Basler-electric DECS-250N User Manual Page 27

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9440500990 Rev D 11
DECS-250N Power Stage
Power Stage
The DECS-250N supplies regulated dc excitation power to the field of a brushless exciter. Excitation
power is supplied at terminals F+ and F.
DECS-250N power stage operating power accepts single- or three-phase ac power from a transformer or
PMG. Power stage operating power is applied at terminals A, B, and C. The GND terminal serves as a
ground connection.
The DECS-250N power stage employs a six-SCR, full-wave bridge that converts the ac operating power
input into dc excitation power. It is capable of two-quadrant operation; it can produce negative as well as
positive output power as long as positive current is flowing in the machine field (in excess of the minimum
holding current required for the SCRs). The DECS-250N power stage is capable of supplying 20 Adc
continuously at nominal voltages of 63, 125, or 250 Vdc.
Nominal DECS-250N operating power levels include 480 Vac for a 250 Vdc continuous exciter field
requirement (style XX3XXXX), 240 Vac for a 125 Vdc requirement, or 120 Vac for a 63 Vdc requirement.
The frequency range of the operating power voltage is determined by the DECS-250N style number. Style
XX1XXXX indicates an operating power frequency of 50 to 60 hertz, style XX2XXXX indicates a
frequency of 61 to 420 hertz, and style XX3XXXX limits the frequency to 50 or 60 hertz (for an operating
voltage of 480 Vac).
Field Transient Protection
Fault conditions such as generator overloading or loss of synchronism can induce transients into the field
circuit. If this energy is not dissipated, it has the potential to damage the power stage SCRs. The
DECS-250N has two modes of field transient protection.
Most voltage transients are less than 150 microseconds in duration and are limited to a safe level by
internal DECS-250N components.
During major system faults, an SCR crowbar circuit protects the DECS-250N power stage. When field
overvoltage is detected, the power stage SCR firing pulses are blocked, the crowbar circuit is energized,
and a crowbar annunciation is made. During a normal shutdown, the same actions are taken but no
crowbar annunciation is made.
Inverting Style Excitation System Compatibility
The DECS-250N power stage can be configured for operation in the negative direction to supply the
control windings of an inverting style excitation system.
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