FIELD OF THE INVENTION

The invention relates to a method, a computer program, a computer-readable medium and a controller for controlling an electrical converter. Furthermore, the invention relates to an electrical converter.

BACKGROUND OF THE INVENTION

For electrical converters model predictive control is used for determining switching states of the converter circuit via determining future states of the electrical converter based on a model of the converter and connected components, such as electrical motors, electrical generators, electrical grids, etc.

In WO 2015 078 656 A1 pre-computed optimized pulse patterns are further optimized with model predictive control by manipulating their switching instants. In such a way, the harmonic content of the produced currents may be reduced.

The harmonic benefit of optimized pulse patterns over other modulation methods is particularly high at low pulse numbers (i.e. the ratio between the switching frequency and the fundamental frequency). At very high pulse numbers and low fundamental frequencies, however, optimized pulse patterns may be difficult to calculate and may use much controller memory. Optimized pulse pattern-based control methods typically solve this issue by switching to other control methods.

In EP 2 469 692 A1 an electrical system is controlled in such a way that switching sequences with switching time instants for the converter that have been determined with respect to a certain first optimization goal are modified in a second step. A resultant flux error is determined. The switching time instants are modified to reduce the flux error.

In the article of JAMES SCOLTOCK ET AL: “A Comparison of Model Predictive Control Schemes for MV Induction Motor Drives”, IEEE TRANSACTIONS ON INDUSTRIAL INFORMATICS, IEEE SERVICE CENTER, NEW YORK, N.Y., US, vol. 9, no. 2, 10 Oct. 2012 (2012 Oct. 10), pages 909-919, several model predictive controls schemes are compared with pulse width modulation and optimized pulse patterns. It is mentioned that a common mode component can be added to the reference voltage.

In the article of NIKOLAOS OIKONOMOU ET AL: “Model Predictive Pulse Pattern Control for the Five-Level Active Neutral-Point-Clamped Inverter”, IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, vol. 49, no. 6, 15 May 2013 (2013 May 15), pages 2583-2592, it is described how optimized pulse patterns can be selected dependent on a fundamental frequency and that reference flux values can be stored together with the optimized pulse patterns.

DESCRIPTION OF THE INVENTION

It is an objective of the invention to provide an electrical converter adapted for producing an output current with low harmonic distortions at low fundamental frequencies. It is a further objective of the invention to provide a simple and computationally less demanding model predictive control scheme, which also may be used at high pulse numbers.

These objectives are achieved by the subject-matter of the independent claims. Further exemplary embodiments are evident from the dependent claims and the following description.

A first aspect of the invention relates to a method for controlling an electrical converter. The electrical converter is adapted for converting a DC voltage into a multi-phase voltage with at least two voltage levels. The electrical converter may comprise a converter circuit with semiconductor switches, which may be arranged in at least two (such as three) phase branches. The phase branches may be connected in parallel to a DC link and/or may be adapted for converting a DC link voltage or more generally a DC voltage into an AC phase voltages. Each phase branch may be adapted for generating at least two voltage levels.

For example, the phase branches may be adapted for generating three voltage levels, such as a positive DC link voltage, a negative DC link voltage and a neutral point voltage. The neutral point voltage may be provided by a split DC link. The electrical converter may be a neutral point clamped converter. However, in general the method also may be performed with other types of converters.

The electrical converter may be connected with its AC side to an electrical machine, such as a generator or motor and/or may supply this electrical machine with electrical power. The stator and/or rotor flux mentioned below may refer to magnetic fluxes in the stator and/or rotor of this electrical machine. It also may be the case that the electrical converter is connected to an electrical grid with its AC side. In this case, also virtual stator and/or virtual rotor fluxes may be defined for the electrical grid.

The electrical converter may be a power converter. Its semiconductor switches may be adapted for switching currents of more than 10 A and/or voltages of more than 1000 V.

The electrical converter may comprise a controller, which performs the method. For example, the method may be performed at each sampling time instant.

