Multilevel converter systems with series connected phase modules may be used in medium voltage drives. These converter systems are mainly used in the range of 3 kV to 15 kV, but may also be used in other voltage ranges. Every phase module may comprise an energy storage element in the form of a direct current capacitor or DC link capacitor. Usually, these elements need to be pre-charged every time the converter system starts to operate.
The charging of these elements is usually an operation that needs care, since a direct connection of the converter system without pre-charged DC link capacitors to a medium voltage grid would cause huge currents on the grid itself.
Several ways to charge the DC link capacitors may be used. Commonly, independent charging unit elements are directly connected to the DC link. These charging unit elements usually comprise voltage transformers connected to a low voltage supply, which transformers are placed in addition to an already present high power medium voltage transformer.
For example, U.S. Pat. No. 8,223,515 B2 shows a medium voltage system with an apparatus that embodies a charging transformer integrated (coupled) to the main transformer, in a way that the low voltage feed windings share the same transformer core. This low voltage feed windings are used as a pre-charge circuit.
In WO2012/140008A2 a medium voltage converter and a method to start up in a way to prevent inrush currents is presented.
In JP2011109801A a low voltage feed transformer element is specially connected to the medium voltage converter to pre-charge DC link capacitors.
U.S. Pat. No. 7,929,323 B2 shows a further example for an apparatus for pre-charging a power converter.
U.S. Pat. No. 6,072,707 discloses in FIG. 2 a converter system with two phase modules, each phase module comprising a first converter leg and a second converter leg interconnected with a DC link, wherein the DC link comprises two capacitors connected in series between a positive point, a middle point and a negative point, each converter leg being adapted for interconnecting its output with the positive point, the middle point or the negative point of the DC link. The phase modules are connected in series via outputs of the converter legs, whereby a second converter leg of a lower phase module is connected with a first converter leg of a higher phase module.
In US 2013/0176014 A1, EP 2 747 269 A2 and EP 2 733 837 A1 further generic converter systems are disclosed.