ade7880 Analog Devices, Inc., ade7880 Datasheet - Page 48

• Manufacturer Part Number: ade7880
• Description: Polyphase Multifunction Energy Metering Ic With Harmonic Monitoring
• Manufacturer: Analog Devices, Inc.

ADE7880
Φ
Between −180 to 0
1
Fundamental Reactive Energy Calculation
Fundamental reactive energy is defined as the integral of
fundamental reactive power.
The fundamental reactive energy accumulation is always a
signed operation. Negative energy is subtracted from the
reactive energy contents.
Similar to active power, the ADE7880 achieves the integration
of the reactive power signal in two stages (see Figure 52).
•
•
Figure 52 explains this process. The threshold is formed by
concatenating the VARTHR 8-bit unsigned register to 27 bits
equal to 0. It is introduced by the user and is common for total
active and fundamental powers on all phases. Its value depends
on how much energy is assigned to one LSB of var-hour registers.
Supposing a derivative of a volt ampere reactive hour (varh) [10
varh] where n is an integer, is desired as one LSB of the VARHR
register, the VARTHR register can be computed using the
following equation:
where:
PMAX =27,059,678=0x19CE5DE, the instantaneous power
computed when the ADC inputs are at full scale.
f
power computed by the DSP at 8KHz is accumulated.
U
the ADC inputs are at full scale.
VARTHR register is an 8-bit unsigned number, so its maximum
value is 2
S
signal; that is, Φ is positive if the load is inductive and negative if the load is
capacitive.
Φ is defined as the phase angle of the voltage signal minus the current
FS
1
= 1.024MHz, the frequency at which every instantaneous
, I
Reactive Energy = ∫q(t)dt
The first stage accumulates the instantaneous phase
fundamental reactive power at 1.024MHz, although they
are computed by the DSP at 8KHz rate. Every time a
threshold is reached, a pulse is generated and the threshold
is subtracted from the internal register. The sign of the
energy in this moment is considered the sign of the
reactive power (see the Sign of Fundamental Reactive
Power Calculation section for details).
The second stage consists in accumulating the pulses
generated after the first stage into internal 32-bit
accumulation registers. The content of these registers is
transferred to the var-hour registers (xFVARHR) when these
registers are accessed. AFWATTHR, BFWATTHR, and
CFWATTHR represent phase fundamental reactive energies.
VARTHR
FS
are the rms values of phase voltages and currents when
8
– 1. Its default value is 0x3. Values lower than 3, that
=
PMAX
U
Sign of Reactive Power
Negative
FS
×
×
f
s
I
×
FS
3600
×
2
27
×
10
n
Rev. PrE | Page 48 of 103
(36)
(37)
n
is 2 or 1 should be avoided and 0 should never be used as the
threshold must be a non-zero value.
This discrete time accumulation or summation is equivalent to
integration in continuous time following the expression in
Equation 37:
where:
n is the discrete time sample number.
T is the sample period.
On the ADE7880, the fundamental phase reactive powers are
accumulated in the AFVARHR, BFVARHR, and CFVARHR 32-
bit signed registers. The reactive energy register content can roll
over to full-scale negative (0x80000000) and continue
increasing in value when the reactive power is positive.
Conversely, if the reactive power is negative, the energy register
underflows to full-scale positive (0x7FFFFFFF) and continues
to decrease in value.
The ADE7880 provides a status flag to signal when one of the
xFVARHR registers is half full. Bit 3 (FREHF) in the STATUS0
register is set when Bit 30 of one of the xFVARHR registers
changes, signifying one of these registers is half full. If the reactive
power is positive, the var-hour register becomes half full when it
increments from 0x3FFF FFFF to 0x4000 0000. If the reactive
power is negative, the var-hour register becomes half full when it
decrements from 0xC000 0000 to 0xBFFF FFFF.
Setting Bit3 in the MASK0 register enables the FREHF
interrupt. If enabled, the IRQ0 pin is set low and the status bit is
set to 1 whenever one of the energy registers, xFVARHR,
becomes half full. The status bit is cleared and the
set to high by writing to the STATUS0 register with the
corresponding bit set to 1.
Setting Bit 6 (RSTREAD) of the LCYCMODE register enables a
read-with-reset for all var-hour accumulation registers, that is,
the registers are reset to 0 after a read operation.
Integration Time Under Steady Load
The discrete time sample period (T) for the accumulation register
is 976.5625ns (1.024MHz frequency). With full-scale sinusoidal
signals on the analog inputs and a 90° phase difference between
the voltage and the current signal (the largest possible reactive
power), the average word value representing the reactive power
is PMAX =27,059,678=0x19CE5DE. If the VARTHR threshold
is set at 3, its minimum recommended value, the first stage
accumulator generates a pulse that is added to var-hour
registers every
ReactiveEn
PMAX
ergy
Preliminary Technical Data
=
3
∫
×
×
q
. 1
2
( )
t
024
27
dt
×
=
10
Lim
T
6
→
0
=



14
n
∑
∞
=
0
.
q
531
( )
nT
µ
sec
×
IRQ0 pin is
T
.



(38)