EPRI Grid Forming

Authors: Nils Wiese (Fraunhofer IEE), Martin Franke (Fraunhofer IEE), Saikrishna Vallabhaneni

Reviewers: Lampros Papangelis (CRESYM)

Context

This model was developed by Electric Power Research Institute (EPRI) and provides a standardized approach for inverter based resources [1]. The original model is described in a report available here.

Model use, assumptions, validity domain and limitations

The model is a positive-sequence RMS model, hence it assumes symmetrical operating conditions and neglects high-frequency dynamics. This type of model is often used in large-scale stability studies, for which it reflects the relevant phenomena. It is not a detailed physical model of the unit. Also for some stability phenomena (e.g. resonance stability) this model is not sufficient and EMT models or other approaches may be necessary. A comparison against EMT is shown in the EPRI documentation. It is also available for PSCAD.

Model description

Difference between grid following and grid forming inverter

A grid-following inverter synchronizes with the power grid’s voltage and frequency using a Phase-Locked Loop (PLL), which tracks the grid’s phase angle and enables the inverter to adjust accordingly. In contrast, a grid-forming inverter can actively establish the grid’s voltage and frequency, allowing it to create or stabilize the grid, particularly in situations where traditional grid infrastructure is weak or absent. Unlike grid-following inverters, grid-forming inverters do not rely on a PLL; instead, they can synchronize with the grid similarly to a synchronous generator. Several approaches can be found in literature, a subset of which is present in the EPRI model.

EPRI GFM operation modes

The Generic EPRI GFM model offers four control modes:

• Droop based, \(\omega_\mathrm{flag}=1\), see Figure 1
• Virtual Synchronous Machine (VSM), \(\omega_\mathrm{flag}=2\), see Figure 2
• Dispatchable Virtual Oscillator (dVOC) based GFM, \(\omega_\mathrm{flag}=3\), see Figure 3
• Phase Locked Loop (Grid following mode), \(\omega_\mathrm{flag}=0\), see Figure 4

Model schema

The different control method diagrams of the GFM is shown below:

[!NOTE]

The xy reference frame is the real-imaginary coordinate frame of the network, to which all voltage angles are typically referenced, while the dq reference frame functions as the coordinate system of the control. The relative angle between these two reference frames is represented by the control variable \(\theta_\mathrm{inv}\).

D and q reference currents calculation

Parameters

In [1], per-unit parameters are based on the inverter’s rated apparent power \(S_\mathrm{r}\) in MVA with a default value of 100 MVA.

\(\omega_\mathrm{base}\) is the nominal angular frequency in rad/s.

\(\frac{K_2^{dvoc}}{\omega_{drp}} = \frac{4 \cdot 100^4}{100^4 - \left( 2 \cdot \left(100 - 100 \cdot Q_{drp}\right)^2 - 100^2 \right)^2} \qquad(1)\)

Inputs

Outputs

Initial equations

\(s_0 = P_\mathrm{ref} \qquad(2)\)

\(s_1 = Q_\mathrm{ref} \qquad(3)\)

\(s_2 = \theta_\mathrm{loadflow} \qquad(4)\) \(s_3 = V_\mathrm{ref} \qquad(5)\) \(s_4 = \omega_\mathrm{grid} - \omega_\mathrm{base} \qquad(6)\) \(s_5 = \theta_\mathrm{loadflow} \qquad(7)\) \(s_6 = E_\mathrm{q,loadflow} - v_\mathrm{q,loadflow} - i_\mathrm{q,loadflow}R_\mathrm{f} - i_\mathrm{d,loadflow}X_\mathrm{f} \qquad(8)\) \(s_7 = E_\mathrm{d,loadflow} - v_\mathrm{d,loadflow} - i_\mathrm{d,loadflow}R_\mathrm{f} + i_\mathrm{q,loadflow}X_\mathrm{f} \qquad(9)\) \(s_8 = E_\mathrm{q,loadflow} \qquad(10)\) \(s_9 = E_\mathrm{d,loadflow} \qquad(11)\)

\(s_{10} = i_\mathrm{q,loadflow} \qquad(12)\) \(s_{11} = i_\mathrm{q,loadflow} \qquad(13)\) \(s_{12} = i_\mathrm{d,loadflow} \qquad(14)\) \(s_{13} = i_\mathrm{d,loadflow} \qquad(15)\)

The index “loadflow” indicates that this is the solution of a loadflow.

Open source implementations

This model has been successfully implemented in:

Table of references & license