Chapter
3
-Functional Description
3 - 2
DCS 500 12-Pulse Planning and Start-Up
Functional descrip-
tion
In control engineering terms, the two power converters are divided
into a master and a slave drive. This is done by setting function-
specific (master/slave) parameters and signals.
For this purpose, the standard software in the DCS 500 additionally
incorporates Parameter Group 36 and Signal Group 136.
Both units work more or less independently, controlling their own
line contactors and the enables.
Switch-on/switch-off
logic
For the DCS 500 in its 12-pulse mode, an additional logic is pro-
vided, which in the event of an error will disable both units through
the INHIBIT input of the drive logic. For the switch-off function, the
two “ready-for-operation” signals RDY_ON (10901) and RUNNING
(10903)- from the power converters are combined using input
pointer (3607) and the output signal (13616) of the 12-pulse logic.
Through the -START_INHIBIT (908)- pointer, this output is used to
influence the drive logic and prevent impermissible switch-on.
Bridge reversal
In the case of a 6-pulse power converter, bridge reversal is initiated
by changing the polarity of the current reference (IREF2). When the
current reference changes its polarity, the actual current is con-
trolled down to zero until the current zero signal is received. This is
followed by a dead time of at least 1 ms (or 2 ms in the case of
power converters larger than 700 A), and then bridge reversal is
initiated. (Depending on the moment involved, “firing” of the new
bridge may take place in the same cycle, but will definitely occur in
the next one at the latest; 3.3 ms at 50 Hz). In addition, reversal
can be delayed by a settable number of cycles using the
REV_DELAY parameter (3601).
In the case of a 12-pulse power converter, (2 x 6-pulse power con-
verters) the logic is the same as in the single mode. In dependence
on the firing angle, the current size, the current alteration, etc., the
dead time is extended by 1 to 2 cycles. The supplement this, the
system waits with bridge reversal until both current reference po-
larities coincide, or until the additional cycles set at Parameter
REV_GAP (3602) have elapsed. This feature can prove useful
when operating large inductances. The system then automatically
changes over to the selected bridge.
If the bridge signals are unequal for the number of cycles settable
in Parameter FREV_DELAY (3603), Error Message 65 will be out-
putted. Parameter (3603) must be greater than the sum of (3601)
and (3602). Parameters (3601) and (3602) must be the same at
each of the two units.
The REV_INDIC (13613) signal indicates that bridge reversal is
running at that moment.
