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REGFM_A1
Description
The REGFM_A1 (Renewable Energy Grid-Forming Model – Type A1) represents a positive-sequence, droop-controlled grid-forming inverter.
The REGFM_A1 model includes a voltage source behind impedance representation (see the figure below)), P-f and QV droop controls, active and reactive power limiting controls, and a transient fault current limiting function. Therefore, this model can be used to study events such as the frequency response, islanding and islanded operation, and typical three-phase faults with a normal clearing time, etc. The following figure shows the interface circuit implemented in this model
The model behaves as a controllable voltage source behind a coupling reactance and contains three main functional parts:
P–f droop control :
Develops the internal voltage angle (and frequency) command and provides active-power sharing (see the diagram below)
Q–V droop control :
Develops the internal voltage magnitude command and provides voltage regulation and reactive-power sharing as shown in the following diagram.
Fault current limiting function:
The following image shows the function that limits inverter current during short-circuit conditions by modifying the internal voltage source.
Mask and Parameters
General
Name | Unit | Description |
|---|---|---|
Vbase | V | Base voltage of the model to be used for the pu calculations |
Sbase | VA | Base power of the model to be used for the pu calculations |
fnom | Hz | Nominal frequency of the model to be used for the pu calculations |
Re | pu | Inverter coupling resistance |
Xe | pu | Inverter coupling reactance |
omega | rad/s | Cutoff frequency for low-pass filters for power filtering |
FilterRL | - | Filter Type (It only impacts the calculation of the model’s initial conditions along with the Load Flow tab) |
Cf | pu | Filter capacitance (It only impacts the calculation of the model’s initial conditions along with the Load Flow tab) |
P-f Droop
Name | Unit | Description |
|---|---|---|
mp | % | P-f droop gain |
Pmin | pu | Minimum power |
Pmax | pu | Maximum power |
kppmax | pu | Proportional gain of the active power controller |
kipmax | pu/s | Integral gain of the active power controller |
Q-V Droop
Name | Unit | Description |
|---|---|---|
mq | % | Q-V droop gain |
Qmin | pu | Minimum reactive power |
Qmax | pu | Maximum reactive power |
kpqmax | pu | Proportional gain of the active power controller |
kiqmax | pu/s | Integral gain of the active power controller |
kpv | pu | Proportional gain of the voltage controller |
kiv | pu/s | Integral gain of the voltage controller |
AWpiv | pu | Anti windup gain of the voltage controller |
Emin | pu | Minimum E |
Emax | pu | Maximum E |
VFlag | - | Voltage control mode selection: 1) External voltage V, after the impedance of the model or 2) Internal voltage of the inverter E, before the impedance of the model |
QVFlag | - | This flag determines which signal interacts with the control: 1) Reactive Power Q, or 2) Voltage V. |
Current Limiter
To make the model more flexible, it is possible to use an external current limiting function instead of the original function mentioned in [1], which is the default and is called the internal current limiting function. When using this internal function, the filter type considered in the inverter must be the RL filter, and the model outputs correspond to the three phase-to-neutral voltages representing the inverter, as well as the active and reactive power delivered at its terminals.
When using the external current limiting function, the model outputs change, as it now requires not only the active and reactive power at the inverter terminals, but also the phase, angular frequency, and all voltages and currents before and after the filter in DQ coordinates. For example, the REGFM_A1 model can use the https://opal-rt.atlassian.net/wiki/pages/createpage.action?spaceKey=pdochs&title=GFM%20Inner%20Control%20Loop&linkCreation=true&fromPageId=3015016526 from the Smart Inverters library to implement some of the current limiting functions available in this block, including the option to change the inverter filter to an RLC, in which case it would need to be manually added to the interface circuit.
An adaptive filter was added to smooth the voltage response during fault conditions. Usually, it is sufficient to adjust the time constant of this filter to a value slightly higher (10%–30%) than the simulation Ts value. This filter only applies if the internal current limiting function is used.
Name | Unit | Description |
|---|---|---|
ImaxF | pu | Inverter maximum trnsient output current |
Current limiting function | - | This flag determines whether the current limiting function to be used is the original one from the model (internal) or one implemented outside the model (external). |
Time constant of the filter | s | Time constant of the adaptive filter implemented to smooth the response of the current-limiting function (only internal function). A time constant slightly larger than the time step Ts is typically used. |
Load Flow
This tab is used to add the IBR initialization values required for steady-state operation. For load flow purposes, REGFM_A1 is generally considered a PV bus, so it is necessary to define the voltage magnitude in pu and the active power P to be delivered by the IBR at the terminals in MW. In the case of the SLACK bus, the voltage magnitude and angle in pu and degrees, respectively, must be defined.
To calculate the initial conditions of the model from the load flow, it is important to know the type of filter used in the model's interface circuit. In the original model described in [1], only the RL filter option can be used, but if an external current limiting function with an RLC (or simply LC) filter is used, this filter type must be selected, and the capacitance value added in the general tab to correctly calculate the initial conditions.
Name | Unit | Description |
|---|---|---|
Bus Type | - | IBR bus type: Dropdown list: Swing (Fixed V and Angle), PV (Fixed P and V), and PQ (Fixed P and Q) |
Voltage | pu | Desired voltage at the terminals of the model. |
Angle | deg | Desired angle of the voltage at the terminals of the model. |
Active power | MW | Desired active power at terminals of the model. |
Reactive power | MVAr | Desired reactive power at terminals of the model. |
Inputs, Outputs and Signal Available for Monitoring
Inputs
Name | Unit | Description |
|---|---|---|
Vref | pu | Reference voltage at the Point of Interconnection (POI). |
Pref | pu | Reference active power injection at the POI. |
Outputs
Name | Unit | Description | Only available when the following current limiting function is selected |
|---|---|---|---|
Po | pu | Active output power of the model at terminals | Internal |
Qo | pu | Reactive output power of the model at terminals | Internal |
Eabc | pu | Reference three-phase output voltage of the internal voltage sources of the model | Internal |
Edq_ref | pu | Reference dq voltage of the internal voltage sources of the model | External |
wt | rad/s | Angular position (phase) of the model | External |
w | pu | Angular frequency of the model | External |
Idq | pu | DQ currents at terminals of the model (after the filter) | External |
Vdq | pu | DQ voltages at terminals of the model (after the filter) | External |
Iinvdq | pu | DQ currents at the inverter output of the model (before the filter) | External |
Acknowledgements
CanmetENERGY at Natural Resource Canada has contributed in the implementation of all the components in the Smart Inverter Modeling Toolbox software.
Reference
[1] W. Du et al., “Model Specification of Droop-Controlled, Grid-Forming Inverters (REGFM_A1)”, PNNL-35110, Pacific Northwest National Laboratory, September 2023.