Medium Voltage Distribution Issue 1

[RonUH]

I work at a large University in Texas. When the electrical deregulation happened here the utility built us our own 138kv/12,470kv substation. Ever since we have had issues with our concentric neutrals burning up on our pole tops and splices burn up in our manholes every few years.

Although my SEL relays only show about 25amps of neutral current, I have measured 200 amps on the pole top jumpers between the concentric neutrals on the cable to the main neutral cable. See attached pictures for examples of the very strange readings.

I recently walked the line and noticed there are still some interconnections between our circuit's neutrals and the utility company (Center Point) city circuits that all originally came from the utility substation that these circuits used to originate in. Could this cause circulating neutral current at these levels?

The SEL relay also does not show excessive harmonics.

These are our main feeders from our substation Delta/Wye 138/12 KV transformer to our main gear on campus which connects to a ring bus. Entire run is about 1.1 miles from Sub to Central Plant. Some underground, some overhead.

We have 3 circuits, 2 of which feed a Main tie Main gear. 1 of which normally only feeds our Central Plant. The 2 that go to the main tie main gear each break out into 8 sub feeders which power our campus buildings. The Sub feeder cables do not have concentric neutrals tape shield only.

I should also mention that our incoming Main Feeders have a Nexus 1500+ power quality meter and although they do not have neutral CT's installed the meters do not show any calculated neutral current. But if I put an amp probe on the concentric neutrals at the back of the gear i can read up to 200 amps.

Anyone ever see this before? Any ideas?

 

[RonUH]

I had trouble interpreting the phaser plots, it’s wasn’t clear to me which labs or was which.

Unfortunately, I am not very experienced with phasor diagram other than to check meter connections are correct. I used a Fluke 435 II on each phase and the pole ground wire for each set separately. The meter is 1kv and under so I could not use the voltage leads but it still shows up on the diagram.

I probably would check whether the utility had significant imbalance in their distribution circuits and if so, see if you can perform a test by getting a utility outage and measuring the neutral current at that time

I agree, but my leadership wants us to do our own investigation before approaching the Utility. Makes no sense to me but it is what it is. It is my belief that these interconnections coupled with the strange way they originally ran the circuits in one of the duct banks is the cause of the induced and circulating neutral currents. But how can I PROVE it?

 

[RonUH]

There is a lot of information in your posts and it would be really helpful if you could possibly insert the details onto a schematic which includes the neutral grounding locations and any ring arrangements. It might provide a better presentation to allow the problem to be assessed more thoroughly.

I will see what I can find or create as it is difficult to describe. When you say Ring arrangements, are you talking about downstream ring bus?

Like wcasey, I’m also having difficulty reconciling your phasor plots. The real and reactive power values seem reasonably balanced between phases. The phasor values are less clear however.
1. Are the "open triangle" arrowhead phasors displaying current magnitude and angle? They are close to 120 degrees apart but with some variability.
2. What are the "line" arrowhead phasors showing? – They are certainly not 120 degrees apart

Unfortunately the Fluke manual doesn't give specific information in regard to which arrows are which values. I am too inexperienced in interpreting phasor diagrams to answer. On a separate note, the Harmonics screen shot doesn't show one thing that was strange. The harmonics were pulsating like a heartbeat, approximately every second. I tried to get video, but the glare was too great.

Finally, the site is adjacent to rail tracks and although I cannot see traction power structures in the images, there does look like a line above the right most track. Is there any chance that there is traction power in the area? If so, it might be worth considering whether traction return currents could be carried in your neutrals.

Not that I am aware of. Not entirely sure what you mean by traction power. Just a normal switching yard with a small shack at one end.

 

[wcaseyharman]

I’m not sure your PQ meter is set up right. I noticed that the frequency is set to 50 HZ, but if you’re in Texas that’s incorrect. Having the fundamental frequency set wrong would pretty much invalidate all the phaser and harmonic results.
I work for a utility (normally in the generation department but do power quality work for distribution customers as well) and customers have approached us with issues with much less evidence than you have. The fact that the neutrals are combined and you have high neutral current without significant imbalance is more than enough evidence to get the utility involved in my opinion. It’s not 100% conclusive as there are possibly other explanations as suggested by others but I think you’d need the utility’s help in testing to resolve the issue.
My $0.02 anyway.

