How to measure the voltage between earth and neutral ?

[smitchell]

Need 3-phase help quick

Hi,

I've already posted a similar question to this recently, but I need to get this sorted pretty quick because i have a uni report due in a couple of weeks.

I'm working on a 3-phase power monitoring system. the power to be monitored is wye connected with an earth and neutral (5 conductors).

The plan so far is to step down each voltage and current, then pass them through AD536 RMS-DC converters and finally into an ADC.

My problem is, I'm not quite sure how to go about measuring the voltage between earth and neutral. Do I connect earth and neutral to the primary side of a 1:1 transformer, then pass the positive side of the secondary into the RMS converter?

Or, should I do something like step down the voltage between earth and phase 1, then at the software end subtract neutral-phase1 from earth-phase1?

Thanks a lot,

Sam

 

[nicleo]

Re: Need 3-phase help quick

Probably the technical documents found at **broken link removed** will help. You can download it free.

By the way, have you ever considered ADE7754? You might wish to take a look on this application note from Analog ([url]http://www.analog.com/UploadedFiles/Application_Notes/108734845343588AN624_0.pdf[/url])

This one is also VERY GOOD (**broken link removed**)


We have a Power Profiler, BMI 3030A. Usually we measure the following voltages:
Line A-to-Neutral, VphA
Line B-to-Neutral, VphB
Line C-to-Neutral, VphC
Neutral-to-Ground, Vn
So, the meter will calculate other values, e.g. Line-to-Line voltage, harmonics, etc., in software. More explanations are given in the attachment.

 

[smitchell]

Re: Need 3-phase help quick

The ADE7754 looks great, but its a bit of an overkill for what i need to to, and it still seems to only support up to 4 wire 'wye' configuration. I need to measure between earth and neutral too.

All I need is voltage and current measurements, and my main concern is how to configure earth and neutral as an input for a measurement circuit such as an RMS-DC converter and ADC.

Can anyone suggest a method for converting the voltage between earth and neutral to a DC equivilent that can be fed to an ADC?

Thanks a lot.

Sam

 

[nicleo]

Need 3-phase help quick

Neutral -> Step Down Transformer -> Full/Half-wave rectifier -> DC -> In1

GND (should be 0V, right?) -> In2

In1 -> diffADCInput1
In2 -> diffADCInput2

Do you think it's possible?

 

[smitchell]

Re: Need 3-phase help quick

The problem is that I have to measure Neutral-Phase voltages using the same ADC. so the ADC's ground reference will be neutral for the other voltages. Can I connect the 3-phase Earth straight to the circuit ground? wont this mess with the measurement's accuracy?

Could I keep neutral as the ground reference for Earth-Neutral, and simply invert it for the ADC (because the voltage from neutral-earth will be negative)

I'm confused...

 

[nicleo]

Need 3-phase help quick

What do you think the following option:

Line-A to GND
Line-B to GND
Line-C to GND
Neutral to GND

And, with little mathematic calculation, you can get the following values:
Line-A to Neutral
Line-B to Neutral
Line-C to Neutral