The present invention relates to the technical field of controlling converters.

Further, the present invention finds its application in the field of traction converters for single- phase applications as it is used by locomotives and rail vehicles. The present invention is based on a control system for traction converters for single-phase applications in accordance with the preamble of the independent claim 1. The present invention is further based on a method for controlling a converter having a control system. A traction converter converts supplied energy and makes it available to drive motors operating railway vehicles such as locomotives or railcars. The conversion of energy for example from a supply power grid with alternative or direct current into a motor operating with three-phase alternating current is made possible by state-of-the-art power electronics. In the particular field of traction applications for railway vehicles the line-side converter of the traction vehicle is usually connected via a transformer and a pantograph to an AC railway grid and feeding a DC-link voltage. The DC-link voltage supplies a load for example a motor drive unit to power the railway vehicle. Usually converters of traction applications are controlled to operate the converter in challenging environmental conditions. In particular, the reliability of converters during operation is often challenged in view of signal disturbances caused by injections from the grid and load-side, e.g. a motor drive. For this reason, various types of controllers for traction technology are known in prior art.

A first solution in prior art used for controlling a converter is disclosed in the document of P. Mattavelli "A Modified Dead-beat control for UPS using Disturbance Oberservers", published in I EEE, 2002. The system and method therein focuses mainly on DC-/DC converters and further, on the elimination of disturbances influencing a voltage and current of a load. A further typical solution in prior art used for controlling traction line converters is for example published in the document of B. Bahrani et al. "Multivariable-PI-based DQ Current control of voltage source converters with Superior Axes Decoupling

Capability", published in IEEE, July 201 1 . The document describes a standard feedback control structure using a Proportional-Integral (PI) control structure to control a traction converter. The feedback control structure consists of two cascaded loops: A first external control loop to regulate a DC-link voltage of the converter to a referenced value and a second internal control loop to regulate a line current to a referenced value. The external control loop generates a d current reference value for the converter. The internal control loop stabilizes the current into the d _q rotating frame to its reference value. In order to control the signal limits, control techniques such as feed forward control and anti-windup are used. However, the described control structure using Pl-techniques has drawbacks. The described prior art control system is limited as it is sensitive to external signal variations influencing the converter system. It does not account for dynamic signal interactions and is bound to a specific parametric controller setting which is sensitive to grid, load changes and disturbances. Therefore, depending on the operation mode of the converter system, constant tuning of the controllers and filters is required resulting in high costs for commissioning. Furthermore, the performance of such a Pl- controlled converter is mainly dependent on the proper decoupling between outer and inner control loop. This might require the control system to be implemented on hardware.

The objective of the present invention is to provide an improved control system for converters used for traction applications that increases the stability of the power flow across the converter in the presence of signal disturbances due to grid and / or load changes entering the converter system. In a more specific way, it is a further objective of the invention to stabilize a DC-link voltage signal and to achieve balanced line currents meaning a current from the grid to which the converter system is conntected and when signal disturbances caused by the grid and / or caused by the load entering the converter system. It is also an objective of the invention to reduce time for adopting the converter settings and parameters to changing applications and operating in dynamically agile technical environments. It is a further objective of the invention to provide for an improved method for controlling a converter having a control system.

The solution is to provide for a control system for converters as defined by the features of independent claims 1 and 12. Preferred examples of the invention are set forth by the appended dependent claims.

In particular, the technical objective is solved by a control system and a method for controlling an AC-/DC converter in particular used for single-phase traction applications. The control system comprising a voltage controller for controlling an output DC-voltage of the AC- /DC converter to a reference output DC-voltage of the AC-/DC converter, a current controller for controlling a line current to a reference line current of the AC-/DC converter, an observer connected to the voltage controller in order to estimate an offset of an output DC-voltage of the AC-/DC converter from a reference output DC-voltage due to disturbances caused by a grid connected to the AC-/DC converter entering the AC-/DC converter. Thehe voltage controller and / or current controller of the control system having a logic model that is Model Predictive Control to predict the dynamic behavior of the AC-/DC converter influenced by the disturbances caused by the grid entering the AC-/DC converter and in order to generate an input voltage or an input current for the AC-/DC converter to compensate for deviations of the output DC-voltage or the line current of the AC-/DC converter from a reference output DC- voltage or a reference line current due to disturbances caused by the grid entering the AC- /DC converter in an operative state of the AC-/DC converter.

