VCB stands for Vacuum
Circuit Breaker, which is a type of electrical switchgear used in power systems
to control and protect electrical equipment. A vacuum circuit breaker is a kind
of circuit breaker where the arc quenching takes place in a vacuum medium. The
operation of switching on and closing of current carrying contacts and
interrelated arc interruption takes place in a vacuum chamber in the breaker
which is called a vacuum interrupter.
A vacuum that is used as the
arc quenching medium in a circuit breaker is known as a vacuum circuit breaker
because vacuum gives high insulating strength due to superior arc quenching
properties. This is suitable for most standard voltage applications because,
for higher voltage, vacuum technology was developed however not commercially
feasible.
The operation of current-carrying
contacts & related arc interruption take place within a vacuum chamber of
the breaker, which is known as a vacuum interrupter. This interrupter includes
a steel arc chamber within the center of symmetrically placed ceramic
insulators. The maintenance of vacuum pressure within a vacuum interrupter can
be done at 10– 6 bar. The vacuum circuit breaker performance mainly depends on
the material used for current-carrying contacts like Cu/Cr.
Construction of VCB:
Vacuum Chamber: The heart of a VCB is the vacuum interrupter, which is a sealed chamber that contains contacts and a vacuum medium. It is responsible for interrupting the electrical current when the breaker operates. The vacuum inside the interrupter ensures efficient arc extinction.
Operating Mechanism: The operating mechanism is responsible for opening and closing the contacts of the VCB. It usually consists of a spring mechanism and a tripping mechanism. The spring mechanism provides the energy to open and close the contacts, while the tripping mechanism controls the operation of the breaker based on external conditions such as overcurrent or fault detection.
Insulating Enclosure: The entire VCB assembly is housed within an insulating enclosure to provide electrical insulation and protect the components from environmental factors. The enclosure is typically made of high-quality insulating materials such as porcelain or composite materials.
Control Panel: A control panel is provided to monitor and control the operation of the VCB. It includes various indicators, switches, and relays for monitoring parameters such as current, voltage, and fault conditions. The control panel also facilitates manual and remote control of the circuit breaker.
Auxiliary Systems: VCBs may
incorporate additional systems for enhanced functionality and safety. These can
include features like current transformers (CTs) for accurate current
measurement, voltage transformers (VTs) for voltage sensing, and protection
relays for detecting faults and initiating breaker operations.
When a fault occurs in the
electrical system, high fault current flows through the circuit breaker
contacts, resulting in the formation of an electric arc. The VCB's primary
function is to interrupt this arc.
The VCB consists of two main
contacts, namely the fixed contact and the moving contact. When the fault
current exceeds a predetermined threshold, the trip mechanism of the circuit
breaker is activated, causing the moving contact to rapidly separate from the
fixed contact.
As the contacts separate, an
arc is initiated between them due to the ionization of the surrounding medium.
In the case of a VCB, the medium is a high-quality vacuum. The vacuum has
excellent insulating properties and high dielectric strength, allowing it to
withstand and extinguish the electric arc.
The vacuum inside the
circuit breaker creates a medium with high dielectric strength, which prevents
the arc from restricting between the separated contacts. The electric field
strength across the contacts is rapidly increased, and the electrons in the arc
are accelerated, leading to the dissipation of energy and cooling of the arc.
Due to the high dielectric
strength of the vacuum, the arc is extinguished when the current passes through
zero during the AC cycle. The rapid increase in voltage across the open
contacts helps in deionizing the medium and preventing re-ignition of the arc.
Once the arc is successfully extinguished, the VCB allows the contacts to close again to restore the normal flow of current in the electrical system. The reclosing operation can be done manually or automatically, depending on the design and control mechanism of the VCB.
Below are the protective relay which are used for VCB safety:
1) Earth Faulty Relay:
Earth fault
relay is used to detect earth leakage in power line & trip the
breaker. To detect earth leakage here CBCT is used which continuously
measures the current of phase & neutral which is zero. When earth leakage happens
then there will be difference found in phase & neutral current. This
condition is detecting CBCT & it provide the signal to earth leakage relay and
on this basis, ELR trip the breaker.
