Design Challenges

Creating a successful microgrid is challenging. It involves understanding basic system requirements, accommodating unique operating characteristics of generation sources, managing and prioritizing power requirements, and seamlessly interfacing with the utility grid. A structured approach to microgrid implementation gives us the ability to support critical infrastructure even in the event of a major power disruption.

A recent project on a military base in the U.S. demonstrates the ability of a microgrid to be isolated at will from the primary grid and achieve uninterrupted power delivery, independent of utility power. The application is designed to maintain mission critical and support functions regardless of the availability of the primary grid.

Early on, an analysis of the system to define the goals and parameters of the microgrid operational characteristics is necessary. The evaluation forms the backbone of the infrastructure requirements and microgrid plan, minimizing long-term operating costs and initial investment by identifying exact system needs. In military applications, a goal may be to maximize the generation content from renewable resources, minimize fossil fuels, and provide secure, local power. Larger government facilities may elect to maximize electrical supply to critical loads at the lowest overall cost.

Establishing system requirements involves identifying critical loads and deciding on the infrastructure that must be operated under islanded conditions. These decisions become much more complex as the gap between available generating capacity and load requirements widens.

Each generation source has unique operating characteristics. For example, photovoltaics are highly intermittent power sources. Combined-cycle units require distinct start-up procedures before they can be counted on to produce steady electricity generation. Available local generation assets need to be analyzed holistically to achieve seamless operation.

Microgrids control the dynamic balancing of varying loads with available generation capacity to maintain system stability. This can be performed with pre-determined rule-based load shedding scheme for simple microgrid applications, while a more dynamic fast demand/response capability coupled with generation optimization strategies is often more suited for larger, more complex designs.

Extensive modeling and analysis needs to be performed to address the protection of the generation assets and subsystems. Fault studies and multiple protection devices are often required to create a robust protection system.

In microgrid applications, many of the control issues are focused on communications and the lack of an integrated systems approach to microgrid controls. Careful consideration needs to be exercised when selecting communication protocols and the data that is available for exchange. An open standard, plug-and-play communication system is essential.

Cybersecurity, password protection, and traceability are key considerations for microgrid applications, especially those serving critical government facilities and functions. Communications in military base applications often require Department of Defense Information Assurance Certification and Accreditation Process certification to communicate with existing infrastructure. Strong consideration should be given to microgrid system capabilities that will make this certification easier and meet utility connection cybersecurity as well.

Further, microgrids control a complex operating environment due to the number and types of generation and load assets. One solution is to require closed loop simulator testing to fully prove the system implementation prior to system shipment. This is the best method to evaluate operating scenarios and validate the safety of the controls.

The system's overall design structure should consider whether it will be easy to expand and change in the future or will considerable custom programming be required. The generation and load assets of microgrids tend to be dynamic as new options become available and load considerations change. Systems designed in a modular building-block fashion adapt well to change with minimal rework.

With the continuing proliferation of distributed energy sources, the grid will continue to become a much more dynamic electrical environment that will require ever-smarter apparatus, faster communications, and automation systems in order to maintain grid stability and power quality.

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