It should be noted that an aspect of the present invention is to provide for an improved control system for a AC-/DC converter comprising a voltage controller for controlling an output DC-voltage of the converter to a reference output DC-voltage of the AC-/DC converter, a current controller for controlling an output current to a reference line current of the AC-/DC converter, an observer connected to the voltage controller in order to estimate an offset of an output DC-voltage of the AC-/DC converter from a reference output DC-voltage due to disturbances entering the AC-/DC converter and that the voltage controller and / or current controller of the control system is having a logic model in order to obtain an input voltage or an input current for the converter to compensate for deviations of the output DC- voltage or line current of the AC-/DC converter from an reference output DC-voltage or reference line current due to disturbances entering the AC-/DC converter in an operative state of the AC-/DC converter.

A further aspect according to the invention is that the control system for an AC-/DC converter is model-based. That means that each of the controllers, the voltage controller and the current controller are having a logic model which preferably uses predictive control strategies. Further, by integrating the characteristics of the AC-/DC converter into a control model, the further advantage is achieved that the dynamic response, reliability and robustness of the converter system can be significantly improved. This is of particular interest, as a traction line converter is not an isolated system, but is embedded in a dynamic agile electrical environment. Thus, the controller of the AC-/DC converter has to perform in different dynamical conditions influenced by signal disturbances caused by a grid connected to the AC-/DC converter. Thus, robustness with respect to grid impedance and harmonics coming from a load for example a motor drive have to be preserved. The inventive control system having a control system for controlling an AC-/DC converter and the method for controlling an AC-/DC converter having a control system achieves the following advantages and positive effects: The voltage controller of the present invention having a logic model that enables to control the output DC-voltage of the AC-/DC converter to a referenced line voltage taking into account grid or load disturbances. In particular, the voltage controller stabilizes a DC-link voltage on the line side. The current controller controls sinusoidal line currents to a line current while advantageously filtering disturbances in form of harmonic noise entering the AC-/DC converter system from grid or load-side. The current controller also having a logic model.

The observer is a classical static gain observer estimating a DC-link offset due to

disturbances in the converter system. Preferably, the observer uses a model of the AC-/DC converter and transformer impedance.

The approach to implement a logic model into the controller allows to account for dynamic changes of grid and load influencing the AC-/DC converter with reduced delay time. This leads to a better prediction of the converter behavior and to a better stabilization of the converter behavior during the operation of the converter.

A further advantage is that a the inventive controller minimizes deviations in form of offsets of the output DC-voltage and line current from their reference output DC-voltage and line current by optimizing a defined cost function meaning to minimize a deviation of the output DC-voltage and / or line current of the AC-/DC converter by defining the calculating the appropriate input voltage and / or input current.

Due to the model-based approach engineering efforts for retuning the AC-/DC converter parameters in case of environmental changes can be reduced, as a recalibration can quickly be done by just changing the parameters in the model.

Furthermore, the inventive controller overcomes delays due to specific system hardware restrictions and delays in signal communication. Hence, the controller is able to provide a fast response to variations in external disturbances due to an improved speed of the current controller. It should be advantageously noted that the entire invention can also preferably be implemented as a software-based solution, reducing maintenance time and hardware costs for a converter system. It should be further noted that the term "converter" in the context of this document mainly focuses on an AC-/DC converter.

According to a preferred embodiment the voltage and / or the current controller are model- based meaning they are having a logic model. In a preferred first control arrangement - modular control arrangement- Model Predictive Control (MPC) is used for such logic model. In general, MPC uses a model for prediction of an system behavior. In context of this invention, converter behavior is predicted when influenced by signal disturbances caused by grid and injected to the converter system. In a further context of this invention, AC-/DC converter behavior is predicted when influenced by signal disturbances caused by the load and injected to the converter system. MPC in general solves an optimization problem, meaning the minimization of deviations of an output signal by calculating an appropriate input signal for a technical system such as an converter. Predictive control methods such as MPC are known in the state of the art. One technical field for which MPC has proven to be effectively applicable is for the control of motor drive variables such as torque and flux in motor drive applications while reducing inverter switching losses.

