Measuring 3 phase voltage in a 3-wire and 4-wire system

[unscripted]

I'd like to understand the voltage sensing circuit when measuring three phase voltages in a 3 wire system (like Delta with no neutral) or a 4 wire system (like WYE). Using the circuit attached as an example, it would work if one has a neutral available. In a delta sytem with no neutral, is it good practice to leave the neutral connection to the opamp floating? I'm not sure if floating is a good idea. What's the practice in the industry?

[IanB]

There's not enough information to give an answer. Where and what is the circuit to be measured? What is the purpose and application?

If you have a transformer secondary wired as delta, then there are only three wires present, and no neutral. So you have to measure voltages phase to phase. If you leave a "neutral" floating, then you can only get phantom voltages. Nothing good could ever come of that.

[TizianoHV]

In a symmetric three phase system you can easily create a "virtual neutral" with some resistors in a star connection:



If you search online there are a lot of examples:

https://www.ti.com/lit/ug/tiduf25/tiduf25.pdf?ts=1717609087820    (pag.8 )

https://www.nxp.com/docs/en/application-note/DRM147.pdf

https://www.analog.com/media/en/technical-documentation/application-notes/2698536550528608457an641_0.pdf

https://www.analog.com/media/en/technical-documentation/data-sheets/78m6631.pdf

[ajb]

If U1 is a standard op amp, the circuit in the OP will be operating as a comparator indicating the polarity of phase A vs neutral, not indicating the relative voltage, because the op amp has no feedback.  I would guess it's meant to be an integrated differential amplifier, in which case the circuit should work fine (with proper resistor values) for phase-to-phase as well as phase-to-neutral measurements.  R1/R2/R5 and R3/R4/R6 scale the Phase A and Neutral voltages respectively relative to VREF (presumably a mid-rail reference to handle positive and negative input voltages), then U1 amplifies the difference in the scaled voltages.  The circuit is entirely symmetrical, other than the sign of the output, so it doesn't matter which way around it's connected. 

Alternatively, the reference point for any circuit is arbitrary, so you could use one of the phases as the reference for all of the voltage measurements.  The measurement circuit can be directly referenced to that phase, with voltage dividers to sense the voltages on the other phases with respect to the reference phase.  The measured results can be sent over an isolated digital link to ground-referenced circuitry if needed -- there are even specialized energy management ADCs designed for that.  Of course it's a bit harder to test and troubleshoot a circuit that is directly referenced to a phase, but it might be worth the effort in some cases.

[unscripted]

In a symmetric three phase system you can easily create a "virtual neutral" with some resistors in a star connection:



If you search online there are a lot of examples:

https://www.ti.com/lit/ug/tiduf25/tiduf25.pdf?ts=1717609087820    (pag.8 )

https://www.nxp.com/docs/en/application-note/DRM147.pdf

https://www.analog.com/media/en/technical-documentation/application-notes/2698536550528608457an641_0.pdf

https://www.analog.com/media/en/technical-documentation/data-sheets/78m6631.pdf

Would this also work if you have 4-wire system, i.e., can you connect your physical neutral to this "virtual neutral"?

By the way, I took a look at the app notes you referenced. Thank you. The one I found useful was the 78M6631 reference design where it explicitly states that it supports both Delta and Wye configurations. Now, I honestly don't understand what the reasoning is for connecting the neutral to the 3.3V potential.

[unscripted]

If U1 is a standard op amp, the circuit in the OP will be operating as a comparator indicating the polarity of phase A vs neutral, not indicating the relative voltage, because the op amp has no feedback.  I would guess it's meant to be an integrated differential amplifier, in which case the circuit should work fine (with proper resistor values) for phase-to-phase as well as phase-to-neutral measurements.  R1/R2/R5 and R3/R4/R6 scale the Phase A and Neutral voltages respectively relative to VREF (presumably a mid-rail reference to handle positive and negative input voltages), then U1 amplifies the difference in the scaled voltages.  The circuit is entirely symmetrical, other than the sign of the output, so it doesn't matter which way around it's connected. 

Alternatively, the reference point for any circuit is arbitrary, so you could use one of the phases as the reference for all of the voltage measurements.  The measurement circuit can be directly referenced to that phase, with voltage dividers to sense the voltages on the other phases with respect to the reference phase.  The measured results can be sent over an isolated digital link to ground-referenced circuitry if needed -- there are even specialized energy management ADCs designed for that.  Of course it's a bit harder to test and troubleshoot a circuit that is directly referenced to a phase, but it might be worth the effort in some cases.

Actually, U1 is an instrumentation amplifier. It was drawn like that for simplification. I also thought of connecting the neutral (or reference point) to one of the phases in the case of a 3-wire system. Could be an option. I was trying to come up with a circuit that doesn't care whether you have a 4-wire or 3-wire system to provide relatively accurate voltage measurements. You either connect your three phases, or your three phases and neutral. And I was avoiding connecting something to the neutral terminal if you don't really have it.

[IanB]

Would this also work if you have 4-wire system, i.e., can you connect your physical neutral to this "virtual neutral"?

No, because it is not a power neutral, it is a reference neutral. It is nothing more than an average of the voltage of the three phase conductors, and it provides no additional information compared to what information is directly available by measuring the three phase to phase voltages directly.

You cannot connect a physical neutral to this because it has a very high impedance, and a true neutral return wire should have a very low impedance.

[TizianoHV]

Would this also work if you have 4-wire system, i.e., can you connect your physical neutral to this "virtual neutral"?

No, because it is not a power neutral, it is a reference neutral. It is nothing more than an average of the voltage of the three phase conductors, and it provides no additional information compared to what information is directly available by measuring the three phase to phase voltages directly.

You cannot connect a physical neutral to this because it has a very high impedance, and a true neutral return wire should have a very low impedance.

There might be some misunderstanding here: it will work in both 3wires and 4 wires systems. If you don't need high accuracy measurement and if the system is symmetric (almost always) you can leave the neutral floating.
But if you are in a 4 wire system you can connect the virtual neutral to the power neutral. By doing that you'll be able to measure all three phase voltages (for example 230V in Italy). Then you can calculate the phase to phase voltage (400V) by difference:
-with the MCU (difference between the ADC samples of two inputs)
-with your INA.

Now, I honestly don't understand what the reasoning is for connecting the neutral to the 3.3V potential.

I'm not 100% sure but I think that they are doing this to add an offset to the measured signal (since the ADC in the microcontroller can measure only positive voltages, I did something similar but using a 0.6V offset -> https://tiziano-bielli-ee.github.io/projects/pk-rms-wattmeter.html)