At a large chemical plant in an industrial area, electricity is the heart of production. The distillation process, chemical pumps, heating reactors—none of them can stop, not even for a second.
At the main distribution center, a 10,000 kVA transformer converts the voltage from 20,000 volts to 3,300 volts to supply various critical loads.
The plant's electrical team realized one crucial thing:
The VCB(VACUUM CIRCUIT BREAKER) installed on the secondary side of this transformer must be selected correctly.
Because one mistake could result in a shutdown, resulting in millions of dollar in losses.
Why is a VCB Important in a Chemical Plant?
A chemical plant is not like a typical factory. Because the electrical system is used:
- Providing power to large motors (pumps, compressors, mixers)
- Driving HVAC systems for chemical temperature control
- Powering highly sensitive control and instrumentation systems
Finding the Right VCB Guard
Technical data for the transformer obtained:
- Capacity: 10,000 kVA
- Primary voltage: 20,000 V
- Secondary voltage: 3,300 V
- Transformer impedance: 8%
Step 1 – Calculate the nominal secondary current:
Inominal = 1750 A
Step 2 – Calculate the short-circuit current:
sc = ZInominal = 21.875 A = 21.9 kA.
The VCB must be able to interrupt currents up to 22 kA, with a safety margin. Therefore, a minimum capacity of 25 kA is recommended, or a larger capacity can be used.
Based on the calculations above, the selected VCB must be capable of:
- Transmitting currents of ≥ 2000 A
- Breaking short-circuit currents of ≥ 25 kA
- Operating in 3.6/7.2 kV systems
- Complying with IEC 62271-100/JIS/the applicable standard
The main, but not limited, criteria for selecting a Vacuum Circuit Breaker (VCB), especially for medium-voltage systems such as 3.3 kV transformers:
1. Rated Voltage
2. Rated Current
3. Breaking Capacity / Short-Circuit Current Rating
4. Operating Frequency
5. Operating Mechanism
6. Operational Durability
7. Used Standards and Certifications
8. Additional Accessories (Optional but important)
Other Related Articles:
Why Does the Contact Resistance Testing Need 100A or Higher?
Hazards and Treatment of Excessive Contact Resistance of Circuit Breakers or HV Switches
How to measure the contact resistance without changing the circuit?
How to Correctly Test the Contact Resistance of HV Switchgear or Circuit Breaker?
Why Does Excessive Contact Resistance Occur In Electrical Secondary Circuits?
What is the Testing Checklist for 110kV/220kV Substation Acceptance and Maintenance Testing?
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