Voltage grade for the cable to be used in 33 kV network having un-earthed neutral

[krisys]

I need to specify the suitable voltage grade for the cable to be used in 33 kV network having un-earthed neutral. [Though the actual network is resistively earthed, we have a mandate to treat the 33 kV network as un-earthed system for the purpose of selection of voltage grade].

By noting the relevant IEC standard for Medium Voltage Cables as below:

IEC 60502-2 : Power cables with extruded insulation and their accessories for rated voltages from 1 kV (Um = 1.2 kV) up to 30 kV (Um = 36 kV);
Part 2: Cables for rated voltages from 6 kV (Um = 7,2 kV) up to 30 kV (Um = 36 kV)

The above standard does not cover the 33/33kV (36kV) voltage grade requirement. The standard covers up to 18/30 kV (36 kV).

Note:
The standard notation for voltage grade is U₀/U(U_m)

I would appreciate if you please share your experience on this.

 

[7anoter4]

In IEC 60502-2 ch.4.1 Rated voltages "Um" is defined as follows:
"Um is the maximum value of the "highest system voltage'' for which the equipment may be used (see IEC 60038)."
It seems that 30 kV and 33 kV rated cable the permissible Um is the same: 36 kV.
In IEC 60038 Table 3 A.C. three-phase systems having a nominal voltage above 1 kV and not exceeding 35 kV and related equipment* Series I for nominal 33 kV Um=36 kV.
Then, in turn, from IEC 60502-2 Table 1 – Recommended rated voltages Uo for Um=36 kV categories A and B Uo=18 kV. Indeed, no recommendation for C category. So, in my opinion, if the system operates with a permanent earth fault, it is recommended to use a cable of sqrt(3)*33=57.15768 kV minimum rated. [According to IEC 60038/2002 66 kV is the rated voltage close to it].

 

[Kiribanda]

krisys,
If you are planning to use an UNEARTHED network, then your cable should have thicker insulation to fight against
transient voltages generated during a an earth fault. Per IEC 60502, you need Cat C cable where you are allowing
the fault to persist for sometime. Therefore, you need to use a cable with 133% insulation to use on your
unearthed network. But in my opinion using a 57kV cable on an unearthed 33kV system is an overkill. This
133% insulated cables are widely used in NA with high resistance grounded networks.

 

[Mbrooke]

How long will the earth fault persist?

 

[7anoter4]

I agree with you Kiribanda, the next 36/60 -36/66 [72.5 kV]-cable it seems to me too exaggerated a bit.
However, the insulation level in % it is according to IEEE [no IEC].
IEEE 141/1993 ch.12.4.1 Voltage rating states as follows:
The insulation level required for up to 1 min. grounding fault duration is 100%.
For more than 1 min. but less than 1 hour the required level is 133%.For more than 1 hour duration the required level is 173% [that means phase-to-phase voltage for phase-to-ground].
IEC 60502-2 ch.4.1 Rated voltages provides also 3 levels [for Um up to 24 kV]: A and B [86.6%] and C [125.6%].For 36 kV only A and B level is recommended.
See, for further information, IEC 60183-1984 Guide to the selection of high-voltage cables.

 

[krisys]

I am convinced with your argument.

The system I am referring to is quite large. It is a petrochemical complex with a power distribution network size of 150 MW, voltage levels from 415V up to 132kV. The power distribution network includes captive power generations at multiple locations amounting to about 95MW. The existing power distribution system will be upgraded to operate the plant with the worst case scenario of all the six (6) generators tripping and complete power is supplied from the Utility grid.

To cope with the new scenario, they will be installing some medium voltage capacitor banks (at 11kV and 33kV) and a 50 MVAR STATCOM. Since these new equipments (capacitor banks and STATCOM) have tendency to generate over voltages in the system, the consensus is to go for the fully insulated [ i.e; 33 kV / 33 kV (36 kV) ] medium voltage cables.

As these cables are not covered in IEC-60502, we may have to refer different IEC ( I am still trying to find out the applicable IEC for this cable). These cables could be 66 kV grade. But the cable termination for the 66 kV cables require more clearances and may need special cable box. i.e; not the commonly used cable box for the 33.kV Gas Insulated SWGR (GIS) or Air Insulated SWGR (AIS).

Do you envisage any other practical difficulties to implement this over conservative requirement? If I have solid justification, I can present to the team and convince to use the 19 kV / 33 kV (36 kV) cable. Also this is an industry wide accepted practice.

Some useful thoughts/experiences will be appreciated.

 

[7anoter4]

In connection with the cable termination: we many times consulted Raychem- one of termination manufacturer. In my opinion you could use Raychem Heat-shrinkable Terminations- IHVT-H/OHVT-H for instance-for your cable of 60 kV.

http://www.ensto.com/download/20928_Raychem_High_Voltage_Cable_Accessories.pdf

The difference of 2 mm of insulation and 5 mm overall cable diameter I think, it will be not a problem. The clearance could stay the same since the rated voltage will remain 33 kV.

 

[Mbrooke]

I am not familiar with the IEC as much as I should be, but why would a 133kv cable be acceptable? A sustained, long duration phase ground fault would cause 1.73 times the voltage on the cable rather than 133%. Would not 133% be insufficient, or do disconnect times eliminate this issue?