Switched Shunt Control

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Switched shunt control parameters can be set from the Switched Shunt Information dialog in either Edit mode or Run mode. They can also be set from the Switched Shunt Display.

 

Parameters (page on the Switched Shunt Information dialog)

Control Mode

Determines whether the switched shunt has a fixed value, or whether the amount of reactive power supplied by the device changes either in discrete steps or continuously in order to maintain its regulated value within the regulation range specified in the Control Regulation Settings. This field can be changed. However, for a switched shunt to be used for automatic control, the following fields must be set correctly:

  1. The Control Mode field must be set to either Discrete, Continuous, or SVC and the shunt must not be controlled by an SVC. If a shunt is being controlled by an SVC but the SVC is currently not on control, the shunt will be controlled based on its own control mode.
  2. The corresponding area's Auto Shunts property must be true
  3. The case-wide Disable Switched Shunt Control option, which can be set on the Power Flow Solution tab of the Simulator Options dialog must not be checked. If the Control Mode is SVC, the case-wide option Disable SVC Control also found on the Power Flow Solution tab of the Simulator Options dialog must not be checked.
  4. The Auto Control option for the switched shunt must be true

Note: Automatic control of switched shunts is disabled if the regulation high value is not greater than the low value; they should not be equal unless in continuous mode.

Note the additional control mode called Bus Shunt (Fixed). This is analogous to the shunt MW and MVAR values that can also be stored at the bus level. The difference is that bus shunts stored directly with the bus cannot be turned on and off in the load flow; rather they are always included in the load flow solution. Bus shunts that are represented as switched shunt objects on Bus Shunt control, however, are mathematically exactly the same and can be turned on or off. The reason for the differentiation of the Bus Shunt versus normal Fixed control is that the Bus Shunt control type is intended to identify the difference between a bus shunt and a switched shunt that MAY have controllability, but is currently turned of off control by being set to a Fixed value.

SVC control mode is intended to simulate controlled VAR devices with one continuous or discrete element that can control up to 8 fixed shunts (these shunts do not have to have a control mode that is fixed but they are treated as fixed as long as they are being controlled by an SVC). Information about these options can be found in the SVC Control Mode.

Multiple shunts at the same bus may have a control mode other than Fixed or Bus Shunt (Fixed). More information about how multiple shunts coordinate control when regulating the same bus can be found in the Switched Shunt Control Coordination section below.

 

Control Regulation Settings

Voltage Regulation

When the switched shunt is on automatic control, its reactive power is changed in discrete steps or continuously to keep the voltage at the regulated bus within the per unit voltage range defined by High Value and the Low Value.

In the case of discrete control, the amount of reactive power supplied by this device changes in discrete amounts, thus the High Value should be greater than the Low Value. The necessary voltage range depends upon the size of the switched shunt blocks. In addition to a voltage range for discrete control, a specific Target Value can be specified as well. The target voltage will try to be met, either approximately under discrete control, or exactly under continuous control (either true continuous or discrete with a continuous shunt correction element.) The number of the regulated bus is shown in the Reg. Bus # field.

Generator Mvar Regulation

This option allows switched shunts to control generator Mvar output to better enable full var range inside the inner power flow loop. During the first inner power flow loop, generator var limit checking is disabled for generators whose Mvar output is controlled by switched shunts. This will enable the use of the full reactive range of the switched shunts before generators hit their reactive limits. This corrects a problem in which the power flow fails to solve because generators are at their Mvar limits even though there is still reactive support available from switched shunts.

The High Value and Low Value specify the Mvar regulation range which limits the total Mvar output of all generators controlling the specified Reg. Bus #. If the regulated value is outside of the regulation range, the switched shunt will attempt to set the regulated value to the specified Target Value.

Custom Control

This option allows switched shunts output to be tied to the results of a Model Expression. It is intended for a Model Expression tied to the MW Output of a single or multiple generators. Otherwise it could lead to wrong solutions if used incorrectly.

Wind Mvar

This option allows switched shunts to control the Mvar output of generators that are on AVR control and are operating with a Wind Control Mode of either Constant Power Factor or Follow Min Mvar Capability Curve. This control type works to control the sum of the Mvar injection for generators on valid Wind Control that are regulating the specified Reg. Bus # plus the output of the controlling switched shunt plus any other switched shunts connected to the same bus as the controlling switched shunt. The allowed range of the Mvar injection sum is specified by the High Value and Low Value. If the Mvar injection sum is outside of the regulation range, the switched shunt will attempt to set the Mvar injection sum to the specified Target Value.

