Converter assembly with an ability to disconnect a fault current and a method for disconnecting a fault current at a converter assembly of this type

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to a converter assembly with an ability to disconnect a fault current as claimed in the independent converter assembly claim and an associated method as claimed in the independent method claim.

Modular multi-stage converters (MMC) are known from high-voltage direct-current transmission. They are fed on the input side from an alternating current network and provide a direct voltage for electrical energy supply in the high-voltage range on the output side. Connection is made through transformers that are connected on the secondary side to parallel-connected two-pole phase modules. The parallel-connected phase modules are each formed of two converter modules and connected to one another at a connection point. Each connection point forms a connection for one of the secondary-side phases of a transformer. The two converter modules are themselves in turn formed of series-connected sub-modules that comprise semiconductor switches that are implemented as a full-bridge circuit or a half-bridge circuit. These are also known in this connection as full-bridge and half-bridge modules (full-bridge and half-bridge sub-modules).

Converters with half-bridge modules are characterized by a high flexibility and efficiency. Due to the rectifier effect of the sub-modules, these converters cannot develop an appropriate opposing voltage in the event of short circuits on the direct voltage side, and must be disconnected from the alternating voltage network to disconnect the DC short circuit.

Due to their bipolar nature, converters with full-bridge modules can also provide direct voltages smaller than the chained alternating voltages, and can thus actively disconnect short circuits on the direct voltage side through the generation of an appropriate opposing voltage. They continue to make services such as, for example, reactive power (reactive current support) to the alternating current network after the disconnection. Due to the larger number of semiconductor switches in the current path, however, significantly higher electrical losses arise.

Summary of the Invention

The object of the invention is to achieve a converter assembly that actively disconnects fault currents on the direct current side, in particular short-circuit currents, within a short time, and which exhibits relatively low electrical losses. The object of the invention is, further, to propose a corresponding method for disconnecting a fault current in a corresponding converter assembly.

The object is achieved in terms of the converter through the features of the independent converter claim, and in terms of the method through the features of the independent method claim; the subsidiary claims present advantageous embodiments.

The solution relates to a converter assembly of multi-phase multi-stage converters that are connected to alternating current networks via primary-side parallel-connected transformers and which provide a total direct voltage (at two direct voltage output connections) for electrical energy supply in the high-voltage range,wherein the converter assembly comprises series-connected partial converters whose partial direct voltages form the total direct voltage (of the total converter) and each of which comprises three parallel-connected (mutually interconnected) two-pole phase modules,wherein each phase module is formed of two series-connected converter modules which are respectively connected to one another at a first connection point that forms one of the phase connections,wherein in each case one transformer is connected to the phase connections of a partial converter,wherein the two converter modules of the phase modules of a first partial converter are formed of series-connected unipolar sub-modules and those of a second partial converter are formed of series-connected bipolar sub-modules and the first and second partial converters are connected to one another at a second connection point,wherein a regulator for adjusting the direct current (that flows through the partial converters) to a predefined target value is provided which each reduce at least the partial direct voltage of the second partial converter when the direct current exceeds the target value, andwherein the partial direct voltage of the second partial converter can be inverted by the regulator at least until compensation of the partial direct voltage of the first partial converter.

The solution of the method for disconnecting a direct current fault current relates to a converter assembly (multi-phase multi-stage converter),wherein alternating current networks are connected to these via primary-side parallel-connected transformers,wherein these provide a total direct voltage (at two direct voltage output connections) for electrical energy supply in the high-voltage range,wherein these comprise series-connected partial converters from whose partial direct voltages the total direct voltage (of the total converter) is formed,wherein the partial converters each comprise parallel-connected (mutually interconnected) two-pole phase modules,wherein each phase module is formed of two series-connected converter modules that are each connected to one another at a first connection point which forms one of the phase connections,wherein respectively one transformer is connected to the phase connections of a partial converter,wherein the two converter modules of the phase modules PM of a first partial converter are formed of series-connected unipolar sub-modules and those of a second partial converter are formed of series-connected bipolar sub-modules, and the first and second partial converters are connected to one another at a second connection point,wherein, in the method, the direct current (that flows through the partial converters) is regulated to a predetermined target value in that respectively at least the partial direct voltage of the second partial converter is reduced when the direct current exceeds the target value, andwherein at least the partial direct voltage of the first partial converter is compensated by the partial direct voltage of the second partial converter (to reduce the direct current), in that an appropriate opposing voltage is generated by the second partial converter.

It is technically relatively simple if the unipolar sub-modules comprise semiconductor switches that are connected as half bridges (HB).

It is technically relatively simple if the bipolar sub-modules comprise semiconductor switches that are connected as full bridges (VB).

The semiconductors of the semiconductor switches of the individual partial converters advantageously belong to different voltage classes in order to adjust the current-carrying capacity of the partial converters to one another.

It is advantageous if the semiconductor switches of the partial converters are implemented in a parallel circuit in order to adjust the current-carrying capacity of the two partial converters to one another.

The solution thus consists of a series connection of in each case two partial converters connected directly in series and interacting, wherein one of the two partial converters is formed only of bipolar sub-modules and the other partial converter only of unipolar sub-modules. The direct voltage of the converter assembly consists of the sum of the direct voltages of the two partial converters. The partial converter with the bipolar sub-modules is here dimensioned such that its direct voltage is greater than (or at least, however, equally as great as) that of the partial converter with the unipolar sub-modules. In the event of a fault current on the direct voltage side, in particular a short-circuit, the direct voltage of the partial converter with the bipolar sub-modules is inverted within a very short time by the converter regulator, so that in total a direct voltage oriented against the direct fault current (short-circuit current) arises that actively disconnects the direct fault current (short-circuit current) through voltage compensation. Both partial converters can remain active here and continue to make, for example, reactive power and the like available to the alternating current network, meaning that the other functionality is retained.

In addition, in comparison to converter assemblies with only bipolar sub-modules for the full direct voltage, the blocking voltage required is approximately halved, which is particularly advantageous for cable applications (XLPE, limited polarity inversion strength). In addition, the proposed converter assembly increases availability since, for example, in the event of failure of one partial converter, the converter assembly can continue to be operated with reduced direct voltage.

The invention is described more closely below with reference to an exemplary embodiment.

