Improved short-circuit-protected converter assembly

Abstract

The invention relates to a converter assembly (1) for transmitting electric power between phases of a three-phase or multi-phase network (4) and a DC bus (5). The converter assembly comprises a DC / AC converter (2) which can be connected to the network (4) via a sine filter (7) and to the DC bus (5) via a split DC-side intermediate circuit, wherein a converter bridge (8) of the DC / AC converter (2) is connected to a central point (M) of the intermediate circuit, and a rectifier (3) which is connected in parallel with the DC / AC converter (2). The DC / AC converter (2) has a controller (10) which is designed to operate the DC / AC converter (2) in an over-modulation mode if the voltage values of the DC bus (5) are less than double the amplitude of one of the phase voltages of the network (4).

Description

[0001] Improved short-circuit protected converter arrangement

[0002] Description

[0003] The invention relates to a converter arrangement for transmitting electrical power between phases of a three- or multi-phase network and a DC bus.

[0004] To supply DC loads connected to a DC bus from an AC grid (DC = direct current; AC = alternating current), DC / AC converters are used. These converters draw electrical power from the AC grid and supply it to the DC bus in order to maintain a DC bus voltage within a permissible range for its operation. For this purpose, a reverse power flow from the DC bus to the AC grid may also be necessary or desired, for example, if, in addition to the DC loads, DC sources and / or storage devices are also connected to the DC bus. In the following, the term "DC / AC converter" also includes such converters.DC / AC converters typically have a converter bridge in which half-bridges with series-connected semiconductor power switches, each with an anti-parallel freewheeling diode, are arranged between a positive DC voltage terminal and a negative DC voltage terminal. Between these half-bridges is a bridge output connected to a phase of the AC mains via a sine-wave filter, also known as a line filter. The freewheeling diodes, viewed in isolation, form a rectifier. If the DC bus connected to the DC voltage terminals has an insufficient DC bus voltage, an uncontrollable current flows through the freewheeling diodes, which can destroy or at least damage the converter bridge. Such a situation occurs, for example, in the event of a short circuit on the DC bus.

[0005] To protect against such damage to the converter bridge of the DC / AC converter, document DE 10 2021 113205 A1 proposes connecting a passive rectifier in parallel with the DC / AC converter to take over the short-circuit current instead of the DC / AC converter. To ensure that the rectifier becomes conductive at a higher threshold than the DC / AC converter, thus effectively protecting it, it was proposed to connect a step-up transformer upstream of the rectifier on the AC side, so that the rectifier is subjected to a higher mains amplitude on the AC side than the DC / AC converter. However, such a transformer represents considerable additional effort.

[0006] Furthermore, the use of such a rectifier as a short-circuit protection device limits the operation of the DC bus to DC bus voltages at which none of the rectifier diodes become conductive, because this creates an uncontrolled current in the phase of the AC network connected to the conductive diode, which can suddenly reach very high values ​​due to the low network impedance. In any case, such an uncontrolled current impairs the network quality because it contains significant, non-mains frequency components. Especially for DC loads with center-grounded interference suppression capacitors, this is already the case if the bus voltage falls below twice the amplitude of one of the phase voltages of the AC network connected to the rectifier. It is therefore recommended for the aforementioned system design that the DC bus voltage maintains a minimum distance from the maximum permissible value of twice the phase voltage amplitude.

[0007] In addition, for pure feeders, the use of overmodulation is known, for example from document EP 2 375 552 A1, to enable standard-compliant feeding of power from a DC source into a grid even when the voltage of the DC source is less than twice the phase voltage amplitude of the grid.

[0008] Accordingly, it is an object of this invention to provide a converter arrangement for transmitting electrical power between phases of a three-phase or multi-phase network and a DC bus, in which a passive rectifier is connected in parallel to an active DC / AC converter as short-circuit protection, and to enable secure operation with an extended DC voltage range in this arrangement, which can be extended to less than twice the value of the amplitude of one of the phase voltages of the network.

[0009] This object is achieved by a converter arrangement having the features of independent claim 1. Preferred embodiments of the converter arrangement are the subject of the dependent claims.

