Further expedient configurations and advantages of the invention are the subject matter of the description which follows relating to exemplary embodiments of the invention with reference to the figures in the drawing, in which the same reference symbols refer to functionally identical components, and in which
FIG. 1 shows the basic construction of a DC network coupling with self-commutated power semiconductor switches,
FIG. 2 shows the feeding converter of the DC network coupling shown in FIG. 1 and the distribution network, in this case realized as an island network, in a schematic illustration, and
FIG. 3 shows the current profile of one phase of a converter as shown in FIG. 2, in a schematic illustration. FIG. 1 shows a DC network coupling 1 for supplying energy to an island network 2 by means of a supply network 3. The supply network 3 is connected to the HVDC bridge 1 via a transformer 4, and the island network 2 is connected to the HVDC bridge 1 by a transformer 5, the switches 6 and 7 being provided for decoupling the HVDC bridge 1 from the respective supply network 3 or from the island network 2.
The DC network coupling 1 has two converters 8 and 9 with self-commutated power semiconductor switches 10 in a 6-pulse bridge circuit. A freewheeling diode 11 is provided in the parallel circuit of each power semiconductor switch 10. The converters 8 and 9 are connected to one another via a DC voltage intermediate circuit 12, which forms a positive DC voltage connection provided with the “+” symbol and a negative DC voltage connection provided with the “−” symbol. Energy stores in the form of capacitors 13 are connected between the positive and negative connection of the DC voltage intermediate circuit 12.
In order to suppress harmonics, which occur on conversion of the current, filter banks 14 are provided which are each connected between the transformers 4, 5 and the converters 8 and 9, respectively, in a parallel circuit. Finally, inductances 15 are connected into each phase in order to provide a smooth current profile.
FIG. 2 shows the DC network coupling 1 shown in FIG. 1, in which the converter 8, which is provided for regulating the voltage in the DC intermediate circuit 12, is only illustrated schematically. In particular, this illustration shows protective devices 16, 17 and 18 which intervene in the energy distribution in a graded manner in terms of their operation and, for this purpose, each interact with a switch 7, 19 and 20, respectively. For current measurement purposes, converters 24 are provided which generate an output signal which is proportional to the respective phase and is sampled and digitized by the respective control unit 16, 17 or 18.
If a short-circuit current is present in a power supply unit region 25 of the island network 2, a short-circuit current fed by the converter 9 flows and is identified by means of the converter 24 both of the protective device 16 and the protective device 17. The protective devices are parameterized such that, initially, the protective device 17 responds and thus the subnetwork 25 is disconnected from the island network 2 via the switch 19 in a targeted manner without the power supply to the subnetwork 26 of the island network 2 being impaired. Once the subnetwork 25 has been disconnected a short circuit and thus disconnection of the entire island network 2 is avoided by the protective device 16. The protective device 16 merely has a safety function and intervenes when the protective device 17 does not trip even after a relatively long period of time, with the result that damage to sensitive components is avoided.
FIG. 3 illustrates one exemplary embodiment of the method according to the invention in a schematic illustration. The current flowing through one phase of the converter 9 in the event of a short circuit is plotted on the axis 27. The time axis is provided with the reference symbol 28. If the absolute value for the current in the phase shown exceeds a threshold value 29, the power semiconductor switches 10 associated with this phase are provided with a pulse inhibitor at time t1. In other words, the power semiconductor switches of the phase are switched off, or, in other words, the power semiconductors are changed over to their inhibiting position. After the end of the pulse inhibiting period, i.e. after the end of the pulse period of the phase, a zero-voltage indicator is generated at time t2 by all of the semiconductor switches 10a, 10b and 10c associated with the positive connection being switched on, the power semiconductor switches 10d, 10e and 10f, on the other hand, remaining switched off. In this manner, soft, gradual decay of the current results, such that severe current fluctuations in the island network 2 are avoided. At time t3, the regulation is taken on by the rated operation regulation, but with a lower driving level. If the subnetwork unit having the short circuit has been removed successfully from the network by means of the protection technique, the current changes over to its rated value owing to the resultant driving level, as is illustrated by the lower arrow 30. If, furthermore, a short circuit is present, the current again rises to above the threshold value 29, as indicated by the arrow 31, with the result that the abovedescribed method is carried out again. Corresponding regulation for negative alternating currents is likewise indicated in FIG. 3.
Patent Application
20080084643
