Direct multi-phase to single-phase, modular multi-level converter, use thereof in railway networks and method for operating same

Abstract

The present disclosure relates to a direct multi-phase to single-phase, modular multilevel converter (MMC) (10) comprising: a first AC interface (40) for connecting it to a multi-phase first electrical network (20), in particular a three-phase electrical network; a second AC interface (50) for connecting it to a single-phase second electrical network (30); two sets of P-phase branches (80, 90), each phase branch comprising N switching cells (60) respectively connected in series between a respective one of the P-phases (L1, L2, L3) of the first AC interface (40) and a single-phase (L) neutral potential (N) of the second AC interface (50), where P > = 2 and N > = 3; and at least one controller configured to control the first set of P-phase legs (80) and the second set of P-phase legs (90) to selectively provide maximum apparent power at the first AC interface (40) and the second AC interface (50), and to switch the MMC (10) to a limited output operating mode in response to detecting at least one failed switching unit (60) of the first phase leg, the MMC (10) provides a reduced maximum apparent power at the first AC interface (40) and / or the second AC interface (50) by controlling at least the first phase leg based on the tuning factor k1ph to output a reduced phase leg voltage # imgabs0 #. The disclosure also relates to a railway network and a method for operating a direct multi-phase to single-phase MMC (10).

Description

Technical Field

[0001] The present disclosure relates generally to multilevel power converters, and more particularly to direct multiphase to single phase, modular multi-level converters (MMCs), three-phase to single phase (3-phase / 1-phase) AC / AC railway networks, and methods of operating direct multiphase to single phase MMCs. Background Art

[0002] Direct multiphase to single-phase converters are used to supply energy from a multiphase grid (e.g., a three-phase power supply network) directly to a single-phase grid and vice versa. In the conversion, the converter can match the voltage and / or operating frequency of the respective grid. For example, energy supplied by a high-voltage three-phase industrial grid with a first frequency of, for example, 50 Hz or 60 Hz can be injected into a single-phase railway power supply grid with a second frequency of, for example, 16.7 Hz or 25 Hz.

[0003] An important aspect of grid converter design is the harmonic distortion of the converter feeding the source grid and / or the target grid. This distortion can be at least partially avoided by using multi-level converters (MLCs), such as modular multilevel converters, where multiple switching cells can be activated or deactivated individually to better match a desired target waveform, such as a sine wave of a given frequency.

[0004] Another important aspect in the design of grid converters is the availability of the converter. Since such converters often form part of critical infrastructure, they should remain operational even in the event of component failure. In the case of MMCs, this is often achieved by providing redundant switching cells in the converter, which can take over the role of a defective switching cell in operation. Summary of the invention

[0005] The object of the present invention is to describe an improved or alternative MMC and methods of use and operation thereof, which limit the amount of harmonic distortion and / or achieve a high level of availability. Preferably, the disclosed MMC should be simple in design, easy to control, and / or have a reduced number of components compared to existing solutions.

[0006] Embodiments of the present disclosure are directed to devices, systems, and methods for their operation that reduce or overcome the need to provide redundant switch units in an MMC.

[0007] According to a first aspect, a direct multi-phase to single-phase, modular multi-level converter (MMC) is provided. The MMC comprises:

[0008] - a first AC interface, for connecting the MMC to a first multi-phase power grid, in particular a three-phase power grid;

[0009] - a second AC interface for connecting the MMC to a single-phase second grid;

[0010] a first group of P phase-legs, wherein each phase-leg of the first group comprises N switching units connected in series between a respective one of the P phases of the first AC interface and a single phase of the second AC interface, wherein P≥2 and N≥3;

[0011] - a second group of P-phase legs, wherein each phase leg of the second group comprises N switching units connected in series between a respective one of the P-phases of the first AC interface and a neutral potential of the second AC interface; and

[0012] at least one controller configured to control the first group of P-phase legs and the second group of P-phase legs to selectively provide maximum apparent power at the first AC interface and the second AC interface, and in response to detecting at least one failed switching unit of the first phase leg, switch the MMC to a limited output operation mode, wherein the MMC is controlled by a tuning factor k 1ph Controlling at least a first phase leg to output a reduced phase leg voltage A reduced maximum apparent power is provided at the first AC interface and / or the second AC interface.

