2.2.2 Phase Calibration Between Voltage Channels

When phase-to-phase vector measurement or manipulation is necessary, it is recommended to correct the phase errors among all voltage channels. One voltage phase, usually Phase-A, is chosen as a reference that all other voltages will be corrected to. The Law of Cosines is 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 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 = 0.5 Lag (measured at 50 Hz), 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
Note: For the best results when phase calibration between voltage channels is desired, first calibrate and apply all magnitude gain factors before measuring and calibrating the voltage phase correction values.