9440300990 10-11
DECS-250 Protection
Figure 10-13. Field Overcurrent Protection Settings
Exciter Diode Monitor
The exciter diode monitor (EDM) monitors the condition of a brushless exciter’s power semiconductors by
monitoring the exciter field current. The EDM detects both open and shorted rotating diodes in the exciter
bridge. EDM settings are illustrated in Figure 10-14. When implementing the EDM, it is imperative that the
user know and specify the number of poles for the exciter armature and the generator rotor. For reliable
open diode detection, the exciter to generator pole ratio should be 1.5 or higher and the level of field
current should be no less than 1.5 Adc. A pole ratio calculator, available in BESTCOMSPlus, can be used
to calculate the pole ratio from the number of exciter armature and generator rotor poles.
If the number of poles for the exciter armature and the generator rotor is unknown, the
EDM function will still operate. However, only a shorted diode can be detected. If the
number of poles is not known, it is best to disable all exciter open diode protection
parameters. In this situation, the generator and exciter pole parameters must be set to
1.0 to prevent false tripping.
All of the EDM setup guidelines presented here assume that the exciter diodes are not
open or shorted at the time of setup and testing.
The EDM estimates the fundamental harmonic of the exciter field current using discrete Fourier
transforms (DFTs). The harmonic, expressed as a percentage of the field current, is then compared to the
pickup level for open diode detection and shorted diode detection. If the percentage of field current
exceeds the open diode or shorted diode pickup level, then the appropriate time delay will begin. After the
time delay for the open diode or shorted diode condition expires and if the percentage of field current
continues to exceed the open or shorted diode pickup setting, the condition is annunciated. EDM pickup
and trip elements in BESTlogicPlus can be used in a logic scheme to initiate corrective action in response
to an open or shorted diode condition.
An EDM disable level setting prevents nuisance annunciations due to low excitation current or the
generator frequency being out of range. A disable level setting can be used to disable both open- and
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