Basler Electric DECS-400 - Manual (Page 54)

42 9369700990 Rev R
Figure 10. V/Hz Characteristic—Time Shown on Horizontal Axis
Exciter Diode Monitor (EDM)
The EDM function in the DECS-400 monitors for an open or shorted brushless, rotating exciter power
semiconductor and can annunciate the condition so that action can be taken to protect the system from
possible damage. An open diode will cause the level of excitation to be drastically increased to maintain
the desired operating level. A shorted diode causes high current to flow through the associated exciter
armature winding which can cause excessive heating.
Exciter diode failures are detected by monitoring the output of the exciter output diodes and measuring
the induced ripple in the exciter field current. The fundamental harmonic of the exciter field current is
estimated by using discrete Fourier transforms (DFTs). The harmonic, expressed as a percentage of the
field current, is then compared to the EDM Open Diode Pickup Level setting and EDM Shorted Diode
Pickup Level setting. If the percentage of field current exceeds either setting, then the appropriate time
delay (EDM Open Diode Delay or EDM Shorted Diode Delay) begins. If the percentage of field current
exceeds the open- or shorted-diode pickup setting at the conclusion of the appropriate time delay, a failed
exciter diode annunciation is issued. The EDM Open Diode and EDM Shorted Diode Pickup Level
settings have a setting range of 0 to 100% with 0.1% increments. The EDM Open Diode Delay setting has
a setting range of 10 to 60 seconds with 0.1 second increments. The EDM Shorted Diode Delay setting
has a setting range of 5 to 30 seconds with 0.1 second increments.
A Disable Level setting prevents nuisance failed diode indications due to low excitation current and the
generator frequency being out of range. The Disable Level setting disables both open and shorted-diode
protection and has a setting range of 0 to 100% with 0.1% increments.
The EDM function is automatically disabled if the field type configuration is Main Field.
Note
The exciter diode monitor may not be able to detect:
•
A shorted diode on a brushless exciter having individually-
fused diodes.
•
An open diode on a brushless exciter having parallel diodes in
each leg of the diode bridge.
Functional Description DECS-400