×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

(OP)
I have a generating system grounded via a 5A NGR. The voltage on the lines are monitored with three VTs connected wye/wye, where both Pri/Sec having their neutral point connected directly to GND.

I understand why you would want to ground both sides of the VT's:
a) Removes harmonic distortion as the triplens have a grounding path at the VT's star point
B) Provides V-G measurements giving you the ability to detect when a Line to Gnd fault occurred as the other two phases will shift up to V L-L

However, when there is a Line-Gnd fault, won't the fault current bypass the NGR because it has a zero sequence back into the system via the VT's solidly grounded neutral point?

I thought about it all night and I can't figure out how this would NOT happen.

Cheers,
Maj
 

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

No
The voltage transformers have a high impedance and hence have almost no appreciable effect.  

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

Hi Majesus.
Maybe I don't understand your Q good, but in all cases, I try explained your problem from my point of view:
1. You have wye/wye VT on the generator side and secondary VT star point is solidly grounded.
2. On the NGR you have also some VT and it's also grounded on the secondary.
In this case you have "good chance" for the unwanted trip in case of L-G fault in the VT circuit, but not in case of L-G primary fault, as right wrote Rodmcm .
for avois this effect, you have grounded B phase of VT or disconnect (possible put additional isolation VT) VT on the NGR.
Best Regards.
Slava

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

The VT has an extremely high impedance (relatively) and can generally be ignored when doing a fault analysis.

 

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

Generally, yes; but always?  What I've seen is that VTs on high resistance grounded generators are often connected wye-wye, with the primary wye grounded and the secondary B phase grounded.  This minimizes the likelihood of a secondary ground fault showing up as a stator ground fault.

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

Isn't the original question regarding fault current path?

The grounding points on the VT don't change the impedance line to neutral impedance of the primary winding, and therefore the amount of fault current through the winding of the VT is not effected.

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

It doesn't take much current to pick up a 100% stator ground system.

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

David-

I can't comment on that. My comment is in regards to "shunting" current around the NGR, which in my opinion, the VT does not do in any appreciable way.

 

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

There is not ground source on the secondary side of the wye-wye VT, so there will be no fault current flowing through the VT for a primary side fault, regardless of the impedance of the VT.  A grdY-grdY transformer is not a ground source.

Most utility 3Ø distribution transormers are connected grdY-grdY and there is no fault contribution from these for a primary distribution ground fault.  This is only a problem for a grdY-delta connection.
 

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

Quote:

This is only a problem for a grdY-delta connection.

Hi jghrist

What is the only problem, can you explain it clearly?  Thanks!

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

jghrist-

In my thinking, the "primary current shunting" mentioned in the original post is not a function of if the secondary wye is grounded or not. It's only a function of the primary winding impedance.

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

(OP)
Thanks Guys, My question was simply more:

Does a grounded WYE PT change the impedance of the Line to Ground on the Power system. I was wondering if the current will be shunted away from the NGR and flow back to the system via a grounded WYE VT. According to info here and what I then was able to look up:

VTs are designed to minimize the current drawn by the burden to reduce voltage drop across the parasitic components of the VT.  The errors are to be kept within the limit defined by the precision required. In order to achieve this a VT is designed and connected to have low leakage reactance, low loss and high magnetizing reactance. Low reactance is achieved by interlacing primary and secondary both on core limb. High magnetizing reactance requires minimum iron path and high permeability steel. Low loss requires low loss steel and very thin laminations.
A typical 2400:120V 60Hz VT would have primary winding resistance: 143Ohm, Secondary winding resistance 154Ohm and Xm = 163kOhm.
During a Phase to Gnd fault, the contribution of the VT would be very small.
 

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

majesus-

I think you maybe meant primary winding resistance 1430 ohms, i.e. you left off a zero. Primary winding resistance of a 5 kV VT should be about 10 times higher than secondary winding resistance. Also, 163kohm would be on the low side.  

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

pwrtran,
A grdY-delta transformer acts as a grounding bank.  A grdY-grdY transformer does not.

scottf,
The short circuit will shunt the grounded VT winding.  If there is no source on behind the VT winding (no source on the secondary), there will be no voltage across the winding and it will not contribute any current to the fault.
 

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

jghrist-

If we're talking about a VT connected across a NGR, then it's a simple current divider. The reactance of the VT reflected to the primary is not a function of secondary grounding. It's that simple IMO.

It's for this reason that VTs do not have any kind of short-circuit or fault duty rating. They never carry enough primary current during a fault condition to matter.

 

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

(OP)
Scottf,

Can you post a link or paste some typical values of PTs? I'm curious.

Thanks,
Maj

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

majesus-

Values of what?

If you mean reactances, that's kind of tough. I don't have all of the values written down for all of my company's designs, but I'm giving you more a feel from experience.

