IDENTIFYING PILOT WIRE CICUITS

PILOT WIRE CIRCUIT: It is a wire circuit used as interconnecting channel for the purpose of
conveying information from one end to another.

(ii) The characteristics of a pilot wire circuit are

• Resistivity
• Capacitance and
• Inductance

PROTECTION SYSTEM: a complete arrangement of protection equipment and other

devices required to achieve a specified function based on a protection principal

PROTECTION EQUIPMENT: a collection of protection devices (relays, fuses, etc.).

Excluded are devices such as CT’s, CB’s, Contactors, etc

PROTECTION SCHEME: a collection of protection equipment providing a defined

function and including all equipment required to make the scheme work (i.e. relays,
CT’s, CB’s, batteries, etc.)

Remote balanced voltage relaying protection over a pilot wire circuit

With primary through current, the secondary e.m.f.’s of the current transformers are
opposed, and provide no current in the interconnecting pilot leads or the series connected
relays. An in-zone fault leads to a circulating current condition in the CT secondaries and
hence to relay operation.

PRIMARY INJECTION TEST

This is the test which involves the entire circuit; current transformer primary and secondary
windings, relay coils, trip and alarm circuits, and all intervening wiring are checked.
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 Instances where Primary Test is done

• It is usually done during installation as the only way to prove correct installation and
operation of the whole of a protection scheme while.
• When the CT ratio is not known and you want to know it
• When you want to verify the CT polarity

CT Ratio Check
The circuit for checking the CT Ratio with a single-phase test set is shown in Figure 1 below.

Figure 1: Current transformer ratio check

 Current is passed through the primary conductors and measured on the test set ammeter,
A1.
 The secondary current is measured on the ammeter A2 or relay display, and the ratio of the
value on A1 to that on A2 should closely approximate to the ratio marked on the current
transformer nameplate.

CT Polarity Chek

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 Fig 1: CT Polarity

If the equipment includes directional, differential or earth fault relays, the polarity of the
main current transformers must be checked. It is not necessary to conduct the test if only
overcurrent relays are used.

A short circuit is placed across the phases of the primary circuit on one side of the current
transformers while single-phase injection is carried out on the other side. The ammeter
connected in the residual circuit, or relay display, will give a reading of a few milliamperes
with rated current injected if the current transformers are of correct polarity. A reading
proportional to twice the primary current will be obtained if they are of wrong polarity.

SECONDARY INJECTION TEST

This is the test whose purpose of secondary injection testing is to check that the protection
scheme from the relay input terminals onwards is functioning correctly with the settings
specified. This is achieved by applying suitable inputs from a test set to the inputs of the relays
and checking if the appropriate alarm/trip signals occur at the relay/control room/CB locations.

SECONDARY INJECTION TEST PROCEDURE

The relay outputs are normally disconnected from the remainder of the protection scheme
by inserting a test plugs, as it is a test carried out to prove correct relay, rather than scheme,
operation.

The top and bottom contact of each test plug finger is separated by an insulating strip, so
that the relay circuits can be completely isolated from the switchgear wiring when the test
plug is inserted.

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 • To avoid open-circuiting CT secondary terminals, it is therefore essential that CT shorting
jumper links are fitted across all appropriate live side’ terminals of the test plug BEFORE
it is inserted.

• With the test plug inserted in position, all the test circuitry can now be connected to the
isolated ‘relay side’ test plug terminals.

• Withdraw the test plug once the tests or readings are completed. This will immediately
restores the connections to the main current transformers and voltage transformers.

Tests that can be done with Secondary Injection Test Set

• Long Time (LT) Pick-Up Test

• Short Time (ST) Pick-Up Test
• Instantaneous (I) Pick-Up Test
• Ground Fault (GF) Pick-Up Test
• Quick-Trip Pick-Up Test

IDENTIFYING SIGNALS
Intertripping Scheme
Intertripping is the controlled tripping of a circuit breaker so as to complete the isolation of a
circuit or piece of apparatus in sympathy with the tripping of other circuit breakers.

Direct Tripping Scheme

In direct tripping applications, intertrip signals are sent directly to the master trip relay.
Receipt of the command causes circuit breaker operation.

