8.1.2.18 GBAL Test
The accuracy is based on the test conditions chosen to measure the longitudinal balance of the transformer under test, and is given as:
AT = AR +0.87(AC+AVX+AVY)
where
AT is the total accuracy (dB)
AR is the basic relative accuracy (dB)
AC is the correction for calibration
AVX is the correction for the X measurement voltage level
AVY is the correction for the Y measurement voltage level
The value of AR is given in the table in section 1.1 of this chapter
The values of the other factors are given below.
AC = 0.1 + (0.0000001% x FM )
AVX = (0.001 + ΔVCMNX) / VMX
AVY = (0.001 + ΔVCMNY) / VMY
where
FM is the measurement frequency (Hz)
VMX is the measured voltage for the X measurement (V)
VMY is the measured voltage for the Y measurement (V)
ΔVCMNX is the error due to the common mode voltage in the X measurement
ΔVCMNY is the error due to the common mode voltage in the Y measurement
The error due to the common mode voltage in the measurement (ΔVCMNX and ΔVCMNY) is given as
ΔVCMN = (VCMN *√1+ 0.00000001 x FM 2 )/ 398
where
VCMN is the common mode voltage measured