Datasheet

REV. 0–24–
ADE7754
INSTANTANEOUS REACTIVE
POWER SIGNAL – p(t)
MULTIPLIER
REACTIVE POWER
SIGNAL – P
I
V
HPF
1
24
LPF
28
–89
Figure 32. Reactive Power Signal Processing
TOTAL REACTIVE POWER CALCULATION
The sum of the reactive powers coming from each phase gives
the total reactive power consumption. Different combinations
of the three phases can be selected in the sum by setting Bits 7
to 6 of the WATMode register (mnemonic WATMOD[1:0]).
Each term of the formula can be disabled or enabled by the
LWATSEL bits of the WATMode register. Note that in this
mode, the LWATSEL bits are also used to select the terms of
the LVAENERGY register. The different configurations are
described in Table III.
The accumulation of the reactive power in the LAENERGY
register is different from the accumulation of the active power in
the LAENERGY register. Under the same signal conditions
(e.g., current and voltage channels at full scale), and if the accu-
mulation of the active power with PF = 1 over one second is
Wh
1
, and the accumulation of the reactive power with PF = 0
during that time is VARh
1
, then Wh
1
= 9.546  VAR
1
.
Note that I
A
*, I
B
*, and I
C
* represent the current channels
samples after APGAIN correction, high-pass filtering, and –89º
phase shift in the case of reactive energy accumulation.
Reactive Energy Accumulation Selection
The ADE7754 accumulates the total reactive power signal in
the LAENERGY register for an integer number of half cycles,
as shown in Figure 31. This mode is selected by setting Bit 5 of
the WAVMode register (Address 0Ch) to Logic 1. When this bit
is set, the accumulation of the active energy over half line cycles
in the LAENERGY register is disabled and done instead in the
LVAENERGY register. In this mode, the accumulation of the
apparent energy over half line cycles in the LVAENERGY is no
longer available. See Figure 33.
ACTIVE POWER
REACTIVE POWER
APPARENT POWER
LAENERGY
REGISTER
LVAENERGY
REGISTER
BIT 5 WAVMODE
REGISTER
0
1
0
1
Figure 33. Selection of Reactive Energy Accumulation
The features of the reactive energy accumulation are the same as
for the line active energy accumulation: each one of three phases
zero-crossing detection can contribute to the accumulation of
the half line cycles. Phase A, B, and C zero crossings, respec-
tively, are taken into account when counting the number of half
line cycles by setting to Logic 1 Bits 4 to 6 of the MMODE
register. Selecting phases for the zero-crossing counting also has
the effect of enabling the zero-crossing detection, zero-crossing
timeout, and period measurement for the corresponding phase
as described in the Zero-Crossing Detection section.
The number of half line cycles is specified in the LINCYC
register. LINCYC is an unsigned 16-bit register. The ADE7754
can accumulate active power for up to 65535 combined half
cycles. At the end of an energy calibration cycle, the LINCYC
flag in the interrupt status register is set. If the LINCYC enable
bit in the interrupt enable register is set to Logic 1, the IRQ
output also goes active low. Thus the IRQ line can also be used
to signal the end of a calibration.
As explained in the Reactive Power Calculation section, the
purpose of the reactive energy calculation in the ADE7754 is
not to give an accurate measurement of this value but to provide
the sign of the reactive energy. The ADE7754 provides an accu-
rate measurement of the apparent energy. Because the active
energy is also measured in the ADE7754, a simple mathemati-
cal formula can be used to extract the reactive energy. The
evaluation of the sign of the reactive energy makes up the calcu-
lation of the reactive energy.
Reactive
Reactive
Energy
sign Power Apparent Energy Active Energy
=
×−()
22
APPARENT POWER CALCULATION
Apparent power is defined as the maximum active power that
can be delivered to a load.
Vrm
s and
Irm
s are the effective voltage
and current delivered to the load; the apparent power (AP) is
defined as V
rms
× I
rms
.
Note that the apparent power is equal to the multiplication of
the rms values of the voltage and current inputs. For a polyphase
system, the rms values of the current and voltage inputs of each
phase (A, B, and C) are multiplied to obtain the apparent power
information of each phase. The total apparent power is the sum
of the apparent powers of all the phases. The different solutions
available to process the total apparent power are discussed below.
Figure 34 illustrates the signal processing in each phase for the
calculation of the apparent power in the ADE7754.
CURRENT RMS SIGNAL – i(t)
0.5V/GAIN1
VOLTAGE RMS SIGNAL –v(t)
0.5V/GAIN2
MULTIPLIER
APPARENT POWER
SIGNAL – P
I
rms
V
rms
00h
00h
D1B71h
AVAG
12
1CF68Ch
1CF68Ch
24
24
24
Figure 34. Apparent Power Signal Processing
The apparent power is calculated with the current and voltage rms
values obtained in the rms blocks of the ADE7754. Figure 3
5
shows the maximum code (hexadecimal) output range of the
apparent power signal for each phase. Note that the output
range changes depending on the contents of the apparent power
gain registers and also on the contents of the active power gain