Danfoss iC7 Series Design Manual page 38

Design Guide | iC7 Series Air-cooled System Modules
In a non-dedicated IT system, it is recommended to leave the ground capacitors connected in each AFE to limit conducted high
frequency disturbances between devices across the system. A non-dedicated IT system is defined here as a network where several
separate DC links are fed from the same AC supply.
If the ground capacitors are connected, continuous operation during an IT ground fault is not allowed, because a large fault current is
going through the capacitors.
In common DC bus installations, it is recommended to have ground capacitors on the DC bus side (DC+ to PE and DC- to PE) to balance
the DC bus voltage against ground. In this case, the AC side ground capacitors should be disconnected.
In common DC bus installations with common mode voltage sensitive energy storages or equipment, it is recommended to have ground
capacitors on the DC bus side (DC+ to PE and DC- to PE) to balance the DC bus voltage against ground. In this case, the AC side ground
capacitors should be disconnected. The ground capacitors should be sufficiently larger than the system parasitic capacitance to ground
to be effective in limiting the common mode voltage peaks.
As a rule-of-thumb:
10 x system parasitic capacitance ~ 100 V common mode voltage to ground
l
100 x system parasitic capacitance ~ 10 V common mode voltage to ground
l
Continuous operation during ground fault when DC side ground capacitors are connected is not allowed due to potentially large fault
currents.
3
A
G
3
Figure 23: AFE Modules in IT System
A
AC grid, floating
C
Filter capacitors
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3
3
3
AFE
C
D
3
3
3
AFE
C
D
3
2 3
2
INU
3
2 2 3
INU
B
3
2 2 3
INU
B
Separate DC links
D
Ground capacitor/capacitors on the LCL Filter side
M
M
M
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