IEDs receive data from sensors and power equipment, and can issue control commands, such as tripping circuit breakers if they sense voltage, current, or frequency anomalies, or raise/lower voltage levels in order to maintain the desired level. Common types of IEDs include protective relaying devices, On Load Tap Changer controllers, circuit breaker controllers, capacitor bank switches, recloser controllers, voltage regulators, etc.
Digital protective relays
are primarily IEDs, using a microprocessor to perform several
protective, control, and similar functions. A typical IED can contain
around 5-12 protection functions, 5-8 control functions controlling
separate devices, an autoreclose function, self monitoring function,
communication functions etc. Hence, they are aptly named as Intelligent
Some recent IEDs are designed to support the IEC61850 standard for substation automation, which provides interoperability and advanced communications capabilities.
Generic Substation State Events (GSSE) is an extension of event transfer mechanism in UCA2.0. Only Status data can be exchanged through GSSE and it uses a status list (string of bits) rather than a dataset as is used in GOOSE. GSSE messages are transmitted directly over IEC/ISO 8802-2 and 8802-3 using a similar mechanism to GOOSE messages (refer IEC 61850-7-1 Clause 12.2, IEC 61850-8-1 Clause 6.4). As the GSSE format is simpler than GOOSE it is handled faster in some devices. GSSE is being progressively superseded by the use of GOOSE and support for it may eventually disappear.
Generic Object Oriented Substation Events (GOOSE) is a control model mechanism in which any format of data (status, value) is grouped into a data set and transmitted within a time period of 4 millisecond. The following mechanisms are used to ensure specified transmission speed and reliability.
- GOOSE data is directly embedded into Ethernet data packets and works on publisher-subscriber mechanism on multicast or broadcast MAC addresses.
- GOOSE uses VLAN and priority tagging as per IEEE 802.1Q to have separate virtual network within the same physical network and sets appropriate message priority level.
- Enhanced retransmission mechanisms - The same GOOSE message is retransmitted with varying and increasing re-transmission intervals. A new event occurring within any GOOSE dataset element will result in the existing GOOSE retransmission message being stopped. A state number within the GOOSE protocol identifies whether a GOOSE message is a new message or a retransmitted message.
- GOOSE messages are designed to be brand independent. Some vendors offer intelligent electronic devices (IED) that fully support IEC61850 for a truly interoperable approach within the substation network without requiring vendor specific cables or algorithms.
Generic Substation Events (GSE) is a control model defined as per IEC 61850 which provides a fast and reliable mechanism of transferring event data over entire substation networks. When implemented, this model ensures the same event message is received by multiple physical devices using multicast / broadcast services. The GSE control model is further subdivided into GOOSE (Generic Object Oriented Substation Events) and GSSE (Generic Substation State Events).
Depending on the purpose of SCL file, it is classified into the following types:
- IED Capability Description (ICD) file: It defines complete capability of an IED. This file needs to be supplied by each manufacturer to make the complete system configuration. The file contains a single IED section, an optional communication section and an optional substation part which denotes the physical entities corresponding to the IED.
- System Specification Description (SSD) file: This file contains complete specification of a substation automation system including single line diagram for the substation and its functionalities (logical nodes). This will have Substation part, Data type templates and logical node type definitions but need not have IED section.
- Substation Configuration Description (SCD) file: This is the file describing complete substation detail. It contains substation, communication, IED and Data type template sections. An .SSD file and different .ICD files contribute in making an SCD file.
- Configured IED Description (CID) file: It is a file used to have communication between an IED configuration tool to an IED. It can be considered as an SCD file stripped down to what the concerned IED need to know and contains a mandatory communication section of the addressed IED.
- Instantiated IED Description (IID) file: It defines the configuration of one IED for a project and is used as data exchange format from the IED configurator to the system configurator. This file contains only the data for the IED being configured: one IED section, the communication section with the IED's communication parameters, the IED's data type templates, and, optionally, a substation section with the binding of functions (LNodes) to the single line diagram.
- System Exchange Description (SED) file: This file is to be exchanged between system configurators of different projects. It describes the interfaces of one project to be used by another project, and at re-import the additionally engineered interface connections between the projects. It is a subset of an SCD file with additional engineering rights for each IED as well as the ownership (project) of SCL data.
An SCL file contains the following parts:
- Header: This part is used to identify version and other basic details of an SCL configuration file.
- Substation: This is the part dealing with the different entities of a substation including various devices, interconnections and other functionalities. The elements include power transformers, Voltage Levels, bays, General Equipment, conducting equipment like breakers. From the Substation part logical nodes that represent functionality related to the object in the Substation are referred.
- Communication: This section deals with different communication points (access points) for accessing the different IEDs of the complete system. This part contains different Sub Networks and access points.
- IED: The IED section describes the complete configuration of an Intelligent Electronic Device (IED). It contains different access points of the specific IED, the logical devices, and logical nodes, report control blocks etc. coming under the IED. It describes what data an IED publish as reports and as Generic Substation Events (GSE; divided into GOOSE and GSSE) and what GOOSE/GSSE data from other IEDs an IED is configured to receive.
- DataTypeTemplates: It defines different logical devices, logical nodes, data and other details separated into different instances. The complete data modeling according to IEC 61850-7-3 & 7-4 are represented in this part of SCL. It is again subdivided into LNodeType, DOType, DAType and EnumType.
System Configuration description Language (SCL) is the language and representation format specified by IEC 61850 for the configuration of electrical substation devices. This includes representation of modeled data and communication services specified by IEC 61850– 7 –X standard documents. The complete SCL representation and its details are specified in IEC 61850-6 standard document. It includes data representation for substation device entities; its associated functions represented as logical nodes, communication systems and capabilities. The complete representation of data as SCL enhances the different devices of a substation to exchange the SCL files and to have a complete interoperability.