Why is there no Circuit Breaker in Bus PT Feeder?
Why there is no Circuit breaker in Bus PT feeder?
In our system, interlock is given such that if any fault in Bus PT, then it opens the Incoming Bus breaker. Why it is done?
Its 11kV busbar. Any economical problem in having breaker for PT or it’s not necessary to have? Kindly clarify.
Answer:
Potential Transformers (PT), also known as voltage transformers, are critical elements in power system operation and protection.
They reduce high voltages to measurable values, providing voltage information for metering and protection devices.
This article will delve into the reasons behind the absence of circuit breakers in Bus PT feeders in a typical 11kV busbar system, the significance of interlocking, and the economic implications of including circuit breakers for PTs.
PT Protection and Interlocking
Circuit breakers are generally designed to protect the electrical system from faults such as short circuits or overloads.
In an 11kV busbar system, faults are usually managed by disconnecting the problematic section to prevent further damage or disruptions.
Given this function, one might question why there are no circuit breakers in Bus PT feeders.
This question arises from the fundamental role of the PT.
The PT is not a power-handling device; rather, it's a measuring device that handles only a small amount of power for measurement and protection relay operation.
The currents flowing through the PT secondary windings are too small to cause significant heating or damage, even in fault conditions.
Hence, the need for a circuit breaker - a device designed to interrupt high currents - is not present.
Furthermore, in the event of a PT fault, the protection mechanism used is different than for power-transformer faults.
Typically, a PT fault will not lead to significant current flow but may produce a voltage difference.
This discrepancy is monitored, and upon detection of an abnormal condition, an interlock system comes into play, causing the incoming bus breaker to open.
This effectively isolates the PT, preventing damage to other system components.
Interlocks are essentially fail-safe devices or systems that ensure a specific sequence of operations occurs, or they prevent operation under certain conditions.
The interlocking system is a logical and cost-effective way to protect the PT and the larger system.
Economic Considerations
Adding a circuit breaker to the Bus PT feeder does not contribute significantly to operational safety or efficiency, considering the low current and power level associated with PT operation.
However, it does add to the complexity and cost of the installation.
Circuit breakers, especially those capable of handling 11kV systems, are not inexpensive.
They also require maintenance, testing, and periodic replacement, adding to the lifecycle costs. When you factor in these costs and consider that a circuit breaker does not substantially improve the PT's protection, it becomes clear that economically, it makes more sense to employ an interlock system rather than a dedicated circuit breaker.
Conclusion
In summary, the absence of circuit breakers in Bus PT feeders is due to the unique characteristics and requirements of PTs and the nature of the faults they might experience. Interlocking is a sensible and cost-effective protection approach that suits the PT's operation, reducing the likelihood of system-wide disruptions due to PT faults. The inclusion of a circuit breaker would unnecessarily inflate costs without significantly improving system protection or efficiency.
In essence, the power industry's practices are grounded in years of experience and technical knowledge, striving for the balance between safety, reliability, and economics. Therefore, while it's always good to question and review these practices, it's also important to recognize the logic and practical considerations that underpin them.