In my research paper," New one cycle microprocessor based differential relay", and ,"New one cycle inrush blocking scheme for differential relay", I measured, instantaneous current(i) and its rate of change(di/dt) for 50Hz.inrush wave of C.T secondary of 220KV and 132KV , you can use same method and circuit. Normally this is achieved by using special beam C.R.O used in H.V lab.

Dear Anil, please clarify if you need to measure voltage or current or both. Note that only the voltage can be 'high voltage' and current cannot be so. If I assume that one terminal of your source is grounded, then current measurement will not involve high voltage isolation needs also.

Dear sijit sir, actualy my circuit over view is like that, a High voltage source of rating 0 to -25 kv hv pulse of rise time 1usec fall time 1 use  pulse width 6usec,duty cycle 0.001 PRF 10 to 200 hz, is connected to a Triode tube and the body of tube is at ground level.

i am tring to measure the current using ct of ratio 1v/amp but the shape is very poor cannot identify that wether it is cirrent or ct capacitance charging ,The max current from tube is 1 amp. suggest any good ct to measure this 0  to 1amp current.

A standard current transformer will not have the bandwidth to accurately measure fast pulses. You need to use either a wideband current transformer (CT) or a Rogowski coil and integrator. Wideband CT's that will do the job are made by Pearson Electronics, Ion Physics Corporation, and Stangenes Industries, Inc. These are designed to terminate into a 50 ohm cable for easy interfacing to an oscilloscope. Terminating the far end will convert the output to half the faceplate output, so a 1 Volt/Amp CT will output 0.5 V/A if the far end of the 50 ohm cable is terminated by 50 ohms. A silicone-insulated 40 kV wire passed through a Pearson Model 2100 or 4100 should work for your 25 kV system. These transformers also use a built-in electrostatic shield to eliminate errors from any fast-changing E-fields.

You can also "wind your own" Rogowski Coil (which generates an output proportional to di/dt) followed by an electrical integrator (sometimes as simple as an RC circuit) so that the output voltage proportional to current. Commercial Rogowski coils are offered by a variety of vendors. Rogowski coils are often used for high voltage high current measurements  - since they are air-core coils they have higher bandwidth than wideband CT's and will not saturate. If you construct your own Rogowski coil, you'll need to calibrate it using a known pulsed or sinusoidal current source, You can obtain good information on commercial Rogowski coil vendors or design/construction information by Googling "Rogowski Coil".