Determining parameters of a filter circuit

Abstract

A converter device comprises a filter circuit, a DC link, and an active rectifier interconnected between the filter circuit and the DC link. A method for determining at least one parameter of the filter circuit comprises switching the active rectifier into an active state in which a first active rectifier leg of the active rectifier connects a first phase of the filter circuit with a positive side of the DC link and a second active rectifier leg of the active rectifier connects a second phase of the filter circuit with a negative side of the DC link, determining a time series of current values by measuring a current through the filter circuit in a plurality of consecutive time steps, and determining the at least one parameter of the filter circuit by calculating a best fit of the at least one parameter from a differential equation of the filter circuit into which the time series of current values is input, wherein the differential equation is a function of the current values at different time steps and the at least one parameter.

Description

Technical Field

[0001] This invention relates to the field of power electronic equipment. Specifically, it relates to a method, computer program, computer-readable medium, and controller for determining at least one parameter of a filter circuit in a converter device. The invention also relates to a converter device. Background Technology

[0002] Understanding the health status of components in converter devices is crucial for the safe and reliable operation of these devices. Specifically, the health status of capacitors is particularly important for two main reasons: capacitors are prone to degradation over time, and failures can lead to a rapid and uncontrolled release of energy due to the potentially large amount of energy stored within them, resulting in potential damage.

[0003] Typically, a dedicated circuit system or at least additional voltage measurement equipment is used to determine the health status of a capacitor. Methods for determining the health status of a capacitor may be based on detecting the resonant frequency, detecting the difference in active and / or reactive power between phases, monitoring the DC charge of the capacitor, or estimating the circuit admittance using positive-sequence decomposition and negative-sequence decomposition.

[0004] The paper "Real-time identification of LCL filters employed with grid converters," published in IEEE Transactions on Industrial Applications, Vol. 56, No. 5, pp. 5158-5169 in 2020, by Pirsto, V, Kukkola, J, Rahman, FMM, and Hinkkanen, M, relates to a system identification technique based on estimating the parameters of components used in power converters using mathematical models.

[0005] EP 3 096 429A1 describes a model-based method for estimating the physical parameters of an LCL filter and a converter connected between a converter and a power network.

[0006] EP 3 232 217 A1 describes the main circuit of a power converter. The inverter includes controllable semiconductor switches and corresponding anti-parallel diodes, wherein two of the semiconductor switches are connected in series with the anti-parallel switch to form inverter branches, and each inverter branch forms a phase of the inverter. Series-connected switching components are connected between a positive voltage bus and a negative voltage bus. An LCL filter is connected to a phase of the inverter and includes an inductor whose first terminal is connected to a phase of the inverter. The inverter is a three-phase inverter, and each phase is connected to a corresponding filter inductor of the filter. The filter also includes a capacitor connected to the second terminal of the inductor and connecting the second terminals of the inductor together. A switching device is provided that enables the filter and inverter to be disconnected from an AC voltage source such as a voltage grid or a rotating motor. The filter capacitor can be determined, wherein first, the switching device connects the filter to the voltage grid, then the switching device is disconnected to disconnect the filter from the voltage grid, and the semiconductor switches of the converter bridge are controlled such that the filter capacitor is short-circuited, thus discharging through the inductor. Two lower semiconductor switches in the lower semiconductor switch are controlled to be turned on, and the filter capacitor is short-circuited through the inductor of the filter. The capacitor and inductor of the filter then form a resonant circuit, and the resonant frequency of the circuit is indicated by the capacitance value of the capacitor. Current is measured in the circuit in which current is flowing, and the condition of the filter capacitor is determined based on the oscillation of the measured current.

[0007] Ren Lei et al., in their article "Modified hybrid model of boost converters for parameter identification of passive components" (frp, XP006066965, ISSN: 1755-4535, DOI: 10.1049 / IET-PEL.2017.0528), published on April 10, 2018 (2018-04-10) in the IET Power Electronics, Volume 11, Issue 4, UK, describe a method for identifying parameters of passive components (including capacitors and inductors) in power electronic converters (specifically, boost converters). Regarding capacitor condition monitoring methods, D2 mentions methods based on the Local Capacitor Model (LCMM) and methods based on the Converter Parameter Model (CPMM), and proposes a hybrid model of LCMM and CPMM.

