Datasheet

ADE7854/ADE7858/ADE7868/ADE7878 Data Sheet
Rev. G | Page 52 of 100
ZERO-
CROSSING
DETECTION
(PHASE A)
ZERO-
CROSSING
DETECTION
(PHASE B)
CALIBR
A
TION
CONTROL
ZERO-
CROSSING
DETECTION
(PHASE C)
LINECYC[15:0]
AWATTHR[31:0]
ZXSEL[0] IN
LCYCMODE[7:0]
ZXSEL[1] IN
LCYCMODE[7:0]
ZXSEL[2] IN
LCYCMODE[7:0]
OUTPUT
FROM LPF2
A
WGAIN
AWATTOS
ACCUMULA
T
OR
WTHR[47:0]
32-BIT
REGISTER
08510-147
Figure 70. Line Cycle Active Energy Accumulation Mode
The line cycle energy accumulation mode is activated by setting
Bit 0 (LWATT) in the LCYCMODE register. The energy accu-
mulation over an integer number of half line cycles is written
to the watt-hour accumulation registers after LINECYC number
of half line cycles is detected. When using the line cycle
accumulation mode, the Bit 6 (RSTREAD) of the LCYCMODE
register should be set to Logic 0 because the read with reset of
watt-hour registers is not available in this mode.
Phase A, Phase B, and Phase C zero crossings are, respectively,
included when counting the number of half line cycles by setting
Bits[5:3] (ZXSEL[x]) in the LCYCMODE register. Any combi-
nation of the zero crossings from all three phases can be used
for counting the zero crossing. Select only one phase at a time
for inclusion in the zero crossings count during calibration.
The number of zero crossings is specified by the LINECYC 16-bit
unsigned register. The ADE78xx can accumulate active power
for up to 65,535 combined zero crossings. Note that the internal
zero-crossing counter is always active. By setting Bit 0 (LWATT)
in the LCYCMODE register, the first energy accumulation
result is, therefore, incorrect. Writing to the LINECYC register
when the LWATT bit is set resets the zero-crossing counter, thus
ensuring that the first energy accumulation result is accurate.
At the end of an energy calibration cycle, Bit 5 (LENERGY) in
the STATUS0 register is set. If the corresponding mask bit in
the MASK0 interrupt mask register is enabled, the
IRQ0
pin
also goes active low. The status bit is cleared and the
IRQ0
pin is
set to high again by writing to the STATUS0 register with the
corresponding bit set to 1.
Because the active power is integrated on an integer number of
half-line cycles in this mode, the sinusoidal components are
reduced to 0, eliminating any ripple in the energy calculation.
Therefore, total energy accumulated using the line cycle
accumulation mode is
( )
∑
∫
∞
=
+
==
1k
kk
nTt
t
IVnTdttpe
cos(φ
k
– γ
k
) (28)
where nT is the accumulation time.
Note that line cycle active energy accumulation uses the same
signal path as the active energy accumulation. The LSB size of
these two methods is equivalent.
REACTIVE POWER CALCULATION—ADE7858,
ADE7868, ADE7878 ONLY
The ADE7858/ADE7868/ADE7878 can compute the total
reactive power on every phase. Total reactive power integrates
all fundamental and harmonic components of the voltages and
currents. The ADE7878 also computes the fundamental reactive
power, the power determined only by the fundamental
components of the voltages and currents.
A load that contains a reactive element (inductor or capacitor)
produces a phase difference between the applied ac voltage and
the resulting current. The power associated with reactive elements
is called reactive power, and its unit is VAR. Reactive power is
defined as the product of the voltage and current waveforms when
all harmonic components of one of these signals are phase
shifted by 90°.
Equation 31 gives an expression for the instantaneous reactive
power signal in an ac system when the phase of the current
channel is shifted by +90°.
∑
∞
=
=
1
2)(
k
k
Vtv
sin(kωt + φ
k
) (29)
( )
k
k
k
γtω
kIti +
=
∑
∞
=
sin2
)(
1
(30)






++=
∑
∞
=
2
sin2)('
1
π
γtωkIti
k
k
k
where iʹ(t) is the current waveform with all harmonic
components phase shifted by 90°.
Next, the instantaneous reactive power, q(t), can be expressed as
q(t) = v(t) × iʹ(t) (31)
∑
∞
=
×=
1
2)(
k
kk
IVtq
sin(kωt + φ
k
) × sin(kωt + γ
k
+
2
π
) +
∑
∞
≠
=
mk
mk
m
k
IV
1,
× 2sin(kωt + φ
k
) × sin(mωt + γ
m
+
2
π
)