Development and calibration of an HV impulse reference divider
Development and calibration of an HV impulse reference divider requires high
technological skills, solid experience in HV measuring and testing techniques,
metrological competence, and high accuracy equipment.
For this reason, only a few companies in the world provide HV impulse reference dividers,
and very few laboratories are able to calibrate them. Also, only quite limited guidance is
given by international standards.
The paper reports the experience in the development and calibration of a 500 kV LI, 400
kV SI reference divider, manufactured by AME Srl for the RSE metrological laboratory,
accredited for calibration according to ISO/IEC 17025 and IEC 60060-2 standards. The
latter standard provides detailed indications for the calibration of approved measuring
systems used in test laboratories during the qualification of HV equipment (insulators,
transformers, disconnectors), but not much guidance is given for the calibration of a
reference divider. Only the requirements for overall uncertainty of the complete Reference
Measuring System (RMS) are provided, but no specifications are given for the individual
uncertainty contributions and related testing methods.
The development of the reference divider was performed taking into account all
metrological requirements. The components were chosen to ensure maximum thermal
stability, highest voltage linearity, and excellent dynamic behaviour. The design solutions
were chosen to minimize stray capacitance and inductance, and to reduce response and
settling time of the divider’s step response. Checks were made during the manufacturing
to find better design solutions and to optimize the values of circuital components.
After the first qualification tests performed in HV laboratories of RSE and INRiM (Italian
National Metrological Institute), the divider, together with the transmission system and the
measuring instrument, was calibrated by VTT MIKES, the Finnish National Metrology
Institute (NMI). The calibration was performed by comparison with a RMS with measuring
uncertainty = 0.5 % for test voltage (Ut) value measurement. This figure is achieved by
correcting the dynamic behaviour of the RMS using deconvolution techniques.
The paper presents the reference divider features, the methodology used by AME during
the development to obtain high metrological performance, and the contribution of INRiM
during the divider qualification tests for the determination of measurement uncertainty.
The paper reports also the main calibration results, together with instrumentation and
methodologies applied by VTT MIKES to obtain low uncertainties.
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