SWA Cable Overheating

[dojedar]

Hi, I am reviewing the calculations for a 200A line with two 3 core 70 sq mm armoured cables, i.e. 2 cables per phase. These cables supplies to a 110KW pump and are installed direct buried along a trench with other 48 power cables in the form of 6 rows of 8 cables each row.

The cables have overheating and losing insulation resistivity. The calculations made had considered 5 cables installed on same row only. I understand this is wrong because there are 8 cables each row and should be applied ather rating factor for each layer. The installer, who made the cable sizing, says that overheating is produced by unbalanced currents on armour.

I have looking for references from BS 5467 standard trying to find the correct rating factors but I did not found anything by the moment. I appreciate if anybody have more information about this issue.

Regards

 

[mechelec]

are both ends of the armour bonded as this can cause circulating currents in the armours.

 

[Knobhead]

There should be no current flowing in the armour, so long as the current flow in the two cables is the same. To check for circulating currents put a clamp meter around each individual cable, if you have one that can show 3[SUP]rd[/SUP] and 5[SUP]th[/SUP] harmonics then you may find some but unlikely.

As an after thought just make sure that each phase is going down each cable, I found a dual feed to a motor that the contractor had wired with:
Phase A connected to cores 1&2 in cable A
Phase B connected to cores 3&3 in cables A&B
Phase C connected to cores 1&2 in cable B
“It was easier to connect up” the cables were roasting!

 

[Engineer54]

Dojedah,

Are you referring to the number of cables or the number of circuits?? You should only be interested in number of circuits mate!!! Don't forget you are using two 3 core cables as ''ONE'' cable. So are you sure that maybe some of these other circuit(s) are not derived in the same way ??

As so many of these cable over heating problems these days, stems from the use of ''inverter'' controllers... I have to ask, .... do you have such controllers in your installation???

 

[dojedar]

Thanks,

Armour is connected in both ends, I have thought that may be better to connect it in one end only


About harmonics, I think is unlikely. The plant have 6 MCC with a dedicated 1,5 MVA transformer each one, and from each MCC there are four 150KW and four 18,5KW VFDs. The VFDs are ACS model of ABB with harmonic filters. II have not checked the motor connections yet, but I have to do it.

Engineer54, number of cables or number of circuits is anything that I do not have clear. The starting system is S-D and the current capacity was calculated for one cable. For this reason I think should be considered the number of cables and not the number of circuits.

In this moment we are planning to install unarmoured cables inside pipes.

 

[Engineer54]

Thanks,

Armour is connected in both ends, I have thought that may be better to connect it in one end only


About harmonics, I think is unlikely. The plant have 6 MCC with a dedicated 1,5 MVA transformer each one, and from each MCC there are four 150KW and four 18,5KW VFDs. The VFDs are ACS model of ABB with harmonic filters. II have not checked the motor connections yet, but I have to do it.

Engineer54, number of cables or number of circuits is anything that I do not have clear. The starting system is S-D and the current capacity was calculated for one cable. For this reason I think should be considered the number of cables and not the number of circuits.

In this moment we are planning to install unarmoured cables inside pipes.

I don't quite understand your reasoning dojedah??? What has anything you have told us about MCC panels and dedicated 1.5MA TXs got to do with anything?? What's far more important in that paragraph is that you haven't checked any of the motor and other related connections!!!!

Wrong!!! You are using 2 identical multi core cables supplying a load.... doesn't matter a jot what the calculated load or measured load is ...you are using 2 parallel cables as your means of supply. Which to anyone is ONE circuit NOT two circuits. The current is being Shared between the two paralleled cables, for gods sake!!! How the hell did you come up with the idea that you count cables and not circuits, ..even when you know they are paralleled cables???

Well i hope the cables on load side of those VFDs etc are suitably sized and are substantially screened, ....or your going to end up in just the same amount of mess as you are now!!!

 

[bomjac]

Pretty sure that ABB recommend an earth cable is run out to the drive along with the cables from their VSD. One we installed recently was fed by 3c 240mm cable and an additional 240mm earth cable run along with it. Check their documentation.

