IEC 61850 - Standards for Substation Automation

IEC 61850 is the global protocol for Substation Automation. It can seamlessly integrate multiple functions like Control, Protection, Measurement and Monitoring. It provides a high speed solution for Substation protection and interlocking schemes by using the most convenient medium of Ethernet .

Remote Terminal Units (RTU) with conventional substation automation protocols have the limitation of low bandwidth and reduced speed of Communication. Though it offered a low cost solution, the time it took for configuration, mapping and documentation lead to lower efficiencies and loss of redundancy and reliability. In modern times with communication speeds in Mbps , a high speed and interoperable protocol provides the framework for the future. IEC 61850 provides following key features :
1)      High speed Device-to-Device Communication.
2)      Completely networkable throughout the utility.
3)      Multi-vendor interoperability.
4)      Support for Current and Voltage based samples data.
5)      Support for File Transfer.
6)      Support for Security.

The Standard describes a Substation Configuration Language (SCL) based on XML to describe various aspects like System Specification, System Capability, Substation Configuration and Configured Device description. The SCL is completely interoperable thus enabling any IEC 61850 client to extract information from a compatible device. By using SCL which fits each user’s requirement, it can be used to restructure the entire power system design. This eliminates manual configuration, no data entry errors, seamless understanding between system capabilities and requirements, enhance interoperability and improve productivity and efficiency .

The model begins with a physical device. This device is connected to the network  and shall be assigned a particular address. The physical device can have multiple logical devices for which the physical device can act as a proxy or gateway. Every logical device will have various logical nodes which can be related to functions like Supervisory control, Automation, Measurement etc. Each logical node can have multiple data elements which shall be named as per the standard. Thus every element can be made to conform to Common Data Class (CDC) which can then be attributed a specific purpose. The data acquisition and communication can be done on protocols like Ethernet or TCP/IP.

The mapping is done through MMS (ISO9506) protocol as it is the only protocol that can support the complex naming and service models in IEC 61850. While you can theoretically map IEC 61850 to any protocol, this mapping can get very complex and cumbersome when trying to map IEC 61850 objects and services to a protocol that only provides read/write/report services for simple variables that are accessed by register numbers or index numbers. The mapping of IEC 61850 object and service models to MMS is based on a service mapping where a specific MMS service/services are chosen as the means to implement the various services of ACSI. For instance, the control model of ACSI is mapped to MMS read and write services .

In case of various current and voltage sensors, there is a need for digitizing values, remote data acquisition as well as setting output controls .IEC 61850 addresses this need through the definition of Sampled Measured Values services and the implementation of a Process Bus. The Process layer of the substation is related to gathering information, such as Voltage, Current, and status information, from the transformers and transducers connected to the primary power system process – the transmission of electricity. Signals from voltage and current sources as well as status information are input into a “Merging Unit”(MU). The Merging Units in a station sample the signals at an agreed, synchronized rate. In this manner, any device can input data from multiple MUs and automatically align and process the data. Depending on the sample data rate, anywhere from 1 to 5 devices can be mapped onto a single 100MB Ethernet link. Multiple 100MB Ethernet data streams can then be combined into a single Ethernet switch with a 1GB backbone. In this configuration, 50 or more datasets can be published to multiple subscribers .

Substation architecture in IEC 61850 can be shown as below where in data acquisition process and communication layers can be observed.

With such a well defined and precise architecture, we can achieve many advantages as below :
.Eliminate Procurement Ambiguity – Definition of any service through SCL gives a precise and unambiguous understanding of the requirement which cannot be misinterpreted.
• Lower Installation Cost – High speed data exchange and complete utilization of Station LAN capabilities reduces need for excess wiring and reduces cost for trenches and conduits.
• Lower Transducer Costs – Merging Units supporting Sampled Measured Values (SMV) can provide multiple values from a single transducer thus reducing transducer requirements.
• Lower Commissioning Costs –Applications can be directly retrieved from compatible devices or imported through SCL file. Thus manual configuration is eliminated
Lower Equipment Migration Costs – As IEC 61850 devices are interoperable, behavioral differences between different brand of devices are minimized.
• Lower Extension Costs – As compatible Intelligent devices can retrieve and import data from existing devices, reconfiguration cost for additional devices is reduced.
• Lower Integration Costs –As IEC 61850 can use existing station VLAN or Ethernet TCP/IP, cost of integrating new devices is reduced.
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