ADE7169F16 datasheet(57/140 Pages) AD

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Preliminary Technical Data

ADE7169F16

Rev. PrD | Page 57 of 140

q(t) = VI sin (θ) + VI sin

)

(

ωt

The average reactive power over an integral number of lines (n)

is given in Equation 26.

∫

)

sin(

)

(

(26)

where:

T is the line cycle period.

q is referred to as the reactive power.

Note that the reactive power is equal to the dc component of the

instantaneous reactive power signal q(t) in Equation 25. This is

the relationship used to calculate reactive power in the

ADE7169F16. The instantaneous reactive power signal q(t) is

generated by multiplying Voltage and Current channels. In this

case, the phase of Current channel is shifted by +90°. The dc

component of the instantaneous reactive power signal is then

extracted by a low-pass filter in order to obtain the reactive

power information – see Figure 45.

In addition, the phase shifting filter has a non-unity magnitude

response. Because the phase-shift filter has a large attenuation at

high frequency, the reactive power is primarily for the

calculation at line frequency. The effect of harmonics is largely

ignored in the reactive power calculation. Note that because of

the magnitude characteristic of the phase shifting filter, the

weight of the reactive power is slightly different from the active

power calculation – see Energy register scaling.

The frequency response of the LPF in the reactive signal path is

identical to that of the LPF2 used in the average active power

calculation. Since LPF2 does not have an ideal “brick wall”

frequency response—see Figure 38, the reactive power signal

has some ripple due to the instantaneous reactive power signal.

This ripple is sinusoidal and has a frequency equal to twice the

line frequency. Because the ripple is sinusoidal in nature, it is

removed when the reactive power signal is integrated to

calculate energy—see the Reactive Power Calculation section.

The reactive power signal can be read from the waveform

the WFSM bit in the Interrupt Enable Register 3 SFR

(MIRQENH, 0xDB). Like the current and voltage channels

waveform sampling modes, the waveform date is available at

sample rates of 27.9 kSPS, 14 kSPS, 7 kSPS, or

3.5 kSPS.

Reactive power gain calibration

Figure 45 shows the signal processing chain for the reactive

power calculation in the ADE7169F16. As explained, the

reactive power is calculated by low-pass filtering the

instantaneous reactive power signal. Note that when reading the

waveform samples from the output of LPF2, the gain of the

reactive energy can be adjusted by using the multiplier and var

gain register (VARGAIN[11:0]). The gain is adjusted by writing

a twos complement 12-bit word to the var gain register.

Equation 11 shows how the gain adjustment is related to the

contents of the watt gain register:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎭

⎬

⎫

⎩

⎨

⎧ +

VARGAIN

Power

active

VARGAIN

Output

(11)

The resolution of the VARGAIN register is the same as the

WGAIN register – see Active power gain calibration section.

VARGAIN can be used to calibrate the reactive power (or

energy) calculation in the ADE7169F16.

Reactive power offset calibration

The ADE7169F16 also incorporates a reactive power offset

bit register that can be used to remove offsets in the reactive

power calculation—see Figure 45. An offset could exist in the

reactive power calculation due to crosstalk between channels on

the PCB or in the IC itself. The offset calibration allows the

contents of the reactive power register to be maintained at 0

when no power is being consumed.

The 256 LSBs (VAROS = 0x100) written to the reactive power

offset register are equivalent to 1 LSB in the waveform sample

Sign of Reactive Power Calculation

Note that the average reactive power is a signed calculation. The

phase shift filter has –90° phase shift when the integrator is

enabled, and +90° phase shift when the integrator is disabled.

Table 41 summarizes the relationship between the phase differ-

ence between the voltage and the current and the sign of the

resulting VAR calculation.

Table 41. Sign of Reactive Power Calculation

Angle

Integrator

Sign

Between 0° to 90°

Off

Positive

Between –90° to 0°

Off

Negative

Between 0° to 90°

Positive

Between –90° to 0°

Negative

Reactive power sign detection

The ADE7169F16 detects a change of sign in the reactive power.

The VARSIGN flag in the Interrupt Status Register 1 SFR

(MIRQSTL, 0xDC) record when a change of sign according to

bit VARSIGN in the ACCMODE register (0x0F) has occurred.

If the VARSIGN bit is set in the Interrupt Enable Register 1 SFR

(MIRQENL, 0xD9), the 8052 core has a pending ADE

interrupt. The ADE interrupt stays active until the VARSIGN

status bit is cleared—see Energy measurement interrupts

section.

When VARSIGN in the ACCMODE register (0x0F) is cleared

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