The document discusses the necessity of numerical relays in power systems for protection against faults to maintain continuity of supply, minimize damage, and ensure personnel safety. It outlines different types of electrical faults, their causes, consequences, and the fundamental requirements of effective protection mechanisms. Additionally, it describes the evolution of numerical relays as advanced protective devices that integrate multiple functions, offering advantages such as self-monitoring and high-speed communication.

1. Numerical Relay

2. Need for protection
 A power system is not only capable to meet the
present load but also has the flexibility to meet
the future demands
 A power system is designed to generate electric
power in sufficient quantity, to meet the present
and estimated future demands of the users in a
particular area, to transmit it to the areas
where it will be used and then distribute it
within that area, on a continuous basis.

3.  To ensure the maximum return on the
large investment in the equipment, which
goes to make up the power system and
to keep the users satisfied with reliable
service, the whole system must be kept in
operation continuously without major
breakdowns

4.  Special equipment is normally installed to
detect such kind of failures (also called
‘faults’) that can possibly happen in
various sections of a system, and to
isolate faulty sections so that the
interruption is limited to a localized area
in the total system covering various area

5.  Hence, use of protective apparatus is very
necessary in the electrical systems, which are
expected to generate, transmit and distribute
power with least interruptions and restoration
time
 It can be well recognized that use of protective
equipment are very vital to minimize the effects
of faults, which otherwise can kill the whole
system

6. Basic Requirements for Protection
 A protection apparatus has three main
functions/duties:
◦ 1. Safeguard the entire system to maintain
continuity of supply
◦ 2. Minimize damage and repair costs where it
senses fault
◦ 3. Ensure safety of personnel.

7. Basic components of protection
 Protection of any distribution system is a function of
many elements and this manual gives a brief outline of
various components that go in protecting a system
 Following are the main components of protection;
 Fuse is the self-destructing one, which carries the
currents in a power circuit continuously and sacrifices
itself by blowing under abnormal conditions
 These are normally independent or stand-alone protective
components in an electrical system unlike a circuit
breaker, which necessarily requires the support of
external components

8.  These requirements are necessary, firstly for early detection and
localization of faults, and secondly for prompt removal of faulty
equipment from service
 In order to carry out the above duties, protection must have the
following qualities:
 • Selectivity:To detect and isolate the faulty item only.
 • Stability: To leave all healthy circuits intact to ensure
continuity or supply.
 • Sensitivity: To detect even the smallest fault, current or
system abnormalities and operate correctly at its setting before the
fault causes irreparable damage.
 • Speed:To operate speedily when it is called upon to do
so, thereby minimizing damage to the surroundings and ensuring
safety to personnel

9.  Accurate protection cannot be achieved
without properly measuring the normal and
abnormal conditions of a system
 In electrical systems, voltage and current
measurements give feedback on whether a
system is healthy or not
 Voltage transformers and current transformers
measure these basic parameters and are capable
of providing accurate measurement during fault
conditions without failure

10.  The measured values are converted into analog and/or
digital signals and are made to operate the relays, which
in turn isolate the circuits by opening the faulty circuits.
In most of the cases, the relays provide two functions
viz., alarm and trip, once the abnormality is noticed
 The relays in old days had very limited functions and
were quite bulky
 However, with advancement in digital technology and
use of microprocessors, relays monitor various
parameters, which give complete history of a system
during both pre-fault and post-fault conditions

11. Faults Types
There are two main types of faults-
 Symmetrical Faults
 Unsymmetrical Faults

12. Symmetrical or Balance Fault
 A balance three phase fault (L-L-L Fault) is
called as symmetrical fault. In this type of fault,
all the three phases are short circuited. Some
times all the three phases are short circuited to
the ground and in other cases all the three
phases are short circuited without involvement
of ground.The three phase fault is considered as
standard fault for determining fault level of
system.

13. Unsymmetrical or Unbalance Fault
 Single line to ground short circuit, double
line to ground short circuit, line to line
short circuit, single phase open circuit and
two phase open circuit are unsymmetrical
faults.

14. Single Line to Ground Fault (L-G Fault)
 When short circuit between any one of
the phase conductor and ground occurs
then this type of fault is called as single
line to ground fault.

15. Double Line to Ground Fault (L-L-G
Fault)
 When short circuit between any two
phase conductors and ground occurs
then this type of fault is called as double
line to ground fault.

16. Line to Line Fault (L-L Fault)
 When short circuit between any two
phase conductors occurs then this type
of fault is called as line to line fault.

17. Open Circuit Phases
 This type of fault is occurred when there
is break in conductor path.This type of
fault caused by breaking of one or more
phase conductors or cable joint or joints
on the overhead line fails. Such faults
occurred when isolators or circuit
breaker are open but fail to close for one
or more phase conductors.

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Winding Faults
 Above mentioned fault also occur on the
alternators, motors and transformers
windings.Also, short circuits of turns
which occur on winding of machine.

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Simultaneous Faults
 Two or more types of faults mentioned above
occurred on the system at same time, then this
type of fault is called simultaneous or multiple
faults. In this type of fault, same or different
types of faults occurred at same or different
locations of the system. For example, single line
to ground fault occur between one conductor
and ground. Breaking of other phase conductor
is second fault. Both faults occur at same time
and same location.

