ADE7754_15 datasheet(24/44 Pages) AD

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REV. 0

–24–

ADE7754

INSTANTANEOUS REACTIVE

POWER SIGNAL – p(t)

MULTIPLIER

REACTIVE POWER

SIGNAL – P

HPF

LPF

–89

Figure 32. Reactive Power Signal Processing

TOTAL REACTIVE POWER CALCULATION

The sum of the reactive powers coming from each phase gives

the total reactive power consumption. Different combinations

of the three phases can be selected in the sum by setting Bits 7

to 6 of the WATMode register (mnemonic WATMOD[1:0]).

Each term of the formula can be disabled or enabled by the

LWATSEL bits of the WATMode register. Note that in this

mode, the LWATSEL bits are also used to select the terms of

the LVAENERGY register. The different configurations are

described in Table III.

The accumulation of the reactive power in the LAENERGY

the LAENERGY register. Under the same signal conditions

(e.g., current and voltage channels at full scale), and if the accu-

mulation of the active power with PF = 1 over one second is

Wh1, and the accumulation of the reactive power with PF = 0

during that time is VARh1, then Wh1 = 9.546

VAR1.

Note that IA*, IB*, and IC* represent the current channels

samples after APGAIN correction, high-pass filtering, and –89º

phase shift in the case of reactive energy accumulation.

Reactive Energy Accumulation Selection

The ADE7754 accumulates the total reactive power signal in

the LAENERGY register for an integer number of half cycles,

as shown in Figure 31. This mode is selected by setting Bit 5 of

the WAVMode register (Address 0Ch) to Logic 1. When this bit

is set, the accumulation of the active energy over half line cycles

in the LAENERGY register is disabled and done instead in the

LVAENERGY register. In this mode, the accumulation of the

apparent energy over half line cycles in the LVAENERGY is no

longer available. See Figure 33.

ACTIVE POWER

REACTIVE POWER

APPARENT POWER

LAENERGY

LVAENERGY

BIT 5 WAVMODE

Figure 33. Selection of Reactive Energy Accumulation

The features of the reactive energy accumulation are the same as

for the line active energy accumulation: each one of three phases

zero-crossing detection can contribute to the accumulation of

the half line cycles. Phase A, B, and C zero crossings, respec-

tively, are taken into account when counting the number of half

line cycles by setting to Logic 1 Bits 4 to 6 of the MMODE

the effect of enabling the zero-crossing detection, zero-crossing

timeout, and period measurement for the corresponding phase

as described in the Zero-Crossing Detection section.

The number of half line cycles is specified in the LINCYC

can accumulate active power for up to 65535 combined half

cycles. At the end of an energy calibration cycle, the LINCYC

flag in the interrupt status register is set. If the LINCYC enable

bit in the interrupt enable register is set to Logic 1, the

IRQ

output also goes active low. Thus the

IRQ line can also be used

to signal the end of a calibration.

As explained in the Reactive Power Calculation section, the

purpose of the reactive energy calculation in the ADE7754 is

not to give an accurate measurement of this value but to provide

the sign of the reactive energy. The ADE7754 provides an accu-

rate measurement of the apparent energy. Because the active

energy is also measured in the ADE7754, a simple mathemati-

cal formula can be used to extract the reactive energy. The

evaluation of the sign of the reactive energy makes up the calcu-

lation of the reactive energy.

Reactive

Energy

sign

Power

Apparent Energy

Active Energy

×−

()

APPARENT POWER CALCULATION

Apparent power is defined as the maximum active power that

can be delivered to a load. Vrms and Irms are the effective voltage

and current delivered to the load; the apparent power (AP) is

defined as Vrms

× Irms.

Note that the apparent power is equal to the multiplication of

the rms values of the voltage and current inputs. For a polyphase

system, the rms values of the current and voltage inputs of each

phase (A, B, and C) are multiplied to obtain the apparent power

information of each phase. The total apparent power is the sum

of the apparent powers of all the phases. The different solutions

available to process the total apparent power are discussed below.

Figure 34 illustrates the signal processing in each phase for the

calculation of the apparent power in the ADE7754.

CURRENT RMS SIGNAL – i(t)

0.5V/GAIN1

VOLTAGE RMS SIGNAL –v(t)

0.5V/GAIN2

MULTIPLIER

APPARENT POWER

SIGNAL – P

Irms

Vrms

00h

D1B71h

AVAG

1CF68Ch

Figure 34. Apparent Power Signal Processing

The apparent power is calculated with the current and voltage rms

values obtained in the rms blocks of the ADE7754. Figure 35

shows the maximum code (hexadecimal) output range of the

apparent power signal for each phase. Note that the output

range changes depending on the contents of the apparent power

gain registers and also on the contents of the active power gain

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