NPTEL Lecture-03.pdf power system protection

POWER SYSTEM PROTECTION AND SWITCHGEAR
FUNDAMENTALS OF PROTECTIVE RELAYING‐III
Dr. BHAVESHKUMAR BHALJA
DEPARTMENT OF ELECTRICAL ENGINEERING
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Tripping Mechanism of Relay
The relay is always connected in the secondary circuit of CT and PT.
The main function of any type of relay is to detect/sense the inception of
fault, whereas the tripping task is carried out by auxiliary relay and circuit
breaker.
Since the relay only does the function of sensing, the speed of the relay is
increased, and hence, it operates instantaneously.
 Auxiliary relay
• It carries high value of trip coil current during a fault.
• It also gives signals to perform certain other functions associated
with relays such as alarms and interlocking.

4
PT
PT
CT
CT
R
R
86
86
Contact of
auxiliary relay
Contact of
auxiliary relay
Relay
contact
Relay
contact
Tripping coil of
circuit breaker
Tripping coil of
circuit breaker
Coil of
auxiliary relay
Coil of
auxiliary relay
PT
CT
R
86
Contact of
auxiliary relay
Relay
contact
Tripping coil of
circuit breaker
Coil of
auxiliary relay
Power circuit Control circuit

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Contacts of Relay
• Normally Open (NO)
• Normally Close (NC)
In control circuit, all relay coils are shown in deenergized condition and all
circuit breakers (CBs) are shown in open condition.

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 Working
• If single input relay is used (current‐based relay or voltage‐based
relay), then the relay receives a signal from the secondary of CT or PT
only.
• Conversely, for two input relays, it receives signals from the secondary
of both CT and PT.
• The relay R senses the fault within a fraction of second (in
millisecond) and gives signal to the auxiliary relay through its contact.
• The contact of auxiliary relay closes owing to energization of the coil
of auxiliary relay.
• This will further energize the trip coil of the circuit breaker.

7
Classification of Relay
Various types of protective relays are used in practice depending on the
function, actuating quantities, or component that is used.
1) According to the quantities by which the relay operates:
These are thermal relays, overcurrent relays, over‐voltage/under
voltage relays, under/over frequency relays, over fluxing relay, and
power relays.
2) According to their construction:
These are attracted armature type relay, induction disc or induction
cup type relays, and balanced beam type relays.

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3) According to the number of sensing quantities:
Protective relays can be classified as single input and multiple input
relays. A single input relay measures (senses) only one quantity, and it
responds when input quantities exceed the predetermine threshold.
A multiple input relay measures two or more than two quantities and
responds when the output of mixing device exceeds the
predetermined threshold.
4) According to its function in protective scheme:
Relay may be divided into main relays, auxiliary relays, and signal
relays.

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5) According to components and devices used:
These are electromagnetic relays (mechanical devices), static relays
(electronic devices), microprocessor relays (sophisticated algorithm),
and digital/numerical relays (fast processor with communication
facilities).
6) According to the characteristic they adopt:
Instantaneous relay, time delayed relay, and inverse time relays are
the best examples of this type.

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Historical Development
1. Electromechanical Relays
• These relays were rugged, reliable and are still used by the utilities.
• Operating Principle: Whenever a current flows through the windings
would on a magnetic core, mechanical force is produced which in turn
energizes the coil of the relay.

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1. Electromechanical Relays
Advantages:
I. They are reliable in nature and still used by the utilities.
II. This relay provides isolation between the input’s and output’s quantities.
III. They are rugged in nature as they can withstand voltage spike due to
surges and can carry substantial currents.
Disadvantages:
I. They consist of moving parts and suffer from the problem of friction.
II. They produce low torque.
III. They suffer from the problems of high burden and high power
consumption for auxiliary mechanisms.