 

[RonUH]

When I first connected the meter, I had it set for Delta. When I corrected that I also checked the HZ. I don't understand why it shows as 50HZ on the screen shot.

 

[wcaseyharman]

Ok, weird. It’s been a bit since I set up a 435.
What would be useful is if you could correlate the utility voltage imbalance or neutral current with your neutral current if you find they have significant imbalance. Can you measure the current where your neutrals connects to the utility neutral? That may help determine the source.

 

[JJ_Roy]

I'm familiar with the Fluke 435. The open arrows are the voltage phasors. But if you had no voltage connection then they'll be somewhat close together (as shown). The frequency that is grayed out (50.000 Hz) on the left hand side just indicates that the analyzer can't determine a frequency from the voltage measurement, because they're not connected, so it defaults to 50Hz. The analyzer is set up for the correct frequency as indicated on the bottom where the time stamp is located.

The current phasors are the closed arrows (triangle heads).

My two cents.... You probably should take this one step at a time. Sit down with a system one-line, and determine how the neutrals are all connected on your system. If you're tied to the local utility somehow, not just at your substation, your neutrals could be a better return path than the utility neutrals. This is especially the case if they have significant imbalance resulting in neutral current. It also looks like you may have some imbalance as well based on the Fluke 435 screen shots with the phasors (screen shot top line: Red, Black, Blue, readings).

 

[JJ_Roy]

Just read your first post and it looks like there is still some neutral connections to the utility. I'd measure the neutral current at those locations and see what you have.

 

[wcaseyharman]

Super helpful, thank, that makes total sense on the phasers. It doesn’t look like it plots the neutral, not sure if that’s an option or not. If not, put a CT around A phase and the B phase CT around the neutral so we can see that phase relationship. Do this for all the neutrals of interest. If the problem is induced currents the phase should be aligned to the current in each cable. If the phase of all neutrals is the same then that would point to an external source like the utility.

 

[JJ_Roy]

The Fluke 435 will take 4 current inputs (3 phases + neutral) but I'm not sure if it will display the neutral phasor or not. I believe it will display the neutral current waveform with the other phase currents in scope mode though.

BTW Fluke has replaced the 435 with the 1777. I'm NOT a salesman for Fluke.... but I highly recommend it if you need to do PQ measurements. It is very user friendly and doesn't need much pre-configuration like the 435 does. The viewer software is really good too.

 

[EnzoAus]

Not entirely sure what you mean by traction power.

Traction power is that supplied for electric railways; either by overhead conductors or ground based rails. Can be AC or DC and can interact unfavourably with adjacent grid systems if not sufficiently electrically isolated. It may not be an issue here but I raised it simply because of the railway tracks in the photo.

Your harmonics graph shows THDi of 18.7%. I believe IEEE Std 519 is your relevant harmonics standard and the attached Table 2 provides Total Demand Distortion (TDD or THDi at 100% load) limits of up to 20% if the fault level is extremely high (Isc/Il > 1000). Lower limits on weaker networks. Neither the fault level or the maximum demand is stated above, but the 18.7% THDi seems quite high compared to the standard. If the utility has set harmonic limits as part of your installation's grid Connection Agreement, it might be worth checking these to see whether they are being exceeded indicating a harmonic issue.

Secondly, the even harmonics in your table appear very high. Table 2 of IEEE 519 limits even harmonics to only 25% of the odd harmonics. Even for an extremely high fault level, the 2nd harmonic current should be less than 3.75% (0.25*15%). Depending on your fault level, it could be lower. The graph above indicates a 2nd harmonic current of between about 15 and 25% of fundamental depending on the phase.

 

[wroggent]

I understand that utilities, when using single conductor cables for three phase circuits, often transpose the neutral or shield connections at equal intervals to cancel induced voltages and sheath or concentric neutral currents.
Have I been misled?

Depending on the distance between terminations and/or joints, what you mentioned is one technique that is sometimes employed to manage voltage or current on shields in 69kV+ transmission cables. Personally I haven't heard of or seen this done by US utilities on a distribution system. Honestly to do so might be dangerous for anyone working on the system because it would be so unusual.

Instead of ABC in one conduit for each parallel run they ran 2 A's and a B or 2 C's and a A.

Seems likely this is part of your problem.

the problem is that there is a 1/2 mile run of overhead where the CenterPoint city circuit does not have its own overhead neutral

Are you saying the blue overhead line doesn't have a neutral but the pink and teal do?

