Installing two different voltage rated one a parallel run feeder

[majesus]

At a site in Canada, we have a 5kv feeder that has four 3c-#500 MCM, shielded, Aluminum armoured, 5kV, 100% cables running in parallel. The run length is 350m. One of the phases on a cable failed and the client wants to replace it. They already have onsite a 400m reel of 3c-#500MCM, shielded, armoured, 8kV, 133% cable that they want to use as the replacement. I've never installed different insulated rated cables on one feeder, but I don't any issues. The cable will be cut to the exact same installation length as all the others. The original and the replacement cables are both rated for the system voltage requirements, and both cables have the same orange outer jacket. I also have looked in my impedance tables and the 3c-500MVM 5kV 100% cable was 0.098ohm/km and the 8kV was 0.1105ohm/km. Does anyone see a problem with this?

Thanks,
Majesus

 

[majesus]

Sorry, I messed up the title grammer and did not notice until after posting. Too bad there is no way to go back and edit after clicking submit.

 

[waross]

The cables will not share the current equally.
For a quick example assume a voltage drop of 10 Volts. The cables will be connected at each end so the voltage drop across each cable must be equal.
For the 5 kV cable at 0.098 Ohms/km and 1 km length the current will be 102 Amps.
For the 8 kV cable at 0.1105 Ohms/km and 1 km length the current will be 90.5 Amps.
When 3 cables are 100% loaded the fourth cable will be 88.7% loaded.
If you can live with that it will work.
As the power factor drops, the sharing ratio may change.
However installing dissimilar cables in parallel is a code violation under the CEC.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[majesus]

Hi Waross,

Thanks for the reply. The run will be 305m. I simulated the circuit in PSIM and I can live with the % of unequal current sharing. This will not be an issue. The question in regards to code violate under the CEC.
The CEC states for parallel runs:
a) free of splices throughout the total length (except a single splice per conductor is permitted) -> [Not applicable in my case]
(b) the same circular mil area; -> [Conductors are the same mil area]
([/color]c) the same type of insulation; -> [Conductors have the the same XLPE insulation, just one cable will have thicker insulation. My interpretation is that I still haven't violated this code rule as the insulation is the same type]
(d) terminated in the same manner; -> [YES]
(e) the same conductor material; and -> [YES]
(f) the same length. -> [YES]


Also in the CEC handbook, it states:
The purpose of these requirements is to
make sure that each conductor of each parallel phase or grounded circuit conductor is
• not less than a No. 1/0 AWG and larger to prevent overheating;
• free from splices between termination points, to prevent overheating and increased resistance at the splice [an
exception in Subrule (2) allows one splice to be permitted to meet the requirements of Rule 4-006];
• the same size, to keep the conductor material resistance the same;
• the same type of insulation, to prevent damage to insulation on one conductor due to lower temperature
rating or different conditions of use; -> [Temperature rating is 90C, the cables have been modeled in ETAP and will run at 60C in the conduit configuration]
• terminated in the same manner at the busbars of the supply and load end equipment, to ensure equal sharing
of current without heating of the termination; -> [Terminations will not overheat, cables are running less 90C]
• the same conductor material, to prevent damage (mechanical or environmental) to one material type; and
• the same length, to keep all conductors’ resistance the same to ensure equal sharing of current.

Specific to on this application, I can't see how the installation is violating the CEC.

 

[waross]

This is my concern:
• the same length, to keep all conductors’ resistance the same to ensure equal sharing of current.
Word play but the impedance is not equal and the load sharing is not equal.
It depends on the interpretation of the AHJ.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Same type of insulation - 5kV 100% XLPE is not the same type of insulation as 8kV 133% XLPE. The 8kV 133% will be much thicker than the 5kV 100%, definitely not the same insulation.