ADE7932 datasheet(59/120 Pages) AD

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Data Sheet

ADE7978/ADE7933/ADE7932

Rev. 0 | Page 59 of 120

SIGN OF REACTIVE POWER CALCULATION

The reactive power is a signed calculation. Table 24 summarizes

the relationship between the phase difference between the voltage

and the current and the sign of the resulting reactive power

calculation.

Table 24. Sign of Reactive Power Calculation

Φ1

Sign of Reactive Power

From 0 to +180

Positive

From −180 to 0

Negative

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.

The ADE7978 has sign detection circuitry for reactive power

calculations; this circuitry can monitor the total reactive powers

or the fundamental reactive powers. As described in the Reactive

Energy Calculation section, the reactive energy accumulation is

performed in two stages. Each time a sign change is detected in

the energy accumulation at the end of the first stage—that is, after

the energy accumulated in 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

Bit 1 (REVRPSEL) in the MMODE register (Address 0xE700)

specifies the type of reactive power that is monitored. When

REVRPSEL is cleared to 0 (the default value), the total reactive

power is monitored. When REVRPSEL is set to 1, the funda-

mental reactive power is monitored.

Bits[12:10] (REVRPC, REVRPB, and REVRPA) in the STATUS0

by Bit 1 (REVRPSEL) in the MMODE register.

Bits[6:4] (CVARSIGN, BVARSIGN, and AVARSIGN) in the

PHSIGN register are set simultaneously with the REVRPC,

REVRPB, and REVRPA bits in the STATUS0 register. The

xVARSIGN bits indicate the sign of the reactive power. When

these bits are set to 0, the reactive power is positive. When the

bits are set to 1, the reactive power is negative.

The REVRPx bits in the STATUS0 register and the xVARSIGN

bits in the PHSIGN register refer to the reactive power of

Phase x and the power type selected by Bit 1 (REVRPSEL) in

the MMODE register.

Interrupts attached to Bits[12:10] (REVRPC, REVRPB, and

REVRPA) in the STATUS0 register can be enabled by setting

Bits[12:10] in the MASK0 register. When enabled, the IRQ0 pin

goes low and the status bit is set to 1 when a change of sign occurs.

To identify the phase that triggered the interrupt, read the PHSIGN

bit is cleared and the IRQ0 pin is returned high by writing a 1 to

the appropriate bits in the STATUS0 register.

REACTIVE ENERGY CALCULATION

Reactive energy is defined as the integral of reactive power.

Reactive Energy =

∫

q(t)

(37)

Similar to active power, the ADE7978 achieves the integration

of the reactive power signal in two stages (see Figure 79). The

process is identical for total reactive power and fundamental

reactive power.

The first stage accumulates the instantaneous phase total or

fundamental reactive power at 1.024 MHz (the DSP computes

these values at an 8 kHz rate). Each time a threshold is reached,

a pulse is generated, and the threshold is subtracted from the

internal register. The sign of the energy at this moment is con-

sidered the sign of the reactive power (see the Sign of Reactive

Power Calculation section for more information).

The second stage consists of accumulating the pulses generated

at the first stage into internal 32-bit accumulation registers. The

contents of these registers are transferred to the var-hour registers

(xVARHR and xFVARHR) when these registers are accessed.

AVARHR, BVARHR, CVARHR, AFVARHR, BFVARHR, and

CFVARHR represent phase total and fundamental reactive

powers. Figure 79 shows this process.

INTERNAL

ACCUMULATOR

VARACC BITS IN

ACCMODE[7:0]

THRESHOLD

DIGITAL SIGNAL PROCESSOR

TOTAL

REACTIVE

POWER

ALGORITHM

APGAIN

AVAROS

AVAR

VARTHR

27 26

32-BIT REGISTER

AVARHR[31:0]

REVRPA BIT IN

STATUS0[31:0]

CURRENT SIGNAL

FROM HPF

VOLTAGE SIGNAL

FROM HPF

Figure 79. Total Reactive Energy Accumulation

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