Keithley and Keysight are industry-standard for accuracy and precision.

To give a qualified answer, we need a bit more information:

• AC or DC?
• Which voltage level?
• What will be the current range?
• Has the voltage measurement to be isolated?

Regards

• DC
• The voltage range is 30 to 100V
• The current range is 8-20Amp
• Yes voltage measurement is isolated   

OK. Not exactly "high power". Seems to become one more PV inverter or alike :)

What I would attempt: Have an "intelligent" subsystem (a µController with the necessary periphery) on the 'HV' side that is dedicated to measuring voltage and current and transmitting the results via some isolation barrier to the rest of the control circuitry. Up to 20 Amps you can still use a shunt resistor to measure the current. And the voltage is measured using a resistive divider. Both concepts are really fast and completely DC-capable.

Alternatively, the current could be measured by means of a "current clamp" which has to be "DC-capable". Eg. LEM has a wide range of such clamps - eg. here

http://www.lem.com/hq/en/content/view/269/206/  Using such a clamp, the current measurement is isolated.

The voltage measurement will remain a challenge: though isolation amplifiers or isolated ADC circuitry is available (and promoted quite well), the performance of such things is rather poor according to my own experience. Thus the approach sketched on top of this answer.

Hope this helps to proceed ...

thank you 

alternate, you can use current transformer(instrument transformer) for  high power measurement.