INTRODUCTION TO THE IEC 61850 STANDARD

The IEC 61850 protocol

The main target of the IEC 61850 standard is:

TO IMPROVE THE AUTOMATION OF THE ELECTRICAL SUBSTATIONS.

Avoiding proprietary protocols and being able to integrate equipment of different manufacturers. This concept is called interoperability.
Using technologies that can reduce the cost in wiring and engineering time.
Seeking for improvements in the commissioning and maintenance tasks.

In order to achieve this goal the standard develops 3 principles:

The definition of a unified information model with a naming hierarchy and specific data structures to use in any compliant device. Vendors are request to identify the same concepts with the same name and they must use a common format to build their information. This characteristic reduces the time looking for information, the errors and the format conversions.
The definition of a communication protocol and unified server functionality. This protocol is the language that all the devices in the system use to exchange data. It has been designed to fulfil all the requirements of the automation of substations keeping in mind the timing and availability requirements. The vendors of protection and control devices, SCADA systems, and remote terminal units must implement this protocol to interoperate.
The requirement of using a common XML based file format with specific rules and the definition of tools and engineering roles to improve the automation and configuration during the engineering process.

IEC 61850 communication protocols

Client-server communication protocols

This is the client-server communication that takes place between the servers (the protection and control devices) and the SCADA and GATEWAY acting as clients.

The protocol used in this exchange use the MMS protocol (Manufacturing Message Specification) that is depicted in the standard ISO 9506. This protocol was developed for the industrial automation and it is one of the first protocols that identify data with hierarchies of names.

The communication is based on the OSI layered model over TCP/IP. A TCP channel is created between each client and each server. Over this channel the client can read data, force settings, request commands or receive spontaneous reporting.

A SCADA system will keep opened as many TCP/IP channels as servers it is monitoring.

Client server communication

Multicast communication protocols

IEC 61850 defines a fast and reliable point to multi point message exchange procedurethat can be used to replace copper wiring using in the data exchange between the cabinets in a substation. These messages are called GOOSE – Generic Object Oriented Substation Event. They use ETHERNET MULTICAST with quality of services and high priority that guarantees low delays in the substation switches and the distribution to all the interested subscriber within 3ms or 20ms depending on the requirements.

Digital wiring with GOOSE messages

IEC 61850 standard defines a repetition mechanism for the GOOSE messages that is useful to check that the sender is alive and it is sending the required information. This is an advantage in comparison with the copper wiring where a broken cable may not be easily detected.

The secondary measurements in the Current or Voltage Transformer can also be digitalized and they can be sent from the switchyard to the control cabinets using optical fibers. This principle has originated the development of new products call the Merging Units that are responsible for the analogue to digital conversion and transmission.

The IEC 61850-9-2lite specification simplifies the possibilities of the information being sent by the merging units improving the interoperability. This specification requires either 80 or 256 samples per cycle. It also defines the information that must be sent in the sampled values message including only 4 voltages and 4 currents measurements at each frame exchanged.

IEC 61850 normalized information model

IEC 61850 defines a hierarchical information model with the following levels:

IED – Intelligent Device.
LD – Logical Device.
LN– Logical Node
DO – DATA OBJECT.
DA – DATA ATTRIBUTE.
BASIC TYPE.

Information model hierarchy

The logical devices are logical containers used to organize the information of an IED splitting it into different categories. Usually several vendors categorize their information with logical device names as PROT(protection), CTRL (control), REC (Recorder). In any case the logical device naming is open to the vendor.

The logical nodes are the functions or the components that automate the system. There are functions related to control, protection, measurement, supervision and more. They are identified by 4 letters; the first one indicates their category. As an example, the logical node PIOC stands for Protection Instantaneous Over Current.

IEC 61850-7-4 include the table with all the categories. The logical nodes starting with D (DER), H (Hydro) and W (Wind) are defined by the standards IEC 61850-7-420, IEC 61850-7-410 and IEC 61400-25-2.

Logical node category

Each logical node includes a set of mandatory and optional data objects to fulfil their tasks. The data objects can represent status information, measurements, set points, controllable points, or descriptive information. Each logical node includes a table in the standard with its related data indicating their name and type. The vendors must select the ones that they can provide and they want to publish in their IEC 61850 model.