2) Master Trip Relay:
Master trip relay receives the signal from protection relays and
outputs tripping command to trip coil of a circuit breaker. Thus, it provides
isolation between the protection relays and the circuit breaker’s trip coil. If
we directly wire protection relays to the trip coil of a circuit breaker and a
fault occurs in a trip coil, then it may cause burning of protection relay
contact. The protection relays are expensive therefore direct interfacing of
the protection relay with the trip coil is not advisable. A master trip relay
is thus used for providing isolation between the protection relays and the breaker’s
trip coil.
The use of a master trip relay reduces the
complexity of wiring. If the master trip relay is not used, then we have to
wire all trip contacts of the protection relays to the circuit breaker’s trip
coil. The output of the protection relays has 110 VDC and thus the DC voltage
routes in the breaker for each protection relay. Thus, there are more chances
of DC voltage leakage. It reduces the reliability of the protection system.
Moreover, a lot of wiring cause more cost. If we use a master trip relay, we
need to carry two wires to the tripping coil.
Master relay has multiple contacts that can be
used for interlocking for tripping other breakers, annunciation, and signaling
to PLC or SCADA. A master trip relay is a high-speed auxiliary tripping relay
and it immediately issues a trip command to the circuit breaker’s trip coil.
The operating time of the relay is 10 milliseconds nominal, at rated
voltage.
We cannot directly wire the protection relays
with a circuit breaker’s trip coil that does not have anti pumping circuit. The
anti-pumping circuit interrupts the tripping command once the breaker gets
tripped. Master trip relay releases a single pulse for tripping and thus, no
anti-pumping circuit is required in a circuit breaker.
3) Supervision Relay:
As shown in the diagram,
the CB trip coil will energize when the trip relay is operated. When the CB is open, the position of the S1 &
S2 contacts are NO & NC respectively and when the CB is closed, the
position of S1 & S2 contacts are NC & NO.
DC(+Ve)
of the trip circuit supervision relay is permanently connected and through the
trip coil and S1/S2 contact, DC (-Ve) is extended to the relay. Under CB open
condition, DC (-ve) is extended through S2 contact, and under CB close
condition, DC(-ve) is extended through S1 contact.
At both the CB open
& close position, DC (-ve) is extended to the trip supervision relay and it
will not operate. Due to the failure of the trip coil or any other issue, DC
(-ve) will not be extended to the supervision relay, and under that condition,
the relay will operate.
The working of this
circuit is based on the principle that “Current always try to flow from the
path of least resistance”. The circuit is normally open and Red LED Glows when
we connect a power source to the input terminal of this circuit. Red LED indicates
a short circuit and Green LED indicates that output power is ON. When we press
the push button coil of the relay becomes active and it switches from normally
close to normally open contact. You can see that even after leaving the push
button the relay stays in a latched condition. The current required to keep
that relay turned on is coming from a normally open terminal. This is how the
circuit comes in on state. We can connect any load to the output terminal the
circuit will work.
When overloading or short circuit occurs or when we short the output terminals of load, a huge current flow through the circuit. The voltage across coil terminals becomes nearly zero, entire current tries to flow from the least resistive path. Relay immediately Switches from normally open terminal to normally closed terminal protecting our power supply or battery. This is how Short Circuit Protection Using Relay works.
5) Shunt Relay:
Basically, this relay can also be called as power relay for air circuit breaker. Shunt coil relay is an automatically controlled relay, which can be operated remotely to open or close a circuit breaker. Just like a PLC operates a relay, this shunt coil relay can also be operated (on or off) by it. The operation of this relay directly opens or closes the circuit breaker.
6) Closing Relay:
Basically, it is same as
shunt coil relay. It can also be controlled remotely via PLC, and it will
automatically open or close the circuit breaker.
This was much needed. Thank you for sharing this
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