CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of International Application No. PCT/EP2018/097093 filed Dec. 28, 2018, which designates the United States of America, and claims priority to EP Application No. 18154369.5 filed Jan. 31, 2018, the contents of which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to converters. Various embodiments include modular multilevel converters and/or methods of operation for the modular multilevel converter.

BACKGROUND

Modular multilevel converters (MMC) are a comparatively recent and attractive solution for power conversion without transformers. Some of their advantages are a high scalability, lesser or no need for AC filters and lesser or no need for DC link capacitors at the DC bus. A disadvantage of MMC is their high complexity which comes from the high number of individual converter cells and the associated switch control and capacitor charge balancing.

SUMMARY

The teachings of the present disclosure describe improved modular multilevel converters with decreased complexity. For example, some embodiments of the present disclosure include a modular multilevel converter (300,400) for converting between a DC voltage and an AC voltage, the modular multilevel converter (300,400) comprising: at least two phase units (102,202) connected in parallel and connectable to a DC terminal, the phase units (102,202) each comprising an upper converter arm (108) and a lower converter arm (112), each converter arm (108,112) having one or more cells (C, CF, CH) being arranged in series, wherein each cell (C, CF, CH) comprises an energy storage element (CP) and a switching arrangement adapted to switching the energy storage element (CP) in or out of the series of cells (C, CF, CH), a control unit being adapted to control the switching arrangements of the cells (C, CF, CH), a middle converter arm (110) arranged in series between the upper and lower converter arm (108,112), the middle converter arm (110) having one or more cells (C, CF, CH) being arranged in series, an upper node (116) between said upper and middle converter (108,110) arm and a lower node (122) between said lower and middle converter arm (112,110), an upper bridging element (118) arranged between said upper node (116) and an AC terminal (120), and a lower bridging element (124) arranged between said lower node (122) and said AC terminal (120).

In some embodiments, each of the bridging elements (118,124) has a voltage rating equal to or higher than that of the middle converter arm (110).

In some embodiments, the energy storage elements (CP) are capacitors (CP) and the capacitors (CP) in the middle converter arm (110) have a smaller capacity than the capacitors (CP) of the lower or upper converter arm (108,112).

In some embodiments, the voltage rating of the middle, upper and lower converter arm (108,110,112) are equal.

In some embodiments, the voltage rating of the middle converter arm (110) is lower than the voltage rating of the upper and/or lower converter arm (108,112).

In some embodiments, the bridging elements (118,124) each comprise one or more switches (802) that are optimized for low conduction losses.

In some embodiments, the bridging elements (118,124) each comprise one or more thyristors with reverse diodes.

In some embodiments, the cells (C, CF, CH) of the upper and lower converter arm (108,112) are full-bridge type cells (CF) and the cells of the middle converter arm (110) are half-bridge type cells (CH).

In some embodiments, there are three or more of said phase units (102,202) being arranged in parallel.

In some embodiments, there are a second upper bridging element (218) arranged between said upper node (116) and an AC terminal (120), and a second lower bridging element (224) arranged between said lower node (122) and said AC terminal (120).

In some embodiments, there is a first disconnector (206) between the middle point (204) of the upper and lower bridging element (118,124) and the AC terminal (120) and a second disconnector (210) between the middle point (208) of the second upper and lower bridging element (218,224) and the AC terminal (120).

As another example, some embodiments include a method for operating the modular multilevel converter (300,400) as described herein, wherein the phase units (102,202) are connected to a DC network and the AC terminals (120) of the phase units are connected to an AC network, the control unit controls the switching arrangements of the cells (C, CF, CH) of the upper and lower converter arm (108,112) to adapt the voltage of the upper and lower converter arm (108,112) to the AC voltage level and the control unit controls the bridging elements (118,124) by turning on the upper bridging element (118) when the lower converter arm (112) voltage is higher than the upper converter arm (108) voltage, and turning on the lower bridging element (124) when the lower converter arm (112) voltage is lower than the upper converter arm (108) voltage.

In some embodiments, the control unit switches the bridging elements (118,124) with a frequency that is twice the AC voltage frequency.

In some embodiments, the control unit controls the switching arrangements of the cells (C, CF, CH) of the middle converter arm (110) such that the individual switches (H1a, H1b, H2a, H2b) are turned on and/or off when their voltage is zero.