However, in developing controllers for traction technology, there are a number of challenges that needs to addressed, which does not allow the direct application of the developed MPC methods. In particular, the reliability of operation of a traction converter is challenged due to signal disturbances coming from the track (slide/slip or polluted rails) side and from the grid side when the grid impedance varies dynamically. Further, the control of traction system is often dependent due to various hardware restrictions such as weights and volume of the electrical components of a converter system. Therefore, in traction applications, in order to produce a desired voltage on the DC link, a modulation unit is used instead of direct control of the switches due to harmonic distortion requirements. Because of the different setting the direct application of MPC methods to traction converters used particularly in single-phase systems is not possible. By integrating the characteristics of the converter into a control model with a logic model using MPC, the dynamic response and reliability of the converter system can be improved. In the modular control arrangement, the controllers are electrically coupled to each other by a cascade-structured circuit. Using a cascade-structure of the inventive controller system results in two linear systems and has the advantage to facilitate controller implementation in case of restrictions due to for example limited CPU computation and storage. Thus, less computation and storage is required, leading to a decreasing hardware costs of the converter system.

Using a MPC-based approach within the voltage and / or current controller of the inventive control system, in particular for controlling single-phase traction converters, has a plurality of advantages. Better prediction of the dynamic behavior and stabilization of an DC-link voltage of the converter is possible which is usually influenced by signal disturbances from the grid or load entering the converter. It should be noted, that the implementation of the logic model using predictive control into the controller is preferably done by a software-based solution, thus reducing engineering and maintenance costs.

A further advantage is that the line converter current is controlled in the way that the line current contains less than about 2% of harmonic content that may be injected for example from the load side, which makes it possible to achieve smooth currents for the converter system. Further, the MPC-based controller shows robustness with respect to unmodeled and undetected signal disturbances coming from current load and grid voltage. Another advantage of using an a logic model using the MPC-approach in the inventive control system is an accurate DC-link voltage and current tracking in case of an inaccurate load current measurement.

Further, the inventive control system is optimized to be operated in a low switching frequency range and thus, implementations for example of sampling times of about 325 microseconds compared to 25 microseconds to prior art system are possible. Further, the performance of the inventive control system is robust with respect to different operating conditions of the converter. Another advantage provided by the MPC-based controller is a dynamically fast response to changes in the current load and grid voltage.

A further advantage of the MPC-based solution is to stabilize the energy flow across the converter in order to enable a large reduction of the energy storage element, the DC-link capacitance. Apart from the result of reduced hardware costs that can be effected by a smaller capacity of about 7 to ^" l OmF, it also increases system safety and decreasing tuning efforts of the capacity when the converter is operated in different operation modes. According to a preferred embodiment the inventive control system uses a multi-criteria optimization approach within the voltage controller to minimize deviations of an output signal of the converter from its reference input signal by a mathematical cost function. Multi-criteria optimization is preferably implemented by an energy balance equation that links the DC-part of the converter to the AC-part of the converter. The multi-criteria optimization adopted for the inventive control system of an converter allows to include the second harmonic link included to the logic model using MPC and a suitable sampling strategy for the voltage controller to avoid introduction of harmonics in the grid current. Moreover, a prediction model tailored to the second harmonic distortion allows an voltage and / or current controller to account for such signal distortions and prevents the controller to pass them to the grid currents. Further, the energy-based approach enables to define a set of requirements when designing a controller usable for various operation modes.

According to a preferred embodiment the inventive control system uses a control unit having a logic model preferably based on predictive control. The control unit combines an integrated control operation to control an output voltage and an output current of the converter. The further arrangement of the inventive controller - the integrated control arrangement - also uses an MPC-based approach. The integrated control allows the controlling of output voltage and output current of the converter without intermediate d-q frame mapping. This guarantees fast dynamic responses of the inventive control system in case of signal disturbances entering the converter. Further, the integrated control structure enables a better converter performance, a high dynamic response of the controller system due to decrease of system- triggered. The integrated controller arrangement, and in a preferred embodiment of the invention, the voltage controller of the integrated controller is connected to an observer. The observer may be operated as a classical static gain observer and uses a model of the converter and, if preferred, a model of the transformer impedance. The observer is also used to compensate for possible signal offsets in the voltage controller and to estimate quantities that cannot be measured such as the voltage and the current of the second harmonic filter. Thus, the observer has primarily a support function for the voltage controller. Further, the observer estimates and accounts for various signal parameters measured that may influence the operation of the converter system, in particular the DC-link voltage of the converter. Such signals parameters which are measured and used as controller inputs may be for example a line current, grid voltage and an offset of the DC-link voltage which may be caused by signal disturbances from the grid or load entering the converter system. In one scenario of the converter for example, in a steady operating state of the converter a current load is constant and the grid voltage has a known frequency, amplitude and phase. In case of a disturbance from outside, influencing the output voltage and output current by a sudden jump of the output voltage and output current, the inventive controller is then able to detect such signals disturbances and delivers an appropriate response.

