I have a similar question to post as well. I have a bunch of 3-phase AC SCR power controllers (CCI) connected as follows:
480 delta supply through SCRs delta output to a 480delta/480wye isolation transformer to a 480 delta quartz lamp load (slightly unbalanced).
I have a single-phase VT and CT in each cabinet, which I can move to any place in the circuit as necessary. I would like to determine how much total (3-phase) power (kW) I am delivering at any point in time, based on some measurements from these single phase devices. In an ideal situation, I would think Volts * Amps * SQRT(3), but I am not so sure about the phase-fired SCRs.
I have taken some V, A, kVA, and kW readings with a Fluke 43 True RMS Power Quality Meter/Scope, but am perplexed by the results. For instance, a typical reading at the output leads from the transformer to the load (Volts T3-T2, Amps on T3) Volts RMS: 323.5 Amps RMS: 84.5 kW: 19.9 kVA: 23.2 PF: 0.86
So based on the meter kW, my three phase power would be 19.9 * 1.732 =
34.5kW. However, based on the meter V, A and PF
323.5
* 84.5 * 0.86 * 1.732 / 1000 = 40.8kW which is about 18% off. Where did I go wrong?
One note, if I read line voltage (i.e. 480V) and amps through one leg of the SCR, I my calculated results using volts and amps are much more consistent with the power readings on the Fluke. Since the SCRs follow the line voltage (when they are on), should I use the line volts (480) and RMS amps to calculate my power?
p.s. If you want, I have some interesting images of pre/post transformer waveforms I could email out if anyone is interested.