ADE7858ACPZ datasheet(39/76 Pages) AD

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Preliminary Technical Data

ADE7858

Rev. PrA | Page 39 of 76

The sign of each phase reactive power may be read in

PHSIGN[15:0] register.

Bits 12, 11, 10 (REVRPC, REVRPB and respectively REVRPA)

in STATUS0[31:0] are set when a sign change occurs in the total

reactive power.

Bits 6, 5, 4 (CVARSIGN, BVARSIGN and respectively

AVARSIGN) in PHSIGN[15:0] register are set simultaneously

with REVRPC, REVRPB and REVRPA bits. They indicate the

sign of the reactive power. When they are 0, the reactive power

is positive. When they are 1, the reactive power is negative.

Bit REVRPx of STATUS0[31:0] and bit xVARSIGN in

PHSIGN[15:0] refers to the reactive power of phase x, x=A,B or

Setting bits 12, 11, 10 in MASK0[31:0] register enables

REVRPC, REVRPB and REVRPA interrupts respectively. If

enabled, the

IRQ 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, PHSIGN[15:0] register is read

immediately after reading STATUS0[31:0]. Then the status bit is

cleared and

IRQ pin is set back high by writing STATUS0

Table 14. Sign of Reactive Power Calculation

Φ1

Integrator

Sign of Reactive Power

Between 0 to +90

Off

Positive

Between −90 to 0

Off

Negative

Between 0 to +90

Positive

Between −90 to 0

Negative

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.

Reactive Energy Calculation

Reactive energy is defined as the integral of reactive power.

dt

Energy

Reactive





(34)

Total reactive energy accumulation is always a signed operation.

Negative energy is subtracted from the reactive energy contents.

Similar to active power, the ADE7858 achieves the integration

of the reactive power signal in two stages (see Figure 45). The

first stage is done inside the DSP: every 125μsec (8KHz

frequency), the instantaneous phase total reactive power is

accumulated into an internal register. When a threshold is

reached, a pulse is generated at processor port 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 Sign of Reactive Power Calculation section for

details).The second stage is done outside the DSP and consists

in accumulating the pulses generated by the processor into

internal 32-bit accumulation registers. The content of these

registers is transferred to var-hr registers xVARHR[31:0], x=A,

B, C when these registers are accessed. AVARHR[31:0],

BVARHR[31:0] and CVARHR[31:0] represent the phase total

reactive powers.

Figure 42 from the Active Energy Calculation section explains

this process. The VARTHR[47:0] 48-bit signed register contains

the threshold and it is introduced by the user. Its value depends

on how much energy is assigned to 1LSB of var-hour registers.

Let’s suppose a derivative of varh [10n varh], n an integer, is

desired as 1LSB of VARHR. Then VARTHR may be computed

using the following expression:

3600

PMAX

VARTHR





where:

PMAX=33,516,139=0x1FF6A6B, the instantaneous power

computed when the ADC inputs are at full scale.

fs=8KHz is the frequency with which the DSP computes the

instantaneous power.

UFS, IFS are the rms values of phase voltages and currents when

the ADC inputs are at full scale.

The maximum value that may be written on VARTHR[47:0] is

247-1. The minimum value is 0x0, but it is recommended to

write a number equal or greater than PMAX. Negative numbers

should never be used.

The VARTHR[47:0] is a 48-bit register. As previously stated, the

serial ports of the ADE7858 work on 32, 16 or 8-bit words.

Similar to the WTHR[47:0] register presented in Figure 43,

VARTHR[47:0] is accessed as two 32-bit registers

(VARTHR1[31:0] and VARTHR0[31:0]), each having 8 most

significant bits padded with 0s.

This discrete time accumulation or summation is equivalent to

integration in continuous time following the description in

expression (34).































Lim

activeEner

(34)

where:

is the discrete time sample number.

is the sample period.

On the ADE7858, the total phase reactive powers are

accumulated in AVARHR[31:0], BVARHR[31:0] and

CVARHR[31:0] 32-bit signed registers. The reactive energy

and continue increasing in value when the reactive power is

positive. Conversely, if the reactive power is negative, the energy

and continue decreasing in value.

Bit 2 (REHF) in STATUS0[31:0] register is set when bit 30 of

one of xVARHR, x=A,B,C registers changes, signifying one of

these registers is half full. If the reactive power is positive, the

var-hr register becomes half full when it increments from

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