[0008] EP 3 637 115 A1 describes a method and apparatus for detecting the capacitance of a filter capacitor in an inverter. To implement the method for detecting the capacitance of a filter capacitor in an inverter, each output terminal of the inverter is connected to a terminal of a first filter capacitor via an inductor, and the other terminals of all the first filter capacitors are interconnected. The inverter controls one output terminal to always output a voltage of zero; controls the other output terminals of the inverter to sequentially output a first voltage; and when one output terminal of the inverter outputs the first voltage, controls the other output terminals to output voltages of zero. When one output terminal of the inverter outputs the first voltage and the other output terminals of the inverter output voltages are zero, the inverter obtains the current of the inductor connected to each output terminal of the inverter. The inverter determines the ratio of the current of the inductor connected to the output terminal in which the output voltage is always zero in the inverter to the current of the inductor connected to each of the other output terminals in the inverter. Furthermore, the inverter determines the capacitance of the first filter capacitor corresponding to the inductor connected to the output terminal where the output voltage is always zero, based on the first voltage, the determined ratio, and the current of the inductor connected to the output terminal where the output voltage is always zero. Therefore, the inverter of D2 can detect the capacitance of the inverter's first filter capacitor by controlling the voltage output by the inverter and obtaining the current of the inductor connected to the output terminal of the inverter.

[0009] CN 102 307 004B describes a method for identifying parameters of a controllable rectifier based on LCL filtering. The method includes the following steps: obtaining the short-circuit current; identifying parameters based on a genetic algorithm; obtaining the initial phase angle of the power grid at startup; estimating the virtual flux linkage value of the power grid under LCL filtering conditions in real time; and obtaining the system orientation angle under LCL filtering conditions. Summary of the Invention

[0010] The purpose of this invention is to provide a simple and reliable method for determining parameters, such as capacitance, of the filter circuitry of a converter device without the need for additional hardware.

[0011] This objective is achieved through the subject matter of the independent claims. Other exemplary embodiments will be apparent from the dependent claims and the following description.

[0012] One aspect of the present invention relates to a method for determining at least one parameter of a filter circuit in a converter device. The at least one parameter may include one or more capacitors, one or more inductors, and / or one or more resistors of components of the filter circuit. The filter circuit may be a passive filter circuit and / or may have at least two phases. For example, the filter circuit may have three phases. The filter circuit may consist of capacitors, inductors, and / or resistors.

[0013] The converter device includes a filter circuit, a DC link, and an inverter interconnected between the filter circuit and the DC link. As the filter circuit, the converter device may include at least two phases. For example, the converter device may include three phases. The DC link may be a simple DC link or a split DC link. The inverter may include inverter branches for each phase of the converter device. The inverter branches may be half-bridge inverter branch topologies, neutral-point clamped inverter branch topologies, full H-bridge inverter branch topologies, or other inverter branch topologies.

[0014] As described below, an inverter is used to generate current through a filter circuit. The current is measured, and at least one parameter is determined by evaluating the transfer function of the filter circuit.

[0015] According to one embodiment of the present invention, the method includes: switching the inverter to an active state, in which a first inverter branch of the inverter connects a first phase of the filter circuit to the positive side of the DC link, and a second inverter branch of the inverter connects a second phase of the filter circuit to the negative side of the DC link. This induces a current in the filter circuit.

[0016] In the case of a three-phase converter device, the third inverter branch of the inverter can be switched so that the corresponding phase of the filter circuit is disconnected from the DC link or connected to the neutral point of the converter device. When the third inverter branch is connected to the neutral point, the active state can be written as [1,-1,0], or when the third inverter branch is disconnected from the DC link, the active state can be written as [1,-1,NC].

[0017] According to one embodiment of the invention, the method includes: after switching to an active state, switching the inverter to a zero state, in which two or more inverter branches of the inverter connect two or more phases of the filter circuit to the same voltage potential of the DC link. For example, the zero state can be achieved by applying a zero vector to the inverter, which may be of the form [0,0,X], where a third phase may or may not be connected (X=0 or X=NC).

[0018] According to one embodiment of the invention, the method further includes determining a time series of current values ​​by measuring the current through the filter circuit at multiple consecutive time steps. During the measurement, the inverter is in a zero state. The measurement indicates the dynamic behavior of components of the filter circuit interconnected with the DC link. The current values ​​can be determined at equally spaced time steps. The measurement can be obtained using a current sensor interconnected between the filter circuit and the inverter (i.e., the current sensor), which is adapted to measure the inverter current.

[0019] Generally, an inverter may include an inverter branch for each phase, which includes a midpoint and an endpoint. The midpoint is connected to the phase of the filter circuit, and the endpoint is connected to the DC link. The inverter branch may be a simple two-level inverter branch or a three-level inverter branch, such as a neutral-point clamped inverter branch.

