Register WTHR Calculation
The WTHR 48-bit signed register contains the threshold. It is introduced by the user and is common for all phase total and fundamental active powers. Its value depends on how much energy is assigned to one LSB of watt- hour registers. When a derivative of active energy (wh) of [10n wh], where n is an integer, is desired as one LSB of the xWATTHR register, then, the xWATTHR register can be computed using the following equation:
where:
PMAX = 33516139 = 0x1FF6A6B as the instantaneous power computed when the ADC inputs are at full scale.
fS = 8 kHz, the frequency with which the DSP computes the instantaneous power.
VFS, IFS are the rms values of phase voltages and currents when the ADC inputs are at full scale.
The maximum value that can be written on WTHR is 247 − 1. The minimum value is 0x0, but it is recommended to write a number equal to or greater than PMAX. Never use negative numbers.
WTHR is a 48-bit register. The WTHR register is accessed as two 32-bit registers (WTHR1 and WTHR0), each having eight MSBs padded with 0s.
Based on the current transformer characteristics and the burden resistors (R1 and R2), compute the full-scale current IFS, that is, the rms of the sinusoidal current that determines a full-scale sinusoid at the current channel ADC inputs as follows:
where:
R1 and R2 are the burden resistors. According to schematic, implemented via a single resistor of 7.59 Ω (E192 series ?).
CTRATIO is the current transformer input-to-output ratio, assumed to be 100A / 50mA = 2000:1
and 0.5 V is the maximum allowed voltage at the ADC inputs.
WTHRCode = 0x1700856016 = 98792988694