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SCADA SYSTEMS

1.0 Introduction

Over the last two or more decades, regulatory

requirements on Electric utilities to maintain high

quality of power supply, high security of power

supply and to supply power at the least possible cost

to its consumers has increased. In order to satisfy

these regulatory requirements, the Electric utilities

have had to deploy advanced and efficient power

system monitoring and control tools. One such tool is

the SCADA system.

The SCADA system is a computer system used to

monitor and control the power process. SCADA is an

abbreviation for Supervisory Control And Data

Acquisition. The abbreviation is derived from the

functions of the system.

• Supervision: the system is used to monitor

many electrical parameters in the power

process. It therefore means that the system is

capable of measuring and determining the

statuses of the electrical parameters.

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• Control: the system is used to control the

electrical parameters and to ensure their values

in the power process are kept within statutory

limits. It therefore means that the system has

an actuating or executing device used for the

purpose of control.

• Data Acquisition: the system is used to acquire

real time electrical parameter data from the

power process and to present the data to the

human user of the system. The human user is

therefore capable of using the acquired data as

a basis to make operational decisions to keep

the system running normally. The acquired data

can also be stored by the system, just incase it

is required in future.

2.0 Components of the SCADA system

The SCADA system has three basic components or

subsystems, namely;

• The local system

• The communication system

• The central system

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2.1 The local system

A local system includes the parts of the control

system installed at, for example, substations and

power plants for remote data acquisition and control.

A local system makes it possible to collect data and

to execute control commands from a central system.

In addition to the fundamental data collection and

executing function, a local system also reports status

and status changes to the central system. The local

system is therefore the part of the control system

where the physical connection to the power process

is made; signals and objects that are being

monitored and controlled are connected by

electronic equipment to the local system. The local

system is like the central system’s eye, ear and

hand. Processed data and status indications are read

and reported and commands from the central

system executed.

The functional content of a local system includes:

• Data acquisition of analog signals and digital

signals.

• Control outputs; on or off control.

• Communication with other computers.

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A local system is distributed to all the parts of the

power system that are going to be monitored and

controlled. Hence, the local system consists of one or

a number of remote units connected to a central

system via a communication system. The remote

terminal units (RTUs) are a common name for those

units. As mentioned above, RTUs read status and

information from the power process, report changes

and information to the central system and execute

commands received from the central system. The

RTU hardware consists of the following main units:

• Central processing unit (CPU)

• Memory

•

Input/Output (I/O) interface• Communication interface

• Power Supply

2.2 The communication system

The communication system is a very important part

of the control system. Without it, the whole idea of

remote data collection and control would be

impossible. The communication system connects the

local system with the central system. The data are

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transmitted in the communication system in serial

form since there is always a physical distance

between the different control subsystems.

The communication media that establish the path for

the movement of data between a local and central

system can take several physical shapes, each with

special characteristics which have some effect on the

communication system. Examples of such media

used to transmit data includes wires, cables, optical

fibers, power line carriers, microwaves, radio links,

satellites and leased telephone lines.

2.3 The Central System

The heart of the control system is all the equipment

that forms the central system. A central system

comprises the equipment in the control centre. This

means that a number of data communication links

from the local systems terminate at the central

system’s location. The central system’s task is to

collect the data or information received from the

power process, analyse it and present the results to

the operator, who is the ultimate decision maker and

is responsible for the operation of the power system.

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The operator must be able to make decisions on the

basis of the information received from the central

system.

The central computer system comprises the

following equipment:

• Central computer

• Mass memory subsystem

• Man Machine computer subsystem

• Equipment for system maintenance and

development

• Front-end computer subsystem

3.0 Data types

The types of data acquired from the power processusing the SCADA system include analog measurands,

indications, digital measurands and accumulators.

The SCADA system also sends control signals into

the power process. These control signals are called

digital outputs or commands.

Analog measurands are the analog variables in the

power process such as system frequency (Hz), active

power (MW), reactive power (MVars), voltage (kV),

current (A), water level (m), etc. Indications show the

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different statuses of the various alarm systems

deployed on equipment and apparatus in the power

process. Indications are basically of two types

namely; single indications and double indications.

Examples of single indications include alarms such

as transformer winding temperature alarm,

transformer oil temperature alarm, door ajar alarm,

protection trip indication, circuit breaker lock out

alarm, circuit breaker SF6 gas pressure low alarm,

etc. Examples of double indications include circuit

breaker open, circuit breaker closed, isolator open,

isolator closed, earth switch open, etc.

Digital measurands are those physical parameters in

the power process whose values changes in steps.

Examples of the digital measurands include

transformer tap positions and fault location data.

Accumulators are the energy values, the integral

sum of the product of the power and time.

