Stray voltage problems with a medium voltage backbone. 1

[CatElectric]

First here is some back ground. 3-phase 4160v system. Each phase feeds 4 2400/120-240 transformers.
The original system had a direct burried conductor with a bare concentric neutral (no jacket). It was a 25 year old system.
The system was upgraded with new 1/0 Al tape shielded cable in conduit. (no concentric neutral). This is per an engineers drawing. There is a #6 copper that was pulled for a ground, but is now acting as the neutral.

The #6 is landed on the H0 of the transformer, which is part of the transformer enclosure. The enclosure is grounded with a ground rod, and there is a bond jumper from the enclosure to the XO. There are multiple secondarys running out to trailer pedistals, which also have a bond jumper from neutral to ground.

This is a master meter trailer park.
After the cable was replaced the park started having strange stray voltage problems. People getting zapped from there trailers, Cable/tv lines burning into. Arcing from cable tv to trailer skins. There was a measured voltage of up to 30volts from the metal of the trailer to the dirt.

At each transformer two more ground rods were diven. One connected to the can on the primary side and the other to the can on the secondary side.

Most of the stray voltage is now under control. But there is still 4volts showing from the ground to the neutral. This voltage still shows up on a pedistal with the jumper removed, and the transformer feeding that section turned off. Primary still live running to pot switch in transformer and out to next one in line.

The bond jumper in the transformers is showing 40 to 60 amps.

I hope this is enough info to help, Ive been scratching my head big time. I think the cause is the #6 neutral causing a high impedance path back to the SES. This causing the voltage to seek lowest imedance path forcing it to ground/ or cable tv system which is grounded to the hilt.

If anyone has any idea to try ? Let me know

 

[DanDel]

You say the system is '3-phase 4160v', and 'Each phase feeds 4 2400/120-240 transformers'. How exactly does each phase feed 4 transformers? How are all the transformers wired?
You then say that the '#6 is landed on the H0 of the transformer'. Which transformer is this?
Is this H0 on a three-phase transformer on the 4160V system, and do you have a primary wye-connected, 4-wire system? Is there no ground from the primary feed, just a neutral?

 

[rbulsara]

I think you know the answer. Install a properly sized Neutral conductor and improve grounding. #6 ground is by no means sufficient for any MV system grounding, never mind for a neutral duty.

Also the grounding may not be effective. Install a ground ring around the xfmr and drive say four rods.

My further comments will raise a few eye browses, but until you install a fully sized neutral back to MV source, you will do well to keep the Ho and Xo electrically isolated. Consider only ONE of the following:

Option 1: Isolate Ho from the transformer casing, if possible. Ground Ho to a separate grounding electrodes not electrically connected to the LV side ground or the xfmr casing. Keep the bond between Xo and the casing and ground it, to separate grounding electrodes.

OR

Option 2: If Ho is inherently bonded to the casing, then isolate Xo from the transformer casing, if possible. Ground Xo to a separate grounding electrodes not electrically connected to the HV side ground or the xfmr casing. Keep the bond between Ho and the casing and ground it, to separate grounding electrodes.

Isolating Ho and Xo will eliminate HV side neutral current trying to flow through the secondary side neutral and ground conductors. (Although connected through earth, the earth resistance wont permit flow of HV side N currents to go through LV side N/G).

Keeping the secondary side Xo grounding isolated is a common practice by some utilities in India that I have worked with.

I am curious also whether the Ho of all the xfmrs are connected by a common conductor? At least each bank of 3 transformers (on different phases) should be, forming a Y connection. This is a whole separate issue.

 

[CatElectric]

Dan, Each phase is a radial that daisy chains to 4 single phase 2400/120-240 transformers.

The transformer set up is the same at all twelve of the transformers, 4 transformers per leg.

 

[CatElectric]

rbul, Thanks I know what should be done, but I am just a middle man trying to make better someone elses mess.

