In this post we are going to learn the practical working of vacuum circuit breaker [VCB] in detail. Vacuum Circuit breaker is used in medium voltage Switchgear for performing switching operations. In Swichgear the VCB is controlled by Electromechanical or Numerical Relays.
Before knowing about Vacuum Circuit breaker first of all, we will see what the circuit breaker is
A device used to close/open a electrical equipment to the power supply is called Circuit Breaker.
They are named by the cooling or arc quenching media used in the device. When an equipment is disconnected from a power supply, the potential strength will be more. This will produce heat in between the surfaces and cause the media to ionize. This is spark. If the spark continues and it will be called as Arc. The heat energy will damage the device.
To reduce the effect of heat generation, the area provided to the breaker will be increased. The time to separate the equipment from power supply will be minimized by using spring. However the arc can be controlled by cooling or using media which can withstand high potential strength.
Air, Oil ,Vacuum and SF6 gas are some of the quenching media used in a circuit breaker. The breaker is named based on them. They are Air Circuit Breaker (ACB), Oil Circuit Breaker (OCB), Vacuum Circuit breaker(VCB) and SF6 Circuit Breaker.
Vacuum Circuit Breaker:
The vacuum circuit breaker interrupts the current in a vacuum. The contacts of a vacuum breaker are enclosed in a ceramic envelope or “bottle” that is evacuated to an extremely low atmospheric pressure, approximately .0023 in. Hg absolute. Although the physics of interruption are quite complex, the vacuum interrupter works because the arc requires a conducting path to sustain it, but within the vacuum bottle there are no gasses to ionize; therefore, for all practical purposes, there can be no conducting path and the arc cannot be sustained.
Figure shows a cutaway view of a single pole vacuum interrupter. The vacuum bottle has a stationary contact firmly mounted on one end of the enclosure, and a moving contact, which travels a very short distance from open to close (1 inch or less), is sealed to the other end of the envelope with a flexible bellow.
Operation of VCB:
The operating mechanism is comprised of the mechanical and electrical components required to:
1. Charge the closing spring with sufficient potential energy to close the circuit breaker and to store opening energy in the opening and contact pressure springs.
2. Mechanisms to release closing and opening actions.
3. Means of transmitting force and motion to each of three pole positions.
4. Operate all these functions automatically through electrical charging motor, cutout switches, antipump relay, close coil, open coil, and auxiliary switch.
5. Provide indication of the circuit breaker status (open/closed), spring condition (charged/discharged), and number of operations.
The Back side of the VCB consists of Three phase Contacts and Vacuum bottle as shown in below figure.
The Vacuum bottle is placed in between incoming and outgoing contacts of each phase. The incoming contacts (upper side in the figure) receive power from the Busbars. The outgoing contacts (lower side in the figure) deliver power to the loads when the breaker is closed.
Protection for VCB: Using Antipumping Relay
VCBs must be protected from multiple closing and tripping operations. If the breaker closing command is given by operator breaker try to close but due to any fault on the protected circuit the breaker has to trip. Since the closing command is still present, there is a chance of breaker closing again and being tripped by the relay. This process is iterative and hence continuous command can damage the closing coil of the breaker.
To prevent this situation the breaker operating mechanism is such that the breaker must close for only one close command given by operator.
For this purpose Antipumping relay is required. It is a device that prevents the Circuit Breaker from unintentional operation caused by standing or too long operation command pulses.
It is placed inside the circuit breaker or externally as auxiliary relay.
Antipumping Relay Operation:
1. Prevents the CB to re-close after a Close-Open when the close command is too long or continuous. The anti-pump relay is reset by releasing the close command pulse. After this the CB can be closed again.
2. Prevents the CB to close when a continuous opening command is applied. To reset the anti-pump relay the opening command has to be removed. When performing a trip-free operation (CO without delay) the open command pulse has to be slightly delayed (10-20 ms) to prevent the anti-pump relay to pick up.