ADE9178 datasheet(46/122 Pages) AD

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Data Sheet

ADE9178

THEORY OF OPERATION

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Rev. A | 46 of 122

The output is given as three registers ANGL_AV_BV,

ANGL_AV_CV, and ANGL_BV_CV for voltage, three regis-

ters ANGL_AI_BI, ANGL_AI_CI, and ANGL_BI_CI for current

and another three registers ANGL_AV_AI, ANGL_BV_BI, and

ANGL_CV_CI angle between voltage and current of corresponding

phases. All angle registers only update when zero crossings are

present. Therefore, if the two relevant phases of a register drop

out, then both zero crossings disappear, and the value in the

angle register remains at the last known value. If only one of the

two relevant phases drop out, then the angle register continues

to update but the value increases indefinitely. This is due to the

nature in which angles are calculated in the ADE9178. Angles

are calculated from time between zero crossings, so if one phase

zero crossing is not present, then the time between zero crossings

increases indefinitely.

To ensure the angle results are valid, monitor the relevant RMS

of One Cycle registers on each phase (voltage and current). If the

RMSONE registers for a particular phase drops out, ignore any

angle output registers related to that phase.

Use the following equation to convert the register to angle in

radians:

Anglerad =2πANGLXY×256

xPERIOD+1

−2π

(33)

For example, for ANGL_AV_BV this equation gives the angle by

which phase B voltage lags phase A voltage expressed as a

negative value. All values are between 0 and −2π.

In Equation 33, xPERIOD corresponds to the period of selected

channels. There is no period register available for current output

and corresponding voltage period values can be used for angle

calculation. If the frequency of the signal is known, the following

formula can also be used to convert angle registers to radians:

Anglerad =2πANGLXY×fLINE

4000×28

−2π (34)

For more details, see the Worked Examples section and refer to the

Calibration and Conversion excel for the calculator, available on the

product webpage.

MEASUREMENTS ONLY FOR VOLTAGE

CHANNELS

Line Period Calculation

The ADE9178 calculates period using zero crossings on voltage

channels and combined voltage channels described in the Zero-

Crossing Detection section. The line period, tL can be calculated

from the xPERIOD register according to the following equation:

tL=xPERIOD+1

4000×216

(35)

If the calculated period value is outside the range of 40 Hz to 70

Hz, or if the negative to positive zero crossings for that phase are

not detected, the xPERIOD register is coerced to correspond to 50

Hz or 60 Hz, according to the setting of the SELFREQ bit in the

CONFIG0 register. With SELFREQ = 0 for a 50 Hz network, xPER-

IOD register is coerced to 0x0050 0000. If SELFREQ = 1, which

indicates a 60 Hz network, the xPERIOD register is coerced to

0x0042 AAAA. PERIOD_AVG_CFG bits in CONFIG0 register con-

figure the level of averaging performed on the period measurement.

It is recommended to configure CONFIG0:PERIOD_AVG_CFG[4:3]

to 0x2 to average the period over 16 samples. This reduces the

period jitter.

Phase-Sequence Error Detection

The ADE9178 monitors the zero crossings of the voltage signals

and raises sequence error if they are not in expected sequence.

If SEQ_CYC is set to 0 or 1, the ADE9178 flags sequence error

after 1 zero crossing errors. If SEQ_CYC is set to more than 1,

the ADE9178 flags sequence error only after getting SEQ_CYC

consecutive zero-crossing errors.

After applying VCONSEL, the expected sequence of voltage sig-

nals for a 3-phase system is shown in Figure 65.

Figure 65. Regular Succession of Zero-Crossing Events: Phase A, Phase B,

and Phase C

Figure 66 shows a sequence with phase-sequence error.

Figure 66. SEQERR Bit Set to 1 When Phase A Voltage is Followed by Phase

C Voltage

Note that phase-sequence error also happens if one of the voltage

channels are missing as shown in Figure 67.

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