MainsPro G99TT 1.0.0 Global Guide
36
8.4 ANSI 47 Voltage unbalance and angle
asymmetry
MainsPro G99TT 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 System (page 54).
8.4.1 Voltage unbalance
In further text, this term refers to the state, when amplitude difference between any 2 phases exceeds the
preset limit V unb, A.V unb [V] (page 58). 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).
Image 8.5 Decomposition of a generic 3-phase voltage to symmetrical components
MainsPro G99TT provides positive and negative sequence voltage evaluation and compares the measured
values with V> neg, A.V> neg [V] (page 59) and V< pos, A.V< pos [V] (page 58) thresholds. In the
perfectly symmetrical arrangement, negative sequence voltage is zero and positive sequence voltage equals
to the measured voltage. If the asymmetry situation occurs, non-zero negative sequence voltage is calculated
and positive sequence voltage drops. When any of the preset thresholds is crossed, the appropriate LED
signal is issued by LED U and the output dU Sig (page 67) moves to fault-indicating position immediately. If
the calculated values keep out of limits for the delay dU del, TRIP (page 31) is issued. As the calculated
values of voltage asymmetry return back within limits, the LED and dU Sig (page 67) output stop to signal the
fault state immediately, regardless of whether TRIP (page 31) was issued or not or Fault reset (page 32)
was performed or not. Some utilities strictly require in their regulations that symmetrical components are
evaluated in the mains-decoupling relay and an appropriate trip is provided. However, the method may also
be used in the areas, where no such requirement is in place, to minimize non-detection zones of detection of
1-phase mains failures. In case that the generator, connected to the mains is operated close to equity-state,
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