[0020] According to one embodiment of the invention, the method further includes: determining at least one parameter of the filter circuit by calculating a best fit for at least one parameter based on a difference equation of the filter circuit, wherein a time series of current values ​​is input into the difference equation, wherein the difference equation is a function of the current values ​​at different time steps and at least one parameter. The difference equation can be derived from the discrete transfer function of the filter circuit. In the active state, small disturbances may be introduced in the inverter current, and the resulting capacitor current through the filter circuit can be analyzed. The method relies on the time series response to the initial disturbance of the filter circuit. The time series of the resulting current values ​​is then analyzed, for example, using a least squares error technique, to determine the best fit for one or more parameters of the observed measurement.

[0021] For the components of the filter circuit interconnected in the active state of the inverter, a transfer function can be determined that describes the dynamic behavior of the filter circuit components in a discrete manner. The transfer function can be viewed as a model of the filter circuit, or at least a model of the components of the filter circuit interconnected with the DC link in the active state. A simplified model of the filter circuit can also be used, for example, where some passive components, such as, for example, small resistors, are neglected.

[0022] The transfer function can be discretized relative to time steps, with time series of current values ​​measured at these time steps. Since the transfer function response may depend only on the current value at the actual time step n, one or more previous time steps n-1, n-2, ..., and at least one parameter, the best fit for at least one parameter can be determined. This can be achieved by minimizing the minimum mean square error. Note that at least one parameter may be time-independent.

[0023] Finally, by means of a method, at least one parameter is determined that when at least one parameter deviates from a given nominal parameter value, at least one parameter can be used to monitor at least one parameter over time (e.g., to monitor the degradation of components in a filter circuit) and / or to generate error or warning messages.

[0024] The above method has been described for two relative states of the filter circuit. This method can be repeated for other relative states. In this way, other parameters of the filter circuit can be determined. The active states of these other relative states can be written as [1,NC,-1] (or [1,0,-1]) and [NC,1,-1] (or [0,1,-1]).

[0025] This method utilizes the inverter's existing semiconductors and current measurement equipment, thus requiring no additional hardware. No additional hardware or measurement equipment needs to be installed. Current measurements can be obtained from the existing inverter current sensor.

[0026] According to one embodiment of the invention, the difference equation equals the current value at time step n plus the current values ​​at previous time steps n-1, n-2, ... multiplied by one or more parameters of the filter circuit. The difference equation can be derived from a continuous transfer function, which can be based on a possible simplified model of the filter circuit. Furthermore, since the inverter remains in a zero state during the measurement, it can be assumed that the inverter voltage is zero.

[0027] According to one embodiment of the invention, the difference equation is of the form g + hb = 0, where generally g is a vector and h is a matrix, the elements of which are linear combinations of current values ​​from time series at different and / or past time points. Generally, b is a vector formed by at least one parameter. In the case where a single parameter needs to be determined, b may be a scalar value.

[0028] According to one embodiment of the present invention, by (h T h) -1 h T g determines the best fit for at least one parameter b.

[0029] In special cases, when b is a scalar, g and h can be derived from the term g. n =y[n]+y[n-2] and h n = a vector consisting of y[n-1], where y[n] is the current value at time step n. However, other statistical techniques can also be used to determine the best fit. Least mean square determination can have the advantage that when determining another measurement y[n], it is possible to use other addends with h. T g and h T At least one parameter is determined recursively by adding h values. There is no need to store individual measurements.

[0030] According to one embodiment of the invention, at least one parameter includes the capacitance of a capacitor in a filter circuit. This method can be used to determine the main capacitance of a filter circuit, which may be an input filter unit (IFU) of a converter device used in conjunction with an inverter, and the IFU may be an active front end (AFE) of the converter device. The purpose may be to determine whether the main capacitor is in a healthy state, thus ensuring safe operation of the converter device.

[0031] According to one embodiment of the invention, the method further includes: switching the inverter to a zero state before switching to an active state, in which the first and second phases of the filter circuit are either disconnected from the same phase or connected to at least one of the same potential, and at least one parameter is determined for the first and second phases of the filter circuit. Thus, the filter circuit and the inverter can enter a defined quiescent state in which no current or almost no current is flowing. The zero state of the first two phases can be implemented as [NC,NC,NC] or [1,1,1] or [0,0,0] or [-1,-1,-1], where NC is not connected, 1 is connected to the positive side of the DC link, 0 is connected to the neutral point, and -1 is connected to the negative side of the DC link.

[0032] According to one embodiment of the invention, the method further includes charging the DC link before switching to the active state. The DC link may be charged optionally before switching to the zero state and after switching to the active state. This can be accomplished using an additional charger for the converter device. The DC link can be charged to different levels as used in normal operation.

