Electricity wants to go to ground?

[don_resqcapt19]

The "primary neutral" you are refering to is little more than a secondary neutral of the upstream system. A substation transformer is just that, a transformer.

That answer has nothing to do with my comment that the unbalanced loads on the secondary of a transformer have nothing to do with the neutral or lack there of on the primary side of that same transformer.

 

[Besoeker]

How can you supply anything other than a dedicated delta load without a neutral? Every three phase line we have has a primary neutral.

It is routine here in UK for transmission to be three phase three wire. Distribution at local level usually from a three wire delta to a four wire star (WYE). The LV side is 400V/230V, the 230V being line to the neutral star point which is earthed. Pretty much all UK residences are supplied with 230V single phase and all appliances are 230V.
Commercial and industrial mostly uses three phase. And many loads simply don't require a neutral like three phase motors and variable speed drives for example. All of which can be operated from the same supply. These are not what I would consider dedicated loads.

 

[Hv&Lv]

What does that have to do with the unbalanced current on the secondary side of a transformer that feeds a building?

Everything. Think of what you are calling primary as the secondary side of a larger system. After all, it is...

Think outside the box...

 

[don_resqcapt19]

Everything. Think of what you are calling primary as the secondary side of a larger system. After all, it is...

Think outside the box...

The primary and secondary are two separate systems. While using two of the 3 line conductors to supply the primary will unbalance the load on the primary supply system, there is no unbalanced current on the primary of that system. Unbalanced current only exists on systems where there is a grounded conductor that is being used as a circuit conductor. If you supply the transformer line to line, there is no unbalanced current on the line side of the transformer, there is just an unbalanced load.

 

[hurk27]

Everything. Think of what you are calling primary as the secondary side of a larger system. After all, it is...

Think outside the box...

I think what Don is trying to say is that the circuits from the sub station to the local pole mounted transformer that feeds the building can be a problem for stray voltages because these are wye circuits, most substations are fed delta and do not cause stray voltages because there is no MGN that shares a current path with Earth.

I agree with Don as to the problem of stray currents being caused by primary side neutrals, and this is a big problem and will get worse as many POCO's get away from local delta circuits feeding local service transformers.

Using the Earth as part of the path back to a substation lowers the cost by allowing a smaller neutral conductor to be used, but at the same time it is increasing the stray voltage problem when to much current on the MGN is being applied or when there is a break in the continuity of the MGN that causes Earth to be the only conductor.

As far as transformers go they are the start of a new source each time we cross one in a circuit, so any neutral current after a transformer only applies a line to line load ahead of the transformer.

 

[Hv&Lv]

I think what Don is trying to say is that the circuits from the sub station to the local pole mounted transformer that feeds the building can be a problem for stray voltages because these are wye circuits, most substations are fed delta and do not cause stray voltages because there is no MGN that shares a current path with Earth.

I agree with Don as to the problem of stray currents being caused by primary side neutrals, and this is a big problem and will get worse as many POCO's get away from local delta circuits feeding local service transformers.

Using the Earth as part of the path back to a substation lowers the cost by allowing a smaller neutral conductor to be used, but at the same time it is increasing the stray voltage problem when to much current on the MGN is being applied or when there is a break in the continuity of the MGN that causes Earth to be the only conductor.

As far as transformers go they are the start of a new source each time we cross one in a circuit, so any neutral current after a transformer only applies a line to line load ahead of the transformer.

A smaller neutral conductor can be used in either the NEC or the NESC. There are sizing requirements, and the neutral is used to carry imbalance currents. I agree that stray voltage can be a problem with the MGN configuration we use. (ladder ground, picture a ladder turned sideways.) BUT, the interconnection of every other device for safety sake and maintanience on the customer side can also contribute to stray voltage.
If the connection to neutral is lost at the customers meterbase, the currents will seek a return to source through any means necessary(interonnected phone, cable, water, etc.)
SWER systems are the only system I can think of that use earth to be the only conductor. I cannot find in the NESC where SWER systems are allowed. I am still looking.

