CIRCUIT BREAKER AND RELAY
Q. Explain different types of faults?
A. Shunt Faults: Basically
shunt faults are short circuits. When the path of the load current is cut short
because of breakdown of insulation, we say that a ‘short circuit’ has occurred
which is nothing but a shunt fault.
Series Faults: Series faults are nothing but a break in
the path of current. It is observed in practice that most of the open conductor
faults sooner or later develop into some or the other short-circuit fault.
Q. What are the various states of operation of a power system?
A. Normal, Alert, Emergency, in extremis and
Restorative.
Q. Explain the working of a Current transformer?
A. The current transformer has two jobs to do.
Firstly, it steps down the current to such a level that it can be easily
handled by the relay current coil. Secondly, it isolates the relay circuitry
from the high voltage of the EHV system. A CT whose primary is in series with
the line in which current is to be measured.
Q. Why the secondary of a CT should not be open?
A. If the secondary of a CT is open current
through it is zero and hence the ampere turns produced by secondary which
generally oppose the primary ampere turns becomes zero. As there is no counter
m.m.f, primary m.m.f produces high flux in the core which leads to excessive
core losses and overheating.
Q. Explain the working of a Potential transformer?
A. The voltage transformer or potential
transformer steps down the high voltage of the line to a level safe enough for
the relaying system (pressure coil of relay) and personnel to handle. A PT
primary is connected in parallel at the point where a measurement is desired.
Q. Explain the working principal of the circuit breaker?
A. The circuit breaker is an electrically
operated switch, which is capable of safely making, as well as breaking,
short-circuit currents. It has two contacts namely fixed contact &
moving contact. Under normal conditions the moving contact comes in contact
with fixed contact thereby forming the closed contact for the flow of current.
During abnormal & faulty conditions (when current exceeds the rated
value) an arc is produced between the fixed & moving contacts &
thereby it forms the open circuit arc is extinguished by the Arc Quenching
media like air, oil, vacuum etc.
Q. What are the different types of circuit breakers?
A. 1. Air Break Circuit
Breaker
2. Oil Circuit Breaker
3. Minimum Oil Circuit Breaker
4. Air Blast Circuit Breaker
5. Vacuum Circuit Breaker
6. SF6 Circuit Breaker
Q. What is SF6 Circuit
Breaker? Its advantages?
A. If SF6 (Sulpher Hexa Fluoride) gas is used
as arc quenching medium in a Circuit breaker, it is termed as SF6 CB. SF6 gas
has superior arc quenching capacity and high dielectric strength. So, they have
very short arcing time and can interrupt high currents.
Q. What is meant by reach point of the relay?
A. The farthest point from the relay location,
which is still inside the zone of protection, is called the reach point.
Q. What is difference between fuse and breaker?
A. Fuses are burnt when over current flows in
the circuit but circuit breakers will just open during over current. Thus fuses
are used for only once but breakers can used by multiple number of times.
Q. What is the
difference between isolators and electrical circuit breakers?
A. Isolators are mainly for switching purpose
under normal conditions but they cannot operate in fault conditions. Actually
they used for isolating the CBs for maintenance. Whereas CB gets activated
under fault conditions according to the fault detected.
Q. Explain the working
of an Over current relay and mention the types of it?
A. An over-current (OC) relay has a single
input in the form of ac current. The output of the relay is a normally-open
contact, which changes over to closed state when the relay trips. The relay has
two settings. These are the time setting and the plug setting. The time setting
decides the operating time of the relay while the plug setting decides the
current required for the relay to pick up.
Types:
1. Instantaneous Over-current Relay
2. Definite Time Over-current Relay
3. Inverse Time Over-current Relay
Q. Explain the working
of an IDMT relay?
A. In an IDMT (inverse definite minimum time)
relay, the operating principle is based on the idea that with the more severe a
fault is, the faster it should be cleared to avoid damage to the apparatus. It
is inverse in the sense; the tripping time will decrease as the magnitude of
fault current increase.
Q. How to select the
pickup value of a relay?
A. We can set the pick-up value of the relay,
keeping in mind that the relay should allow normal load as well as a certain
degree of overload to be supplied. Thus the pick-up value of the relay should
be more than the allowable maximum load. At the same time, the relay should be
sensitive enough to respond to the smallest fault. Thus, the pick-up value
should be less than the smallest fault current.
Q. How to set the operating time of a relay?
A. A relay must get an adequate chance to
protect the zone under its primary protection. Only if the primary protection
does not clear the fault, the back-up protection should initiate tripping. Thus
as soon as the fault takes place, it is sensed by both the primary and the
back-up protection.
Naturally the primary protection is the first
to operate; it’s operating time being less than that of the back-up relay. We
have to allow for the overshoot of the primary relay, so that there is proper
coordination between the primary and the back-up.
Q. When do we use IDMT relays and DTOC relays?
A. IDMT relays have better fault clearing
times over DTOC relays. But, DTOC relays are used for lines which are short in
length as Zs>>Zl and If remains almost same.
