Method for switching an operating current

Abstract

A method for switching an operating current in a meshed DC voltage network enables operating currents in a DC voltage network to be switched economically in both directions. At least one converter connected to the DC voltage network is controlled in such a way that a zero current is generated in a switching branch having a mechanical switch and the mechanical switch is actuated in accordance with the generated zero current.

Claims

1. A method for switching an operating current in a meshed DC voltage network, the method comprising the following steps: connecting converters to respective AC voltage networks; connecting the converters together on their DC voltage sides using the meshed DC voltage network; transmitting electrical power using each converter between the AC voltage network to which it is connected and the DC voltage network; providing the DC voltage network with a switching branch having a mechanical switch; regulating at least one of the converters to generate a zero current crossover in the switching branch; actuating the mechanical switch in dependence on the generated zero current crossover; causing the zero current crossover by a voltage drop generated at a DC voltage terminal of at least one of the converters; inducing a first voltage drop by using at least one of the converters and subsequently detecting and evaluating a curve of a switching current flowing in the switching branch; and then inducing a second voltage drop using the same at least one of the converters having a magnitude determined in dependence on the evaluation of the curve of the switching current.

2. The method according to claim 1, which further comprises carrying out the step of actuating the mechanical switch before reaching the zero current crossover.

3. The method according to claim 1, which further comprises: connecting at least one power semiconductor switch in series with the mechanical switch in the switching branch; holding the at least one power semiconductor switch in normal operation continuously in its conductive state; and transferring the at least one power semiconductor switch into its non-conducting blocking state to switch an operating current of the power semiconductor switch.

4. The method according to claim 1, which further comprises: detecting a switching current flowing in the switching branch by using measuring sensors; and carrying out the step of regulating at least one of the converters in dependence on the detected switching current.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) Further expedient embodiments and advantages of the invention are the object of the following description of exemplary embodiments of the invention with reference to the figures of the drawing, wherein the same reference signs refer to components that have the same effect, and wherein

(2) FIG. 1 schematically shows a meshed DC voltage network,

(3) FIG. 2 shows a switching branch of the DC voltage network according to FIG. 1 with a mechanical switch,

(4) FIG. 3 shows an idealized current curve for a zero current crossover in the switching branch according to FIG. 2,

(5) FIG. 4 shows a realistic current curve for the generation of a zero current crossover in the switching branch according to FIG. 2 and

(6) FIG. 5 shows the mechanical switch as well as the switch-off branch in more detail.

DESCRIPTION OF THE INVENTION

(7) FIG. 1 shows an exemplary embodiment of a DC voltage network 1. The DC voltage network 1 connects converters 2 together on the DC voltage side. The DC voltage network here forms network nodes 3. In the DC voltage network 1, DC voltage power switches, not shown in the figure, are arranged, being capable of switching fault currents in one direction. Only mechanical switches are provided for switching operating currents, and these also are not shown in FIG. 1. Each converter is connected to an AC voltage network not shown in the figure.

(8) FIG. 2 shows an enlarged section of the DC voltage network 3 according to FIG. 1. A switching branch 4 can be seen here, in which a mechanical switching unit 5 is arranged. The mechanical switching unit 5 comprises a mechanical switch along with thyristors arranged in series with it as power semiconductor switches. The switching branch 4 extends between two DC voltage network nodes 3a and 3b, each of which is connected directly to the converter 2a and 2b respectively. It should be noted at this point that, in the figures of the drawing, the DC voltage network 1 is only illustrated as a single-pole network. This is, however, only for the purposes of clarity. The DC voltage network expediently comprises in the context of the invention two oppositely polarized lines, for example a positive pole and a negative pole.

(9) The voltage at the first DC voltage network node 3a is largely determined by the output voltage on the DC voltage side of the converter 2a, while the voltage at the second DC voltage network node 3b is largely determined by the voltage output of the second converter 2b. In normal operation, the voltage drop U1 at the first network node 3a with respect to ground potential is somewhat larger than the corresponding voltage U2 at the second DC voltage network node 3b. The current I thus flows in the direction shown in FIG. 2 from the first DC voltage network node 3a to the second DC voltage network node 3b through the switching unit 5. If now a voltage drop is generated by the converter regulation, not shown in the figure, of the first converter 2a at the DC voltage output of the converter 2a, the voltage U1 at the first DC voltage network node drops. If this voltage drop is large enough, it results in a zero current crossover.

(10) FIG. 3 shows by way of an example an idealized zero current crossover. At time point 0 the operating conditions U1 and U2, usual in normal operation, are present, and the current flows in the direction shown in FIG. 2. After 10 seconds, the voltage drop is initiated by the first converter 2a. After 25 seconds a zero current crossover, with a flow of current in the opposite direction then occurs.

(11) FIG. 4 shows a more realistic current curve, wherein it is assumed that the voltage drop of the first converter 2a only occurs for a short period of time, so that then the first converter 2a can again be operated with normal operating parameters. As a result there are two zero current crossovers after about 16 and 24 milliseconds. If the mechanical switch of the switching unit 5 is triggered at, for example, time point 0, then after 16 milliseconds an arc between its switching contacts is extinguished, as they have reached such a large distance from each other that a sufficiently high voltage resistance is provided, and re-ignition of the arc is avoided. FIG. 5 shows a preferred embodiment of the switching unit 5, wherein it can be seen that the switching unit 5 comprises a mechanical switch 6 and two thyristors 7 and 8 connected in series with it as power semiconductor switches, which are connected in parallel with one another with opposite polarities. An arrester 9 is connected in parallel with the two thyristors 7, 8. The two thyristors 7 and 8 are continuously triggered in normal operation, so that an operating current can flow in both directions through the thyristors 7 and 8 and the mechanical switch 6. In the case illustrated in FIG. 5, the operating current I flows from left to right and thus through the thyristor 8 as well as then through the mechanical switch 6.

(12) A zero current crossover is generated in order to switch off the operating current I. The continuous triggering of the thyristor 8 is suppressed. If the current I flowing through the thyristor 8 falls below its holding current, the thyristor 8 changes into its blocking state. A flow of current through the thyristor 8, and of course also through the thyristor 7, in the direction shown is thus no longer possible. The mechanical switch 6 can now be opened with zero current. The arrester 9 serves to protect the thyristors 7 and 8 from overvoltage. As a result of the serial arrangement of the thyristors 7, 8 and of the mechanical switch 6, it is possible to make use of a less precise synchronization between the actuation of the mechanical switch 6 and the voltage drop induced by the regulation of the converter 2.