According to an embodiment of the invention, the method comprises: determining a modulating signal vector from a stator flux reference vector. The stator flux reference vector may be provided by an outer control loop, which based on measurements and on a reference speed and/or reference frequency determines the stator flux reference vector.

The modulating signal vector is a signal indicative of the output voltages to be generated by the converter. For example, the modulating signal vector may be a normalized voltage vector determined by differentiating the stator flux reference vector.

The invention is also applicable to grid-side converters, i.e. the stator flux vector may be a virtual stator flux vector. Also, a rotor flux vector may be a virtual rotor flux vector. For example, in the case of a converter supplying an electrical grid, the stator flux vector may be a virtual converter flux vector, and the rotor flux vector may be a grid flux vector, which may be defined at a point of common coupling (PCC).

It has to be noted that here and in the following, vectors may be quantities with two or three components.

According to an embodiment of the invention, the method further comprises: determining a switching pattern from the modulating signal vector via pulse width modulation, the switching pattern comprising a sequence of switching transitions, wherein a switching transition defines a switch position, at which a phase of the converter is switched from one voltage level to another voltage level, and a transition time instant at which the phase of the converter is switched.

The modulating signal vector may be indicative of the actual output voltages to be generated. Together with an actual frequency and/or actual speed, the movement of the modulating signal vector may be extrapolated into the future and therefrom, a sequence of future switching sequences may be determined. For example, the trajectories of the components of the extrapolated modulating signal vector may be compared with one or more carrier signals (which may be triangular signals with a carrier frequency). The switching transitions may be determined from the trajectories and the carrier signal(s) with pulse width modulation. For example, a crossing point of a trajectory with a carrier signal may determine a switching time instant and/or the direction of the crossing may determine, whether the voltage level is raised or lowered at the switching time instant.

It has to be noted that these calculations all may be performed online during a sampling time interval, i.e. the time between two sampling time instants, and/or may be performed numerically.

According to an embodiment of the invention, the method further comprises: determining a stator flux error by subtracting from the stator flux reference vector an estimated stator flux vector, which is estimated from measurements in the electrical converter. The estimated stator flux vector may be provided by an outer control loop. The difference of the stator flux reference vector and the estimated stator flux vector or the magnitude of the difference may be seen as error, which may be minimized by the following step of the method and/or with model predictive control.

According to an embodiment of the invention, the method further comprises: modifying the switching pattern by moving transition time instants of switching transitions of the switching pattern, such that the stator flux error is minimized. The transition time instants may be moved forward and backward in time, such that the switching pattern, which was generated by pulse width modulation, is modified. The transition time instants may be moved such that the modified switching pattern results in a stator flux, which is closer to the stator flux reference. In such a way, the modified switching pattern may result in lower amplitudes for the higher order harmonics than the unmodified switching pattern.

It may be the case that constraints are present on the movement of the switching time instants. For example, the order of the switching transitions in one phase or all the phases may not be changed.

Furthermore, it may be the case that additional switching time instants are included in the unmodified switching pattern, which, for example, would result in pulses of zero length. After optimization, these pulses may have a nonzero length and may contribute to the compensation of the stator flux error.

The optimization of the switching pattern may be performed online by solving a quadratic program, by minimizing an objective function subject to constraints, etc.

According to an embodiment of the invention, the method further comprises: applying at least a part of the modified switching pattern to the electrical converter. The switching transitions may be translated into switching times and switching states of the semiconductor switches of the electrical converter. Corresponding gate signals for the semiconductor switches may be generated.

It may be the case that the switching pattern and/or the modified switching pattern is longer than the sampling time interval. In this case, only a part of the switching pattern may be converted into switching times and switching states.

According to an embodiment of the invention, a stator flux correction is defined as the sum of products of a voltage level difference and a time difference, the voltage level difference of a switching transition being the difference of the voltage level before and after the switching transition, and the time difference being the difference between a moved and an unmodified transition time instant. This stator flux correction may be determined per phase or for all phases. The switching pattern may be optimized such that the stator flux correction at least partially compensates the stator flux error. The stator flux correction may be optimized by moving the transition time instants, such that the stator flux correction is equal to the stator flux error.

