Instrumentation & Measurement Magazine 24-3 - 13

The VT calibrations are performed at voltages up to 500
kV in-house and up to 250 kV on-site. The type B uncertainties of current-comparator-based VT calibrations are better
than 10 μV/V, at power frequencies. The CT calibrations are
performed at currents up to 60 kA in-house and up to 2 kA
on-site. The type B uncertainties of current-comparator-based
CT calibrations are better than 2 μA/A for transformers with
errors below 100 μA/A and lower than 6 μA/A for transformers with errors above 100 μA/A, at power frequencies. These
calibrations are compliant with the ISO/IEC Standard 17025.
At higher frequencies and significantly smaller currents, the
CT calibrations are performed using audio-frequency current
comparators for frequencies up to 16 kHz developed at the
NRC [9], which have uncertainties within a few parts in 106.
The type B uncertainties (k = 2) of VT/CT calibrations based
on digital sampling are better than 100 μV/V or μA/A, at
frequencies up to a few kilohertz. As the current generating capability decreases with frequency increase, the test currents on
the order of 100 A-200 A can be generated at frequencies of up
to 6 kHz or 10 kHz, depending on the dimensions of the CT under test and the test setup.
Calibrations of VTs at higher frequencies represent larger
challenges since the voltage generating capability decreases
rapidly as the frequency increases. The test voltage is generated by an arbitrary waveform generator in conjunction with
a high-voltage amplifier directly or with a power amplifier
driving a voltage step-up transformer. Low-loss gas-dielectric capacitors serve as references in the calibrations, and their
capacitive currents increase proportionally with frequency.
These currents increase further in calibrations of Capacitor
(Coupled) Voltage Transformers or resistive-capacitive dividers, which add significant capacitive burdens. The attainable
test voltages are reduced by capacitive currents loading the
source. Although the upper test voltage limits vary with the
available voltage generators, test voltages on the order of 20 kV
are achieved at frequencies of several kilohertz.

Fig. 2. Frequency calibration of a high-voltage VT by a digital capacitance
bridge.
May 2021

The calibration of a high-voltage VT in the presence of
harmonics is performed as shown in Fig. 2. Two low-loss
gas-dielectric capacitors (a high-voltage capacitor CH and a
standard capacitor C S), two current-comparator-based active voltage dividers (AVDs) with feedback capacitors (CF),
a digital sampling system (DSS), and a computer, collecting
sampled data and performing processing (DSP), form a digital capacitance bridge similar to that in [11], which operates
with in-phase and quadrature uncertainties of less than 50
μV/V under high-voltage conditions. Another system based
on digital sampling for determining frequency response of
medium-voltage VTs under actual waveforms with harmonic
frequencies on the order of 10 kHz is presented in [12].

Calibration System for Instruments
with Harmonic-measurement
Capability
A simplified block diagram of the NRC system for calibration
of power analyzers under nonsinusoidal conditions with implemented recent improvements [2] is shown in Fig. 3. In it,
AWG represents a programmable arbitrary waveform generator; VA-a voltage amplifier; TCA-a transconductance
amplifier; VT-a (step-up) voltage transformer; CT-a (stepdown) current transformer; R-an ac current shunt; T-a
special high-accuracy voltage transformer for referencing the
voltage across the shunt to the ground potential; DUT-a device under test that has voltage and current inputs, V and I,
respectively; VD-a voltage divider; B1 and B2-buffer-amplifiers isolating the voltage divider and shunt output voltages
from loading; DS1 and DS2-digital samplers; GPS-a Global
Positioning System (GPS) receiver (or other high-accuracy frequency source) providing a reference frequency of, usually, 10
MHz; SG1 and SG2-signal generators providing reference
clock and sampling (Smp) signals, at necessary voltage levels,
for the digital samplers.
A programmable two-channel arbitrary waveform generator, AWG, with high short-term stability of output voltages
generates the low voltage test signals that are amplified and
converted into test voltage and current.
Two wideband voltage amplifiers are used as VA, one for
voltages up to 120 V, and the other up to 600 V. Both have implemented error feed-forward technique. The error is derived
from the deviations of the amplifier input and output voltages
from the ratio set by a resistive voltage divider with shielded
and phase-compensated resistors. The error is then fed forward to the output of the voltage amplifier by means of an
electronic amplifier driving a voltage injection transformer
connected to the voltage amplifier output.
A wideband custom-built transconductance amplifier,
TCA, is used for currents up to 1 A and 5 A. For currents up to
20 A and 100 A, two commercial TCAs that can operate at frequencies up to and above 10 kHz are used.
Two step-up voltage transformers are used as VT. One is a
high-accuracy two-stage VT 120 V/240 V with a load current
compensation. The other is a single-stage multiratio VT 120 V/
120 V-240 V-360 V-480 V-600 V with a stable ratio.

IEEE Instrumentation & Measurement Magazine 13

Instrumentation & Measurement Magazine 24-3

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