Symmetrical Components: 3-phase-to-ground fault vs 3-phase fault?

[Powerman888]

Using symmetrical components and looking at sequence networks (positive, negative, and zero), a 3-phase fault is said to be a balanced fault, therefore only positive sequence network applies (might think of it kind of like normal conditions, except load is "way excessive").

Now considering a 3-phase-to-ground fault (as opposed to just a 3-phase fault), would the ground be like a neutral that nominally has no current flowing through it for balanced load conditions (in this case, the balanced load being a fault that ties all 3 phases together)? That is, are we again talking about a balanced fault with only positive sequence networks applicable (and no negative sequence network and no zero sequence network)?

The answer to the question may be obvious, but never see any literature taking the time to state that a 3-phase-to-ground fault is for all practical purposes the same as a mere 3-phase fault (if that is indeed the case).

Have not looked at it in a while, but believe I recall that a 2-phase-to-ground fault "is" different from a mere 2-phase fault. So want to feel comfortable that a 3-phase-to-ground fault "is not" different from a mere 3-phase fault (again, if that is the case). Appreciate any thoughts on the matter.

 

[Ohmly]

Theoretically speaking, a 3 phase fault and a 3 phase to ground fault are the same thing.

The software we use to calculate fault levels only gives us the 3ph-G option, however when the calcs are run there is no ground fault current.

There is only positive sequence components throughout the network.

In practice, I believe that some current will flow to ground, the calculations rely on every phase of every piece of equipment having the exact same value of impedance.

A 2 phase - ground fault is a different fault level to a Ph-Ph fault.

 

[jghrist]

A 3Ø fault is the same as a 3Ø-grd fault. The three phase currents add to zero.

For the difference between a Ø-Ø and a Ø-Ø-grd fault, consider loads like you did for a 3Ø fault. A load connected between phases is different from two loads connected phase to neutral on different phases.

 

[odlanor]

I guess the difference of 3-phase-to-ground fault or 3-phase fault is merely physical.
For instance, imagine an isolated bus duct 3-phase system. You will never have a 3-phase fault!
But if you decide to ground busbar, you will connect ( at the same section) each phase to ground grid.
A busbar energization will carry a 3-phase-to-ground fault .

 

[karalahana]

dear odlanor why dont u get a three-phase fault with isolated bus duct 3 phase system? do u mean 3 -phase- ground fault maybe??

 

[rcw retired EE]

"why dont u get a three-phase fault with isolated bus duct "

Isophase busduct has each conductor completely surrounded by a grounded metal enclosure. (A bus bar inside a metal pipe). There is no way to connect any phases together without first going to ground. A phase-to-phase fault is impossible (except inside the generator termination compartment).

One reason isophase bus is used on large generators is the generator cannot be designed economically to withstand a phase-phase or 3-phase fault at its terminals. Isophase bus makes a phase-phase fault very unlikely and the generator neutral grounding limits the phase-ground fault level so the generator is protected from itself.

 

[pwrtran]

Same reason as rcwilson explained, you don't get Ph-Ph fault in a SF6 switchgear too.

 

[odlanor]

Old standard for hydrogenerator: ANSI C50.12-1982
6.1-short circuits
A generator is able to withstand, without damage, within 30 seconds, a short phase in their terminals when operating in kVA and power factor at nominal 5% overvoltage, with fixed excitation.

 

[rcw retired EE]

odlanor - You are correct. Let me clarify:

IEEE C50.13-2005 section 4.2.4.1 "Sudden short circuits at the generator terminals" says,

"A generator shall be designed so that it can be fit for service after experiencing a sudden short circuit of any kind at its terminals while operating at rated load and 1.05 rated voltage, provided that the fault is limited by the following conditions:

- The maximum phase current does not exceed the phase current obtained form a three-phase sudden short circuit.

-The stator winding short-time thermal requirements are not exceeded."

If the generator neutral is not high impedance grounded, the ground fault ( 3Io ) can be higher than the 3-phase fault, violating the first condition. The negative sequence currents from a phase-phase fault could violate the second condition. But the rotor capability is usually the limiting factor on the negative sequence withstand and this C30.13 paragraph is referring to the stator windings.

So you are correct that the generator should withstand a phase-phase fault at its terminals.

 

[odlanor]

rcwilson ,
What about duration 30sec for short circuit?
Was kept in standard later?

 

[rcw retired EE]

C50.13 is for Cylindrical Rotor Generators 10 MVA and larger, as normally applied on gas turbiens and steam turbines. It isn't for the typical lower rpm hydro units.

There is no specific short-circuit time limitation but paragraph 4.2.1 "Stator winding short-time thermal requirements" says:

"The generator stator shall be capable of operating at 150% of rated stator current for at least 30 seconds starting from stabilized temperatures at rated conditons."

It goes on to give an equation and a table:

Time 10 sec, Current 218%.
30 sec, 150%
60 sec 127%.

A caveat follows that during certain unbalanced faults, the negative-sequence thermal capability of the rotor may be exceeded before the stator limits are reached.

Bottom line, trip fast and use isophasebus to minimize the chances of a fault between the generator and the breaker.

 

[cuky2000]

The general simplified voltage and current relation for a three phase with or without ground fault is:

Va=Zf.Ia + Zg.(Ia+Ib+Ic)

i. For 3 phase fault without ground fault Zg = 0; Va=Zf.Ia
ii. For 3 phase fault with ground fault Zg>0 and |Ia+Ib+Ic|>0 In general. However, if the following assumptions are met:
a. The phase voltages and current are equal and 120 degrees apart
b. The electrical system is symmetrical with equals phase & mutual impedances and equal shunt admittances.

Then, |Ia+Ib+Ic| = 0 and also Va =Zf.Ia _{(same as above for 3phasefault without ground fault.}

Fortunately, most power system with transmission line fully transposed, with symmetrical rotating machines and transformers operating with phase voltage within close tolerance this assumption could be considered valid for practical application.

See the enclosed attachment for additional information.