HV Cable Single Point Bonding within a substation
HV Cable Single Point Bonding within a substation
(OP)
G'day,
I need to run a 3.3kV cable from a 10MVA 33/3.3kV transformer to a prefabricated substation within the same switchyard. At 1750A, with derating factors due to the heating effects of parallel cables, it looks like I need 4 parallel runs of 630mm^2 1C cables (12 cables); presenting some installation difficulties.
Using single point bonding, the datasheets say I can instead do this with 3 parallel cables instead, however my understanding is that a voltage will be induced at the non-bonded end. I have seen numerous other posts on a similar subject, but these deal with long distances using cross bonding techniques. In my case however, the entire cable length is very short - only about 25m. What I would like to know is, what would the expected potential be? Will this be low enough that the cable can be safely run without making any typical installation modifications to a normally run cables?
The cable will be run undeground in conduit, coming up bottom-entry to a switchroom, 1.8m off the ground.
Many thanks in advance.
I need to run a 3.3kV cable from a 10MVA 33/3.3kV transformer to a prefabricated substation within the same switchyard. At 1750A, with derating factors due to the heating effects of parallel cables, it looks like I need 4 parallel runs of 630mm^2 1C cables (12 cables); presenting some installation difficulties.
Using single point bonding, the datasheets say I can instead do this with 3 parallel cables instead, however my understanding is that a voltage will be induced at the non-bonded end. I have seen numerous other posts on a similar subject, but these deal with long distances using cross bonding techniques. In my case however, the entire cable length is very short - only about 25m. What I would like to know is, what would the expected potential be? Will this be low enough that the cable can be safely run without making any typical installation modifications to a normally run cables?
The cable will be run undeground in conduit, coming up bottom-entry to a switchroom, 1.8m off the ground.
Many thanks in advance.






RE: HV Cable Single Point Bonding within a substation
As another note, if the screen is only bonded at one end, that infers to me that there is no direct ground connection. Do I need to run another separate earth cable? If so, do I just use the one 120mm^2 earth cable or do I need one per parallel run? i.e. three cables?
RE: HV Cable Single Point Bonding within a substation
So if you have no metallic sheath (screen), then no problems with circulating currents or induced voltages.
As far as the earthing goes, I am no expert on earthing so a bit hesitant to offer any advice.
If your earthing grid is design to avoid any touch and step potentials at both ends of the cable then I cant see any reason why you need to run a separate earth cable with the 3.3kV's. You should check with your earthing designer though.
RE: HV Cable Single Point Bonding within a substation
check this thread
RE: HV Cable Single Point Bonding within a substation
RE: HV Cable Single Point Bonding within a substation
Even if you know cable separations calculating system parameters with more than one cable per phase is a tedious task.
Cheers,
RE: HV Cable Single Point Bonding within a substation
Let's take General Cable 3.3/1.9 kV of 630 SQR.MM copper conductor .As indicated by Manufacturer ampacity of 3*single core cables in a p.v.c. duct [one single duct occupied] will be 551A per cable[1.2 Cm/W earth thermal resistance and 25oC earth temperature ].See:
http
But, it is for shield both ends solid grounded[earthing ].If only one end of shield will be grounded then for 4 ducts filled with 3*1/c 630 3.3 kV cables[three phases per one duct] XLPE insulated ampacity will be 460 A.[ thermal resistance 1.2 Cm/W, earth temperature 25oC, distance between duct center lines 250 mm.]
The built-up voltage for 437.5 A per cable will be 3.2 V for 3 *1/c per duct[4 conduits] or 4.45 V for single core cable, each in separate p.v.c. conduit, 250mm apart[3*4=12 conduits].
RE: HV Cable Single Point Bonding within a substation
RE: HV Cable Single Point Bonding within a substation
As pointed out by WhiteyWhitey in his post,why cannot you use Aluminium (or non magnetic)armoured single core cables for your application to nullify all induced currents?
RE: HV Cable Single Point Bonding within a substation
RE: HV Cable Single Point Bonding within a substation
First of all you are right: the General Cable table shows only one cable per phase ampacity.
Further, you are right also: one cable per phase ampacity is always more than ampacity of each one of four cables parallel in the same run.
You are right also: the duct bank is more expensive than simple underground buried cables.
But, some time you need to cross over a street or a highway and you have to prepare the place before. You can close then a half of the street permitting circulation on the other side.
There are other advantages of duct bank as, for instance, possibility of extracting of a damaged cable and put another in its place. Also you could provide a reserve duct for future expansion.
In order to calculate the ampacity of parallel cables in a duct bank one could follow the proceeding of IEC Standard 60287 or IEEE/ICEA S-135/P-46-426[J.H. Neher and M.H. McGrath AIEE Paper 57-660].See:
h
or:
http://neher-mcgrath.com/Heating_Calculations.pdf
In order to calculate the built-up voltage in the shield you may follow EPRI-EL5036 Power Plant Electrical ref. series VOL.4-WIRE AND CABLES.
There are some national standards as NEC [NFPA 70] indicating ampacity. For instance in
APPENDIX B single core cable of 1250 MCM [633sqr.mm] for only 9 duct bank, the ampacity of copper conductor insulated for 2000 V and 75oC maximum temperature it is 536 A.
RE: HV Cable Single Point Bonding within a substation
Thanks for the addresses on behalf of all participants of this thread.
However, the duct vs direct buried could be discussed. In the first post of this thread ahandle was connecting a transformer with switchgear. I would use rather cable bus than duct but he may have a different reason. Also using screened cable at this voltage may not be necessary. And again this is his choice.
Now, the duct bank is popular in NA. Sometimes there is no other way of doing it as you should not install direct buried circuits in couple of layers. This is for distribution circuits. Apart from this, the duct installation is not the best solution from the thermal point of view. "rho' of air in the conduits is ... 40 [K*m/W]. On the top of it there is a duct bank liner made of PVC with resistivity of 6.5. There are more "pros and cons" to this subject. In any case, I would not recommend using "standard rho" or standard soil temperature or backfill material without testing or circuit load factor taken from "the space". One can get a big surprise or loose a lot of money or both.
Going back to the extracting of damaged cable ... Yes, but you have to have quite a few meters of cable to sacrifice ... while mane repairs of direct buried circuits take just another joint.
Going again to the sheath voltage - everything can be calculated but it is a matter of complication and I doubt if there are many engineers that will be eager to do that for two cables per phase. You can check IEEE 575 or ELECTRA (forgot the numers ... there were two issues).