Fault Calculations - t=8ms (60Hz) 1

[simonfr]

We are using the ETAP computer modelling for our power system and are calculating the fault levels for the 600VAC busbars which are connected to the main GTG's

My understanding is that the value always used to determine the fault level is the t – 8ms time, which for a 60Hz sytem is the half cycle time.
Apparently his is the standard for fault calculation studies.

However I am not an expert in this field, and am wondering why a time of t=0 is selected, as would this not be a valid fault level which the boards can ‘see’.

Cheers,

Simon

 

[magoo2]

Normally a time corresponding to the interrupting time of the circuit breaker (or protective device) is used to check coordination.

I'd expect to see a time of 1 cycle or so to use for 600 V breakers. You might find the following article to be of interest.

For a typical L-C network, you'll find that a time corresponding to 1/2 cycle will result in the highest assymmetrical duty (greatest amount of dc offset) on the devices involved. This result should be compared to the breaker close and latch rating.

 

[dpc]

This depends on the purpose of the short circuit calculation.

In the ANSI world:

Low Voltage Breaker:

Momentary Current (first 1/2 cycle)

Medium & High Voltage Breakers:

Close & Latch (Momentary) Rating - Momentary Current (first 1/2 cycle)
Interrupting Rating - 3 to 5 cycles

For relay coordination, the 30 cycle current is often more meaningful.

For the total asymmetrical current, the peak will always occur within the first 1/2 cycle.

 

[EddyWirbelstrom]

In the IEC world :

LV Breaker ( IEC 947-2 )
The rms value of the initial symmetrical 3 phase short-circuit current - I"k3 specifies breaker interrupt rating within the constraints due to the X/R ratio which are specified in Table 2 of above Standard.
ie. If the initial symmetrical short-circuit current >= 50kA, then the power-factor shall not be less than 0.2 ( or X/R not greater than 5 ).
Note: In locations close to large transformers the X/R can exceed 5.

HV Breakers ( IEC 62271-100 )
The rated short-circuit breaking current is characterised by two values :
• rms value of the a.c. component
• percentage of the d.c. component
If the d.c. component does not exceed 20% of the peak value of the symmetrical a.c. component at the instant of breaker contact separation, then the rated short-circuit breaking current is characterised only by the r.m.s. value of its a.c. component.
The Standard states that for most applications the X/R will not exceed 14 which corresponds to a d.c. time-constant of 44.6 msec. For a d.c. time-constant of 44.6 msec, the d.c. component would decrement to 20% in 71.7 msec.
Note: Although the Standards specifies a breaker make rating of 2.5 times the initial short-circuit current, calculations are recommended to ensure the actual peak current does not exceed 2.5 times initial short-circuit current.