I am running Martin's PLL sketch on an EmonTX with 3 CT's. Having calibrated the voltage & 3 current constants I would now like to tackle the phase alignment. As far as I can tell the best way to do this is to tune the CT lead time for each CT channel to obtain the maximum power reading when measuring a purely resistive test load (constant and stable) equal to the average or typical load to be monitored by that circuit. I hope I've got that right and not over simplified it.
So first question - Is that right?
and secondly if that is right - when calibrating the CT channel that drives the diverter ie the grid CT, should I not target a load as small as possible (avoiding circuit noise and interference etc) so that phase alignment is most accurate around the "sweet spot" to maximise the control element accuracy rather than the reporting accuracy.
Paul
Re: A question (or 2) about calibrating EmonTX
Yes, you are right to maximise the real power with a purely resistive load. That was the easy bit.
The hard bit is the phase error of the voltage transformer (ac-ac adapter) varies with voltage, and the phase error of the current transformer varies with current. So you will never get it right across a wide range of conditions. I'm not sure that you should aim to adjust at minimum current though. It might be representative of your PV generating current, but your dump load is always 10 - 12 A (albeit the average over time is less), and in general the c.t. phase error increases rapidly at low currents. I would recommend adjusting for reporting accuracy, i.e. at 'typical' current, on the grounds that Robin Emley says he can detect no practical difference when adjusting the phase correction over quite a range.
Re: A question (or 2) about calibrating EmonTX
Okay so my intention was to use a 2Kw heater load to calibrate CT2 (Solar) for a 4Kw array and to use the 3Kw immersion heater itself to calibrate CT3 (Diverted power) obviously with the diverter manually overriden. It was just the grid CT I was unsure of where to target and am pleased to hear it isn't critical.
Since my original post I have tried to calibrate CT3 using the immersion heater but my results were too erratic to make use of.
Is there a recommended procedure eg min number of samples, average over set time or ignore initial few results to allow circuit to settle ?
Using standard EmonTX and CT's in what region would you expect the lead values to be ?
In another discussion Martin mentions 158 microseconds maybe a good starting point for this value based on it being approximately the same as a phasecal of 1.7, the equivalent setting when using emonlib I believe, so is that for all the CTs or just the first one?
Thanks for you help again Robert
Paul