Alternating current and direct current microgrid systems are not very common yet, but with all the new energy sources being developed, they may become important energy sources in the future.

While DC microgrids are the most common source of energy for these systems, with photovoltaic (PV) panels, wind turbines and energy storage systems being the primary sources of energy, do not discount the more common AC systems. AC and DC power sources will be required to supplement our normal power grid, but new power systems under development may be totally separate from the grid and can be switched from utility to stand-alone power.

To become more familiar with DC microgrids, electricians, electrical contractors and electrical engineers must know Article 712, covering DC microgrids that are not designed to interface with the utility company power. Article 705 is also important because it deals with primary sources of power, including utility company power supplies, which are often called interconnected electrical power production sources.

What is a microgrid?

The definition of a DC microgrid is a power distribution system consisting of more than one interconnected DC power source, supplying DC-to-DC converters and DC loads, as well as AC loads powered by DC-to-AC inverters. A DC microgrid is usually not connected to an AC primary source of electricity, but it can be connected by one or more DC-to-AC bidirectional converters (commonly called rectifiers) or DC-to-AC inverters. A transfer switch can be used on the AC side to provide transfer of power to either supplement the utility company power or replace it when not available.

A DC microgrid can either be an ungrounded system based on Part VIII of Article 250 or can use a “reference grounded system,” which is defined as not solidly grounded but having a low-resistance electrical reference that maintains voltage to ground in normal operation. This reference system is often functionally grounded through a ground-fault detection system that senses a ground fault and opens all the conductors to clear the fault. Another option is to use a two-wire or three-wire grounded system. Either system can be solidly connected to ground or can use the reference grounded system.

Turning to the NEC

All DC microgrids must be grounded based on Section 250.162, which states that “a 2-wire, DC system supplying premises wiring and operating at greater than 60 volts but not greater than 300 volts shall be grounded” and that “the neutral conductor of all 3-wire, DC systems supplying premises wiring shall be grounded.”

Section 712.52(B) states that microgrid systems operating in excess of 300V must be reference-grounded or functionally grounded DC systems. Either of these systems operating at greater than 60V DC must have ground-fault detection, and this equipment also must be marked in accordance with 250.167(C), as follows: “Direct-current systems shall be legibly marked to indicate the grounding type at the DC source, or the first disconnecting means of the system. The marking shall be of sufficient durability to withstand the environment involved.”

Where required elsewhere in the NEC , specific DC systems in the microgrid must have listed arc fault protection. This protection will ensure arcing will be detected and shut down.

The components

All electrical components of a DC microgrid must be listed and labeled for use in a DC circuit. The output of each DC source in the system must have a readily accessible disconnecting means that is lockable in the open position, based on the requirements in 110.25, and must be located immediately adjacent to the DC source.

In solidly grounded two- or three-wire DC systems, the disconnecting means must simultaneously open all ungrounded conductors. In an ungrounded, resistively grounded (where installing a resistor to increase the impedance of the system to ground similar to that permitted in 250.20(D)) or reference-grounded system, the disconnecting means must open all current-carrying conductors. For any directional current devices, such as magnetically quenched contactors or semiconductor switches, the disconnecting means shall be listed, marked for use in a single current direction and only be used in the designated current direction.

The DC branch and feeder circuit conductors must be identified in compliance with 210.5(C)(2) for branch circuits and 215.12(C)(2) for feeders. The available DC fault current on the microgrid must be field marked and include the date the available fault current calculation was performed. When any system modifications occur, recalculations must be done to ensure the equipment ratings are sufficient for the available fault current.

Installing and maintain a microgrid system can be complex, and ensuring safety is of the utmost importance.

Header image by Wikimedia Commons.