# Investigation of Nanogrids for Improved Navy Installation Energy Resilience
## 2 Background and Related Research
### Nanogrid Architectural Configurations from a Resilience Perspective  
A number of major architectural configurations have been proposed in the literature and some have been implemented on a limited basis. This section discusses some of the considerations of nanogrid architectural configurations from a resilience perspective and from the perspective of other important military requirements such as efficiency and renewable energy. The conflict is that many authors design nanogrid infrastructure differently: with centralized and decentralized control systems, the sole use of DC power, or a hybrid of DC-AC\(\backslash\)AC-DC power conversions. Currently no actionable guidance exits on potential nanogrid configurations that may improve resilience of critical loads to outages on military installations. However, commercial solutions that stem from the demand for electrical power for space applications have led to similar refinement of existing technologies for nanogrid-like solutions. Similar to nanogrids, these new technologies address power conversion from PV arrays with management, regulation, and monitoring of electrical demand. Though these power systems are not specifically called nanogrids, their basic elements are similar: energy storage, power conversion, power management and distribution, and use by spacecraft systems [34]. Further, the details of critical loads vary from installation to installation; thus, there is no one size fits all solution. Research has investigated the benefits and drawbacks of implementing both AC and DC nanogrids which highlights the difference in cost between DC nanogrids (high up-front costs) and AC nanogrids (low up-front costs) [18]. Though we explicitly do not consider cost within our research, it is important to note the added cost to augment existing military microgrid infrastructure with DC nanogrids. Some research has recently been focused on understanding the cost of increasing energy resilience from a military perspective [26; 35]. While some military energy organizations are primarily driven by cost, we expect that cost will soon be balanced with energy resilience to better align with the high-level desire to assure that important national security missions can continue in spite of disruptions to grid and microgrid infrastructure. Safety is another concern with military requirements and therefore protection concerns arise when choosing DC nanogrid architecture. Short circuit line and ground faults are more common at output terminals for DC nanogrid architecture then Alternating Current (AC) nanogrid [18]. Examples of this are seen in Okinawa, Japan with experimentation done by researchers to stabilize DC power on nanogrids with three sets of bidirectional DC to DC converters (used in current and voltage regulated mode) to maintain a constant bus voltage [17]. In addition, mitigation of these potential failure modes can occur with the use of arcing-type circuit breakers or more advanced strategies [18]. Control strategies and control system design can have a large impact on a variety of important nanogrid requirements. Nanogrid architectures using either centralized or decentralized control provide different solutions to optimize power production and consumption to better match a load'ssupply and load curves, and reduce the negative effects of intermittency [18]. Centralized control at the microgrid level (controlling the microgrid plus any constituent nanogrids) enables a cohesive control strategy of system dynamics but provides a potential single point of failure. Though cohesion is important, militaries are generally more concerned about reliability and resilience. In addition, when a microgrid is under stress, the purpose of the nanogrid is to independently disconnect from the microgrid at its PCC and operate as an independent system in island mode.