Example of optional data objects of a logical node

IEC 61850 standard configuration file. SCL language

The equipment integration has been always one of the most complex tasks in the automation process. This integration required specific processes to convert proprietary formats of the vendors or the utilities. A standardized format that is able to define aspects as the information model, the equipment functionality and the connection establishment parameters improves notably this process.

The SCL file format improves the integration process. It allows the exchange of descriptive information and the information exposed by the device and its communication capabilities.

In order to work with SCL files, IEC 61850-6 defines the functionality that must be implemented in two different tools: the IED Configuration Tool (ICT) and the System Configuration Tool (SCT).

The ICT tool is provided by the equipment vendor and it allows to modify parameters and to load the created IEC 61850 configuration into the devices.

The SCT tool is used to integrate the IED configuration files and to define the communication parameters and the data exchanges needed to automate the system.

IEC 61850-6 defines different extensions for the SCL files in order to make it easy to understand the information contained:

SSD: System Specification Description. This file describes the topology of the electrical system to automate and the required functions (logical nodes) without including the specific IED models that implement these functions.

61850 Graphical view of the substation section of the SCL file

Figure-6- Graphical view of the substation section of the SCL file

Substation section example in the SCL file

ICD: IED Capabilities Description. Template file provided by the vendor describing the device information model and capabilities. It is the base for automate the IEC 61850 system.
SCD: Substation Configuration Description. File with the complete configuration of the system including the information model of the real equipment selected, the network parameters, the data flows and even the relationship with the electrical topology.
CID: Configured IED Description. File only one configured IED including the information published in GOOSE messages and the information available for the different clients.
IID: Instantiated IED Description. Configuration file generated with the IED engineering tool generated after the configuration of GOOSE subscriptions and the assignation of these signals to the internal logics.
SED: System Exchange Description. File format used to exchange configuration information between two different projects that need to configure any data exchange between IED located at different locations.

The configuration of the control and protection equipment can be achieved by the used of the CID/IID files of the devices that publish GOOSE messages. The configuration of the clients as SCADA or GATEWAY usually uses the SCD file with the complete configuration of the system.

Basic process of SCL files exchange

How IEC 61850 influences in the devices of a substation?

Protection and control equipment

The integration of the MMS protocol and the required functionality of IEC 61850-7-2 in the protection equipment forced several manufactures to add new communication boards or even to use platforms or chipset with more computing power.

At the same time the vendor specific configuration tools have include new modules in order to:

be able con configure IEC 61850 specific information that is sent to the SCADA and GATEWAY clients, and also the GOOSE messages published and subscribed;
configure the signals used from the subscribed GOOSE messages, so they are accessible in the control and protection logics in the same way that the digital inputs are used.

Local SCADA

The SCADA systems must include an IEC 61850 client module. Their configuration tools shall be able to import the standard SCL files and to select the monitored information matching the standard references with the internal descriptors used to show the information in the different SCADA views.

Remote terminal unit using DNP3 or 60870-5-101/104

The remote terminal units used in IEC 61850 systems must include an IEC 61850 client module. Their configuration tools shall be able to import the standard SCL files and to select the monitored information matching the standard references with the remote control addressing information.

Merging units

These are new equipment that converts the analogue current and voltage measurement to digital samples and they sent it over optical fibers to the control cabinets.

Remote input output modules

These devices only send and receive GOOSE messages. The can publish the status of some digital inputs by publishing a GOOSE message and they can activate digital output by the values received in a subscribed GOOSE.

They can be used to replace physical wiring used for interlocking or even opening and closing commands of breakers and disconnectors.

Breaker signals wired

The breaker trip is probably the most critical action in a substation. Right now there are few companies using remote breakers tripped by GOOSE messages. Some time will be needed to get confidence in the technology behind the remote circuit breaker for their extensive use in this task.

Digital breaker signals

How the IEC 61850 standard affects to the users?

Manufacturers

The different equipment manufacturers have to include the communication protocols required by the IEC 61850 standard and they have to adapt their tools to work with the normative files.

System integrators

They need to know how to identify the capabilities of the IEC 61850 equipment (the communication services available and the information exposed in the logical nodes) to be able to automate the system to fulfil the required specification. They also need to know how to identify if a device provides the required functionality.

