three phase voltage measurements

[smitchell]

Hi, Im working on a project where i want to monitor voltages and currents in a 3 phase distrobution system.

I need to measure the following voltages and currents on a Y connected supply.

Phase to Phase
Neutral to each phase
Neutral to Earth
Current on each phase

I have to convert the readings to digital then send them through ethernet, so i want to keep the number of measurements to a minimum.

Is there any reliable way to calculate any of these voltages at the software end if certain other voltages are known? for example, if the voltage between phase 1 and 3 could be calculated if the voltage between phase 1 and 2, and 2 and 3 was known?

I dont know much about 3 phase theory, so any help would be much appreciated.

I'd also love to here if anyone knows how to build a digital voltmeter type circuit that can handle up to 500V.

Thanks,

Sam

 

[korek]

To measure high voltage you have to use voltage divider at the input

 

[batdin]

If you know the three voltages to neutral,you can calculate the inter-phase voltages using the "cosine theorem" knowing that the phase difference is 120 deg.
As I can see it you need seven measurements for phase voltages,phase currents and ground to neutral voltage.For measuring the voltages a divider will be enough.For current measurement a current transformer is a good idea because it also isolates the ADC from hot phases.The "ground" of ADC should be neutral,not ground.Otherwise the dividers will not work right.

 

[smitchell]

Can you give me a quick example of calculating the phase to phase voltage using the cosine theorem? or a reference to good info on it?

Will that method give an acurate phase to phase measurement even if the neutral-phase voltages differ by 10-20V?

Also, I'm not sure I quite get what you mean by the ADC not working when connected to ground. I'm planning on having a seperate power supply for the measurement circuits, so should I keep the earth connection out of the circuits all together?

Thanks heaps,

Sam

 

[smitchell]

Actually dont worry about it, i figured it out. had to go back to high school maths..

is this method accurate and reliable though? can we count on the phase difference always being 120 degrees? even from a generator?

thanks,

 

[batdin]

Look now,the phase difference is determined by the physical angle between windings of a stator.It should not vary.
What I meant about the ADC ground is that the phase voltages are measured with respect to neutral "star-center" , that's why if you use a divider the analogue ground of ADC must be this same neutral.If you measure the phase voltages with respect to (earth) ground,you will have an error.This is what I was talking about

 

[flatulent]

On the phase angle, there is an impedance, usually inductive reactance, in the lines to the load. If the load draws vastly different currents there may be some phase differences from 120 degrees. It will probably be very small and not make a significant error in your calculations.

 

[smitchell]

Thanks heaps for all that. I should be able to get started now. It's good to know i can cut my number of measurements back to 7.

I was thinking of using hall effect current transducers to measure the line currents, as i wont have break the circuit to take the measurements. Is there any problems with using these?

Thanks again,

Sam

 

[korek]

It depends on the accuracy and dynamic range you need. Professional energy meter manufacturers use current transformers for up to 0.1 class meter (with some kind of correction of course)

 

[smitchell]

If i was to use current transformers, would there be any possiblity of the transormer failing and causing a short or open circuit?

It's absolutely essential that my circuit doesn't effect the power distrobution, even if any components fail..

Thanks,

Sam

 

[batdin]

The well-designed current transformer will not influence the circuit no matter if its secondary winding is open or short-circuited.There is no need to break the circuit to install a current transformer ; you can make it as a disjointable toroid through which the power conductor goes.Effectively,this is about half turn primary winding.