Method and system for system identification of a modular multilevel converter

Abstract

The present invention relates to a method for system identification of a modular multilevel converter (1) with a number N Phase Phases, which are formed by respective strands with a number N Module on battery modules (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34). A vector of module currents flowing in the battery modules results from the matrix-vector product of a matrix C for a respective switching state combination and a vector of all phase currents. During operation, the system identification for the current distribution of module currents for a respective switching state combination is carried out by a central control unit (2) for each of the respective switching state combinations for a number of N Phase linearly independent combinations of module currents and phase currents measured values (61, 62, 63) of a respective module current sensor and measured values (51, 52, 53) of N PhasePhase current sensors (41, 42, 43) are determined, and an updated matrix C is calculated for the respective switching state combination from the respective measured values. A scheduler calculates current distributions for the respective switching state combinations, which are incorporated into a cost function, using the respective updated matrix C. Furthermore, a system is presented on which the system identification is carried out.

Description

[0001] The present invention relates to a method for system identification of a modular multilevel converter. Furthermore, a system on which the system identification is carried out is presented.

[0002] Modular multilevel converters, abbreviated to MMC, exhibit increased complexity with regard to controlling the respective switches, such as MOSFETs, due to the multitude of switching options. This complexity is increased even further when parallel connection options are also provided in addition to serial connection, as is particularly the case with the modular multilevel series / parallel converter, for example, as described in Goetz, SM; Peterchev, AV; Weyh, T., "Modular Multilevel Converter With Series and Parallel Module Connectivity: Topology and Control," Power Electronics, IEEE Transactions on, vol. 30, no. 1, pp. 203-215, 2015. doi: 10.1109 / TPEL.2014.2310225.

[0003] To improve the efficiency of the MMC system and enable active balancing, it is essential to know how the current is distributed when modules are connected in parallel. For example, a network analysis, in which the current distribution of a specific switching configuration is calculated and saved offline, can be used to determine the current distribution for a specific switching configuration.

[0004] In a balanced system, i.e. the same state of charge or SoC (English: “State of Charge”) of all modules, the distribution of a respective phase current between the battery modules in each string would always correspond to a linear combination of the phase currents. However, during operation, important parameters of the current distribution change. For example, the on-state resistance of the MOSFETs is temperature-dependent and also changes with the ageing of the component. In addition, the internal resistance of battery modules depends on the temperature, the state of charge and the ageing of the battery. This can be compensated for by saving characteristic maps of these variables as a function of temperature, state of charge and ageing and by regularly recalculating the current distribution depending on these parameters. A disadvantage of this method, however, is that a lot of memory is required to save a parameter list. Some factors, such as e.g.The influence of aging is difficult to determine. However, inaccuracies in the parameter list lead to lower efficiency of the overall system and poorer active charge balancing, as a scheduler calculates the target variables incorrectly or inaccurately.

[0005] The document DE 10 2015 117 464 A1 discloses a real-time capable computing unit of an electric vehicle, by means of which a dynamic adjustment of the electrical current between two electrical machines can be carried out depending on the vehicle status.

[0006] From the document DE 10 2020 129 131 A1 a method for monitoring the state of charge of a modular multilevel converter is known, wherein a state of charge observer monitors the state of charge of each individual energy storage device and, based on a continuous evaluation of measured values, a respective current flow at a respective energy storage device is estimated and, by processing the respective current flows, a conclusion is drawn about the state of charge of the respective energy storage device.

[0007] The document CN 111505365 A describes a current sensing device for a modular multilevel inverter.

[0008] In the publication by E. SPECHT; CH. KORTE; M. HILLER: Reducing Computation Effort by Parallel Optimization for Modular Multilevel Converters; in: IECON 2018 - 44th Annual Conference of the IEEE Industrial Electronics Society, Year: 2018, Conference Paper, a control method for a modular multilevel converter is explained. This method uses dynamically updated tables with optimized module switching states. It demonstrates how to estimate the current distribution in the modules from phase currents and module switching states and, based on these currents, determine an optimal current distribution with respect to losses using a cost function.

[0009] Against this background, it is an object of the present invention to present a method that enables system identification in a multilevel converter, i.e., identification of respective current distributions for respective switching configurations. The method should be capable of this, particularly in the case of parallel connections of battery modules, so that a scheduler can calculate the effects of different switching configurations on respective energy storage devices in battery modules and make a suitable selection. Furthermore, a system that enables such identification should be presented.

