Patent ID: 12218607

DETAILED DESCRIPTION OF THE INVENTION

FIG.1depicts a power converter system1. The power converter system1comprises a modular multilevel converter (MMC)2, which, in the depicted example, is used for converting an AC voltage of an AC voltage grid3, to which the MMC2is connected by means of a grid transformer4, into a DC voltage Udc.

The MMC2comprises six power converter branches5to10, which are interconnected in a double star connection. Each of the similarly configured power converter branches5to10comprises two arm inductors11,12and a series connection of two-pole switching modules SM. In the exemplary embodiment depicted inFIG.1, all switching modules SM have the same configuration, but this is generally not necessary. The number of switching modules SM in each power converter branch5to10is also basically arbitrary and may be adapted to the respective application. The switching modules SM may, for example, be full-bridge switching modules or half-bridge switching modules, the configuration of which will be described in greater detail inFIGS.2and3below. Each switching module SM comprises controllable semiconductor switches, for example, IGBTs or the like, an energy storage device, and a control module, by means of which the semiconductor switches can be actuated.

The power converter system1further comprises a central control device13, which is configured for controlling the MMC2and for actuating the switching modules SM. From a superordinate instance, the control device13receives specifications with respect to the required active power and reactive power, which are converted by the control unit into setpoints of some control parameters. The control parameters may, for example, be a voltage Uac on the AC-voltage side, a current Iac on the AC-current side, a current Idc on the DC-current side, and/or a voltage Udc on the DC-current side. In power converter systems which are designed as a symmetrical monopole, a voltage between the positive DC-voltage pole and the ground potential, Udc+, and a voltage between the negative DC-voltage pole and the ground potential, Udc−, are significant.

FIG.2depicts a first switching module SM1, which is suitable as a switching module SM for the power converter ofFIG.1, and which is connected in a half-bridge circuit. A parallel connection of a first semiconductor switch S1and a capacitor C is arranged in a capacitor branch. A second semiconductor switch is arranged in a bridge branch between two terminals X1, X2of the first switching module SM1. A flyback diode F is respectively connected in antiparallel to the two semiconductor switches S1, S2. By means of suitable actuation of the two semiconductor switches S1, S2, a switching module voltage USM1which corresponds to the capacitor voltage Uc, or a zero voltage, may be generated at the terminals X1, X2.

FIG.3depicts a second switching module SM2which is suitable as a switching module SM for the power converter ofFIG.1, and which is connected in a full-bridge circuit. The switching module SM comprises a deactivatable first semiconductor switch H1to which a first flyback diode D1is connected in antiparallel, a second deactivatable semiconductor switch H2to which a second flyback diode D2is connected in antiparallel, wherein the first and second semiconductor switches H1, H2are connected to one another in a first semiconductor series circuit and have the same forward direction. The switching module SM2furthermore comprises a third deactivatable semiconductor switch H3to which a third flyback diode D3is connected in antiparallel, and a fourth deactivatable semiconductor switch H4to which a fourth flyback diode D4is connected in antiparallel, wherein the third and fourth semiconductor switches H3, H4are connected to one another in a second semiconductor series circuit and have the same forward direction. The two semiconductor series circuits are arranged in parallel to one another and to an energy storage device C in the form of a capacitor, to which a capacitor voltage Uc is applied. In addition, the switching module SM2furthermore comprises a first terminal X1which is arranged between the semiconductor switches H1, H2of the first semiconductor series circuit, and a second terminal X2which is arranged between the semiconductor switches H3, H4of the second semiconductor series circuit. By means of suitable actuation of the semiconductor switches H1to H4, a switching module voltage USM2may be generated at the terminals X1, X2, which corresponds to the capacitor voltage Uc, corresponds to the negative capacitor voltage −Uc, or a zero voltage.

FIG.4depicts a control device13for the power converter system ofFIG.1. The control device13comprises a conventional control component14, which receives measured values M from the measuring device of the power converter2and transmits control signals S to the power converter2. In addition, the control device13comprises a module15for temperature modeling or temperature calculation. The module15receives the measurement values M on the input side. On the output side, the module15provides a set of limiting values B which is sent to a prioritization module16for determining the prioritization of the control parameters or their setpoints. At the same time, the prioritization module16is connected on the input side to a distributed control and protection system17, which derives setpoints Soll for the relevant control parameters from the measured values M, and transmits them to the prioritization module. By means of the prioritization module16, it is determined which setpoints are limited in a prioritized manner. The setpoints, along with the limiting values, are then transmitted to the control component14.

FIG.5depicts an example of temperature modeling which may be carried out in a method according to the present invention.

On the input side, a power loss model component VK receives a capacitor voltage Uc of a switching module, a duty cycle of a power converter branch in which the relevant switching module is arranged, a switching state a of a semiconductor of the switching module, the semiconductor temperature of which is to be determined or estimated, and a current Iconv in the relevant power converter branch, as input parameters. On the output side, the power loss model component VK provides a power loss L and transmits it to a temperature model component TK which also provides a coolant temperature Tv as an input parameter. By means of a pre-assigned thermal model T, the temperature model component TK determines a semiconductor temperature T(Tv, L) on the basis of the input parameters Tv, L. The semiconductor temperature T(Tv, L) is transmitted on the output side of the temperature model component TK to further components of the control system for further processing.

FIG.6shows a flow chart illustrating a method for controlling a power converter. In step S1, a setpoint of at least one control parameter is obtained. In step S2, a temperature of the power converter is obtained. In step S3, several limiting values for several control parameters are determined at predetermined time intervals as a function of a power converter temperature. In step S4, at least one limiting value is selected in a prioritized manner. In step S5, the setpoint of the selected control parameter is limited based on the at least one limiting value selected. In step S6, the power converter is controlled to limit at least one of an active power or a reactive power of the power converter based on the setpoint of the control parameter limited by the at least one limiting value.