Communications in power system protection (medias, protocols and network topologies)

Communication system

A communication system consists of a transmitter, a receiver and communication channels. Type of medias and network topologies in communications provide different opportunities to advance the speed, security, dependability, and sensitivity of protection relays.

There are a several types of communication media such as micro wave, radio system, fiber optic, etc. The advantages and disadvantages in communication medias which are currently in operation (both analog and digital) and different network topologies are summarized below, respectively.

Contents:

1. Comparison of Communication Medias
2. Comparison of Different Communication Network Topologies
   1. Point-to-Point network
   2. Star network
   3. Bus network
   4. Linear Drop and Insert network
   5. SONET Path Switched Ring
   6. SONET Line Switched Ring
3. Description of Different Communication Protocols
   1. Physical-based protocol
      1. RS232 Protocol
      2. RS485 Protocol
   2. Layered Based-protocols
      1. DNP 3.0
      2. ModBus
      3. IEC 61850
      4. LON
4. Comparison of Relay Characteristics between Different Vendors

1. Comparison of Communication Medias

Let’s start with brief description of seven most known and most used communication medias used in power system communications (in terms of protection and automation):

1.1 Transmission Power Line Carrier

1.2 Microwave

1.3 Radio System

1.4 Satellite System

1.5 Spread Spectrum Radio

1.6 Leased Phone

1.7 Fiber Optic

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2. Comparison of Different Communication Network Topologies

2.1 Point-to-Point network

Point-to-Point network is the simplest configuration with channel available only between two nodes.

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2.2 Star network

Star network consists of multiple point-to-point systems with one common data collector.

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2.3 Bus network

Bus network has single communication path which runs throughout the system to connect nodes.

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2.4 Linear Drop and Insert network

Linear Drop and Insert network consists of multiple paths for relays to communicate with each other. Information between two non-adjacent nodes can be transferred directly passes through intervening node(s).

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2.5 SONET Path Switched Ring

SONET Path Switched Ring comprises of two separate optical fiber links connecting all the nodes in counter rotating configuration.

In normal case, the information is transferred from A to C through outer ring (via B) which is the primary route (left figure). However if channel failure occurs, the information is transferred through inner ring which is secondary route (right figure).

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2.6 SONET Line Switched Ring

SONET Line Switched Ring has the same structure as SONET Path type however one path is active and other is a reserved one.

Under normal condition, the active path transfers information via outer ring (left figure). However in case of channel failure, the inner ring is activated to reverse and transmit information through another direction (right figure).

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3. Description of Different Communication Protocols

Communications protocols are sets of rules by which communication over a network is achieved. Communications protocols are responsible for enabling and controlling network communication.

Communication protocols can be categorized into two groups which are (1) Physical-based protocols and (2) Layered-based protocols. Both types of protocol are briefly discussed in this technical article.

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3.1 Physical-based protocol

Physical Based-protocols have been developed to ensure compatibility between units provided by different manufacturers, and to allow for a reasonable success in transferring data over specified distances and/or data rates.

In addition, these physical-based protocols are also included in the “Physical layer” of the Open Systems Interconnection (OSI) model that will be explained in Layered-based protocols, section below.

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3.1.1 RS232 Protocol

The RS232 Protocol is the most basic communication protocol which specifies the criteria for communication between two devices.

A single twisted pair connection is required between the two devices. Figure 7 shows the RS232 protocol configuration.

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3.1.2 RS485 Protocol

This protocol is similar to the RS232 protocol which allows multiple relays (up to 32) to communicate at half-duplex. This half duplex scheme authorizes one relay either to transmit or receive command information.

This means that the information is handled by polling / responding.

There are two communication modes in RS485 protocol:

1. Unicast mode and
2. Broadcast mode

In the unicast mode, the “Master unit” sends polling commands, and only one “Slave unit” (assigned by an unique address) responds to its command accordingly. The “Master unit” will wait until it obtains a response from a “Slave unit”or abandon a response in case a pre-defined period expires.

In the broadcast mode, the “Master unit” broadcasts message to all “Slave units”. Figure 8 and 9 show a simple RS485 protocol configuration in the unicast and the broadcast mode, respectively.

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3.2 Layered Based-protocols

Other protocols mentioned in Anex table are developed by the Open Systems Interconnection (OSI) model.

This model is a product of the Open Systems Interconnection (OSI) effort at the International Organization for Standardization.

Figure 10 demonstrates data communication using OSI model where “H” represents “headers”.

Table 1 below describes function of each layer.

