high voltage cables

[sokkerdude]

We have just installed the conduit system for the primary to a 750 KVA pad mount transformer. It was 3- 4 inch PVC with rigid steel long sweep 90's. The utility company has pulled 1 - 25 KV cable in each conduit. The primary voltage is 22 KV. My question is that not the same as us putting only one phase in a conduit and with rigid steel 90 ? If it is, will the primary wires be subject to heating or is there something about 25 KV wire that would keep it from heating. Thank you

 

[don_resqcapt19]

The same problems occur no matter what the voltage. The magnitude of the problme is a function of current.
Don
[FONT=&quot][/FONT]

 

[kingpb]

Seems a little overkill seeing how the current on the high voltage side of the transformer is only 20A. The fact that the smallest conductor size for this voltage class is usually a #6AWG or larger, I doubt there is going to be much worry of overheating.

 

[jtester]

With PVC as a conduit, you will have much less heat with one phase wire in a conduit than with 3. There are numerous reasons to run all circuit conductors together, including decreasing circuit impedance, ease in identifying the conductor use, etc. Utilities that run each phase wire in a separate conduit usually do that for short ug laterals where impedance is insignificant.

Additionally with ferrous conduits like RGS or emt, will be heat generated by the conduit due to the magnetic field surrounding a single phase wire that isn't there with a complete set of circuit conductors. Obviously pvc won't present that problem.

The installation you observed is relatively common, and shouldn't present a problem.

Jim T

 

[don_resqcapt19]

I understand that the Candian Electrical code does not considder this type of installation to be a problem unless the current is 200 amps or greater.(I could be wrong on this statement...just something that I think that I recall from an old thread in one of the forums) Can anyone comment on this from an technical standpoint as to the amout of heat expected to be created with an isolated phase installation in ferrous raceways.
Don

 

[RayS]

I'd say the 90's will act as chokes during a fault, as well as warm up during use. The utility may not be concerned due to the small magnitude of the problem as posted earlier.

Another factor is the cable shielding, but I don't think that is going to affect the field.

 

[ggdennfru]

Some Utility companies keep a minimum wire size in stock which they will use instead of ordering small cables.

 

[ramsy]

The utility company has pulled 1 - 25 KV cable in each conduit. The primary voltage is 22 KV. ..is there something about 25 KV wire that would keep it from heating.

Did you watch the utility guys terminate that cable?

Once helped a utility install a pad mounted xfmr, and was amazed by the concentric cabling. The outermost concentric ring (grounding conductors), in the single line-side cables, are tailored for emf canceling, but the splicing process demands a custom kit that MD's could mistake for a circumcision with the rubber booties, dielectric glues, & crimping.

So, line-side emf should not be an issue with concentric cables, but using metalic conduit underground is also strange, since irrigation or alkaline soils can corrod rigid into dust within a few months.

 

[don_resqcapt19]

Roger,

The outermost concentric ring (grounding conductors), in the single line-side cables, are tailored for emf canceling,

How does that change the magnetic field that causes the problems with isolated phase installation in ferrous raceways?
Don

 

[jtester]

The outer concentric cable around the high voltage cable is probably the concentric neutral. It will only cancel the magnetic field if the run is single phase. The magnetic field will be there in a 3 phase circuit.

Don, your question about how much heat is a good one. It will be dependent on the current flow. I've seen paint burned off enclosures and insulation melted when 600 volt cables were paralleled with all A in one conduit, B in a second, etc. Clearly we are not dealing with that magnitude of current here, but there are plenty of installations where that current exists.

I don't know how to calculate the heat in the conduit as a function of conductor current flow, lets get Rattus to work on it.

Jim T

 

[ramsy]

The outer concentric cable around the high voltage cable is probably the concentric neutral. It will only cancel the magnetic field if the run is single phase. The magnetic field will be there in a 3 phase circuit.

Jim describes whats also known as bipolar transmission with concentric cables.

Maximum neutralisation of the electromagnetic field is achieved by a concentric cable, in which an outer hollow conductor encases an inner conductor, separated from it by high-voltage insulation. The outer conductor is also encased in both insulation and metal shields. The coincidence of the radial magnetic lines of force results in the absolute neutralisation of the forward and return fields. Offshore wind ..a New Powerhouse for Europe

A 3-PH implement of Superconducting Triax HTS cable also appears to optimize concentric cable for EMF canceling between phases.

 

[ramsy]

How does that change the magnetic field that causes the problems with isolated phase installation in ferrous raceways?

In the Power Quality forum, about two weeks ago, we discussed inductive heating and established ferrous conduit or enclosures absorb all EMF. So, without being a CCC itself, the outer conductor ring of concentric cables would probably need to be ferrous to act on EMF like ferrous conduits.

The last part of this GENERATING AND TRANSMISSION PROJECT describes it as follows:

4.13.5.2 Substation Power Feed
Electric Field. Since the power conductors from the generator transformer to the substation will be installed within conduit, the cables will not produce an electric field external to the concentric cable shield and metallic sheath.

 

[don_resqcapt19]

Roger,
In both of those cases, the only way the magnetic fields can cancel is if all of the conductors of the circuit are run togehter. If the phases are not in the same raceway, there is nothing that can cancle the magnetic field.
Don

 

[ramsy]

Thats right, no interference pattern exists, EMF is not canceled. As it is well documented and referenced above, EMF can also be absorbed by outer shields so that no external field exists beyond the ferrous barrior.

 

[jtester]

In my past utility work, we dealt with E fields and B fields. E fields are controlled by the use of shielding on power cables fore example. They evenly distribute the E field and limit the decay of insulation. They are present whether there is current flow or not.

B fields are cancelled when there is an equal and opposite current present. A concentric neutral around a single phase power cable will have almost equal and opposite current and will cancel the field generated by the phase current.

In the OP, each single phase cable is part of a 3 phase connection and there will be little or no current in the concentric neutral. I believe there will be no cancellation of B fields in that installation. If all 3 phase conductors are in close proximity, the B field will be essentially eliminated.

Jim T

 

[don_resqcapt19]

Roger,

As it is well documented and referenced above, EMF can also be absorbed by outer shields so that no external field exists beyond the ferrous barrior.

But the effect of this is that the magnetic energy is turned in to heat energy in the ferrous material. Any time the current is high enough, there will be enough heat to damage the installation.
Don

 

[ramsy]

But the effect of this is that the magnetic energy is turned in to heat energy in the ferrous material. Any time the current is high enough, there will be enough heat to damage the installation

Unless of course EMF-absorbing sheilds from all phases cancel any magnitude build-up at the wiring method's main-bonding junction.

Not sure how the utilities are getty away with this, or how inductive heating works on cookware, but have no doubt it could increase ambient beyond conductor-insulation limits with similar, unbonded-EMFshields. Maybe inductive heating is dampened somehow by bonding EMF-shields for each phase to main bondings.

 

[steve66]

After reading 310.60(C)(1), it sounds like the field and heat is confined to the ground shield if both ends of the sheild are grounded.

Steve

 

[jtester]

The field that generates the heat will not be contained by the shield we commonly call a tape or drain wire shield associate with shielded power cables. The voltage field will be distributed, but the field caused by current flow won't be affected.

Jim T