 

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

(OP)
Scottf,
No probs.
I was curious to know some typical VT's transformer characteristics (
Primary winding resistance, primary leakage,
secondary winding resistance, secondard leakage and Magnetizing reactance)

I'm glad you did give some input. Always good to know ;)
Thanks

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

Majesus-

Since most of those values are typically not provided to customers, I'd have to look them up in the design records (which I don't have here at home :)

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

jghrist-

Sorry I don't follow you. What is your point if a primary grdy/secondary delta transformer connected to distribution system when a fault on the distribution system but there is no source on the secondary level?  

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

pwrtran,
I was using a distribution system as an analogy.  A grdY-grdY connected VT is no different conceptually than a grdY-grdY distribution transformer.  These don't feed ground faults - neither would a VT.

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

Thanks jghrist, I agree with you. Waht I didn't understand is your comments on a grdY-delta connection, did you mean a grdY-delta connection will feed ground fault on primary?

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

Quote:

Thanks jghrist, I agree with you. Waht I didn't understand is your comments on a grdY-delta connection, did you mean a grdY-delta connection will feed ground fault on primary?
Yes.

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

How? If it is a radial line and a grdy/delta transformer connects to a load customer, how come the grdy/delta transformer will contribute to a primary fault?

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

May I try to field this one?
Consider a three phase distribution circuit with only two phases energized, "A" and "B".
Now let's look at a transformer on "C" phase. If this transformer is energized on the secondary with full secondary voltage, it will back feed full primary voltage into "C" phase primary.
Now lets consider the bank of three transformers when the primary "C" phase is lost for some reason:
"A" and "B" are still energized and form an open delta secondary. The open delta voltage is full voltage at the same phase angle as phase "C". But transformer "C" is connected across this open delta and back feeds full voltage into the "C" phase primary.
What is a closed delta when energized with three phases becomes an open delta back feeding the third transformer if a primary phase is lost.
What happens next depends on the transformer size relative to the other loads on the "C" phase primary.
There are about 4 possible scenarios:
1> The loads are within the capability of the transformer and even though the circuit has lost one phase from the supply, the circuit stays energized on three phases due to the back feed, but with a slightly lower voltage on the back fed phase due to voltage drops in the extra length of distribution line and the voltage drops in the transformer.
2> The loads exceed the transformer capacity but not the fuse capacity and the transformer becomes over heated.
3> The loads exceed the transformer capacity and the fuse capacity and the fuse blows.
4> These banks are problematic and the servicemen tend to become frustrated with frequent fuse failures and start over sizing the fuse every time they replace it.
As a result, the fourth scenario is a burned up transformer.
In the event of a ground fault, the wye:delta bank will back feed fault current limited by the impedance and X:R ratio of the transformers until the fuses blow. Remember that the fuses often have been grossly over sized for the transformer capacity and may not blow instantly.

History>- Generations ago, many distribution circuits were 2400V delta. As the loads grew it became common for utilities to increase capacity by running a primary neutral and reconnecting the transformers in wye. The voltage was jumped up to 4160V but as the line to neutral voltage was 2400V the same transformers would still serve.
By the addition of one conductor and either low voltage insulators or uninsulated metal support brackets they had just increased the circuit capacity 73%.
At that time, however, the standard industry voltage was 440V delta. We now had the 4 wire wye:delta connection with all it's foibles. Old text books warn that a fault often resulted in extensive fuse blowing throughout the system.
 This transformer connection is not recommended by REA (Rural Electrification Administration) or its successor. Legacy installations of delta services are still in service but mostly run with the primary wye poit floating rather than tied to neutral. The delta secondary stabilizes the voltages and phase angles.
(Draw the primary vector sketch, a wye, superimposed on  the secondary vector sketch, a delta.
This is a very rigid construction when you consider that if the magnitude of a primary vector changes, the corresponding secondary vector must also change, but remain at the same phase angle as the primary.)
The floating wye point is not a [perfect solution as you may now generate switching transients. Some utilities install four fuse cutouts on the primaries of wye:delta banks. A fuse link is first installed in the neutral connection. The three phases are then energized. Then the neutral fuse is removed and take away. This avoids switching transients when field work is done on the bank.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

Hi waross, very well explained! Further questions extended on this - what if a 3-leg, 4-leg and 5-leg core 3Ph grdy/delta transformers vs. a secondary delta connected three single phase bank under same conditions?

RE: Will a L-G fault bypass the NGR through a Grounded Wye connected VT?

Back to the original post. The VT's are an impedance to ground in normal operation. The the primary current of a VT is very low. A ground fault will be in parallel with the existing impedance to ground of the PTs and will lower the impedance to ground.
Secondly, the PT currents will cancel and only any harmonic currents will flow in the NGR. This is a small fraction of a very small quantity. Under fault conditions the fault will drop the voltage to one PT and the resultant vector sum of the currents of the other two CTs will add to the fault current.
With a burden of 50 VA on 480V, The PT current will be  0.104A plus the magnetizing current. Not enough to affect your NGR. It will probably change its current more than that with temperature changes.

Bill
--------------------
"Why not the best?"
Jimmy Carter

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources