Frequency converter with shortened pre-charging time

Abstract

The invention relates to a frequency converter, which has a rectifier (1) on the input side and at least one supporting capacitor (3) arranged downstream of the rectifier (1). The rectifier (1) has a plurality of half bridges (6), through which the corresponding input side phases (U, V, W) of the rectifier (1) supply power to the supporting capacitor (3). The half bridges (6) have active switching elements (8), so that the rectifier (1) is designed as a converter capable of regenerative feedback. The input side phases (U, V, W) are connected to the grid side phases (L1, L2, L3) of a multiphase power supply network (5) via an upstream circuit (4). The grid side phases (L1, L2, L3) are respectively connected to one of the input side phases (U, V, W) via corresponding phase capacitors (9) within the upstream circuit (4). After the support capacitor (3) reaches a first charging state (Z1), the control device (11) of the frequency converter drives the active switching element (8) to apply an input-side phase voltage (UU, UV, UW) to the input-side phase (U, V, W) via the active switching element (8), which operates inversely to the grid-side phase voltage (U1, U2, U3) applied to the grid-side phase (L1, L2, L3) connected to the input-side phase (U, V, W) via the phase capacitor (9).

Description

Technical Field

[0001] The present invention relates to a frequency converter.

[0002] wherein the frequency converter has a rectifier on the input side and at least one supporting capacitor arranged downstream of the rectifier,

[0003] wherein the rectifier has a plurality of half-bridges, via which the respective input-side phase of the rectifier is supplied with power to the supporting capacitor,

[0004] - wherein the half-bridge has active switching elements, so that the rectifier is designed as a regenerative converter,

[0005] - wherein the input side phases of the rectifier are connected to the grid side of the multiphase power supply network via an upstream circuit,

[0006] In this case, the grid-side phases are each connected within the upstream circuit via a corresponding phase capacitor to one of the input-side phases. Background Art

[0007] Mains-powered frequency converters usually require a filter circuit and a pre-charging circuit between the supply network and the rectifier on the input side of the frequency converter. The filter circuit is used to limit grid interference. The pre-charging circuit is only required when the frequency converter is connected to the supply network. The pre-charging circuit is used to limit the pre-charging current for charging the support capacitor. As soon as the support capacitor is charged, the current limitation can be terminated. For this purpose, the pre-charging circuit can be bridged by a relay, for example.

[0008] A simple and widely used precharging circuit is designed to arrange a resistor in the line from the grid-side phase of the power supply network to the input-side phase of the rectifier, which resistor is bridged by the switch contacts of the relay after the precharging of the supporting capacitor. The disadvantage of this precharging circuit is that a large power loss occurs in the resistor during precharging, so the resistor is heated significantly. In particular, in the case of large capacitances of the supporting capacitor, the resistor must also be designed to be large. Alternatively, relatively large resistance values ​​can also be selected. However, in this case, the time required for precharging increases. In addition, since the resistor takes a relatively long time to cool down again, repeated precharging can usually only be carried out to a limited extent.

[0009] Alternatively, it is known to use separate power semiconductors to precharge the support capacitor. However, this measure is complex and expensive in terms of circuitry.

[0010] A frequency converter of the type mentioned above is known from EP 2 533 409 A1. In this frequency converter, switches are also arranged in the line including the phase capacitors. The phase capacitors are therefore not used for filtering, but for limiting the charging current. In addition, filter capacitors are provided, which are respectively arranged between two phases on the grid side or between phases on the input side.

[0011] The frequency converter known from EP 2 533 409 A1 is advantageous compared to conventional prior art frequency converters because only small power losses occur during precharging. However, the overall volume is still large due to the need for both the precharging circuit and the filter circuit. Therefore, the costs are still high. In addition, a relatively long time is required to precharge the support capacitor.

[0012] From the technical paper “Control of Active Front-End Rectifier of the Solid-State Transformer with Improved Dynamic Performance during Recharging” by HD Tafti et al., a frequency converter is known, which has a rectifier on the input side and a support capacitor arranged downstream of the rectifier. The rectifier has a half-bridge, via which the corresponding input-side phase of the rectifier supplies power to the support capacitor. The half-bridge has active switching elements, so that the rectifier is designed as a converter capable of regenerative feedback. The input-side phase of the rectifier is connected to the grid side of the power supply network via an upstream circuit. The rectifier has a control device, which controls the active switching element when the support capacitor reaches a first charging state, so that the support capacitor continues to be charged at a constant charging rate. Summary of the invention

[0013] The object of the invention is to design a frequency converter of the type mentioned in the introduction such that precharging can be achieved in a relatively short time.

[0014] According to the present invention, a frequency converter of the type described in the opening part is designed so that the frequency converter has a control device, which drives the active switching element after the supporting capacitor reaches a first charging state to apply an input side phase voltage to the input side phase through the active switching element, and the input side phase voltage operates in the opposite direction to the grid side phase voltage applied to the grid side phase connected to the input side through the phase capacitor.

[0015] As a result, the effective potential difference across the phase capacitor can be increased, so that the phase capacitor delivers more charge to the supporting capacitor in each period of the grid voltage.

[0016] In an advantageous design, it is proposed that

[0017] - the grid-side phase is also directly connected to the corresponding other input side via a corresponding switch inside the upstream circuit, so that the grid-side phase is short-circuited to the input-side phase when the switch is closed, and the phase capacitors connect the two grid-side phases or the two input-side phases respectively, and

[0018] The control device keeps the switch open while the supporting capacitor is precharged and closes the switch when a predetermined second charge state of the supporting capacitor is reached, wherein the second charge state is assumed after the first charge state during the precharging of the supporting capacitor.

[0019] As a result, the same capacitor, the phase capacitor, can in a simple manner on the one hand achieve current limiting during precharging and on the other hand act as a filter capacitor for current smoothing during normal operation.

[0020] Interchanging the connected grid-side phases and input-side phases poses no major problems, and in particular the control of the active switching elements can be easily adjusted accordingly by the control device, since only the individual control signals have to be distributed to the active switching elements. Such control can remain unchanged.

[0021] Advantageously, in this case the frequency converter is designed so that when a predetermined second charging state is reached, the control device

[0022] - firstly the active switching element is driven so that an input-side phase voltage is applied to the input-side phase via the active switching element, which input-side phase voltage corresponds in amplitude and phase to a grid-side phase voltage applied to the grid-side phase connected to the input-side phase via the phase capacitor,

[0023] - then driving the active switching elements so that the input-side phase voltage applied to the input-side phase via the active switching elements is continuously converted into the grid-side phase voltage applied to the grid-side phase connected to the input-side phase via the switch, and

[0024] The switch is closed only after the input-side phase voltage has been converted to the grid-side phase voltage present at the grid-side phase connected to the input-side phase via the switch.

[0025] This significantly reduces current peaks and voltage jumps that may occur when the switch is closed.

[0026] Advantageously, the upstream circuit has an inductance in the line to the grid-side phase and / or to the input-side phase. This embodiment enables particularly simple and effective filtering.

[0027] Advantageously, the inductor is arranged in the line so that the phase current supplied by the supply network and / or flowing through the half-bridge flows through the inductor both when the switch is open and closed. As a result, the implementation of the combination of the two effects of the phase capacitor (i.e. one for filtering and one for current limiting) is particularly simple and effective. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The characteristics, features and advantages of the present invention and their implementation modes and methods will become clearer and easier to understand with the following description of the embodiments, which are described in detail with reference to the accompanying drawings. Herein, a schematic diagram is shown:

[0029] Figure 1 shows a frequency converter,

[0030] Figure 2 The network connections, upstream circuits and input-side rectifier are shown.