We have discussed isolating the HO from the frame and providing a seperate ground.
My question with this is what could be some of the down falls. IE if the #6 should fail, potential safety issues.

Real question do you think the #6 could cause the problems we are seeing. Current measured on the #6 is around 40 amps.

 

[busbar]

One primer is ANSI/IEEE C62.92.4-1991 …Application of Neutral Grounding in Electrical Utility Systems, Part IV—Distribution. It sounds like what is referred to is a multigrounded-neutral system. §4.4 and Annex B may be applicable.

Although primarily intended to address agricultural concerns, there is the somewhat pedestrian

http://www.strayvoltage.org

 

[CatElectric]

rbul, Each radial (phase) has a seperate #6 daisy chaining to each of the 4 transformers, all three #6's are landed on the neutral bus in the SES.

 

[busbar]

The grounding-electrode system at the 4160V source may bear close inspection and testing. “…#6 copper that was pulled for a ground, but is now acting as the neutral.” implies that the bare #6 was not originally intended to be a neutral for the 4160Y/2400V system.

 

[jbartos]

Suggestion: Normally, the 240/120 single phase 3wire systems, e.g. overhead transformers, are grounded at one point that ties the ground conductor to ground rods and neutral. If there are more of those transformers and their secondaries are not in parallel, then the single grounding point still applies for each transformer. The transformer equipment ground may be connected together and connected to one point at which the neutrals are grounded.
Visit

http://ww1.oppd.com/lib/meter/pdf/CH6.pdf

http://www.srpnet.com/power/service/pdf/2.pdf

http://www.srpnet.com/power/service/pdf/8.pdf

(for grounding and bonding)

http://www.srpnet.com/power/service/

etc. for more info

 

[rbulsara]

Cat:

#6 is part of the problem. Its normally rated for 60A but its long length (you did not mention the length) may be a problem. Regardless of that, you are providing parallel path throuh your LV system for the return current which you need to eliminate.

To your other piece of information:

You should be daisy chaining the Ho of the three transformers on different phases, creating (4) Y systems and at the mobile park end (load end) combine all N before going to the source. This will further reducce the amount of the current flowing through, as return of 3 phases will cancel out.

They way you have it now adds up return current of foru transforemrs in each #6 wire. In fact if they are all more or less equal, and if you combine them at the park end, you will alomost eliminate current going back to source. (Balanced 3 phase system).

You still need to isoltate Ho. (its not different than having a delta primary which is always isolated)

 

[DanDel]

CatElectric, my understanding of the system(please correct me if I'm wrong) is that you have 12 single-phase, 2400V x 240/120V transformers, all at different locations, 4 transformers on each leg of a 4160/2400V 3-phase, 4-wire system. The phase conductor(#1/0AWG) and the neutral(#6AWG) are paralleled from the first of each group of 4 transformers to the last on each phase. The X0 is grounded at each transformer.

The H0 is part of each transformer enclosure, so it is also grounded, and cannot be ungrounded, correct?

Grounding and bonding of H0 and X0 together at the transformer is common practice, similar to pole-mounted transformers for residential systems.

I believe the problem is in the flow of current through the ground between the primary H0 connections of each set of transformers on each phase, partially using the secondary transformer neutral. Remember, there is a potential between the neutrals on each phase of the primary, and current flow between them to equalize it. Part of the 40 to 60A in the X0 bonding jumper is primary current flowing to the grounded secondary neutral. This raises the potential of the secondary neutral to some value above that of the ground, therefore you have the problems mentioned. A good ground connection at each pedestal should have alleviated this problem. Are there any ground rods at these pedestal sites, or just bonding?

Installation of more ground rods at the transformers helped because this lowered the impedance to ground at this point, lowering the potential difference. Installation of ground rods at each pedestal will help more by lowering the potential difference at each load, where the problems are.

I would check the ground connections at each pedestal and test the ground rods(if there are any) to earth there.

Changing out the primary cable to the correct configuration(with concentric neutral and separate ground wire) is also recommended.