According to an embodiment of the invention, the switching pattern is regularly determined at each sampling time instant over a prediction horizon. The prediction horizon may be longer than a difference between consecutive sampling time instants. For example, only a part of the modified switching pattern shorter than the prediction horizon, such as a difference between consecutive sampling time instants may be applied to the electrical converter. In other words, the method may predict a future behavior of the electrical converter and/or the supplied electrical machine for a prediction horizon that may be longer than a sampling time interval. At each sampling time instant, the future behavior for more than the sampling time interval may be determined.

According to an embodiment of the invention, the switching transitions of the switching pattern are determined over a prediction horizon by: determining future samples of the modulating signal vector over the prediction horizon by rotating the modulating signal vector in the αβ0-coordinate system with an actual angular frequency to a time instant of the respective future sample; and generating a switching transition when a carrier signals crosses a value of the future sample during a time interval of the future sample. The future samples may determine a trajectory and/or function of the modulating signal over time. The future samples may be determined by extrapolating the components of the modulating signal vector into the future. This may be done by translating the modulating signal vector into the αβ0-coordinate system, for example with a Clarke transformation. After that the modulating signal vector may be rotated with the actual angular frequency, which may be determined from the actual speed and/or actual frequency, to the time instant of the future sample. This rotation may be a rotation of the αβ-components by the actual angular frequency times the time instant of the future sample. Thus, it may be assumed that the modulating signal vector is rotating with constant angular frequency. After that rotation, the rotated modulating signal vector may be transformed back into the system, in which it was provided, for example an abc-system. This back transformation may be performed with an inverse Clarke transformation.

The carrier signal may be represented as a triangular signal, and intersections of the trajectories of the extrapolated components of the modulating signal vector may be determined with the carrier signal. For example, the trajectories may be step (or piecewise constant) trajectories over time, whose values are determined from the values of the future samples at the time instants of the future samples. Each crossing may determine a switching time instant for a switching transition. Depending on the direction of the crossing, it may be determined, whether the voltage level increases or decreases at the intersection.

According to an embodiment of the invention, the stator flux reference vector is a three-component vector in the αβ0-coordinate system. A 0-component of the stator flux reference vector may be used for controlling a neutral point potential. Also, the stator flux error may be a three-component vector in the αβ0-coordinate system.

According to an embodiment of the invention, a common-mode component is added to the modulating signal vector. A common-mode component may be the sum of the phase components of the modulating signal vector. With this common-mode component, the output voltages of the electrical converter may be influenced without changing the output current, assuming that a star point of a load floats. This may be beneficial for influencing further optimization goals of the electrical converter and/or for complying with physical restrictions of the electrical converter.

According to an embodiment of the invention, when a modulation index determined from a magnitude of the stator flux reference vector is smaller than a threshold, the common-mode component is at least the modulation index plus a minimal pulse width divided by a carrier interval length of a carrier signal, such that pulse lengths of pulses generated by pulse width modulation with the carrier signal are longer than a minimal pulse width.

The modulation index may indicate the ratio of the magnitude of the actual output voltage with respect to a possible maximal output voltage, such as (half the DC voltage provided by a DC link. When the modulation index is rather small, it may be necessary to make sure that pulse lengths, i.e. intervals of the switching pattern with nonzero output voltage levels, are longer than a minimal pulse length. This minimal pulse length may correspond to a minimal time for which the semiconductor switches must remain in the on state before they may be turned off.

According to an embodiment of the invention, the common-mode component is controlled with a hysteresis controller, which sets the common-mode component to a first value, when a neutral point potential is larger than a maximal value and which sets the common-mode component to a second value, when the common-mode component is smaller than a minimal value. The second value may be of opposite sign with respect to the first value, for example, the negative first value. The first value may be in a direction of the power flow, which may be positive in motoring operation and negative in generating mode.

For example, in the case when the modulation index is higher than the threshold, the common-mode component may be used for controlling the neutral point potential of a split DC link. Since a common-mode component may increase and/or decrease the neutral point potential, the neutral point potential may be controlled to stay within bounds by inverting the neutral point potential each time the bound is exceeded.

