Specifying HV/MV breakers to IEC 72271.100 using IEC 60909 prospective short-circuit current duties

[EddyWirbelstrom]

The attached spreadsheet SKM-PTW and IEC 60909-0 Manual Calc.xlsx includes data for a ABB VD4 1212-40 circuit-breaker.
The calculations on Tab ABB VD4 of attached spreadsheet confirms that the breaker has following IEC 62271.100 short-circuit withstand ratings:

Rated short-circuit breaking current ( Ics ) = 40 kA at contact parting time => 72.4 msec
Rated d.c. component of 20% ie. 0.20 * ( sqrt(2)*40 ) = 11.31 kA See Note 1.
Rated peak withstand current (Ip ) or Make current (Im) = 2.5 * 40 = 100 kA

Note 1. IEC 62271.100 Figure 8 demonstrates the a.c. current wave offset due to the apparent presence of d.c. current.
The ratio between breaker Isc and d.c. component ensure sufficent time between a.c. current zeros for the breaker to deionize the arc between breaker contacts.

 

[EddyWirbelstrom]

Does anybody out there in IEC land have experience in specifying MV breakers to IEC 62271.100 ?
The breakers are ‘near’ to large generators.
The IEC 60909.0 calculated values for Ib sym, Ib asym, i p and i dc are high.
I need to determine if a 11kV, breaker with following short-circuit withstand ratings is adequate:
 Ib sym at breaker contact parting = 20 kA
 d.c. component at contact parting = 0.2
- Breaker contact parting time = 0.06 sec

IEC 60909-0 C – PTW calcualted short-circuit current duties are:
 I”k3 = 23.125 kA
 ip 3 = 57.641 kA
 Ib sym at contact parting = 17.857 kA
 i d.c. at contact parting = 9.475 kA
My SKM/PTW IEC 60909 results and calculations are attached.
I am not confident with the PTW calculated value of i d.c. which uses the IEC 60909-0 formula 64 where:
i d.c. = √2 I”k e^-2πftR/X
The value of I”k will quickly decrement before contact parting at 0.06 sec due to the changing subtransient, transient and d.c. time constants.
Can anyone comment on my attached calculations which conclude that the following breaker IEC 62271.100 specifications are required:
Assume the breaker rated percentage d.c. component is 20%.
 Rated short-circuit breaking current ( Isc ) 25 kA
 Rated percentage d.c. component 20%
 Rated peak withstand current (Ip) 100 kA