Basics for IEC61850

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What is data modelling?

Data Modeling -- Primary process objects as well as protection and control functionality in the substation is modelled into different standard logical nodes which can be grouped under different logical devices. There are logical nodes for data/functions related to the logical device (LLN0) and physical device (LPHD).

 

Generic Substation Events (GSE):

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).

 

Generic Object Oriented Substation Events (GOOSE):

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 assure 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

 

Generic Substation State Events (GSSE):

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.

Setting Groups -- The setting group control Blocks (SGCB) are defined to handle the setting groups so that user can switch to any active group according to the requirement.

 

 

MODBUS

Modbus is a serial communications protocol published by Modicon in 1979 for use with its programmable logic controllers (PLCs). The main reasons for the extensive use of Modbus over other communications protocols are:

It is openly published and royalty-free

Relatively easy industrial network to deploy

It moves raw bits or words without placing many restrictions on vendors

Modbus allows for communication between many devices connected to the same network.

Protocol versions

Versions of the Modbus protocol exist for serial port and for Ethernet and other networks that support the Internet protocol suite. Most Modbus devices communicate over a serial EIA-485 physical layer[1]. There are many variants of Modbus protocols

 

Modbus RTU — This is used in serial communication & makes use of a compact, binary representation of the data for protocol communication. The RTU format follows the commands/data with a cyclic redundancy check checksum as an error check mechanism to ensure the reliability of data.  A Modbus RTU message must be transmitted continuously without inter-character hesitations. Modbus messages are framed (separated) by idle (silent) periods.

 

Modbus ASCII — This is used in serial communication & makes use of ASCII characters for protocol communication. The ASCII format uses a longitudinal redundancy check checksum. Modbus ASCII messages are framed by leading colon (':') and trailing newline (CR/LF).

 

Modbus TCP/IP or Modbus TCP — This is a modbus variant used for communications over TCP/IP networks. It does not require a checksum calculation as lower layer takes care of the same.

Data model and function calls are identical for the first 4 variants of protocols; only the encapsulation is different. However the variants are not interoperable as the frame formats are different.

 

STP and RSTP

 

• Fast detection of failure over links
• Generate a topology change, after a link stats change
• Time delay
• Forwarding, Filtering and Aging
• Recommended link cost
• BPDU
• Root address deciding parameter (switch add and MAC add)
• Designate Bridge

 

Features of 61850

Interoperablity

IEC 61850 supports the free allocation of functions to devices (IEDs)

The standard contains an object-oriented data model that groups all data according the common user functions in objects called Logical Nodes (LN), e.g. for switch control (LN CSWI) or distance protection (LN PDIS). All related data attributes are contained and defined in these Logical Nodes. The access to all the data is provided in a standardized way by the services of the standard, which are defined to fulfill the performance requirements.

• Data modeling
• Data transmit at high data rate
• GOOSE, GSSE
• Reliable and Security and long term stability

 

Switch

Manageable and Non-Manageable

VLAN, RSTP, Port mirroring, Port Truncking

Link Aggregation Control Protocol

Internet Group Multicast Protocol (IGMP) Snooping

 

Classes of Ethernet Switches

127 - loopback

 

OSI Model

 

Internet Protocol (connection and connectionless)

IP is a connectionless protocol, which means that IP does not exchange control  information (called a handshake) to establish an end-to-end connection before transmitting data. In contrast, a connection-oriented protocol exchanges control information with the remote computer to verify that it is ready to receive data before sending it. When the handshaking is successful, the computers are said to have established a connection. IP relies on protocols in other layers to establish the connection if connection-oriented services are required. The Internet Protocol (IP) is a network-layer (Layer 3) protocol that contains addressing information and some control information that enables packets to be routed.

 

User Datagram Protocol

The User Datagram Protocol (UDP) is a connectionless transport-layer protocol (Layer 4) that belongs to the Internet protocol family. UDP is basically an interface between IP and upper-layer processes. UDP protocol ports distinguish multiple applications running on a single device from one another.

Unlike the TCP, UDP adds no reliability, flow-control, or error-recovery functions to IP. Because of UDP’s simplicity, UDP headers contain fewer bytes and consume less network overhead than TCP. UDP is useful in situations where the reliability mechanisms of TCP are not necessary, such as in cases where a higher-layer protocol might provide error and flow control.

UDP is the transport protocol for several well-known application-layer protocols, including Network File System (NFS), Simple Network Management Protocol (SNMP), Domain Name System (DNS), and Trivial File Transfer Protocol (TFTP). The UDP packet format contains four fields, as shown in Figure. These include source and destination ports, length, and checksum fields.

 

Transmission Control Protocol (TCP)

The TCP provides reliable transmission of data in an IP environment. TCP corresponds to the transport layer (Layer 4) of the OSI reference model. Among the services TCP provides are stream data transfer, reliability, efficient flow control, full-duplex operation, and multiplexing. With stream data transfer, TCP delivers an unstructured stream of bytes identified by sequence numbers. This service benefits applications because they do not have to chop data into blocks before handing it off to TCP. Instead, TCP groups bytes into segments and passes them to IP for delivery.

