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Data Sheet
Sign of Fundamental Reactive Power Calculation
Note that the fundamental reactive power is a signed calculation.
ADE7880
Similar to active power, the ADE7880 achieves the integration
of the reactive power signal in two stages (see Figure 77).
between the voltage and the current and the sign of the resulting
reactive power calculation.
The ADE7880 has sign detection circuitry for reactive power
calculations that can monitor the fundamental reactive powers.
section, the reactive energy accumulation is executed in two
stages. Every time a sign change is detected in the energy
accumulation at the end of the first stage, that is, after the energy
?
?
The first stage accumulates the instantaneous phase
fundamental reactive power at 1.024 MHz, although they
are computed by the DSP at 8 kHz 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
Power Calculation section for details).
The second stage consists in accumulating the pulses
PMAX × f s × 3600 × 10 n
U FS × I FS × 2 27
accumulated into the internal accumulator reaches the VARTHR
register threshold, a dedicated interrupt is triggered. The sign of
each phase reactive power can be read in the PHSIGN register.
Bits[12:10] (REVFRPC, REVFRPB, and REVFRPA, respect-
tively) in the STATUS0 register are set when a sign change
occurs in the fundamental reactive power.
Bits[6:4] (CFVARSIGN, BFVARSIGN, and AFVARSIGN,
respectively) in the PHSIGN register are set simultaneously with
the REVFRPC, REVFRPB, and REVFRPA bits. They indicate the
sign of the fundamental reactive power. When they are 0, the
reactive power is positive. When they are 1, the reactive power
is negative.
Bit REVFRPx of the STATUS0 register and Bit xFVARSIGN in
the PHSIGN register refer to the reactive power of Phase x.
Setting Bits[12:10] in the MASK0 register enables the REVFRPC,
REVFRPB, and REVFRPA interrupts, respectively. If enabled,
the IRQ0 pin is set low and the status bit is set to 1 whenever a
change of sign occurs. To find the phase that triggered the
interrupt, the PHSIGN register is read immediately after read-
ing the STATUS0 register. Next, the status bit is cleared and the
IRQ0 pin is set to high by writing to the STATUS0 register with
the corresponding bit set to 1.
Table 17. Sign of Reactive Power Calculation
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.
concatenating the VARTHR 8-bit unsigned register to 27 bits
equal to 0 and it is introduced by the user. 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 n
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:
VARTHR =
(37)
where:
PMAX = 27,059,678 = 0x19CE5DE, the instantaneous power
computed when the ADC inputs are at full scale.
f S = 1.024 MHz, the frequency at which every instantaneous
power computed by the DSP at 8 kHz is accumulated.
U FS , I FS are the rms values of phase voltages and currents when
the ADC inputs are at full scale.
VARTHR register is an 8-bit unsigned number, so its maximum
Φ 1
Between 0 to +180
Between ?180 to 0
Sign of Reactive Power
Positive
Negative
value is 2 8 ? 1. Its default value is 0x3. Values lower than 3, that
is 2 or 1 should be avoided and 0 should never be used as the
threshold must be a non-zero value.
1
Φ is defined as the phase angle of the voltage signal minus the current
signal; that is, Φ is positive if the load is inductive and negative if the load is
capacitive.
This discrete time accumulation or summation is equivalent to
integration in continuous time, shown in Equation 38:
ReactiveEn ergy = ∫ q ( t ) dt = Lim ? ∑ q ( nT ) × T ?
Fundamental Reactive Energy Calculation
Fundamental reactive energy is defined as the integral of
fundamental reactive power.
Reactive Energy = ∫ q(t)dt
The fundamental reactive energy accumulation is always a
(36)
? ∞ ?
T → 0 ? n = 0 ?
where:
n is the discrete time sample number.
T is the sample period.
(38)
signed operation. Negative energy is subtracted from the
reactive energy contents.
Rev. A | Page 51 of 104
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