The method of communication must be reliable and secure because any signal detected at
the receiving end will cause a trip of the circuit at that end.

For direct tripping Protection relay is bypassed. Therefore, fault clearance time = Signalling
Time + Trip operating Time + Circuit Breaker operating time

Permissive Tripping Scheme

Permissive trip commands are always monitored by a protection relay. The circuit breaker is
tripped when receipt of the command coincides with operation of the protection relay at the
receiving end responding to a system fault.

Overall fault clearance time = Signalling time + Protection Relay operating Time + Trip
operating time + Circuit breaker operating time.

Blocking Scheme
Blocking commands are initiated by a protection element that detects faults external to the
protected zone. Detection of an external fault at the local end of a protected circuit results
in a blocking signal being transmitted to the remote end. At the remote end, receipt of the

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 blocking signal prevents the remote end protection operating if it had detected the external
fault.

Blocking Scheme is highly suitable in situations where power line is used for carrier signal
transmission. This is achieved by using a carrier coupling equipment at both ends
(terminating points) of a power line. The high voltage capacitor is tuned by a tuning coil to
present a low impedance at the signal frequency. The complete arrangement is designed as
a balanced or unbalanced half-section band pass filter, according to whether the
transmission is phase-phase or phase-earth

It is necessary to minimize the loss of signal into other parts of the power system, to allow
the same frequency to be used on another line. This is done with a 'line trap' or 'wave trap',
which in its simplest form is a parallel circuit tuned to present a very high impedance to the
signal frequency. It is connected in the phase conductor on the station side of the injection
equipment.

Plain high frequency signals (Plain Tone Signal) can then be used successfully for the
signalling of blocking information over a power line.
Methods of Signalling
D.C. VOLTAGE SIGNALLING: A d.c. voltage step or d.c. voltage reversals may be used
to convey a signalling instruction between protection relaying points in a power
system, but these are suited only to private pilot wires, where low speed signalling is
acceptable, with its inherent security

PLAIN TONE SIGNALS: Plain high frequency signals can be used successfully for the
signalling of blocking information over a power line. Plain tone power line carrier
signalling systems are particularly suited to providing the blocking commands often
associated with the protection of multi-ended feeders.

FREQUENCY SHIFT KEYED SIGNALS: Frequency shift keyed high frequency signals can
be used over a power line carrier link to give short operating times for all
applications of protection signalling. Frequency shift

keyed voice frequency signals can be used for all protection signalling applications
over all transmission media. Frequency modulation techniques make possible an
improvement in performance, because amplitude limiting rejects the amplitude
modulation component of noise, leaving only the phase modulation components to
be detected.

DATA TRANSMISSION EQUIPMENT

Data Transmission Equipment
The communications equipment used in direct support of data processing

Transmission medium and their advantages

WIRELESS: Easy implementation and reachability.
COAXIAL CABLE: Less electromagnetic interference.
FIBER CABLE: High bandwidth and no Electromagnet interference
COPPER WIRE CABLE: In a telephone line, two signal are sent from the Central Office. One
is the DC current to active the handset. This eliminates the need for an extra power source
to activate the telephone set (receiver) in the remote area.

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 The other one is an ac current for speech and calling.

Digital transmission over an analogue line.

The transmission of digital signals over an analogue line can be achieved with the use of
modem. A modem receives digital signals from a computer. The signals received are
converted to an analogue signal then sent over an analogue PSTN line. When these signals
reach the other end, the modem on that end receives these analogue signals and convert
them back to digital signals before they are sent to a computer.

Fig 3.1: Data transmission over an analogue wire

TYPES OF TRAFFIC
Simples: Simplex refers to one-way communications where one party is the transmitter and the
other is the receiver. An example of simplex communications is a simple radio, which you can
receive data from stations but can't transmit data.

Half- duplex: This is the transmission of data in just one direction at a time. For example, a
walkie-talkie is a half-duplex device because only one party can talk at a time

Full- duplex: This is the transmission of data in two directions simultaneously. For example, a
telephone is a full-duplex device because both parties can talk at once.