[0033] According to one embodiment of the invention, the method further includes: disconnecting a circuit breaker between the power grid supplying power to the converter device and the filter circuit before switching to the active state. The method can be performed when the power grid and the filter circuit are disconnected, for example, when the circuit breaker between the power grid and the filter circuit is open. The circuit breaker can be disconnected only after a considerable period of time (e.g., after the converter device has been shut down). The circuit breaker can be closed only when at least one parameter indicates that the filter circuit is healthy. The method can be run automatically before the circuit breaker is closed.

[0034] According to one embodiment of the invention, the filter circuit includes an inductor and a capacitor for each phase, the inductor being connected to the phase and the capacitor connecting the phases to a common point or connecting two phases together. The capacitors can be star-connected or delta-connected.

[0035] According to one embodiment of the present invention, the method further includes: periodically determining at least one parameter. For example, at least one parameter may be automatically determined when the converter device is started.

[0036] According to one embodiment of the invention, the method further includes: storing at least one parameter each time it is determined, thereby generating a history of at least one parameter indicating the degradation of components of the filter circuit. The at least one parameter may be stored, along with time, in the memory of the controller of the converter device. For example, when the at least one parameter includes the capacitance of a capacitor in the filter circuit, the parameter may decay over time, thus allowing the determination of future times when the capacitor must be replaced.

[0037] According to one embodiment of the invention, the method further includes: predicting the lifetime of components of the filter circuit based on the history of at least one parameter. The history of the at least one parameter can be extrapolated to the future. When the at least one parameter exceeds a threshold in the future, this can indicate the end of the lifetime. Generally, the history can be used to monitor parameter degradation over time. Degradation over time can be used to predict component lifetime and / or issue warnings.

[0038] According to one embodiment of the invention, the method further includes automatically determining at least one parameter before closing the circuit breaker of the converter device. As already mentioned, this method can be performed automatically when the converter device is started to test whether the filter circuit is healthy. It should be noted that this method can be performed with the circuit breaker open, and is therefore particularly suitable for such testing.

[0039] According to one embodiment of the present invention, the method further includes: outputting a warning message when at least one parameter exceeds a threshold. The warning may indicate that maintenance must be completed soon.

[0040] According to one embodiment of the present invention, the method further includes: preventing the operation of the converter device when at least one parameter exceeds a threshold. The method can also be used to automatically stop the converter device from starting when at least one parameter indicates that the filter circuit is unhealthy.

[0041] Another aspect of the invention relates to a computer program comprising instructions that, when executed by a processor, cause the processor to perform the methods described herein. Such a computer program may be stored in the memory of a controller of a converter device and / or may be executed in the processor of the converter device.

[0042] Another aspect of the invention relates to a computer-readable medium in which such a computer program is stored. The computer-readable medium may be a hard disk, a USB (Universal Serial Bus) storage device, RAM (Random Access Memory), ROM (Read-Only Memory), EPROM (Erasable Programmable Read-Only Memory), or FLASH memory. The computer-readable medium may also be a data communication network, such as the Internet, which allows the download of program code. Generally, the computer-readable medium may be a non-transitory medium or a transient medium. For example, the memory of the controller of a converter device may be such a computer-readable medium.

[0043] Another aspect of the invention relates to a controller for a converter device, adapted to perform the methods described herein. These methods may be implemented at least partially in hardware (e.g., RPGA).

[0044] Generally, a controller can be adapted to control the state of the semiconductor switches of an inverter and, optionally, another converter, analyze the resulting inverter and / or filter currents, and determine at least one parameter of the filter circuit based on a best-fit method. This may mean discovering one or more parameters that minimize the least-squares error of the equation (i.e., the difference equation) describing the expected circuit current.

[0045] Another aspect of the invention relates to a converter device including a filter circuit, an inverter connected to the filter circuit, a DC link provided by the inverter, and a controller adapted to perform the methods described herein.

[0046] The controller can also be adapted to control the inverter to convert AC current from the filter circuit into DC current to be supplied to the DC link. The inverter can be controlled as an active rectifier.

[0047] The converter device may also include one or more other inverters connected to a DC link for powering one or more motors. The controller may also be adapted to control the other converter to convert DC current from the DC link into AC current to be supplied to the one or more motors. Alternatively or additionally, the one or more other inverters may be connected to other electrical systems, such as microgrids, power grids with different frequencies, electrical loads, or combinations thereof.

[0048] The converter device may also include a transformer for transforming the voltage from the power grid into voltages of different amplitudes to be supplied to the electrical filter. The transformer may be interconnected between the power grid and the filter circuit. The transformer may also have the same voltage on both its primary and secondary windings.

[0049] The converter device may also include a DC link charger for charging the DC link. The controller may be adapted to control the DC link charger to charge the DC link from another power source, such as another transformer connected to the power grid.