4.0 The RTU input/output interface.

The equipment for the acquisition of accumulators,

indications, analog and digital measurands as well as

the equipment for the control outputs is connected

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to the control system through the RTU input/output

interface. The input/output interface is perhaps the

most important part of the entire control system.

Without a perfectly operating input/output interface,

the data received will be erroneous and a control

action may not function properly.

The primary function of the input/output interface is

to serve as an input for analog values or digital

signals from contacts, transducers, and other signal

sources from the power process. It also executes the

operation of relays, breakers, tap changers or

motors. A number of specialized electronic cards or

modules, each of which handles the data types

mentioned in 3.0 above, make up the RTU

input/output interface. These specialized electronic

cards include the analog measurand input board, the

indications input board, the digital measurand input

board, the accumulator pulse counter input board

and the digital output or command board.

The analog measurand input board is used to acquire

analog values and are connected to the outputs of

current, voltage, active power, reactive power,

frequency or water level transducers. Each analog

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measurand board has 16 input channels. Each board

can therefore be used to acquire data from 16

transducer outputs or point objects in the power

process.

The indications board is used to acquire status

indications and is connected to an auxiliary contact.

Each indication board has 336 input channels. Each

indication board can therefore be used to acquire

status indications of 336 different point objects of

single indications types or 168 different point objects

of double indications types.

The digital measurands input board is used to

acquire tap positions on transformers and fault

location data and is connected to a number of

contacts which generate a digital word such as an

analogue to digital converter (ADC). Each digital

measurand board has 16 input channels. Each board

can be used to acquire tap positions data from two

tap changers or from two fault locator devices.

The accumulator pulse counter input board is used to

acquire energy values and is connected to energy

meters capable of generating a digital pulse. Each

accumulator pulse counter board has 14 pulse input

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channels. Each board can therefore be used to

acquire energy values from 14 point objects or pulse

sources.

The command board is used to send an executive or

command signal to the power process and is

connected to command relays. Each command board

has 256 command output channels and can

therefore be used to send control commands to 128

double command objects or to 256 single command

objects.

Each input and output channel in these specialized

electronic cards, except the channels in the analog

measurand input and the accumulator pulse counter

input boards, is supplied by a -48 Vdc voltage,

sourced from the RTU power supply unit. The voltage

supplied to the input and output channels in the

specialized boards is basically used in the acquisition

of each of the respective data types as well as the

execution of commands. This therefore makes the

RTU power supply unit very important as it

determines the basic reliability of the control system.

5.0 How the data acquisition is technically

realized

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Different techniques are employed to acquire the

different data types as described below.

5.1 Data acquisition of Currents.

Power system apparatus such as lines, generators

and transformers carry currents in order of

magnitudes of hundreds to thousands of amperes.

These apparatus are equipped with current

transformers, which transform the primary currents

in order of magnitudes of hundreds to thousands of

amperes to lower secondary values of currents in

order of magnitudes of 1 ampere to 5 amperes. The

secondary winding of the current transformer is

connected to the input terminals of a current

transducer. It should be noted here that the physical

property, which is transduced in this case, is the

variation of the current. The current transducer

outputs a corresponding value of current that ranges

between 0 mA to a maximum of 10 mA. The current

produced by the current transducer is analog. It is

fed into an input channel in the analog measurand

board.

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It is important to note that the data is transmitted

through the communication system in digital format.

The current fed into the input channel of the analog

measurand board must therefore be converted to

digital format. This is done through the process of

sampling, quantizing and coding.

5.2 Data acquisition of Voltages.

The voltage levels at which electric power is

generated, transmitted, distributed and consumed

varies, but generally ranges between 250 V to as

high as 800 kV. Power system apparatus such as bus

bars, lines, generators and transformers are

therefore equipped with voltage transformers, which

transforms the primary voltages of 250 V, 415 V, 11

kV, 33 kV, 66 kV, 132 kV, 220 kV, etc to a lower

value of secondary voltage of 110 V. The secondary

winding of the voltage transformer is connected to

the input terminals of a voltage transducer. It should

again be noted that the physical property transduced

in this case is the variation of the voltage. The

voltage transducer outputs a corresponding value of

current that ranges between 0 mA to a maximum of

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20 mA. The current produced by the voltage

transducer is analog. It is fed into an input channel in

the analog measurand board. The current is

converted to digital format as explained in 5.1

above.

5.3 Data acquisition of Frequency.

The system frequency data is acquired from the

secondary winding of the voltage transformer. The

secondary winding of the voltage transformer is

connected to the input terminals of a frequency

transducer. The physical property transduced in this

case is that variation of the system frequency. The

frequency transducer outputs a corresponding value

of current that ranges between 0 mA to 20 mA. The

current produced by the frequency transducer is

analog. It is fed into an input channel in the analog

measurand board. The current is converted to digital

format as explained in 5.1 above.

5.4 Data acquisition of Active and Reactive

Power.

