Though there are many specific definitions of a microgrid, at its core a microgrid means the ability of a distributed energy resource, typically solar PV and battery energy storage, to both interact with the utility grid and stand alone with no utility. From a technical perspective, what are the key elements to achieve this functionality?
1) Managing the point of interconnection (POI)
2) Power equipment mode shifting
Managing the point of interconnection
The point of interconnection is the specific point where the facility’s power network touches or interacts with the utility company’s centralized grid. This is typically within or ahead of the main utility service equipment, though it could also be to isolate a sub panel of priority loads from the overall facility load. Managing the POI involves monitoring and actioning a complete disconnection of the facility from the grid.
Monitoring is critical so the components “know” what the state of the utility grid is. Ideally, a system monitors not just whether the grid has failed completely before taking action but will take action when voltage and or frequency stability drifts outside of allowable values. Once monitoring has determined the grid is no longer stable for facility loads, then an action must occur. This is most commonly opening a contactor, creating an electrical break – an isolation – with the failing utility grid.
One other application where monitoring the point of interconnection is important is when there is a grid restriction on exporting site-produced power to the utility. This can be an outright prohibition, so-called zero export systems, or a limit on exported power, say 50 kW. In these applications, the actionable process will not be to disconnect from the utility but to either charge the battery energy storage system or to curtail PV production to stay under the restricted value.
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Power equipment mode shifting
Once the facility has been isolated from the grid, then the energy storage system must change mode of operation. Advanced power conversion devices (inverters) attached to the battery system are multi-mode devices. This means they can run in parallel with the utility grid, in Grid Following mode, or they can run as a standalone power source, in Grid Forming mode.
To avoid issues with voltage source synchronization, the simplest rule of thumb is to only have one grid forming device in a system at a time. When the utility grid is present, then all other components, PV inverters and battery energy storage system inverters, will interact in a Grid Following mode. When the facility is in a standalone state, then the battery inverter will change to Grid Forming mode, creating the voltage and frequency for the isolated grid.
This changing of modes of battery inverters typically does cause a slight cessation of power. Different solutions offer differing time lags to bring the facility back up to stand alone operation. Remember that each link in the actionable chain adds time to the equation.
First, monitoring captures the failing grid and communicates to a smart relay, which opens a contactor. This first chain typically takes 30-100 milliseconds. Then the battery inverter must change mode of operation, and this typically takes from 20-500 milliseconds. Some solutions will change inverter modes proactively when the utility grid starts to lose stability to minimize this transition time. Generally, this transition will occur in less than one second, typically far faster than the time needed to start and load a back-up combustion generator.
When the utility grid returns, then the order of operations is typically reversed. The monitor at the point of interconnection records a stable grid, for at least 5-minutes. Then the battery inverter will synchronize its voltage and frequency signal to the utility grid, and once achieved, the isolation grid contactor will close. With the utility grid back online, carrying the facility loads, the battery inverter will drop out and change modes back to Grid Following.
Now is the microgrid moment
As one can see, there are a fair amount of actions that must be executed to successfully manage a microgrid. The point of interconnection must be monitored and grid connection managed based on utility stability. And battery inverters must offer multi-mode operation, from grid following to grid forming to manage a power outage. The industry has made excellent progress on the technology side of the microgrid equation, making now better than ever to implement grid-optional systems that are often more safe, reliable and cost effective than traditional utility power in many parts of the world.
Wes Kennedy is a System Design Specialist at Blue Planet Energy.
— Solar Builder magazine