[0013] Among other things, the inventors have found that it is possible to operate a multiphase to single-phase direct modular multilevel converter without redundancy even in the presence of some defective switching cells. This is achieved at least in part by providing a limited output mode with reduced power capability, in particular a so-called N-1 operating mode. 1ph Controlling the switching cells of at least the affected phase leg or even all phase legs to output a reduced voltage can avoid damage to other parts of the converter and can limit harmonic distortion in the source and / or target network. This in turn enables the MMC to operate with a reduced maximum apparent power capability, e.g. Figure 6 and Figure 7 until the defective switching unit is repaired or replaced, at which time the maximum apparent power rating can be reached again, e.g. Figure 4 and Figure 5 As no or fewer redundant switch units are required, the disclosed MMC has a smaller number of components, a simpler design, and a lower manufacturing cost.

[0014] In at least one embodiment, the MMC is configured to selectively provide maximum apparent power at the first AC interface and the second AC interface using all 2NP switch units in a normal operating mode without a failed switch unit, for example, Figure 4 and Figure 5 This alleviates the need to provide any redundant switching units in the MMC.

[0015] In at least one embodiment, the MMC is configured to selectively provide a reduced maximum apparent power at the first AC interface and the second AC interface using a subset of the switch units other than the at least one failed switch unit in a limited output operation mode in which at least one failed switch unit exists, particularly in an N-1 operation mode in which at least one failed switch unit exists, e.g. Figure 6 and Figure 7 This allows the switching voltage to be distributed across all remaining, operational switching cells.

[0016] According to another aspect, a three-phase to single-phase (3-phase / 1-phase) AC / AC railway network is provided. The railway network comprises at least one MMC according to the first aspect. Such a railway network can be implemented with fewer components at a lower cost and still provide a high degree of reliability, for example, at least for providing a reduced amount of power to the railway power supply grid.

[0017] According to another aspect, a method for operating a direct multi-phase to single-phase MMC (in particular an MMC according to the first aspect) comprising a plurality of phase legs, each phase leg comprising a plurality of switching units, is provided. The method comprises the following steps:

[0018] - in a normal operating mode of the MMC, using all switch units of all phase legs, providing a maximum apparent power to a multi-phase first AC grid at a first AC interface, and providing a maximum apparent power to a single-phase second AC grid at a second AC interface;

[0019] - detecting whether at least one switching unit of a first phase leg of a plurality of phase legs fails;

[0020] -Based on the tuning factor k 1ph determining at least one control parameter for operating the MMC in a limited output operating mode having a reduced maximum apparent power compared to a maximum apparent power rating; and

[0021] - In response to detecting a fault in at least one switch unit of the first phase leg, controlling at least the remaining switch units of the first phase leg in a limited output mode based on at least one determined control parameter to provide a reduced maximum apparent power at the first AC interface and / or the second AC interface.

[0022] The above-described method is particularly suitable for operating the MLC reliably without redundant switching units.

[0023] The present disclosure includes several aspects as described above.Each feature described with respect to one aspect is disclosed herein even if the corresponding feature is not explicitly mentioned in the context of a particular aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The accompanying drawings are included to provide further understanding. In the accompanying drawings, elements of the same structure and / or function may be represented by the same reference symbols. It should be understood that the embodiments shown in the accompanying drawings are illustrative representations and are not necessarily drawn to scale.

[0025] Figure 1 A three-phase to single-phase (3-phase / 1-phase) AC / AC direct MMC in the form of a railway network is shown.

[0026] Figure 2 A possible design of a bipolar switching cell is shown.

[0027] Figure 3 A schematic diagram showing the control loop of the MMC.

[0028] Figure 4 and Figure 5 Typical example PQ diagrams for the three-phase side and the single-phase side of a 3-phase / 1-phase MMC in normal operation mode are shown respectively.

[0029] Figure 6 and Figure 7 Typical example PQ diagrams for the three-phase side and the single-phase side of a 3-phase / 1-phase MMC in N-1 operating mode are shown.

[0030] Figure 8 A schematic diagram showing a method of determining a control signal for operating an MMC in a normal operating mode and a limited output mode, respectively. DETAILED DESCRIPTION

[0031] Before describing the specific details of the disclosed invention, reference is first made to Figure 1 and Figure 2 The configuration of an exemplary MLC is described.

[0032] Figure 1 The topology of the MMC 10 is shown, which can be used in a railway network, for example. As shown, it involves the interconnection of a first power grid 20 (for example, a three-phase industrial power grid at 50 Hz or 60 Hz) and a second power grid 30 (for example, a single-phase railway power grid running at 16.7 Hz or 25 Hz).

[0033] like Figure 1As shown, the MMC 10 includes three upper phase branches 80 and three lower phase branches 90 in a full bridge configuration. Each upper phase branch 80 is connected between a corresponding phase L1, L2, or L3 of the first AC interface 40 and a single phase L of the second AC interface 50. Correspondingly, each lower phase branch 90 is connected between a corresponding phase L1, L2, or L3 of the first AC interface 40 and a neutral potential N of the second AC interface 50.

[0034] although Figure 1 1 and 2. Although not shown in the drawings, the first AC interface 40 and / or the second AC interface 50 may include other components for coupling the corresponding phases L1, L2, and L3 and L and N to the corresponding terminals of the first power grid 20 and the second power grid 30, respectively. In particular, the first AC interface 40 may include a transformer for converting the input high voltage from the three-phase industrial power grid, i.e., a voltage exceeding 1 kV, such as a voltage of 30, 50, or 130 kV, to an intermediate voltage for the operation of the MMC 10. Similarly, the second interface 50 may also include a transformer for voltage matching. In addition, the neutral line N of the second AC interface 50 may be coupled to the ground potential. That is, the second power grid 30 may only include a single conductor directly or indirectly connected to the overhead line of the railway network.

[0035] Each of the phase branches 80 and 90 includes a plurality of switch units 60 and an inductor 70 connected in series. Each switch unit 60 can be selectively switched to the corresponding phase branch and used as a voltage and current source to form a desired waveform at the output side of the MMC10 (e.g., at the second AC interface 50). In the illustrated embodiment, each of the three upper phase branches 80 and the three lower phase branches 90 includes four switch units 60, i.e., the MMC10 has a total of 24 switch units. Of course, fewer or more switch units 60 may be used depending on the specific design or application.

[0036] like Figure 1 As shown, each phase leg in the phase legs 80 and 90 provides a corresponding voltage u1a, u2a, u3a, u1b, u2b, and u3b, as shown in FIG. Figure 1 As shown, they add up to correspond to the single-phase output voltage uR on the second AC interface 50. Similarly, as shown, in terms of current, currents i1a, i2a, i3a, i1b, i2b, and i3b flow through each of the phase legs 80 and 90. Figure 1 When added as shown, they correspond to the three (input) currents iL1 , iL2 , and iL3 of the MMC 10 provided by the first AC interface 40 and the (output) current iR provided by the MMC 10 to the second AC interface 50 .

[0037] Figure 2An exemplary design of one of the switch cells 60 in the form of a bipolar switch cell is shown. As shown, the switch cell 60 includes four reverse conducting integrated gate-commutated thyristors (RC-IGCT) 62 in a bridge rectifier configuration, a storage capacitor 64, and a clamp circuit 66, which includes a clamp diode 68. By applying a corresponding control signal to the control gate of the RC-IGCT 62, the switch cell 60 can be selectively activated and deactivated, that is, the storage capacitor can be switched to the corresponding phase branch 80 or 90. In a specific embodiment, the switch cell 60 has four possible modes, +Ucap (switching the storage capacitor 64 to the corresponding phase branch 80 or 90 in a first direction), -Ucap (switching the storage capacitor 64 to the corresponding phase branch 80 or 90 in the opposite second direction), zero vector (bypassing the storage capacitor 64), and diode rectifier (deactivating the switch cell 60, which means that the pulse is blocked). Other designs of the switch cell 60 are known to those skilled in the art and are therefore not described in detail at this stage.

[0038] In conventional MMC systems, so-called N+1 redundancy is used. This means that Figure 1 In the converter design shown, the MMC 10 includes more switching cells 60 than are required to enable the MMC 10 to maintain its maximum apparent power. During normal operation, the switching cells 60 are only charged to a fraction of their maximum rated voltage. If a switching cell fails, the defective cell is shorted, for example using a bypass switch or bypass thyristor ( Figure 1 and Figure 2 (not shown) and charges the remaining healthy cells to a higher voltage. Figure 1 In the illustrated embodiment, in normal operation, all four switch cells 60 of each of the phase legs 80 and 90 can be charged to 75% of their nominal peak voltage. In the event of a unit failure, the remaining three switch cells 60 are charged to 100% of their nominal peak voltage. In this way, the phase leg 90 can maintain its peak voltage output even in the event of a switch cell 60 failure.

[0039] According to one aspect of the present disclosure, it is proposed that no redundant switch units 60 are provided in the MMC 10. Instead, the MMC 10 and its switch units 60 are designed so that all switch units 60 are fully utilized during the normal operating mode of the MMC 10, so that the MMC 10 can provide its maximum apparent power at the first AC interface (40) and the second AC interface (50). In other words, it is decided not to build N+1 redundancy into the system. Among other things, this reduces the number of parts, complexity, and converter system costs. In the event of a failure of one or more switch units 60 of the MMC 10, the MMC 10 operates in a so-called limited output mode with reduced power capability. Specifically, in the limited output mode, the MMC 10 provides at least a reduced maximum apparent power, for example by reducing the reactive power at the first AC interface 40 and the second AC interface 50. In particular, under overvoltage conditions of the corresponding network AC network, the MMC 10 can also provide reduced active power at the first AC interface 40 and the second AC interface 50. Reducing its maximum power eliminates the need to trip the MMC 10 when a switch unit 60 fails. The limited output mode may be maintained until the defective unit 60 is repaired or replaced. Thereafter, by switching the converter back to its normal operating mode, the maximum apparent power based on, for example, full reactive power may be reached again.

[0040] Figure 3 The MMC (e.g. Figure 1 Schematic diagram of a control loop 100 of an MMC 10 of FIG. 1 . The control loop 100 includes an application control portion 110, a valve control portion 120, and a converter branch portion 130. Typically, one valve control portion 120 and one converter branch portion 130 will be provided for each phase branch of the MMC 10, although some components may also be shared by multiple phase branches. For simplicity of representation, Figure 3 Only a single valve control portion 120 and a single converter branch portion 130 are shown.

[0041] The application control portion 110 can be implemented by one or more first, high, or converter level controllers. It queries one or more valve control portions 120 and / or converter branch portions 130 for the number of switch units 60 in operation. Based on this information, it determines various system parameters for operating the MMC 10 in one of a plurality of operating modes (including, for example, a normal operating mode and a limited output operating mode) in a closed-loop manner. Such system parameters can be calculated in real time by the controller, or can be selected from a corresponding predetermined parameter set (e.g., calculated at design time). Among them, the application control portion 110 determines a reference voltage for each converter branch. Reference voltage The sum of the reference voltages of the two converter interfaces to the respective grid corresponding to a particular operating point. It should not exceed the maximum phase leg voltage corresponding to the sum of all cell (or capacitor) voltages, as described below.

[0042] The valve control section 120 may be implemented by one or more second, intermediate, or phase leg level controllers. It does this by determining the number of positive or negative switching cells 60 cellRef required to produce the desired voltage on each phase leg. Leg to modulate the reference voltage received from the application control section 110 Based on the determined number of cells, a subset of the switch cells 60 in operation of each converter branch is selected by providing one or more corresponding control signals to each switch cell 60. The switch cells 60 may be specifically selected to balance the cell capacitor voltages. For example, if the required level commands charging of the cell capacitors, the cell with the lowest voltage is taken, taking into account the actual current sign. Furthermore, a second priority loss distribution may also be used as a selection criterion. Furthermore, in Figure 3 In the illustrated embodiment, the valve control portion 120 is also responsible for monitoring the operating status of each switch unit 60 and reporting the number of switch units 60 in the operating state to the application control portion 110 .

[0043] The converter branch section 130 comprises the actual switching cells and the corresponding bypass circuits 132, as well as low-level cell control and monitoring circuits (not shown). It can be implemented by discrete electrical components and / or one or more third, low, or cell level controllers. Based on the control signal for cell selection provided by the valve control section 120, it selectively connects the corresponding switching cell 60 to the corresponding phase branch 80 or 90 or disconnects it from the corresponding phase branch 80 or 90. In addition, in the event that the switching cell 60 is considered defective, it is bypassed by the corresponding bypass switch. For example, this can be determined by the valve control section 120 based on the detection of an overvoltage across the corresponding switching cell 60.