[0031] Figure 3 shows a flow chart,

[0032] Figure 4 shows a timing diagram, and

[0033] Figure 5 Another timing diagram is shown. DETAILED DESCRIPTION

[0034] according to Figure 1 , the frequency converter has a rectifier 1 on the input side. A further device 2 is arranged downstream of the rectifier 1 on the output side. The frequency converter also has at least one supporting capacitor 3, which is arranged between the rectifier 1 and the further device 2. The further device is usually designed as an inverter. In this case, it is an intermediate current converter. However, the further device can also be designed as another type, for example a DC power consumer, a DC grid, a photovoltaic system or an energy storage device.

[0035] The rectifier 1 has phases U, V, W on the input side, which are connected to the phases L1, L2, L3 of the power supply network 5 via an upstream circuit 4. Therefore, both the rectifier 1 and the power supply network 5 are multi-phase, that is, they have a plurality of phases U, V, W or L1, L2, L3, respectively. The phases U, V, W of the rectifier 1 are referred to below as input-side phases U, V, W to distinguish them from the phases L1, L2, L3 of the power supply network 5. Likewise, in order to distinguish the terminology from the input-side phases U, V, W of the rectifier 1, the phases L1, L2, L3 of the power supply network 5 are referred to as grid-side phases L1, L2, L3.

[0036] The number of input side phases U, V, W is usually three. However, regardless of the number of input side phases U, V, W, the number of input side phases U, V, W is equal to the number of grid side phases L1, L2, L3.

[0037] The grid-side phase L1 may be 120° ahead of the grid-side phase L2 in terms of electricity, and similarly, the grid-side phase L2 is 120° ahead of the grid-side phase L3 in terms of electricity, and the grid-side phase L3 is 120° ahead of the grid-side phase L1 in terms of electricity. However, it is preferred that the grid-side phase L1 is 120° behind the grid-side phase L2 in terms of electricity, and similarly, the grid-side phase L2 is 120° behind the grid-side phase L3 in terms of electricity, and the grid-side phase L3 is 120° behind the grid-side phase L1 in terms of electricity.

[0038] like Figure 2 As shown, the rectifier 1 has a plurality of half bridges 6. One of the input side phases U, V, W of the rectifier 1 supplies power to the support capacitor 3 via each half bridge 6. The half bridge 6 is as shown in FIG. Figure 2 The rectifier 1 is shown with a diode 7. In addition, it has an active switching element 8 connected in parallel with the diode 7. The switching element 8 is an electronic semiconductor switch, for example a MOSFET. Due to the active switching element 8, the rectifier 1 is able to regenerate. They are controlled by corresponding control signals C1 to C6. The diode 7 can be a separate component. Alternatively, the diode 7 can be an integrated component of the active switching element 8.

[0039] like Figure 2 As shown, the grid side phases L1, L2, L3 are connected to one of the input side phases U, V, W via corresponding capacitors 9 inside the upstream circuit 4. Specifically, the grid side phase L1 is connected to the input side phase U, the grid side phase L2 is connected to the input side phase V, and the grid side phase L3 is connected to the input side phase W via one of the capacitors 9. However, other designs are also possible. In particular, the upstream circuit 4 can also be designed as in EP 2 533 409 A1. Figure 5 The capacitor 9 is referred to below as the phase capacitor 9 in order to distinguish it from the supporting capacitor 3 in terms of language.

[0040] Furthermore, the grid-side phases L1, L2, L3 are connected to one of the input-side phases U, V, W via a corresponding switch 10 within the upstream circuit 4. Figure 2 , the grid side phases L1, L2, L3 are connected to the input side phases U, V, W connected via the phase capacitors 9 relative to the grid side phases L1, L2, L3 through the corresponding switches 10, respectively, with respect to the other input side phases U, V, W. Specifically, the grid side phase L1 is connected to the input side phase V, the grid side phase L2 is connected to the input side phase W, and the grid side phase L3 is connected to the input side phase U. The connection between the grid side phase L1 and the input side phase V, the connection between the grid side phase L2 and the input side phase W, and the connection between the grid side phase L3 and the input side phase U are directly connected. When the switch 10 is closed, the grid side phases L1, L2, L3 and the input side phases U, V, W are short-circuited. In accordance with Figure 2In the case of the design scheme, the phase capacitor 9 connects two grid-side phases L1, L2, L3 or two input-side phases U, V, W. In particular, preferably, the phase capacitor 9 is used to connect two grid-side phases L1, L2, L3 or two input-side phases U, V, W.

