STPM33/34 RMS Current Calculation Error

[bsahan]

Hi everyone,

I met a problem when I measured the current rms value with stpm33 ic.I'm using this IC with resistive loads and other load profiles with low crest factor. But I met a case with a high crest factor load like as SMPS type loads. 

I collected the momentary current datas from STPM33eval board over SPI lines. And than calculate the RMS current value using with point by point datas. The calculated value asme as real value. I compared the calculated value with true RMS values that are measured with other power analyzers as called as Vitrek PA900 and Schneider PA module.

I compared the current values with resistive and refrigerature type loads. They are same for all measurement device.

Is it possible to make any mistake when I config the STPM?

You can find my measured values and voltage-current graph at below.

Blue: Actual Voltage Orange: Actual Current

Load	STPM33 Current RMS(mA)	STPM33 Current Calculated RMS(mA)	Vitrek PA900 RMS(mA)	Scheneider PM2200 RMS(mA)
High Crest Factor SMPS Load	119,9	199	198	198
Low Crest Factor Load (Resistive Load)	1033	1033	1033	1033

[bsahan]

Hi again,

I opened a ticket explaining my issue for this error. However, the STM team stated that it’s not possible to resolve this issue with the STPM33. They pointed out that the reason is due to the current signal having a high peak. Based on their recommendation, I should handle this with raw data. But this is not possible with my design, as I am using UART communication. I didn’t expect an IC that performs RMS readings to make such a fundamental error. If I have to perform RMS and all other calculations with raw data, I don't fully understand how the STPM differs from an ADC with a high input voltage.