According to an embodiment of the invention, the flux reference vector is determined from a reference angle and a reference magnitude. The reference angle and the reference magnitude may be provided by an outer control loop, which determines them from a reference speed and/or reference frequency and optional measurements of currents and/or voltages in an electrical machine supplied by the electrical converter.

According to an embodiment of the invention, a pulse number for the switching pattern is determined from a fundamental frequency and a maximum allowed switching frequency. The pulse number may be the ratio of the maximum allowed switching frequency and the fundamental frequency rounded to the closest lower integer. The fundamental frequency may be based on the reference speed and/or the reference frequency for the electrical converter. The pulse number may determine the number of pulses for the optimized pulse pattern, which may be used, when the pulse number is smaller or equal to a threshold.

According to an embodiment of the invention, when the pulse number exceeds a threshold, the switching pattern is determined with (for example carrier-based) pulse width modulation and otherwise, the switching pattern is determined from a table of optimized pulse patterns.

In general, for low pulse numbers (i.e. lower or equal to the threshold), optimized pulse patterns may be used as switching patterns, while for higher pulse numbers, (for example carrier-based) pulse width modulated switching patterns may be used, as described above and below. Thus, the same outer control loops and/or the same switching pattern optimizer may be used in both cases.

The optimized pulse patterns may have been optimized offline and may have the advantage that they may result in a lower total harmonic distortion than the (for example carrier-based) pulse width modulated switching patterns. However, for high pulse numbers, it may be difficult to store large optimized pulse patterns and/or to determine them offline.

The pulse width modulated switching pattern may be used to extend an optimized pulse pattern based control method to high pulse numbers. The pulse width modulated switching patterns may be generated online and may be fed to the same switching pattern optimizer as the optimized pulse pattern. In a variable speed drive, for example, this extension may make the switching pattern optimization, which moves switching time instants, applicable to all operating points, ranging from standstill operation to field weakening at high speeds.

According to an embodiment of the invention, the optimized pulse patterns are indexed in the table by a pulse number and a modulation index. Contrary to the pulse width modulated switching patterns, the optimized pulse patters are determined from these two quantities and not from the stator flux reference and/or modulating signal vector. The pulse number and the modulation index may be provided by an outer control loop.

According to an embodiment of the invention, in the case, when an optimized pulse pattern is used as switching pattern, the stator flux reference vector is determined from the optimized pulse pattern. The stator flux reference may be determined by integrating a voltage waveform of the optimized switching pattern. For performing the integration, a flux reference angle and a reference magnitude, which may be provided by an outer control loop, may be needed.

A further aspect of the invention relates to a computer program, which when being executed on a processor is adapted for executing the steps of the method as described in the above and in the following. For example, the controller of the electrical converter may provide such a processor and the method is performed by software running in the controller. However, it may also be possible that the method is at least partially implemented in software. For example, at least parts of the methods may be performed by an FPGA and/or DSP.

A further aspect of the invention relates to a computer-readable medium on which such a computer program is stored. A computer-readable medium may be a floppy disk, a hard disk, an USB (Universal Serial Bus) storage device, a RAM (Random Access Memory), a ROM (Read Only Memory), an EPROM (Erasable Programmable Read Only Memory) or a FLASH memory. A computer readable medium may also be a data communication network, e.g. the Internet, which allows downloading a program code. In general, the computer-readable medium may be a non-transitory or transitory medium.

A further aspect of the invention relates to a controller for controlling an electrical converter, which controller is adapted for executing the steps of the method as described in the above and in the following. As already mentioned, such a controller may comprise a processor running software, which performs the method. Furthermore, the controller may comprise an FPGA and/or DSP at least partially implementing the method.

A further aspect of the invention relates to an electrical converter, which comprises a converter circuit adapted for converting a DC voltage into a multi-phase voltage with at least two voltage levels and such a controller adapted for controlling semiconductor switches of the converter circuit.

It has to be understood that features of the method as described in the above and in the following may be features of the computer program, the computer-readable medium and the electrical converter as described in the above and in the following, and vice versa.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