POCO's use the neutral as a low Z path to source, not the earth. The earth becomes the return path only when this wire is stolen, broken, or connections are bad creating higher resistance.

 

[mivey]

POCO's use the neutral as a low Z path to source, not the earth. The earth becomes the return path only when this wire is stolen, broken, or connections are bad creating higher resistance.

The neutral and earth share the current on the MGN system.

 

[Hv&Lv]

The neutral and earth share the current on the MGN system.

Proportional to resistance...

 

[mivey]

Proportional to resistance...

You will find that in most it is about a 50/50 split or thereabouts.

 

[don_resqcapt19]

...

POCO's use the neutral as a low Z path to source, not the earth. The earth becomes the return path only when this wire is stolen, broken, or connections are bad creating higher resistance.

Some studies in dairy farm areas have shown that the utility undersizes the primary grounded conductors and as much as 30% of the grounded conductor current travels in the earth.

 

[don_resqcapt19]

Proportional to resistance...

That is correct, but the impedance of the path via the earth is very low on a multi grounded neutral system. The earth itself, given its huge cross sectional area, is a very good conductor. Most of the resistance on a earth ground return path is from the grounding electrode to earth connection. The multi grounded neutral system provides many grounding electrode to earth connections resulting in a low impedance and a substantial amount of grounded conductor current traveling via the earth.

 

[don_resqcapt19]

A smaller neutral conductor can be used in either the NEC or the NESC. There are sizing requirements, and the neutral is used to carry imbalance currents. I agree that stray voltage can be a problem with the MGN configuration we use. (ladder ground, picture a ladder turned sideways.) BUT, the interconnection of every other device for safety sake and maintanience on the customer side can also contribute to stray voltage. ...

Of course problems with broken conductors or poor quality connections on either side of the transformer can result in excessive neutral to earth voltages, but the most common source of neutral to earth voltage (stray voltage) is the normal and unavoidable voltage drop on the utility system grounded conductor. If the utility would feed all transformers line to line this voltage drop on the primary neutral would go away. Once that is done the only voltage drop that would be available to create neutral to earth voltage would be the voltage drop on the secondary grounded conductor. This will normally be a lot less voltage than the drop on a current carrying primary grounded conductor.

 

[realolman]

Here is a real good thread re: the impedance of the earth.

http://forums.mikeholt.com/showthread.php?t=116358&highlight=impedance+of+the+earth

 

[kwired]

Getting back to the OP questions.

I have always read that it wants to go back to the source to complete the circuit. Like back to the transformer etc.

I was talking to a guy at work, and he said that's not true because look at POCO's wires. There is no neutral, just three hots. So that got me thinking, if POCO's power is coming from generator's and one of there wires shorts, does it trip a breaker? Are there wires even protected by breakers or anything at all? Is POCO using the earth as a return path to complete the circuit? Thank you for your help.

If the utility grounds a conductor in a system then current will seek that path during a ground fault. If they actually have a grounded conductor run with the system conductors the majority of fault current would flow back to source on that conductor, but there will be current traveling on every path possible.

Same is true if the grounded conductor is a normal current carrying conductor of the system, except that current is flowing pretty much all the time instead of just during a ground fault.

Yes they do use overcurrent devices, some of them are one shot, some of them recycle a couple of times in case the fault is cleared to help eliminate outages, but if the fault is not cleared it will remain open.

The transformers are only different from what we see under 600 volt from a theory perspective in the applied voltage. A wye connected winding is still a wye connected winding and a delta is still a delta. All that is different is the numbers are higher in the voltage part of calculations.

 

[mbrooke]

I have always read that it wants to go back to the source to complete the circuit. Like back to the transformer etc.

I was talking to a guy at work, and he said that's not true because look at POCO's wires. There is no neutral, just three hots. So that got me thinking, if POCO's power is coming from generator's and one of there wires shorts, does it trip a breaker? Are there wires even protected by breakers or anything at all? Is POCO using the earth as a return path to complete the circuit? Thank you for your help.