[0041] - the grid-side phase L1 and the input-side phase V (which are short-circuited to one another in this case by a switch 10 ) are connected to the grid-side phase L2 and the input-side phase W (which are short-circuited to one another in this case by another switch 10 ),

[0042] The grid-side phase L1 and the input-side phase V are connected to the grid-side phase L3 and the input-side phase U (which are likewise short-circuited to one another via a further switch 10 in this case), and

[0043] - The grid side phase L2 and the input side phase W are connected to the grid side phase L3 and the input side phase U.

[0044] according to Figure 1 The frequency converter also has a control device 11. The control device 11 generates control signals C1 to C6 in particular. In addition, the control device 11 keeps the switch 10 open while the supporting capacitor 3 is precharged. As a result, the supporting capacitor 3 is slowly charged via the phase capacitor 9. Keeping the switch 10 in the open state can be achieved, for example, in that the control device 11 does not load the relay 12 that actuates the switch 10 with current. However, at a later point in time, the control device 11 closes the switch 10. For example, a corresponding control signal for the relay 12 is Figure 2 In this state (ie when the switch 10 is closed), the normal operation of the frequency converter is realized. In normal operation, the control device 11 also controls the other device 2. For example, Figure 1 As shown, electrical energy can be supplied by the inverter to the load 13. This state can be defined, for example, by the fact that the support voltage UZK dropped across the support capacitor 3 has reached a predetermined percentage of its set value, for example at least 90%.

[0045] exist Figure 1 and Figure 2 In particular, Figure 2In the figure, many advantageous designs are also shown. In particular, the upstream circuit 4 can have an inductance 15 in the line 14 to the grid-side phases L1, L2, L3. If the inductance 15 is present, they are preferably arranged in the line 14 as follows, that is, the inductance is not only traversed by the phase currents I1, I2, I3 (grid-side phase currents I1, I2, I3) fed in from the power supply network 5. Alternatively or additionally, the upstream circuit 4 can have an inductance 16 in the line 14 to the input-side phases U, V, W. If the inductance 16 is present, they are preferably arranged in the line 14 as follows, that is, the inductance is not only traversed by the phase currents IU, IV, IW (input-side phase currents IU, IV, IW) flowing by the half-bridge in the case of the open switch 10 but also in the case of the closed switch. The nodes 17, 18 at which the switch 10 is connected to the line 14 are therefore arranged between the phase capacitors 9 and the inductances 15, 16.

[0046] Combine the following Figure 3 The operating mode of the frequency converter is described. In this context, two further advantageous embodiments are also described. These embodiments are interrelated. However, they can be implemented regardless of whether the inductors 15 and 16 are arranged in the line 14.

[0047] First, the basic design according to the present invention is explained.

[0048] according to Figure 3 , when the rectifier 1 is connected to the power supply network 5, the control device 11 checks in step S1 whether the support capacitor 3 has reached a charging state Z1, hereinafter referred to as a first charging state Z1. In particular, the control device 11 can check in step S1 whether the support voltage UZK reaches or exceeds a suitable predetermined value, which is, for example, between 30% and 70% of the rated value of the support voltage UZK, in particular between 40% and 60%.

[0049] If this is not the case, the control device 4 returns to step S1 via step S2. In step S2, the control device 11 keeps both the switch 10 and the active switching element 8 open. Figure 3 This is indicated by the fact that there are no outputs of the control signals C0 to C6 (denoted by the symbol “ . / ."express).

[0050] When the supporting capacitor 3 reaches the first charging state Z1, the control device 11 controls the active switching element 8 in step S3. This control causes the input side phase voltage UU to be fed to the corresponding phase capacitor 9 via the active switching element 8, through which the input side phase U is switched, and the input side phase voltage runs inversely (in opposite phase) to the grid side phase voltage U1. Figure 4 In the left region thereof, the correlation curves of the grid-side phase voltage U1 and the input-side phase voltage UU as a function of time t are shown.

