As published by Consulting-Specifying Engineer
Microgrids have been around for quite some time, some as early as 1882. Microgrids were generally specified and installed on the electrical utility side of the industry. However, as building owners, governments and designers push for more sustainable, resilient building designs in the commercial and industrial sectors, a customer-owned microgrid may be an option.
For many consulting engineers that work in the commercial or industrial sectors, the design tasks may be daunting. So where does the electrical engineer begin? It is important to understand that there are a number of other types of microgrid contracts in use, such as utility owned or third-party owned (power as a service, or PaaS). These contracts are outside of the scope of this article.
Setting the stage, the electrical engineering team has worked through the upfront legwork of a resiliency study resulting and determined that a microgrid is the right solution for the project. The team is prepared to start the design and approvals process and think begins with codes and standards.
In general, the local building codes and ordinances will govern the design and installation of commercial and industrial facilities, however which code applies depends on the location, type and size of the facility or installation.
In this case, we will focus on NFPA 70: National Electrical Code (2020 edition), as that will be applicable to most designs, with few exceptions for utility-scale generation sources. But first, to understand where to start within the NEC, we need to clearly define what the parts of a microgrid are and how a microgrid connects to the building.
The Department of Energy Microgrid Exchange Group defines a microgrid as “a group of interconnected loads and distributed energy resources within clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid. A microgrid can connect and disconnect from the grid to enable it to operate in both grid-connected or island-mode.”
So what does that mean to a commercial building? That means that some or in most cases, all of the power production equipment on a facility’s site is interconnected within the site distribution on the customer’s side of the point of common coupling. The controls system within the building then operates the sources or distributed energy resources, to either modulate the flow of energy based on the utility agreement or to create a stand-alone system or “island.”
It is important to coordinate with the local utility company through this design process to ensure that any modifications to the utility distribution are discussed and accounted for. In many cases, this process can take several months if not years to complete.
As an example, Figure 1 shows a facility that is integrating DERs such as photovoltaics, wind turbines, generators, electric vehicles and a battery storage system.
There have been many buzz-worthy words that have popped up recently around the industry regarding microgrids. It is important to highlight the differences between the terms grid-connected and grid-integrated as it relates to DERs.
The term “grid-interactive” is also used in regard to buildings as a whole to describe the process of shifting demand to off-peak times through the use of energy storage and/or power production sources.
The electrical installation of the microgrid and all of its components are still governed by Chapters 1 through 4. Chapter 5 may apply in specific applications and all communications cabling shall be in conformance with Chapter 8. So that leaves us to discuss Chapters 6 and 7.
As you the team working through the design of the electrical distribution system, Articles 705 and 710 within the NEC are at the forefront of microgrid principles:
Article 705: Interconnected Electric Power Production Sources — This article is extremely important for both a microgrid design and for buildings looking to back-feed power onto the grid or provide parallel production with the utility source. In Article 705, the code delineates two types of connections: supply side connections and load side connections.
Supply side connections are those made ahead of the service disconnecting means and typically tap from the service conductors. Load side connections are those that are made downstream of the service disconnecting means. Load side connections could occur anywhere on the facility’s electrical distribution, it is not limited to being located adjacent to the service disconnecting means.
There are different requirements for both types of connections and the specific code should be referenced depending on the location and connection of each DER. Additionally, Article 705.13 allows some leeway to right-size the common electrical bus through the use of a power control system. This can be a useful tool for adding to existing installations.
When designing a microgrid, all of the DERs will interact in some form or fashion and the code allows for interactive devices to communicate through a PCS or other interactive means to island the system from the utility source. This is an important concept because the code also requires that, should the primary utility source lose power, an interconnected power production source must be capable of disconnecting all phase conductors from the primary source.
The primary reason for this is safety. Back-feeding onto the service conductors of a facility while the utility works to fix the problem could cause a shock hazard for any lineman working to restore power.
Additionally, when one source drops out, the remaining sources may shift out of synchronization. Should the dropped source restore while connected, the out of sync source would cause electromechanical stresses on the system, potentially resulting in a catastrophic failure. To solve the synchronization issue, interactive devices must be used and the utility source monitored.
In regard to safety, identification and access are addressed in NEC Article 705 for interconnected sources and sources connected on the load-side of the service disconnect. Identification is required to use specific verbiage for service equipment labelling. Be sure to work through access restriction requirements with the facility owner to meet the requirements of the NEC.
Lastly, a key engineering concept to understand when designing an interconnected source of any kind is how the source will contribute to a fault in the system. Article 705 highlights this as a concern in 705.16 — all interconnected power sources shall be considered in the contribution of fault currents when sizing interrupting and short-circuit ratings for equipment and devices. In most systems this is an additive effect and can increase the ratings required considerably. Be sure to take this into account when specifying the equipment.
Article 710: Stand-Alone Systems — In some cases, interconnection with the utility source is not allowed by the utility. When this happens, designing the distribution as a stand-alone system or isolated microgrid through open transitions is an option. Article 710 is a relatively short article, but it sets the requirements for sizing and protection of the supply conductors.
