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National Measurement Institute

Power Systems Capabilities

NMI's High Voltage LabNMI provides calibrations of high voltage measurement apparatus for direct, alternating and impulse voltages. These calibrations can also be performed away from the laboratory using our Mobile Calibration Unit as long as a suitable regulated power supply is available.

Dielectric withstand tests using any of the three types of voltages can also be carried out. Tests may be carried out at potentials up to 1 MV dc, 600 kV rms ac, 2.0 MV lighting impulse and 1.1 MV switching impulse. Radio influence voltage measurements can be performed using alternating voltage on transmission line hardware and insulator assemblies. Calibration of voltage transformers, current transformers and instrument transformer testing sets can be performed. Measurements of relative permittivity and loss factor can be made on dielectric materials. Click here:

Before consigning an instrument for calibration always consult us to discuss your requirements. For further information contact

High Voltage Testing and Calibration

DC High Voltage

The Australian standard for direct current high voltage measurement is maintained, up to 500 kV, using modular high voltage resistive dividers. The resistance values of the constituent resistors at low voltage, are traceable to the national standard of resistance. The deviation of the high voltage ratio from the low voltage ratio is estimated to be less than ±0.001% at 100 kV or below, and ±0.02% between 100 kV and 700 kV.

AC High Voltage

The Australian standard for alternating high voltage is maintained by two compressed-gas capacitors and a ferro-magnetic voltage transformer. The capacitance values at low voltage are traceable to national Australian standards. The voltage transformer ratio is calibrated in an absolute sense by the buildup of low voltage capacitors. The change of capacitance of the high voltage standard, from low to maximum voltage, has been estimated to be less than 2 ppm for each capacitor.

AC high voltage testing and calibration that is available include high voltage capacitors (e.g. compressed-gas or paper-oil), voltage transformers, capacitor voltage transformers, voltmeters and dividers, radio-influence voltage and dielectric withstand (wet and dry).

The test voltage supply is high-quality 50 Hz mains, of single phase and has a rated capability of 600 kV, at 0.5 A (1 A short-time rated).

Non-standard calibration or testing, for compliance or development purposes, may be arranged, after consultation.


High voltage capacitors are calibrated using a current-comparator method. The typical least uncertainties, up to 550 kV rms, are 15 ppm for capacitance and 10 ppm for dielectric losses (i.e. dissipation factor or tan δ).

Voltage Transformers

Above about 100 kV, voltage transformers can be calibrated using a current-comparator method up to 550 kV rms, with typical least uncertainties of 60 ppm for voltage error and 0.006 crad for phase displacement.


AC voltmeters, in rms-mode, peak or peak/√2-mode, can be calibrated up to 550 kV rms, with typical least uncertainties of 0.1%, by comparison with calibrated voltmeters.

Radio Influence Voltage

Radio influence voltage and the determination of corona onset and extinction voltages can be measured up to 400 kV rms, with a typical least uncertainty of 5 dB. The conducted radio noise is measured using a test circuit in accordance with NEMA and ANSI C36.2 specifications. The normal centre frequency is 1 MHz, with quasi-peak detection of the noise. A CISPR 18-2 detector capability is being developed.

Dielectric Withstand

Wet or dry testing of the insulation integrity of materials and structures can be carried out, up to 600 kV, with typical least uncertainties of 3%. 

Impulse Voltage

The Australian standard for lightning impulse voltage is maintained by two identical reference resistive voltage dividers. Three identical, non-inductively wound high voltage resistors are provided for each of the divider sets. One, two or three of the high voltage resistors can be installed in the high voltage arm of the divider, to construct reference dividers with voltage ratings ranging from 350 kV and 1 000 kV.

Attenuators made of networks of non-inductive resistors are also maintained for further scaling down the lightning impulse voltage to levels appropriate to the voltage ratings of the impulse reference digitiser.

The Australian standard for switching impulse voltage is maintained by a damped capacitor divider with a switching impulse voltage rating of 1 000 kV.

