve: Set to YES to use the capability curve which can be specified by entering point for ReactiveCapability objects
• UseLineDrop : Set to YES to set a generator to use Line Drop compensation.
• Rcomp: this is the resistance of the line drop impedance used when UseLineDrop = YES.
• Xcomp: this is the reactance of the line drop impedance used when UseLineDrop = YES.
• WindControlMode: Can be set to either None, Boundary Power Factor, Constant Power Factor, or Follow Min Mvar Capability. All of these features simply impact how the MvarMin and MvarMax are calculated.
  • When set to None the generator simply uses the Mvar limits as normal.
  • When set to Boundary Power Factor then the maximum and minimum Mvar automatically follow a constant power factor line as defined by the WindControlModePF value.
  • When set to Constant Power Factor, the generator behaves as a fixed Q during the power flow solution as the Mvar output is simply a function of the MW output and the input value WindControlModePF. A positive value means that positive values of MW result in positive values of Mvar.
  • When set the Follow Min Mvar Capability, the generator behaves as a fixed Mvar output during the inner power flow solution as the Mvar output is simply a function of the MW according to the capability curves minimum Mvar outputs.
• WindControlModePF: This is the power factor used in conjunction with the WindControlMode field.
• VoltageDroopControl: This is the name of the VoltageDroopControl object to which the generator belongs.

VoltageDroopControl Objects

• Enabled: Set this to YES to enable the Voltage Droop Control for generators assigned to the object. When performing voltage droop control, Simulator will automatically group together generators that (1) share the same VoltageDroopControl AND (2) regulate the same bus (or buses connected by very low impedance branches that are below the ZBR Threshold and discussed in the advanced power flow options). See the Power Flow: Voltage Droop Control with Deadband topic for more information on how this works.
• Qauto: NO means to use the Qdb, Qmax, and Qmin values directly. If set to YES, then the values of Qmax and Qmin are automatically calculated based on the summation of generator Qmax and Qmin, and Qdb is assumed to be 0.0
  • If both Qmax and Qmin are the same sign, then Qdb is assumed to be either Qmax or Qmin depending on which value is closer to 0.0.
• Vdeviation: NO means to use the values of Vlow, Vdblow, Vdbhigh, and Vhigh directly. If set to YES, then the input values of Vlow, Vdblow, Vdbhigh, and Vhigh are interpreted as deviations away from the voltage setpoints of the generators
• Qdb, Qmax, Qmin, Vlow, Vdblow, Vdbhigh, and Vhigh: These fields all define the QV characteristic curve against which the reactive power and voltage are dispatched when performing voltage droop control.

Effect of the ZBR Threshold on Voltage Regulation

Sometimes when looking at the actually achieved voltage at a regulated bus (even when not using a tolerance) you will notice that the voltage is not exactly what the Voltage setpoint was. You may see 1.05021 instead of 1.05000 for instance. Because our equations are enforcing the voltage setpoint exactly, that is actually not expected behavior in general, but it may occur due to regulated buses being connected by very low impedance branches.

When performing voltage control, due to numerical stability different generators can not attempt to regulate buses extremely close to one another even if the same voltage setpoint is used. This will result in numerical problems and would cause trouble in a real control systems as well. To avoid this situation, PowerWorld automatically detects groups of buses that are connected by very low impedance branches. There is an advanced power flow option that specifies a ZBR threshold impedance which defines what a very low impedance branch is. By default the value is 0.0002 (so really small!). Using this ZBR threshold PowerWorld internally builds groups of buses connected by sets of very low impedance branches. From the list of buses that are internally found, a Primary bus is automatically chosen. Any device that regulates a bus within these groups will automatically regulate this internally chosen primary bus instead. This avoids any control conflicts.

Information about choices PowerWorld is making can be seen under the Model Explorer under Solution Details\Bus Zero-Impedance Branch Group case information display. There is a column of a bus called ZBRPrimary which shows which bus is chosen. In addition there is a bus field called ZBRNeighbors which gives a list of all the buses in the local ZBR group. Finally, on the generator case information displays there is a field for "Regulated Bus\Number (used due to ZBR)" which is called RegBusNumUsed which shows the number of the primary bus related to the respective generator's regulated Bus.