high voltage utility lines

[Cleveland Apprentice]

I had a discussion with a co-worker the other day regarding high voltage utility lines. I understand that high voltage utility lines are 3 phase AC. If the utility can use the earth as the neutral conductor, then why do utility companies run a neutral on top of towers with the 3 phases? Wouldn't this be considered redundant and an unnecessary added expense of stringing an extra line miles and miles long? Just looking for other's opinions. Thanks.

 

[mivey]

I had a discussion with a co-worker the other day regarding high voltage utility lines. I understand that high voltage utility lines are 3 phase AC. If the utility can use the earth as the neutral conductor, then why do utility companies run a neutral on top of towers with the 3 phases? Wouldn't this be considered redundant and an unnecessary added expense of stringing an extra line miles and miles long? Just looking for other's opinions. Thanks.

It is for lightning and ground faults. The transmission is balanced so we don't really use it for a neutral like we do on distribution since its load current is so minor.

On multi-grounded neutral distribution, the earth does act as a return for parts of the line but getting to the earth is the problem. Since we can't put a huge low-resistance earth connection at each point, the connection to earth is paralleled over many poles and it takes some distance for the current to split into the earth (about 50% of it at mid-feeder). The neutral conductor is more reliable for current. Some distribution does not use the neutral for load (loads are all phase-phase instead of some phase-neutral) and most of the current flows on the phase conductors. Even without intentional connections to earth we still get capacitive coupling.

 

[kwired]

Using the earth for a return is going to create some potential problems at times, as the exact current path is not obvious and may be pretty wide compared to channeling same current through a specific conductor.

Disregard that for a moment and think about how much impact just 3-5, or even up to 10 ohms of resistance will have on say a 7200 volt circuit Vs a 120 volt circuit and you will see why it does work in the places where it is done.

 

[Cleveland Apprentice]

Thanks for the replies.

So basically a neutral is ran with the utility lines because the neutral conductor has less impedance than earth besides using for lightening protection and clearing ground faults? If a neutral is not ran with utility lines then how would the utility clear ground fault between phase to ground?

 

[Cleveland Apprentice]

So 10 ohms of load on a 7200 volt line will have a return current of 720 amps through earth if earth is used?

Disregard that for a moment and think about how much impact just 3-5, or even up to 10 ohms of resistance will have on say a 7200 volt circuit Vs a 120 volt circuit and you will see why it does work in the places where it is done.[/QUOTE]

 

[gar]

130803-1922 EDT

Cleveland Apprentice:

At my home the primary lines on the utility pole are three phase, no neutral, and no ground, just three wires.

This spring from a storm one of the primary wires was broken by a branch and fell to the ground. There was a lot of arcing. This implies the three phase source at the substation was probably a wye.

I was supplied by that phase wire and one of the others. The arcing only caused minor voltage variations at my panel. Obviously I was closer to the substation than the arcing point. Rather than cut the wire up near the insulator to eliminate the arc the power company broke the line somewhere between the substation and me. Now my voltage fluctuated between around 20 V and 90 V, and I pulled my main fuse. I know the cause, Can you figure out why?

.

 

[Cleveland Apprentice]

I am not 100% sure. Is it because the primary side of xfmr requires 2 different phases or a phase and a neutral to transform let's say 7200 v to 240 v. You lost one of the two phases on the primary side? Is this correct?

 

[gar]

130803-2333 EDT

I will give you some more background in the morning.

.

 

[domnic]

high voltage

high voltage

A NEUTRAL only comes from a transformer.

 

[eHunter]

A NEUTRAL only comes from a transformer.

Or a generator/alternator or a power supply.

 

[templdl]

If the earth were to be used for a neutral wouldn't there be a concern for a step voltage? Since the earth is really not a good conductor for electric current just the mere fact of walking on the ground could subject a person to a lethal shock, the longer the stride, the higher the voltage. As I recall this can be a very serious concern for farm animals.

 

[kwired]

So 10 ohms of load on a 7200 volt line will have a return current of 720 amps through earth if earth is used?

True, but not exactly what I was getting at.


Say you had a 25KVA transformer fed with 7200 volts, for simplicity lets disregard power factor and efficiency for a moment. At full load that transformer draws about 3.47 amps of primary current, from this we can calculate it has an impedance of about 2074 ohms. If you place 10 ohms of earth resistance in series because you used the earth as the return conductor you only change the total impedance of the circuit to 2084 ohms and will have a current flow of 3.45 amps, voltage drop across the transformer is 7165.5 volts (still nearly 7200) and little performance change is noticeable.

Put the same 10 ohms of resistance in the secondary side and you have a very noticeable problem. Lets say for some reason we have half the 25KVA capacity all on one secondary line to neutral for some reason. That means 12,500 VA @ 120 volts which draws about 104 amps. 120 volts / 104 amps means we have 1.15 ohms resistance in this circuit. Now if we try to use the earth with 10 ohms of resistance as part of the neutral conductor we will see the total current drop to about 10.7 amps and the original 120 volt load will only see 12.3 volts across it.

 

[gar]

130804-0822 EDT

Cleveland Apprentice:

First, definitions. In an electrical distribution circuit "upstream" means toward the power source, and "downstream" means away from the power source.

My location is possibly 10 customers downstream from the substation, and there are possibly 20 customers further downstream after me. Somewhere at or between the substation and my location there are parallel circuits. The single downed wire was about 250 ft downstream from my transformer. The distribution to me is like an ungrounded delta, except at the substation this is supplied by a wye transformer. Thus, each of the primary phase wires has the same voltage magnitude to earth, and can not be moved much from that relationship.

The two primary wires to my transformer I shall call A and B. The various customers on this this three phase system will be somewhat uniformly distributed between the three phases. Thus, some are A to B like me, and others are B to C and A to C. This is important to the analysis.

