Medium Voltage Power Distribution Cables, TRXLPE vs EPR insulation 1

[nonsinusoidal]

We are currently in the process of replacing ancient types of Underground Distribution cables within our electric underground distribution system.I would like to know if some of you, Gentlemen, can provide feedback on which cable to select for the electric distribution system we have. The voltages for the electric distribution system are within the 5kV to 24kV Grounded Wye and some Delta system. I have two types of power cables which I have selected based on research and other means. The two types are:

A. TRXLPE,concentric Neutral,Jacketed Underground Power Distribution Cable.

B. EPR,underground Power Distribution Cables.

My questions are as follows:

1. Which one of those two cables should I select(which one is better overall)?

2. Which one is better when it comes to having long distribution circuits(Power losses and other facts which I might be missing)?

3. What type of insulation level (100, 133, and 173 percent level) should be selected when selecting the power cable?

2. If EPR cable is selected, do we need to incorporate a bare neutral along with the cables so lightning can be diverted away from the EPR insulation? There are regions, where lightning enjoys releasing its energy more severely than other areas. The power cables will be installed in conduit and direct buried to deliver the electrical power.

Please advice and than you in advance.

 

[dpc]

This is a hotly debated topic, especially amongst the cable sales folks.

FWIW, here's my short version:

EPR - historically much better insulation life - it does not "tree". More expensive. Most industrial facilities use only EPR.

XLPE - less expensive. Older cable had significant problems with insulation failures due to treeing. Newer material is apparently better.

EPR has higher dielectric losses then XLPE, but unless you some extremely long circuits (30-50 miles), the difference is not a major issue.

If you accept the TR-XLPE claims for expected cable life, it will be the lower cost option on any type of life-cycle cost analysis. But EPR remains very popular and is what we specify unless the client insists otherwise.

Let the flame wars begin.

"The perfect is the enemy of the good." -- Voltaire

 

[TurbineGen]

I'll second dpc's post, although I don'tknow about the dielectric loss of EPR vs cross linked polyethylene. I tend to prefer EPR despite is higher cost. It just doesn't fail as often. Of course, it hasn't been around quite as long as XLPE either.

As for what insulation level, it would depend on local codes and what your companie's "preferred practices" are. We use 133% EPR. Of course all of our cables are jacketed as well, even our older XLPE ones. I do not see where a bare neutral would make that much of a difference in EPR vs XLPE.

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[Zogzog]

Very good anf fair summary dpc, I hope Ben chimes in on this.

 

[jghrist]

One more pro for EPR, especially for large conductor. It is more flexible.

On the other hand, I have seen EPR where it was difficult to strip the semicon from the insulation without damage or leaving bits of semicon.

EPR is not a specific compound and varies from manufacturer to manufacturer. I'd be a bit more careful in selecting quality manufacturers when using EPR.

Overall, I would favor EPR for large conductors (over 500 kcmil) and favor TR-XLPE for smaller conductors.

The higher dielectric losses of EPR can become important at higher (>=34.5 kV) voltages.

 

[nonsinusoidal]

Can someone expand on the topic of why there are higher dielectric losses in EPR than TRXLPE at voltages above 34.5kV? Is the dielectric constant the main reason for the dielectric losses being of concern? What is the definition of dielectric losses?

 

[alehman]

See

http://books.google.com/books?id=Mh...ts=xEWyxXf3G_&sig=hULJ2mshYdYei0x4FSpGTXDdnaU

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[dpc]

The dielectric losses are higher at 15 kV as well, AFAIK. It can be visualized as a leakage resistance across the insulation.

For your purposes, it doesn't really matter why - it's just the way it is. It is an intrinsic function of the insulating material.

But you have to put it in context. The dielectric losses will be a small fraction of the resistive losses in the conductor itself, especially at higher loading factors. The difference between EPR and XLPE is an even smaller fraction.

It generally only becomes signficant for very long circuits that are lightly loaded much of the time. Then the dielectric losses become a larger fraction of the total losses, since the conductor losses are function of the square of the current. The dielectric losses are constant, independent of load and 24/7.

"The perfect is the enemy of the good." -- Voltaire