kwired
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Well lets just say spoken words may be more obvious than written words on this one.
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K8MHZ
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Earthworms may disagree.
kwired
Electron manager
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- NE Nebraska
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- EC
Where does the majority of that stray current flow? Certainly not in the top few inches of soil, that are most likely the least conductive part of the earth most of the time. That current is not injected into earth via a single ground rod either, it is put in a little at every rod.
An inaccurate generalisation.
At some voltages here the distribution is three wire delta. There is no neutral.
And just to compound the issue, not all distribution is overhead.
Then you have HVDC, 400kVdc in UK IIRC. DC and a neutral?
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I think that Sahib may have been referring to the U.S. MGN system.
There is still a wire neutral that will be carrying most of the current as long as it is intact.
In practice, most bad results occur in the immediate vicinity of substations. Some POCOs seem to have a problem understanding how to correct or mitigate those problems.
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Quite possibly so.
India, having been a British colony, has been influenced by British practices.
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I have read in a number of sources that about 1/3 of the primary grounded conductor current on our MGN system flows via the earth.
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I guess that where really long distances are involved the zero resistance in the long haul component overrides the high resistance at individual electrodes.
That would leave the current/ voltage drop at an individual electrode pretty small.
As a practical matter, are the touch / step potentials high enough to be a concern anywhere outside a substation?
Tapatalk!
That would leave the current/ voltage drop at an individual electrode pretty small.
As a practical matter, are the touch / step potentials high enough to be a concern anywhere outside a substation?
Tapatalk!
mbrooke
Batteries Included
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Doesn't your country use a primary delta connection on the transformer? Or is the LV neutral that is earthed?
mbrooke
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Even if the MV is delta, which is the norm for California and Europe, the LV neutral often ends up grounded all over the world even if the primary is delta.
HVDC comes in mono-polar and bipolar, whether to use the earth in that case is up for debate.
IMO, utlites that use multiply earth current carrying conductors are doing a huge disservice to the public. Both from the EMFs and the stray current hazards. Saves money in the short term? You bet. Long term benefits? NO way. Even if under normal conditions its a none issue I have personally seen multi grounded neutrals break open on utility poles without any protective devices tripping. Its not until the smoke or stray voltage gives it away.
mbrooke
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POCOs know little about how grounding works or at least what they want to admit about it. The MGN system was developed as a cheap method of increasing distribution capacity while reusing equipment. Often most distribution systems were 3 wire 2.4kv or 4.8kv Delta in 50s and earlier. Eventually pocos decided to add a 4th neutral wire and ground it repeatedly so it could be treated as a bare wire. The system could be raised to 4160 volts or 8300 volts in the case of 4.8 delta and the same transformers could be reused by connecting them phase to neutral. Capacity could be about doubled and voltage drop reduced by reusing the same equipment. And because the MGN has so many redundant water pipes and ground rods lightning related damages were reduced, especially on then fragile already decade old transformers. The practice continued as the insulation cost of transformers dropped and pocos decided to build new higher voltage lines or turn existing sub transmission lines into distribution lines. With 12 22 and 34 kv sub transmission lines a MGN was added, and single bushing 7200, 13200, and 19900 volt wound single bushing transformers could be used. Same went for brand new lines and the cost of an MGN was seen justifiable as compared to the extra cost from double busing transformers and extra cutouts. And if no MGN was run at all lightning arrestors away from the substation would have to be sized for a phase to phase voltage since a grounded down phase may cause a voltage rise before a breaker or fuse clears the fault.
It was a good idea back then when load was light, balanced and non harmonic rich. Today with the capacity of lines being pushed to the limit and with deteriorating equipment everywhere the disadvantages of the MGN is making itself known. This is something no poco wants to admit whos having a problem with stray voltage/current.
One major draw back of the MGN that I have seen (and I have personally seen this well more than once) is that downed conductors go unnoticed. Because the MGN and the ground are a return path for electrical loads, distribution breakers and re closers cant be set with low ground fault pick up values. That means a breaker or recloser GFI logic cant be set to trip the line at say 5 amps but rather hundreds of amps often 2/3 of the phase value. Often that means a downed conductor has to draw close to 600 to 800amps for some time for a trip to take place.
I had a phase come down further down the road a few years back from a fallen tree branch during an ice storm. The wire burned for litterally 2 and a half hours leaving a huge pothole in the street. Wasn't until poco got there and manually tripped the re closer to stop it. Same scenario plays out frequently when a pole comes down without hitting the MGN, especially on lower voltage systems.
Usually its about half and half, but it can vary depending on the size of the neutral, load balance, harmonics, soil resistance, water pipe grounding, ect. It can be as low s 10% or as high as 90%.
Same in the USA especially in rural systems.
