Protocol Choices for the Substation

Transmission & Distribution World May 2000

By Chuck Newton, Automation Editor

It seems that everyone has an opinion on distributed network protocol (DNP) and utility communications architecture (UCA), but just hearing the acronyms seems to turn many people off. Just what is it with these protocols, and why should you care? Well read on, because if you have any involvement in your utility's substations, this issue is sure to affect you - even if indirectly.

Three protocols are vying for attention in North America, with another two competing in the international market. The list includes DNP, UCA, and IEC 870-5, along with two found in substations using programmable logic controllers (PLC) - typically Modbus in North America and Profibus elsewhere. The DNP is maturing most rapidly and is more widely used as a de facto standard.

For some utilities, DNP is positioned as an interim choice until the now decade-long development of UCA becomes more finalized and accepted. UCA certainly has its supporters and a growing number of user adherents. Furthermore, UCA development is managed and backed by the still-powerful and persuasive EPRI, Palo Alto, California, U.S. IEEE is working to standardize elements of substation protocols around both DNP and UCA.

According to the DNP User Group, "DNP is not being positioned as an alternative to UCA. The consensus by many vendors is that one or two years remain before UCA at the substation level is fully defined in order to support 'plug and play.'"
A feature comparison chart available at the DNP User Group Web site (www.dnp.org) indicates that UCA/MMS was not specifically designed for the utility industry and that no active user groups or technical committees currently are in place. Neither defined nor independent protocol verification programs are available for UCA at this time.

Andrew West, of Triangle MicroWorks and chairman of the DNP User Group's technical committee, says, "I believe that the DNP3 has evolved from an electric-power supervisory control and data acquisition (SCADA) tradition where security and performance were paramount over low-bandwidth, low quality communications systems. UCA seems to have evolved from a design concept for equipment coordination and integration of data within a substation and between sites. Each borrows from elements of relevant international standards and builds on these. Each meets its target goal reasonably well. I believe these goals are somewhat different, and hence, it is not always appropriate to consider DNP3 and UCA in an adversarial manner. Not 'one or the other' but 'the right one for the job' and sometimes 'both.'"

West summarizes his thoughts by saying, "I believe in the near term DNP will remain a dominant standard for SCADA communication, and UCA will take on a role of intrasubstation coordination and automation. Later, UCA will migrate to intrasubstation or system coordination and automation."

On the UCA side of this protocol discussion, keep in mind that UCA was developed to provide an integrated, open-systems protocol for real-timer information exchange among all major utility data communications systems. UCA Version 2 expands on these capabilities by including Internet compatibility and specifying a common interface for electric, gas and water utility systems. Heavy hitters in the standards promulgation process, including the IEC and IEEE and a few major manufacturers, are already on board with EPRI's shepherding of UCA development.

William Blair, EPRI's manager of information and automation technology, says, "UCA is the glue to tie together various components of a utility operating system. UCA is the only means today to integrate utility hardware and software from the control center to the meter, using common language, semantics and services."
EPRI now has 24 major utilities helping to fund its demonstration program, along with 20 participating vendors that have announced products conforming to the UCA 2.0 substation protocol.

Perhaps the most important contribution made during the decade-long UCA demonstration initiative has been the development of a straightforward and practical substation object-modeling and data structuring environment. Using this tool, known as GOMSFE (Generic Object Models for Substation and Feeder Equipment), a developer can map objects between all required protocols, not just the UCA protocol.
This substation protocol situation is a plus for all forward-thinking utilities and suppliers, with two very active developments moving ahead, down from the scores of proprietary protocols found in many substations less than a decade ago.
DNP usage and development are going well, and many vendors and utilities are moving ahead with substation automation programs. Nonetheless, the importance and future role of UCA will continue to unfold, and this protocol will become more widely used - especially among larger utilities such as the funding partners in its development. By 2004, expect to see more than a handful of substations using both protocols.