Gen Ground Fault C50.13

So what does the industry typically do?

Well, let's see:

Your specification: "...told him to comply with ANSI C50.13 which will limit the ground fault current Isc SLG to a value less than or equal to the 3 phase fault of the generator."

If you went to a restaurant and ordered crab legs, and the waiter brought you a tuna sandwich, your order would not be as specified. You would deserve a replacement item, as specified. In lieu of that replacement, you might find the supplied material that is not in compliance acceptable, if there is a price adjustment.

I am not certain of other sectors of industry, but if I order crab legs and get tuna, I do not accept it. If, when I ordered crab legs the waiter told me "we do not have crab legs, please make another selection", I could at that time make another selection from the offering here, or select another supplier of dinners where they do have crab legs.

If you do not get what you want, reject it. If your vendor accepted the order, indicating it will be within your specifications, it is time for him to produce for the stipulated amount.

This soliloquy assumes standard ANSI C50.13 is applicable... if it is not, then I guess I wasted a soliloquy.

First of all, there is no clause in ANSI/IEEE C50.13 that says about the requirement of 'limit the ground fault current Isc SLG to a value less than or equal to the 3 phase fault of the generator'. The only information it has regarding ground fault is in article 4.2.4.4 (Rapid reclosure following system faults), which is "Switching operations involving three-phase faults and phase-to-phase faults are more serious than switching operations involving line-to-ground faults".

Secondly, the title of ANSI/IEEE C50.13 is "Cylindrical-Rotor 50 Hz and 60 Hz Synchronous Generators Rated 10 MVA and Above" which clearly indicates this standard applies to generators 10 MVA and above. So, in terms of standard, the vendor's response is correct.

If you find the requirement is specified in any other standard, specify it to the vendor or if it is your absolute requirement, mention it to the vendor without mentioning the standard.

- MS

Interesting. I was reading the IEEE buff book and it says "To comply with ANSI C50.13, generators shall be grounded so that the maximum phase-to-ground fault current is limited to a magnitude less than or equal to the thre phase fault current. So I guess there is no clause in there.

Generator short circuit current are limited by the respective system (positive, negative and zero) impedances. Under three phase short circuit (I presume sudden short as you have specified x_d^" ) first peak of short circuit current would be about twice the value equal to 1/(X_d^") taking account of worst moment of short when dc component would be maximum. This would work out in your case a bout 14 times the rated current. In case of single line to ground fault the current would be 1/(X_d^"+X₂+X₀). This works out to about 3 times the rated current without any neutral grounding additional impedance. So the current under single line to fault would be less than that under three phase fault which meets your requirement. But this all about short time (sub transient) condition which last for only first one or two cycles. I think one should specify the continuous fault current withstand capability. One should not go by short circuit current in relative term but it should be specified in terms of absolute steady state value like 5Amp or 10 Amp. So you may like to specify the value accordingly.

The zero sequence impedance of a generator is usually much less than the positive sequence impedance as observed in this instance.As such if system is solidly grounded then the terminal ef current could be greater than terminal three phase current(Truths are stranger than fiction).Since generators need be secured only for 3ph fault currents and it is very very difficult to increase zero sequence impedance it is prudent to resistance ground generator circuits.

It is agreed that zero sequence impedance is less than positive sequence impedance. But single phase ground fault current would be controlled by all the three sequence impedances whereas three phase fault being symmetrical fault would be controlled by only positive sequence impedance. Therefore three fault current is always more than single phase line to ground fault. In my reply I have only tried to say that SLG fault current would be less than three phase fault current. I did not mean that generator neutral should not be grounded. I said that SLG fault current should be limited to some absolute value like 5A or any other value for that matter. This requirement would call for addition of some grounding resistance in neutral circuit.

From memory-for terminal short circuits: 3ph sc current=E/(Z+ )and for the 1ph-e sc current=3E/((Z+) + (Z-) + (Z0))

Please look into unbalance electrical system analysis. What you think (-Z) is not negative value of (+Z) but it is negative sequence reactance which is entirely different than positive sequence reactance and no way equal to minus value of positive sequence.

for terminal sc. in this instance I3ph = E/0.13 p.u. and Ief= 3E/0.34 p.u. with zero neutral circuit impedance. Trust this explains.ABB protection systems hand book i checked and find the formulae i am using are correct.