Utilities

The utilities need to know the main features of the standard to apply it in the specification of their substations. There are different degrees of commitments:

Option 1: IEC 61850 for automate the communication with the local SCADA and GATEWAY.
Option 2: IEC 61850 with GOOSE automatisms. The equipment used shall be able to publish and subscribe to GOOSE messages.
Option 3: IEC 61850 with GOOSE and digital measurements. The equipment will need specific digital measurement cards to accept the IEC 61850-9-2lite measurements.

The use of Ethernet makes it also possible to synchronize the system using PTP (Precision Time Protocol). The same optical fiber used to communicate allows removing the IRIG-B wiring if the switches and the equipment use the IEEE-1588v2 specification.

At the same time the use of the IEC 61850 standard and the GOOSE messages affects to the electric diagrams. The GOOSE messages must be documented and the devices that use that information identified to check their status during the maintenance tasks.

IEC 61850 includes a test mode used to check the equipment functionality without disturbing its normal operation. The utilities need to evaluate if the test mode fulfils their requirements and to specify its inclusion in the required equipment. Of course, its use should be included in the maintenance procedures.

IEC 61850 standard evolution

New application fields

Originally the IEC 61850 standard applied only to the automation of the electrical substations. Nowadays several groups within IEC are working to define an information model to automate several other energy system areas as:

Hydroelectric and wind power plants.
Distributed generation.
Electrical vehicle.

Current standard edition: edition 2

In 2017 we are using IEC 61850 edition 2 and working in its first amendment 2.1. Edition 2 includes:

Improvement of interoperability in comparison with edition 1, as many optional configuration parameters are mandatory in edition 2.
Better SCL based engineering with additional attributes.
Network redundancy.
Better description of the equipment test mode for maintenance tasks.
More precise information model with many mistakes solved and with the inclusion of logical nodes and common data classes for domains outside of the substation automation.

The amendment to the edition 2 (edition 2.1) includes the resolution of mistakes that cannot wait for the edition 3 and also some improvements in the engineering processes.

Towards the edition 3

The edition 3 is supposed to include:

Engineering process improvement. The engineering tools provided by the manufactures have been one of the issues that the users considered that need several improvements. Mainly the capability to exchange files between tools. The analysis in the information needed in the SCL files that has been added to the standard and the higher degree of experience make us to think that this issue should be solved in the near future.
Automated exchangeable logics between devices based on the use of IEC 61131 and the logical nodes GAPC.
Basic Application Profiles (BAP) to define the minimum requirements, services and functions needed to automate specific functions within the substation. It is expected that in the future the vendors could provide a list of the BAP that their devices support making it easier for the utilities to select the appropriate devices.
New communication mapping (IEC 61850-8-2) based on the exchange of XML messages for distribution generation. This profile is using XMPP transport; a communication protocol used for instant messaging system with multiple intermittent connections with hundreds of devices, being the latest the ones that connect with a central server once they want to be online.

IEC 61850 protocol COMMON TERMS

DNP3 – Distributed Network Protocol version 3. Protocol used for automation and remote control communication with serial and TCP-IP capabilities that is used in substation automation and the communication with control centers.
GOOSE – Generic Object Oriented Substation Event. Fast and safe messages sent in the substation to exchange data among intelligent devices. These messages can replace copper wires.
HMI– Human Machine Interface –Interface used to modify the device behaviour or requesting actions from a SCADA system.
IEC – International Electrotechnical Commission – International organization that develops standards related to the energy sector.
IEC 60870-5-101/104 – Protocol serial or TCP/IP to exchange data from a substation to the control centre.
IED – Intelligent Electronic Device – Any equipment with communication capabilities used to automate a system.
RTU – Remote Terminal Unit – Device that gather the information of a whole system and send it to the control centre using protocols as DNP3 or IEC 60870-5-101/104.
SCADA – Supervisory Control and Data Acquisition – A system used to monitor and control an industrial process as a factory or the energy distribution as in a substation. The communication protocols used for the automation are usually specific of the sector.

ENSOTEST is a company that develops test automation tools for IEC 61850 protocol. UCA International Users Group.

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Categories: IEC standards , Electric power, Smart grid