 

[jghrist]

Each trailer should have a grounding electrode at its service. The metal of the building should be bonded to the grounding electrode. This should prevent any appreciable voltage between the building and earth, even if some of the primary return current is flowing through the secondary neutral.

If you get 30 volts between the trailer and earth under non-fault conditions, you should consider a ground loop around each trailer. The voltage during fault conditions could be dangerous.

 

[peebee]

It's not quite clear to me why the primary side would be bonded to neutral or ground.

If you let the primary float, and you have proper grounding and neutral-ground bonding on the secondary, the stray voltages should go away.

Am I missing something?

 

[CatElectric]

Peebee, Most of the new single phase transformers (50-75 Kva) padmounts the neutral (HO) is internally bonded to the can. There is always a bond from the can to the XO. So the concentric neutral, and ground have to bonded together in that circumstance.

The transformers in this situation have the HO lug factory bolted to the frame of the transformer.

We are discussing "rbul" suggestions of isolating either the HO or the XO.

I know that the pedistal grounding is Insufficient. Is seems that the original system of 20-30 years ago used the water pipes and possibly a ufer of some sort for grounding. Many of the pedistals have been replaced by someone over the years and donot seem to have sufficient grounding. IE 2 rods 6' apart.

I am wondering If the old cable, which had direct buried concentric neutral, was acting as a giant ground grid. now that it is gone and everything is in pvc all of these other issues arose, despite the #6 neutral.

Rbul if we do isolate the HO or XO, Where would you recomend we land the tape shield of the cable. Right now it is probably acting as a parrellel path for the neutral current.

Thanks again for any help given.

 

[jbartos]

Suggestion: Visit

http://www.hq.usace.army.mil/cemp/e/Et/transfor.pdf

etc. for more info
The wye primary neutral grounded - wye secondary neutral grounded will propagate the zero sequence current. The network transformers use these connections under an assumption that the zero sequence current will not be high. This implies a high power supply voltage and current symmetries in that part of the power distribution.

 

[alehman]

I second jghrist's comments. It is very important that the frame of the trailer be bonded to the grounding system at the pedestal. If it is properly bonded and you still see voltage between the trailer and earth, your pedestal grounding electrode system is inadequate. You must correct that situation. Verify the N-G bonds really exist at each pedestal, if you haven't.

As stated by others, the #6 neutral you describe is totally inadequate. Also the share of neutral current in the cable shield may cause heating problems for the cable. I think your conclusion about the old direct burried CN cable is correct. I don't like the idea of isolating X0 and H0, even if it can be done.

 

[busbar]

Sounds like this a recognized problem. Excerpt from ANSI/IEEE C62.92.4-1991 “On multigrounded neutral systems, it is common practice to interconnect the primary and secondary neutrals and to use a single ground for these neutrals as well as for the transformer tank and surge protection… This technique effectively parallels all the primary grounds with the secondary grounds and provides lower grounding resistance for both the primary and secondary systems as well as for the transformer surge protection. A disadvantage to this practice is the occasional occurrence of abnormally high neutral-to-earth voltage (stray voltages) on the secondary system emanating from the primary neutral…”

“Stray voltage is an undesirable byproduct of the increased use and complexity of electrical equipment. When a stray voltage problem exists, it often has a multiplicity of causes. For this reason, it is a time-consuming and tedious process to track down and eliminate, often involving the power supplier, the local electrician, and various equipment manufacturers.”

The text discusses dedicated-purpose spark-gap and surge-arrester isolators for primary and secondary neutrals, but seem more applicable to overhead distribution.

 

[jbartos]

Suggestion: The stray voltages are caused by the asymmetry of the power distribution system variables, i.e. voltage and current, which may be caused by asymmetrical loading or faults. The zero sequence current and zero sequence voltage are causing higher voltages in the wye neutral connections. To avoid these problems, the zero current path is often interrupted by the delta winding connections.