2.2.2 Phase Calibration Between Voltage Channels
When phase-phase vector measurement or manipulation is required, it is recommended that the phase errors between all voltage channels be corrected. One voltage phase, usually Phase-A, is chosen as a reference that all other voltages will be corrected to. The “Law of Cosines” is, then, used to determine voltage phase correction factors, using the fundamental only integration accumulator values.
CAL_PH_VA=0x00000000 (reference phase)
The following formulas are used to calibrate the phase errors between the remaining voltage phases:
To phase-calibrate VB (relative to VA):
Where:
VA = RMS voltage of Phase-A
VB = RMS voltage of Phase-B
VAB = Inter-phase RMS voltage between Phase-A and Phase-B
V_A_F = Integration period accumulator value for Phase-A voltage (fundamental)
V_B_F = Integration period accumulator value for Phase-B voltage (fundamental)
V_AB_F = Integration period accumulator value for Phase-A to Phase-B voltage (fundamental)
θVV = Phase angle between the two voltage phases [in degrees]
Likewise, to phase-calibrate VC (relative to VA):
Then, the total phase corrections for the current channels now become:
CAL_PH_IA = CAL_PH_IAIV + CAL_PH_VA
CAL_PH_IB = CAL_PH_IBIV + CAL_PH_VB
CAL_PH_IC = CAL_PH_ICIV + CAL_PH_VC
Example:
An integration period of 1 second is used to gather the following accumulator values used for calibration, with conditions: 120V, 30A, PF=0.5Lag (measured at 50Hz), K_V = 11557, K_I = 308.642
- ACC_V_A_F = 0x144BD23C9BDE = 22315882290142
- ACC_V_B_F = 0x1454C5C79B64 = 22354328001380
- ACC_V_C_F = 0x144FAD879049 = 22332446314569
- ACC_V_AB_F = 0x 3CE0EC510A7B = 66936735074939
- Acc_V_CA_F = 0x 3CFB87060C83 = 67050999778435
