HFCT installation in HV cables

I wish to know the difference in installation of high frequency current transformers (HFCT) to detect PD in 11 and 33 KV and above
cables. In 11 KV , HFCT is clamped in the ground strap.Based on the information from others I heared that HFCT installation is different in 33 KV and above cables.Can anyone help Please
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