[0044] In the control loop 100 shown in the figure, the normal operation mode may correspond to N switch units 60 in operation for each phase leg. In the corresponding limited output mode, only N-1 switch units 60 in operation can be used for power storage and conversion. Therefore, this mode is also called N-1 mode, because all switch units of the phase leg are operational except one switch unit 60.

[0045] In order to prevent any damage to the remaining switch units 60, as Figure 3 As shown, the application control section 110 sets the reference voltage Reduced, for example, from the full range of -22 to +22 kV in normal operation to a reduced range of -20 to +20 kV in N-1 operation. Limiting the reference voltage This can prevent the MMC 10 from being overmodulated due to additional unwanted harmonics. is applied to all phase legs 80 and 90, not just the converter leg 80 or 90 comprising the defective switching cell 60, as described in further detail below. Thus, for all remaining phase legs 80 or 90, i.e. phase legs without a damaged switching cell 60, the capacitor voltage of its cell 60 is reduced accordingly.

[0046] During normal operation of the converter, i.e. when all switching units 60 are in operation, all converter phase legs are charged to their maximum voltage levels. Figure 4 and Figure 5 The PQ capabilities generated by the converters on the three-phase side and the single-phase side of the 3-phase / 1-phase MMC 10 are shown respectively.

[0047] Among them, different common coupling point (PCC) voltages of the three-phase interface 40 are as follows: Figure 4 As shown, for example, the 5% undervoltage (u PCC,3ph =0.95), nominal voltage (u PCC,3ph =1.00), and 5% overvoltage (u PCC,3ph =1.05). Similarly, Figure 5 Different PCC voltages of the single-phase interface 50 are shown in FIG. 5 , for example, a 5% undervoltage (u PCC,1ph =0.95), nominal voltage (u PCC,1ph =1.00), and 5% overvoltage (u PCC,1ph =1.05). Among them, Figure 4 and Figure 5 It shows that the PQ diagram becomes flatter in the overvoltage case, i.e. the capacitive reactive power decreases in the case where the actual grid voltage exceeds the nominal grid voltage.

[0048] Regarding converter size in terms of voltage, the maximum phase branch voltage (Considering overmodulation) the calculation is as follows:

[0049]

[0050] in, is the maximum single-phase side root mean square (rms) voltage, It is the maximum phase-to-phase root mean square (rms) voltage on the three-phase side.

[0051] This can be achieved by the minimum available DC voltage in the phase leg To prevent:

[0052]

[0053] in, is the minimum cell voltage based on provided factors (e.g. inherent low frequency ripple). Based on the equation, the constraints can be obtained and the minimum number of converter cells required can be calculated:

[0054]

[0055] It is clear from (2) that when a cell fails, it is no longer possible to maintain the full DC voltage in the phase leg unless the remaining, healthy individual cells 60 are charged to a higher voltage. However, this is only possible up to a certain level if the design margin and semiconductor safe operating area (SOA) limits allow.

[0056] In the case of a 3-phase / 1-phase MMC 10, such as a railway network, if no redundant converter cells are provided, the impact of cell loss on converter performance will be considerable. This is because the voltages on both sides of the grid are given, i.e. fixed by external factors.

[0057] To mitigate negative effects such as converter damage, harmonic distortion, and / or low reliability, according to one aspect of the present disclosure, when a unit fails, the affected phase leg no longer provides its maximum peak voltage level. To achieve symmetrical operation, all other five phase legs also reduce their maximum peak voltages.

[0058] In order to not have only one side experience voltage loss, a tuning factor k is introduced in the converter design. 1ph , which can be freely chosen to tune the voltage capability between the two converter sides. The resulting reduced peak voltage at the three-phase interface 40 is then It can be expressed as follows:

[0059]

[0060] in, is the maximum peak voltage capability at the single-phase interface 50 when the MMC 10 operates with N switches. 1ph Controls how much active power and / or reactive power the MMC 10 can lose in limited output operating mode. 1ph Typical values ​​are in the range of 0.87-0.95. The value of divides the voltage difference between the two sides (ie, between the two AC interfaces 40 and 50) into two halves.

