Short Version-
I wish to have a meter about the same size with a color touch-screen. I want to read in three phase current simultaneously and display the magnitudes and phase angles on a graph. Is there a current sensor that can also provide phase angle measurement?
Eventually I would like everything to be wireless. My question is... is that even possible? Can clamp on CTs be wireless? In theory, it wouldn't actually have to send the actual current back to the meter, just the data. How would each individual clamp on CT know what the phase angle is without a reference?
A current transformer develops a small AC voltage in phase with the current through a single wire, with the instantaneous voltage being proportional to the instantaneous current (typically 5-10 mV per ampere). What you do with that AC voltage is up to you.
If you had two CTs on two wires they would be independent, but if you measured their output voltages simultaneously, you could define one signal as the zero phase reference and compute a phase difference between the two. The same consideration would hold for three phase circuits.
To make the signals wireless, they would have to be sampled quite frequently and converted into a digital signals for transmission. To get good accuracy digitizing with an Arduino, you may have to amplify the CT signal. The instrument you linked claims an accuracy of 0.1% and that level of accuracy would be quite difficult for an experimenter to achieve.
jremington:
A current transformer develops a small AC voltage in phase with the current through a single wire, with the instantaneous voltage being proportional to the instantaneous current (typically 5-10 mV per ampere). What you do with that AC voltage is up to you.
This is dangerously wrong!! A current transformer develops a secondary current proportional to the primary current. This current can be converted to a voltage by a resistor. If left open the secondary winding on a current transformer can develop lethal voltages. It is therefor VERY important to ensure that the secondary never is left open. Also fuses in secondary circuits of ct's are prohibited
I will be able to gather the information necessary to compute the phase angles of the currents in relation to each other. Now how will I be able to plot them on a graph? How will I know where to plot the angles? I will be able to tell that all three phases are 120 degrees apart in a balanced system, but how will I actually know what the angle is?
For example, if A phase was at 120 degrees, wouldnt that angle drift uncontrollably without a reference?
I am thinking I am going to need to also read in a voltage (wireless hopefully) to use for that very reason.
I don't think there is any useful meaning to the concept absolute phase angle; it is just the angle corresponding to the time when your particular measurement clock starts. Why do you think it is important?
The local power company wants to measure certain aspects of the transmision & distribution system. During routine maintenance it is good practice to check all the secondary current & voltage inputs to the protective relaying. These inputs are from PTs, CTs, and CCVTs for the most part. If these currents and voltages are not correct the protective relaying will not operate correctly.
That being said, it is important to check all three current phase angles and plot them against all 3 voltage phase angles in referance to a steady (known) voltage, to compare the new to the old.
any good recommendations for some small clamp-on CTs?
That kind of measurement should preferably be made with calibrated equipment like the one you linked to.
Search for power quality analysers.
Or maybe something like this will do: http://www.powerstandards.com/PQube.php
Its doubtful that the arduino has enough processing power for what you want to achieve. Maybe using a due.
Here are some ct's
I see no real challenge with measuring the actual phase difference between three legs of a 3 phase AC power circuit. Each leg has a zero crossing every 8.333 milliseconds and there for 2.777ms between zero crossings of the 3 phases. A little signal conditioning via opto-isolators, followed with 3 channels of comparitors, and wire or the three outputs into a single interrupt input pin and have the ISR measure the time between the past interrupt and the current interrupt. I see no reason this would require a high speed 32 bit microcontroller chip.
zero crossing isn't accurate in the presence of harmonics though - you should use
a proper method like a windowed FFT to get the phase information for the fundamental,
and it will really help to sample at a proper multiple of the nominal power frequency,
so a multiple of 300Hz would be good (works for both 50 and 60Hz).
Failing that a correlation against a sine wave will produce meaningful results if the
sample rate is high enough.
Measuring 3-phase supplies involves even higher voltages than normal single-phase
mains as the difference between two phases is 1.73 times more than a single phase itself.
For instance for 240V 3-phase that means a peak difference between two phases of 600V,
so you really do need to think about safety for voltage probes and check all the insulation
is rated for it.