IEC 61850 – A new standard that will change the industry
A. It is now a globally accepted international standard, and in the few years since its publication several hundred substations around the world have been built based on this standard.
The original scope of the standard was to achieve interoperability by standardised information exchange between intelligent electronic devices (IEDs) from multiple vendors in an electrical substation. The concepts used in the standard are generic enough, however, to be used in other application domains of the power industry. It can therefore be foreseen that the standard will be the basis for a global communication infrastructure for operation and management of power systems.
In this article we will introduce the scope and basic concepts of IEC 61850. This important standard consists of 14 parts that describe the communication aspects (protocol and services), and also include information models of the applications and a standardised, XML-based language to exchange system configuration information. With regard to the communication services, the standard goes beyond traditional protocols used in the past for substation automation by including services to transmit sampled values from sensors as a stream of data, thus enabling the connection of new optical current and voltage transformers to the protection and metering equipment with a digital interface.
While the first edition has been published and has already been used in many projects, the standardisation bodies are working on extensions that will lead to a second edition. These extensions include information models for new application domains like:
- Power quality metering
- Control of wind power generation
- Control of distributed energy resources
- Control of hydro power plants.
IEC 61850 OVERVIEW
The international standard IEC 61850 consists of a total of 14 parts, as shown in Figure 1. While parts 1 to 5 describe generic requirements and procedures and part 10 describes conformance testing, the other parts contain the essential specifications.
The specifications defined in IEC 61850 are based on a layered approach (see Figure 2). In parts 7-3 and 7-4, the information model of the substation equipment is specified. This includes models for primary devices like circuit breakers or instrument transformers, as well as models for other functions like protection or metering.
In part 7-2, the models used for the information exchange and the related communication services are described in an abstract form, independent from any communication protocol. This is called the Abstract Communication Service Interface (ACSI). Finally parts 8-x and 9-x specify how real communication protocols are used to transmit the information specified in IEC 61850-7-3 and -7-4, using the services of IEC 61850-7-2. In the terminology of IEC 61850, this is called “specific communication service mapping” (SCSM). With this approach, the use of newer communication technologies in the future can be easily achieved without influencing the information models and the information exchange models by adding additional mappings for these new technologies.
CONCEPTS OF THE INFORMATION MODEL
The core element of the information model is the logical node. A logical node can be considered as a container for functionrelated data. The name of a logical node class is standardised and comprises four characters. Basically two kinds of logical nodes exist:
- Logical nodes representing information about the high voltage equipment (e.g. circuit breaker (XCBR) or current transformer (TCTR)). These logical nodes implement the interface between the equipment and the substation automation system.
- Logical nodes representing information about substation automation functions. Examples are any kind of protection functions (e.g. distance protection (PDIS) or the measurement unit (MMXU)).
As an example, the logical nodes involved in the acquisition of measured information are shown in Figure 3. TCTR is the logical node for a current transformer, TVTR the one for a voltage transformer. Note that the logical nodes representing high voltage equipment are typically single phase, and we therefore need three instances of a logical node TCTR or TVTR to represent a three phase system.
In the example of Figure 3, the instance name includes the phase identification as a prefix (e.g. A for phase A). The output of the logical nodes TCTR and TVTR are the samples of the measured waveform. Several logical nodes representing substation automation functions are using that waveform; in the example these are MMXU – measurement unit (the function to calculate e.g. rms values), MHAI – harmonic calculation, MSQI – sequence calculation, Pxxx – any kind of protection function, RSYN – synchrocheck and RADR – waveform recording, analogue channel.
INFORMATION EXCHANGE AND COMMUNICATION PROTOCOLS
The major information exchange models defined in IEC 61850- 7-2 are used to:
- Read and write data
- Control equipment
- Report event driven data
- Distribute event information between IEDs (GOOSE – generic object oriented system event)
- Transmit the digital samples of an analogue waveform (sampled value transmission).
The first three models are based on a client/server relationship, where the server is the component that contains the information and the client is the component accessing that information. Read and write services are used to access data or data attributes – these services are typically used to read and change configuration attributes. Control model and services are a specialisation of a write service. The typical use for the control model is to operate disconnectors, earthing switches and circuit breakers. The reporting model is used for event-driven information exchange; the information is spontaneously transmitted when the value of the data changes.
The last two models are based on a publisher/subscriber concept. In IEC 61850 this concept is called peer-to-peer communication, and the publisher/subscriber models are used for the exchange of time-critical information. The device that is the source of the information publishes it, and any other device that needs that information can then subscribe to it. These models use multicast communication (the information is not directed to one single receiver).
GOOSE is a model to quickly transmit status information to multiple devices. Instead of using a confirmed communication service, the information exchange is repeated regularly. Applications of GOOSE services are the exchange of position information from switches for the purpose of interlocking, or the transmission of a digital trip signal.
