MainsPro G99TT 1.0.0 Global Guide
33
8 Protective features
The following protective functionalities, referred also by their ANSI number, are available in MainsPro G99TT
unit:
8.1 ANSI 59 Overvoltage, ANSI 27 Undervoltage 33
8.2 Floating 10 minutes average overvoltage 34
8.3 ANSI 81H Overfrequency, 81L Underfrequency 34
8.4 ANSI 47 Voltage unbalance and angle asymmetry 36
8.4.1 Voltage unbalance 36
8.4.2 Positive sequence undervoltage, Negative sequence overvoltage 36
8.5 ANSI 78 Vector shift 37
8.5.1 Measuring principle 37
8.6 81R Rate Of Change Of Frequency (ROCOF) 38
8.7 Phase rotation, incorrect phase polarity 39
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8.1 ANSI 59 Overvoltage, ANSI 27 Undervoltage
The RMS value of measured voltage is compared with the preset limit of overvoltage or undervoltage. When
any of the preset thresholds is crossed, the appropriate LED signal is issued by LED U and the output U Sig
moves to fault-indicating position immediately. If voltage in the given phase keeps out of limits for the delay of
the appropriate stage, TRIP (page 31) is issued. As the voltage returns back within limits in all measured
phases, the LED and U Sig (page 66) 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. Both overvoltage and
undervoltage protective stages provide possibility of setting 2 levels with independent delay assigned to each
level.
To avoid any unwanted oscillation around the undervoltage or overvoltage limit, there is a possibility to set up
a voltage hysteresis. The principle is shown in the pictures below. After overvoltage situation, voltage must
decrease under the hysteresis limit to clear the fault. After undervoltage situation, voltage must exceed the
hysteresis limit to clear the fault.
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