PM8000 I4 THDi value differs between front display and Webpage/Modbus

[Stuart_Cheng]

Hi All,

My customer has observed a discrepancy in the PM8000 meter I4 THDi measurement.

For example, the front display shows I4 THDi = 112.6%, while the webpage and Modbus (ModScan) continuously read 0%. Communication is normal, and other measurements are updating correctly.

Wondering has anyone encountered a similar situation? What could be the possible cause or explanation for this behavior? Thank you.

[Mehran_Mehrnia]

Hi @Stuart_Cheng 

In Advanced mode of ION Setup, could you check the Data Mapping PQ module by holding the Shift key and clicking on the Input Registers to verify the "I4 THD 3S" value?

The Data Mapping modules define the Modbus data as specified in the device's Modbus register map, and they also determine the values displayed on the meter's display screens and web pages.

Best Regards,

-Mehran

L3 Expert Advanced metering

[Charles_Murison]

Hello @Stuart_Cheng ,

The front panel is hard coded to read from Harmonic measurement module #13 ("I4 Harmonics") while the web page and Modbus will read from Data mapping module as highlighted by Mehran. Would recommend confirming in ION that what you see at the data mapping module matches what you are seeing in web page and Modbus. Then confirm if what you see at data mapping module matches what you see on the front panel display.

Regards,

Charles

[Stuart_Cheng]

Thanks @Charles_Murison @Mehran_Mehrnia for the advice. 

We will go onsite and check the data from ION Setup to confirm the issue. 

Thank you!

[Ray_Chang_EPEC]

Hi @Mehran_Mehrnia @Charles_Murison @Stuart_Cheng 

I'm this case's site engineer.
I just try this on my demo panel PM8000
As you say these two matchs.

And at modbus 21306 = I4 THD 3s = Web I4 THD 3s Present

We test this Meter at enduser's site today and when ACB just close.
We got this situattion like picture.

Meter's THD shows 10.5% But all other shows 0.

And After some time, Maybe 5min  Maybe 30min.

It will come back normal.

Is there any situations may shows like this?

Many thanks for answers

 

[Mehran_Mehrnia]

Hi @Ray_Chang_EPEC 

Thank you for sharing the images.

Because current THD (THDi) is calculated as a percentage relative to the fundamental current (I1), an extremely low fundamental current immediately after the ACB is closed, with I1 close to 0 A, can cause the calculated THD percentage to appear relatively high due to very small residual harmonic components, measurement noise, or CT-related noise. The individual harmonic values displayed as 0.00 may also be the result of display resolution and rounding.

I believe a delay of 5 to 30 minutes is too long to attribute solely to the meter settling or completing its post-energization calculations. If the measurement updates normally within a few seconds, this suggests that the observed delay may instead be related to the behavior of the downstream load.

Once the ACB is closed, some connected loads may take time to energize, ramp up, or reach their steady-state operating conditions. During this period, if the fundamental current remains very low, the calculated current THD percentage may remain relatively high. As the load increases and the fundamental current reaches its normal operating level, the THDi value should become more representative of the actual harmonic distortion of the load.

Best Regards,

-Mehran
L3 Expert Advanced metering

[Charles_Murison]

Hello @Ray_Chang_EPEC ,

Regarding the display verses Modbus during no load, have you tried reading the 10 minute harmonic values?

Regards,

Charles

[Ray_Chang_EPEC]

Hi @Charles_Murison :

Sorry about that End user has start useing the site so it's can't be test after these picture was taken.

Many thanks for reply

[Charles_Murison · Accepted solution]

Hello @Ray_Chang_EPEC ,

The Meter's display for Harmonic information calls APIs internal to the meter. The web page and Modbus report information linked to the Data mapping module. If you were to delete the Harmonic measurement module, the display would still provide data however web pages and Modbus would give N/A.

This slightly different source of data can explain why display and web pages/Modbus may not always align.

Regards,

Charles