High Voltage Measurement
HVAC is often measured with the cable interface insulators called bushings ( usually porcelain , glass or polycarbonate) that have  built-in electrodes with an specified low capacitance value , supplier to large diameter coaxial jacks and have the same high BIL ratings Basic Insulation Level, to a low voltage higher capacitance load that acts as a capacitance voltage transformer of 10k:1 or cascaded to the desired end voltage range.

HVDC can be measured with E field sensors using a known impedance current loop as a voltage divider. These must be clean dry air gapped E Field sensors designed to withstand the HVDC breakdown in a very clean environment . Although I am not up to speed on laser E field tracking and other modern methods.