The model for the transmission of sampled values is used when a waveform needs to be transmitted using digital communication. In the source device, the waveform is sampled with a fixed sampling frequency. Each sample is tagged with a counter representing the sampling time and transmitted over the communication network. The model assumes synchronised sampling – that is, different devices are sampling the waveform at exactly the same time. The counter is used to correlate samples from different sources. The advantage of this approach is that it creates no additional requirements regarding compensation for variations in the transmission time.
The model for the transmission of sampled values as specified in IEC 61850-7-2 is quite flexible. The different data included in one message, the number of individual samples that are packed within one message, and the sampling rate can all be configured. While this flexibility makes the concept future-proof, it adds configuration complexity. That is why the UCA users’ group has prepared the “Implementation guideline for digital interface to instrument transformers using IEC 61850-9-2”. Basically, the implementation guideline defines the following items:
- A dataset comprising the voltage and current information for the three phases and for the neutral.
- Two options for the sampling rate and the number of samples packed within one message – the first one for a sample rate of 80 samples per period where for each set of samples an individual message is sent, and a second option for 256 samples per period, where 8 consecutive sets of samples are transmitted in one message. The first one is intended for protection applications, the second for power quality metering.
- The use of scaled integer values to represent the information, including the specification of the scale factors for current and for voltage.
IEC 61850 does not define new communication protocols – it is using existing protocols instead. How to use these protocols is defined in the specific mappings (IEC 61850-8-1, IEC 61850-9-1 and IEC 61850-9-2). The mappings define how the abstract models and services according to IEC 61850- 7-2 are implemented using the models and services of the communication protocols employed.
The mappings currently defined in IEC 61850 differentiate between the client/server services and the publisher/subscriber services. The client/server services use the full seven-layer communication stack using MMS and TCP/IP; the publisher/subscriber services are mapped on a reduced stack, basically directly accessing the Ethernet link layer with specific, standardised EtherTypes.
Client/server-based services use the following communication protocols:
- Application layer: MMS [ISO 9506] and association control service elements [ISO/IEC 8649 and ISO/IEC 8650].
- Presentation layer: Connection oriented presentation [ISO/IEC 8822 and IEC/ISO 8823-1] ASN.1 using basic encoding rules (BER) [ISO/IEC 8824-1 and ISO/IEC 8825-1].
- Session layer: Connection oriented session [ISO/ IEC 8326 and ISO/IEC 8327-1].
- Transport layer: ISO transport on top of ICP [RFC1006], Internet control message protocol (ICMP) [RFC 792] and transmission control protocol (TCP) [RFC 793].
- Network layer: Internet protocol [RFC 791] and address resolution protocol (ARP) [RFC 826].
- Data link layer: Transmission of IP datagrams over Ethernet [RFC 894] and CSMA/CD [ISO/IEC 8802-3].
- Physical layer: Electrical 10Base-T/100Base-T or fibreoptic transmission system 100Base-FX [ISO/IEC 8802-3]. Publisher/subscriber based services use the following communication protocols:
- Presentation layer: ASN.1 using basic encoding rules (BER) [ISO/IEC 8824-1 and ISO/IEC 8825-1].
- Data link layer: Priority tagging / VLAN and CSMA/CD [IEEE 802.1Q and ISO/IEC 8802-3].
- Physical layer: Electrical 10Base-T/100Base-T or Fiber optic transmission system 100Base-FX [ISO/IEC 8802-3].
THE CONFIGURATION OF AN IEC 61850-BASED SYSTEM
IEC 61850-6 defines the substation configuration language (SCL). The original purpose of SCL is to define a file format that can be used to exchange configuration information between tools. The principle is shown in Figure 4. The SCL supports the design, engineering and commissioning of the substation during its whole lifecycle. It starts with the formal specification of the substation (single line diagram, functionality in terms of logical nodes) in the system specification description or SSD file. The capability of the IEDs (logical nodes and communication services that can be supported) that will be used to implement the substation automation system is described in the IED capability description or ICD file. The SSD and ICD files are used as input for the system configuration tool. With the system configuration tool, the complete design and engineering of the future substation is then developed.
The output of the system configuration tool is the substation configuration description or SCD file, which contains the complete configuration information for the substation. This file can be used as input to the IED configuration tool. With the IED configuration tool, additional IED specific information can be added and the configuration download for a specific IED is created.
IEC 61850 is a true international standard that can be applied in multiple domains, thanks to its open and standardised concepts. Several IEC working groups are using the concepts of IEC 61850 to define standards for the wind power industry, hydro power plants and distributed energy resources; more domains are expected to follow.
IEC 61850 supports future-proof and interoperable multi-vendor systems that allow both vendors and end-users to optimise their designs and lower system lifecycle costs by standardisation. It is therefore an excellent alternative to existing technologies that are used today.