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2. Static Relays
• Came in 1950’s. They have many advantages such as low burden,
precise and complex characteristic and small size.
• However, their cost is little high as compared to electromechanical
relays. They may mal‐operate in case of temperature variations,
mechanical vibrations etc.
• To operate all assembled electronic devices, static relays require
separate DC power supply.

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3. Microprocessor‐based Relays: Came in 1970s
Advantages:
I. They provide many functions such as multiple setting groups,
programmable logic, adaptive logic, self‐monitoring, self‐testing,
and ability to communicate with other relays and control computers.
II. The cost per function of microprocessor‐based relays is lower as
compared to the cost of their electromechanical and solid‐state
counterparts.
III. Allow users to develop their own logic schemes, including dynamic
changes in that logic.

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3. Microprocessor‐based Relays
Advantages:
IV. Microprocessor‐based relays place significantly less burden on
instrument transformers than the burden placed by the relays of the
previous technologies.
V. Microprocessor‐ based protection systems require significantly less
panel space than the space required by electromechanical and solid‐
state systems.
VI. Reporting features, including sequence of event recording and
oscillography are another feature of microprocessor‐based
protection systems.

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Shortcomings:
I. These relays will always remain more susceptible to problems like
EMI, RFI etc.
II. They have short life cycles. While each generation of
microprocessor‐based systems increases the functionality compared
with the previous generation, the pace of advancements makes the
equipment obsolete in shorter times. This makes it difficult for the
users to maintain expertise with the latest designs/versions of the
equipment.

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Shortcomings:
III. They have a significant number of settings. This may pose problems
in managing the settings and in conducting functional tests. Special
testing techniques, specifically the ability to enable and disable
selected functions, are generally used when microprocessor‐based
relays are tested. This increases the possibility that the desired
settings may not be invoked after testing is completed. Proper
procedures must be followed to ensure that correct settings and
logic are activated.

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4. Digital Relays: Came in 1975s
• Digital relays can realize some very useful functions which are not
possible with electromechanical or analog circuits, such as
mathematical functions, long‐term storage of pre‐fault data.
• They also inherit all the features of microprocessor based relays.

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5. Adaptive Relaying: Philosophy presented in 1989
• All the settings are usually selected on the basis of worst case and
changed only when a major change in the system configuration is
made. This requires high degree of professionalism on the part of the
user to decide as to when and what changes to make in the settings.
• Relay settings which are selected for the worst case would generally
give slow speed, low sensitivity or poor selectivity on other conditions
in the protected system.

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5. Adaptive Relaying
• A fixed operating characteristic of a given relay may not be able to give
the requisite speed, selectivity and sensitivity on all the operating
conditions of the protected system.
• Relay engineers have dreamed that relay could adapt to the system
changes.
• With the development of high speed microprocessors, new tools for
signal processing and digital communication techniques, this dream is
fast turning true.

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• The idea of modifying relay settings was first proposed by DyLiacco in
1967. Thereafter, different researchers have given different definition
of adaptive protection. All these definitions narrate the same facts in
different forms.
• It is defined as changing relaying parameters or functions
automatically depending upon the prevailing system condition or
requirements.
• The adaptive relaying philosophy can be made fully effective only with
digital computer based relays.

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• However, adaptive relays are not manufactured by any manufacturers.
They are under research. Now a days, utilities are using
Numerical/Digital relays.
• Digital/Numerical relays take (n+2) samples.
• Operating time of CBs is 2.5‐3.5 cycles.
• Hence, even though, relay operates less than a cycle or half a cycle,
overall operating time (with breakers) remains 3‐5 cycles.

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6. Intelligent Electronic Device (IED)
• These relays have capabilities of Protection, Monitoring, Control,
Measurement and Communication.
• IEDs are designed to support the IEC61850 standard for substation
automation, which provides interoperability and advanced communications
capabilities.
• A typical IED can contain many protection functions and control functions
controlling separate devices, an auto‐recloser function, self monitoring
function, communication functions etc.

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