# DC Modeling in Dynamic Stability

No specialty effort has been done to handle DC components in this version of the Dynamic Stability feature. Please note the following:

- Inverter output is fixed voltage source with AC symmetrical short circuit equal to that specified.
- Rectifier output is fixed voltage source with DC short circuit equal to that specified.
- DC Motors are loads.
- DC Generators are fixed voltage source with DC short circuit equal to that specified.
- DC Utilities are fixed voltage source with DC short circuit equal to that specified.
- DC Batteries are fixed voltage source with DC short circuit equal to that specified.
- DC Photovoltaics are fixed voltage source with DC short circuit equal to that specified.

Note however that specialty Wind Turbine and Photovoltaic models (both which only connect to the AC system) are offered under the EasyPower AC Generator component. Add an AC generator to your system to simulate the Wind Turbine or Photovoltaic, and then select either:

- For the Photovoltaic model:
- Generator Model: Gen Inv PV1
- Exciter Model: Exc Inv QCntrl

- For the Wind Turbine model:
- Generator Model: Gen Inv WT4
- Exciter Model: Exc Inv QCntrl

## Inverter and Photovoltaic Iteration Methods

Additional power flow solution logic and adjustments have been included for both Inverters and Photovoltaics, as they play off of each other if built into a model together. On each iteration, that logic is shown in the following chart. Note that only pertinent power flow solution steps are included.

1. Set DoingInvAdjust = false

2. Do Power Flow Iteration Code

3. Update Inverter and Rectifier

Inverter:

Check input load voltage

if ( VDC < V Low Limit )

{Reduce specified output kW

Set bDoingInvAdjust = true

}

Transfer AC source load on output to input load considering efficiency

Rectifier:

Use DC Equations to calculate input kW and kVar

Transfer DC source load on output to input load

Use DC Equations to calculate output DC Voltage from loading and AC input voltage

Update DC Output voltage

4. Update Photovoltaics

if ( VTerm > VOC)

{Force I Output to zero

}

if ( VTerm < VOC AND VTerm > VMP )

{Set output according current to VI slope

}

if ( VTerm < VMP AND NOT bDoingInvAdjust )

{Force I Output to ISC

if ( VTerm < V Min Soln Limit )

{Terminate the PF solution

}

}

5. Either GoTo (2) or Exit if Solved

Comment 1: The method shown above thus enables some amount of interaction between Inverters and Photovoltaics, but it enables the Inverter, if there is one, to perform its action first. This is done on purpose, as the Photovoltaic action on an overload condition is to drop voltage. This action has a stronger (even overriding) control action than the load reduction of the inverter. We really want the inverter to first attempt to do all of its load dropping (dropping load until it meets it low voltage limit) before we even attempt any voltage reduction of the Photovoltaic.

Comment 2: Voltage reduction on the Photovoltaic is applied in hopes that the load that it is feeding is a constant impedance load. In this case, there will eventually be a valid power flow solution, even as the voltage plummets to what would be considered unacceptable levels, as the Photovoltaic attempts to hold its current output to ISC. If a constant power or constant current load is specified in the DC system with the Photovoltaic, it is the sole source of power, and if the load is specified such that the Photovoltaicâ€™s bus voltage drops below VMP, then the system will collapse into a severe low voltage, and will terminate the solution once we have dropped below V Min Soln Limit.

Comment 3: An inverter specified with a Photovoltaic system cannot accomplish a load reduction if it is specified in a SWING or Stand-Alone mode. In this mode, the Inverter accumulates all load that it is serving, and applies it to its DC input. If that load causes the Photo-Voltaic to have a voltage below VMP, then the same voltage collapse discussed in Comment 2 occurs.

## More Information

Dynamic Stability |