Direct Current Microgrids

  1. Voltage and Frequency Control. The stand-alone supply shall be controlled so that voltage and frequency remain within suitable limits for the connected loads.

Part I. General

  1. Scope. This article applies to direct current microgrids.
  2. Definitions.Direct Current Microgrid (DC Microgrid). A direct current microgrid is a power distribution system consisting of more than one interconnected dc power source, supplying dc- dc converter(s), dc load(s), and/or ac load(s) powered by dc- ac inverter(s). A dc microgrid is typically not directly connected to an ac primary source of electricity, but some dc microgrids interconnect via one or more dc-ac bidirectional converters or dc–ac inverters.ARTICLE 712 — DIRECT CURRENT MICROGRIDS 712.65
    Informational Note: Direct current power sources include ac-dc converters (rectifiers), bidirectional dc-ac inverters/converters, photovoltaic systems, wind generators, energy storage systems (including batteries), and fuel cells.Grounded Two-Wire DC System A system that has a solid connection or reference-ground between one of the current carrying conductors and the equipment grounding system.Grounded Three-Wire DC System. A system with a solid connection or reference-ground between the center point of a bipolar dc power source and the equipment grounding system.Nominal Voltage. A value assigned to a circuit or system for the purpose of conveniently designating its dc voltage class.Informational Note: The actual voltage at which a circuit oper‐ ates can vary from the nominal voltage within a range that permits satisfactory operation of equipment.Reference-Grounded DC System. A system that is not solidly grounded but has a low-resistance electrical reference that maintains voltage to ground in normal operation.Resistively Grounded. A system with a high-resistance connec‐ tion between the current carrying conductors and the equip‐ ment grounding system.Primary DC Source. A source that supplies the majority of the dc load in a dc microgrid.Ungrounded DC System. A system that has no direct or resis‐ tive connection between the current carrying conductors and the equipment grounding system.
  3. Other Articles. Wherever the requirements of other articles of this Code and Article 712 differ, the requirements of Article 712 shall apply. DC microgrids interconnected through an inverter or bi-directional converter with ac electric power production sources shall comply with Article 705.
  4. Listing and Labeling. Any equipment used in the dc circuits of a direct-current micro grid shall be listed and labeled for dc use.

712.10 Directory. A permanent directory denoting all dc elec‐ tric power sources operating to supply the dc microgrid shall be installed at each source location capable of acting as the primary dc source.

Part II. Circuit Requirements

712.25 Identification of Circuit Conductors

  1. Ungrounded circuit conductors in dc microgrids shall be identified according to the requirements of 210.5(C)(2) for branch circuits and 215.12(C)(2) for feeders.
  2. Ungrounded conductors of 6 AWG or smaller shall be permitted to be identified by polarity at all termination, connection, and splice points by marking tape, tagging, or other approved means.

712.30 System Voltage. The system voltage of a dc microgrid shall be determined by one of the following methods:

  1. The nominal voltage to ground for solidly grounded systems
  2. The nominal voltage to ground for reference-grounded systems
  3. The highest nominal voltage between conductors for resistively grounded dc systems and ungrounded dc systems.

Informational Note: Examples of nominal dc system voltages include but are not limited to 24, 48, 125, 190/380, or 380 volts.

Part III. Disconnecting Means

  1. DC Source Disconnecting Means. The output of each dc source shall have a readily accessible, disconnecting means that is lockable in the open position and adjacent to the source.
  2. Disconnection of Ungrounded Conductors. In solidly grounded two- and three-wire systems, the disconnecting means shall simultaneously open all ungrounded conductors. In ungrounded, resistively grounded and reference-grounded systems, such devices shall open all current-carrying conduc‐ tors.

712.37 Directional Current Devices. Disconnecting means shall be listed, be marked for use in a single current direction, and only be used in the designated current direction.

Informational Note: Examples of directional current devices are magnetically quenched contactors and semiconductor switches in overcurrent devices.

Part IV. Wiring Methods

712.52 System Grounding.

  1. General. Direct-current microgrids shall be grounded in accordance with 250.162.
  2. Over 300 Volts. DC microgrids operating at voltages greater than 300 volts dc shall be reference-grounded dc systems or resistively grounded dc systems.

712.55 Ground Fault Detection Equipment. Ungrounded, reference grounded, or resistively grounded dc microgrids operating at greater than 60 volts dc shall have ground fault detection that indicates that a fault has occurred. The ground fault equipment shall be marked in accordance with 250.167(C).

712.57 Arc Fault Protection. Where required elsewhere in this Code, specific systems within the DC microgrid shall have arc fault protection. The arc fault protection equipment shall be listed.

Informational Note: Section 90.4 applies when suitable equip‐ ment for arc fault protection is not available.

Part V. Marking

712.62 Distribution Equipment and Conductors. Distribution equipment and conductors shall be marked as required else‐ where in this Code.

712.65 Available DC Short-Circuit Current.

  1. Field Marking. The maximum available dc short-circuit current on the dc microgrid shall be field marked at the dc source(s). The field marking(s) shall include the date the short-circuit current calculation was performed and be of suffi‐ cient durability to withstand the environment involved.
  2. Modifications. When modifications to the electrical instal‐ lation occur that affect the maximum available short-circuit current at the dc source, the maximum available short-circuit current shall be verified or recalculated as necessary to ensure the equipment ratings are sufficient for the maximum available short-circuit current at the line terminals of the equipment. The required field marking(s) in 712.65(A) shall indicate the new maximum available short-circuit current and date.

Part VI. Protection

712.70 Overcurrent Protection. Equipment and conductors connected to more than one electrical source shall have over‐ current protective devices to provide protection from all sour‐ ces.

712.72 Interrupting and Short-Circuit Current Ratings. Consideration shall be given to the contribution of short-circuit currents from all interconnected power sources for the inter‐ rupting ratings and short-circuit current ratings of equipment in the dc microgrid system(s). Overcurrent protective devices and equipment used within a dc microgrid shall have an inter‐ rupting rating at nominal circuit voltage or a short-circuit current rating sufficient for the available short-circuit current at the line terminals of the equipment.

Part VII. Systems over 1000 Volts

712.80 General. Systems with a maximum voltage between conductors of over 1000 volts dc shall comply with Article 490 and other requirements in this Code applicable to installations rated over 1000 volts.