11 kV cable insulation rating 1

[krisys]

I have an 11 KV offshore power distribution system. This system has only onsite power generation and operating in Island mode. I am replacing the old 11 kV cables. Problem is that we do not have the data sheet of old cables. Now we are facing problem in in selecting the cable voltage grade.

There is a group of people who says 6.35/11 kV (Um = 12kV) is adequate; another group who says the voltage rating shall be 8.7/15 kV (Um = 17.5kV).

Please note that the 11 kV system is resistively grounded (earthed) and the installation is in the IEC world.

My own observation in this whole scenario is it appears that there is a small discrepancy between BS (British Standards) and IEC standards. Ultimately the cable having a declared rating of 6.35/11 kV(Um = 12kV) and 8.7/15 kV (Um = 17.5kV) looks to be the the same or at least equivalent in performance.

I would appreciate if if anyone can give a sensible justification for selecting 6.35/11 kV (Um = 12kV) in place of 8.7/15 kV (Um = 17.5kV).

Reference standards are IEC 60502-2 and its equivalent BS-EN.

 

[7anoter4]

It depends on how the neutral is connected to Earth and how long you intend to work with an earthed phase. See IEC 60502-2 4.1 Rated voltages:
NOTE 2 It should be realized that in a system where an earth fault is not automatically and promptly isolated, the extra stresses on the insulation of cables during the earth fault reduce the life of the cables to a certain degree. If the system is expected to be operated fairly often with a permanent earth fault, it may be advisable to classify the system in category C.
The values of U0 recommended for cables to be used in three-phase systems are listed in Table 1 – Recommended rated voltages U0
For Highest system voltage (Um) =12 kV Uo=8.7 kV [type C] [15/8.7 kV]

 

[cuky2000]

The cable insulation rated for a max. Operating voltage Um=12kV is adequate for a resistance grounded since 1<COG<1.73. Therefore, the cable insulation is rated to withstand an overvoltage in the healthy phase in te event of a ground (earth)fault.

NOTE:COG= Coeficiente of Grounding = Earth Fault Factor (EFF)/1.73

 

[Kiribanda]

krisys,
You have mentioned that your 11kV system is resistively earthed.
Normally in the IEC world the 11kV systems are not HR grounded as we see (13.8kV) in the ANSI world.
Therefore, your 11kV system should be Low Resistance earthed (LRE) with EFF=1.38 or lower and
and hence it will trip within one minute during an earth fault.
That means IEC cables designated as 6/10(Um=12kV) is the correct spec.
Pl. note that 6.35/11(Um=12kV) is the old IEC designation.Cable manufacturers
would quote for 6/10(Um-12kV).

 

[ppedUK]

@ Krisys
A daft question.......have you checked the markings on the sheaths of the existing cables (or get someone to send a photo)? I can't think of an occasion where I have come across a HV cable with no voltage grade imprinted in the sheath.

In a "normal" 11kV system (in the UK anyway) the 6.35/11kV grade would be used, providing that it is a grounded star point system and has earth fault protection fitted to all circuits, unless there are other constraints (maybe unusually high impulse rating for the switchgear, or excessive voltage rise on sudden loss of load within the island?), so not sure why you would choose the higher grade in this instance, but you need to carry out further investigations before making your choice. The choice of the higher voltage grade would result in a slightly larger cable O.D. which may cause you other problems (bending radius etc).

Some other things to think about when replacing the old (presumably PILC) cables is that older switchgear might not be suitable for the higher conductor operating temperature of XLPE cables (90^oC), and it is wise in this instance to size the new cables based on a lower conductor operating temperature appropriate to the switchgear. Also, if the "old" cables are PILC, then they will more than likely be terminated into compound filled cable boxes, which are often cast iron. If so, these should be replaced with sheet steel cable boxes, suitable for dry terminations (required for XLPE cable, unless you use a modern non-pitch based filling compound). If changing to dry terminations, also check the Phase - Phase and Phase - Earth clearances within the cable box.

@ Kiribanda
The OP doesn't state whether they have a LR or HR grounded system, but HR grounded 11kV systems are not unusual in IEC land, particularly on embedded generator or islanded systems, where E/F current is typically limited to <25A to minimise core damage for a stator earth fault.

6.35/11kV cable is still common here in the UK and is constructed to BS6622 or BS7835. 10KV systems are common in the rest of Europe, but I would not accept 6/10kV grade cable in an 11kV system, no matter where it was in the world.

 

[Kiribanda]

If I am not mistaken,
Per BS6622,a 11kV cable suitable for a 11kV nominal system voltage is designated as 6.35/11(Um=12kV) whereas the same 11kV cable
suitable for a rated system voltage of 11kV is designated as 6/10(Um=12kV). But both cables have the same insulation thickness
which is the dominating factor during an earth fault etc.

 

[krisys]

Additional information/clarifications:

The system is Low Resistance (LR) grounded. 11 kV system neutral earthed to limit the current to 200A.

This is an offshore oil processing platform. Like a marine environment. Power system is an island system and the power generation is at 11 kV. These cables will be installed in the Switchboard which is directly connected to the generators.

As I understand, normally for the island power systems like this, a slightly higher insulation grade (i.e. the next available voltage grade) is generally preferred. i.e. in this case 8.7kV/15kV, which is the immediate higher insulation grade. The technical reason behind this is as below:

In the marine environment conventionally it is believed that the bus voltage the island systems are prone to have wider variation (voltage excursion) as compared to the grid system. This is due to the relatively lower system inertia owing to the lower generation capacity. So to cover this risk, it is not uncommon to use the 8.7kV/15kV cable.

In the few recent past projects, 8.7kV/15kV cable has been used.

With this background, I would like to know "Am I justified in demanding for the 8.7kV/15kV cable?"

 

[Kiribanda]

krisys,
I donot know how you say that your operating voltage is subjected to a wider variation than standard. Also you have mentioned that lower
inertia is responsible to that voltage variation.But it is my understanding that the operating voltage of the system
irrespective of marine or on-shore,is controlled by the AVR of your generators.I donot think you can operate a system with
nominal system voltage 11kV higher than 12kV because the equipment on your 11kV system are rated for 12kV maximum.

 

[krisys]

Kiribanda,

There is no such continuous over voltage on the bus. But the any disturbances on the power system may yield relatively higher degree of voltage disturbances.

Otherwise I am not finding any technical reason to select the higher voltage grade cables. But in reality it has been the practice in many offshore installations to select one step higher rating for the MV cables.