[0050] The converter device may also include a circuit breaker that can be interconnected between the power grid and the filter circuit. Furthermore, a controller can operate the circuit breaker.

[0051] The converter device may also include a current measuring device adapted to sense the inverter current and / or the current between the filter circuit and the inverter device.

[0052] It should be understood that the features of the methods described above and below can be features of the computer programs, computer-readable media, controllers, and converter devices described above and below, and vice versa.

[0053] These and other aspects of the invention will become apparent from the embodiments described below. Attached Figure Description

[0054] The subject matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments shown in the accompanying drawings.

[0055] Figure 1 A schematic diagram of a converter device according to an embodiment of the present invention is shown.

[0056] Figure 2 A schematic circuit diagram of the filtering circuit of a converter device according to an embodiment of the present invention is shown.

[0057] Figure 3 A schematic circuit diagram of the filtering circuit of a converter device according to another embodiment of the present invention is shown.

[0058] Figure 4 A schematic circuit diagram of the inverter of a converter device according to another embodiment of the present invention is shown.

[0059] Figure 5 A schematic circuit diagram of the inverter branch of a converter device according to an embodiment of the present invention is shown.

[0060] Figure 6 A schematic circuit diagram of the inverter branch of a converter device according to another embodiment of the present invention is shown.

[0061] Figure 7 An equivalent circuit of the filtering circuit of a converter device according to another embodiment of the present invention is shown.

[0062] Figure 8 A flowchart of a method according to another embodiment of the present invention is shown.

[0063] The reference numerals used in the accompanying drawings and their meanings are listed in summary form in the reference numeral list. In principle, the same parts have the same reference numerals in the accompanying drawings. Detailed Implementation

[0064] Figure 1 A converter device 10 is shown, which includes a circuit breaker 12, a transformer 14, a filter circuit 16, an inverter or active rectifier 18, a DC link 20, and another inverter 22. AC current is supplied to the transformer 14 via the power grid 24. The other inverter 22 supplies a different AC current of a different voltage and frequency to the motor 26. Components 12, 14, 16, 18, 20, and 22 are cascaded. The converter device 10 has three phases A, B, and C.

[0065] The converter device 10 also includes a DC link charger 28 for charging the DC link 20, specifically, charging one or more of its capacitors. The DC link charger 28 can be powered via another transformer connected to the power grid 24.

[0066] The current sensor 30 is configured to sense the inverter current between the filter circuit 16 and the inverter 18.

[0067] In addition, the controller 32 of the converter device 10 is configured to control the active components 12, 18, 22, and 28.

[0068] During normal operation, circuit breaker 12 is closed, controlling inverter 18 to convert AC current from filter circuit 16 into DC current to be supplied to DC link 20, and controlling another inverter 22 to convert DC current from DC link 20 into AC current to be supplied to motor 26.

[0069] During the shutdown operation, inverters 18 and 22 are stopped and circuit breaker 12 is disconnected.

[0070] During startup, the DC link 20 is charged using charger 28, as described above and below, to determine the health status of the filter circuit 16. If the filter circuit 16 is healthy, the circuit breaker 12 is closed and normal operation is initiated.

[0071] Figure 2 An embodiment of the filter circuit 16 is shown. One or more inductors 34 are interconnected in each phase A, B, C. Each phase A, B, C is connected to a common point 38 via a capacitor 36. This point can be floating, or directly or via a network of passive components connected to ground or neutral.

[0072] Figure 3 Another embodiment of the filter circuit 16 is shown. (Compared to...) Figure 2 Conversely, each of the opposite A, B, and C is interconnected via capacitor 36, thus they are delta connected.

[0073] Figure 4 An embodiment of inverter 18 is shown. Inverter 18 includes three inverter branches 40, each having a midpoint 42 connected to one of phases A, B, and C. The outer ends of inverter branches 40 are connected to the positive side 44 and the negative side 46 of DC link 20.

[0074] Figure 5 A two-level inverter branch 40 in half-bridge configuration is shown. The two-level inverter branch 40 includes two semiconductor switches 48 connected in series between the two sides 44, 46 of the DC link 20 and providing a midpoint 42 between them.

[0075] Figure 6 A three-level inverter branch 40 in the form of a neutral-point clamped H-bridge is shown. This branch includes four semiconductor switches 48 connected in series between two sides 44 and 46 of a DC link 20. A midpoint 42 is located between two internal switches 48. A neutral point 50 is connected via two diodes 52 to the midpoint 54 between the external and internal switches 48. The neutral point 50 is connected to the midpoint of a discrete DC link 20, which includes two capacitors 56 connected in series.