TCP offers reliability by providing connection-oriented, end-to-end reliable packet delivery through an internetwork. It does this by sequencing bytes with a forwarding acknowledgment number that indicates to the destination the next byte the source expects to receive. Bytes not acknowledged within a specified time period are retransmitted. The reliability mechanism of TCP allows devices to deal with lost, delayed, duplicate, or misread packets. A time-out mechanism allows devices to detect lost packets and request retransmission. TCP offers efficient flow control, which means that, when sending acknowledgments back to the source, the receiving TCP process indicates the highest sequence number it can receive without overflowing its internal buffers. Full-duplex operation means that TCP processes can both send and receive at the same time. Finally, TCP’s multiplexing means that numerous simultaneous upper-layer conversations can bemultiplexed over a single connection.

 

CAT 5 cable

Category 5 Cable Quality Category 5 distributed cable that meets ANSI/EIA/TIA-568-A building wiring standards can be a maximum of 328 feet (ft.) or 100 meters (m) in length, divided as follows: 20 ft. (6 m) between the hub and the patch panel (if used) 295 ft. (90 m) from the wiring closet to the wall outlet 10 ft. (3 m) from the wall outlet to the desktop device The patch panel and other connecting hardware must meet the requirements for 100-Mbps operation (Category 5). Only 0.5 inch (1.5 cm) of untwist in the wire pair is allowed at any termination point. A twisted pair Ethernet network operating at 10 Mbits/second (10BASE-T) will often tolerate low-quality cables, but at 100 Mbits/second (10BASE-Tx) the cable must be rated as Category 5, or Cat 5, by the Electronic Industry Association (EIA). This rating will be printed on the cable jacket. A Category 5 cable will meet specified requirements regarding loss and crosstalk. In addition, there are restrictions on maximum cable length for both 10- and 100-Mbits/second networks.

 

Abstract Communication Service Interface

ACSI

virtual interface to an IED providing abstract communication services, for example connection, variable access, unsolicited data transfer, device control and file transfer services, independent of the actual communication stack and profiles used.

 

Bay

a substation consists of closely connected subparts with some common functionality. Examples are the switchgear between an incoming or outgoing line and the busbar, the buscoupler with its circuit breaker and related isolators and earthing switches, the transformer with its related switchgear between the two busbars representing the two voltage levels. The bay concept may be applied to one and a half breaker and ring bus substation arrangements by grouping the primary circuit breakers and associated equipment into a virtual bay. These bays comprise a power system subset to be protected such as a transformer or a line end, and the control of its switchgear has some common restrictions such as mutual interlocking or well-defined operation sequences. The identification of such subparts is important for maintenance purposes (which parts may be switched off at the same time with a minimum impact on the rest of the substation) or for extension plans (what has to be added if a new line is to be linked in). These subparts are called bays and may be managed by devices with the generic name “bay controller” and have protection systems called “bay protection”. The concept of a bay is not commonly used all over the world. The bay level represents an additional control level below the overall station level.

 

Data object

part of a logical node object representing specific information, for example, status or measurement. From an object-oriented point of view, a data object is an instance of a data object class. Data objects are normally used as transaction objects; i.e., they are data structures.

 

Device

Mechanism or piece of equipment designed to serve a purpose or perform a function, for example, breaker, relay, or substation computer

 

Functions

Tasks, which are performed by the substation automation system, i.e. by application functions. Generally, functions exchange data with other functions. The details are dependent on the functions in consideration. Functions are performed by IEDs (physical devices). Functions may be split in parts residing in different IEDs but communicating which each other (distributed function) and with parts of other functions. These communicating function parts are called logical nodes. In the context of this standard, the decomposition of functions or their granularity is ruled by the communication behavior only. Therefore, all functions considered consist of logical nodes that exchange data.

 

Intelligent Electronic Device

IED

Any device incorporating one or more processors with the capability of receiving or sending Data/control from or to an external source (for example, electronic multifunction meters, digital relays, controllers)

 

Interchangeability

Ability to replace a device supplied by one manufacturer with a device supplied by another manufacturer, without making changes to the other elements in the system

 

Interoperability

Ability of two or more IEDs from the same vendor, or from different vendors, to exchange information and use that information for correct execution of specified functions

 

Logical Node

LN

Smallest part of a function that exchanges data. A LN is an object defined by its data and methods.

Open protocol

Protocol whose stack is either standardized or publicly available

Physical Device

PD

Equivalent to an IED as used in the context of this standard

 

PICOM

Description of an information transfer on a given logical connection with given communication attributes between two logical nodes (Piece of Information for Communication). It also contains the information to be transmitted and, in addition, requirement attributes such as performance. It does not represent the actual structure and format for data that is exchanged over the communication network. The PICOM approach was adopted from CIGRE working group 34.03.

 

Protocol

Set of rules that determines the behavior of functional units in achieving and performing communication

 

Self-description

A device contains information on its configuration. The representation of this information has to be standardized and has to be accessible via communication (in the context of this

Standard series).

 

System

Within the scope of this standard, system always refers to substation automation systems unless otherwise stated

Process level functions
all functions interfacing to the process, i.e. basically binary and analogue I/O functions such as data acquisition (including sampling) and issuing of commands. These functions communicate via the logical interfaces 4 and 5 to the bay level.

 

Station level functions
station level functions refer to the substation as a whole. There are two classes of station level functions; i.e. process related station level functions and Interface related station level functions

 

Process related station level functions
process related station level functions functions using the data of more than one bay or of the complete substation and acting on the primary equipment of more than one bay or of the complete substation. Examples of such functions are station-wide interlocking, automatic sequencers or busbar protection. These functions communicate mainly via the logical interface 8.

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