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Protection DECS-250
shorted-diode protection when the field current drops below the user-defined percentage of rated. EDM
protection can be disabled and enabled by the user without altering the individual protection settings.
Applying EDM Protection
It is especially difficult to detect open diode conditions when the number of generator and exciter poles is
unknown. For this reason, the ratio of the number of brushless exciter armature poles to the number of
generator rotor poles should be entered to ensure detection of both open and shorted diodes.
Finding the Maximum Field Ripple Current
To set the open diode pickup level and shorted diode pickup level, the maximum ripple current on the
field must be known. This can be accomplished by running the generator unloaded and at rated speed.
Vary the generator voltage from minimum to maximum while monitoring the EDM ripple level on the HMI
display. Record the highest value.
Setting the Pickup Level—Number of Generator Poles Known
Multiply the highest EDM ripple value, obtained in the preceding paragraph, by 2. The result is the open
diode pickup level setting. The multiplier can be varied between 1.5 and 5 to increase or decrease the trip
margin. However, reducing the multiplier could result in nuisance open diode indications.
Multiply the highest EDM ripple value, obtained in the preceding paragraph by 50. The result is the
shorted diode pickup level setting. The multiplier can be varied between 40 and 70 to increase or
decrease the trip margin. However, reducing the multiplier could result in nuisance shorted diode
indications.
The DECS-250 has fixed EDM inhibit levels to prevent nuisance failed-diode indications while the
generator frequency is less than 40 hertz or greater than 70 hertz. EDM operation is also inhibited when
the level of field current is below the disable level setting.
Figure 10-14. Exciter Diode Monitor Protection Settings
Setting the Pickup Level—Number of Generator Poles Unknown
The DECS-250 can detect shorted diode conditions when the number of generator poles is not known. To
provide this protection, disable open diode protection, set the pole ratio to 1.0, and enable shorted diode
protection. Multiply the maximum EDM ripple level, obtained under Finding the Maximum Field Ripple
Current, by 30. The multiplier can be varied between 20 and 40 to increase or decrease the pickup
margin. However, reducing the multiplier could result in nuisance shorted diode indications.
Testing the EDM Settings
Start the generator from rest and increase the speed and voltage to the rated value. Load the machine to
its rating and confirm that no failed diode annunciations occur. All of the EDM setup guidelines presented
here assume that the exciter diodes were not opened or shorted at the time of setup and testing.
Power Input Failure
A power input failure condition exists when any one of the following occurs:
1-phase operating power
When operating power decreases below 30 Vac, a power input failure condition exists.
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