[0010] In one aspect of the invention, a converter arrangement for transmitting electrical power between phases of a three- or multi-phase network and a DC bus comprises a DC / AC converter that is connectable to the network via a sine-wave filter and to the DC bus via a DC-side split intermediate circuit, wherein a converter bridge of the DC / AC converter is connected to a center point of the intermediate circuit. The converter arrangement further comprises a rectifier connected in parallel with the DC / AC converter. The DC / AC converter has a control device that is configured to operate the DC / AC converter in overmodulation at voltage values ​​of the DC bus below twice the amplitude of one of the phase voltages of the network.In this case, the center point of the DC link and thus the potentials of a positive and a negative connection of the DC link, or the positive and negative potentials of a connected DC bus, are shifted such that the positive DC bus potential is greater than the highest instantaneous phase potential of the AC network, while at the same time the negative DC bus potential is smaller than the lowest phase potential of the AC network. Consequently, even for DC bus voltages below twice the amplitude of the phase voltage of the AC network, an uncontrolled current through one of the rectifier diodes due to the operation of the DC / AC converter in overmodulation can be avoided, at least within a limited voltage range.

[0011] In this way, the occurrence of a fault current due to the charge reversal of interference suppression capacitors connected to the DC bus is accepted. However, since this fault current flows through the DC / AC converter, its amplitude, and especially its higher-frequency components, are significantly reduced by the DC / AC converter's sine-wave filter compared to the case where one of the diodes of the parallel-connected rectifier becomes conductive. Any fault current that occurs in this case bypasses the DC / AC converter and is therefore not attenuated by the sine-wave filter.

[0012] Due to operating-state-related voltage drops across the sine-wave filter during operation of the DC / AC converter, it is advantageous to shift the potential of the intermediate circuit center point so that a suitably selected minimum distance exists at all times between the positive or negative bus potential and the highest or lowest phase potential of the AC network. This can be achieved through flat-top modulation and increases operational reliability and improves the EMC (electromagnetic compatibility) behavior of the converter arrangement. At the same time, the fault current generated by overmodulation is reduced.

[0013] Overmodulation can be generated by exciting a zero-sequence system at three times the grid frequency, with the center point of the intermediate circuit being shifted sinusoidally. However, other forms of overmodulation with different time profiles of the potential shift are also conceivable, ensuring that there is always a sufficient distance between the positive or negative bus potential and the highest or lowest phase potential of the AC grid. For example, the center point potential can be shifted only by the smallest necessary amount to ensure the minimum distance.

[0014] A further advantage of using overmodulation is that the switching losses of the converter bridge can be reduced by reducing the number of switching operations during overmodulation compared to operation without overmodulation. The converter arrangement can therefore be operated particularly energy-efficiently in the extended voltage range.

[0015] It should be noted at this point that operating the DC / AC converter in overmodulation mode generates a capacitive fault current that is three times the grid frequency. However, this fault current has a lesser impact on grid quality than the uncontrolled current through the rectifier diodes.

[0016] In an advantageous embodiment, the fault current generated by the overmodulation is either disregarded, at least when monitoring the fault current of the transformer arrangement by a fault current monitor, or is actively compensated for when detecting the fault current. This can be achieved, for example, by an additional conductor that is routed alongside the phase conductors and the neutral conductor through a fault current sensor of the fault current monitor and through which a suitably generated compensation current flows. It is also conceivable to correct the signal from the fault current sensor by the known fault current component generated by the overmodulation before evaluating the fault current signal. In an advantageous embodiment, a frequency component of the fault current at three times the mains frequency is not taken into account at all when assessing whether a fault current has occurred, or is eliminated from the signal before evaluation.

[0017] In the following, the invention is illustrated by means of figures, of which

[0018] Fig. 1 shows an embodiment of a converter arrangement according to the invention,

[0019] Fig. 2 shows a time course of voltages and earth currents without overmodulation,

[0020] Fig. 3 shows a time course of voltages with a first type of overmodulation, and

[0021] Fig. 4 shows a time course of voltages with a second type of overmodulation.

[0022] Fig. 1 shows an embodiment of a converter arrangement 1 according to the invention, comprising a DC / AC converter 2 and a rectifier 3, which are connected to a three-phase network 4 at a common connection point. The network 4 provides, at the connection point of the converter arrangement 1 with phases L1, L2, L3, respectively assigned phase voltages UL1, UL2, UL3, which have an equal amplitude aligned symmetrically to a ground potential PE.

[0023] The DC / AC converter 2 has a sine-wave filter 7 on the grid side, to which a converter bridge 8 is connected in order to exchange power in both directions between the grid 4 and a DC bus 5 connected to a DC connection of the DC / AC converter 2. A split intermediate circuit is arranged between the converter bridge 2 and the DC bus 5, comprising a first intermediate circuit capacitor C1 and a second intermediate circuit capacitor C2, which are connected in series via a center point M. During normal operation, a voltage UDC of the DC bus 5 is divided equally between the intermediate circuit capacitors C1, C2. The voltage distribution can be changed in a known manner by controlling bridge switches of the converter bridge 2 and thus regulated as required.

