Tek-Tips is the largest IT community on the Internet today!
Members share and learn making Tek-Tips Forums the best source of peer-reviewed technical information on the Internet!
-
Congratulations cowski on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!
stray current
- Thread starter atysh2024
- Start date
- Moderator
- #2
"..What would happen if the neutrals of secondary wye connected trasnformers were both connected to ground through a Neutral Ground Resistor but had their neutrals tied together on the line side of each ground resistor, or esentially at the neutral bushing of each transformer Could there be circultating currents that would result between this connection? What would cause these circulating currents? The secondaries are connected Delta loads. T1 and T2 Pri are NOT paralleled."
I have the following opinion for your consideration.
1. T1 and T2 can be of different kVA, Pri Sec voltage ratings, % Z, loading etc....
2. Each Trafo can be loaded with its own balance or unbalance L-N load, irrespective of the NGR. There is no current flowing though NGR. This has nothing to do with the other Trafo.
3. If a L-G GF from say T1 line will result to current flow from faulted Line to the Ground, to NGR to the T1 Neural only. No current flows to the T2 Neutral. T2 is NOT affected, even T1 and T2 Neutrals are connected.
Che Kuan Yau (Singapore)
I have the following opinion for your consideration.
1. T1 and T2 can be of different kVA, Pri Sec voltage ratings, % Z, loading etc....
2. Each Trafo can be loaded with its own balance or unbalance L-N load, irrespective of the NGR. There is no current flowing though NGR. This has nothing to do with the other Trafo.
3. If a L-G GF from say T1 line will result to current flow from faulted Line to the Ground, to NGR to the T1 Neural only. No current flows to the T2 Neutral. T2 is NOT affected, even T1 and T2 Neutrals are connected.
Che Kuan Yau (Singapore)
You will find a current in the transformer's neutral connection to the substation's earth mat. When measured by a tong tester, this current was up to 100A in power and substations. There will also be current in the tank-to-ground earthing connection. The maximum current that came to my notice was 800 A in a rectifier transformer in an aluminium smelter. When one of the two tank earthing leads was removed, the current reached 400 A. The reason for the death of people while stealing this earthing copper strip is due to the arc flash from breaking this current.
Typically on a Y connected secondary transformer winding, the neutral current should be equal to three times the zero sequence current on the feeders, so yes there will be current. But I would not call it stray current. It is the current to serve the unbalanced single phase loads on the distribution.
Opening the X0 lead will make the voltage rise on the X0 bushing, and what ever is attached to it. This voltage will be up to the voltage on the phases, depending on how unbalanced the load is.
This is the danger point, because we expect there to be some current on this.
On rectifier transformers, the current will be there because of the harmonics from the rectifier, as in many cases the transformer is part of the harmonic suppression.
The current between the transformer case, or core ground and the ground grid is actually stray current. And should be small.
Opening the X0 lead will make the voltage rise on the X0 bushing, and what ever is attached to it. This voltage will be up to the voltage on the phases, depending on how unbalanced the load is.
This is the danger point, because we expect there to be some current on this.
On rectifier transformers, the current will be there because of the harmonics from the rectifier, as in many cases the transformer is part of the harmonic suppression.
The current between the transformer case, or core ground and the ground grid is actually stray current. And should be small.
@ Mr prc (Electrical)8 Jul 24 16:22
"... that tank current was 800 A ! ".
What is that tank current 800 A doing ? Is it used for heating up the earthing conductor and to melt the NGR resistor grids? A Fire ! for sure.
Che Kuan Yau (Singapore)
"... that tank current was 800 A ! ".
What is that tank current 800 A doing ? Is it used for heating up the earthing conductor and to melt the NGR resistor grids? A Fire ! for sure.
Che Kuan Yau (Singapore)
- Status
Similar threads
- Locked
- Question
- Question