Characterisation of High Voltage Dividers for X-ray Measurements

Invasive high voltage dividers are usually used for the measurements of high voltage x-ray tube and x-ray generators. Because of the diversity of the pulses it is usually difficult to establish a measurement set up for all the waveforms without determining a test procedure for the calibration and characterization of the measuring system. For example in X-ray diagnostic application, the voltage level is up to 150 kV, the time duration is varying between 50 µs (fast transient over voltages) to several seconds (temporal over voltages), the rise time is varying between 10 µs and 500 µs, the waveforms are usually superposed by DC voltage, spikes and a high frequency ripples up to 200 kHz. To overcome these difficulties, LNE (The National laboratory of Metrology in France) and LCOE (the Designated Institute for High Voltage measurements in Spain) have developed, in the frame of 15NRM02 UHV EMPIR European project, a technical procedure to measure with accuracy this large type of pulses using one reference system. An invasive resistive divider (with bandwidth adapted to the measured waveforms) combined with a storage calibrated digitizer is used as a reference standard for the evaluation of the impulse parameters (PPV, Average peaks value, Rise time, duration of the pulse and the fall time). Firstly, high voltage measurements technics have been used to determine the linearity of the reference system for known, traceable and standardized waveforms. Secondary, to ensure a best accuracy while performing the calibration of pulse parameters, we have decided to use a FFT and inverse FFT based technique (deconvolution technics) before the main calibration to correct the frequency non-linearity of the measuring system. The uncertainties of measurements are 0.4% for Voltage measurements and 2% for time parameters measurements.