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.
Replies
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neo23Can you upload a sld of your system....that would help a lot. -
Vinodh PanchaSorry if I have asked you in wrong manner. ie. I referred to 11kV Switchgear panels of Unit, Station Transformer.Kindly find below the snapshot of the part of feeder.Hope it is enough for you to rectify my doubt.
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neo23it is very difficult to read . can you just simply explain clearly what you really want to know? are you referring to the coupler circuit breaker used in bus bar or what. please explain everything
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Vinodh Pancha
Simple!!!neo23it is very difficult to read . can you just simply explain clearly what you really want to know? are you referring to the coupler circuit breaker used in bus bar or what. please explain everything
In the fig, u can see two feeders...one is ID fan feeder and other is Bus PT feeder.U can see VD4 1250A- Breakerfor ID fan feeder whereas same is absent in Bus pt FEEDER.Y is it so??? Y no breaker is installed for Bus Pt feeder for its protection???This is ma doubt.
Can u get me ??? -
neo23Yeah i got you.
Now see clearly there is a PT with two cores on the Bus bar side.
One core is used for metering .
And the second core i think is used for protection . Read the digits in the box and do tell me ( i am unable to read them) . As far as i know these must be the protection scheme codes , resulting in the feeding of certain relays. and definitely these relays must be driving some breaker circuit or an alarm circuit.
Please confirm and revert. after that we shall continue our discussion -
aarthivgthe potential transformer from feeder should not have any circuit breaker, because pt is just a measuring equipment. just consider if there is a circuit breaker before pt, we can interrupt the supply and change the pt itself, or make the meter reading standstill, there is lot of such chances to misuse the supply. so there should not be any circuit breaker.
with respect to protection of pt, fuse will be provided. ground fault protection will be provided. -
neo23What about the Bus Ground Protection? What breaker shall trip?
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aarthivgfrom feeder, the supply will be given to bus that ht bus which has 3 runs. from ht bus only supply will be given to pt. there will be separate protection to bus
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neo23Plus , this is an 11 kv busbar. What misuse of power can you do on an 11 kv busbar?
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aarthivgPT is just a meter to measure voltage. if circuit breaker is there, we can isolate the pt. and we can even stop the rotation of meter reading. but the supply can be feed from the feeder
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aarthivgcircuit breaker helps to isolate the equipment
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neo23
suppose a fault comes and the pt trips the breaker on other run of the bus bar as you are saying. In that case also the meter rotation stops , and it also stops if it is on the same run.aarthivgpt is just a meter to measure voltage. if circuit breaker is there, we can isolate the pt. and we can even stop the rotation of meter reading. but the supply can be feed from the feeder -
aarthivghow can a pt trip a cb? whats that other run which u have specified?
can u be clear please. -
neo23I am sorry to be unclear, but i meant the fault is sensed by a relay which furthur trips the breaker. And the other run thing i said because i thought you were reffering to
from feeder, the supply will be given to bus that ht bus which has 3 runs. from ht bus only supply will be given to pt. there will be separate protection to bus
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aarthivg
runs are nothing bur R,Y,B. three runs. since you have mentioned it has a 11 kv feeder, most probably it will be in distribution side. so from the feeder, the supply is given to ht line which has three runsneo23I am sorry to be unclear, but i meant the fault is sensed by a relay which furthur trips the breaker. And the other run thing i said because i thought you were reffering to -
Vinodh Pancha
Hi Neo,neo23Yeah i got you.
Now see clearly there is a PT with two cores on the Bus bar side.
One core is used for metering .
And the second core i think is used for protection . Read the digits in the box and do tell me ( i am unable to read them) . As far as i know these must be the protection scheme codes , resulting in the feeding of certain relays. and definitely these relays must be driving some breaker circuit or an alarm circuit.
Please confirm and revert. after that we shall continue our discussion
as u said,
1st core for metering and 2nd core for protection.
Inside the box, its
ABB make REU610-VOLTAGE PROTECTION RELAY with 27(Undervoltage),27N(No volt),2(timer).
I found that as @aarthivg stated, CB is not provided for PT as its just a measuring device and just a MCB is provided to isolate the fault from transformer. Some said Since the output of pt is just 110V for measurement purposes, it doesnt have any effect.
But I think its not the right reason....
Also, as aarthivg stated, there is no chance of misuse in power????what you exactly mean to say -
neo23Now can you tell me which circuit the voltage protection relay makes or breaks , by reading the drawing?
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aarthivg
I am sorry, if it is wrong. but this explanation was given by my hod, who had more than 30 years of experience in transmission field. anyway i will get you a clear explanation soon.Vinodh PanchaHi Neo,
as u said,
1st core for metering and 2nd core for protection.
Inside the box, its
ABB make REU610-VOLTAGE PROTECTION RELAY with 27(Undervoltage),27N(No volt),2(timer).
I found that as @aarthivg stated, CB is not provided for PT as its just a measuring device and just a MCB is provided to isolate the fault from transformer. Some said Since the output of pt is just 110V for measurement purposes, it doesnt have any effect.
But I think its not the right reason....
Also, as aarthivg stated, there is no chance of misuse in power????what you exactly mean to say -
Vinodh Pancha
- neo23Now can you tell me which circuit the voltage protection relay makes or breaks , by reading the drawing?
neo23Hi neo, as per the drawing,
As the relay gets activated on undervolt or no volt, it opens the MCB which protects the secondary of transformer where the voltage will be 110V. thereby transformer is protected from any fault.
Hope am right.
Hereby I have attached clear view of feeder again. Sorry for my previous unclear attachment.
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