Short Circuit Considerations for Shunt admittances of non-rotating loads and Line Capacitances 2

[Wfg42438]

Hello,

I wanted to confirm with others if there are any ANSI standards which recommend to consider the following items for L-G SC calculations:

[ul]
[li]Shunt admittances of non-rotating loads[/li]
[li][/li]
[li]Line Capacitances[/li]
[/ul]

So far i was able to see only IEC60909 2016 has the following description:

Shunt admittances of non-rotating loads shall be neglected in the positive-, the negative and
the zero-sequence system.

Line capacitances shall be neglected in the positive- and negative-sequence system. Line
capacitances in the zero-sequence system shall be taken into account in low-impedance
earthed networks having an earth-fault factor (see IEC 60027-1) higher than 1,4.


Please let me know if you know of any ANSI standards that provide some guidance on this item, thanks!

 

[jghrist]

From IEEE Std 141-1993 (Red Book, 4.2 Sources of Fault Current):

"Fundamental frequency currents that flow during a short circuit come from rotating electric machinery. (Charge power capacitors can also produce extremely high transient short-circuit discharge current, but they are of natural frequency much higher than power frequency and usually of such short duration that the calculate power frequency short-circuit duty current is not significantly increased by adding the capacitor discharge...)"​

 

[Wfg42438]

jghrist,

I am not 100% sure of how to interpret that statement as a definitive recommendation is not made


On another note I noticed that IEEE 399-1997 has the following statement in section 7.5.3:

7.5.3 Neutral grounding
For faults necessitating the inclusion of zero sequence data, i.e., line-to-ground faults, double
line-to-ground shunt faults, and series faults, the flow of fault currents is appreciably affected
by the system grounding conditions. Of particular concern is the presence of multiple
grounding points and the values of system grounding impedances. Grounding impedances
can be used, to various degrees, to limit the value of the ground fault current to a minimum
value, to suppress resulting overvoltages, and to provide “handles” for ground protection.
System grounding can also play an important role in the proper simulation of the system zero
sequence response. More specifically, for solidly, or low-impedance grounded systems, it is
sufficient to include in the study only the occasional current limiting transformer and or gen-erator
grounding impedances, while disregarding zero sequence line/cable charging shunts.
For high-impedance grounded, floating, and/or resonant-grounded systems, however, the latter
will have to be taken into account (per IEC 60909 (1988), since the assumption that
neglecting it yields conservative (higher) fault currents is no longer valid.

Based on the statement in IEEE 399 it seems there's a recommendation to neglect the zero seq line/cable charging shunt impedance, would you agree with that?
Should the same be assumed for Shunt admittances of non-rotating loads?

 

[jghrist]

I would interpret it to mean that shunt capacitance of any source could be neglected for solidly grounded systems, but for ungrounded, high impedance grounded, and resonant grounded systems, you should include inrush from shunt capacitance to ground.

 

[Wfg42438]

Ok I agree with what you are saying

Its interesting to see that IEC60909 recommends the opposite for these cases

 

[jghrist]

I didn't read your description of the IEC 60909 to be opposite - maybe I'm interpreting it wrong. I'm not familiar with IEC 60909; I only know what I read in this thread.

 

[Wfg42438]

The statement I was referring to is the following:

Line capacitances shall be neglected in the positive- and negative-sequence system. Line
capacitances in the zero-sequence system shall be taken into account in low-impedance
earthed networks having an earth-fault factor (see IEC 60027-1) higher than 1,4.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1556058282/tips/IEC_ik776b.tiff[/url]

 

[cuky2000]

Interpretation:
1) For effectively grounded system [highlight #FCE94F]EFF<1.4[/highlight] (or equivalent per IEEE Std C62.92.1) Coefficient of Grounding, [highlight #FCE94F]COG<80%=EFF/SQRT(3)[/highlight]), the line capacitance has a negligible impact on the SC modeling.
2) For non-effectively grounded system, only the zero sequence impedance has a significant impact on the accuracy of the SC modeling.
3) SLG>60% of 3 Phase fault current for a In an effectively grounded system.
4) Adding capacitive in the neutral grounding increases the SLG fault currents compared to solidly grounded case, due to negative reactance in the zero sequence networks.
5) The fault current ratio I_SLG/I_3phase > than 1.0. for a circuit with sizable neutral capacitance. Careful should be exercised to avoid power frequency resonance causing extremely high SLG fault currents

 

[Wfg42438]

cuky2000 and jghrist ,

Thank you for your feedback on this topic I have a better understanding now