Datasheet

ADE7758 Data Sheet
Rev. E | Page 38 of 72
Integration Time Under Steady Load
The discrete time sample period (T) for the accumulation
register is 0.4 µs (4/CLKIN). With full-scale sinusoidal signals
on the analog inputs, a 90° phase difference between the voltage
and the current signal (the largest possible reactive power), and
the VAR gain registers set to 0x000, the average word value from
each LPF2 is 0xCCCCD.
The maximum value that can be stored in the reactive energy
register before it overflows is 2
15
− 1 or 0x7FFF. Because the
average word value is added to the internal register, which can
store 2
40
− 1 or 0xFF, FFFF, FFFF before it overflows, the
integration time under these conditions with VARDIV = 0 is
calculated as
sec0.5243s0.4
0xCCCCD
FFFFFFFF,0xFF,
=×=Time
(36)
When VARDIV is set to a value different from 0, the time
before overflow are scaled accordingly as shown in Equation 37.
Time = Time(VARDIV = 0) × VARDIV (37)
Energy Accumulation Mode
The reactive power accumulated in each VAR-hr accumulation
register (AVARHR, BVARHR, or CVARHR) depends on the
configuration of the CONSEL bits in the COMPMODE register
(Bit 0 and Bit 1). The different configurations are described in
Table 13. Note that IA
’/IB’/IC’ are the current phase-shifted
current waveform.
Table 13. Inputs to VAR-Hr Accumulation Registers
CONSEL[1, 0] AVARHR BVARHR CVARHR
00 VA × IA’ VB × IB VC × IC’
01 VA (IA’ – IB’) 0 VC (IC’ – IB’)
10 VA (IA’ – IB’) 0 VC × IC’
11 Reserved Reserved Reserved
Reactive Power Frequency Output
Pin 17 (VARCF) of the ADE7758 provides frequency output for
the total reactive power. Similar to APCF, this pin provides an
output frequency that is directly proportional to the total
reactive power. The pulse width of VARPCF is 64/CLKIN if
VARCFNUM and VARCFDEN are bot h equ al. If VARCFDEN
is greater than VARCFNUM, the pulse width depends on
VARCFDEN. The pulse width in this case is T × (VA RC FDE N/2),
where T is the period of the VARCF pulse and VARCFDEN /2
is rounded to the nearest whole number. An exception to this
is when the period is greater than 180 ms. In this case, the pulse
width is fixed at 90 ms.
A digital-to-frequency converter (DFC) is used to generate the
VARCF pulse output from the total reactive power. The TERMSEL
bits (Bit 2 to Bit 4) of the COMPMODE register can be used to
select which phases to include in the total reactive power calcu-
lation. Setting Bit 2, Bit 3, and Bit 4 includes the input to the
AVARHR, BVARHR, and CVARHR registers in the total
reactive power calculation. The total reactive power is signed
addition. However, setting the SAVAR bit (Bit 6) in the
COMPMODE register enables absolute value calculation. If the
active power of that phase is positive, no change is made to the
sign of the reactive power. However, if the sign of the active power
is negative in that phase, the sign of its reactive power is inverted
before summing and creating VARCF pulses. This mode should
be used in conjunction with the absolute value mode for active
power (Bit 5 in the COMPMODE register) for APCF pulses.
The effects of setting the ABS and SAVAR bits of the COMPMODE
register are as follows when ABS = 1 and SAVAR = 1:
If watt > 0, APCF = Watts, VARCF = +VAR.
If watt < 0, APCF = |Watts|, VARCF = −VAR.
INPUT TO BVARHR
REGISTER
INPUT TO
A
V
A
RH
R
REGISTER
INPUT TO CVARHR
REGISTER
+
+
+
INPUT TO BVAHR
REGISTER
INPUT TO AVAHR
REGISTER
INPUT TO CVAHR
REGISTER
+
+
+
0
1
VARCF
VARCFNUM[11:0]
VARCFDEN[11:0]
÷
DFC
VACF BIT (BIT 7) OF
WAVMODE REGISTER
÷4
04443-073
Figure 74. Reactive Power Frequency Output
The output from the DFC is divided down by a pair of frequency
division registers before sending to the VARCF pulse output.
Namely, VARCFDEN/VARCFNUM pulses are needed at the
DFC output before the VARCF pin outputs a pulse. Under
steady load conditions, the output frequency is directly
proportional to the total reactive power.
Figure 74 illustrates the energy-to-frequency conversion in the
ADE7758. Note that the input to the DFC can be selected between
the total reactive power and total apparent power. Therefore,
the VARCF pin can output frequency that is proportional to the
total reactive power or total apparent power. The selection is
made by setting the VACF bit (Bit 7) in the WAVMODE register.
Setting this bit switches the input to the total apparent power.
The default value of this bit is logic low. Therefore, the default
output from the VARCF pin is the total reactive power.
All other operations of this frequency output are similar to that
of the active power frequency output (see the Active Power
Frequency Output section).
Line Cycle Reactive Energy Accumulation Mode
The line cycle reactive energy accumulation mode is activated
by setting the LVAR bit (Bit 1) in the LCYCMODE register. The
total reactive energy accumulated over an integer number of
zero crossings is written to the VAR-hr accumulation registers
after the LINECYC number of zero crossings is detected. The
operation of this mode is similar to watt-hr accumulation (see
the Line Cycle Active Energy Accumulation Mode section).