[0076] On the contrary, Figure 5 The DC link 20 includes a single capacitor 56, which is connected to sides 44, 46.

[0077] Figure 7 As shown Figure 2 The equivalent circuit of one phase of the filter circuit 16 shown is used to describe how to derive the discrete transfer function for the filter circuit 16. Generally, it can also be derived from... Figure 3 The filter circuit 16 shown, or any other passive filter circuit, derives this discrete transfer function.

[0078] Figure 7 The equivalent diagram refers to a portion of the filter circuit 16, which is connected to the DC link 20 in a specific active state of the inverter 18, wherein one phase of the filter circuit 16 is connected to the positive side 44 of the DC link 20, and the other phase is connected to the negative side 46 of the DC link.

[0079] When circuit breaker 12 is open, the transfer function describing the inverter current through filter circuit 16 as a function of inverter voltage is given by the following equation:

[0080]

[0081] Where L = L1 + L2 is the sum of the two inductors L1 and L2. Note that for this transfer function G(s), the equivalent resistance R is ignored, but it can be included. The transfer function for other types of filter circuits 16 also has the form of a fractional polynomial in units of s.

[0082] Using a bilinear approximation and a sampling time T s Discretizing the transfer function yields the following:

[0083]

[0084] in

[0085]

[0086] a1 = 0

[0087]

[0088]

[0089] b2 = 1

[0090] The discrete transfer function G(z) can be written as a difference equation that describes the inverter current y[n] at time step n as a function of the inverter voltage u[n].

[0091] y[n]=-b1y[n-1]-b2y[n-2]+a0u[n-1]+a1u[n-2]+a2u[n+2]

[0092] After the inverter voltage u[n] is set to zero, it waits for two or more sampling times T. s The input terms u[n-1], u[n-2], and u[n+2] are zero and can be ignored. Using the definition and rearrangement of b2 above, we can derive:

[0093] 0 = y[n] + y[n-2] + b1y[n-1]

[0094] Note that the expression above is valid for all n samples. Specifically, for n samples, we can write:

[0095] 0 = y[n] + y[n-2] + b1y[n-1]

[0096] 0 = y[n-1] + y[n-3] + b1y[n-2]

[0097] 0 = y[n-2] + y[n-4] + b1y[n-3]

[0098] 0 = y[n-3] + y[n-5] + b1y[n-4]

[0099]

[0100] 0 = y[n-N+1] + y[nN-1] + b1y[nN]

[0101] Or it can be written in vector form:

[0102] g+hb1=0

[0103] In general, that is, for a more general transfer function, h is an N×P matrix, b1 is a vector of length P, and g is a vector of length N. Note that this system of difference equations is overdetermined. The best fit for parameter b1 is given by the following equation:

[0104] b1=(h T h) -1 h T g

[0105] Figure 8 A flowchart is shown of a method, for example, that is automatically and / or periodically executed by the controller 32 during the startup of the converter device 10.

[0106] In step S10, ensure that DC link 20 is charged and circuit breaker 12 is open. DC link 20 can be charged using charger 28. Circuit breaker 12 may have been opened during the final shutdown of converter device 10.

[0107] At the end of step S10, a zero vector is applied to inverter 18 to bring the combination of filter circuit 16 and inverter 18 into a zero state, and the inverter current is waited for to decrease, such as to less than 1% of the inverter's nominal current. The actual inverter current can be measured using current sensor 30.

[0108] The zero state can be where phases A, B, and C of filter circuit 16 are connected to the same potential, such as positive side 44, negative side 46, or neutral point 50. The zero vector applied to the inverter can be indicated as [1,1,1], [0,0,0], or [-1,-1,-1].

[0109] In step S12, the inverter 18 is switched to an active state, in which the first inverter branch 40 of the inverter 18 connects the first phase (such as A) of the filter circuit 16 to the positive side 44 of the DC link 20, and the second inverter branch 40 of the inverter 18 connects the second phase (such as B) of the filter circuit 16 to the negative side 46 of the DC link 20.

[0110] The switching vectors of phases A and B, which are applied by inverter 18 to achieve the active state, can be indicated as [1; -1; 0] or [1, -1, NC].

[0111] A switching vector can be applied for 50μs, during which time the current flows through the filter circuit and partially charges the filter capacitor.

[0112] Subsequently, inverter 18 is switched to a (possibly second type) zero state, in which two or more inverter branches 40 of inverter 18 connect two or more phases of filter circuit 16 to the same voltage potential of DC link 20, such as the positive side 44 or negative side 46 of DC link 20. For the second type of zero state, a second type of zero vector can be applied to inverter 18. The second type of zero vector may be different from the first type of zero vector applied in step S10. For example, the second type of zero vector may be [1,1,NC], [0,0,NC], or [-1,-1,NC].

