Medium Voltage Distribution Issue

[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..

Anyone ever see this before? Any ideas?

 

[RonUH]

Picture of pole top readings

 

[synchro]

Can you measure the current on the neutral wire going between poles? Or if you've already done so, what did you measure?

What kind of transformer is at the other end of the MV cables? If it's a wye-wye, then excessively unbalanced L-N loads (and maybe even one line open) could result in significant current on the neutral. Also, if it's a wye-wye with a tertiary winding, then perhaps the neutral was connected to the primary H0 and it's causing current to flow on the concentric neutrals when the line voltages are unbalanced.

If you can put a Rogowski coil around all of the MV cables then you can see whether there is a net current that might be coming through other circuit paths or cross connections.

Is this part of a main-tie-main arrangement?

 

[RonUH]

Thanks for your reply.

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.
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 am not familiar with a " Rogowski coil", Is this a special type of flexible CT ?

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.

 

[synchro]

I am not familiar with a " Rogowski coil", Is this a special type of flexible CT ?

A flexible CT is sometimes called a Rogowski coil because Mr. Rogowski invented this type of CT which doesn't need a magnetic core. But I didn't mean anything special by it, just a CT that is physically large enough to surround all of the conductors that are to be measured.

And so a suggestion is to make CT probe measurements around all 6 of the MV cables (as I understand it). Also, preferably make CT probe measurements around each pair of MV cables on the same phase, as well as around each individual MV cable of the pair. All of these measurement would be around the entire cable, including its concentric neutral.

 

[RonUH]

I do not have a CT that could go around all 6. Not even sure if my flexible CT is large enough for both of each phase, but i will attempt it today with my Fluke 435 II.
My understanding is that if I measure the entire cable including the Concentrics, the neutral current will subtract for the Amp reading on my meter.

 

[synchro]

I do not have a CT that could go around all 6. Not even sure if my flexible CT is large enough for both of each phase, but i will attempt it today with my Fluke 435 II.

If you can't get the flexible CT around each pair on the same phase, then I suggest putting a CT around each cable of the pair. One CT to channel A and the other to channel B. Then you can make a reading in Scope Phasor mode, which will give you the magnitude of each current and the phase angle between them. Then you can do a save screen to store the measurement. I suggest numbering the cables and keeping track of which measurements are associated with a given pair of cables.
Actually, doing this phasor measurement will give you more information than if you had a single CT around the pair of cables.
By the way, I haven't used that particular instrument and so I don't know its operational details.

My understanding is that if I measure the entire cable including the Concentrics, the neutral current will subtract for the Amp reading on my meter.

The CT around the cable will measure the vector sum of the currents on the inner conductor and the concentric outer conductor. Depending on the angle between the currents, the neutral current could make the magnitude of the measured current larger or smaller.
Normally the neutral currents should be noticeably smaller than the line currents if the system is reasonably balanced. The phase of this smaller neutral current is normally not very important. But since your neutral currents are so large, their relative phase may provide a clue about where these currents are coming from.

 

[RonUH]

The best i could do today is below.

 

[RonUH]

 

[synchro]

Are the CT probes and analyzer that provided the data below capable of measuring at DC (0 Hz), or is the DC content shown just an artifact that might not be meaningful?

Also, there is a significant level of second harmonic present, which indicates that the current waveform is not symmetric between plus and minus polarities of the current waveform. Such asymmetry is sometimes associated with DC content.
If DC is actually present, then that could possibly result in some amount of transformer saturation during parts of the voltage waveform.