Basler Electric DECS-400 - Manual (Page 55)

9369700990 Rev R 43
Exciter Diode Monitor Settings
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 poles for the brushless exciter armature to the
generator rotor is entered to ensure proper operation for both open and shorted diode protection.
Setting the Pickup Level
To set the pickup level of the EDM OD value (open diode) ripple and EDM SD (shorted diode) ripple
parameters, 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 voltage while monitoring the EDM OC (open circuit) and EDM SC (short circuit) % ripple on the
DECS-400 HMI metering screen. Record the highest value for each. See the Human-Machine Interface
chapter for more details on displaying metering quantities.
With Number of Generator Poles Known
Multiply the highest EDM OC (open circuit) value, obtained under Setting the Pickup Level by 3. The
result is the Exciter Open Diode % Ripple Level (EDM OD % Ripple). The multiplier can be varied
between 2 and 5 to increase or decrease the trip margin. However, reducing the multiplier could result in
nuisance EDM OD indications. A time delay is also available and adjustable from 10 to 60 seconds.
Multiply the highest EDM SC (short circuit) value, obtained under Setting the Pickup Level by 50. The
result is the Exciter Shorted Diode % Ripple Level (EDM SD % Ripple). The multiplier can be varied
between 40 and 70 to increase or decrease the trip margin. However, reducing the multiplier could result
in nuisance EDM SD indications. A time delay is also available and adjustable from 5 to 30 seconds.
The DECS-400 has fixed EDM inhibit levels to prevent nuisance EDM indications while the generator
sensing voltage is less than 45 Hz, greater than 70 Hz, or when the field current is less than 1 Adc.
Although the user can adjust the field current inhibit level from 0 to 100%, the fixed EDM inhibit levels
take priority. Pole ratios must be in the range of 1 to 40 with 0 used if the ratio is unknown.
With Number of Generator Poles Unknown
The DECS-400 can detect shorted diode conditions when the number of generator poles is not known. To
provide this protection, disable EDM OD protection and set the pole ratio to zero. Enable EDM SD
protection. Multiply the maximum EDM SC (short circuit) % ripple value, obtained under Setting the
Pickup Level by 30. The multiplier can be varied between 20 and 40 to increase or decrease the trip
margin. Reducing the multiplier could result in nuisance EDM SD indication.
Test the EDM Settings
Start the generator from a dead stop condition and increase its speed and voltage to the rated value.
Load the machine to its rating and confirm no EDM alarm indications occur. All of the EDM setup
guidelines presented here assume the exciter diodes were not opened or shorted at the time of setup and
testing.
Voltage / Current Unbalance Detection
If the voltage or current unbalance increases above 40% for 18 half-cycles, (about 150 milliseconds at
60 Hz), the power system stabilizer (PSS) is disabled. Voltage and current balance is determined by the
ratio of negative-sequence quantities to positive-sequence quantities. When an unbalance condition
decreases below the 40% threshold for 18 half-cycles, the stabilizer is enabled again. This feature is
active only when the positive-sequence voltage magnitude is above 10% of the Rated Voltage setting
(System Configuration screen, Rated Data tab). The PSS does not have to be enabled for this function to
operate.
Limiter Functions
DECS-400 limiter functions consist of a stator current limiter, an overexcitation limiter, and underexcitation
limiter, an underfrequency limiter, and a volts per hertz limiter.
DECS-400 Functional Description