[0010] To achieve the above-mentioned object, a method for system identification of a modular multilevel converter is proposed, in which in at least one string, through which a single phase from a number N Phase phases, a number N ModuleA respective phase current sensor is arranged on battery modules and on each output connection. At least three semiconductor switches, at least one energy storage device, which is connected in series or parallel to at least one neighboring battery module by the semiconductor switches, and a module current sensor are arranged in each battery module. For each switching cycle, a central control unit, which has a scheduler, sends a respective switching state combination specified by the scheduler, i.e. switch positions of all semiconductor switches of all battery models, synchronously to all battery modules via a communication bus connected to the battery modules, which can be a high-speed bus, for example. A vector of module currents flowing in the battery modules results from the matrix-vector product of a matrix C for the respective switching state combination and a vector of all phase currents.During operation of the modular multilevel converter, the central control unit repeatedly carries out the system identification for the current distribution of the module currents for the respective switching state combination by. • from the central control unit for each battery module in continuous sequence to the respective switching state combination for a number of N Phase linearly independent combinations of module currents and phase currents measured values of the respective module current sensor and measured values of the N Phase Phase current sensors are determined simultaneously at a respective switching cycle, and • for the respective switching state combination, a matrix C updated according to the continuous sequence is calculated from the respective measured values. The continuous sequence is to be understood as meaning that after the measured values have been added to N Phase linearly independent current combinations are present (i.e. at least N Phase-times respective combinations of module currents and phase currents, which are linearly independent of each other, were measured, whereby the module currents and phase currents are determined simultaneously during each measurement), for each additional linearly independent current combination, the corresponding measured values replace the respective measured values of the respective oldest linearly independent current combination. This results in a continuous updating of the matrix C. The scheduler calculates current distributions for respective switching state combinations, which are included in a cost function, using the respective updated matrix C.

[0011] For example, when communicating between the control unit and the battery modules, no dedicated measurement bit is used. The respective battery modules are controlled directly, allowing simultaneous measurement of the module and phase currents.

[0012] Linearly independent current combinations for a respective switching state combination are provided, for example, by different phase currents. Measurements are made as long as (minimum: N Phase times) until you get N Phase linearly independent combinations exist.

[0013] A switching state of an individual battery module, which affects the switch positions of the semiconductor switches arranged in the battery module, connects the at least one energy storage device of the battery module, for example, in series or in parallel with a respective adjacent battery module (or its at least one energy storage device). A bypass of the at least one energy storage device is also possible.

[0014] Based on the calculation of the cost function, an optimal switching state combination is specified for each switching cycle. Its associated current distribution—compared with other current distributions for other switching state combinations—leads to an extreme value depending on the selection criterion in the cost function. The cost function is geared, for example, toward uniform discharge or charge equalization as the respective selection criterion, with a maximum or minimum extreme value being sought depending on the mathematical version of the cost function.

[0015] The vector i→ from module currents flowing in the battery modules results as a matrix-vector product of a matrix C for the respective switching state combination and a vector y→ from all phase currents. The matrix C has a dimension of (N Module * N Phase ) times N Phase on. (i1,1⋮i1,NModulei2,1⋮i2,NModule⋮iNPhase,1⋮iNPhase,NModule)=C⋅(j1⋮jNPhase), and C=(C1,1⋯C1,NPhase⋮⋱⋮CNPhase⋅NModule,1⋯CNPhase⋅NModule,NPhase).

[0016] Each module current i p,m for a respective phase p and for an m-th module of the respective strand, which forms the respective phase p, is given according to equation (1) by ip.m=c(p−1)⋅NModule+m,1⋅j1+⋯+c(p−1)⋅NModule+ m,NPhase⋅jNPhase⋅

[0017] Therefore, to calculate a respective module current i p,m , which flows through the m-th module of the p-th strand, the determination of a number of k=1, ..., N Phase Variables c( p-1 )·N Module +m,k necessary.

[0018] In one embodiment of the method according to the invention, an additional measurement bit is sent from the central control unit of the respective module current sensors to provide measured values. Upon receipt of the additional measurement bit, a measurement of the respective module current is performed in the battery modules, and a measured value is stored in the associated cycle. The respective measured values are then transmitted to the central control unit. Upon transmission of the set measurement bit, the phase currents are also measured accordingly, and the respective measured values are transmitted to the control unit. Due to the synchronous reception of the measurement bit, the respective measurements in the battery modules are also carried out simultaneously, so that all measured values are determined synchronously, which is of direct necessity for the calculation of matrix C.