A digital oscilloscope with an 8-bit resolution and a maximum sampling rate of 2 GS/s is used as the reference impulse digitiser. The digitiser performance has been evaluated according to IEC 61083-1, with its dynamic scale factors determined with an uncertainty of 0.16%. Associated impulse measurement software has been developed and evaluated according to IEC 61083-2.

Calibration of lightning impulse measurement systems up to 2.0 MV and switching impulse measurement systems up to 1.1 MV can be performed, with a least scale factor uncertainty of 0.4% for lightning impulse and 0.5% for switching impulse. The calibrations are performed at several voltage points or at one voltage point and supplemented by a voltage linearity test over the complete voltage range.

The step response of impulse voltage dividers can be measured with a maximum input step voltage of 500 V. A service is also available, for improvement of the step response performance of damped capacitor dividers, by adjusting the divider’s low-voltage arm components.

Calibration is also available for impulse digitisers, impulse voltage testers and impulse voltage calibrators, with peak voltage ratings up to 5 kV.

Partial Discharge Calibrators

The charge values and rise times of the voltage steps of partial discharge calibrators can be calibrated. The range of charge values are from 0.5 pC to 10 nC and the lower limit of the rise times is 1 nS.

Dielectric Material Properties

The dielectric material properties of solid or liquid samples can be carried out between 10 Hz and 100 kHz. The recommended sample size for liquid samples is 100 mL. Solid samples can be in sheet or film form, up to 2 mm in thickness and up to 80 mm in diameter or linear edge measurement.

Instrument Transformers

When enquiries are made regarding a test, the following information should be supplied:

  • test frequency
  • secondary burden
  • ratios on which the transformer is to be tested
  • voltage or current levels at which the transformer is to be tested

When an indicating instrument or other instrument constitutes the burden, it may be forwarded with the transformer, which will then be tested with the instrument as burden. (The test will not include the calibration of any such instrument.)

Alternatively, the values of the resistance and inductance of the instrument may be supplied and an equivalent burden will be used in the tests. In the case of a current transformer, the leads used to connect the instrument to the transformer should be forwarded with the instrument or the value of their resistance and reactance stated.

Current Transformers

Transformers which have been calibrated by an absolute buildup method serve as NMI's standards. Clients' current transformers are calibrated by a differential method by comparing against these standard transformers. Least uncertainties range from 3 ppm for a 1:1 ratio up to about 10 ppm for a 4000:1 ratio.

Tests are carried out on current transformers with rated secondary currents of 1 and 5 A. Transformers are demagnetised before testing unless a request is made to the contrary. Measurements are typically made at current settings of 5, 10, 20, 40, 60, 100 and 125% of rated primary current although, on request, measurements may be made at any current setting between 2.5% and 200% of rated primary current.

Voltage Transformers

Voltage transformers are calibrated against a gas dielectric capacitive divider which itself is calibrated using a buildup method. Least uncertainties range from 4 ppm for a 1:1 ratio up to about 5 ppm for a 1000:1 ratio.

When a voltage transformer with fuses is submitted for test, the test will be made with fuses included in the circuit unless a request is made to the contrary. Measurements are typically made at voltage settings of 5, 10, 20, 60, 90, 100 and 110% of rated primary voltage, although on request, measurements may be made at any voltage setting between 2.5 and 200% of rated primary voltage.

Tests at primary voltages exceeding 100 kV using this method may be carried out by special arrangement.

Instrument Transformer Testing Sets

Instrument transformer testing sets (current, voltage or combined) can be calibrated with typical least uncertainty in the reported results of 3 ppm for ratio error and 0.000 3 crad for phase displacement.

The NATA-recommended calibration interval for current transformer testing sets can be extended from 5 years to 10 years, by use of a special adjustable current transformer. The testing set is used to measure the errors of this current transformer and the results are used to assess the calibration of the testing set. The transformer, which has settable current errors and phase displacements, is available for hire, for this purpose, or the current transformer testing set may be calibrated at NMI, using this transformer.