As the IEC 61850 standard evolved, other features such as the definition of two time-critical services—the fast transmission of trip-type signals and sampled analog current and voltage values—were added.
These time-critical services enable the extension of the serial links to be used between any intelligent electronic device (IED) and the electronic interfaces near the switchyard equipment. Demanding market requirements, such as the shortening of transfer times down to 3 ms and time synchronization in the order of 1 μs had also to be considered….
Station and process buses
The station bus connects the IEDs for protection, control and monitoring (ie, bay units) with station level devices (ie, the station computer with HMI and the gateway to the network communication center (NCC)) using whatever services are required by the applications. The process bus connects the bay units with the switchyard devices, and the communication of status information, commands and trips is the same as for the station bus (see Fig. 9). However, getting synchronized samples of current and voltage to the relevant protection IEDs using the SV service is quite challenging.
The conversion of proprietary signals from nonconventional instrument transformers for current and voltage or of the analog values from conventional instrument transformers to IEC 61850 telegrams is done using an IED called a merging unit (MU). An MU merges the 3-phase currents and voltages, including the zero-components of one bay high-precision time-synchronized by definition. The process bus functionality for the switchgear is provided by the so-called breaker or switch IEDs (BIED, SIED). The free allocation of functions allows the creation of IEDs with both BIED and SIED, and MU functionalities.
The station bus may be configured in a ring topology with ring redundancy, a redundant star for IEDs with dual port redundancy or any solutions which fulfill the requested performance and reliability requirements. The process bus may also adopt a ring or even a star topology, but at the very least one or more point-to-point connections.
To allow the exchange of data between tools from different manufacturers, IEC 61850 introduces a basic engineering process: Based on the system specification and the description of the IEDs, the required device types are selected and their formal description, in the form of an ICD file, is loaded into the system configuration tool. The system configuration tool then defines the meaning of IED functions in the context of the switchyard by allocating LNs to elements of the switchyard single-line diagram. The data flow between all IEDs is then defined, and all IED names and communication related addresses and parameters are configured.
The resulting system configuration description (SCD) file is a comprehensive description of the entire system in the context of IEC 61850. This file is then imported into the device tools of the different IEDs to complete their individual configuration in the context of the system. The principles of engineering with SCL files are shown in Fig. 10.
As the entire IED data model is visible via the communication system, including possible configuration and setting parameter values, and all this can be described in SCL, the SCD file is also a medium usable by other applications in the life-cycle of the system, such as the archiving of the system configuration in a standardized form and the transfer of protection parameters to protection system configuration tools. It may be used in simulation and testing tools or to check the configuration (version) state of the running system against the intended state. While these applications are outside the scope of IEC 61850 as a communication standard, they are of additional benefit for the user of the standard.
- Renee Robbins Bassett, Managing Editor
Link to the full version of this white paper at bit.ly/awtech003
This white paper is an ABB Review Special Report on IEC 61850. Publication Date: February 2012