Lecture 3

Remote Terminal Units

(RTU)

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Building blocks of SCADA systems

• The SCADA system has four components, the first being the remote terminal unit (RTU) or

data concentrator, which is the link of the control system to the field, for acquiring the data

from the field devices and passing on the control commands from the control station to the

field devices.

• Modern-day SCADA systems are incomplete without the data concentrators and intelligent

electronic devices (IEDs) which are replacing the conventional RTUs with their hardwired

input and output (I/O) points.

Components of SCADA systems

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• The second component is the communication system that carries the monitored data from

the RTU to the control center and the control commands from the master station to the RTU

or data concentrator to be conveyed to the field.

• The communication system is of great significance in SCADA generally and in power

automation specifically, as the power system field is widely distributed over the landscape,

and critical information that is time bound is to be communicated to the master station and

control decisions to the field.

• The third component of the SCADA system is the master station where the operator

monitors the system and makes control decisions to be conveyed to the field.

• The fourth component is the user interface (UI) also referred to as the human-machine

interface (HMI) which is the interaction between the operator and the machine.

• All automation systems essentially have these four components, in varied proportions

depending on the process requirements. Power system SCADA systems are focused on the

master stations and HMI is of great significance, whereas process automation is focused on

controllers, and master station and the HMI has less significance.

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Remote terminal unit (RTU)

• The RTU is the eyes, ears, and hands of the SCADA system.

• In older days, RTU was a slave of the master station, but now RTUs are equipped with

internal computational and optimization facilities.

• RTU collects data from the field devices, processes the data, and sends the data to the

master station through the communication system to assist the monitoring of the power

system as “eyes” and “ears” of the master station.

• At the same time, the RTU receives control commands from the master station and

transmits these commands to the field devices, thus justifying the comparison to the

“hands” of the master station.

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The new RTU systems had the following advantages:1. Modular system development capability2. Largely preprogrammed user interface system that is easy to adapt to the

individual process3. Preprogrammed menu-driven software (final programming using a few buttons

on keyboard)4. Wide selection of control algorithms with preprogrammed menu

5. Data highway with transmission and communication capabilities between separate units ,

wideband, redundancy

6. Relatively easy communication with the control room for supervisory control

7. Extensive diagnostic scheme and devices for easy maintenance and replacement of circuit

board (card level)

8. Redundancy at any level to improve the reliability

9. Industry standard communication protocols (IEEE 1815 or DNP3, IEC 60870-5-101 and

103)

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Components of RTU

RTU has the following major components to monitor and control the field devices:

1. Communication Subsystem: Is the interface between the SCADA communication network

and the RTU internal logic. This subsystem receives messages from the master, interprets

the messages, initiates actions within the RTU which in turn initiates some action in the

field. RTU also sends an appropriate message to the master station on the completion of the

task. It also collects data from the field, and processes and conveys relevant data to the

master station. RTU may report to a single master or multiple masters.

2. Logic Subsystem: The logic subsystem consists of the main processor and database and

handles all major processing, time keeping, and control sensing. The logic subsystem also

handles the analog-to- digital conversions and computational optimization, in most of the

cases.

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3. Termination Subsystem: The termination subsystem provides the interface between RTU

and external equipment such as the communication lines, primary source, and substation

devices. RTU logic needs to be protected from the harsh environment of the substation.

4. Power Supply Subsystem: The power supply converts primary power, usually

from the substation battery, to the supply requirements of the other RTU

subsystems.

5. Test/HMI Subsystem: This subsystem covers a variety of components, built-in

hardware/firmware tests, and visual indicators, within the RTU, and built-in or

portable test/maintenance panels or displays.

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Components of RTUTypical RTU in a substation

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The RTU communication subsystem handles the following functions:

1) Communication protocols

A large variety of communication protocols exist in the power system, and the RTU

communication system is designed to format and interpret the data in the required

protocol. SCADA communication protocols generally “report by exception” or give

information on the points that have changed since the last scan, to reduce the

communication system load.

2) Message security

The data handled by the SCADA system are critical, and any corruption in the data

can lead to serious consequences. Parity check is the simplest method, where a

single bit is added to the message so that the sum is always odd. Cyclic redundancy

check (CRC) is another error-checking mechanism used, which is more reliable.

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3) Multi-port communication

Modern RTUs have to communicate to the higher SCADA hierarchy to more than one master

station, and at the same time, communicate with peer RTUs and IEDs in a variety of protocols.

The communication subsystem should be designed to handle this capability.

The primary functions of the RTU (Logic subsystem) are time keeping and data acquisition

and processing

1) Time keeping

Sequence-of- events (SOE) logging or time tagging of events is of great significance in power

systems, and the logic subsystem handles this task in the RTU. The RTU also has to perform

many functions on a time basis. The RTU supports time synchronization in addition to time

keeping. Time synchronization of the RTU and the master station takes place through the GPS

receivers which ensure perfect synchronism (1 ms resolution).

