CT I dynamic

[Sargardani]

The short circuit current at the branch bus duct for excitation system has been calculated as 130kA. But at this location we have CTs with Idynamic as 50kA.

Need the forums response on whether the CTs will with stand the calculated short circuit current? What could be the repercussions?

Thanks


Sarg

 

[scottf]

The dynamic current rating is the maximum peak current that the CT can withstand without mechanical damage. This is an instantaneous peak current and is normally thought of as being 2.55 times the thermal current rating, which is an rms rating with a duration (normally 1 second).

So, if the 120 kA you calculate is a peak current, then the CT needs to have a dynamic current rating of 120 kAp. If the 120 kA calculation is an rms value, then the CT needs to have a thermal current rating of 120 kArms and a dynamic rating of 2.55 x 120 kA = 306 kAp.

 

[mc5w]

There is also a maximum short time current that will pass from primary to secondary without producing excessive error. How much is acceptable depends on the application.

For revenue metering the secondary current only needs to be accurate up to full load, maybe a bit more. You do not really care if the cor saturates during a ground fault.

For a simple ammeter or relaying application you do not realy care what happens after you reach full scale for the ammeter or relay. If your relay is something along the lines of a motor overload relay you can tweak the relay setting to compensate for error. If the application is instantaneous trip on short circuits the CT only needs to be accurate up to 2x the instantaneous trip rating.

For differnetial bus protection you really do need CTs that are accurate at the short circuit current that a fault outside of the bus will draw so that the bus protection will not do a false trip.

 

[Sargardani]

scottf: do you know of any links to sites that would help?

Thanks

Sarg

 

[scottf]

Sarg-

You need to get a copy of IEC44-1 (assuming IEC is your norm). It defines the terms basically as I have written above.

Bottom line is that the CT needs to be rated to handle the fault current duty you expect for the installation.

As for the 2.55 multiplier (some use 2.7), that is not covered under the standard, but is a general rule of thumb that assume a fully offer asymetrical current wave to calculate the maximum peak current. Your X/R ratio may be such that you could use a lower number.