today I have an issue regarding the meter accuracy over frequency variation.
For realizing an energymeter which accuracy meets EN 50470 requirements, it is (amongst other things) necessary to keep the additional error lower than 0,3% (class C meter) for pf=1 and pf=0,5ind when varying the frequency by +/-2% (49Hz and 51Hz).
Unfortunately, our energy meter does not meet this requirement.
For PF=1, the accuracy is pretty good (additional error smaller than 0,03%).
For PF=0,5ind., the additional error is at about 0,7...0,8% (49Hz) and at about 0,3...0,4% (51Hz)!
I guess the reason is to find in the phase calibration. Since we use a dc-tolerant current transformer (to meet the dc tolerance test for EN 50470), a phase calibration of about 4,5° has to be done.
Since the phase calibration is done by applying a fixed time shift in the AFE, the compensated angle is varying when varying the line frequency, while the angle applied by the current transformers is supposed to be constant.
I read out the ANGLE1 register at various line frequencies and the delay between voltage and current actually does vary when varying the frequency.
But with this behaviour it seems to be impossible to meet EN50470 accuracy requirements for class C meter?
Is there a possibility to solve this issue which I have overlooked?
Thanks for your help!
The error over frequency at PF=1 does seem to match the error shown in typical performance characteristic. However, the variation at PF=0.5 does not match the results in the typical performance characteristics (fig 11 in the datasheet)
This possible reason could be the current transducer phase shift is high enough to cause these errors.
However, before going that route it will be good to know the error you get at PF=0.5 and 50Hz. What is the phase shift introduced by CT at 50Hz?
Can you do calibration in the following order and check the performance:
xIGAIN and xVGAIN for RMS readings
xWGAIN and xWATTOS compensation
Thanks for your answer.
The current transducer phase shift at 50 Hz is about 4.5 degrees (because it is a dc-tolerant CT).
The measured error value at 50 Hz is at about 0.2% at PF=0,5 and Iref (5A).
The calibration process was rather simple (VGAIN, IGAIN, PHCAL, IOFFS) but the measurements were done by a certification institute.
In my understanding it should not influence the ADDITIONAL error how accurate the calibration was.
Unfortunately I do not have the resources to do further tests at the moment but the problem is not dependent on calibration accuracy but is rather to be discussed basically.
Thanks for your help