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AN-750
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: 781/329-4700 • Fax: 781/326-8703 • www.analog.com
INTRODUCTION
The ADE7758 is a polyphase, multifunction energy
metering IC that performs kWh, kVAh, kVARh, and rms
current and rms voltage measurements. This application
note describes how phase dropout affects VAR calcula-
tion in the ADE7758, the solution to detect phase dropout
with the ADE7758, and how to avoid VAR errors.
PHASE DROPOUT AND THE ADE7758
The ADE7758 measures the line frequency of one phase
at a time. If a phase drops out, the line frequency
measurement on that phase can be incorrect due to
offset or noise causing erroneous zero-crossing detection.
Compensation for reactive power calculation is based
on the line frequency measurement of one phase. If the
phase used for frequency measurement drops out, this
can induce error to the reactive power calculation of the
other phases. To solve this problem, the meter needs
to detect the phase dropout and switch the frequency
measurement to an active phase. The phase used for
frequency measurement can be changed using Bit 0 to
Bit 1 in the measurement mode register (MMODE [7:0],
Address 0x14). Table I shows the setting of the two
bits, MMODE [1: 0], for different phase frequency
measurement.
Table I. Phase Selection for Frequency Measurement
MMODE [1] MMODE [0] Phase
0 0 A
0 1 B
1 0 C
VAR CALCULATION
Reactive power is dened as the product of the voltage
and current waveforms when one of these signals is
phase shifted by 90°. There is a phase-shift lter in the
ADE7758 that introduces a 90° phase shift in the current
channel of the reactive energy datapath. The magnitude
and phase response of the phase-shift lter is shown
in Figure 1 and Figure 2, respectively. Because the
ADE7758 Phase Dropout Detection for VAR Calculation
by Rachel Kaplan
phase-shift lter has a nonunity magnitude response, a
compensation based on line frequency is implemented
to achieve accurate reactive power calculation around
fundamental frequency.
40 706560555045
5
–1
0
1
2
3
4
FREQUENCY (Hz)
MAGNITUDE (dB)
Figure 1. Magnitude Response
40 706560555045
89.80
90.10
90.05
90.00
89.95
89.90
89.85
FREQUENCY (Hz)
PHASE (Degrees)
Figure 2. Phase Response
ZERO-CROSSING TIMEOUT
The ADE7758 measures the line frequency using zero-
crossing detection circuits. Refer to the Zero-Crossing
Detection section in the data sheet for more details
about zero-crossing detection.
REV. 0
