Technical Article
Microgrid Sequence of Operations Documentation Explained
If you ask five people to define a “microgrid," you might get five different answers. But one universally required function that cuts across all the nuances of what can make a microgrid a microgrid is the ability to “island” from the grid while continuing to serve onsite electrical loads. The process of disconnecting and later reconnecting to the grid is complex and specific to each microgrid project, and a document developed to aid in system design, called the Sequence of Operations, clarifies how a microgrid is intended to behave. In this article, we will define common modes of operation for solar-plus-storage microgrid systems, explain the transitions from one mode to another, and provide a short list of key questions to ask early in the development process.
Defining Microgrid Sequence of Operations
For the purposes of this article, let’s consider a hypothetical microgrid consisting of a PV solar array and battery energy storage system (BESS) designed to meet resiliency goals by providing full backup during a grid outage and otherwise supply as many of the onsite loads as possible with electricity generated from the PV array. Based on the project goal (resilience) and equipment (solar array plus BESS) we can derive three main modes of operation:
Normal Operation - Our microgrid is connected to the grid, which is operating within the expected voltage and frequency ranges. Since we want to be ready for a resiliency scenario, the energy storage system is programmed to maintain above a reserve state of charge. Any excess electricity generation from the PV array will be used to meet instantaneous loads, charge the BESS further, or export to the grid.
Grid Disconnection - Our microgrid detects a grid outage and undergoes the process of “islanding.” Once fully isolated from the grid, the solar array and BESS will become the primary power source for the microgrid. One of the BESS inverters will enter “grid-forming” mode to provide a reference voltage and frequency for the entire microgrid system.
Grid Reconnection - Grid power returns, and the microgrid transitions from being a self-sufficient energy island to a parallel connection to the broader electrical grid, after detecting grid stability. The solar and BESS inverters all enter “grid-following” mode, relying on the grid as a voltage and frequency reference.
Starting from these three core operating modes, we can document how the components of the microgrid are intended to work together. This forms the basis of a “Sequence of Operations” document.
Sequence of Operations Documentation
Microgrid development is highly collaborative: The end user and system owner should kick off the project by defining performance goals. The developer and engineer of record will determine the best path to meet those goals while also adhering to AHJ requirements and technical constraints. The best way to ensure goals are met – and any changes are addressed – is through documentation.
When Mayfield Renewables designs a microgrid system, we assume the system will perform certain functions. Our Conceptual Sequence of Operations (CSOO) document captures those to verify alignment between us, the client, and other project stakeholders. We include the CSOO document as an attachment with the 30% preliminary design, the point at which the major system components should all be specified. At later stages of the development process, the client may choose to share the CSOO document with the energy management system (EMS) manufacturer who would review and revise the CSOO and program the EMS accordingly.
Returning to our hypothetical solar-plus-storage microgrid system, a CSOO for the three primary operating modes could include:
- Grid Reconnection - Method of detection of a stable grid; operations undertaken to reconnect to the grid.
- Normal Operation - Export of excess PV to the grid vs. curtailment; routine BESS maintenance cycling; reserve state of charge for backup power requirements; and method of monitoring the grid signal for voltage and frequency anomalies.
- Grid Disconnection - Method of grid isolation, including whether the transition is closed or open (see next section); when the BESS inverter will enter “grid-forming” mode; and energy storage state of charge reference points.
Engineering Considerations
Sequence of operations documents are customized to each microgrid project, but there are a few common variables to keep in mind.
Is an open or closed transition required? In an open transition, the microgrid must fully break its connection to the grid before making a connection to the other onsite generation sources. There will be a brief moment (on the order of a few milliseconds to a few seconds) during which the microgrid loses power during the transition. In contrast, in a closed transition the microgrid establishes onsite generation sources as the primary power source (i.e., the BESS inverter enters grid-forming mode) before breaking its connection to the grid. A closed transition is seamless but the equipment required is far more complex and expensive. Closed transitions are required for “emergency” backup loads. Open transitions are much more common for everyday backup loads, because the equipment required is cheaper, easier to procure, simpler to program, and will not risk backfeeding between sources.
Are any of the backup loads considered “emergency” loads? The 2023 National Electrical Code defines an Emergency System as “a system intended to supply, distribute, and control power and illumination essential for safety to human life.” If the backup loads for your microgrid fall into this category (e.g., lighting, heating, and cooling loads in a hospital) you’ll have to comply with more stringent Code requirements, and your microgrid will require a “closed transition” to ensure power is never interrupted.
What are the microgrid interconnection requirements? The IEEE 1547 standard sets baseline requirements for grid-interconnected distributed energy resources such as solar PV and BESS. Utilities can apply their own regulations on top of the IEEE 1547 standard, such as Electric Rule 21 in California. All interconnection requirements must be adhered to during normal operation and must be factored into the grid disconnection and reconnection processes.
Mayfield Renewables provides design and engineering services for solar-plus-storage systems, and feasibility studies for microgrid systems. Contact us today for a consultation.
