ADE7758 datasheet(35/68 Pages) AD

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ADE7758

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Phase A, Phase B, and Phase C zero crossings are, respectively,

included when counting the number of half-line cycles by

setting ZXSEL[0:2] bits (Bit 3 to Bit 5) in the LCYCMODE

phases can be used for counting the zero crossing. Only one

phase should be selected at a time for inclusion in the zero

crossings count during calibration (see the Calibration section).

The number of zero crossings is specified by the LINECYC

can accumulate active power for up to 65535 combined zero

crossings. Note that the internal zero crossing counter is always

active. By setting the LWATT bit, the first energy accumulation

result is therefore incorrect. Writing to the LINECYC register

when the LWATT bit is set resets the zero-crossing counter, thus

ensuring that the first energy accumulation result is accurate.

At the end of an energy calibration cycle, the LENERGY bit

(Bit 12) in the STATUS register is set. If the corresponding

mask bit in the interrupt mask register is enabled, the IRQ

output also goes active low; thus, the IRQ can also be used to

signal the end of a calibration.

Because active power is integrated on an integer number of half

line cycles in this mode, the sinusoidal component is reduced to

0. This eliminates any ripple in the energy calculation. Therefore,

total energy accumulated using the line-cycle accumulation

mode is

()

IRMS

VRMS

(14)

where t is the accumulation time.

Note that line cycle active energy accumulation uses the same

signal path as the active energy accumulation. The LSB size of

these two methods is equivalent. Using the line cycle accumula-

tion to calculate the kWh/LSB constant results in a value that can

be applied to the WATTHR registers when the line accumulation

mode is not selected (see the Calibration section).

REACTIVE POWER CALCULATION

A load that contains a reactive element (inductor or capacitor)

produces a phase difference between the applied ac voltage and

the resulting current. The power associated with reactive

elements is called reactive power and its unit is VAR. Reactive

power is defined as the product of the voltage and current

waveforms when one of these signals is phase shifted by 90°.

Equation 17 gives an expression for the instantaneous reactive

power signal in an ac system when the phase of the current

channel is shifted by +90°.

()

(

–

sin

)

(15)

()

⎟

⎠

⎞

⎜

⎝

⎛

′

sin

(16)

where V = rms voltage, I = rms current, θ = total phase shift

caused by the reactive elements in the load. Then the

instantaneous reactive power q(t) can be expressed as

( )

()

⎟

⎠

⎞

⎜

⎝

⎛

⎟

⎠

⎞

⎜

⎝

⎛

′

–

cos

–

cos

where ( )

i′

is the current waveform phase shifted by 90°. Note

that q(t) can be rewritten as

( )

( )(

)

–

sin

(17)

The average reactive power over an integral number of line

cycles (n) is given by the expression in Equation 18.

()

( )

∫

sin

(18)

where T is the period of the line cycle.

Q is referred to as the average reactive power. The instantaneous

reactive power signal q(t) is generated by multiplying the voltage

signals and the 90° phase-shifted current in each phase.

The dc component of the instantaneous reactive power signal in

each phase (A, B, and C) is then extracted by a low-pass filter to

obtain the average reactive power information on each phase.

This process is illustrated in Figure 71. The reactive power of

each phase is accumulated in the corresponding 16-bit VAR-

hour register (AVARHR, BVARHR, or CVARHR). The input to

each reactive energy register can be changed depending on the

accumulation mode setting (see Table 17).

The frequency response of the LPF in the reactive power signal

path is identical to that of the LPF2 used in the average active

power calculation (see Figure 65).

VRMS

× IRMS × sin(φ)

00000h

CURRENT

i(t) = 2

× IRMS × sin(ωt)

VOLTAGE

v(t) = 2

× VRMS × sin(ωt–θ)

INSTANTANEOUS

REACTIVE POWER SIGNAL

q(t) = VRMS

× IRMS × sin(φ) + VRMS × IRMS × sin(2ωt+θ)

AVERAGE REACTIVE POWER SIGNAL =

VRMS

× IRMS × sin(θ)

Figure 71. Reactive Power Calculation

The low-pass filter is nonideal so the reactive power signal has

some ripple. This ripple is sinusoidal and has a frequency equal

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