Could someone explain X/R in relation to interruption

[faulkma]

We typically have to interupt breakers with a X/R between 6.6 and 8 with X and R in series but our lab gives us around 11.25. Which one is more severe on the breaker interruption a higher X/R or a lower one?

 

[DanDel]

The X/R ratio indicates the level of reactance (X) compared to resistance (R) in the system. The higher the reactance, the longer the fault current will stay at it's highest value.
A higher X/R usually indicates a higher level of fault current at the interrupting time, which is typically 3-8 cycles after the momentary fault value.

 

[dpc]

Higher X/R is worse because of the dc offset in the asymmetrical fault current. The higher the X/R ratio, the long it takes the dc portion of the fault current to decay, so the higher the peak current when the peak of the ac wave occurs. The peak current always occurs in the first 1/2 cycle.

 

[Marmite]

In SF6 and vacuum switchgear arc extinction occurs as the current passes through zero on the AC sinewave. The X/R ratio is a measure of the phase angle displacement between the voltage and current. By way of illustration, if you assume a theoretical purely inductive circuit with no resistance, the current lags the voltage by 90 degrees. This means that the voltage across the opening interrupter is reaching a peak as the current passes through zero. This increases the possibility of the arc restriking. That is why X/R is important when specifying switchgear.

Regards
Marmite

 

[Marmite]

Press submit in haste repent at leisure.
Whilst what I said is true in respect of the voltage, DPC has the right explanation in terms of the effect of X/R on breaking fault current.The breaker has to interrupt the symmetrical current (Normal AC fault current)with the asymmetrical current caused by the release of stored energy in the inductance superimposed on top. The mechanical analogy would be a spring (Inductance) attached to a damper (resistor). If you increase the proportion of resistance in respect of inductance (reactance) you apply more damping and the stored energy dissipates more quickly.

Regards
Marmite

 

[faulkma]

Dumb question but what angle would X/R = 6.6? In other words, a purely inductive angle of 90 degrees would be what X/R?

thanks much

 

[tem1234]

a purely inductive impedance would have a X/R ratio infinite, or very large

the angle is arctan(X/R)
arctan(6.6) = 81 deg

 

[jghrist]

tan([θ])=X/R
[θ]=arctan(X/R)

 

[faulkma]

thanks for all of your help!