Surge Protection for Buried HV cables

[coppabox]

Hello,

I am seeking guidance on when and where to use cable surge arresters for buried 33kV cable.

The situation is as follows:

1. The HV cable is buried for its entire length except at each end where it is fully enclosed inside a switchroom. There is no exposure to a direct lightning strike (also see point 2 below)
2. The cable is buried at depth of 1m. There are many steel structures on the surface. Hence we are not considering a direct lightning strike to the cable through the 1m depth of soil (statistically its too low to consider).
3. There is no bare earth conductor above or near the HV cable. The HV cable screen is bonded both ends.
4. The terminations are via C-type connectors, these can be fitted with 5kA (or 10kA) surge arrester units
5. The cable lengths are very long, up to 6000m.
6. The switchboard vendor does not mandate that surge arresters are used at the cable terminations.

I am struggling to find a criteria and/or calculation method, to determine if the cable or connected equipment impulse rating will be exceeded due to an indirect lightning strike propagating via the buried cable.

Generally speaking, where HV cables are not exposed to direct lighting strikes, I see people add lighting surge arrester units to the 33kV switchboard termination but they cant really explain why.

If someone can point me to a simple criteria and/or calculation method it would be very much appreciated. Also, because the cable lengths are long, I am seeking guidance on whether when surge arresters would be required at both ends.

Regards,

Chris

 

[RRaghunath]

With 6km long 33kV cable, what is the voltage expected at the receiving end! The cable capacitance could cause substantial rise in the voltage at the receiving end I believe!
If you have to use surge arresters, it will be at the receiving end in order to prevent damage to the cable due to sustained overvoltages.
The cables are not exposed at either end and do not warrant protection against lighting strike. Typically, if cable is terminated to a overhead line, in order to protect the cable termination / cable against surges coming from the overheadline, surge arresters are installed at the termination.
If Vacuum Circuit Breakers (VCBs) are used and the manufacturer recommends surge arresters at the cable termination in the switchgear, that would be a requirement to safeguard the cable termination against switching surges due to current chopping effect of VCB. However, I don't think any of the 33kV switchgear manufacturers currently recommend this, with the advancements in the technology.

 

[cuky2000]

Surge arrester (Sheath Voltage Limiters -SVL)it is not required at the receiving end bonded points since the voltage is virtually at the earth potential (V~0).
The application of the SVL is mainly to limit induce voltages across the cable jacket during short circuits events. It should be noted that, there is no explicit method in IEC or IEEE Standards for this type of protection.

^{SUGGESTIONS:
1) Verify the induced shield voltage at the mid-point (3,000 m) during SC event exceed the allowable safe voltage.
2) Verify if the losses are acceptable due to the circulation current with shield both-end grounded.
3) If the allowable voltage is exceeded, consider using Surge Arrester (SVL) at the intermediate splicing points.}

 

[stevenal]

Most underground systems at 33kV are not isolated/off grid. The overhead systems feeding them are susceptible to lightning. Unlike the air insulating the overhead system, your UG insulation does not recover following an event. Surge arresters should be placed at every reflection point if any part of the system is overhead.