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Series Faults
 Series faults represent open conductor and occur when
unbalanced series impedance conditions of the lines are
present. Consider these examples of series fault are
when the system holds one or two broken lines, or
impedance inserted in one or two lines. In the real
world a series faults takes place, for example, when
circuit breakers controls the lines and do not open all
three phases, in this case, one or two phases of the line
may be open while the others is closed . In series faults
voltage and frequency are increase and current
decreases in the faulted phase.

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Shunt Faults
 The shunt faults are the most familiar
type of fault happening in the power
system network.They involve power
conductors or conductor-to-ground or
short circuits between conductors.

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Causes of Faults
Faults on distribution and transmission lines are due to
overvoltage caused by lightning or switching surges.
Overvoltages due to switching surges or lightning surges
cause flashover on insulator’s surface. It causes a short circuit
between phases.
Faults are caused by broken conductors falling on the ground.
Failure of joints on cables or overhead lines are also causes a
short circuit.
Faults on overhead lines are also caused by aircraft, snakes,
ice and snow loading, storms, earthquakes, creepers.

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Faults in cables, generators and transformers are due to
failure of solid insulation caused by aging, moisture, heat,
overvoltage, mechanical damage, accidental contact with
the ground.
Faults are also caused by flashover due to
overvoltage.Poor quality of power system equipments,
improper maintenance of equipments and errors in
the power system design causes a fault.
Defect in protective devices like circuit breakers and
relays, error in switching operation of circuit breaker,
errors in it’s testing and maintenance causes a fault.

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Faults on power system are caused by insulation failure
of conductor or failure of conducting path.
Failure of insulation of conductors causes short circuit
between conductors.This type of short circuits is very
harmful as it damage to power system equipments.
Faults are also caused by tree branches falling down on
overhead conducting lines. In some cases, birds are
responsible for occur a fault on overhead lines.
If body of birds touch to the conducting phases then it
may cause a short circuit.

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Effect of Fault
 Fault causes loss of stability of power system, loss of synchronism
of generators. It causes complete shutdown of power plant. Short
circuit causes to arcs production.
 arcs are responsible for fire hazard.This fire may destroy faulty part
of the power system. Fault causes reduction in supply voltage of
healthy feeder. So, there may be loss of industrial loads. Due to
faults, there may be interruption of power supply to consumers. It
causes revenue loss.
 Large fault current can damage to the equipment or any other
component of the power system due to creating of high
mechanical forces and overheating. Faults causes unbalance of
supply current and supply voltage, it results in heating of rotating
equipments.

26. General Relaying Requirement
 In order to fulfil the requirements of protection with the optimum
speed for the many different configurations, operating conditions
and construction features of power systems, it has been necessary
to develop many types of relay that respond to various functions of
the power system quantities
 For example, observation simply of the magnitude of the fault
current suffices in some cases but measurement of power or
impedance may be necessary in others
 Relays frequently measure complex functions of the system
quantities, which are only readily expressible by mathematical or
graphical means

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NUMERICAL RELAYS
 In utility and industrial electric power transmission and
distribution systems, a Numerical protective relay is a
computer-based system with software-based protection
algorithms for the detection of electrical faults.
 Such relays are also termed as microprocessor type
protective relays. They are functional replacements for
electro-mechanical protective relays and may include
many protection functions in one unit, as well as
providing metering, communication, and self-test
functions.

30. Non-Simultaneous Sampling Scheme
 A multiplexer selects the analog channel
sequentially. Typically, power system
applications involve more than one analog
input.To reduce the cost of the hardware,
multiple channels are multiplexed through
analog multiplexer to asingle ADC. An
analog multiplexer permits a single output
line to mirror the signal at the selected
input, say one of the 3 voltages/ 3 currents.

31.  Thus, multiplexer is a collection of analog
switches. Each channel can be selected by
supplying appropriate binary code to the
multiplexer e.g. for 8-channel multiplexer, 3 bit
address space is required. A chip disable line
permits parallel expansion if external logic is
used to select desired multiplexer.A multiplexer
has two inputs (terminals) for a single channel. It
provides better noise immunity.Accuracy of the
analog multiplexer depends on load impedance
at the output terminal.

32. Non-Simultaneous Sampling Scheme

33. Simultaneous Sampling Scheme
 In this scheme, all S&H amplifiers are set
to hold state simultaneously. This
preserves the relative phase information
between multiple analog signals.Then, the
multiplexer selects the channel
sequentially. Typically, digital relays use
successive ADC which have a conversion
time of 15-30 s.
μ

34. Simultaneous Sampling Scheme

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ADVANTAGES
 Numerical protection provides low burden on instrument
transformer (current transformer and potential transformer).
 It is programmed to detect fault in instrument transformer.
 It has self testing and self monitoring facility.
 It has standard hardware structure and standard features.
 Numerical relays have metering facility and high speed
communication facilities. Numerical protection system has
reporting facility and it can record sequence of events.

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Numerical relays - issues
 SoftwareVersion Control – Same
problem as for all software systems
 Relay Data Management – Large amounts
of parameters – Vendors specific vs.
standardisation
 Testing & Comissioning – Complex
equipment needed for testing – Too
complex for field repairs