use shunt type of sensor on high side, then high accurate sigma-delta ADC with serial 1-2-3 wire interface transduced data through optocouplers. Supply can be organized as shunt regulator with measuring system ground (low point of shunt, ADC ground, around circuitry ground) connected to external ground through high ohmic current limiting/protection resistor. So your measuring system ground will be at ~1.5KV above external ground. Optocoupler should witstand at least 2x1.5KV and PCB have to have thin air gape between In/Out to avoide breakdown.