According to a preferred embodiment the voltage controller embodies a DC-link voltage tracking. This functionality allows the inventive controller to account for signal distortions for example 2 ^nd harmonic distortions, so that the signal distortions are not passed to the grid currents.

According to a preferred embodiment the inventive controller is synchronized with measurements of signals to detect and register grid and load changes affecting the behavior of the converter. In particular, measurements such as line currents, grid voltages and DC-link voltage may be used to continuously update the control action of the inventive controller. In particular, the voltage controller and current controller are supplied with continuously updated information about grid and load changes or signal changes such as output voltage and output current of the converter resulting from grid and load changes. For example, if a desired value for a DC-link output voltage of the converter is required due to a certain operational mode that needs to applied to the converter, then the essential input signal parameters for the converter are computed using a steady state behavior of the converter system. This allows to deliver a fast response of the converter in case of grid and / or load changes. Thus, the inventive controller system leads to a recognizable speed increase of the current and voltage controller compared to hardware-based solution.

According to a preferred embodiment the data required by the inventive controller can be computed offline by using predefined data stored in a look-up table. The look-up table preferably enables to save computational load on a CPU-board. Therefore, the controller may also be operated in environments, where restricted computational availability affects the calculation capabilities of the controller.

According to a preferred embodiment the controllers and observer of the inventive control system are implemented on a CPU board. A hardware-based solution allows to implement a fast current controller, so errors are reduced which do occur due to delays of system and communication components of the converter. Further, a fast converter response due to variations in the reference signals and grid/load disturbances is provided. According to a preferred embodiment the MPC-based control system is implemented in a cascade control structure having a first loop for voltage control and a second loop for current control of the converter. The cascade structure is easy to implement and a preferred solution, when CPU computation and storage space is limited.

According to a preferred embodiment the inventive control system uses a pulse-wide- modulation (PWM) module that is implemented between the controllers and the converter. This guarantees constant switching frequencies and also minimizes harmonic injections into the grid. Further, the semiconductor switches are controlled by an PWM module. In this context it should be further noted, that for measurements used by the current controller, these are obtained only at a pick of the carrier of the Pulse-wide-modulation module (PWM), while receiving continuously updated measurements of the grid voltage. In between 2 picks, it is possible to compute the control action of the converter and then apply it to the next carrier pick. This allows a choice between the measurements when to compute the control action in between 2 carrier picks. Thus, it is achieved to compute the control action for the converter in a very short time period to be able to react quickly to changes in the grid voltage.

It should be noted that the inventive control system for a converter may preferably be used in vehicles used in traction applications. Vehicles as such may be for instance railway vehicles of all kind.

Apart from the inventive control system for an AC-/DC converter, a method for controlling a converter having a control system is claimed as described as follows. All advantages and effects described for the control system for controlling an AC-/DC converter may also applied to each of the following methods for controlling an AC-/DC converter having a control system described as following.

According to a preferred method for controlling an AC-/DC converter having a control system the method comprising the following steps: o controlling an output DC-voltage of the AC-/DC converter to a reference DC- output voltage of the AC-/DC converter by a voltage controller;

o controlling an line current to a reference line current of the AC-/DC converter by a current controller for estimating an offset of an output DC-voltage of the

AC-/DC converter from a reference output DC-voltage due to disturbances caused by a grid connected to the AC-/DC converter entering the AC-/DC converter by an observer connected to the voltage controller; and o generating an input voltage or an input current for the AC-/DC converter to compensate for deviations of the output DC-voltage or the line current of the AC-/DC converter from a reference output DC-voltage or reference line current due to disturbances caused by a grid connected to the AC-/DC converter entering the AC-/DC converter by that the voltage controller and / or current controller of the control system containing a model logic that is Model Predictive Control to predict the dynamic behavior of the AC-/DC converter influenced by the disturbances caused by the grid entering the AC-/DC converter in an operative state of the AC-/DC converter.

