Location

This example model can be found in the software under the category Renewable Energy  with the file name Microgrid_DroopControl.ecf.

Description

This example shows the islanded operation of an inverter-based microgrid using the droop control technique and it is based on a recent example available in Matlab 2021b using  specialized components and algorithms inside Specialized Power Systems libraries of Simscape Electrical.

The microgrid consists of three parallel inverters, with power ratings of 500 kW, 300 kW and 200 kW respectively, connected to the PCC (Point-of-Common-Coupling) bus. A dynamic load model is used to dynamically change the microgrid total load. A Microgrid Supervisory Control is also included, so that when enabled, it will slowly modify the inverters P/F and Q/V droop set points in order to bring back the microgrid frequency and voltage at their nominal values (60 Hz and 600 V respectively).

In addition, a three-phase two-level converter (Switching Function Model), an LC filter, a 480 / 600 V transformer as well as an ideal DC source (used to represent the DC link of a typical renewable energy generation system) are also modeled. Each inverter also includes a control system and a PWM generator feeding the inverter.

The figure below shows the modeling of the main components of the inverter control system used in this example:

• Measurements block: Based on the frequency value given by the Droop control block, the Measurements block computes the active and reactive power generated by the inverter. It also computes the d-q components of the three-phase voltages and currents at the microgrid PCC bus.
• Droop Control block: The droop P/F is set to 1%, meaning that microgrid frequency is allowed to vary from 60.3 Hz (inverter produces no active power) to 59.7 Hz (inverter produces its nominal active power). The droop Q/V is set to 4%, meaning that the microgrid voltage at the PCC bus can vary from 612 Vrms (full inductive power) to 588 Vrms (full capacitive power). In addition, Qmax is  half of the nominal active power Pnom.
• Voltage Regulator block: Reference voltage Vout given by the Droop Control block is fed to the Voltage Regulator block. The regulator processes the measured d-q voltages and reference voltage Vref to generate the reference currents Id_ref and the Iq_ref.
• Current Regulator block: The Id_ref and the Iq_ref reference currents are fed to the Current Regulator block. The regulator processes the measured and reference currents to produce the required d-q voltages (VdVq_conv) for the inverter. The regulators dynamics benefits from a feed-forward calculation.
• Reference Voltage block:  The required d-q voltges from produced by the Current Regulator block VdVq_conv are scaled and transformed to a three-phase signal Vref feeding the PWM modulator generating pulses to the inverter.

Simulation and Results

Before to start the simulation, please make sure that the Activate iterative method option (General tab in the Simulation Settings window) and the Solve control inputs before solving power option (Avanced tab in the Simulation Settings window) are both enabled. Start the simulation and use the SCOPEVIEW template to see the resulting signals of the following programming sequence:

1. At 1 s, the total microgrid load is increased from 450 kW / 100 kvar to 850 kW / 200 kvar.
2. At 3 s, the droop control is enabled on all inverters. See how the microgrid load is now shared equally among the three inverters.
3. At 5 s, the supervisory control is enabled. The frequency is then being slowly increased to 60 Hz and the line voltage to 600 V.

The PCC bus measurements are shown in the figure below. The model was tested at a time-step of 50 us. At the beggining of the simulation the frequency at PCC bus is 60.3 Hz while the line voltage is 600 Vrns. See how the line voltage decreases while the microgrid load is increased at 1 s. Once the droop control is enabled the frequency drops from 60.3 Hz to 59.8 Hz. Finally, when the supervisory control is enabled (5 s), see how both the frequency and the line voltage are slowly increased to 60 Hz and 600 Vrms respectively.

The resulting power sharing based on the programmed events is shown in the figure below.  Observe at the beggining of the simulation all the inverters start at different active powers (Inverter 1 = 0.52 pu, Inverter 2 = 0.31 pu and Inverter 3 = 0.41 pu). Once the droop control is enabled (at 3 s) the inverters slowly share the same active power (0.83 pu).

References

MathWorks, Islanded operation of an Inverter-based Microgrid Using Droop Control Technique https://www.mathworks.com/help/physmod/sps/ug/power-Microgrid-IslandedOperation-DroopControl.html.

See Also

Examples | 2 MW Grid-Connected PV array,