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04.05
3
Description
3.1
Field of Application
The SICROWBAR DC surge suppressor was specially designed to protect the excitation
equipment (field windings and converters) of synchronous motors and generators with slip rings
against DC overvoltages. High-speed de-excitation can also be initiated by means of an additional
option.
An appropriate field discharge/discharge resistor must always be provided in order to convert the
energy stored in the field winding into heat during high-speed de-excitation or to reduce the
overvoltage (see also Section 3.4).
Generally speaking, the SICROWBAR DC surge suppressor was developed to specifically
discharge capacitances and inductances and to provide protection against overvoltage. This takes
place electronically.
3.2
Structure
The device comprises the following components:
• Two disc-type thyristors V1 and V2 connected back to back.
• Ignition circuit, which depending on the version can ignite the blocking thyristor at a specific
voltage (possible igniting voltages: 800V, 1200V, 1600V, 2200V, 2600V and 3000V).
• Voltage sensing module for field discharge/discharge resistor (449404.9210.50) with signaling
outputs
• Power connections (copper bars) C, D
• Terminal XEW1 (for connecting the sense cable).
•
An optional high-speed de-excitation module (option G11). This module enables the thyristors to
be ignited at any time by energizing three independent quick-acting relays
Refer to the dimension drawing in Section 4 for details of the mechanical structure of the device.
Refer to the block diagram in Section 3.6 for the electrical structure.
3.3
Functional Description
Functional Description of the Thyristor Switch
The two thyristors connected back to back between connections C and D can carry the current
specified on the rating plate (see technical data) for short periods (approx. 5 seconds). In the event
of an overvoltage, the ignition circuit ignites only the blocking thyristor via a break-over diode and
diverts the igniting current past the blocking thyristor via a diode which is anti-parallel to its
gate/cathode. The BOD is always used in its direction of operation by means of a bridge rectifier
and the igniting current is limited to 6 to 8 A by means of series resistors. The thyristor breaks
through within a few microseconds and the voltage collapses to the forward voltage (1 to 1.5 Volts).
The load current heats the disc-type thyristor in just a few seconds and the heat energy is
absorbed by the clamped thyristor assembly. The load cycle can, therefore, only be repeated after
a cooling time has elapsed (refer to the technical data in Section 3.5).
High-Speed De-Excitation Option
The high-speed de-excitation option is connected to the ignition circuit in such a way that the series
resistors for the igniting current are located in the current path. Three quick-acting relays, which are
energized independently of each other and whose contacts are connected in parallel, allow the
thyristors to be ignited at any time providing sufficient voltage is present (approx. 5% of the tripping
voltage). The control inputs for the three relays are 24 V, 110-125 V and 220-240 V DC with a
common reference potential for each relay (see diagram in Section 5.2).
Siemens AG 7VV3003-8TA74
SICROWBAR 7VV3003 DC-Surge Suppressor Operating Instructions
3 - Description
ENGLISH
9
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