In a further preferred method for controlling an AC-/DC converter having a control system, the logic model is Model Predictive Control to predict the dynamic behavior of the converter influenced by the disturbances entering the AC-/DC converter.

According to another preferred method for controlling an AC-/DC converter having a control system, the control system uses a multi-criteria optimization to minimize deviations from the output DC-voltage or line current of the AC-/DC converter from the reference output DC- voltage or reference line current due to disturbances entering the AC-/DC converter.

In another preferred method controlling an AC-/DC converter having a control system, the control system is implemented in a cascade control structure having a first loop for controlling the output DC-voltage and a second loop for controlling the line current of the AC-/DC converter.

In another preferred method for controlling an AC-/DC converter having a control system, the control system comprising a control unit having the logic model and combining an integrated control operation to control the output DC-voltage and line current of the AC-/DC converter.

According to another preferred method for controlling an AC-/DC converter having a control system, the voltage controller embodies a DC-link voltage tracking.

In another preferred method for controlling an AC-/DC converter having a control system, the voltage controller and current controller are supplied with updated information about grid and load changes influencing the behavior of the AC-/DC converter. In another preferred method for controlling an AC-/DC converter having a control system, the control system uses predefined data stored in a look-up table in order to be operated offline.

In another preferred method for controlling an AC-/DC converter having a control system, a pulse-wide-modulation module is implemented between the controllers and the AC-/DC converter.

In another preferred method for controlling an AC-/DC converter having a control system, the method for controlling an AC-/DC converter having a control system is preferably used in single-phase traction applications for converters.

The further inventive objects, alternatives and features of the present invention will become apparent from the following detailed description of preferred embodiments of the invention in conjunction with the drawing.

In the drawing:

Fig. 1 shows an example of a prior art traction converter arrangement; Fig. 2 shows a prior art standard Pl-control structure for a converter,

Fig. 3 shows a modular arrangement of the inventive control system of a converter, Fig. 4 shows an integrated arrangement of the inventive control system of a converter.

The reference symbols used in the drawing and their meanings are listed in summary form in the reference list. In principle, the same parts are provided with the same reference symbols in the figures. Any described embodiment represents an example of the subject-matter of the invention and does not have any restrictive effect.

Fig. 1 . shows an an example of a traction converter arrangement commonly used in prior art. An AC-/DC converter 1 containing a control system 2 is connected via a transformer 3 with an LC-Filter 4 to a power grid 6 with an LC-Filter 5. The AC-/DC converter 1 is producing output signals for an energy storage element 8 which is usually a capacitance C. A DC-link voltage U _DC at the energy storage element 8 is feeding a load 9 which could be for example a drive unit in form of a motor drive or the like.

Fig. 2 shows an example of a standard Pl-control structure for a line converter known in prior art. The modular control system consists of two coupled control loops in cascade structure - a first outer control loop 16 and a second inner control loop 17. The first slow outer control loop 16 regulates a DC-link voltage U _D c to a reference value and the second fast inner control loop 17 regulates the line current to a reference value. The control loop 16 generates the d current reference for the converter 1 . In detail, the figure 2 shows a DC-link voltage reference signal U _D c-ref 20 that is the input signal for the control system 2. The first control loop 16 having a PI or PID voltage controller 10 and a reactive power controller 13. The controller 10 regulates an active power which defines the i _d reference 21 . The reactive power controller 13 regulates an reactive power and defines the i _q reference 21 . The second control loop 17 having a PI or PID controller 1 1 , a Pulse-wide-Modulator (PWM) modulator 12 and a converter 1 for regulating a output signal of the converter which is usually the DC-link voltage U _DC 26. The modulator 12 guarantees constant switching frequency and minimizes harmonic injections into the grid by controlling the semiconductor switches. The output signals of the converter are the currents I convener 25. Fig. 3 shows a modular arrangement of the inventive control system 2 of a line converter 1 . The structure is in principle identical to the control structure 2 shown in figure 2. However, the control structure 2 distinguishes from Fig. 2 in the way that the voltage controller 18 and the current controller 19 are model-based. This means in the context of the inventive control system that both controllers 18, 19 contain a model logic that operates model-based. The model could preferably be a predictive control model (MPC). In general, the predictive character of by implementing such methods to the control system allows to anticipate the converter behaviour caused by signal disturbances which are caused by a grid 6 that is connected to the converter 1 and to compensate for delays in the traction converter system. It should be noted that signal disturbances may also be caused by the load 9.

