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Integrating Substation IT Devices with MMS and SNMP

For conformity's sake—and to future-proof their automation systems—the most up-to-date smart substations use Intelligent Electronic Devices (IEDs) as defined by IEC 61850, to monitor and control their switchgear and transformers. IEC 61850, however, calls for the use of the Machine Messaging Standard, or MMS, as the substation’s device monitoring and control messaging system. This results in an unfortunate conundrum for power substation engineers, because today’s substations also use a lot of IT equipment as key parts of the system, and those devices use SNMP as their messaging protocol. Ideally, to achieve the highest level of interoperability and compatibility in IEC 61850 substations, engineers should be able to call upon IT equipment capable of supporting MMS communications. The upside offered by IT devices that support MMS is clear: a fully integrated management platform utilizing a single communications and networking paradigm. While it is possible to use only SNMP-capable IT equipment without MMS support, for IEC 61850-compliant systems that translates into more complex management architectures, with MMS and SNMP comprising distinct, incomplete subsystems within the station: SNMP for IT devices, and MMS for IEDs and everything else.

IEC 61850 and the Future of Substation Automation Systems

With such a variety of substation designs and purposes, and an even greater variety of legacy devices using proprietary communications protocols, putting together a cost-effective automation system that efficiently meets the needs of a particular substation—and doing so without running the risk of rapid obsolescence—can be an extremely difficult task. In large part, the IEC 61850 standard was established to rectify this situation. The primary purpose of IEC 61850 is to standardize substation automation technology for future generations of substation devices, as well as the immediate next generation. This IEC technology model is expected to keep automated substation hardware standardized and interoperative well into the distant, unforeseen future. Unfortunately, in an industry where core machinery is commonly 70 years old or more, IEC 61850-compliant devices are quite new and have yet to hit a price equilibrium. Smaller, established substations often find full upgrades largely unnecessary and expensive, while larger substations may only require partial upgrades that must fit a patchy system of older, competing technologies. Still more substation operators simply wish to move cautiously, and make the change in slow stages. Cases such as these present station operators and system integrators with a conundrum: how can one upgrade a substation with targeted IEC 61850 compliant devices without forcing the customer to refit the entire system, and yet still maintain the possibility for meeting full IEC 61850 compliance at some point in the future?

A key area where this tension comes into play is in messaging subsystems for automated monitoring and control. Traditionally, an electrical substation’s process layer has been composed of very large, dumb mechanisms that are automated through physical means, rather than electronic; one of the big steps that IEC 61850 is pushing are intelligent electrical devices, or IEDs, which integrate smart devices with process layer machinery such as transformers, switches, and merging units. This allows process layer information to be communicated to the station level, while also allowing these smart devices to reliably react to process events in a coordinated fashion, with millisecond reaction times. While MMS is a powerful tool for monitoring the process and device data generated by IEDs, it has one significant drawback: IT equipment is also a critical component of today’s substation automation systems, and generally enterprise IT only uses SNMP for monitoring and control.

Messaging from Process to Station: MMS

The messaging model IEC 61850 uses for soft real-time communications between the process, bay, and station layers is called the Manufacturing Messaging Specification, or MMS. This is a relatively old specification (est. 1990) that has been adapted from its previous role as a catch-all messaging specification for automated manufacturing. MMS is, however, not a protocol: theoretically, MMS can be ported to any protocol, as has already been demonstrated by its move from serial interfaces to TCP/IP, over which it is communicated in IEC 61850 systems. MMS allows IEDs to transmit reports both on their own device status (such as temperature, network readiness, and other self-diagnostics) as well as process data like voltage, current, and surge events. This is in contrast to Generic Object Oriented Substation Events (GOOSE) and sampled measurement values (SMV), which form, in combination, a hard real-time control model that can communicate (and trigger) critical substation events within 4 milliseconds or less. GOOSE and SMV are communicated directly at the Ethernet link layer (layer 2) on the network, while MMS is carried on TCP/IP, at the top of the network stack.