 

[Knobhead]

Dojedar, in your first post you didn’t mention the cables were for a star/delta motor, setting people off on the wrong track. Have you checked the actual loading on each cable core? In fact what tests have you done? You say the cable IR is failing, to what extent? Give all the information you have and we may be able to help.

As to replacing the cables, that seems to be the knee jerk reaction of someone that’s out of their depth.

2 X 70mm are well on top of the job so there’s a fundamental problem somewhere.

Information needed:

• Cable IR readings. Original readings as well if you have them.
• Cable CR readings.
• Motor IR & CR readings.
• Ph – Ph voltages while running at both the motor terminals and the starter incoming.

Replacing cables is an expensive option when you don’t know what the fault is yet!

Are any of the other 150KW drives showing the same symptoms? If not then cable replacement is not the answer. To be honest with you with the 1.5MVA supply I can't understand the use of star/delta, the transformers would handle DOL with no problems.

 

[dojedar]

Dojedar, in your first post you didn’t mention the cables were for a star/delta motor, setting people off on the wrong track. Have you checked the actual loading on each cable core? In fact what tests have you done? You say the cable IR is failing, to what extent? Give all the information you have and we may be able to help.
sorry for the error, was my mistake do not mention the S-D system. The loadings on each core were very similars about 191, 188, 189 A. I say that IR is failing becouse it was measured and values are coming down about 430, 398, 760MOhm each Ph-Gr, and 305MOhm 3Ph-Gr.

As to replacing the cables, that seems to be the knee jerk reaction of someone that’s out of their depth. May be, I just got to plant, the before electrical engineer left the work, there are missing information, if pumps goes shutdown the plant stops and is imperative the plant production, so I have to avoid to stop the plant production at any cost while I am looking for the cause

2 X 70mm are well on top of the job so there’s a fundamental problem somewhere. I'm not sure, 48 power cables direct buried on same trench, in midle east... the calculations made for design shows a current capacity of 112A for each 70 sq mm cable and current demand is 110A, i.e. any small variation on derating factors and it would be undersized, derating factor for 5 cables on same trench is correct?!...spacing between cables considered on calculations is lower than the one showed on trench layout, etc.

Information needed:

• Cable IR readings. Original readings as well if you have them.
• Cable CR readings.
• Motor IR & CR readings. Were made before my arrival and it seems that were good
• Ph – Ph voltages while running at both the motor terminals and the starter incoming. Pending

Replacing cables is an expensive option when you don’t know what the fault is yet!

Are any of the other 150KW drives showing the same symptoms? If not then cable replacement is not the answer. To be honest with you with the 1.5MVA supply I can't understand the use of star/delta, the transformers would handle DOL with no problems. The power cables that supplies VFD panels have the same symptoms as all the cables that goes by the same trench (other pumps, instrumentation). In addition there are many water overflows over the trench that increases the problem. I would not have use S-D, but I would had used soft starters. The current peaks of DOL affects the other equipment conneted to same transformer.

Thanks for the answer

 

[maddfridge]

hi there

think plumber

just a passing thought is the mechanical seal on the pump motor a sealed on or does it require greasing could expalin hi currents if not correctly lubricated

cheers
maddfridge

 

[Engineer54]

dojedah,

I'm begining to understand some of your reasoning now. Not that i agree with much of what your saying here. I take it your working in the Jeddah area of Saudi, and English is not your first language.


Your current loadings seem basically fine. As are your IR readings, nothing wrong with those figures at all. Remember IR readings will differ with temperature, and if the original Ir readings were taken during the cooler months, you could well see a substantial difference than those taken in the summer months... I can well understand your predicament as regards to your plants continuous operation by the way!!!

