Siemens SINAMICS G130 Engineering Manual page 103

SINAMICS S120 Smart Infeed comprising a Smart Line Module and a line reactor with v
The IGBTs for regenerative operation of the Smart Infeed are always switched on at the natural firing point and
switched off after 120º (electr.) independently from the direction of the energy flow. As a result of this, a current resp.
energy flow from the supply to the DC link or vice-versa is possible at any time. The direction of the actual current
resp. energy flow is only determined by the voltage ratios between the supply and the DC link. In steady-state motor
operation, the DC link voltage during the possible current-flow phase is always smaller than the supply voltage, so
that the current flows from the supply to the DC link via the diodes. In steady-state regenerative operation, the DC
link voltage during the possible current-flow phase is always larger than the supply voltage, so that the current flows
from the DC link to the supply via the IGBTs. This control principle offers the advantage that the Smart Infeed can
react relatively fast to load variations and can also change the direction of the current resp. energy flow at any time.
However, a characteristic of the control principle described is that a harmonic reactive current flows on the line side in
no-load operation. The cause is the sinusoidal supply voltage on the one hand, and an almost perfectly smoothed DC
voltage in the DC link during no-load operation at the other hand. Consequently, directly after the firing of the IGBTs,
a short-term current flows from the DC link to the supply because, at this time, the supply voltage is slightly lower
than the DC link voltage. If the supply voltage then reaches its peak value, the voltage ratios are reversed and thus
the current direction is also reversed. This reactive current decreases as the load on the Smart Infeed increases and
disappears completely under full load.
The reactive current under no load can be prevented by a regenerative operation disable command (parameter
p3533: Infeed, inhibit generator mode).
It can also be helpful to inhibit regenerative operation of the Infeed to obtain greater stability in operation when the
unit is operating in motor mode if one or more of the following conditions apply:
·
Operation of the Smart Infeed on a weak, unstable supply system with frequent line voltage dips.
·
Operation of the Smart Infeed within a DC configuration with very high DC link capacitance C
·
12-pulse operation of the Smart Infeed on a three-winding transformer.
·
Mixed Smart Infeed / Basic Infeed operation.
Regenerative operation can be inhibited and enabled by the higher-level automation system, for example, if the
process steps in which regenerative operation can occur are clearly defined in the process control system.
Since the Smart Infeed is a line-commutated Infeed, it generates an unregulated, load-dependent DC link voltage
V from the three-phase line voltage V
DCLink
In motor operation, the DC link voltage decreases by a slightly larger amount than on the Basic Infeed, because the
voltage drop across the 4% reactor is higher than across the 2% reactor on the Basic Infeed. Under partial load in
motor operation, V ≈ 1.32 • V
DCLink
The DC link voltage is higher in regenerative operation than in motor operation, because the direction of current flow
reverses and thus also the voltage drop across the 4% reactor. Under partial load in regenerative operation, V
1.38 • V , while the figure for full load in regenerative operation is
Line
Because the DC link voltage is not regulated, fluctuations in the line voltage and changes in the operating state
(motor mode / regenerative mode) cause it to fluctuate accordingly.
Fundamental Principles and System Description
.
Line
, while the figure for full load in motor operation is
Line
V ≈ 1.30 • V
DCLink Line
V ≈ 1.40 • V
DCLink Line
SINAMICS Engineering Manual – November 2015
Engineering Information
= 4%
k
(motor mode).
(regenerative).
.
DCLink
DCLink
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≈