Var Regulation Sharing

Determines the priority for how a shunt should operate within its control group. Discrete shunts are operated in the order of highest to lowest sharing value, while continuous shunts are operated in proportion to their sharing parameter. More information is provided in the Switched Shunt Control Coordination section below.

Reg. Bus PU Voltage to Mvar Sensitivity

This field is only available while in Run mode. This provides information about how sensitive the voltage is at the regulated bus, Reg. Bus #, to a Mvar change at the switched shunt. This field gives an indication if the shunt can be effective in controlling within the desired regulation range.

Switched Shunt Blocks

The amount of shunt reactive power (susceptance) is specified in the Switched Shunt Block field. The columns in this field correspond to different blocks of reactive power. The first row indicates the number of steps in each block, and the second row gives the amount of nominal Mvars per step (assuming 1.0 per unit voltage). You may model both capacitors and reactors. The reactors should be specified first, in the order in which they are switched in, followed by the capacitors, again in the order they are switched in. The sign convention is such that capacitors are positive and reactors negative. Shunt blocks are switched in order from left to right.

Status Branch Added in Version 20

Line shunts can be modeled as controllable switched shunts by linking their status to the status of a branch. The Status Branch field specifies a branch whose status will affect the status of a switched shunt. If specified, a switched shunt can only be closed if is has a status of closed and its Status Branch also has a status of closed. If the Status Branch has a status of open, the switched shunt will also have a status of open.

 

Control Options: Advanced Options (page on the Switched Shunt Information dialog)

 

Single Largest Step

This option only applies when a switched shunt is set on discrete control. If checked the switched shunt will switch in EITHER all of the available reactor blocks OR all of the capacitor blocks at once when the voltage falls outside the given range. Whether the reactor or capacitor blocks switch is determined by which limit is violated. A switched shunt with this option checked will only switch ONCE during a load flow solution, and then remains fixed at the new output for the remainder of the same solution calculation.

Allow switching in the inner power flow loop

This option is available for individual discrete shunts only. If this option is set, discrete shunts are treated as continuous in the inner power flow loop. This means that they are treated as PV buses in the inner power flow loop. After the first inner power flow loop, the shunt nominal Mvar setting is rounded to positive infinity to the next discrete step. If any shunts exist that are allowed to switch in the inner power flow loop, the inner power flow loop is repeated again with the shunts being treated as discrete in the subsequent inner power flow loop. Shunts that are allowed to switch in the inner power flow loop will only switch if the global option to Disable Treating Continuous SSs as PV Buses is not checked.

Shunts that are allowed to switch in the inner power flow loop must not be part of a switched shunt control group (see Switched Shunt Control Coordination section below). This means that a particular shunt can be the ONLY shunt regulating its regulated bus and only one shunt at the shunt's terminal bus can be on control.

Use Continuous Element

If this option is checked, then Simulator will use a continuous element to fine-tune a discrete controlled switched shunt by injecting or absorbing additional MVARs to try and obtain the target voltage of the controlled bus.

Minimum and Maximum Susceptance

The minimum and maximum susceptance range for the continuous correction element.

Use High Target Voltage 

Check this box to use the target value specified in the High Target Value edit box when the regulated point goes above the High Value. This option can be used when regulating either voltage or Mvar. This will give a different target value if the regulated value goes out of range on the high end than the low end. If the regulated value goes out of range on the low end, the original Target Value on the Parameters page will be used. If this box is unchecked, only the target value on the Parameters page will be used, whether the violation is high or low.

 

Switched Shunt Control Coordination

 

Multiple switched shunts may regulate the same bus. When this occurs, control groups are formed internally in Simulator to share the control amongst all of the switched shunts in the group. A control group is determined based on switched shunts that regulate the same bus or buses that are the same due to being connected via very low impedance branches (X <= 0.0002 pu). This grouping of buses can be referred to as a ZBR group. Only shunts that are on either discrete or continuous control will be included in control groups.

There is a requirement that all of the switched shunts in a control group be contained within the same ZBR group. If there are multiple shunts that control the same bus, multiple control groups will be created if these shunts are contained in different ZBR groups. Within a ZBR group, only one control group can exist, i.e. all switched shunts in the same ZBR group must regulate the same bus. If multiple control groups exist within a ZBR group, automatic control will be turned off for all of the control groups within that ZBR group. All switched shunts within the same control group must also have the same regulation type, e.g. Voltage, Generator Mvar, or Wind Mvar. If not, automatic control will be turned off for the control group.