Table 1 – Functions of Open Systems Interconnection (OSI) model

Some of protocols, mentioned in Anex table, that are derived from OSI model are described below:

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3.2.1 DNP 3.0

The Distributed Network Protocol (DNP) 3.0 is a protocol developed to achieve interoperability standard between substation computers.

This protocol adopts layers 1, 2 and 7 from the OSI model for basic implementation. A fourth layer (a pseudo-transport layer) can be added to allow for the message segmentation.

This protocol does not wait for data as TCP/IP. If a packet is delayed, after a while, it will be dropped. This is because the protocol consists of embedded time synchronization (timetag) associated with messages.

This timetag’s accuracy is on the order of milliseconds. It is feasible to exchange messages asynchronously which is shown in a function of the polling/
response rate.

The typical processing throughput rate is 20 milliseconds.

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3.2.2 ModBus

ModBus is also a three-layer protocol that communicates using a “master-slave” technique in which only one device (the master) can initiate transactions (called queries).

The other devices (slaves) respond by supplying the requested data to the master, or by taking the action requested in the query.

This protocol does not consist of embedded time synchronization as in case of DNP 3.0 that each message is stored in an internal buffer.

In general, IRIG provides accuracy in the 100 microsecond range but it requires dedicated coaxial cable to transport the timing signals which can be limitation for the number of connected devices (depending on cable length and device load).

On the other hands, GPS provides higher accuracy (in the range of 1 microsecond) compare to IRIG but cost and complications of antennas installation to every device are the restriction for the GPS deployment.

Nevertheless, the choice of time synchronization protocol is usually dictated by the number and type of power system devices as well as the physical arrangement of the equipment. The typical processing throughput rate of ModBus protocol is 8 milliseconds.

The protocol can be categorized into three frame formats which are American Standard Code for Information Interchange (ASCII), Remote Terminal Unit (RTU), and Transfer Control Protocol and Internet Protocol (TCP/IP) format.

The ModBus ASCII and ModBus RTU are both used in serial communication. The difference between these ASCII and RTU frames is the format of communication message. In the ASCII format, two ASCII characters are used in each 8 bit byte message whereas two 4 bit hexadecimal characters (or 8-bit binary) are used in case of the RTU format.

The advantage of ASCII format is that it allows time intervals of up to one second to occur between characters without causing an error.

Figure 11 shows the Protocol Data Unit (PDU) for ASCII and RTU frame formats.

Meanwhile, the ModBus TCP/IP is modified from the PDU frame with the Ethernet-TCP/IP as an additional data transmission technology for the ModBus Protocol.

First, an “Error Check” algorithm at the end of frame is removed and the Address Field (address of slave) is replaced by a new header called the ModBus APplication (MBAP) Header. This header consists of (i) Transaction Identifier, (ii) Protocol Identifier, (iii) Length Field, and (iv) Unit Identifier.

Figure 12 shows the Application Data Unit (ADU) for TCP/IP frame format (compare with PDU message).

The difference between ModBus and DNP 3.0 is the communication purpose. ModBus is suitable for communication within substations that are used for communicating with devices meant for protection control and metering.

Meanwhile DNP 3.0 is suitable for communicate outside the substations (communication of data from substation to master control centers). This is because the ModBus protocol has limited function codes while the DNP 3.0 supports the specific data objects that provide more flexibility, reliability and security.

For example, the DNP 3.0 has ‘Control Function Code’ to perform specific function.

In addition, the ModBus protocol is a prototype for proprietary protocols such as K-BUS and SPA protocols which are of Areva-Alstom and ABB, respectively.

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3.2.3 IEC 61850

IEC 61850 is an electrical substation standard promoted by the International Electrotechnical Commission (IEC).

The protocol includes time tags and also messages that can be exchanged asynchronously.

The typical processing throughput rate is 12 milliseconds.

IEC 61850 provides many advantage over other protocols such as programming can be done independent of wiring, higher performance with more data exchange, or data is transmitted multiple times to avoid missing information.

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3.2.4 LON

The Local Operating Network (LON) protocol equates all seven layers of the OSI Model. It is capable of establishing network communications not only for power system applications, but also for factory automation, process control, building networks, vehicle networks etc.

This may be considered as a drawback in relay communication perspective since the LON protocol occupies seven layers in order to transfer information, thus it provides lower data exchange rates compare to the EPA model such as DNP 3.0.

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ANEX

Comparison of Relay Characteristics between Different Vendors

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Reference // Power System Protective Relaying: basic concepts, industrial-grade devices, and communication mechanisms by Rujiroj Leelaruji and Dr. Luigi Vanfretti (KTH Royal Institute of Technology – Electric Power Systems Department)