[0061] During the limited output operating mode, the remaining unaffected phase legs' switch cells 60 in the operating state can be temporarily disabled so that the number of remaining cells 60 is equal for all phase legs. However, in the described solution, all available switch cells 60 are kept in the operating state so that a correspondingly lower cell voltage is applied to the switch cells 60 of the phase legs in which the defective cell 670 is not present.

[0062] Figure 6 and Figure 7 PQ diagram showing the limited output mode for a fault example case (N-1 operation mode) with one unit loss using the proposed control method, with converter topology and PCC values ​​similar to Figure 4 and Figure 5 The PQ diagram for the normal operating mode shown is the same.

[0063] As shown therein, in the event of a loss of one unit, the MMC 10 injects energy from the three-phase grid 20 into the single-phase grid 30, and in the N-1 operating mode, the apparent power on both sides of the MMC 10 is significantly reduced.

[0064] Assuming that the unit average voltage is designed with a certain margin, the virtual converter DC link (average voltage of all converter units) margin It can be defined as follows:

[0065]

[0066] in, is the maximum permissible average cell voltage, is the nominal average cell voltage:

[0067] For a general implementation, refer to the Virtual DC Link Real-time calculation in units:

[0068]

[0069] in, It is the minimum number of cells in operation in all phases at any given time. is the minimum number of cells required to be in operation in order to obtain a complete PQ diagram for a particular design.

[0070] when Below 1 per unit, the maximum value of the safe operating area (SOA) is adjusted accordingly to protect the converter components by reducing their maximum apparent power. Typically, due to the loss of one unit, the capacitive reactive power (overexcitation) is limited, resulting in a reduction in the maximum voltage capability. When the point of common coupling (PCC) voltage is high, inductive reactive power may have to be injected (underexcited) to avoid overmodulation.

[0071] Figure 8 A schematic diagram of a possible control implementation of the limited output mode is shown. As shown in the figure, three different parameters are output for the operation of the MMC 10 according to the number of switch units 60 in operation, etc.

[0072] like Figure 8 As shown in the lower part, in the first step S1, the number of units in operation is The minimum number of cells provided in each converter branch to obtain a complete PQ diagram (eg, the maximum apparent power of the MMC 10) The ratio between the two times the DC margin Calculated based on the current status of MMC 10 The maximum value of .

[0073] In the second step S2, according to the above equation (7), the value calculated in step S1 is selected. Values ​​and reference values The minimum value in, for example 1 per unit, is used as the DC link reference The current value of the DC link reference The current value of can be used to directly control the total energy of the MMC 10.

[0074] In the third step S3, the DC link reference Current value and reference value If the DC link reference The current value is less than the reference value Then the first control signal sel_u1 is generated to operate the MMC 10 in the limited output mode. Otherwise, that is, when the cell margin is sufficient to operate the MMC 10 at the required power, no control signal is generated or a second control signal, such as sel_u2, is generated to operate the MMC 10 in the normal operation mode.

[0075] In case the first control signal sel_u1 is provided in step S3, a predetermined reduced maximum converter voltage sel_u1 on the three-phase side is selected in steps S4 and S5, respectively. and the predetermined reduced maximum converter voltage on the single-phase side Alternatively, in case no control signal or the second control signal sel_u2 is provided in step S3, in steps S4 and S5, a predetermined normal or maximum converter voltage on the three-phase side is selected, respectively. and a predetermined standard or maximum converter voltage on the single-phase side The selected value as parameter and output, and define three-phase and single-phase SOA for MMC 10.

[0076] References in the above Figure 8 In the control implementation described, various input parameters can be pre-calculated during the design phase of the MMC 10, including the minimum number of cells in each converter branch required to obtain a complete PQ diagram. DC Margin Reference value Reduced maximum three-phase converter voltage Reduced maximum single-phase converter voltage Maximum three-phase converter voltage and the maximum single-phase converter voltage and can be based on the number of units in operation These input parameters are selected in real time. Specifically, the reduced maximum single-phase converter voltage may correspond to the maximum single-phase converter voltage Multiply by the tuning factor k chosen during design 1ph , as defined in equation (4).

[0077] In other implementations, some or all of these values ​​may be calculated dynamically, ie, in real time (eg, during operation of the MMC 10). This approach may provide greater flexibility, such as handling failures of multiple switch cells 60 in the same or different phase legs 80 or 90.