The total desired Mvar output for the control group is divided among the shunts in the control group by first assigning Mvar output to all discrete shunts in the order of highest Var Regulation Sharing to lowest. When specifying the order, all values of Var Regulation Sharing are acceptable whether they be positive, negative, or zero. The shunt with the highest priority will have its Mvar output set to the remaining desired amount until it hits either its low or high limit. If there is any remaining desired Mvar amount, the shunt with the next highest Var Regulation Sharing is then set. This is continued until there are no more discrete shunts with available Mvar output or the total desired Mvar output for the group has been met. If after all discrete shunts have been processed and the total desired Mvar output has not been met, the remaining desired Mvar is proportioned to the continuous shunts based on their Var Regulation Sharing. In this process the Var Regulation Sharing acts as a participation factor, and only values greater than zero will cause a continuous shunt to be set to an output other than zero. If continuous shunts hit limits during this process, their remaining proportion will be distributed to the other shunts in the group that are not already at limits.

Within a control group, the output of individual shunts will either be all reactive or all capacitive based on the total desired Mvar amount, i.e. if the total desired Mvar output is capacitive only capacitive blocks will be used.

Shunts that are allowed to switch in the inner power flow loop, i.e. continuous shunts or discrete shunts that are allowed to switch in the inner power flow loop, cannot be part of a switched shunt control group. This means that a particular shunt can be the ONLY shunt regulating its regulated bus and only one shunt at the shunt's terminal bus can be on control.

 

Closing Breakers to Energize Switched Shunts

 

If using the Integrated Topology Processing add-on, the Close Breakers to Energize Switched Shunts option can be found on the Simulator Options dialog on the Power Flow Solution Common Options page. Using this option will attempt to close breakers in order to energize switched shunts and allow them to actively be on control. Only breakers will be closed for switched shunts that are on either discrete or continuous control and are needed to meet the total desired Mvar output of the control group or regulated value of the individual shunt if the shunt is not part of a group.

When selecting which breakers can be closed to energize a particular switched shunt, breakers are only selected if in the process of closing them they would ONLY energize the particular switched shunt. Breakers that would energize additional devices will not be selected as valid for energizing the shunt. If no appropriate breakers can be found, a switched shunt will remain de-energized.

Branches with Branch Device Type of Breaker and Load Break Disconnect are included when searching for switching devices to energize switched shunts. Disconnects are not included.

See the Close Breakers Overview topic for more information on how the close with breakers process works.

 

Voltage Control Groups Added in Version 19

Switched shunts can be assigned to a voltage control group. Each voltage control group has two fields

Voltage Control

Group Field

Description of the Field

Name

The name of the control group which will be used to refer to it from the switched shunt records.

Status

 

 

 

This field determines how switched shunt control will behave for this group.

ON : Normal behavior where the Control Group acts as described above as long as the global options for moving shunts is enabled.

OFF : Means that the control group is ignored and the individual shunts in the group revert back to their own individual control behavior

FORCEON : Ignore the global option (or Area record option) to disable switched shunt control and always force control enabled for this group. The FORCEON status makes it easy for the user to disable switched shunt control globally in the contingency analysis tool and then override this disabling by setting the status to FORCEON for the Voltage Control Group.

There will also be a new field added for a switched shunt object called Voltage Control Group. A switched shunt will only be permitted to belong to one voltage control group, but it may also not have a voltage control group assigned. A switched shunt without a Voltage Control Group will simply obey the default behavior and will behave the same as shunts have always behaved in power flow contingency analysis tools.

Within power flow solution software, discrete capacitor switching is already modeled outside of the internal solution of the power flow equations in the controller loop (: Green Loop). This is where switched shunt control is implemented.

When voltage control groups are present, the following process is done.

  1. Process shunts in each “Voltage Control Group” as follows
    1. Determine the switched shunt that has the largest deviation below Vlow (call this shunt “LowShunt”)
    2. Determine the switched shunt that has the largest deviation above Vhigh (call this shunt “HighShunt”)
    3. Measure largest deviation in kV not per unit (thus regulated buses with a higher nominal voltage have a higher precedence)
    4. Only switched shunts that are marked as Control Mode = Discrete participate in these control groups. Any marked as Continuous, SVC, Fixed, or BusShunt will be ignored and will not switch at all (including in Step 2 below)
    5. If the terminal bus of the switched shunt or the regulated bus of the switched shunt is being regulated by a generator or a generator that is on AVR control is connected to the terminal bus of the switched shunt , that shunt will be ignored
    6. If LowShunt was found, then move that switch shunt UP by one step
      Else if HighShunt was found move this shunt DOWN by one step
  2. Once all voltage control groups are processed simply perform the rest of the switched shunt as has always been done, but ensure we don’t move any shunts that are part of a Voltage Control Group.

Note you can use the FORCEON feature of the Voltage Control Group to force shunts on control even when all of the global options to disable shunts are chosen.