Microgrids have been around for as long as the electric generator. Indeed, before we built a highly centralized grid, electricity was generated, distributed, and used in small microgrids.
And interestingly enough, these very first microgrids were DC microgrids.
They were built by Thomas Edison in New York City, prior to Tesla’s introduction of multiphase alternating currents (AC) that changed the electricity generation, distribution, and consumption for good.
Today’s microgrids are very different. They are driven by our society’s quest for sustainable and renewable power generation, the need for a more flexible, versatile, and resilient power system, and the ability to effectively control power flow with power electronics converters.
There are three main types of microgrids:
- Customer microgrids (what we all consider as “standard” microgrids)
- Utility distribution microgrids
- Remote microgrids.
Customer microgrids are true microgrids that are self-controlled subsystems connected to the grid downstream from a point of common coupling (PCC).
Utility distribution microgrids are distribution subsystems, part of the regulated grid, that are owned and operated by a utility with the goal of helping the utility manage DER portfolios and improve reliability on the network.
Remote microgrids (power systems) are islanded power systems that never operate in grid tied mode. Here we focus on the grid tied microgrids that can operate both in the islanded and grid-tied modes.
Microgrid control requirements
Microgrids are complex systems since they have to provide all the functions of a large grid, including dynamic control and stabilization, yet with a much simpler control infrastructure, smaller number of generators, and significantly smaller system inertia.
The dynamics of modern microgrids span time constants ranging from microseconds to seconds and all the way to minutes and hours.
While the utility grid has a much larger set of resources available for grid control and stabilization, microgrids are expected to achieve the same power quality with a drastically smaller number of energy resources and often with large penetration of intermittent power sources (PV, wind etc.).
Microgrids are expected to provide a subset of the 6 key functions:
- Frequency and voltage regulation
- Spinning reserve
- Standalone operation
- Seamless transition from grid-tied to islanded modes
- Peak shaving
- Load shifting
All of these 6 functions are enabled by a microgrid controller.
The microgrid controller architecture
Microgrid controllers are both temporally and spatially distributed, due to the nature of the underlying microgrid that is controlled.
Temporal distribution of a microgrid controller comes from the fact that microgrid controllers control the system dynamics spanning from millisecond-second range (voltage and frequency control) all the way to minutes and hours (load shifting and dispatching) as depicted in the previous graph.
Spatial distribution of a microgrid controller stems from the fact that most microgrid devices (that are by definition distributed) have lower level controllers (fast) on the devices, while higher level controllers (slower) are either: centralized, distributed, or a hybrid between the two.
A central microgrid controller topology has one master controller communicating directly with microgrid devices which is responsible for control and coordination of all the devices. The problem with central control is that there is a single point of failure, but this can be mitigated by duplicating the controller.
A distributed control architecture means that control tasks are performed on multiple controllers and inherently implies robustness to control and communication faults. This approach provides more flexibility and fault tolerance, but comes at a price of more complex control design and hence more complex test and validation.
In practical microgrids centralized controllers are still predominant, however there are more examples of distributed controls emerging and we are seeing significant research focus on distributed controls.
This is an edited version of a blog post that originally appeared on Typhoon HIL's Blog.
Rest of the article you can read here: "Simplify Microgrid Control Design, Testing, and Commissioning".