Siemens SINAMICS DCM 6RA80 Faq page 33

The converter generates a variable direct voltage a function of a variable firing angle by
rectification of the line voltage. The firing angle determines the firing time of the thyristor and
thus the magnitude of the variable output direct voltage.
The figure below shows the system line-to-neutral voltages and the magnitude of the
converter output voltage Ud.
The number defines the firing sequence for the thyristors (valve), 1 - 6, if a clockwise rotating
field exists for the line supply.
One valve must always be conducting in two phases for each to permit current flow in the DC
circuit.
The figure shows: At the first instance, valve 5 and 6 are conducting, then valve 6 is
conducting and 1 is fired; at the next firing instant, valve 1 is still conducting and valve 2 is
fired etc. As is shown, each valve must remain conducting for two subsequent firing
instances.
A thyristor remains conducting as long as there is a current flow, on current zero, it returns to
the off state. Therefore suitable measures are needed to ensure that the valve, i.e. the first
thyristor to be fired, remains conducting for continued current flow now via the next thyristor.
For this, the firing pulse duration of the thyristor must be greater than 60 degrees, because
when stationary, the next thyristor is fired every 60 degrees. Because the firing pulse
duration on the 6RA80 is however only 0.89 ms (60 degrees correspond to 3.3 ms at 50 Hz
line frequency), to keep the first valve to be fired conducting for a further 60 degrees,
another firing pulse, the second pulse is triggered from it. This is above all necessary for
discontinuous current.
Figure applies to the clockwise rotating field of the network
Circuit B6C
U
Phase
U U U
U
V
W U
U
U -U -U
U V W
ud = u - u
U V
Id
Iv
Ud
The momentary value of the output direct voltage can be read from the voltage characteristic
above for the delay angle = 30 degrees.
See example: Arrow of direct voltage magnitude Ud in pink.
An overlap o occurs during commutation, i.e. on transfer of current conducting from one
valve to the next. At this point in time both thyristors involved are conducting at the same
time (commutation short circuit). o lasts longer for longer commutation inductance. o results
in a loss of the voltage-time area and thus a loss in the mean direct voltage.
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