Modem: Short for modulator-demodulator. A modem is a device or program that enables

a computer to transmit data over, for example, telephone or cable lines. Computer information
is stored digitally, whereas information transmitted over telephone lines is transmitted in the
form of analogue waves. A modem converts between these two forms.

Universal Asynchronous Receiver/Transmitter (UART):

• a piece of computer hardware that translates data between parallel and serial forms or
• an integrated circuit used for serial communications over a computer or peripheral
device serial port.
• The UART performs serial-to-parallel conversion on data characters received from a
peripheral device or a MODEM and parallel-to-serial conversion on data characters
UARTs are commonly used in conjunction with communication standards such as EIA, RS-
232, RS-422 or RS-485

SCADA principle
Telemetry: A process by which remote measurement and monitoring is achieved. Measuring
instruments or units are remotely located and the data collected is sent over the
transmission network to the server for interpretation and reading.

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 Telecontrol: A process by which remote control is achieved. Signals are sent over the
transmission line to the remote actuator to execute an instruction.

SCADA application
• Nuclear reactor
• Gas chambers
• Furnace
• Liquid or gas pipes to monitor leakages
• Power Plants
• Dams to monitor and control the water flow.

Main conponents of SCADA

• Remote Terminal/ Telemetry Unit (RTU)
• Network
• Master Terminal Unit (MTU)
• Human Machine Interface (HMI)/ Man Machine Interface(MMI)

RTU/PLC: One or more field data interface devices, usually RTUs, or PLCs, which interface to field
sensing devices and local control switchboxes and valve actuators.

Network: Transfer data between field data interface devices and control units and the computers in
the SCADA central host. The system can be radio, telephone, cable, satellite, etc., or any
combination of these.

MTU: A central host computer server or servers (sometimes called a SCADA Center, master station,
or Master Terminal Unit (MTU). The computers process the information received from and sent to
the RTU sites and present it to human operators in a form that the operators can work with.

HMI/MMI: A collection of standard and/or custom software [sometimes called Human Machine
Interface (HMI) software or Man Machine Interface (MMI) software] systems used to provide the
SCADA central host and operator terminal application, support the communications system, and
monitor and control remotely located field data interface devices.

The communication protocols used by SCADA are RS-232, RS-485, MODBUS4, and DNP

SCADA threats or attacks

• Smurf attack
• Spoofing attack
• Worm or virus attack
• Man-In-the-Middle (MIM) attack

Mitigations of attacks
• Border Router and Firewalls
• Snooping
• Antivirus
• Authentication server
• Policies and Procedures
.

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 USING CARRIER METHODS

Antenna

Modulati Modulator Power Power Demodul

L.F source ng Signal Amplifier Amplifier ator

Oscillator L.F
Signal
Fig 3.2:RF Block diagram
A low frequency signal also known as a modulating signal is fed into a modulator. A modulator can
be of any modulating method; Frequency modulation, Amplitude modulation, Phase Modulation,
etc. The oscillator provides a carrier frequency. The output from the modulator or modulated signal
is fed into a power amplifier and finally to the antenna which propagates the signal into the air.

AMPLITUDE MUDULATION
Amplitude modulation is a type of modulation where the carrier signal’s amplitude is varied in
accordance with the information bearing signal.

b) The general expression for a carrier wave is

v = Vcsin(ωct + θ)

where v = instantaneous carrier voltage

Vc = peak value
ωc = 2πf
θ = phase of the carrier at t = 0 hence θ will be taken as being equal
to zero.

The modulating signal is given by vm = Vmsinωmt

With amplitude modulation

v = (Vc + vm) Vmsinωct
= (Vc + Vmsinωmt) Vmsinωct
= Vcsinωct + Vmsinωmtsinωct eq 1

Using trigonometry identity

2sinAsinB = cos (A - B) – cos (A + B)
Eq 1 may be rewritten as
V = Vcsinωct + [( Vm/2)cos(ωc - ωm)t] – [[( Vm/2)cos(ωc + ωm)t],

From the equation above the frequency components are

- Original carrier frequency, fc = ωc/2π
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 - lower sidefrequency, fc – fm = (ωc - ωm)/ 2π
- upper sidefrequency, fc + fm = (ωc + ωm)/ 2π

The figure above shows a double sideband A.M. The transistor is biased to operate over the
non-linear part of the mutual characteristics. The carrier and modulating signal voltages are
introduced into the base/emitter circuit of T1 by means of Transformer TR1 and TR2
respectively. The collector current contains the wanted carrier and side-frequency
components plus various other, unwanted components. The collector circuit is tuned to the

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 carrier frequency and has a selectivity characteristic such that the required amplitude
modulated waveform appears across it. The various unwanted frequencies are at
frequencies well removed from resonance and do not develop a voltage across the collector
load.