By chance do you live in California? When a single phase transformer is installed it can be either connected phase to phase or phase to neutral. The neutral is usually below the the 3 phases. Same with 3 phase units, it can be other wye or delta. If all the transformers are phase to phase and delta connected the utility usually does not run a neutral since it is not needed. In this case the earth is used as the equipment grounding conductor for sensing faults. In a 3 wire system the substation transformer can either be wye solidly grounded, resistance earthed, peterson earthed, unearthed or the secondary can be Delta. In a 4 wire MGN it is solidly earthed wye. Most utlitys are 4 wire MGN.

In California to combat stray current and EMFs utilitys can not ground or wire there system in a manner that forces excessive current to flow across the earth. Thus 3 wire systems are the most common along with 5 wire systems that have 2 neutrals, one kept on insulators only for load imballance and a second one connected to ground rods only for fault conditions. (Think NEC standards)

 

[realolman]

.......systems that have 2 neutrals, one kept on insulators only for load imballance and a second one connected to ground rods only for fault conditions. (Think NEC standards)

how does that work?:?

 

[kwired]

how does that work?:?

Just like it does in all other NEC installations. Neutral is separate from EGC except for bonding jumper at/near source.

It would eliminate a lot of stray voltage issues, but also requires one more conductor run with every circuit. You would not be able to have neutral bonded to transformer tanks (some are designed that way I believe, others may have a remvable bonding jumper) or it will defeat the purpose, but equipment ground would bond the tanks instead.

Add: This is about the neutral on the primary system not the secondary system.

 

[mbrooke]

Just like it does in all other NEC installations. Neutral is separate from EGC except for bonding jumper at/near source.

It would eliminate a lot of stray voltage issues, but also requires one more conductor run with every circuit. You would not be able to have neutral bonded to transformer tanks (some are designed that way I believe, others may have a remvable bonding jumper) or it will defeat the purpose, but equipment ground would bond the tanks instead.

Add: This is about the neutral on the primary system not the secondary system.

Thats correct. More utlitys should do this. It would eliminate many hazards and reduce injurys to linemen by broken grounds too. But sadly utilitys are always stingy with money. And yes most utility transformers have both the primary and secondary neutrals bonded to the case EEEk, however they can be ordered other wise.

 

[mivey]

Thats correct. More utlitys should do this. It would eliminate many hazards and reduce injurys to linemen by broken grounds too. But sadly utilitys are always stingy with money. And yes most utility transformers have both the primary and secondary neutrals bonded to the case EEEk, however they can be ordered other wise.

Utilities are not "stingy with money", so to speak. It does not matter if the cost to serve doubles or triples as they are simply going to pass the cost along to the consumer. It is the consumer who complains about the costs and so the utility is forced to go with the cheaper supply method. Our state IOU is given a rate of return by the governing agency. If the governing agency said "don't use neutral grounded transformers", they would comply. They will get their cost back plus a margin.

It is not a "greedy utility" issue as much as it is a "how much safety do you want to pay for" issue.

 

[Hv&Lv]

Utilities are not "stingy with money", so to speak. It does not matter if the cost to serve doubles or triples as they are simply going to pass the cost along to the consumer. It is the consumer who complains about the costs and so the utility is forced to go with the cheaper supply method. Our state IOU is given a rate of return by the governing agency. If the governing agency said "don't use neutral grounded transformers", they would comply. They will get their cost back plus a margin.

It is not a "greedy utility" issue as much as it is a "how much safety do you want to pay for" issue.

I have to agree with this statement! Also some here fail to understand that utilities are regulated by the NESC. This standard says where and how a system is to be grounded. The standard is property of IEEE. Have the rules changed in the governing standard if it is such an issue. There are just as many causes of stray voltage caused by customer equipment issues. A faulty neutral connection in a meter base will cause problems with stray voltage, just as bad as a faulty MGS.