Woodward MRM4, Manual

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Protective Elements
heavy-current earth fault in a solidly-earthed system. Minimize the saturation problem by minimizing the burden.
Use the shortest and heaviest leads possible between the earth CT and the relay. The relay itself has very low
burden, usually much lower than the connecting wiring.
A residual connection – the wired summation of the phase CT circuits through the earth CT input – requires a
much higher earth fault time setting to avoid false tripping. Thus, sensitivity is not nearly as good as with a
separate flux-canceling CT.
If the relay is installed where a residual connection is used, XCT should be set to the same value as CT Pri. The
User must then set the earth fault trip level at a high value to avoid nuisance tripping from CT ratio errors, third
harmonic and certain higher harmonics, or other measurement errors producing false residual currents. Monitor
the metered earth current during various loading conditions to ensure a good margin between these error
currents and the earth fault trip current setting 50R [x]. Also, watch out for phase CTs that saturate during motor
starting. The saturation produces a large residual current and a earth fault trip.
IG> - Earth Fault [50N/G, 51N/G]
Available elements:
IG[1] ,IG[2] ,IG[3] ,IG[4]
If you are using inrush blockings the tripping delay of the earth current
protection functions must be at least 30 ms or more in order to prevent
faulty trippings.
All earth current elements are identically structured.
This module offers Adaptive Parameter Sets.
Parameters can be modified within parameter sets dynamically by means of
Adaptive Parameter Sets.
Please refer to chapter Parameter / Adaptive Parameter Sets.
The following table shows the application options of the earth overcurrent protection element.
Applications of the IE-Protection Module Setting in Option
ANSI 50N/G – Earth overcurrent protection,
non directional
Device Planning menu
Setting: non directional
Measuring Mode:
Fundamental/TrueRMS
ANSI 51N/G – Earth short circuit protection,
non directional
Device Planning menu
Setting: non directional
Measuring Mode:
Fundamental/TrueRMS
Measuring Mode
For all protection elements it can be determined, whether the measurement is done on basis of the
» Fundamental« or if » TrueRMS« measurement is used.
For each element the following characteristics are available:
•
DEFT (UMZ);
•
NINV (AMZ);
•
VINV (AMZ);
Page 322 EN MRM4 01/11