instantaneous current setting 1

[ciy]

Hi,

There is a relay setting question need to ask for help.

That is related to the instantaneous current setting (50).

For example, the known fault current value is 10000A, the said setting should be larger or smaller than 10000A?
Which one is better protective philosophy?

Please find the illustrated curve as attached for reference.

Thanks!
CIY

 

[DiscoP]

It would depend on whether you want the relay to operate or be stable for this fault level

 

[AMBMI]

If 10 kA is largest possible calculated fault current (i.e. 3phase short circuit) the real fault current will be always smaller.
So if you want to have the F50 element trip you could set it to a smaller threshold (i.e. 8 kA).
Of course the situation can be more complicate (i.e. close to the generators, subsubtransitory current?)

 

[ciy]

DiscoP and AMBMI, thanks for your reply.

In my understanding, the 50 setting should be smaller than real fault current.

However, sometimes I see some settings are larger than fault current.

At this situation, the 50 tripping maybe never do forever. This make me confuse.

Please provide more your great opinions. THANKS!

Replenish missing attachment as attached.

CIY

 

[waross]

Is your fault current symmetrical or a-symmetrical.
Calculations based on transformer impedance voltages yield a steady state value of symmetrical current. (Available Short Circuit Current.)
Calculations based on the transformer X:R ratio, point on wave of occurrence and the available short circuit current may be over twice the available short circuit value. The a-symmetric or fully offset current value is an initial transient and decays to the available short circuit value. Time to decay varies widely depending on circuit parameters.
Interrupting ratings for breakers and switchgear is given in available short circuit current. The actual interupting capability of the switch or breaker is higher than the stated rating to allow for a-symmetrical fault currents.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[burnt2x]

In my understanding, the 50 setting should be smaller than real fault current.

However, sometimes I see some settings are larger than fault current.

At this situation, the 50 tripping maybe never do forever. This make me confuse.

What you wanted to coordinate will determine how you set your Inst, STPU, and LTPU. If you wanted to defeat the Instantaneous trip and allow the branch breaker to pickup first, you set your main breaker Inst pickup just above the real fault level. As long as the Inst trip setting is lower than the rated breaking capacity of your breaker, you're okay.

 

[DiscoP]

There are 2 fault levels to consider when setting an IOC element.

The fault level you don't want the relay to operate at, and the one that you do.

One you know these values, working out the setting becomes almost self evident.

It is sometimes not practical to apply IOC.

And other times you are forced into it, you need to be satisfied that discrimination is lost.

You are correct, if it set above any possible prospective fault current, it will never operate.

Sometimes this is deliberate so that it doesn't ever operate, or it could be so that it is ready for any future increases in fault level which the equipment cannot handle.

There is a lot of work that goes into setting protection correctly, and it is difficult to offer exact reasons without knowing a lot of details.