Basler DECS-250, Instruction Manual

Page: 67 / 352

9440300990 Rev J 51
DECS-250 Protection
Notes
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 at
zero to prevent false tripping.
An open exciter diode may not be detected if the generator frequency
and operating power frequency are the same and the DECS-250
operating power is supplied by a single-phase source. Three-phase
operating power is recommended for reliable open diode detection.
Open diode detection will also be impaired when a permanent magnet
generator (PMG) is supplying DECS-250 operating power and the
PMG frequency is the same or lower than the generator frequency.
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
B
and shorted diode detection
C
. 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
D
or shorted diode
E
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 BESTlogic
Plus 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
F
can be used to disable both open- and
shorted-diode protection when the rated field current drops below the user-defined percentage. EDM
protection can be disabled and enabled
G
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.