IEC 60909 Multiplier - ANSI equivalent 1

[NickParker]

Where can I find out the actual X/R ratio in ETAP? to compared it with the tested ratio of X/R=14?
What should be done if the actual X/R ratio is greater than that of tester X/R ratio (14)? I know that in ANSI, the breaker to be derated by the multiplying factor. Is there any multiplier in IEC 60909?

 

[cuky2000]

The breaker derating is governed by the IEC Std 909. At different of the IEEE/ANSI method is governed by the standard C37.010, IEC Std. does not use the X/R multiplication factors method. Instead, IEC uses the Thevenin impedance at the fault point, with the recommended corrections. Then, the decaying dc current is computed based on the breaker’s opening time and the relay operation time. The dc component at breaker opening is added to the initial current to get the actual interrupting current. This current is compared with the breaker’s interrupting capacity.

[highlight #FCE94F]DC%(t) = 100% ∙ e^{− t/T_dc}[/highlight]​

Although there is homogenization progress between the IEEE and IEC, still there are differences between the standards regarding the circuit breaker rating methods. For example, IEC type breakers are not derated for reclosing duties other than normal while IEEE does.

_{PS: We have a case with an HV circuit breaker with X/R =48 and the breaker manufacturer has wide dissimilar derating values using the IEEE and IEC method. They had a hard time understanding the rationale of the ANSI derating and some conflict with actual test values from the lab data. Appears, that that the ANSI multiplication factor was developed back with old oil technology and may need to use with caution for modern breaker technology until the standard is updated. It is a good practice consulting with the breaker manufacturer for an SF6 breaker with a dc decay ≠45 ms (X/R>17 at 60Hz) and/or use of a very fast relay and interrupting devices.}