AN3254

Converting readings into measured values

Doc ID 17783 Rev 1

21/24

From these values the following scaling parameters are calculated for Rogowski Coil

sensor:

Kf = frequency / 50

Kdspu = 1

Kdspi = Kf

and for CT or Shunt current sensors:

Kdspu = Kdif * Kint

Kdspi = Kdif

The uN RMS value (12 bits in DEN register) conversion formula is:

u = x_u * 2 / len_u

For the sum of RMS currents sI RMS (12 bits in DMN register) the value is obtained as

follows:

si = x_i * Vref/(Ks * Kisum * Ai * len_isum * Kint * Kint_comp *

Kdspi)

For the data from the momentary registers, as the related momentary voltage and current

parameters are signed (they can be positive or negative), it is necessary to evaluate the sign

with the following task. This does not apply to the RMS values:

if (x_i & (len_i>>1)) // positive current

x_i = x_i & ((len_i>>1)-1);

else

// negative current

{

x_i = (len_i>>1) - x_i;

x_i = x_i * (-1);

}

if (x_u & (len_u>>1)) // positive voltage

x_u = x_u & ((len_u>>1)-1);

else

// negative voltage

{

x_u = (len_u>>1) - x_u;

x_u = x_u * (-1);

}

Both RMS and momentary current and voltage conversion formulas then are:

u = (1+R1/R2)* x_u * Vref /(Kut * Ku * Au * len_u * kint_comp*

Kdspu)

i = x_i * Vref/(Ks * Ki * Ai * len_i * Kint * Kint_comp * Kdspi)