[0113] When the inverter is in a zero state, a time series y of current values ​​can be generated by measuring the current through the filter circuit 16 at multiple consecutive time steps.

[0114] The time series y includes the measured value y[n] of the inverter current at time step n.

[0115] In step S14, the measured current data, i.e., the time series y, is evaluated. Specifically, at least one parameter b of the filter circuit 16 is determined by calculating the best fit of at least one parameter b according to the difference equation of the filter circuit 16, and the time series y of the current value is input into the difference equation.

[0116] As described above, the difference equations are functions of the current values ​​at different time steps n, n-1, ... and at least one parameter b. Specifically, the system of difference equations has the following form:

[0117] g+hb=0

[0118] The difference equation equals the current value y[n] at time step n plus the current values ​​y[n], y[n-1], ... at previous time steps multiplied by the parameter b of filter circuit 16. The parameter b can be provided in vector form, but it can also be a scalar.

[0119] g and h are vectors formed by linear combinations of current values ​​from a time series at different points in time, such as...

[0120] and

[0121] The best fit for at least one parameter b can be determined in the following ways:

[0122] b = (h T h) -1 h T g

[0123] Note that the equation can be implemented recursively without storing several measurements y[n] in a buffer.

[0124] The capacitance C of capacitor 36 in filter circuit 16 can be determined based on parameter b. For example, for the example above, capacitance C can be retrieved using the following equation:

[0125]

[0126] In step S16, the current can be waited for to decrease again, and the determination can be repeated for other pairs (such as (B, C) and (A, C)). Steps S12 and S14 can be repeated for these pairs. In these steps, the activity vectors [0; 1; -1] and [-1; 0; 1] (or [NC; 1; -1] and [-1; NC; 1]) can be applied to inverter 18.

[0127] Finally, for each relative phase, an estimate of the average parameter b of the two effective phases is provided.

[0128] In step S18, post-processing of parameter b can be performed. This post-processing ensures that all phases A, B, and C have approximately the same parameters and / or that parameter b is close to the desired value.

[0129] For example, a warning message can be output when at least one parameter b exceeds a threshold. As another example, operation of converter device 10 can be prevented when the deviation from the expected parameter value of at least one parameter b exceeds a threshold.

[0130] At least one parameter b can also be stored each time it is determined, for example, each time it is started. This generates a history of at least one parameter b, which can indicate the degradation of components 34, 36 of the filter circuit 16. Based on this history, the lifespan of components 34, 36 of the filter circuit 16 can be predicted.

[0131] In summary, this method can be used to evaluate the health of capacitors 36 in filter circuits 16 (such as inverter filter units). The entire method may take less than a second. A short inverter voltage pulse of approximately 50 μs can be applied, and the resulting inverter current can be used to determine the value C of capacitor 36. If the identified capacitance C differs significantly from the nominal capacitance, a warning or fault may be generated. When the inverter is operating in normal operating mode, the proposed method may require very little energy, such as approximately 5% of the energy temporarily stored in each capacitor 36 of filter circuit 16.

[0132] Although the invention has been described in detail with reference to the accompanying drawings and the foregoing description, such description should be considered illustrative or exemplary rather than restrictive; the invention is not limited to the disclosed embodiments. Other variations of the disclosed embodiments can be understood and implemented by those skilled in the art and by practice upon studying the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude a plurality. A single processor or controller or other unit may perform the functions of several items as described in the claims. The fact that certain measures are recited in mutually different dependent claims does not mean that combinations of these measures cannot be advantageously used. Any reference numerals in the claims should not be construed as limiting the scope.

