Mains pro 1.6.1 Global Guide
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Image 8.4 Frequency hysteresis for underfrequency
8.4 ANSI 47 Voltage unbalance and angle
asymmetry
MainsPro provides 3 independent methods for evaluation of voltage symmetry failures. All of these protections
are only active in case that 3-phase system is selected by the setpoint
8.4.1 Voltage unbalance
In further text, this term refers to the state, when amplitude difference between any 2 phases overreaches the
preset limit
. I.e., it refers to the amplitude unbalance of the measured voltage.
8.4.2 Positive sequence undervoltage, Negative sequence
overvoltage
These two methods provide very good sensitivity also to angle asymmetry of the measured voltages. The
evaluation is based on the mathematical principle of evaluation of the symmetrical components of measured
voltage. Any 3-phase system in any asymmetrical arrangement may be decomposed to 3 perfectly symmetrical
components:
Positive sequence - system of 3 phases with 120° phase-shift between the system vectors and the same
phase-order as the original system.
Negative sequence - system of 3 phases with 120° phase-shift between the system vectors and opposite
phase-order as the original system.
Zero sequence - system of 3 conphase vectors (with 0° phase-shift between the phases).