By Yasmin Ali, Microgrid Knowledge
‘Air gap microgrids’ — DC microgrids electrically separated from the utility — open up the opportunity for installing and adapting microgrids with fewer permitting requirements and less strain on the utility’s grid system.
North American microgrids are normally connected to the grid with AC coupling. Each item of equipment in the microgrid, like solar PV or a battery, is connected to the main grid as well as being interconnected within the microgrid. Although behind the meter economically, it’s all part of the same system electrically.
“When you connect in the AC coupled manner, which the whole industry is doing, you’re connecting those resources on someone else’s grid,” Aleksey Toporkov, president of ARDA Power, told Microgrid Knoweldge.
Simpler permit requirements
As each item of an AC microgrid is grid-connected, the responsible utility must issue a permit accounting for the maximum export and import capacity of every piece of equipment. A separate permit is required for islanding the microgrid, the process of physical disconnection from the grid to allow independent operation.
DC coupling creates an ‘air gap’, making the microgrid invisible to the utility. A single power converter, set to the desired export or import limits, connects the air gap microgrid to the grid. The microgrid’s individual elements connect only with each other.
The DC microgrid can be designed to have an import only grid interface, if there is no value in exporting to the grid.
“When you have a DC microgrid, you have one DC world managed by that local entity. The AC world continues to be managed by the big utility grid,” Toporkov explained.
Supporting electronics manufacturing with air gap microgrids
ARDA Power provides the architecture for DC microgrids. Burlington phase 1, a DC microgrid in Ontario, Canada, supporting an electronics manufacturing facility, was the first full deployment of ARDA’s solution in the commercial and industrial sector. Federal and provincial authorities funded the project.
Phase 2 of the project will extend the microgrid, making it what ARDA describes as the world’s first electric vehicle (EV) charging DC microgrid. Upon completion, the microgrid will have 50 kW of solar photovoltaics, a 100 kW/120-kWh battery, a 10-kW natural gas generator, 50 kW of EV fast charging, and 12 kW LED lighting. But to the utility, the picture is different.
“When we connected that microgrid, we didn’t ask the utility a single question. We didn’t need to get any permits…It’s only a 15-kW load as far as the utility is concerned. What’s behind it is quite a sophisticated microgrid,” Toporkov said.
ARDA Power have announced a similar DC microgrid project at the University of Toronto, with 30 kW of solar PV, a 150-kW/180-kWh battery, 10-kW LED lights, high capacity EV fast charging, and a 30-kW grid connection.
Solar PV, batteries, and EV chargers operate on DC. A power converter is required for each of these when connecting with the grid, to convert the power to AC. In a DC microgrid, these connect with each other on the DC side.
Only one converter connects the system to the AC side, sized for the import and export, saving on equipment costs and preventing unnecessary power conversions. This offers increased system efficiencies.
Turning solar PV installations into DC microgrids
Many solar PV installation on rooftops in North America export power to the grid, from the DC to AC side. If the grid is down, these systems no longer operate.
According to Toporkov, adding power converters to existing installations can transform them into DC microgrids, without changes to the grid connection permit.
“If you like, you can extend the solar capacity. You can add a different source, like a natural gas generator, or wind, or diesel. You can also use this resource to cover other loads in the building.”
Many commercial and industrial facilities are beginning to think about EV fast charging. A DC microgrid could be a way of locally managing the additional loads, without asking the grid for more capacity.
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