Introduction
Today, the use of vacuum circuit breakers (VCBs) is increasing. Due to its maintenance-free operation, VCB is the best solution for MV networks. VCBs are more efficient than other types of circuit breakers for two main reasons.
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High dielectric strength
The vacuum inside the VCB is a superior dielectric medium when compared to various other types of insulating media used in circuit breakers.
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High arc extinction capability:
Current interruption occurs at the first zero of the sinusoidal waveform of the fault current. When the arc is interrupted, the dielectric strength of the VCB increases by up to a factor of thousands in comparison with other circuit breakers.
When a fault occurs in the system, high current flows through the circuit-breaker contacts. The protection relay sends a trip command and the moving contact of the VCB starts to move. An arc occurs between the moving contact and the fixed contact as it separates. This arc raises the temperature of the contacts and Ionization occurs inside the VCB. Due to the high capacity of the VCB, the fault current is chopped before it reaches zero. The amplitude of the chopped current is dependent on the material used in VCB contacts.
Disconnecting highly inductive loads with VCBs
VCBs disconnect the fault current at a time before zero, as explained in the previous part. This causes high voltage due to di/dt in inductive loads and causes the equipment to fail. MV motors and transformers are highly inductive equipment. VCBs can be used for the de-energizing of these devices, with the knowledge of the problem involved.
When inductive loads are disconnected by VCB, multiple re-ignition occurs. High voltage with high frequency is generated between the terminals of the equipment, causing insulation failure, as shown in Figure 1.
Fig1. Multiple reignition and voltage escalation in VCB
The real case is simulated in an EMTP software. The voltage and current waveforms of the VCB are shown in Fig. 2. These waveforms are related to the de-energizing of a no-load transformer by the VCB.
Fig 2. Waveforms of the voltage and current of the VCB
Surge arresters or RC snubbers?
Surge arresters are normally used to attenuate transient surges in the power network. It reduces the amplitude of the surge to a level that a device can withstand. However, the nature of the high voltage that occurs in the operation of a VCB is that it has high-frequency peaks. An induction motor and transformer can be modeled as an RLC network with natural resonances at different frequencies. The overvoltage amplitude inside the device will increase if the overvoltage frequency is equal to one of the natural resonance points. An external arrester cannot attenuate this internal overvoltage. The high-frequency peaks of the overvoltage must therefore be attenuated. The best way to do this is with a well-known RC filter.
Real case
In the metal melting industry, electric arc furnace (EAF) transformers are used. Due to the high current and low maintenance requirements, VCBs are used to de-energize the transformer. Our customer was experiencing a problem with this type of network. When they wanted to disconnect the system and shut it down, they heard an arc inside the transformer. When they opened the transformer, they found that the windings and insulation were burnt. To find out what had happened, they asked our ESFA (Electrical System Failure Analysis) team.
Figure 3 shows the schematic of the network. Exactly the current chopping caused by the VCB was the reason. The voltage waveform across VCB contacts with and without RC snubber is shown in Fig4.
Fig 4. Voltage across VCB contacts a) without RC snubber b) with R=100 ohm and C=100 nF.
Conclusion
The VCB is an experienced circuit breaker. However, some transients occur when VCB interrupts a fault, which needs further analysis. Using RC snubbers alongside VCBs archives the best performance.