[0019] In a further embodiment of the method according to the invention, a secondary supply connection with a current sensor is provided at a double star point of the modular multilevel converter. The vector of module currents flowing in the battery modules results from the matrix-vector product of a matrix C NV to the respective switching state combination and a vector of all phase currents and an auxiliary supply current. The number of linearly independent combinations of module currents, phase currents, and the auxiliary supply current, which are measured simultaneously, is limited to (N Phase + 1) to the matrix C NV to be able to determine.

[0020] For a system with auxiliary consumer tap, the matrix C NV a dimensioning of (N Module * N Phase ) times (N Phase + 1). This modifies the above equations (1) and (2) to (i1,1⋮i1,NModulei2,1⋮i2,NModule⋮iNPhase,1⋮iNPhase,NModule)=CNV⋅(j1⋮jNPhasejNV) and CNV=(c1,1⋯c1,NPhasec1,NV⋮⋱⋮⋮cNPhase⋅NModule,1⋯CNPhase⋅NModule,NPhasecNPhase,⋅NModule,NV).

[0021] A respective module current i p,m for a respective phase p and for an m-th module of the respective strand, which forms the respective phase p, is in this case given by ip.m=c(p−1)⋅NModule+m,1⋅j1+⋯+c(p−1)⋅NModule+m,NPhase+c(p−1)⋅NModule+m,NV⋅jNV.

[0022] Accordingly, a number of (N Phase + 1) Variables of linearly independent (and synchronously executed) measurements of the module currents and phase currents / auxiliary consumption current are necessary to solve equations (6) unambiguously.

[0023] In yet another embodiment of the method according to the invention, four half-bridges are connected in parallel to the at least one energy storage device in the respective battery module. A module input connection is formed at two of the four half-bridges by a respective center tap, while a module output connection is formed at two other half-bridges by a respective center tap. This topology corresponds to a multilevel converter with serial and parallel connection options, for example, described in the publication US 2014 / 0226377 A1 and ibid. Fig. 11 shown.

[0024] In another embodiment of the method according to the invention, the respective battery module, which comprises three semiconductor switches, is connected either in series or in parallel with a respective adjacent battery module. Such a so-called three-switch topology is shown, for example, in the document DE 10 2016 112 250 A1.

[0025] Furthermore, a system is claimed, wherein the system comprises a central control unit having a scheduler, a communication bus, and a modular multilevel converter. The modular multilevel converter has at least one phase through which a single phase from a number N Phase phases, with a number N Moduleon battery modules and a respective phase current sensor at a respective output terminal. Each battery module has at least three semiconductor switches, at least one energy storage device, and a module current sensor and is designed to connect the at least one energy storage device in series or in parallel with at least one adjacent battery module. A vector of module currents flowing in all battery modules results from the matrix-vector product of a matrix C for a respective switching state combination and a vector of all phase currents. The central control unit is configured to • to send a switching state combination specified by the scheduler synchronously to all battery modules for each switching cycle via the communication bus connected to the battery modules, • During operation of the modular multilevel converter, the central control unit repeatedly carries out a system identification for the current distribution of the module currents for the respective switching state combination by ◯ from the central control unit for each battery module in continuous sequence to the respective switching state combination for a number of N Phase linearly independent combinations of module currents and phase currents measured values of the respective module current sensor and measured values of the N Phase Phase current sensors are determined simultaneously at a respective switching cycle, and ◯ for the respective switching state combination, a matrix C updated according to the continuous sequence is calculated from the respective measured values.

[0026] The scheduler is configured to calculate current distributions for respective switching state combinations using a cost function using the respective updated matrix C.

[0027] In one embodiment of the system according to the invention, the central control unit is configured to send an additional measurement bit to provide measured values from the respective module current sensors and simultaneously initiate measurements of the phase currents or the auxiliary supply current. Upon receiving the additional measurement bit, the battery modules are configured to perform a measurement of the respective module current and transmit the respective measured values to the central control unit.

[0028] In a further embodiment of the system according to the invention, a secondary supply connection with a current sensor is provided at a double star point of the modular multilevel converter. The vector of module currents flowing in the battery modules results from the matrix-vector product of a matrix C NV to the respective switching state combination and a vector of all phase currents and an auxiliary supply current. The number of linearly independent combinations of module currents, phase currents, and the auxiliary supply current, which are measured simultaneously, is limited to (N Phase + 1) to the matrix C NV to be able to determine.