[0078] It should be noted that the above-mentioned limited output operation mode can also be implemented in an MMC 10 including at least one redundant switching unit, for example in the case of N+1 redundancy. In this case, the failure of the first unit can be compensated without reducing the maximum apparent power of the MMC 10. However, unlike conventional N+1 converters, such an MMC 10 can continue to operate in the limited output operation mode in the event of a failure of the second switching unit 60 in the same phase leg as described above.

[0079] Furthermore, while this description focuses on a three-phase to single-phase converter mode commonly used in railway networks, it is worth noting that the same approach can also be applied to a single-phase to three-phase converter mode, or another multi-phase to single-phase or single-phase to multi-phase converter.

[0080] Compared to other approaches, the proposed solution is easy to implement since it reduces the number of components of the MMC 10 and does not require the introduction of a neutral point shift at the first interface 40 and / or the second interface 50 .

[0081] like Figures 1 to 8The illustrated embodiments represent exemplary embodiments of the improved multi-level converter design and method of operation thereof. Therefore, they do not constitute a complete list of all embodiments according to the improved multi-level converter design and method of operation thereof. For example, the actual converter circuit and method of operation may differ from the illustrated embodiments in terms of the number of phase legs, the switch cells per phase leg, the switch cell configuration, the operating voltages, the design parameters, and the exchanged control signals.

[0082] Reference mark

[0083] 10 MMC

[0084] 20 (First) Power Grid

[0085] 30 (Second) Power Grid

[0086] 40 (first) AC interface

[0087] 50(Second)AC interface

[0088] 60 (switch) units

[0089] 62 RC-IGCT

[0090] 64 Storage capacitor

[0091] 66 Clamp Circuit

[0092] 68 Clamping diode

[0093] 70 Inductor

[0094] 80(upper) branch road

[0095] 90(down) branch road

[0096] 100 Control loop

[0097] 110 Application Control Section

[0098] 120 Valve control section

[0099] 130 Converter branch section

[0100] 132 Bypass circuit

Claims

1. A direct multi-phase to single-phase, modular multi-level converter MMC (10), comprising: A first AC interface (40) for connecting the MMC (10) to a multi-phase first power grid (20), in particular a three-phase power grid; A second AC interface (50) for connecting the MMC (10) to a single-phase second power grid (30); A first group of P-phase branches (80), wherein each phase branch (80) of the first group comprises N switch units (60) connected in series between a corresponding one of the P-phases (L1, L2, L3) of the first AC interface (40) and a single phase (L) of the second AC interface (50), wherein P≥2 and N≥3; a second group of P-phase legs (90), wherein each phase leg (90) of the second group comprises N switching units (60) connected in series between a corresponding one of the P-phases (L1, L2, L3) of the first AC interface (40) and a neutral potential (N) of the second AC interface (50); and At least one controller is configured to control the first group of P-phase legs (80) and the second group of P-phase legs (90) to selectively provide maximum apparent power at the first AC interface (40) and the second AC interface (50), and in response to detecting at least one failed switch unit (60) of the first phase leg, switch the MMC (10) to a limited output operation mode, wherein the MMC (10) is controlled by a tuning factor k 1ph At least the first phase leg is controlled to output a reduced phase leg voltage A reduced maximum apparent power is provided at the first AC interface (40) and / or the second AC interface (50).

2. The MMC (10) according to claim 1, wherein: The MMC (10) is configured as follows: In a normal operation mode in which no failed switch unit (60) is present, using all 2NP switch units (60) to selectively provide the maximum apparent power at the first AC interface (40) and the second AC interface (50); as well as In a limited output operation mode in which at least one failed switch unit (60) exists, in particular in an N-1 operation mode in which one failed switch unit exists, a subset of the switch units (60) other than the at least one failed switch unit is used to selectively provide the reduced maximum apparent power at the first AC interface (40) and the second AC interface (50).

3. The MMC (10) according to claim 1 or 2, wherein: The at least one controller is specifically configured to control the switch unit (60) of each phase leg (80, 90) so that the maximum peak output voltage at the first AC interface (40) does not exceed in, is the minimum unit voltage, corresponds to the maximum peak voltage at the second AC interface (50) in the absence of a lossy unit, and Corresponding to a reduced maximum peak voltage at the first AC interface (40) produced in the presence of a loss cell.

