(Part V in a series)
As outlined in the previous four articles (links to the four previous articles at end of this article), grid architecture provides a set of methodologies for analyzing and understanding the structures of the electric grid to enable superior decision-making, and among other outcomes, reduce the risk of poor functionality and stranded assets. This is not a simple process for a utility to undertake. Developing and executing a grid architecture from scratch requires significant dedication and resources. A more practical approach for a utility is to adopt and adapt an existing reference grid architecture to meet their specific needs.
To facilitate utility adoption of the grid architecture methodologies, the U.S. Department of Energy Grid Modernization Initiative is developing reference architecture packages covering a range of scenarios and electric power sector segments. These span a set of identified key issues and focus on:
- High Resilience Grid
- High Distribution Automation and High DER (Distributed Energy Resources) with High Storage option
- Advanced Bulk Energy Systems
- Variable Structure Grids
- Urban Converged Networks
The reference architectures focus on different aspects of the grid allowing utilities to choose relevant parts and pieces as needed. The models and structures contained in these packages have been developed and vetted by industry experts and represent a baseline upon which to build the next-generation architecture for a utility. They apply the grid architecture process to complex grid scenarios and provide a set of high-level specifications that are intended to be insightful but not necessarily prescriptive.
Reference Architecture Packages
Each of the reference architecture packages contains a specification library that includes a core principles document, a master specification containing structure views, component class models, quality/property mapping, and other supporting documents. Along with the specification, the package also includes documents that describe the inputs used during the architecture process such as emerging trends and systemic issues, reference models, stakeholder expectations, regulatory issues, and key use case scenarios.
The grid architecture specification is the core technical document that defines a set of grid architecture views. It references other documents in the specification library including background theory and principles, grid entity definitions and structure models. The documents included in a specific architecture package will vary based on its scope.
High Resilience Grid Reference Architecture
For this reference architecture, resilience and reliability have been examined from the perspective of their relationship to structure. This leads to expanded definitions for both. Grid resilience and reliability are related concepts but for this reference grid architecture we found it necessary to draw some distinctions. Grid resilience is the ability of the grid to avoid or withstand grid stresses without suffering outages. Once a sustained outage occurs, recovery efforts are undertaken which focus on isolating faults and restoring service. (see diagram 1) Grid resiliency includes avoiding stress, resisting stress and adjusting for the resulting strain up to the point of fracture. Grid reliability focuses on the fracture and recovery. Improving resiliency results in fewer sustained outages thereby improving overall grid reliability. Grid resilience is best understood in terms of grid vulnerability, where resilience elements represent countermeasures to those vulnerabilities.
The objective of the high resilience reference architecture is to modify existing grid structure and specify new grid structure to improve grid resilience. These structures are aimed at improving resistance to stresses and the capability to adjust to strain, and do not address process improvements.
There are numerous work products of the grid architecture process in the high resilience grid architecture package. For this article we’ll highlight four of these outputs.
The “Distribution Layer Structure and Observability Platform” (see diagram 2) uses layering concepts to define a platform structure for resilient electric distribution operations that treats sensing and communications as an infrastructure layer. This results in the decoupling of applications and reduces brittleness and enhances configurability, functional flexibility and functional extensibility. This structure defines the following three layers; electric infrastructure, sensing and communications, and applications.
“Logical Energy Networks” (LENs) (see diagram 3) virtualize distribution systems to improve the accessibility of DER assets for resilience support. Virtualization consists of creating a uniform logical structure that exists on top of physical structure. The uniform logical structure allows for standardization of interfaces and interactions across a variety of underlying physical components and structures. Virtualization is accomplished by segmenting the distribution system into “resilience cells” that can operate autonomously when needed, but cooperatively in groups as well. These are roughly analogous to bulk system balancing areas and may be thought of as virtual microgrids that may have, but do not require, islanding capability. Each cell of a primary feeder has devices and loads along with energy sources such as two physically diverse primary distribution feeders; distributed generation and/or storage that is utility controlled or coordinated; along with non-utility-controlled DER.
LENs operate in a cellular fashion, functioning autonomously when necessary and coordinating globally via laminar coordination networks when connected electrically and via communications. Under conditions of grid strain, a distribution system based on LENs can separate into segments that continue to operate without the need for either centralized or global coordination and control.
“Coordinated Storage Networks” improve resiliency by adding buffering to the system, which serves the key purpose of decoupling volatilities. While some storage exists in transmission systems, electric distribution typically has little or none. Fast bilateral energy storage (energy flows in from the grid and back out to the grid at high rates) added to electric distribution systems as core infrastructure can be used for resilience and operational purposes such as generation/load decoupling, outage ride-through, volatility export suppression, and cyber-security improvement. This structure specification describes interconnection topologies and operational considerations.
“Distributed Intelligence Platform” (see diagram 4) provides integrated computing and communications throughout the grid at each level in the grid hierarchy. This provides the operating environment that enables the application of laminar coordination frameworks that support some of the other new grid architecture structures. This platform includes the application layer, tools layer, and the network and compute layer.
Putting Grid Architecture to Use: Adopting and Adapting
Reference structure models and specifications contain broad views of the grid which can apply to any sized utility’s transmission and distribution systems. The first step in adopting and adapting a reference architecture tailored to a specific utility involves understanding and analyzing the documents. Experts within a utility need to review and analyze these documents to separate relevant from non-relevant information with the goal of creating a customized grid architecture package that can be easily shared within an organization.
Step 1: Review the Objective, Emerging trends, Systemic Issues and Principles
Review the objective of the package as described in the architecture specification along with the emerging trends, systemic issues and principles to understand if these align with a specific utility. If they do, then the reference architecture can provide a blueprint for the structural evolution of the utility. If these are partially aligned, then an effort needs to be undertaken within the organization to review, modify and extend them as needed. An analysis then needs to be undertaken to assess the impact of these revisions on the reference architecture.
Step 2: Review and Adapt the Structure Diagrams and Component Class Specifications
The reference package contains several industry, market and regulatory structure diagrams that should be reviewed and selected based upon their relevance to a specific utility. The review process should validate or update the relevant structures. This provides the overall context for developing a grid architecture adapted to a utility. Component class specifications describe the black-box components used in the architecture.
Step 3: Review and Adapt the Grid Architecture Specification
The first two steps provide the context for a specific grid architecture. Once they have been reviewed and adapted, the grid architecture specification itself can be reviewed and updated based upon any changes or enhancements made in the previous steps. The result of this process is a grid architecture specification that is tailored to the unique context and organization of a given utility.
Grid architecture reference packages enable utilities to leverage the expertise and knowledge of industry experts prior to planning and developing robust grid modernization programs. These packages provide a foundation upon which to construct a next-generation grid architecture that aligns with the needs of a specific utility. Their primary purpose is to provide insight into structural grid modernization issues.
The first reference architecture package is the high resilience grid. This package is available for download here. The remaining reference architecture packages will be posted on the same web page as they become available.
Next-Gen Grid Architecture Series:
- Part I – Next-Gen Grid Architecture: Laying the foundation for a next gen grid architecture
- Part II – Next-Gen Grid Architecture: A prerequisite for grid modernization
- Part III – Next-Gen Grid Architecture: A peek into industry structure
- Part IV – Next-Gen Grid Architecture: A grid coordination primer