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This article is excerpted from EPRI's "Assessment of the Case for IEC 61850 in Advanced Distribution Automation"

The utility industry is rapidly moving to modernize their distribution systems, including wider use of advanced distribution automation (ADA^TM). The smart distribution system of the future will be based on ADA that includes two key aspects:

• ADA will enable new system configurations and reconfiguring capabilities, which will increase the flexibility and reliability of the distribution system, as well as aid in preventing outages or recovering from outages that do happen.
• ADA will enable integration and strategic use of new intelligent electric devices (IEDs) embedded in power electronic components, advanced volt amperes reactive (VAR) management systems, power quality enhancement equipment, distributed generation, and fault anticipators and locators. These IEDs not only enable the more flexible electrical architecture mentioned above, but also provide the means for expanded customer service options. These IEDs also act as components of a widespread real-time monitoring system capability. Integration of advanced metering infrastructure (AMI) will also be a key component in the monitoring system infrastructure for ADA.

A conceptual view of future distribution systems is given in Figure 1-1.

Figure 1-1: Conceptual view of advanced distribution automation characteristics. For purposes of this paper, a smart distribution system (alternatively known as advanced distribution automation) is the distribution system portion of a so-called "smart grid" (the overall power system). The following are some key elements of a smart distribution system [1-1]:

• Widespread real-time monitoring of the distribution system
• State-of-the-art protection and control systems
• Automated reconfiguring and outage mitigation technologies
• Real-time or predictive simulation of operations
• Communication system to allow information exchange with a wide variety of system components and cyber security
• Incorporation of AMI data into real-time monitoring to support automation
• Integration of distributed generation into automated systems
• Integration of demand management and customer systems into automated operations

For this vision of ADA to be workable, the intelligent electronic devices must be capable of being used in a "plug and work" approach, which means that they would be easily added to the distribution system in large quantities, have information models that are universally understood and maintained by standards working groups, and are therefore easily integrated into an increasingly complex distributed control infrastructure for distribution systems. Achieving this level of interoperability will require the use of widely adopted information model standards for the intelligent devices in the communication and control system infrastructure.

Power System Engineering and Requirements for Adding Smart Devices Rapidly

ADA systems will be evolved as a collaborative effort of the power system engineering and information/communication system groups at distribution utilities. However, the power system engineering will drive the information modeling needs of the communication systems. In other words, the information models adopted must reflect the actual information exchange requirements of the power system engineers for electrical design and protection systems in building up the distribution system infrastructure. The information models should in turn be developed into industry standards to facilitate easy and rapid assimilation of many different types of devices, each of which may have multiple vendors.

Communications, Plug and Work, Building Communications Media and Networks

As stated above, the build-up of ADA systems will require rapid additions of many smart devices into an environment of advanced distribution control systems. A "plug and work" capability is needed to achieve rapid interoperability of smart devices in this context. Utilities need to be able to specify a standardized information model in the devices they will procure for ADA and vendors need to supply products that meet this standard. Otherwise, the integration of devices into the distribution system will require unnecessarily complicated custom engineering and much jury-rigging in using protocol translator capabilities to allow multiple vendors' products to be used in whatever core communication protocol the utility has chosen.

Hence, international standards are needed. Otherwise, the build-up of communications media and networks will unnecessarily become a limiting factor on what can be done in automating systems. The question is not whether standards are needed, but rather what the standards should be.

Applications

To date, automation progress has been limited to systematically automating specific existing component types -- usually one type at a time. The sequence and extent of automation varies from one utility to another. Substation automation has progressed most rapidly. Distribution feeder and circuit automation and customer systems automation have progressed more slowly.

Communications

Communication systems for automation have been developed, based on a number of alternative communication protocols. Physical communications media availability, coverage and cost have been a significant design constraint for distribution systems that cover thousands of square miles and all types of terrain and environments. This limitation has lead engineers to austere solutions with the communications protocols to keep any extraneous "overhead" in communications message payloads to the absolute minimum. This has resulted in systems put together with minimal message content with little more than the direct data values from the remote device.

The meaning of the messages is in effect designed into or "hardcoded" into the devices and applications sharing the data. While this can work and can be quite successful, it drives the need for good "manual" documentation of the data stream content. These systems can be challenging to upgrade, or add equipment, if the documentation is weak or the original design engineer is no longer available.

Historically, at one point there were over 50 Supervisory Control and Data Acquisition (SCADA) protocols for intelligent systems and equipment. To break away from a constant adaptation of vendor proprietary representations of data in intelligent equipment, some utilities developed their own communication protocols. In some cases, the utility was a big enough market to influence the vendor(s) into making the data available in the utility homemade protocol. This approach had some success if the number of utilities was limited, but ultimately this is not a long-term workable solution for the vendor community to support nor for the industry. What the industry needed was something that was common that several utilities and vendors could all support. Enter Distributed Network Protocol.

Distributed Network Protocol 3 (DNP3) was one of the first "open systems" standards to emerge to support communications with intelligent equipment that has become successful. It assists in the above process by providing standardization for key elements of communications. Utilities are to be commended for moving to DNP since it was able to provide a path away from proprietary systems. DNP is the most widely used protocol for distribution automation today and represents the pinnacle of the old forms of integrating intelligent equipment. While DNP3 (and its related standard IEC 61870-5) have helped immensely in this process, they fall short of the long-term needs and emerging requirements of ADA.

IEC 61850 offers new sets of functionality that assist with scaling up intelligent equipment and applications for ADA including the ability to integrate with enterprise information systems through harmonization with another IEC Standard: IEC 61968. IEC 61850 represents the next generation of intelligent equipment communications in that it uses a rich library of well-defined "objects" for integrating intelligent equipment. This structure for application level communications can be thought of as the "language" used by intelligent equipment.

Reference

[1-1] Technical and System Requirements for Advanced Distribution Automation, EPRI Report 1010915, Electric Power Research Institute, 2004.