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Chapter 2 Functional Description GEH-6632 EX2100 User’s Guide
Auxiliary Cabinet
The auxiliary cabinet is located next to the power conversion cabinet and contains
modules to protect the generator and provide startup dc power. Modules for filtering
the incoming ac power, for de-excitation, shaft voltage suppression, and field
flashing are mounted in this cabinet.
Ac Line-to-Line Filters
Fuse protected line-to-line series RC filter circuits (snubbers) are provided to damp
the ac system to prevent voltage spikes at the completion of SCR commutation.
There are two styles of filters employed depending on the voltage. The 600 V filter
uses RC circuits and MOVs. The 1000 V filter uses the 600 V version with
additional RC circuits. Refer to Chapter 4 for details and connections.
De-excitation Module (EDEX)
During any shutdown, the energy stored in the generator field must be dissipated. In
a normal shutdown, a stop is initiated by an operator. The bridge is fired at retard
limit and sufficient time is allowed for the field to decay before the field contactors
are opened. During an abort stop (trip), the field contactors are opened immediately.
The stored field energy must be dissipated through some other means.
SCR De-excitation Module (EDEX)
For customers requiring a rapid de-excitation, an SCR de-excitation module is
provided. In the EDEX module, an SCR is fired to provide a conduction path
through the field discharge resistor (or inductor) for the field current to flow and
dissipate the field energy.
The de-excitation module has dual independent firing control circuits. Each is
activated by a parallel combination of auxiliary contacts representing the status of
the field contactor(s), bridge ac supply breaker, and exciter bridge operating state.
Any one of these paths can gate the de-excitation SCR which does not conduct
unless the field voltage is inverted. If neither firing control circuit can fire the SCR, it
is fired on overvoltage when the anode to gate voltage on the SCR exceeds the break
over voltage of the breakover diode string connected between the anode and gate.
De-excitation modules can be paralleled for larger excitation systems.
Thyrite
In systems that do not use the standard de-excitation module, a thyrite is connected
across the dc output buses of the thyristor bridge. This protects the thyristors from
high peak inverse voltages, which may occur as a result of abnormal generator
operation. These are typically only supplied on salient pole generators.