[0133] List of reference numerals

[0134] 10 Converter Equipment

[0135] 12 Circuit Breakers

[0136] 14 Transformers

[0137] 16. Filtering Circuit

[0138] 18 Inverters / Active Rectifiers

[0139] 20 DC Link

[0140] 22 Other Inverters

[0141] 24 Power Grid

[0142] 26 motors

[0143] Phases A, B, and C

[0144] 28 DC Link Charger

[0145] 30 Current Sensor

[0146] 32 controllers

[0147] 34 Inductors

[0148] 36 Capacitors

[0149] 38 Public Points

[0150] 40 Inverter Branch

[0151] 42 Midpoint

[0152] 44 Front and Side Views

[0153] 46 Negative side

[0154] 48 Semiconductor Switch

[0155] 50 neutral point

[0156] 52 diode

[0157] 54 Midpoint

[0158] 56 DC Link Capacitor

[0159] L1 equivalent inductance

[0160] L2 equivalent inductance

[0161] R equivalent resistance

[0162] C Equivalent capacitance

[0163] b parameter

[0164] Time series of inverter current measurements

Claims

1. A method for determining at least one parameter (b) of a filter circuit (16) of a converter device (10), the converter device (10) including the filter circuit (16), a DC link (20), and an active rectifier (18) interconnected between the filter circuit (16) and the DC link (20), wherein the active rectifier (18) includes an active rectifier branch (40), the method comprising: The active rectifier (18) is switched to an active state, in which the first active rectifier branch in the active rectifier branch (40) connects the first phase of the filter circuit (16) to the positive side (44) of the DC link (20), and the second active rectifier branch in the active rectifier branch (40) connects the second phase of the filter circuit (16) to the negative side (46) of the DC link (20). Subsequently, the active rectifier (18) is switched to a zero state, in which two or more active rectifier branches (40) of the active rectifier (18) connect two or more phases of the filter circuit (16) to the same voltage potential of the DC link (20). The time series (y) of the current value is determined by measuring the current through the filter circuit (16) at multiple consecutive time steps. The at least one parameter (b) of the filter circuit (16) is determined by calculating the best fit of the at least one parameter (b) according to the difference equation of the filter circuit (16), wherein the time series of the current value is input into the difference equation, wherein the difference equation is a function of the current value at different time steps and the at least one parameter (b).

2. The method according to claim 1, The difference equation therein is equal to the current value at time step n plus the current value at the previous time step multiplied by the parameters of the filter circuit (16).

3. The method according to claim 1 or 2, The difference equation is of the form g + hb = 0; Where g is a vector, and h is a matrix formed by a linear combination of the current values ​​of the time series at different time points; Where b is a vector formed by the at least one parameter.

4. The method according to claim 1 or 2, Wherein the best fit b for at least one parameter is given by b = (h T h) -1 h T g determines, Where g and h are derived from term g n = y[n] + y[n - 2] and h n = a vector consisting of y[n - 1], where y[n] is the current value at the time step n.

5. The method according to claim 1 or 2, The at least one parameter (b) mentioned therein includes the capacitance (C) of the capacitor (36) of the filter circuit.

6. The method according to claim 1 or 2, further comprising: Before switching to the active state, the active rectifier (18) is switched to the zero state, in which the first phase and the second phase of the filter circuit (16) are either disconnected from the same potential or connected to at least one of the same potentials, and the at least one parameter (b) is determined for the first phase and the second phase of the filter circuit (16).

7. The method according to claim 1 or 2, further comprising: Before switching to the active state, charge the DC link (20); and / or Before switching to the active state, disconnect the circuit breaker (12) between the power grid (24) supplying power to the converter device (10) and the filter circuit (16).

8. The method according to claim 1 or 2, The first active rectifier branch (40) and the second active rectifier branch (40) are two-level active rectifier branches; or The first active rectifier branch (40) and the second active rectifier branch (40) are neutral point clamped.

9. The method according to claim 1 or 2, The filter circuit (16) includes an inductor (34) and a capacitor (36) for each phase, the inductor (34) being connected to the phase and the capacitor (36) connecting the phase to a common point (38) or interconnecting two phases to each other.

10. The method according to claim 1 or 2, further comprising: Periodically determine at least one parameter (b); Each time it is determined, the at least one parameter (b) is stored, thereby generating a filter circuit history that indicates the degradation of the at least one parameter (b) of the components (34, 36) of the filter circuit (16); The lifespan of components (34, 36) of the filter circuit (16) is predicted based on the history of at least one parameter (b).

11. The method according to claim 1 or 2, further comprising: Before the circuit breaker (12) of the converter device (10) is closed, the at least one parameter (b) is automatically determined; and / or When at least one parameter (b) exceeds the threshold, a warning message is output; and / or When at least one parameter (b) exceeds a threshold, the operation of the converter device (10) is prevented.

12. A controller (32) for a converter device (10), the controller including a memory and a processor, and adapted to perform the method according to any one of claims 1 to 11, specifically for controlling the state of the semiconductor switch of the active rectifier (18) of the converter device (10), analyzing the resulting active rectifier and / or filter current, and determining at least one parameter (b) of the filter circuit (16) of the converter device (10) based on a best-fit method.

13. A computer program product comprising instructions that, when executed by a processor of a controller according to claim 12, cause the processor to perform the method according to any one of claims 1 to 11.

14. A computer-readable medium storing a computer program product according to claim 13.

15. A converter device (10), comprising: Filtering circuit (16); An active rectifier (18) is connected to the filter circuit (16). DC link (20) is powered by the active rectifier (18); The controller (32) according to claim 12.

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