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File: LIQ User Manual V2.0
Page 12 of 37
2.4.2 LIQ-60 Sensing
The LIQ-60 measures conductor current, conductor voltage, ambient and conductor temperature, and
calculates Displacement Power Factor.
Current
Current is sensed with a Rogowski coil, which is automatically closed around the conductor during the
installation process. The inherent nature of a Rogowski coil (which does not have a magnetic core)
means that the coil does not saturate, allowing the LIQ-60 to measure currents over a wide dynamic
range, from 1 A up to 25 kA.
Voltage
LIQ-60 measures voltage by sensing the capacitive current which flows between the LIQ-60 body and
ground/earth. This capacitance varies with atmospheric conditions (temperature, humidity, air pressure,
rain, snow etc.) and also with the physical arrangement of the LIQ-60 and nearby grounded/earthed
structures and energized conductors. This means that it is not possible to make high-accuracy voltage
measurements. The LIQ-60 therefore scales the voltage measurements so that a reading of 100%
corresponds to the normal energized conductor voltage.
Temperature
Conductor and ambient temperatures are sensed using platinum resistance thermometers (PRT). One
PRT is automatically placed in contact with the overhead conductor when the LIQ-60 is i n s t a l l e d . Ambient
temperature is measured using a shaded PRT mounted on the upper part of the LIQ-60 housing.
Temperatures are sampled regularly, then averaged and recorded at the Log Average Interval.
Power Factor
The voltage and current data are low pass filtered to eliminate harmonic information, and then the zero
crossings of the voltage and current signals are detected and processed over a one minute period. This
allows calculation of the phase angle of the current with respect to the voltage. The sign of phase angle
measurements are taken into account, so that the measurements can be assigned to the correct quadrant.
Displacement Power Factor (DPF) is calculated from the measured phase angle,
φ
, where DPF = cos (
φ
).
For voltages and currents with low distortion, as is normally the case on high voltage systems, this
calculated DPF is a good approximation to the True Power factor (TPF).
Power factor measurements are averaged and recorded at the Log Average Interval. Recorded data
includes phase angle, sign (leading or lagging) and DPF.
See section
2.4.5 Power Factor Calculation
for a more detailed description of the power factor
calculations.