The arcing short to earth was a high impedance compared to the source impedance of the substation transformer, and the wire from the substation to the point of arcing. This is identified by my oberservation that when arcing was present that my voltage did not drop much.

Assume the arcing is from the phase A wire. Downstream from my location there is no customer connection to my wire A because the wire is broken. Upstream there are some customers connected between A and B or C.

When DTE opened wire A upstream from my location, at the substation or some intermediate point (most likely), then I had large voltage flucuations. Some of the 10 or more upstream neighbors, and I are connected to primary wire A with no connection to anything except some of my neighbors and me. What is the cause of the voltage I saw and the large swings in value?

.

 

[Cleveland Apprentice]

I don't know, sorry. This is a hard concept for me. It's probably obvious and I'm not picking up on it.






What is the cause of the voltage I saw and the large swings in value?

 

[gar]

130804-1931 EDT

Cleveland Apprentice:

Don't give up. I don't just want to give you an answer. I want to try to help your thinking process along.

All the single phase customers whose transformers were connected between wires B and C saw no interruption in their service because these lines were intact. All the customers connected between A and B or A and C saw a disruption. Everyone downstream from the open in line A closest to the substation had problems.

Whatever power customers on AB or AC were actually getting was power derived from lines B to C. Draw the circuit on paper, and what does the load look like?

.

 

[Cleveland Apprentice]

Thanks gar! I think I understand it. So "A" line is getting fed through the coil from other phases?

 

[gar]

130805-1216 EDT

Cleveland Apprentice:

Yes you are beginning to see what is happening.

From the substation to the first break in wire A a 3 phase system exists. Downstream after the break wires B and C constitute a single phase source.

There are transformers and loads on the transformers from all locations that are connected from A to C and A to B. All loads between A and C are in parallel with each other, same for A and B. But loads A to C and A to B are not in parallel, but these two groups are in series when the energy supply to A is nonexistent, and you can describe wire A as floating with its potential determined by the impedances connected to it.

Suppose I replace all those loads with resistors such that R(A-C) = R(A-B), then what is the voltage drop across each of those two resistors?

.

 

[mbrooke]

130804-0822 EDT

Cleveland Apprentice:

First, definitions. In an electrical distribution circuit "upstream" means toward the power source, and "downstream" means away from the power source.

My location is possibly 10 customers downstream from the substation, and there are possibly 20 customers further downstream after me. Somewhere at or between the substation and my location there are parallel circuits. The single downed wire was about 250 ft downstream from my transformer. The distribution to me is like an ungrounded delta, except at the substation this is supplied by a wye transformer. Thus, each of the primary phase wires has the same voltage magnitude to earth, and can not be moved much from that relationship.

The two primary wires to my transformer I shall call A and B. The various customers on this this three phase system will be somewhat uniformly distributed between the three phases. Thus, some are A to B like me, and others are B to C and A to C. This is important to the analysis.

The arcing short to earth was a high impedance compared to the source impedance of the substation transformer, and the wire from the substation to the point of arcing. This is identified by my oberservation that when arcing was present that my voltage did not drop much.

Assume the arcing is from the phase A wire. Downstream from my location there is no customer connection to my wire A because the wire is broken. Upstream there are some customers connected between A and B or C.

When DTE opened wire A upstream from my location, at the substation or some intermediate point (most likely), then I had large voltage flucuations. Some of the 10 or more upstream neighbors, and I are connected to primary wire A with no connection to anything except some of my neighbors and me. What is the cause of the voltage I saw and the large swings in value?

.

Back feed via transformers across the unfaulted phase and the open one. BTW, your substation secondary may or may not be grounded wye, cross faults such as A phase on one circuit and a fault on B phase on another will produce very similar conditions to a solidly grounded system.


One a side not, one area around me has a 4.8kv ungrounded Delta system no ground or neutral. One night after a serve snow storm there were downed lines and broken phases all over the system. The sight was something to see, half of the 8 circuits and dozens of cutouts never opened from the high impedance ground fault conditions. One store I went in to shop was fed from a circuit with the C phase down. 1/3 of the lights were full brightness and normal, the other 2/3 were flickering the exact way a downed wire would, varying in intensity, I latter calculated an open phase in Delta gives 50% the normal voltage. The 3 phase motors in the area were trashed in the process. One line had A&B phase crossed, but only one cutout blew. Hence 120 120 0 volts with the 2 120s being in phase, smoked the 3 phase motor good. Interesting stuff to say the least took place. BTW, because the storm was so large and neighboring towns had similar outcomes it took a few days to fix the whole mess.

 

[fmtjfw]

In the middle of nowhere in Canada

In the middle of nowhere in Canada

In areas of sparse population in Canada, they use a single primary wire and an earth return for the other primary conductor. Consider a primary voltage of 13kv (wild guess) and a 10kw load, the current in the primary would be 10/13 or less than an amp. Now with an earth ground resistance of 20 ohms, the step potential voltage drop would be less than 20v. This saves them half the cost of primary conductor.

 

[kwired]

In areas of sparse population in Canada, they use a single primary wire and an earth return for the other primary conductor. Consider a primary voltage of 13kv (wild guess) and a 10kw load, the current in the primary would be 10/13 or less than an amp. Now with an earth ground resistance of 20 ohms, the step potential voltage drop would be less than 20v. This saves them half the cost of primary conductor.

Step potential depends on how long the step is. 20 volts around dairy cows is way too high, in fact it is more than a production issue at that level, it is likely dead cows everywhere at that level.

If same system were used near a marina, or even where private docks with power exist, there would likely be dead swimmers found around the docks quite frequently. I don't care how much primary conductor it saves it is a poor design IMO.