[0051] In a similar manner, the control device 11 controls the active switching element 8, and switches the input side phase V by the active switching element, so that the corresponding phase capacitor 9 is configured with the input side phase voltage UV, which runs inversely (opposite in phase) to the grid side phase voltage U2. Similarly, the control device 11 controls the active switching element 8, and switches the input side phase W by the active switching element, so that the corresponding phase capacitor 9 is configured with the input side phase voltage UW, which runs inversely (opposite in phase) to the grid side phase voltage U3. As a result, a relatively large potential difference is applied to the phase capacitor 9, so that a relatively large phase current I1, I2, I3, IU, IV, IW flows in the line 14. As a result, the supporting capacitor 3 is quickly charged. Since the grid side phase voltages U1, U2, U3 vary over time, the input side phase voltages UU, UV, UW also vary. The control of the active switching element 8 must meet these conditions. However, the corresponding determination of the control of the active switching element 8 is generally known to those skilled in the art and does not need to be explained in detail.

[0052] During the precharging of the support capacitor 3, the second charge state Z2 is adopted, but only after the first charge state Z1. As already mentioned, the second charge state Z2 can be defined, for example, as follows: the support voltage UZK dropped across the support capacitor 3 has reached a predetermined percentage of its rated value, for example at least 90%. Therefore, the control device 11 checks in step S4 whether the support capacitor 3 has reached the second charge state Z2. If this is not the case, the control device 4 returns to step S3.

[0053] When the second charge state Z2 is reached, the basic principle of the invention is completed. If the second charge state Z2 is determined accordingly (in particular, the support voltage UZK dropped across the support capacitor 3 has reached a sufficiently high percentage of its rated value) and the upstream circuit 4 is designed accordingly, normal operation can be carried out directly (see later step S7). However, Figure 2 Two advantageous embodiments are also shown. These will be described in conjunction below. This occurs because the two embodiments are interrelated. In particular, they relate to measures taken before and shortly before closing the switch 10.

[0054] In this case, the second charge state Z2 corresponds only to a partial charging of the supporting capacitor 3. For example, as already mentioned, it may be required that the supporting voltage UZK dropped across the supporting capacitor 3 has reached a predetermined percentage of its rated value, for example at least 90%.

[0055] In the context of this advantageous embodiment, the control device 11 does not immediately close the switch 10, but rather proceeds to step S5 in advance. In step S5, the control device 11 controls the active switching elements 8 for the phase U so that an input-side phase voltage UU is applied to the input-side phase U via the associated active switching elements 8, which phase voltage corresponds in amplitude and phase to the grid-side phase voltage U1. Figure 4 The right side thereof shows the correlation curves of the phase voltage U1 and the phase voltage UU as a function of the time t. Figure 5 In its left-hand region, the correlation curves of the phase voltage U1 and the phase voltage UU as a function of the time t are likewise shown.

[0056] Figure 4 It also shows how and in what way the curve running in reverse (left area) is switched to the curve running in phase (right area). Figure 4 As shown, switching is then preferably effected at the zero crossing of the phase voltages U1 , UU.

[0057] In a similar manner, in step S5, the phase voltage UV of the input-side phase V is also switched from a curve running inversely to the curve of the phase voltage U2 to a curve in phase with the curve of the phase voltage U2. The same applies to the switching of the phase voltage UW of the input-side phase W from a curve running inversely to the curve of the phase voltage U3 to a curve in phase with the curve of the phase voltage U3.

[0058] Then in step S6, the control device 11 controls the active switching elements 8 for the phase U so that an input-side phase voltage UU is applied to the input-side phase U via the associated active switching elements 8, which corresponds in amplitude and phase to the grid-side phase voltage U3. Figure 5 As shown, a continuous conversion is performed. This is achieved, for example, as follows: Figure 5 As shown, the input side phase voltage UU is constantly maintained at a specific voltage value from time point t1 until the grid side phase voltage U3 reaches a specific voltage value at time point t2. Once the specific voltage value is reached, the phase voltage UU follows the grid side phase voltage U3.

