Why are there differences between the MicroGrid Islanded Operation example in Simulink R2024b and the referenced paper?

I am following the MathWorks example about Micro-grid Islanded Operation Droop Control.
I noticed two discrepancies in the example model and model in the referenced IEEE paper: H. Karimi-Davijani and O. Ojo, “Dynamic operation and control of a multi-DG unit standalone Microgrid,” IEEE PES Innovative Smart Grid Technologies Conference, pp. 1–7, Apr. 2011, doi: 10.1109/ISGT.2011.5759177.
Specifically, in the following subsystems:
"MicroGridIslandedOperation/Inverter 1 500 kW/Control System/Voltage Regulators"
"MicroGridIslandedOperation/Inverter 1 500 kW/Control System/Current Regulator"
The differences:In the paper, "Vq" passes through a "PI controller" block to generate "Iqref" and "Vd" becomes "Idref." However, in the example model, "Vq" is passed through the "PI controller" block and becomes "Idref" and "Vd" becomes "Iq."In the paper, the output transmission line inductance "Lsi" is multiplied by the measured currents "Id" and "Iq", but in the example model, the respective inductance "Lff" is multiplied by the reference currents "Id" and "Iq."
Are there any explanations for these changes in the control structure?

Answer 1

[MathWorks Support Team]

The model in the example uses the model in the reference paper as just a reference. The original author of this example likely did not intend to copy the paper's model in full entirety. For questions related to the reference paper, we recommend reaching out to the authors of that paper. Below are explanations for the design justifications in regards to the two differences:

For the first difference:

The active power and reactive power (in per unit) flowing into the AC grid are given by the following equations:

P = Vq*Iq + Vd*Id

Q = Vq*Id – Vd*Iq

For the ‘Voltage Regulators’ subsystem, the d-axis voltage reference "Vd_ref" is set to be 0. The q-axis voltage reference "Vq_ref" is related to the required AC grid voltage magnitude (i.e., 600 Vrms, line voltage). 

Assuming that the dq-axes voltages are perfectly controlled, then Vd = 0, and "Vq" is around 1 per unit. As a result, the above P and Q equations are simplified as follows:

P = Vq*Iq 

Q = Vq*Id

Now, you may see that the d-axis current "Id" is proportional to the reactive power flowing into the AC side (i.e.,Q) and the q-axis current "Iq" is proportional to the active power flowing into the AC side (i.e., P). So, to control the AC grid voltage magnitude, we need control the amount of reactive power flowing in the AC side (Q), in other words, we need control "Id." This is why the output of the q-axis voltage controller is the d-axis current reference "Id_ref."

For the second difference:

Using either dq-axes current measurements or dq-axes current references in the feedforward calculation is feasible. Users can pick one approach depending on their preference. In the example model, the current references are used.