4. The MMC (10) according to claim 3, wherein: The tuning factor k 1ph Corresponds to in, Corresponding to the minimum number of switch units (60) required to provide the maximum apparent power at the first AC interface (40) and the second AC interface (50).

5. The MMC (10) according to any one of claims 1 to 4, wherein: The tuning factor k 1ph In the range of 0.7 to 1.0, preferably in the range of 0.87 to 0.

95.

6. The MMC (10) according to any one of claims 1 to 5, wherein: The at least one controller is specifically configured to calculate a virtual DC link reference voltage in real time in, Corresponding to the number of remaining switch units (60) in operation, corresponding to the minimum number of switch units (60) required to provide the maximum apparent power at the first AC interface (40) and the second AC interface (50), corresponds to the cell average voltage reference, and Corresponds to the voltage margin in the switch in normal operation.

7. The MMC (10) according to any one of claims 1 to 6, wherein: The at least one controller is specifically configured to: Based on normal and reduced Calculating Single-Phase Converter Voltages in, and The ratio corresponds to the tuning factor k 1ph ; and / or Based on normal and reduced Calculating three-phase converter voltages 8. The MMC (10) according to any one of claims 1 to 7, comprising: A converter level control unit configured to determine the number of cells (60) in an operational state Based on the number of units in operation Determine the maximum phase leg voltage of each phase leg of the first and second groups and At least one phase branch level control unit is configured to modulate the maximum voltage And at least a subset of switches (60) that are in an operational state are selected.

9. A three-phase to single-phase, 3-phase / 1-phase, AC / AC railway network comprising at least one MMC (10) according to any one of claims 1 to 8.

10. A method for operating a direct multi-phase to single-phase, modular multi-level converter MMC (10), the MMC comprising a plurality of phase legs (80, 90), each phase leg (80, 90) comprising a plurality of switching units (60), in particular an MMC (10) according to any one of claims 1 to 8, the method comprising: In a normal operation mode of the MMC (10), all switch units (60) of all phase branches (80, 90) are used to provide a maximum apparent power to a multi-phase first AC power grid (20) at a first AC interface (40), and to provide the maximum apparent power to a single-phase second AC power grid (30) at a second AC interface (50); Detecting whether at least one switch unit (60) of a first phase branch among the plurality of phase branches (80, 90) fails; Based on the tuning factor k 1ph determining at least one control parameter for operating the MMC (10) in a limited output operating mode having a maximum apparent power reduced from a maximum apparent power rating; as well as In response to detecting a fault in at least one switch unit (60) of a first phase leg, at least the remaining switch units (60) of the first phase leg are controlled in the limited output mode based on at least one determined control parameter to provide a reduced maximum apparent power at the first AC interface (40) and / or the second AC interface (50).

11. The method according to claim 10, wherein: Before a failure of the at least one switching unit (60) is detected, the tuning factor k is determined 1ph , and the control of the switch unit (60) is based on real-time determination of at least one of the following parameters: the maximum converter voltage on the single-phase side Maximum converter voltage on the three-phase side and DC link reference 12. The method according to claim 10 or 11, comprising the following steps: Count the number of active cells (60) in each converter branch and the minimum number of cells (60) provided in each converter branch to obtain the maximum apparent power of the MMC (10) The ratio between The calculated ratio is compared with the DC margin Multiply to get the running Maximum DC link reference for units (60) Select the maximum DC link reference obtained and reference DC link reference The smaller one; as well as If the maximum DC link reference Less than the reference value A first control signal (sel_u1) is generated to operate the MMC (10) in the limited output mode.

13. The method according to claim 10, wherein: The tuning factor k 1ph is determined in real time and depends on the number of failed switch units (60) detected.

14. The method according to any one of claims 10 to 13, wherein: In response to detecting a fault in at least one switch unit (60) of a first phase leg, remaining switch units (60) of the plurality of phase legs are controlled to limit capacitive reactive power thereof.

15. The method according to any one of claims 10 to 14, wherein: In response to detecting the at least one failed switch unit, the remaining switch units (60) of the plurality of phase legs (80, 90) are controlled to inject induced reactive power.

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