Based on the definitions of a microgrid and Article 710 and 705, it can be concluded that a primary function of a microgrid is to integrate multiple sources together to function as a cohesive system. Let’s discuss some potential sources and how they are governed by the NEC. Figure 2 provides a quick reference to the applicable sections.
Article 690: Solar Photovoltaic Systems and Article 691: Large-Scale Photovoltaic Electric Power Production Facilities — When solar PV systems are used as a DER, the requirements within these articles will need to be followed. In recent years, Article 690 has grown and many adjustments have been made to the language to clarify the intent. Article 691 was added in 2017 as an expansion to Article 690 to better address systems 5,000 kilowatts and greater. As with most code sections, the emphasis here is safety, including a requirement for a rapid shutdown function for firefighter use.
Article 700: Emergency Systems, Article 701: Legally Required Standby Systems and Article 702: Optional Standby Systems — Standby generation sources are typically a necessary part of a microgrid in that they can provide a reliable source of continuous power when there is otherwise no alternate source. It is important to understand the distinction between the three sections that govern the design and installation of standby systems.
Article 692: Fuel Cell Systems — Fuel cells use a chemical reaction to convert hydrogen or a hydrogen-rich fuel such as natural gas to produce electricity. This electricity is produced as direct current power and normally uses an inverter to convert the power from DC to alternating current. Article 692 of the NEC focuses on the requirements for electrical connections to the fuel cell system and the distribution components needed to distribute that power from the fuel cell to the microgrid or other premise wiring systems. It covers the output circuit conductors, overcurrent protection, grounding and labeling requirements.
Article 694: Wind Electric Systems — As with other DERs listed here, a wind electric system can be part of a microgrid with interconnectability to the utility or it can be a stand-alone system, powering immediate loads and/or charging a battery system. Article 694 addresses the electrical installation of the components needed for both types of systems. It provides the required locations of disconnects within the system, sizing of conductors and overcurrent devices, signage requirements, grounding requirements and working clearance requirements.
Article 625: Electric Vehicle Power Transfer System — These systems are no longer just being used to charge electrical vehicles, but also for storage and export of power from electrical vehicles back to the grid or microgrid. Article 625 provides the installation requirements for the electrical vehicle supply equipment, whether it is for charging, exporting power or providing bidirectional current flow.
Although it is important to read and follow the entire article, there are a couple of sections that are specifically for electrical vehicle power export equipment. Paragraph 625.48 addresses listing requirements for systems that can export power and points to other NEC articles that are applicable. Article 625.60 addresses the requirements for AC receptacles used for this type of export equipment.
Article 750: Energy Management Systems — Article 750 provides requirements for how energy management systems control both the energy produced by DERs as well as how loads are controlled. Article 750 requires that when a DER is used to supply power to certain loads related to the protection of life and property, means of egress and other critical building functions the EMS cannot override the controls necessary for the DER to provide continuous power for the loads.
For example, if a facility uses a diesel generator to serve emergency loads per Article 700 and other loads as part of a microgrid, the EMS cannot have controls that would prevent the generator from starting and supplying the emergency loads if normal power is lost. Similarly, Article 750 has requirements that prevent the EMS from reducing the electrical system’s capacity below the minimum needed to serve critical loads and from disconnecting power to critical loads as part of the load balancing process.
Article 706: Energy Storage Systems — One of the issues with sustainable DERs such as PV and wind systems, is that they are not available 24/7, year-round and availability of these sources may not match peak usage times for electricity. Energy storage systems — usually using some type of battery storage — can be used to store and extend availability of power to better match need. Article 706 provides the requirements for installation of the storage system, input connections to energy sources and output connections to the circuits that are powered from the energy storage system. It covers disconnect and overcurrent protection requirements within the system for both DC and AC circuits and controls of the charging process.
Article 712: DC Microgrids — Although most of the microgrids are currently AC to match current utility distribution, DC power is gaining popularity. Many of the DERs are natively DC producing, including PV, wind turbines, and fuel cells, and many of the devices are DC receptive. By using DC power, engineers can eliminate conversions between DC and AC power, eliminating the efficiency losses caused by these conversions.
Article 250: Grounding and Bonding — Although it is outside of chapters 6 and 7, it is important to highlight the requirements of Article 250 as it relates to interconnected sources. Many of the sections mentioned reference these requirements and expand upon the need for a stable, interconnected grounding and bonding system.
Ultimately, the NEC guides the design and installation of all aspects of building electrical systems and application of its articles applies to designing a microgrid. As the technologies and system materials evolve, the code must also evolve and adapt.
The technologies are ever-changing and the NFPA updates the NEC to ensure the safety of the building occupants and the personnel that maintain the systems. Because some states delay in code adoption, a number of the technologies discussed may not be governed by the applicable codes for a project. By using sound judgment and working closely with the authority having jurisdiction, a safe and functional design can be provided.
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