Islanded Operation of Remote Microgrid Using Droop Controllers
This example shows islanded operation of a remote microgrid modeled in Simulink® using Simscape™ Electrical™ components. This example demonstrates the simplest grid-forming controller with droop control.
Remote Microgrid Model
A remote microgrid is often used to serve electric loads in locations without a connection to the main grid. Because the main grid is not available to balance load changes, controlling such a low-inertia microgrid is challenging.
The microgrid in this example consists of two inverter subsystems connected to two different points of common coupling (PCC) buses. The microgrid originally reaches power balance with the fixed loads while a switchable load also connects to the microgrid. A microgrid typically has a preplanned load shedding strategy to reach balanced operations. In a remote microgrid, instant load shedding is difficult to implement. In this example, there is no high-level energy management system, so the microgrid frequency and voltage are kept around their nominal values (60 Hz and 380 Vrms, respectively) using droop control.
In this microgrid diagram, each inverter subsystem interfaces an ideal DC source to represent the DC link of a typical renewable energy generation system, such as a photovoltaic array, wind turbine, or battery energy storage system. Each subsystem includes a droop controller to calculate the d-axis and q-axis reference voltages. The voltage controller regulates voltages by generating the switching sequences feeding to the inverter. The loads originally connected consume a total of 175 kW AC power with a power factor of 0.95.
The droop P/F is set to 2.5%, meaning that microgrid frequency is allowed to vary 1.5 Hz with 1 pu change of real power injected from an inverter. The droop Q/V is also set to 2.5%, meaning that the microgrid voltage at each PCC bus is allowed to vary over a range of 9.5 Vrms around the nominal 380 Vrms with 1 p.u. change of reactive power.
Open the model.
mdl = 'scd3busMicrogridDroopControl'; open_system(mdl)
There is a total of 175 kW load in the microgrid at the beginning of simulation. At 0.5 seconds, a load consuming 75 kW real power with a power factor of 0.98 is connected into the microgrid through a breaker.
Display the frequencies and voltages in the microgrid using the scopes.
open_system([mdl,'/Scopes/Inverter Freq']); open_system([mdl,'/Scopes/Voltage']);
After the abrupt load increase at 0.5 seconds, the microgrid maintains its frequency around 60 Hz and voltages around 1 p.u. at the PCC of each inverter. The increased real and reactive power loads are shared between both sources, as you can see in the scope. The inverter real and reactive powers adjust without using any high-level supervisory control.
open_system([mdl,'/Scopes/Active & Reactive Power']);
Control Design Considerations
In the scopes, note oscillations in both frequency and voltage at each PCC. This result is not surprising as the droop control technique is often considered to be the simplest grid-forming controller for microgrids. Such oscillations might be even worse if you take into account the dynamics of energy storage devices and renewable energy resources.To improve the power quality in the microgrid, more advanced approaches are available, such as synchronous machine emulation and virtual oscillator control. You can implement many of these grid-forming controllers based on droop controller architecture.
The inverter controller also contains voltage controllers. You can further tune the voltage PI controllers to achieve better tracking performance of the d-axis and q-axis reference voltages. For an example on how to tune controllers using the PID Tuner app, see Design PID Controller Using Simulated I/O Data.
Close the model.