Back to your trench, who the hell dreamt up that installation?? If were talking about a manufacturing plant that relies on these pumps and other equipment fed by cables in this trench, that are direct buried, someone has seriously got the installation method wrong!! All critical supply and feed cables should have been installed in a ducted system, like a buried multi storey concrete encased system that uses buried marshaling pit/rooms (can't think of the correct term, ...lol!!) Which then makes any of the installed cables ''rewirable/replacable''

When you say, ''de-rating factor for 5 cables is correct'' What are you talking about here, where did you get this 5 cables thing from?? Were talking about 48 cables in this trench are we not ?? Still totally in the dark on your reasoning of counting cable numbers instead of circuit numbers?? If you are using a current carrying capacity of 112A for a 70mm cable, and your load current is 110A then you will have a current carrying capacity of around 225A when ran in parallel.

A couple of questions for you, ....What is the depth and width of this single trench??
You say that you have many ''water overflowings'' ..are you saying that this trench is continuously being flooded?? What de-rating factors have been applied to these cables.
What is the design ambient temperature your working too, ...i would have thought 50C, as that was the general standard of most installations when i worked in Saudi.

You my friend are at a very real disadvantage, as you were not around during the laying down installation of these trench cables, so you have no idea of just how well (or not so well) this has be done. I hope that none of these cables sufferd any sheath damage during pulling in/laying in, if they are subject to all this on-going water flooding....

 

[dojedar]

dojedah,

I'm begining to understand some of your reasoning now. Not that i agree with much of what your saying here. I take it your working in the Jeddah area of Saudi, and English is not your first language. The installation is near, in Oman


Your current loadings seem basically fine. As are your IR readings, nothing wrong with those figures at all. Remember IR readings will differ with temperature, and if the original Ir readings were taken during the cooler months, you could well see a substantial difference than those taken in the summer months... I can well understand your predicament as regards to your plants continuous operation by the way!!!

Back to your trench, who the hell dreamt up that installation?? If were talking about a manufacturing plant that relies on these pumps and other equipment fed by cables in this trench, that are direct buried, someone has seriously got the installation method wrong!! All critical supply and feed cables should have been installed in a ducted system, like a buried multi storey concrete encased system that uses buried marshaling pit/rooms (can't think of the correct term, ...lol!!) Which then makes any of the installed cables ''rewirable/replacable'' Totally agree

When you say, ''de-rating factor for 5 cables is correct'' What are you talking about here, where did you get this 5 cables thing from?? Were talking about 48 cables in this trench are we not ?? Still totally in the dark on your reasoning of counting cable numbers instead of circuit numbers?? If you are using a current carrying capacity of 112A for a 70mm cable, and your load current is 110A then you will have a current carrying capacity of around 225A when ran in parallel. derating factor for 5 circuits on same trench is the factor taked in the original design. Ok, as you say we will count circuits. On same trench there are 20 circuits with 2 cables per phase and 8 circuits with 1 cable per phase. From this, the derating factor for only 5 circuits on same trench seems not correct. On manufacturers specifications I have seen until 12 circuits derating factor but no more. For this reason I'm looking for the sizing BS standard for these cables.

A couple of questions for you, ....What is the depth and width of this single trench?? bottom cables: 1.1m, top cables: 0,680m. There are not information about what circuits are on trench bottom, middle or top. Original design considered 0.8m for calculations. Width: 0.850m
You say that you have many ''water overflowings'' ..are you saying that this trench is continuously being flooded?? not continously but quite frequently
What de-rating factors have been applied to these cables. Grounf Temp: 35ºC, Soil Thermal Resist: 2.5ºC cm/w, Depth: 0,8m, Grouping: 5 circuits spaced 0.15m. Total derating factor: 0.5224. Derating factors taked for these conditions taked from manufacturer specifications.
What is the design ambient temperature your working too, ...i would have thought 50C, as that was the general standard of most installations when i worked in Saudi. 50ºC with 100% hum

You my friend are at a very real disadvantage, as you were not around during the laying down installation of these trench cables, so you have no idea of just how well (or not so well) this has be done. I hope that none of these cables sufferd any sheath damage during pulling in/laying in, if they are subject to all this on-going water flooding.... I have request by information, records, testing and other information about installation but nothing for the moment, only comments of plant people

Thanks