ADE7878 datasheet(46/92 Pages) AD

46 / 92 page

ADE7878

Rev. 0 | Page 46 of 92

Energy Accumulation Modes

The active power accumulated in each watt-hour accumulation

32-bit register (AWATTHR, BWATTHR, CWATTHR,AFWATTHR,

BFWATTHR, and CFWATTHR) depends on the configuration of

Bit 5 and Bit 4 (CONSEL bits) in the ACCMODE[7:0] register. The

various configurations are described in Table 14.

Table 14. Inputs to Watt-Hour Accumulation Registers

CONSEL

AWATTHR

BWATTHR

CWATTHR

VA × IA

VB × IB

VC × IC

VA × IA

VC × IC

VA × IA

VB × IB

VC × IC

VB = −VA − VC

VA × IA

VB × IB

VC × IC

VB = −VA

Depending on the polyphase meter service, choose the appropriate

formula to calculate the active energy. The American ANSI

C12.10 standard defines the different configurations of the

meter. Table 15 describes which mode should be chosen in

these various configurations.

Table 15. Meter Form Configuration

ANSI Meter Form

Configuration

CONSEL

5S/13S

3-wire delta

6S/14S

4-wire wye

8S/15S

4-wire delta

9S/16S

4-wire wye

Bits[1:0] (WATTACC[1:0]) in the ACCMODE[7:0] register

determine how the CF frequency output can be generated as a

function of the total and fundamental active powers. Whereas

the watt-hour accumulation registers accumulate the active

power in a signed format, the frequency output can be

generated in signed mode or in absolute mode as a function

of the WATTACC[1:0] bits. See the Energy-to-Frequency

Conversion section for details.

Line Cycle Active Energy Accumulation Mode

In line cycle energy accumulation mode, the energy accumula-

tion is synchronized to the voltage channel zero crossings such

that active energy is accumulated over an integral number of

half-line cycles. The advantage of summing the active energy

over an integer number of line cycles is that the sinusoidal compo-

nent in the active energy is reduced to 0. This eliminates any

ripple in the energy calculation and allows the energy to be

accumulated accurately over a shorter time. By using the line

cycle energy accumulation mode, the energy calibration can be

greatly simplified, and the time required to calibrate the meter

can be significantly reduced. In line cycle energy accumulation

mode, the ADE7878 transfers the active energy accumulated in the

32-bit internal accumulation registers into the xWATHHR[31:0] or

xFWATTHR[31:0] register after an integral number of line

cycles, as shown in Figure 64. The number of half-line cycles is

specified in the LINECYC[15:0] register.

ZERO

CROSSING

DETECTION

(PHASE A)

ZERO

CROSSING

DETECTION

(PHASE B)

CALIBRATION

CONTROL

ZERO

CROSSING

DETECTION

(PHASE C)

LINECYC[15:0]

AWATTHR[31:0]

ZXSEL[0] IN

LCYCMODE[7:0]

ZXSEL[1] IN

LCYCMODE[7:0]

ZXSEL[2] IN

LCYCMODE[7:0]

OUTPUT

FROM

LPF2

AWGAIN

AWATTOS

ACCUMULATOR

WTHR[47:0]

32 BIT

Figure 64. Line Cycle Active Energy Accumulation Mode

The line cycle energy accumulation mode is activated by setting

Bit 0 (LWATT) in the LCYCMODE[7:0] register. The energy accu-

mulation over an integer number of half-line cycles is written

to the watt-hour accumulation registers after LINECYC[15:0]

number of half-line cycles are detected. When using the line

cycle accumulation mode, the Bit 6 (RSTREAD) of the

LCYCMODE[7:0] register should be set to Logic 0 because

the read with reset of watt-hour registers is not available in

this mode.

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

included when counting the number of half-line cycles by setting

Bits[5:3] (ZXSEL) in the LCYCMODE[7:0] register. Any combi-

nation of the zero crossings from all three phases can be used

for counting the zero crossing. Select only one phase at a time

for inclusion in the zero crossings count during calibration.

The number of zero crossings is specified by the LINECYC[15:0]

16-bit unsigned register. The ADE7878 can accumulate active

power for up to 65,535 combined zero crossings. Note that the

internal zero-crossing counter is always active. By setting Bit 0

(LWATT) in the LCYCMODE[7:0] register, the first energy

accumulation result is, therefore, incorrect. Writing to the

LINECYC[15:0] 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, Bit 5 (LENERGY) in

the STATUS0[31:0] register is set. If the corresponding mask bit

in the MASK0[31:0] interrupt mask register is enabled, the

IRQ0 pin also goes active low. The status bit is cleared and the

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