Basler Electric DXCB-2 De-Excitation/Crowbar Module

INTRODUCTION

The DXCB-2 De-Excitation/Crowbar Module protects the generator field

and exciter circuits by providing a low impedance path for currents that

would otherwise cause excessively high transient voltage or damaging

arcs. A DXCB-2 module can be operated in one of two modes: de-

excitation (DX) or crowbar (CB).

In DX mode, the DXCB-2 module provides a path for the generator field

current when its field breaker is opening or when the normal path is not

available.

In CB mode, the DXCB-2 module provides a low-resistance path for large

positive voltages across the generator field and exciter bridge. Generator

pole-slip is one source of high voltage.

The enclosure of the DXCB-2 offers electric shock protection from incidental

contact. An identical footprint and functional compatibility make for simple

replacement of a DXCB with a DXCB-2.

DXCB-2 Models

Three versions of the DXCB-2 module are available. Part number 9500900100

is rated for up to 600 Vdc nominal field voltage and can conduct up to

2.000 Adc for one second. Part number 9500900101 is rated for up to 1.000

Vdc nominal field voltage and can conduct up to 5.000 Adc for one second.

Part number 9500900102 has identical ratings to part number 9500900101

but has a higher range of jumper-selectable trip voltage values.

Reference Table 3 for jumper-selectable trip voltages for each model.

SAFETY

As with all electrical equipment, appropriate safety measures must be taken

when dealing with excitation equipment. High voltage is present within the

excitation cubicles; their magnitudes depend upon the particular system.

Opening the sources (e.g., disconnecting device 41) does not completely

disconnect the high voltages.

As long as the machine is still physically connected to the system, there is

a possibility that a safety hazard exists.

Every precaution must be taken when working at the excitation control

switchgear to ensure that all high voltages are isolated and avoided by

test personnel. In addition to the machine terminal voltage (460 Vac or

greater) at the excitation transformer, there may be other sources of power

entering the cubicles, such as 125/250 Vdc and/or 120/240 Vac used to

feed certain devices. A good rule to follow is to assume that all connections

are live and dangerous until proven otherwise and to avoid all body contact

with any of these voltages.

INSTALLATION

The DXCB-2 is typically preinstalled in the excitation control system. If

replacement is necessary, carefully label and document each of the

connections to the DXCB-2 in order to properly reconnect the new module.

The DXCB 2 must be monitored and adjusted to fit the specific application

at initial startup, whether it is a new unit or a replacement.