Semi-Contactless Power Measurement Method for Single-Phase Enclosed Two-Wire Residential Entrance Lines

This article presents a semi-contactless power measurement method aimed at offering power measurements of residential homes, which are power supplied by the single-phase two-wire entrance lines, for studying the nonintrusive load monitoring (NILM) technique. The active power is calculated using the waveforms of the line current and voltage. To measure the line current, a magnetic sensor array consisting of seven tunnel magnetoresistance (TMR) sensors is utilized to detect the magnetic fields around the entrance lines. The single-phase time-domain current is then reversely calculated on real time by matrix multiplication using the time-domain output signals of the TMR sensor array once the entrance line positions are identified through solving the formulated nonlinear least square (NLLS) problem. In terms of the line voltage, a capacitively coupling voltage sensor is proposed to achieve almost in-phase tracking with the line voltage waveform. The voltage ratio of the voltage senor is calibrated using the measured rms value of line voltage from the outlet sockets or power strips in situ by a multimeter (thus called semi-contactless) due to the main disadvantage for capacitively coupling voltage sensing. That is, accuracy in voltage amplitude measurement is low because of the great difficulty in accurately determining the coupling capacitances. Numerical integration method for calculating the active power based on quasi-synchronous algorithm is proposed to eliminate the errors caused by the noninteger period sampling. Prototype instrument is designed and fabricated. Frequency response characterization shows that the TMR sensor (voltage sensor) has an almost constant gain factor (voltage ratio) and an approximate zero-phase shift (<1 degrees) over a frequency range of 50-3.05 kHz. Laboratory tests for both fundamental current phasor and active power measurement along with their measurement uncertainty evaluations are first conducted to verify the feasibility of the proposed method. Field tests in a residential home are subsequently performed to further demonstrate the effectiveness of the proposed method. A power measurement error of less than 5% can be obtained, which is sufficient for home energy tracking.