In contrast to a networking protocol, MMS merely establishes a set of objects and standardized messages and then sets coding rules for transmitting those messages, leaving the communications medium neutral with regard to protocols and interfaces. It is for these reasons MMS was chosen for substation automation systems: because it is protocol- and interface-neutral, MMS is an ideal means for future-proofing today’s substation automation systems. Regardless of how technology develops over the next fifty or hundred years, MMS will be able to continue to serve reliably, and—at least from a messaging standpoint—continue to remain backwards compatible with whatever is in place today.

The rather large drawback to all of these MMS advantages is that up until a short while ago, almost no IT hardware could use it. On enterprise IT hardware, messaging for monitoring and control was largely restricted to the Simple Network Messaging Protocol, or SNMP. Today, however, Moxa’s new line of PowerTrans substation IT switches come with fully integrated MMS support. PowerTrans IEC 61850-compliant switches give substation engineers the option of bringing their IT devices into the same SCADA overview as the IEDs, or any other IEC 61850 device that uses MMS as its device-to-device messaging model.

This screenshot from the management console of Moxa's PowerTrans PT-7528 shows the system displaying the status of Ethernet ports managed and monitored using MMS.

The Industrial Infiltrator: Refreshing Conventional Systems with SNMP

Of course, one of the reasons MMS no longer reigns supreme in manufacturing automation is because of the IT upstart, SNMP (Simple Network Messaging Protocol). Adapted from enterprise IT networks, SNMP has in recent years steadily gained ground in industrial manufacturing systems, and taken over much of the role MMS was originally intended for. Unfortunately, when it comes to substation automation, SNMP is simply incapable of performing with the same timing accuracy and reliability as MMS. Yet that has not stopped it from making inroads to substation automation systems, as well. Today, SNMP is used extensively in electrical substations for integrating network switches, computers, and other IT devices into SCADA systems at the station layer. Yet without extensive customization, it is extremely difficult to extend SNMP into the process layer.

Nevertheless, smaller substations have been known to push SNMP to precisely that extreme; this may be done because purchases of IEDs are outside the operator’s budget, or simply because a full IEC 61850 system isn’t necessary. Using SNMP, it is possible to kit out process layer mechanisms with sensors that communicate traps and respond to regular polling from the process layer, and which may then be integrated with automated events managed by SCADA. The solution is more roundabout and offers fewer capabilities than IEC 61850-compliant systems do, but it has the advantage of being easy to deploy and, in most instances, much cheaper. For these reasons, Moxa’s PowerTrans switches also support standard SNMP messaging, as well, giving substation engineers the best of both worlds.

Dual SNMP/MMS for Comprehensive Messaging Capabilities

In the current environment, today’s electrical substation environments are best served by IT devices like the PowerTrans series, which may use either MMS or SNMP according to the requirements of the situation. For those systems seeking full IEC 61850 compliance, switches, computers, and other IT equipment that support full MMS communications offer system integrators and substation operators the opportunity to fully integrate IT equipment within the IEC 61850 vision, future proofing their system while also integrating the full stock of substation hardware under a single messaging standard (rather than the current workaround, where SNMP is used for IT equipment and MMS for IEDs and everything else). Last but not least, since the IEC 61850 is slated to include IT switches that support MMS in its next revision, MMS support on all IT equipment is clearly a substation engineer’s best choice for their next generation of switches. Towards this end, Moxa is already working towards implementing MMS on our substation computers, gateways, and other substation-specific devices.

To get more information about Moxa’s dual MMS/SNMP-capable electrical substation switches and computers, check out our latest white paper on power substation advances: Using MMS and SNMP to Integrate IT Management for Substation Automation.

Or, download our other white paper: Making Smart Substations Even Smarter: Enhancing Substation Reliability, Availability, and Maintainability.

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