1. Power system protection
Power system protection is a branch of electrical engineering that deals
with the protection of electrical equipment (or component) in a power
system network by removing the faulty part.
Power system protection deals with protecting electrical power systems
from faults by disconnecting faulty components from the rest of the
network. Power system protection is a branch of electrical engineering.
For example, a tree falling or touching an overhead transmission line
may cause a fault. There will be overloading of a motor due to worn out
of the bearing. A lightning strike on the overhead insulator can cause
insulation failure. Pollution may result in degradation in the performance
of insulators which may lead to a breakdown. Under frequency or over
frequency of an alternator may result in mechanical damage to its
turbine requiring tripping of an alternator. Even otherwise, the low-
frequency operation willreduce the life of a turbine, and hence it should
be avoided.
Protective systems disconnect the faulty part of the system and ensure
that the rest of the system is still powered, and protect the system from
further damage caused by the fault. In order to generate, transmit, and
distribute power with the least amount of interruptions and restoration
time, it is extremely important to make use of protection systems. There
is no doubt that protective systems are very important in order to
minimize the negative effects of faults, which, otherwise, can shut down
the whole system. Hence there is a need of protective system.
Function of protective system
The function of the protective system is to protect the system from
abnormal conditions and faults by isolating the faulty element as quickly
as possible. This will ensure the working of a healthy section of the
network. In short, the function of a protective system is as follows.
1. To maintain continuity of supply by safeguarding the entire system.
2. 2. It minimizes damage and repair costs.
3. The safety of personnel must be ensured.
Components of protective system
A protection system consists of Current & Voltage Transformers,
protective relays, circuit breakers, and batteries.
1. Current transformers (CTs) & Potential Transformers (PTs): The
purpose of these transformers is to reduce the current and voltage so
that it could be used by relays for operation.
2. Protective Relays: The purpose of relays is to detect the fault and
initiate a trip sequence when an electrical quantity (example, voltage,
current, frequency, phase-angle, etc.) goes beyond the normal range.
Hence it will send a command to the circuit breaker to disconnect the
faulty part.
3. Circuit Breakers: The purpose of the circuit breakers is to operate on
the trip sequence initiated by the relays in order to open the circuit.
4. Batteries: The batteries are used as back up power supply in the event
of mains supply failure.
Zones of Protection in Power
System
A power system deals with generation, transmission, and distribution of
electrical power. Transformers, generators, bus bars, transmission &
distribution lines, circuit breakers, etc. are the elements or equipment of
a power system. Each element or equipment of the power system has its
own protective scheme. For example, transformer protection, generator
protection, bus bar protection, transmission line protection, etc. In this
3. way, a power system is divided into several protection zones. We study
the topic “zones of protection in power system” under the power
system protection.
A protective zone refers to a zone established around each system
element. Any fault occurring within a protective zone will cause the relays
to trip, which will open all the circuit breakers within that zone.
One or at most two elements of a power system are covered by a
protective zone. The protective zones are planned in such a way that
they cover the entire power system collectively, no portion of the power
system will be left out of protection. Figure 1 illustrates the various
protective zones of a typical power system.
4. Primary and Backup Protection in
Power System
n the previous article on “zones of protection in power system“, we see
that in order to ensure the safety of the power system, it is divided into
different zones. For each zone, there is an appropriate protective
scheme. In this article, we will learn about two types of protection in a
power system i.e. primary protection and back up protection.
Primary protection
The relays used in primary protection are called primary relays. The
primary relays of a zone are responsible for isolating the faulty
component in the event of a fault occurring in that zone. The primary
relay serves as the first line of defense. If the primary relay fails to
operate then back up protection is used to isolate the faulty component
from the system.
Back up protection
When the relay used in primary protection fails to operate then back up
protection is used to clear the fault. The relays used in back up
protection are called back up relays. It is important to note that backup
relays are independent of factors that might cause primary relays to fail
to operate. In order to give the primary relay sufficient time to operate, a
backup relay operates after a time delay. In the event that a back up
relay is operated, a large part of the electrical system will be
disconnected from the local power source and it cannot be avoided. The
back up relay serves as the second line of defense.
Types of back up protection
There are three types of back up protection i.e.
5. 1. Remote back up
2. Relay back up
3. Breaker back up
1. Remote back up
As the name implies, there is a backup relay located at a nearby station.
The function of remote back up is to back up the primary protective
scheme (equipment such as relays, circuit breaker, CT, VT, bus bar, etc.) in
case of a failure of the primary protective scheme. It’s the most cost-
effective and easiest type of back up protection. This is a widely used
backup protection system for transmission lines.
2. Relay back up
In this scenario, an additional relay is used to provide backup protection
in the form of a local backup. When the primary relay fails, the additional
relay trips the same circuit breaker, and this operation occurs
immediately. It is desirable that additional relays operate according to
different principles than the primary protection. The additional relays
should be powered by separate current transformers and potential
transformers. The relay back up protection is costly. The relay back up
protection is recommended in situations where a remote backup is not
feasible.
3. Breaker back up
This serves also as a backup on a local level similar to relay back up.
Generally, this type of backup is utilized for bus bars that are connected
to a number of circuit breakers. In the event that a protective relay
operates in response to a fault, but the circuit breaker does not trip, it is
treated as a bus bar fault. This is the case when it is necessary to trip all
other circuit breakers within the bus bar. If the appropriate circuit
breaker fails to trip in the specified time period after its trip coil is
energized, the main relay closes the contact of a back-up relay, which
trips all other circuit breakers on the bus.
Current Limiting Reactor
6. A current limiting reactors is also called as series reactors. It is an inductive coil having large
inductive reactance compared to the resistance and is used to limit the short circuit current during
fault conditions. These reactors are connected in feeders and ties, in generator leads and between
the bus sections to reduce the magnitude of the short circuit current. The reactor allow free
interchange of power under normal condition. however during fault condition the disturbance is
confined to only faulty location. As the resistance of the reactors are small compared to reactance
the efficiency of the system is not affected much Short circuit current is reduced by an increase
in the reactance of the system. Short circuit current depends on the generating capacity, voltage
at the fault point and the total reactance between the generating point and the fault location.
Breaking capacity of the circuit breaker depends on the magnitude of the fault current. If the
fault current is beyond the designed limit of the breaking capacity of the breaker, the fault cannot
be extinguish. Therefore in large interconnected power system large number of generators and
motors feed the fault on occurrence of the fault. Therefore at times the magnitude of the short
circuit current exceeds the breaking capacity of the breaker. Therefore it is necessary to limit the
fault current by some means. By including a reactor or few reactors at some strategic locations,
short circuit currents at different points can be reduced. The current limiting reactor is an
inductive coil having large inductive reactances in comparison to their resistance and is used for
limiting short circuit currents during fault conditions. Current-voltage reactors also reduced the
voltage disturbances on the rest of the system. It is installed in feeders and ties, in generators
leads, and between bus sections, for reducing the magnitude of short circuit currents and the
effect of the respective voltage disturbance.Current reactor allows free interchange of power
under normal condition, but when the fault occurs the disturbance is restricted by the current
reactor to the faulty section. As the resistance of the system is very small as compared to their
reactance. Hence, the efficiency of the system is not much affected.