[0024] A load 6 is connected to DC bus 5, symbolized by a load resistor R1 and comprising series-connected interference suppression capacitors C5 and C6 to improve electromagnetic compatibility. The center point of the interference suppression capacitors C5 and C6 is connected to ground PE. Additional loads, which optionally also include interference suppression capacitors, can be connected to DC bus 5, as can additional feeders, such as battery converters.

[0025] The voltage on the DC bus 5 depends on the current operating conditions and can fluctuate within a permissible voltage range or even leave this range, for example due to undesirable operating conditions. For example, a fault in the form of a short circuit or an impermissibly high temporary power draw can occur, as a result of which the voltage UDC of the DC bus 5 drops below the rectified value of the phase voltages UL1, UL2, UL3 of the mains 4. In this case, the rectifier 3 should generate a current flow from the mains 4 into the DC bus 5, bypassing the DC / AC converter 2, which current is sufficient to trigger the fuses provided in the DC bus 5. This measure prevents damage to or destruction of the freewheeling diodes connected in anti-parallel to the bridge switches of the converter bridge 8 or integrated therein.

[0026] However, if the voltage UDC of the DC bus 5 approaches the rectified value of the network 4, and the positive potential UDC_P with respect to earth PE becomes slightly smaller than an instantaneous value of a phase voltage UL1, UL2, UL3 of the network 4 with respect to earth or the negative potential UDC_P with respect to earth PE becomes slightly larger than an instantaneous value of a phase voltage UL1, UL2, UL3 of the network 4 with respect to earth, one of the diodes of the rectifier 3 changes to a conductive state and triggers a current flow, which, for example, is shown in Fig. 1 as a dashed curve starting from diode D1 of the rectifier 3 via interference suppression capacitor C5 to earth PE and from there via phase L1 back to diode D1. To detect a fault current, a fault current monitor 9 can be provided in the converter arrangement 1. The residual current monitoring 9 can be communicatively connected to a control device (10) of the converter arrangement 1.

[0027] Fig. 2 shows a measured time profile of one of the phase voltages of the network, here as an example the phase voltage UL1, the voltage 11 of the same phase between the sine-wave filter and the converter bridge, the fault current 12 flowing via a selected rectifier diode (here diode D1 in Fig. 1) and via earth PE, and a value of the positive DC bus potential UDC_P. It can be seen that the moment the phase voltage UL1 exceeds the value of the positive DC bus potential UDC_P, a peak value of the fault current 12 occurs, which only completely subsides when the phase voltage UL1 reaches its peak value. The maximum value of the fault current 12 is essentially only limited by the level of the network impedance of the affected phase. However, this does not necessarily lead to the fuses of the DC bus 5 being triggered if the excess is only brief.However, since the current peak has impermissibly high frequency components above the grid frequency and thus degrades grid quality, operation of the converter arrangement under these conditions violates grid connection standards. In addition, a comparable current peak is generated via the other diodes of the rectifier whenever a phase voltage UL1, UL2, UL3 is exceeded or undershot compared to one of the DC bus potentials UDC_P, UDC_M, so that in a three-phase grid, up to six current peaks can occur during one grid period and impair grid quality. Such a converter arrangement would therefore have to be taken out of service if a minimum distance between the positive or negative DC bus potential and one of the phase voltages is undershot, which limits the permissible voltage range.

[0028] In addition to the fault current described above, which can also be understood and described as the generation of a zero-sequence system by the rectifier, a deviating zero-sequence system generated by the DC / AC converter can cause a circulating current flowing between the DC / AC converter and the rectifier, leading at the very least to losses and heating of the energized components. This circulating current can significantly exceed the fault current and also shorten the service life of the converter assembly, but it has no impact on power quality, at least not directly.

[0029] Therefore, without the use of a converter arrangement according to the invention, protecting the DC / AC converter via a parallel rectifier results in the disadvantage that the lower limit of the permissible voltage range for the DC bus 5 must be higher by a safety margin than the rectified value of the mains voltage. This disadvantage is overcome by the converter arrangement according to the invention in that the converter arrangement can also be operated at voltage values ​​UDC of the DC bus 5 that are lower than the rectified value of the mains 4, reliably preventing the above-described impairment of the mains quality.

