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REGFM_B1
Description
The REGFM_B1 (Renewable Energy Grid-Forming Model – Type B1) represents a positive-sequence, virtual synchronous machine controlled (VSM) grid-forming inverter.
The REGFM_B1 model includes a voltage source behind impedance representation (see the figure below)), a primary control block which regulates the P-f control loop, a Q-V voltage regulation control, 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:
VSM control :
Develops the internal voltage angle (and frequency) command and provides active-power sharing (see the diagram below)
Voltage control :
Develops the internal voltage magnitude command and provides voltage regulation as shown in the following diagram.
PLL :
In the REGFM_B1 model, the PLL does not serve as a synchronization mechanism — that role belongs to the VSM control. Instead, it acts as a network observer, providing a terminal voltage angle (δPLL) used exclusively for dq-frame transformations of measured signals. This ensures measurement consistency during transients, when the VSM internal angle (δVSM) may temporarily diverge from the actual terminal voltage due to virtual inertia dynamics
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) |
VSM
Name | Unit | Description |
|---|---|---|
mp | % | P-f droop gain |
H | s | Inertia time constant |
D1 | pu | Damping |
D2 | pu | Transient damping |
wD | pu | Angular frequency of the washout filter |
Dwmin | pu | Lower limit of delta wm |
Dwmax | pu | Upper limit of delta wm |
wFlag | - | A flag to select wheter to use the measured frequency (wm) or the PLL frequency (wPLL) as the input for the P-F droop |
Voltage Control
Name | Unit | Description |
|---|---|---|
mq | % | Q-V droop gain |
kpv | pu | Proportional gain of the voltage controller |
kiv | pu/s | Integral gain of the voltage controller |
VdrpFlag | - | This flag determines which signal interacts with the control: 0 = Reactive Power Q, or 1= Q-axes current iq as the input of Q-V droop |
PLL
Name | Unit | Description |
|---|---|---|
VPLLfrz | pu | Voltage threshold to freeze the PLL state variables |
kpPLL | pu | Proportional gain of the PLL controller |
kiPLL | pu/s | Integral gain of the PLL controller |
DwPLLmin | pu | Lower limit of the PLL output |
DwPLLmax | pu | Upper limit of the PLL output |
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_B1 model can use the https://opal-rt.atlassian.net/wiki/pages/createpage.action?spaceKey=pdochs&title=GFM%20Inner%20Control%20Loop&linkCreation=true&fromPageId=3014230242 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.
Name | Unit | Description |
|---|---|---|
ImaxSS | pu | Steady-state current limit |
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) |
ki | pu/s | Integral gain for the active current limit loop |
kf | - | A factor to determine Iqmax or Idmax |
ESFlag | - | A flag to determine if the model represents a battery source (=1) or a non-battery source (=0) |
PQFlag | - | A flag to determine whether P priority or Q priority should be selected |
Load Flow
This tab is used to add the IBR initialization values required for steady-state operation. For load flow purposes, REGFM_B1 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] Du, Wei, Sebastian Achilles, Deepak Ramasubramanian, et al., 2024. Virtual Synchronous Machine Grid-Forming Inverter Model Specification (REGFM_B1). UNIFI-2024-6-1