Example
(i) A variable capacitance diode of an a.m oscillator has a
characteristic given by the table 8.1. Plot a graph of a diode capacitance
against diode voltage.

Reverse Voltage (V) -1 -2 -3 -4 -5 -6

Diode Capacitance (pF) 12.5 7.5 6.0 5.0 4.3 3.8

(ii) A 90MHz oscillator employs a parallel tuned circuit to control the

carrier frequency. The circuit consists of a coil of inductance 0.2µH in
parallel with a 10 pF capacitor, across which is connected a variable
capacitance diode described in (i) above. Using the given characteristic,
determine the voltage which must be applied to the diode for oscillation
to occur at 90MHz.

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FRQUENCY MODULATION.
Frequency modulation (FM) is the method of conveying informations over a carrier
wave by varying its frequency.

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1. The most important feature of frequency modulation is its resilience to signal
level variations. The modulation is carried only as variations in frequency. That is,
any signal level variations will not affect the audio output, provided that the signal
does not fall to a level where the receiver cannot cope.
2. FM wave has property of resilience to noise and interference. It is for this reason
that FM is used for high quality broadcast transmissions.
3. Another important feature is related to FM transmission. It is possible to apply
the modulation to a low power stage of the transmitter, and it is not necessary to
use a linear form of amplification to increase the power level of the signal to its
final value.
4. For FM transmission, it is possible to use non-linear RF amplifiers to amplify FM
signals in a transmitter. This is more efficient than the linear RF amplifier
Therefore, for a given power output, less battery power is required.

• Time-division multiplexing (TDM) is a method of putting multiple data streams in a single

signal by separating the signal into many segments, each having a very short duration.
Each individual data stream is reassembled at the receiving end based on the timing.

• The circuit that combines signals at the source (transmitting) end of a communications
link is known as a multiplexer. It accepts the input from each individual end user, breaks
each signal into segments, and assigns the segments to the composite signal in a thus
contains data from multiple senders. At the other end of the long-distance cable, the
individual signals are separated out by means of a circuit called a demultiplexer, and
routed to the proper end users

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• Time-division multiplexing (TDM) is a method of putting multiple data streams in a single
signal by separating the signal into many segments, each having a very short duration.
Each individual data stream is reassembled at the receiving end based on the timing.

• The circuit that combines signals at the source (transmitting) end of a communications
link is known as a multiplexer. It accepts the input from each individual end user, breaks
each signal into segments, and assigns the segments to the composite signal in a thus
contains data from multiple senders. At the other end of the long-distance cable, the
individual signals are separated out by means of a circuit called a demultiplexer, and
routed to the proper end users

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FILTER CIRCUITS

Figure 5.3: reasonant band-pass filter

• At very low frequency the inductor will act like a pure conductor but the capacitor will act
like an open circuit. The current on RL will be very small.
• As you increase the frequency the Inductive reactance on L1 will start increasing while on C1
will be decreasing resulting in an increase of current flow in RL.
• At some point the inductive reactance will become equal to the capacitive reactance which
will result in the peak voltage on RL. This will occur at resonance frequency.
• As you continue increasing the frequency further, the inductive reactance will continue
increasing to a point where L1 will appear like an open circuit. This action will correspond to
the decrease in the current through RL.

Series resonant circuit plot of current I(v1).

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Fig 5.1 Capacitive Low pass filter

The capacitor’s impedance decreases with increasing frequency. This low impedance in
parallel with the load resistance tends to short out high-frequency signals, dropping most of
the voltage across series resistor R1.

The response of a capacitive low-pass filter falls off with increasing frequency

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