[0029] In yet another embodiment of the system according to the invention, four half-bridges are connected in parallel to the at least one energy storage device in the respective battery module. A module input connection is formed by a respective center tap on two of the four half-bridges, and a module output connection is formed by a respective center tap on two of the other four half-bridges.

[0030] In another embodiment of the system according to the invention, the respective battery module, which comprises three semiconductor switches, is connected either in series or in parallel with a respective adjacent battery module.

[0031] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0032] It is understood that the features mentioned above and those to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

[0033] Fig. 1 schematically shows a modular multilevel converter for an embodiment of the system identification according to the invention.

[0034] In Fig. 1 schematically shows a modular multilevel converter 1 for an embodiment 10 of the system identification according to the invention. The modular multilevel converter 1 shown as an example comprises three strings corresponding to N Phase =3 phases with N each Module=4 battery modules 11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34 and supplies a load 4 with alternating current. A secondary supply connection 3, which provides a direct voltage, is arranged at the double star point of the modular multilevel converter 1. Phase current sensors 41, 42, 43 are arranged at the string connections forming the three phases, and a current sensor 44 is also arranged at the secondary supply connection 3. A central control unit 2 sends a respective switching state combination specified by a scheduler to all battery modules 11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34 for each switching cycle synchronously via a command line 60 of a communication bus. The central control unit 2 receives directly from the current sensor 44 at the auxiliary supply connection 3 and the phase current sensors 41, 42, 43 respective measured values 50, 51, 52, 53 for the auxiliary supply current j NV and the phase currents j U , j V , j W, while measured values 61, 62, 63 correspond to the module currents flowing in the respective battery modules 11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34 l→ from the battery modules 11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34 via the respective signal lines of the communication bus to the central control unit 2. For the modular multilevel converter 1 shown, the module currents for a given switching state combination can be calculated as follows: (iU,1⋮iU,4iV,1⋮iV,4iW,1⋮iW,4)=C1⋅(jUjVjWjNV), where C1 is a 12 by 4 matrix, for the determination of which according to Eq. (6) four measurements of independent combinations of module currents and phase currents / auxiliary supply current are necessary per matrix row.

[0035] If no auxiliary supply connection 3 for a low-voltage DC supply were active, the corresponding entry in equation (7) would be deleted and, in the example with three phases and four modules per phase, the following matrix-vector equation would result for calculating the module currents: (iU,1⋮iU,4iV,1⋮iV,4iW,1⋮iW,4)=C2⋅(jUjVjW).

[0036] C2 is a 12 by 3 matrix, for the determination of which, according to Eq. (3), three measurements of independent combinations of module currents and phase currents are necessary per matrix row. List of reference symbols 1 Modular multilevel converter 2 Central control unit 3 Auxiliary supply connection 4 Multiphase electric machine 10 Overall system 11 Battery module 12 Battery module 13 Battery module 14 Battery module 21 Battery module 22 Battery module 23 Battery module 24 battery module 31 Battery module 32 battery module 33 Battery module 34 Battery module 41 U-phase current sensor 42 V phase current sensor 43 W phase current sensor 44 Current sensor 50 Measured value of auxiliary supply current 51 Measured value phase current first phase 52 Measured value phase current second phase 53 Measured value phase current third phase 60 Command line in the communication bus 61 measured values module currents first string 62 measured values module currents second string 63 measured values module currents third string