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2) Data acquisition and processing

SCADA data come in analog and digital forms. The logic subsystem data acquisition

processing collects and reports both types of data. The analog values are acquired

from transducers connected to the field devices, an example being the current and

voltage values from the transmission lines or transformers. The earlier generation of

RTUs had the analog-to- digital conversion module as part of the RTU, requiring

hardwires to be brought in from the field to the RTU. With the advances in analog-

to- digital conversion techniques and communication networks, field devices are

becoming “intelligent” and can supply digital data directly to a LAN which in turn

can be acquired by the RTU.

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Digital data acquisition is done in four ways:

1. Current status

2. Current status with memory detect number of contact changes since last report

3. Sequence of events (SOE) with time tag

4. Accumulator value a count of the number of contact closures over a period of time

(generally used for energy meter pulse generators)

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Analog data acquisition

• The analog signals, generally a voltage or current that changes over a period of

time and also within a certain range, are generally converted to a 4 to 20 mA

signal by the appropriate transducers. Some utilities also use –1 to +1 mA.

• The analog-to- digital converter circuit converts these signals into binary values

for further transmission or analysis by the RTU.

• The analog signals should be free of noise and electromagnetic interference.

• The 4 to 20 mA current loop signals are generally immune from electrical noise

sources and are the most preferred standard input to the A/D converters.

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Termination subsystem

• The termination subsystem is the interface between the RTU, which is an

electronic device, and the physical world, which is hazardous for the RTU.

• The main function of the termination subsystem is to protect the RTU from the

hostile field environment.

• The substation environment is hostile due to many factors such as surges,

lightning, over voltage and reverse voltage, electrostatic discharge (ESD), and

electromagnetic interference (EMI).

• In the case of process industry, the hazardous environment will include temperature,

humidity, and fumes. The actual provisions of isolation between the RTU logic subsystem

and the field will depend upon individual manufacturer; however, the bottom line is the

RTU will have to be protected from the hazardous environment.

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Digital terminations

• The digital inputs to the RTU are from the various forms of switches in the field and

originate from the electromechanical contacts in switchgear and metering devices.

• Generally the contact sensing is done by an interposing relay powered by a battery, which

provides isolation from the field. Optical isolators are also prevalent, which provide

complete isolation. If the contact input comes from a metering device, then the firmware

contains change detection and pulse accumulation logic.

Analog terminations

• Analog inputs are from the transducers, sensors, transmitters, thermocouples, and resistance

devices, which themselves provide electrical isolation.

• The 4 to 20 mA signals from these devices reach the analog-to- digital conversion unit

through fuses and are grounded at the RTU.

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Testing and human-machine interface (HMI) subsystem

• RTUs located at remote locations are generally unmanned and may not have a

display system or HMI associated with them.

• The panel of the RTU will have a number of LEDs which indicate the status of the

various cards and functionalities of the RTU, which give the personnel an idea

about the status of the RTU.

• The RTU will have its own built-in routines that can test the hardware and

software and give indications on the panel.

• The test results and related information will be passed on to the master.

• Continuous monitoring of the firmware and software of the RTU is done so that

faults and problems can be identified and rectified instantly.

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• With the availability of low-cost LED and LCD displays, RTUs can be fitted with

such display panels that will give the values measured by the RTU to convey

information to the personnel present in the plant floor or substation if necessary.

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Power supplies

• The RTU will have a separate power supply unit, which is powered from a suitable

DC source.

• The most common voltage levels in use are 24 VDC, 48 VDC, and 125 VDC.

Sometimes even 250 VDC may be used in systems.

• RTUs in the transmission and distribution system are located in the substation and

are powered from the substation battery.

• These batteries are floating so that a single fault on either side of the battery to

ground will not cause malfunction, equipment damage, or danger to human

beings.

• Many premises have two voltage levels, say 24 VDC and 48 VDC, the RTU supply

can be easily switched from one to the other, making the system more reliable.

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Advanced RTU functionalities

With the advent of microprocessor technology and with the integrated circuit–based

devices, the RTUs also gained in functionality and versatility. The CPUs became

faster, with more memory and advanced computations possible. In the power

industry, the major advancements have been in the following aspects, as shown in the

figure below.

Advanced RTU functionality of the logic subsystem

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1. Multi-port and multi-protocol operation: RTU at a substation in a transmission

SCADA hierarchy will report to the local master station as well as the regional control

center.

2. Digital interface to other electronic devices: RTU has to be equipped to handle these

digitized data. Generally serial interfaces are used which include RS 232and RS 485 for

such communication.

3. Closed-loop control, computation, and optimization at the RTU level: Complex

computations include using measured values of many parameters to compute a value,

which will be the set point and the resulting control action (controlling the capacitor

banks).

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4) Interface to application functions: The operator can initiate the load reduction, demand

response and other activities using the same system, and the RTU has to establish and

interface with such application programs.

5) Advanced data processing: when a circuit breaker operates, numerous analog point

alarms can be generated (low voltage and low current for all three phases). The important

message to give the system operator is the fact the circuit breaker operated (primary

alarm), but present systems also provide the analog point alarms (secondary alarms), too.

6) Time tagging of analog and digital values for sequence of events recording is

implemented in RTUs. Other functions that can be implemented in the RTU

include distribution automation, volt-ampere reactive (VAR) control and fault

detection, isolation, and service restoration