Implement Droop Controllers for Islanded Operation of Remote Microgrids
Learn how to design grid-forming controllers with droop control for an islanded operation of a remote microgrid. A microgrid typically has a preplanned load shedding strategy to reach balanced operation. However, instant load shedding is difficult to implement with the absence of a main grid to balance load changes. Droop control is used for such a scenario, keeping the microgrid frequency and voltage around their nominal values without the energy management capability of the main grid.
Published: 25 May 2022
In this video, we look at developing a simple grid-forming control for a remote AC microgrid using droop control. A microgrid is a type of power network used to facilitate the incorporation of renewable energy resources. The AC microgrid in Simulink has been modeled with Simscape blocks.
The DC cells here represents DC things of typical renewable energy generation systems such as PV arrays, wind turbines, or battery energy storage systems. The concept of microgrid is enabled by controllers associated with power electronics interfaces such as these inverters here between the cells and the network.
Along with inverters and DC cells, there are three fixed loads consuming 175 kilowatts of AC power from the microgrid with a power factor of 9.95. There is also a switchable 75-kilowatt real power load with a power factor of 4.98 that connects to the microgrid through a circuit breaker. A microgrid typically has a pre-planned load shedding strategy to reach balanced operations.
Instant load shedding, however, is difficult to implement without a high-level energy management system. This is where a simple droop controller can help maintains AC microgrids frequency and voltages around their nominal values. Each of the inverter subsystems in this model includes a droop controller to maintain a voltage of 380 volts and a frequency of 60 Hertz.
In this droop controller design, the active power frequency droop coefficient is set to 2.5%, meaning that microgrid frequency is allowed to vary 1.5 Hertz around 60 Hertz with one per unit change of real power injected from the inverter. The reactive power voltage droop coefficient is also set to 2.5%, meaning that's the voltage at each point of common coupling bus is allowed to vary over a range of 9.5 volts around the nominal 380 volts with one per unit change of reactive power.
The resulting frequency and the voltage commands are used to calculate the D and Q axes reference voltages, which are positive to the voltage control loop. This PID controller regulates the voltages through the generated three-phase PWM switching sequences fitting to the inverter. Let's just now run simulation and examine how the droop controller performs while an abrupt 75-kilowatt load is brought up online.
This is the voltage scope of the connected inverters. We can see that both inverters voltages are maintained around one per unit after the load switches on at around 0.5 seconds. Likewise, the microgrid frequency is also maintained at 60 Hertz. Looking at the active and reactive power scopes, we see that the increase in the real and reactive power lows are shared between both cells without any high-level supervisory control. In summary, we saw how we can design a simple grid-forming controller with droop control for remote AC microgrid using Simulink.