Claims

[1] Method for system identification of a modular multilevel converter, in which in at least one string through which a single phase from a number N Phase phases, a number N Moduleon battery modules (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34) and at a respective output terminal a respective phase current sensor (41, 42, 43) is arranged, in which in a respective battery module (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34) at least three semiconductor switches, at least one energy storage device, which is connected by the semiconductor switches in series or in parallel with at least one adjacent battery module (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34), and a module current sensor are arranged, in which from a central control unit (2), which has a scheduler, for each switching cycle via a communication bus connected to the battery modules (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34), a respective switching state combination specified by the scheduler is sent synchronously to all battery modules (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34), wherein a vector is formed from the battery modules (11, 12, 13,14, 21, 22, 23, 24, 31, 32, 33, 34) flowing module currents as a matrix-vector product of a matrix C for the respective switching state combination and a vector from all phase currents, in which during the ongoing operation of the modular multilevel converter (1) the central control unit (2) repeatedly carries out the system identification for the current distribution of the module currents for the respective switching state combination by, • by the central control unit (2) for each battery module (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34) in continuous sequence to the respective switching state combination for a number of N Phase linearly independent combinations of module currents and phase currents at a respective switching cycle simultaneously measured values (61, 62, 63) of the respective module current sensor and measured values (51, 52, 53) of the N Phase Phase current sensors (41, 42, 43) are determined, and • for the respective switching state combination from the respective measured values (51, 52, 53, 61, 62, 63) a matrix C updated in accordance with the continuous sequence is calculated, and in which current distributions for respective switching state combinations, which are included in a cost function, are calculated by the scheduler with the aid of the respective updated matrix C. [2] Method according to claim 1, in which an additional measurement bit is sent from the central control unit (2) to provide measured values (61, 62, 63) of the respective module current sensors, in which a measurement of the respective module current is carried out in the battery modules (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34) upon receipt of the additional measurement bit and respective measured values (61, 62, 63) are transmitted to the central control unit (2). [3] Method according to one of the preceding claims, in which a secondary supply connection (3) with a current sensor (44) is provided at a double star point of the modular multilevel converter (1), wherein the vector of module currents flowing in the battery modules (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34) is a matrix-vector product of a matrix C NV to the respective switching state combination and a vector of all phase currents and an auxiliary supply current, and in which the number of linearly independent combinations of module currents, phase currents and the auxiliary supply current, which are each measured simultaneously, is limited to (N Phase + 1) to obtain the matrix C NV to be able to determine. [4] Method according to one of the preceding claims, in which in the respective battery module (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34) four half-bridges are connected in parallel to the at least one energy storage device, and in which a module input connection is formed on two of the four half-bridges by a respective center tap and a module output connection is formed on two other half-bridges by a respective center tap. [5] Method according to one of claims 1 to 3, in which the respective battery module (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34), which comprises three semiconductor switches, is connected either in series or in parallel with a respective adjacent battery module (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34). [6] System comprising a central control unit (2) having a scheduler, a communication bus and a modular multilevel converter (1) having at least one string through which a single phase from a number N Phase phases, with a number N Moduleon battery modules (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34) and at a respective output terminal a respective phase current sensor (41, 42, 43), wherein a respective battery module (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34) has at least three semiconductor switches, at least one energy store, and a module current sensor and is designed to connect the at least one energy store in series or in parallel with at least one adjacent battery module (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34), wherein a vector is formed from all battery modules (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34) flowing module currents as a matrix-vector product of a matrix C for a respective switching state combination and a vector from all phase currents, wherein the central control unit (2) is configured to • to send a respective switching state combination specified by the scheduler synchronously to all battery modules (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34) for each switching cycle via the communication bus connected to the battery modules (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34), • during operation of the modular multilevel converter (1) by the central control unit, repeatedly carry out a system identification for the current distribution of the module currents for the respective switching state combination by o by the central control unit (2) for each battery module (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34) in continuous sequence to the respective switching state combination for a number of N Phase linearly independent combinations of module currents and phase currents measured values (61, 62, 63) of the respective module current sensor and measured values (51, 52, 53) of the N PhasePhase current sensors (41, 42, 43) are determined simultaneously at a respective switching cycle, and o for the respective switching state combination, a matrix C updated in accordance with the continuous sequence is calculated from the respective measured values (51, 52, 53, 61, 62, 63), and wherein the scheduler is configured to calculate current distributions for respective switching state combinations, which are included in a cost function, with the aid of the respective updated matrix C. [7] System according to claim 6, wherein the central control unit (2) is configured to send an additional measurement bit to provide measured values (51, 52, 53) of the respective module current sensors, wherein the battery modules (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34) are configured to carry out a measurement of the respective module current upon receipt of the additional measurement bit and to transmit respective measured values (51, 52, 53) to the central control unit (2). [8] System according to one of claims 6 or 7, wherein a secondary supply connection (3) with a current sensor (44) is provided at a double star point of the modular multilevel converter (1), wherein the vector of module currents flowing in the battery modules (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34) is a matrix-vector product of a matrix C NV to the respective switching state combination and a vector of all phase currents and a secondary supply current, and wherein the number of linearly independent combinations of module currents, phase currents and the secondary supply current, which are each measured simultaneously, is limited to (N Phase + 1) is increased to the matrix C NV to be able to determine. [9] System according to one of claims 6 to 8, wherein in the respective battery module (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34) four half-bridges are connected in parallel to the at least one energy storage device, and wherein a module input connection is formed on two of the four half-bridges by a respective center tap and a module output connection is formed on two other half-bridges by a respective center tap. [10] System according to one of claims 6 to 8, wherein the respective battery module (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34), which comprises three semiconductor switches, is connected either in series or in parallel with a respective adjacent battery module (11, 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34).

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