The rapid growth and diversity of distributed resources is driving changes to the traditional electric system operating model. This emerging hybrid system, comprised of a complementary mix of centralized and distributed energy resources, has created multi-directional power flows while also expanding the number of transactions. Additionally, these resources are increasingly owned and controlled by various parties, including utilities, merchant distributed generators, merchant energy storage, demand aggregators, energy services like solar PV firms, and customers. Operating such an electric system safely and reliably requires an evolution of the distribution system operator (DSO) role and responsibilities. This also necessitates changes in the coordination and duties between transmission system operators (TSO) and DSOs.
Traditionally, the distribution system delivers power from the transmission grid to customers. Utility distribution companies are responsible for maintaining safety and reliability of the system. As such, a primary focus for distribution operators is outage restoration and system reconfiguration (switching) for maintenance and construction. A more distributed system with multi-direction power flows adds new engineering and operating conditions to consider.
The power system will continue to rely on centralized resources and leverage regional diversity of resources to maintain overall efficiency and system reliability. However, the evolution to hybrid system also creates operational interdependencies that requires rethinking the traditional centralized operating paradigm. Currently, industry attention has largely focused on value determination, system planning and infrastructure design. However, it is necessary to expand the scope of industry discussion to include system operations. This more holistic approach is what EPRI calls the Integrated Grid.
In this broader context, DSOs will be required to coordinate the operation of distributed resources and micro-grids to maintain safety and reliability. So, in addition to the functions performed by the traditional distribution operator, the DSO role will be expanded by a new set of minimal responsibilities:
The distributed reliability coordination function refers to a federated reliability system in which DSOs coordinate with distributed energy resources, independent micro-grids and self-optimizing customers to maintain reliable, real-time operation of the distribution grid. In order for the system to maintain reliability, it will require new integrated operational processes and distributed control systems. This system will also include differentiated services provided by distributed resources to support grid operations. The types of participating distributed energy resources will vary based on their performance characteristics and operational need. The DSOs ability to use locally-provided reliability services will also enable it to maintain a more stable and predictable interchange with the TSO at the T-D interface.
The TSOs role will remain to provide open-access transmission services, a duty which includes maintaining supply-demand balance, transmission reliability by scheduling and dispatching resources, along with interchange transactions with other balancing authorities. The DSOs responsibility would then need to include T-D Interface Reliability Coordination. This function would ensure distributed resource-provided services to distribution operations, transmission operations or power markets are properly coordinated, scheduled and managed in real-time. The TSO needs predictability and assurance that distributed resources committed to provide transmission services will actually deliver those services across the distribution system to the T-D interface. The DSO would address this need by also being responsible for forecasting net load in each local distribution area and net power flows across each T-D interface.
The growth of merchant distributed resources and excess energy available for resale by customer-side resources owned by energy services firms will create distribution-to-transmission transactions, as well as transactions within each local distribution area. This creates a need for energy transaction coordination across the T-D interface. Only the physical aspects of these energy and capacity transactions need to be coordinated by the DSO in order to ensure operational integrity of the distribution system. System-wide supply-demand balancing will remain the sole responsibility of the TSO.
It is important to clarify that these minimal DSO functions do not require the DSO to be inserted into the economic transactions between distributed resources and wholesale markets. Rather, the core operational safety and reliability based DSO activities confine it to managing real and reactive power flows across the distribution system. These activities require tight integration of the people, resources, processes and technology used to operate the distribution system. This is due to the highly dynamic and multi-directional nature of power flows in a distributed electric system along with the constantly changing local distribution feeder configurations due to routine and emergency switching.
Beyond these minimal functions, however, it is conceivable that a DSO could take on additional roles, such as dispatch coordinator. The dispatch coordinator function provides a single point of interface on the distribution side of a T-D interface in order to coordinate the operational dispatch of flexible distributed resources in the local area that participates in wholesale energy and ancillary services markets. Such participation may extend beyond TSO managed areas to include broader regional markets as are increasingly available through arrangements such as dynamic scheduling across multiple TSOs. In this role the DSO acts as the dispatch coordinator providing a consolidated offer to the TSO at each T-D interface. Thus, the DSO provides an efficient interface with the TSO along with an organized wholesale markets to ensure alignment between distributed energy transactions and grid reliability.
It is increasingly clear that the industry must begin to specify the operational functions of the minimal DSO as described above in order to achieve large scale integration of distributed resources. Rome wasnt built in day and neither will we wake up tomorrow to a new fully functioning integrated distributed energy system. However, the efforts underway in California, Hawaii and New York, as well as EPRIs Integrated Grid initiative, are developing useful frameworks to realize the vision of a distributed and customer-centric electric system at scale.
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