Noise levels on high voltage power lines
There are a few standards which seek to describe what electromagnetic immunity is necessary for equipment in power/substation environments.  I would recommend looking to IEC 61000-6-5 for generic power station/substation EMC immunity requirements.  This will in turn point you to the appropriate levels in other IEC 61000-4-x standards.

IEC standards are copyrighted, so unfortunately I assume I cannot provide example values here.

Other standards describing immunity in substations for various specific circumstances:


IEC 60870-2-1 
IEEE 1613 (which specifies many of the IEEE C37.90.x standards)
IEC 60255-26