Utilities are allowed and do use earth as a conductor intentionally. On MGM conductor it saves the utilities millions of dollars as the earth return has far lower impedance than any conductor could possible be. In Alaska and some extremely remote area a system called SWER (Single Wire Earth Return) is used. A single high voltage line is connected to a transformer high side and a ground rod(s) driven used for the return.
That would be incorrect. The MGN wire itself has a higher impedance than the earth return. The further away you get from the distribution sub, the majority of the return current is via Earth. The MGN is bonded to the pole butt ground every 4th pole. Once you get a mile or more away, the ground impedance is lower than the conductor.
Transmission on the other hand is Delta and no current flows in earth. HVDC transmission on the other hand does use Earth as a conductor.
mbrooke
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Copper, gas, and phone lines are even better than the ground rods. Its funny how telco guys are told bond to the MGN its the most important thing yet have no clue as to why. Every water bond is a free conductor as well as every gas line to a lesser extent.
kwired
Electron manager
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- NE Nebraska
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- EC
Around here I believe you will find there is an electrode at nearly every pole or other structure that has over 600 volt lines attached to it, and most of the time even if under 600 volts.
It has been said on this site before but I can't remember who to credit, that the earth has a very low impedance, but it is hard to make a low impedance connection to earth. But when a utility has so many points connected to earth you eventually get a good connection between all of them together, and an occasional loss of one of those connections may have an impact in the local vicinity of that electrode, but will not have much impact at all on the current flow through the rest of the system.
I believe you as you will see that in dense populated area.
It has been said on this site before but I can't remember who to credit, that the earth has a very low impedance, but it is hard to make a low impedance connection to earth. [/QUOTE]Well that is tue and part of the reason for all the confusion. The earth as a whole is an excellent conductor, but a resident or most businesses cannot get enough surface contact to use it. At low voltages under 600 volts is useless as a conductor.
An Electric utility on the other hand takes full advantage of earth for many reasons both economic and safety. They can so because they operate at very high voltages and build the generation plants and sub-stations with massive ground electrodes and test them yearly. I know because I was a sub-station engineer for the first ten years of my career. Transmission is all 3-phase Delta and really long line transmission is by-polar DC. Distribution is a mixed bag, Residential is Wye connected and thus a grounded circuit conductor must be used. POCO uses Earth as a parallel conductor to the MGN. Commercial can be either Delta or Wye, but residential by the nature of the beast is Wye. Rural is always Wye using MGN as it has to be.
Here is the USA we can belly ache all we want about our electrical architectural, but it cannot be changed. We made a huge mistake early on when we decided to use a unbalanced grounded system. It causes all kinds of problems but cannot be changed and we are stuck with it. It some ways at least on a house by house basis could be fixed somewhat by wiring everything 240, and that eliminates some of the problems on the user end, but will not address the distribution end. But where are you going to find all the gizmos in your house that can use 240?
mbrooke
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I disagree that it cant be changed. It can easily be done so over time. California has been fixing it without issue. Yes some places would be harder than others, but for example on trunk lines which are 3 phase to start with, all it would involve would be gradually phasing in double bushing transformers as the single bushing units fail. CSPs would probably be cheaper since you eliminate the extra cutouts. Single phase lines would be harder, however a neutral on an insulator could gradually be phased in along with double bushing units. Rural lines where customers are between miles would be last and the most hardest to change over, however, fortunately those lines tend to be less of an issue as appose to densely populated areas. Under ground URD direct burial cable installs would be changed over as cables are replaced, which to start with usually have a 40 year life.
Homes would actually be the easiest. Best solution would be a 4th half sized wire which would be an insulated neutral all the way up to the XO. This solution can take place even if the transformers a single bushing and an MGN is used. It will of course not fix the MV stray voltage alone but will fix the LV issue.
Of course if we took the straight 240 option (my favorite for sack of ease of wiring and reusing what is there) it would not be to hard since 240 volt appliances can easily be made, they are all over the world as is.
Of interesting note, if we did convert all 120/240 and 120/208Y services to straight 240 we can easily get away with using an MGN and grounded Y primaries as long as those services are fed from a 3 phase bank. Similar to what is done with transmission lines all connected via grounded Y grounded Y transformers but since the transformers feeding the MV distribution system are usually Delta primary the load neutral current doesn't transfer over to the transmission system.
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At least one virtue of the MGN SYSTEM, IMO: The resistance of a ground rod becomes very small, if not zero, once it is connected to the MGN. So if there is a phase to gound fault in an equipment connected with it, there is no dangerous touch potential for the duration the concerned OCPD takes to operate to clear the fault.
Don't bet on that. The rapid rise and fall times of a fault represent a very high frequency for which there is no such thing as a low impedance.
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