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In the acquisition of active and reactive power, the

secondary windings of both the current and voltage

transformers are connected to the inputs of active

and reactive power transducers, respectively. Again

the physical property transduced is the variation of

the active and the reactive power. These transducers

output corresponding values of current that range

between -20 mA to a maximum of 20 mA. The

current produced by the active and reactive power

transducers is analog. It is fed into an input channel

in the analog measurand board. The current is

converted to digital format as explained in 5.1

above.

5.5 Data acquisition of Indications.

Indications are acquired from the closing and

opening of auxiliary contacts corresponding to the

changes in the status of circuit breakers, isolators,

alarm signals, etc. These auxiliary contacts are

connected to each of the channels in the indications

input board and are supplied with a -48 Vdc supply.

When the status of an alarm system changes, for

example from normal to alarm, or when the status of

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a breaker changes, for example from closed to open,

corresponding opening or closing of the auxiliary

contacts associated with such an alarm system or

such a breaker, takes place. When a contact closes,

current will flow through the auxiliary contact to the

common. The flow of current is detected by the

galvanic isolation circuitry in the indications board

and a logical 1 is generated in the associated

channel. When a contact opens, no current flows and

the galvanic isolation circuitry generate a logical 0 in

the associated channel.

In the single type of indications, only one auxiliary

contact is used to acquire the status of an object

example door ajar alarm. In the double type of

indications, two auxiliary contacts are used to

acquire the status of an object example circuit

breaker open or close status. Double indications are

used in those objects where the failure of a single

auxiliary contact is likely to impact on safety.

5.6 Data acquisition of Tap Positions in

Transformers.

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A typical power transformer has 17 tap positions with

tap 9 as the nominal tap. The closing and opening of

a combination of several auxiliary contacts

associated with changes in the tap positions on each

transformer enables the acquisition of tap position

data. A binary code generator (an analogue to digital

converter) converts analog currents of 4 mA to 20

mA obtained from a resistor ladder or potentiometer

into digital word. The digital measurand board

acquires the tap indications by literally summing

binary codes values as illustrated below.

Channel 1

________________________________________ 20

Channel 2

________________________________________ 21

Channel 3

________________________________________ 22

Channel 4

________________________________________ 23

Channel 5

________________________________________ 10х20

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Channel 6

________________________________________ 10х21

Channel 7

________________________________________ 10х22

Channel 8

________________________________________ 10х23

5.7 Data acquisition of Energy values.

Like the ordinary watt hour meters installed in

homes, and has a rotating metallic disc which make

for instance 200 revolutions for every kWh of energy

consumed, depending on the design, energy meters

installed in the power system generate a digital

pulse for every 50 kWh of energy transferred or a

digital pulse for every 100 kWh of energy

transferred, depending on the choice of the user. The

digital pulses generated are input to the accumulator

input board. The board has circuitry which counts the

number of pulses. The pulse counting normally

involves two registers: a continuous counter and a

time interval register. The generally accepted time

interval is one hour. When the hour expires, a

transfer is initiated from the counter to the time

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interval register. Then the counter continues on for

the next hour and so on.

6.0 How commands are executed.

Commands sent from the central system are handled

by the command board in the RTU. Each channel in

the command board is connected to a command

relay. When a command is sent to an object from the

central system, the channel in the command board

associated with the object generates a -48 Vdc,

which energizes the coil of the command relay,

causing the relay contacts to make. The making of

this relay contact causes the circuit breaker trip coil,

supplied with 110 Vdc, to be energized and

consequently the operation of the breaker. The same

applies to commands to motors that drive isolators,

tap changers, etc.

7.0 Data transfer from the local system to

Central System.

The central system receives data from a number of

RTUs. The process communication unit (PCU), which

is part of the central system, manages the transfer of

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data from the RTUs to the Central system. This is

done through a process called polling. The PCU polls

the RTUs, one RTU at a time and it does the polling

at a very fast rate. When an RTU is polled by the

PCU, the central system receives updates of the

changes that have occurred in the power process,

exclusively from the polled RTU. Other functions

performed by the PCU include the transfer to the

acquired data to central computers for display to the

operators and storage.

The acquired data, which is in digital format at the

RTU, is modulated onto a carrier using frequency

shift keying, amplitude shift keying or phase shift

keying modulation techniques before transmission.

At the receiving end demodulation of the data takes

place. The sources of digital data at the RTU are also

many. As such time division multiplexing (TDM) also

takes place before the modulation. A communication

protocol therefore exists between the central system

and the RTUs to govern transfer of data.

To avoid overloading the communication link through

unnecessary transfer of data, only those indications

whose status have changed since the last data

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transfer are re-transmitted. For analog measurands,

a local dead band exists, and data transmission to

control centre is initiated only when the analog

measurand changes by a value more than the

defined dead band.

SCADA SYSTEM (2)[1]

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