We have 3 circuits, 2 of which feed a Main tie Main gear.

Is the tie breaker operated closed or open?

 

[RonUH]

Seems likely this is part of your problem.

Absolutely, we have corrected one circuit (CGR3) and hope to correct 1 other in the next few months.

Are you saying the blue overhead line doesn't have a neutral but the pink and teal do?

The colored lines indicate overhead neutrals. It is difficult to describe, the circled numbers are our circuits (CGR1,CGR2,CGR3). The ones marked CP are CenterPoint city circuits. Originally all these circuits were fed from the same CenterPoint substation. 2 of our circuits have an overhead neutral connected to the respective Concentrics. CGR2-Pink and CGR3-Teal. The Blue is a CenterPoint city circuit neutral that also connects to our CGR1 in their sub at one end but the other end of CGR1 at the rail yard is connected to the CGR3 overhead neutral and also connected to a different CenterPoint city circuit.

Is the tie breaker operated closed or open?

All ties are Normally open. At the moment we are using a tie (CA1-2 in the pics below) in our central plant since CGR1 is still out of service.

 

[jghrist]

I absolutely agree with this point, the problem is that there is a 1/2 mile run of overhead where the CenterPoint city circuit does not have its own overhead neutral.

The colored lines indicate overhead neutrals. It is difficult to describe, the circled numbers are our circuits (CGR1,CGR2,CGR3). The ones marked CP are CenterPoint city circuits. Originally all these circuits were fed from the same CenterPoint substation. 2 of our circuits have an overhead neutral connected to the respective Concentrics. CGR2-Pink and CGR3-Teal. The Blue is a CenterPoint city circuit neutral that also connects to our CGR1 in their sub at one end but the other end of CGR1 at the rail yard is connected to the CGR3 overhead neutral and also connected to a different CenterPoint city circuit.

Is there a section of CenterPoint line with no neutral? If there is and there are connections on each side to the university system neutral, then the university neutral could be carrying unbalanced CenterPoint neutral current. Either get CenterPoint to install a neutral or disconnect all of your neutrals from their system. Disconnecting your neutrals may leave the CenterPoint system with no metallic path for unbalance current (load or fault) and may cause voltage problems on their system. Disconnecting your neutrals could be hazardous if the CenterPoint circuits are energized and carrying load.

 

[RonUH]

Secondly, the even harmonics in your table appear very high. Table 2 of IEEE 519 limits even harmonics to only 25% of the odd harmonics. Even for an extremely high fault level, the 2nd harmonic current should be less than 3.75% (0.25*15%). Depending on your fault level, it could be lower. The graph above indicates a 2nd harmonic current of between about 15 and 25% of fundamental depending on the phase.

My SEL relay doesn't show the same harmonics as the fluke meter. Could the harmonics I measured with the Fluke meter be picking up the harmonics from either the CenterPoint city circuit or possibly the induced circulating current?

 

[JJ_Roy]

I believe the Fluke 435 really wants voltage reference when making harmonic measurements (even harmonic current). You might be getting some odd readings if you didn't have the voltage connected. You also noted that the harmonics were "pulsing" as well when you were taking the readings. Additionally, a high 2nd harmonic is fairly odd so I don't think the fluke measurements are accurate.

I think you'd be having a lot more other issues if the current THD was truly at 18%. But I could be wrong as I usually am.

 

[EnzoAus]

RonUH,

Thanks for the additional drawings. I’ve taken the liberty of putting together a sketch which may or may not properly collate your information. Please feel free to tag it as incorrect or otherwise correct any mistakes as appropriate if you think they will assist discussion. Other queries that I have, which may be relevant are

1. Where on the sketch have the 200A neutral currents been measured?
2. Where on the sketch have the neutrals been burning? (Closer to uni might point to uni sourced currents whilst closer to CentrePoint s/s might point to currents from other sources)
3. Have the neutral failures been associated with faults; either utility logged or university logged?

Given JJRoys comments about the Fluke’s inability to measure harmonics without a voltage source I’m inclined to think that other sources of neutral current returning to the utility substation could be worth considering. I'm not a protection expert, but the attached link to a short summary of cable screening shows a "cross-bonded cable system" that might apply to your installation. It gives various reasons why the technique compromises between circulating currents and induced voltages in the screen.