[0059] In a similar manner, the phase voltage UV of the input-side phase V is also converted from a curve in phase with the curve of the phase voltage U2 to a curve in phase with the curve of the phase voltage U1 in a step S6. The same applies to the conversion of the phase voltage UW of the input-side phase W from a curve in phase with the curve of the phase voltage U3 to a curve in phase with the curve of the phase voltage U2.

[0060] The conversion of the phase voltages UU, UV, UW just described can be performed simultaneously. In this case, it is particularly advantageous if the grid-side phase L1 lags behind the grid-side phase L2 by 120° in terms of electricity, and likewise if the grid-side phase L2 lags behind the grid-side phase L3 by 120° in terms of electricity, and if the grid-side phase L3 lags behind the grid-side phase L1 by 120° in terms of electricity. This is because the corresponding input-side phase voltages UU, UV, UW only have to maintain a constant value for a period of one-third of the grid-side phase voltages U1, U2, U3. Otherwise, the input-side phase voltages UU, UV, UW would have to maintain a constant value for two-thirds of the period of the grid-side phase voltages U1, U2, U3.

[0061] The individual input side phases U, V, W can be controlled independently of each other by the corresponding half bridge 6. As an alternative, the conversion of the phase voltages UU, UV, UW just described can therefore be performed successively. In this case, it is even possible to switch the input side phase voltages UU, UV, UW from a curve in phase with one of the grid side phase voltages U1, U2, U3 to a curve in phase with another of the grid side phase voltages U1, U2, U3 without causing a voltage jump and then switching continuously as before. However, in this case, for example, for phase U, the switching must be performed at a time point when the grid side phase voltages U1 and U3 have the same value. In a similar manner, the switching of phase V must be performed at a time point when the grid side phase voltages U1 and U2 have the same value. In this case, the switching of phase W must also be performed at a time point when the grid side phase voltages U2 and U3 have the same value.

[0062] After the just described conversion of the phase voltages UU, UV, UW, the control device 11 closes the switch 10 in step S7. The switch 10 should be closed as soon as possible after the time t2. With the closing of the switch 10, the precharging of the support capacitor 3 and the precharging of the entire frequency converter is completed. This is followed by normal operation, in which the further device 2 is operated in the usual manner. In order to close the switch 10, the control device 11 can, for example, control the relay 12 in such a way that the switch 10 is closed.

[0063] In summary, the present invention relates to the following facts:

[0064] The frequency converter has a rectifier 1 on the input side and at least one supporting capacitor 3 arranged downstream of the rectifier 1. The rectifier 1 has a plurality of half bridges 6, through which the respective input-side phases U, V, W of the rectifier 1 supply the supporting capacitor 3. The half bridges 6 have active switching elements 8, so that the rectifier 1 is designed as a regenerative converter. The input-side phases U, V, W are connected to the grid-side phases L1, L2, L3 of the multiphase power supply network 5 via an upstream circuit 4. The grid-side phases L1, L2, L3 are connected to one of the input-side phases U, V, W via a corresponding phase capacitor 9 within the upstream circuit 4. After the support capacitor 3 reaches the first charging state Z1, the control device 11 of the frequency converter drives the active switching element 8 to apply input side phase voltages UU, UV, UW to the input side phases U, V, W through the active switching element 8. The input side phase voltages are inversely proportional to the grid side phase voltages U1, U2, U3. The grid side phase voltages are applied to the grid side phases L1, L2, L3 connected to the input side phases U, V, W through the phase capacitors 9.

[0065] The invention has numerous advantages. In particular, a fast and low-loss precharging of the support capacitor 3 is possible. Furthermore, the upstream circuit 4 is greatly simplified. In particular, the precharging resistors and precharging capacitors required therein can be saved compared to the prior art. Thus, the frequency converter according to the invention can be realized more cheaply and efficiently compared to the frequency converter of the prior art. In particular, the costs of the upstream circuit 4 can be reduced by more than 50% compared to the prior art. The capacitance values ​​required for the phase capacitors 9 can be matched in such a way that a fast precharging and a good filtering effect are achieved.

