Calculating zero sequence current compensation factor (k0) for distance protection in cable system

[EBDRUP]

Hi

I am working on determining the settings for a distance relay in a nearshore wind farm (turbines in the water, substation on land). The customer has chosen to use distance protection for the primary protection of the array cables.
Each array consists of 5km 1200 mm2 land cable, 5 km 1200mm2 submarine cable and 5 km 300mm2 submarine cable so the array is 15 km long with 3 different cable types.
I am in doubt how to assess/calculate the k0 factor for the ground fault unit in the distance relay. My idea is to calculate it based on the positive and zero sequence impedances stated in the data sheet form the cable manufacture – by simply summing the positive sequence impedance and summing the zero sequence impedances and thereby having an array positive sequence and an array zero sequence from these two impedances I then calculate the k0 as:

K0 = (Z0-Z1)/(3*Z1)

Taking the zero sequence impedance from the cable data sheets will to my knowledge represent a return of the single phase fault current in both screen and ground/sea bead/ sea water.
How do the above approaches sound?

I know that measuring will provide a much better knowledge of the system zero sequence but that is not an option now.

Do anyone in here have experience with distance protection of cables? Distance protection in wind farms?

Any good references?

Any sparring, comments or thoughts is highly appreciated!

Best Regards
Ebdrup

 

[HamburgerHelper]

Model the cables and get Z0. Adjusting k0 makes the distance calculation more or less sensitive to zero sequence components, which you might care about if you have mutual coupling. Whatever you set K0 to, you check the reach to make sure that it isn't overreaching. k0 will change the reach of your ground distance relaying. The k=3 default is based on common values of Z0/Z1 with normal transmission spacing. Z0/Z1 should be much higher for cables due to the tight spacing.

This is some stuff I think will help you.

https://www.techrentals.com.au/wiki...e-More-Specific-About-Equipment-Grounding.pdf

http://www.nzccpts.co.nz/publications/Cable Sheath Bonding Application Guide Companion.pdf

You might be best just to call up your cable vendor and get the values from them.

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