[0030] Fig. 3 shows a first variant of an overmodulation of the converter arrangement 1 according to the invention. The time profiles of the phase voltages UL1, UL2, UL3 are shown in comparison to the mean positive DC bus potential IIDCP and the mean negative DC bus potential IIDCM, respectively, on which a common sinusoidal potential shift in the form of a zero-system NS with three times the mains frequency is superimposed, so that the superimposed positive DC bus potential UDCP+NS is always higher and the superimposed negative DC bus potential UDCM+NS is always lower than each of the phase voltages UL1, UL2, UL3. This ensures that the diodes of the rectifier 3 remain blocked even when the phase voltages exceed or fall below the limit values ​​given by the mean DC bus potentials IIDCP, IIDCM.By superimposing the DC bus potentials with a zero-sequence system, designated UDCP+NS, UDCM+NS, a fault current is also caused by the charge reversal of the interference suppression capacitors connected to DC bus 5. However, this fault current has a much lower amplitude than the fault current 12 in the case described in Fig. 2 and, furthermore, only has a frequency component at three times the mains frequency without higher frequency components. This has a much lesser impact on the power quality, and the fault current caused by the overmodulation can be easily compensated for in a known manner via the fault current during insulation monitoring of DC bus 5.

[0031] Fig. 4 shows a second variant of an overmodulation of the converter arrangement 1 according to the invention. The time profiles of the phase voltages UL1, UL2, UL3 are also shown here in comparison to the mean positive DC bus potential UDCP and the mean negative DC bus potential UDCM, respectively, on which a common potential shift in the form of a flat-top modulation FT with three times the mains frequency is superimposed, so that the superimposed positive DC bus potential UDCP+FT is always higher and the superimposed negative DC bus potential UDCM+FT is always lower than each of the phase voltages UL1, UL2, UL3.Specifically, with flattop modulation, the DC bus potentials UDCP+FT, UDCM+FT are maintained at the values ​​of the average DC bus potentials UDCP, UDCM if the average DC bus potentials UDCP, UDCM are spaced from all phase voltages UL1, UL2, UL3 by a distance greater than a specified minimum distance, and otherwise shifted so that the specified minimum distance is maintained. In other words, the minimum shift of the DC bus potentials UDCP+FT, UDCM+FT is applied, which is necessary to maintain the specified minimum distance. This minimizes the magnitude of the fault current caused, although the fault current then also has frequency components that are several times higher than the mains frequency. Other forms of overmodulation with different time profiles of the shift of the DC bus potentials are also conceivable without departing from the technical teaching of the invention.

[0032] List of reference symbols

[0033] 1 converter arrangement

[0034] 2 DC / AC converters

[0035] 3 rectifiers

[0036] 4 Network

[0037] 5 DC bus

[0038] 6 Last

[0039] 7 sine filters

[0040] 8 converter bridge

[0041] 9 Residual current monitoring

[0042] 10 Control device

[0043] 11 Tension

[0044] 12 Residual current

[0045] UL1 , UL2, UL3 phase voltage

Claims

Patent claims:

1. Converter arrangement (1) for transmitting electrical power between phases of a three- or multi-phase network (4) and a DC bus (5), comprising: - a DC / AC converter (2) which can be connected to the mains (4) via a sine-wave filter (7) and to the DC bus (5) via a DC-side, split intermediate circuit, wherein a converter bridge (8) of the DC / AC converter (2) is connected to a center point (M) of the intermediate circuit, and - a rectifier (3) connected in parallel with the DC / AC converter (2), wherein the DC / AC converter (2) has a control device (10) which is designed to operate the DC / AC converter (2) in overmodulation at voltage values of the DC bus (5) below twice the amplitude of one of the phase voltages of the network (4).

2. Converter arrangement (1) according to claim 1, wherein the rectifier (3) is connected in parallel with the DC / AC converter (2) without a transformer.

3. Converter arrangement (1) according to claim 1 or 2, wherein the converter arrangement (1) further comprises a fault current monitor (9), wherein the fault current monitor (9) is designed to disregard a fault current caused by the overmodulation during the monitoring or to compensate for it.

4. Transformer arrangement (1) according to one of the preceding claims, wherein the converter arrangement (1) has a fault current monitor (9), wherein the fault current monitor (9) is designed to disregard a frequency component of the fault current at three times the mains frequency during the monitoring.

5. Converter arrangement (1) according to one of the preceding claims, wherein the overmodulation is designed such that a positive DC bus potential to earth is always greater than the maximum phase voltage of the network (4) to earth and a negative DC bus potential to earth is always smaller than the minimum phase voltage of the network (4) to earth.

6. Converter arrangement (1) according to one of the preceding claims, wherein the overmodulation is a sinusoidal shift of the DC bus potentials relative to ground.

7. Converter arrangement (1) according to one of the preceding claims, wherein the overmodulation is a flattop modulation of the DC bus potentials relative to ground.

Images