Siemens sinamics dcp - 7.11.11 Pulse width modulation (PWM)

Operation
7.11 Closed-loop control
SINAMICS DCP
346 Operating Instructions, 09/2019, A5E34382853K
7.11.11 Pulse width modulation (PWM)
See function diagram 6830 in the SINAMICS DCP List Manual.
The PWM unit generates the control signals for the power semiconductors in the power unit.
Further, it controls the switchover of the direction of the current.
In the closed-loop current control, switching over the direction of the current can be delayed
(p55065).
In the current control, a delay (p55065) can be set for switching over the current direction.
During this delay time, the current setpoint is set to zero.
The individual power sections are clocked with the switching frequency; however, the switch-
on instants are phase shifted with respect to one another:
● 30 kW device: 2 power units, the power units are clocked with a 180° phase shift
● 120 kW device: 3 power units, the power units are clocked with a 120° phase shift
Depending on the modulation depth (r55250, r55251[0], r55251[1]), the power units are
operated as follows:
200 % ... 100 %
Step up
pos. current (side 1 -> side 2)
100
% ... 0 %
Step down
pos. current (side 1
-> side 2)
0 % ... -100 %
Step down
neg. current (side 2 -> side 1)
-100 % ... -200 %
Step up
neg. current (side 2 -> side 1)
The parameters for limiting the modulation depth (P55305 and p55306) allow the possible
operating range of the SINAMICS°DCP to be limited (FP6825).
Examples:
● Modulation depth limited to 0 % ... +200 %: only positive current possible
● Modulation depth limited to -200 % ... +0 %: only negative current possible
● Modulation depth limited to -100 % ... +100 %: only stepping down possible
Transition range between stepping up and stepping down
In the transition range between stepping up and stepping down, approximately the same
voltages are present at both sides of the SINAMICS DCP. Modulation depths would be
required, which would result in very short switch-on durations of the power semiconductors.
Short switch-on durations such as these would not be permitted in order to protect the power
semiconductors.
The response of the PWM units in this range can be set using parameter p55321 (Optimized
modulator step-down/step-up transition):
● p55321 = 0 (deactivated):
In the transition range, the two sides of the DCP are directly connected through the power
semiconductors without any clocking. This state is maintained until the current controller
intervenes, which means that the system exits the transition range.
If the system still operates close to the transition range, then operation alternates
between being directly connected and being clocked.
This results in a slight low-frequency oscillation of the current, comparable with the
response of a 3-level control system.