Q. Explain different types of faults?
A. Shunt Faults: Basically
shunt faults are short circuits. When the path of the load current is cut short
because of breakdown of insulation, we say that a ‘short circuit’ has occurred
which is nothing but a shunt fault.
Series Faults: Series faults are nothing but a break in
the path of current. It is observed in practice that most of the open conductor
faults sooner or later develop into some or the other short-circuit fault.
Q. What are the various states of operation of a power system?
A. Normal, Alert, Emergency, in extremis and
Restorative.
Q. Explain the working of a Current transformer?
A. The current transformer has two jobs to do.
Firstly, it steps down the current to such a level that it can be easily
handled by the relay current coil. Secondly, it isolates the relay circuitry
from the high voltage of the EHV system. A CT whose primary is in series with
the line in which current is to be measured.
Q. Why the secondary of a CT should not be open?
A. If the secondary of a CT is open current
through it is zero and hence the ampere turns produced by secondary which
generally oppose the primary ampere turns becomes zero. As there is no counter
m.m.f, primary m.m.f produces high flux in the core which leads to excessive
core losses and overheating.
Q. Explain the working of a Potential transformer?
A. The voltage transformer or potential
transformer steps down the high voltage of the line to a level safe enough for
the relaying system (pressure coil of relay) and personnel to handle. A PT
primary is connected in parallel at the point where a measurement is desired.
Q. Explain the working principal of the circuit breaker?
A. The circuit breaker is an electrically
operated switch, which is capable of safely making, as well as breaking, short-circuits
currents. It has two contacts namely fixed contact & moving contact.
Under normal conditions the moving contact comes in contact with fixed contact
thereby forming the closed contact for the flow of current. During abnormal
& faulty conditions (when current exceeds the rated value) an arc is
produced between the fixed & moving contacts & thereby it forms
the open circuit arc is extinguished by the Arc Quenching media like air, oil,
vacuum etc.
Q. What are the different types of circuit breakers?
A. 1. Air Break Circuit
Breaker
2. Oil Circuit Breaker
3. Minimum Oil Circuit Breaker
4. Air Blast Circuit Breaker
5. Vacuum Circuit Breaker
6. SF6 Circuit Breaker
Q. What is SF6 Circuit Breaker? Its advantages?
A. If SF6 (Sulpher Hexa Fluoride) gas is used
as arc quenching medium in a Circuit breaker; it is termed as SF6 CB. SF6 gas
has superior arc quenching capacity and high dielectric strength. So, they have
very short arcing time and can interrupt high currents.
Q. What is meant by reach point of the relay?
A. The farthest point from the relay location,
which is still inside the zone of protection, is called the reach point.
Q. What is difference between fuse and breaker?
A. Fuses are burnt when over current flows in
the circuit but circuit breakers will just open during over current. Thus fuses
are used for only once but breakers can used by multiple number of times.
Q. What is the difference between isolators and electrical circuit breakers?
A. Isolators are mainly for switching purpose
under normal conditions but they cannot operate in fault conditions. Actually
they used for isolating the CBs for maintenance. Whereas CB gets activated
under fault conditions according to the fault detected.
Q. Explain the working of an Over current relay and mention the types of it?
A. An over-current (OC) relay has a single
input in the form of ac current. The output of the relay is a normally-open
contact, which changes over to closed state when the relay trips. The relay has
two settings. These are the time setting and the plug setting. The time setting
decides the operating time of the relay while the plug setting decides the
current required for the relay to pick up.Types:
1. Instantaneous Over-current Relay
2. Definite Time Over-current Relay
3. Inverse Time Over-current Relay
Q. Explain the working of an IDMT relay?
A. In an IDMT (inverse definite minimum time)
relay, the operating principle is based on the idea that with the more severe a
fault is, the faster it should be cleared to avoid damage to the apparatus. It
is inverse in the sense, the tripping time will decrease as the magnitude of
fault current increase.
Q. How to select the pickup value of a relay?
A. We can set the pick-up value of the relay,
keeping in mind that the relay should allow normal load as well as a certain
degree of overload to be supplied. Thus the pick-up value of the relay should be
more than the allowable maximum load. At the same time, the relay should be
sensitive enough to respond to the smallest fault. Thus, the pick-up value
should be less than the smallest fault current.
Q. How to set the operating time of a relay?
A. A relay must get an adequate chance to
protect the zone under its primary protection. Only if the primary protection
does not clear the fault, the back-up protection should initiate tripping. Thus
as soon as the fault takes place, it is sensed by both the primary and the
back-up protection.
Naturally the primary protection is the first
to operate, it’s operating time being less than that of the back-up relay. We
have to allow for the overshoot of the primary relay, so that there is proper
coordination between the primary and the back-up.
Q. When do we use IDMT relays and DTOC relays?
A. IDMT relays have better fault clearing
times over DTOC relays. But, DTOC relays are used for lines which are short in
length as Zs & gt; & gt;Zl and If remains almost same.