[0066] Although the present invention has been illustrated and described in more detail by means of preferred embodiments, the present invention is not limited to the disclosed examples and other variations may be derived therefrom by those skilled in the art without departing from the scope of protection of the present invention.

Claims

1. A frequency converter, -in, The frequency converter has a rectifier (1) on the input side and at least one supporting capacitor (3) arranged downstream of the rectifier (1). - wherein the rectifier (1) has a plurality of half-bridges (6), via which the respective input-side phases (U, V, W) of the rectifier (1) supply the supporting capacitor (3), wherein the half bridge (6) has active switching elements (8), so that the rectifier (1) is designed as a regenerative converter, - wherein the input side phases (U, V, W) of the rectifier (1) are connected to the grid side phases (L1, L2, L3) of the multiphase power supply network (5) via an upstream circuit (4), - wherein the grid-side phases (L1, L2, L3) are each connected to one of the input-side phases (U, V, W) via a corresponding phase capacitor (9) within the upstream circuit (4), It is characterized in that The frequency converter has a control device (11) which, after the support capacitor (3) reaches a first charging state (Z1), controls the active switching element (8) to apply an input-side phase voltage (UU, UV, UW) to the input-side phase (U, V, W) via the active switching element (8), the input-side phase voltage operating inversely to a grid-side phase voltage (U1, U2, U3) applied to a grid-side phase (L1, L2, L3) connected to the input-side phase (U, V, W) via the phase capacitor (9).

2. The frequency converter according to claim 1, characterized in that: - the grid side phase (L1, L2, L3) is also directly connected to the corresponding other input side through the corresponding switch (10) inside the upstream circuit (4), so that the grid side phase (L1, L2, L3) is short-circuited with the input side phase (U, V, W) when the switch (10) is closed, and the phase capacitor (9) is respectively connected to two grid side phases (L1, L2, L3) and to two input side phases (U, V, W), and - the control device (11) keeps the switch (10) open when the supporting capacitor (3) is precharged and closes the switch when a predetermined second charging state (Z2) of the supporting capacitor (3) is reached, the second charging state being adopted after the first charging state (Z1) during the precharging of the supporting capacitor (3).

3. The frequency converter according to claim 2, characterized in that: When the predetermined second charging state (Z2) is reached, the control device (11) - firstly, the active switching element (8) is driven to apply an input-side phase voltage (UU, UV, UW) to the input-side phase (U, V, W) via the active switching element (8), the input-side phase voltage corresponding in amplitude and phase to a grid-side phase voltage (U1, U2, U3) applied to a grid-side phase (L1, L2, L3) connected to the input-side phase (U, V, W) via the phase capacitor (9), - then driving the active switching element (8) so that the input side phase voltage (UU, UV, UW) applied to the input side phase (U, V, W) through the active switching element (8) is continuously converted into a grid side phase voltage (U1, U2, U3) applied to the grid side phase (L1, L2, L3) connected to the input side phase (U, V, W) through the switch (10), and The switch (10) is closed only after the input-side phase voltage (UU, UV, UW) is converted into a grid-side phase voltage (U1, U2, U3) applied to the grid-side phase (L1, L2, L3) connected to the input-side phase (U, V, W) via the switch (10).

4. The frequency converter according to claim 2 or 3, characterized in that: The upstream circuit (4) has inductances (15, 16) in the lines (14) to the grid-side phases (L1, L2, L3) and / or to the input-side phases (U, V, W).

5. The frequency converter according to claim 4, characterized in that: The inductors (15, 16) are arranged in the line (14) so ​​that the phase currents (I1, 12, 13, IU, IV, IW) supplied by the power supply network (5) and / or flowing through the half bridge (6) flow through the inductors both when the switch (10) is opened and closed.

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