Known converters, such as inverters or rectifiers, can have three DC poles. In addition to positive and negative DC poles, they have a neutral DC pole. For example, three-level converters have three DC poles. Examples of three-level converters are given in T. Brückner, S. Bernet and H. Güldner, “The Active NPC Converter and Its Loss-Balancing Control”, IEEE Transactions on Industrial Electronics, Vol. 52, No. 3, June 2005. For example, examples of three-level neutral-point-clamped (NPC) converters are given.
FIG. 1 shows a circuit diagram of a first three-phase three-level inverter according to a known implementation. The inverter can be supplied by one or more photovoltaic panels 10 connected between its positive and negative direct current poles UDC,P, UDC,N as illustrated. The inverter further supplies a three phase power system connected to its alternating current poles AC1, AC2, AC3.
A possible problem related to the use of three-level inverters is that in normal operation a high-frequency voltage is formed between the DC circuit and earth, e.g., a common-mode voltage ucm includes a high-frequency component. This high-frequency component can stress, e.g. in case of photovoltaic applications, insulations of the photovoltaic panels and further cause harmful high-frequency currents in the vicinity of support structures of the photovoltaic panels. The following equation applies to the system of FIG. 1:UDC>Ug√{square root over (6)}+Umargin  (1)whereUg=phase voltage of the three phase power system supplied by the inverter; andUmargin=voltage value depending on the system control and component values.
FIG. 2 shows a circuit diagram of a second three-phase three-level inverter according to a known implementation. The formation of the common-mode voltage ucm can be prevented by earthing the neutral direct current pole M of the inverter DC circuit as illustrated in FIG. 2. This solution eliminates the common-mode voltage ucm completely and thus also the voltages of the positive and negative direct current poles UDC,P, UDC,N against earth are pure DC voltages, in this case UPM and UMN respectively. A possible problem related to this solution is that the voltage UDC of the DC circuit should be considerably higher. The following equation applies to the system of FIG. 2:UDC>Ug√{square root over (8)}+Umargin  (2)
FIG. 3 shows a circuit diagram of a third three-phase three-level inverter according to a known implementation. Because in known implementations only the high-frequency components of the common-mode voltage ucm cause problems, these problems can be resolved by removing the high-frequency components. This can be accomplished by connecting the neutral direct current pole M of the inverter DC circuit to a virtual neutral point of the power system supplied by the inverter. The virtual neutral point of a three phase power system generally refers to a star point of three star-connected impedances in the three-phase power system. An example of such a virtual neutral point is the star point of star-connected capacitors of an AC output filter of the inverter as shown in FIG. 3. This procedure is also called virtual earthing. The virtual earthing makes the operation possible with a lower voltage UDC of the DC circuit than in the case of an earthed system (as in FIG. 2). However, a possible problem with this solution, when compared with the system of FIG. 1, is that the current stress of the semiconductor switches can be greater and the operation can call for a higher voltage UDC of the DC circuit. The following equation applies to the system of FIG. 3:UDC>Ug√{square root over (6)}+Umargin+Uadd  (3)whereUadd=additional voltage depending on the system control and modulation methods and component values.
In practice Uadd can be about 20 to 50 V, which is 5 to 12% if a desirable voltage variation of a maximum power point tracking of the inverter is 400 V. This 5 to 12% in the DC voltage range can be a significant amount when possible operating environments of the system are evaluated.