The Engine Driving Distributed DERMS
by Gary Ockwell, P.E.
Implementation of renewable generation within the smart grid can be complex. The complexity does not arise from the detached use of renewable electric energy generation serving a native load. The complexity arises through the coordination of all available feeder resources, including renewable generation injection, to optimize the operation of the entire feeder for the benefit of all loads. Without an interconnected and reliable network, all loads are isolated and vulnerable.
The complexity is expedient since it amplifies the effectiveness of renewable deployment, thereby maximizing its benefits to the grid. DERMS should focus on its ability to contribute and interact with all the other smart grid functions that exist to operationally improve reliability and power quality while lowering the cost of delivery. Therefore, DERMS operation should not be a standalone feeder resource; it must be integrated with other smart grid automation technologies.
An integrated set of smart grid functions operating within DERMS necessitates certain capabilities. The most basic requirement is for a common operational and real-time model of the network. All applications will derive their information from this model to coordinate the optimum real-time response to the network.
A common roadblock for many utilities who seek to deploy a renewable implementation is that few legacy centralized distribution control center systems can support a large dynamic network load flow model, nor are they equipped to manage the size of a complete model database. The effective DERMS implementation should consider "what is the simplest and lowest cost approach to deploy DERMS without the model complexity dwarfing the DERMS itself?"
Distributed DERMS Architecture
The selection of the physical architecture with the commensurate model of a DERMS will influence a utility's cost, as well as the complexity and schedule of deployment. A characteristic of DERMS growth is that it is generally localized in small areas throughout the network. Accordingly, it is preferable to avoid modeling the entire network in favor of implementing DERMS incrementally. A decentralized, distributed architecture greatly simplifies the project cost and, in particular, reduces the size of the network model, which is generally the most complex and costly aspect of a centralized DMS/ADMS.
A generic application processor that manages and controls the DERMS island is the heart of each distributed island of automation' in the DERMS architecture. The island' is defined as the contiguous feeders that are to be automated. The user defines the island size. The application processor may be deployed as a virtual or physical platform which can reside in a single rack, or it can be geographically distributed to reside local to each island. Three potential levels of growth should be considered when deploying a distributed DERMS automaton architecture:
Initial solution: the distributed system must support the ability to easily expand an island as the number of feeders within it become automated or when the number of disconnected islands grow.
Ultimate solution: it will become important to centralize the solution; therefore, a roadmap plan must exist to effectively merge islands into a single or centralized network.
Hybrid solution: distributed, autonomous DERMS islands may coexist within a centralized system. By operating autonomously, they effectively distribute the processing load during crisis events and storms.
Regardless of the level of deployment and growth, each distributed island must report the telemetry, messages, and control actions to the centralized system. To reduce communications and avoid costly changes to the legacy system, the island emulates the physical RTUs as virtual RTUs, using the same protocols and configuration as the physical units.
Distributed DERMS Model
The purpose of a common network model is to support the operation of various basic fully-automated application technologies such as self-healing, optimal feeder reconfiguration and Volt/VAr control with renewables. The real-time coordinated control solution, which meets the optimized balance of all objective functions of the island's feeders, is sent as control signals to the relevant feeder network devices and energy resources.
To maximize the benefits of a flexible, low cost, incremental deployment of renewables, it is important that the network model supports distributed, centralized or hybrid architectures. Two aspects of the model should be considered: the as-built' model maintenance and the real-time topology' model.
Model maintenance must easily support the creation and addition of objects and changes to the as-built' network. New islands are incrementally added using the same process. A DERMS model definition tool must be provided to field technicians so that they can create a model and commit it to operation within an hour. The key feature of this tool is the efficient, flexible creation of the model defining line segments, switches, regulators, circuit ties, capacitors, voltage regulators, LTCs and DERs. The island's configuration is defined at the same time by selecting communication ports, protocols, IED point mapping and profiles. The list of potential devices and configuration settings should be selected easily from a drop down list. To simplify model maintenance for hybrid configurations, the island's model can be created as a subset of the centralized system which is pushed to the island.
The real-time topology model reflects the operational state of the network from which all applications derive their analysis. The real-time telemetry of all island IEDs and RTUs are consolidated in the application processor and passed to the centralized systems. The software that runs within each island should be identical, where only the model definition defines the distinctiveness between islands.
An important feature of the model creation is the automatic creation of a graphical representation built from the model. Field crews and potential control center users should be able to view the graphical colorized depiction of the island's real-time topology. The user can change the state of manual switches. In a centralized or hybrid architecture, the manual switch states can be downloaded to reflect the non-telemetered normal and abnormal states. Similarly, the user interface should enable the user to insert temporary operational (not as-built) network cuts, jumpers, switches, and fuses. Following the restoration of outages and events causing the DERMS to perform network reconfiguration, the island can implement a switching plan to return the network to its normal state.
Renewable deployment offers greater benefits than just serving a native load as a gentler form of load control. The feeder resiliency can be greatly improved. To adopt these benefits, key and complex technologies are needed. However, the centralized approach to meet these challenges is not easily adaptable to the field. A lower cost and faster solution meeting the same functionality is achievable using a distributed approach to DERMS.
Gary L. Ockwell, P.E., holds a B.S., EE Degree from University of Saskatchewan. From 1973 to 1985, he worked for SaskPower Corporation as a manager of the control department and project manager for the gas and electric system control project. From 1985 to 1995, he worked as a product manager for Harris Controls Division, working with transmission EMS. He joined Advanced Control Systems in 1995, working on both EMS and DMS. From 2007 to the present, he has held the position of Chief Technology Officer. Mr. Ockwell has authored and co-authored two dozen papers and articles for industry conference and publications over the last 10 years. He is an IEEE/PES member.