Size of the breaker

[payam.ansari19@gmail.com]

Hello;

I'm working on a substation project in school for my senior design class. My instructor keeps saying, we need to know the following items to size our breaker.
1. Power Flow
2. Steady State Current
3. Fault Current
4. the ratio of(X/R)

I don't know why we need them? What I'm thinking I just need to know the voltage and say for example I need 138KV Circuit breaker.
Would you please help me figure out what is going on? what are all those other information for?

Thanks,
Payam

 

[Dennis Alwon]

Any engineers out there that can help the op.

 

[topgone]

Hello;

I'm working on a substation project in school for my senior design class. My instructor keeps saying, we need to know the following items to size our breaker.
1. Power Flow
2. Steady State Current
3. Fault Current
4. the ratio of(X/R)

I don't know why we need them? What I'm thinking I just need to know the voltage and say for example I need 138KV Circuit breaker.
Would you please help me figure out what is going on? what are all those other information for?

Thanks,
Payam

Any engineers out there that can help the op.

Lemme try.

1. You need the steady state current level to choose your breaker size.
2. You need to know the fault current that may pass through your breaker to set your protections properly and to know what withstand ratings your breaker should be.
3. You need to know the x/r ratio because it affects your breaker's interrupting capacity; if the x/r ratio of your point of installation is higher than your breaker's x/r rating, that would mean that the initial fault current (with dc component) will be beyond what your breaker is designed to withstand.

That's the simplest I can say about it. Perhaps, it would be of much help if you search this forum using "x/r" as keyword.

 

[ggunn]

Hello;

I'm working on a substation project in school for my senior design class. My instructor keeps saying, we need to know the following items to size our breaker.
1. Power Flow
2. Steady State Current
3. Fault Current
4. the ratio of(X/R)

I don't know why we need them? What I'm thinking I just need to know the voltage and say for example I need 138KV Circuit breaker.
Would you please help me figure out what is going on? what are all those other information for?

Thanks,
Payam

138kV at what amperage? Breakers are designed to open a circuit when their rated amperage is exceeded; the voltage rating is to ensure that the flow of current does not arc across the contacts when the switch is opened. The current rating needs to be higher than the steady state current so that it can supply the current needed without tripping and less than the fault current so that it will open in the event of a fault.

 

[kingpb]

That information is the standard info needed, or determined by every utility for designing substations, selecting the appropriate equipment, and setting their relays.