In specific terms, the MPC-based approach for both controllers 18, 19 allows minimizing deviations of an output DC-voltage and an line current from their reference output DC- voltage and reference line current by multi-criteria optimization with the help solving a cost- function. Further, it should be noted, predictive control used in the inventive control system 2 helps to obtain adequate input parameters for the converter 1 to compensate offsets of the DC-link voltage U _D c 26 and delays in the controller application. The voltage controller 18 uses predictive control model which includes a second harmonic link and accounts for harmonic disturbances injected by the grid or load to the converter system 1 . Using the MPC-based approach in a cascade loop structure allows an easier implementation on hardware, when dealing with limited CPU computation and storage. Using the model-based approach for the controllers 18,19 in the control system 2, advantageously implemented in software, decreases engineering costs for retuning the controllers 18, 19 as the inventive control system 2 allows simply recalibrating instantaneously by changing the parameters or settings of the model. A further difference to the prior art control structure of Fig.2 is an observer 14 that is for example connected to the voltage controller 18. The electrical connection of the observer 14 to either the voltage controller 18 or the current controller 19 may be implemented by a cable connection or a wireless connection. The observer 14 uses a model of the converter and transformer impedance. The observer 14 estimates a filter current and voltages and an offset of the DC-link voltage U _D c 26 due to signal disturbances in the converter system. Usually, the observer 14 uses a static gain model to detect the dynamic behavior of the converter.

The voltage controller 18 stabilizes the DC-link voltage U _DC 26 on the line side. The current controller 19 guarantees that the converter currents lconverter 25 do not contain more than about 2% of harmonic distortions. Both the controllers 18, 19 and the observer 14 use measurements of the line current, grid voltage and DC-link voltage U _DC 26 to regularly update the control action of the control system 2.

Fig. 4 shows an integrated arrangement of the inventive control system of a converter. The main difference to the modular arrangement shown in Figures 2 and 3 is this embodiment includes a control unit 15 having a logic model and combining an integrated control operation to control the output DC-voltage and line current of the AC-/DC converter 1 in a non-PI-based controlling-structure. A model-based control unit 15 means in the context of the inventive control system 2 that it contains a model logic which could preferably be predictive model control (MPC), and thus, having a prediction strategy as well as optimization criteria to minimize signal deviations, for example an output DC-voltage or line current of the AC-/DC converter, caused by the grid and / or load side by minimizing a cost function. The cost function may minimize a deviation or offset of an output DC-voltage or an line current from a reference output DC-voltage or line current. This enables to calculate the appropriate input voltage or input current for the converter to compensate the offset. Thus, the functionality of the embodiment of figure 4 is identical to the embodiment shown in figure 3. The control unit 15 is electrically connected to an observer 14. The observer is used to compensate for possible signal offsets in the voltage controller and to estimate quantities that cannot be measured such as the voltage and the current of the second harmonic filter. As an another example, the observer is also able to estimate a filter current and voltages and an offset of the DC-link voltage U _DC 26 due to signal disturbances in the converter system.

For both arrangements of the inventive control system, the controllers 18, 19 containing the logic unit using predictive control methods in order to also enable to stabilize the DC-link voltage U _D c 26 of the converter 1 to a given reference value. Changes in grid voltage and current load allow a much faster dynamic response that control system of converters in prior art using standard Pl-techniques. A main advantage due to the stabilization of the energy flow across the AC-/DC converter is also that it is possible to reduce the energy storage element 7 - the DC-link capacitance C _D c to about 70% of their original value usually used in control systems known in prior art. It should be further noted that the inventive controller solution with the MPC-based approach can be implemented as a complete software-based solution. Therefore, no modification of an FPGA is required, as the implementation of the controller structure is usually done by systems known in prior art. It further allows a quick adaption of the converter system with respect to parameter changes when used in different operating modes of the AC-/DC converter.

It should be further noted that both arrangements of the inventive control system 2 - modular and integrated approach, may use offline solutions to calculate the parameter inputs for the converter 1 . One offline solution is to use look-up tables. The look-up tables contain various pre-calculated settings of controller output parameters which are relative to specific sets of input parameters of voltages and current for the controller 2. This allows to operate the converter 1 in traction applications with limited space and computation availabilities.

Further it should be noted that both controller 18, 19 of the inventive control system 2 are preferably operated at a specific set of frequency range in order to avoid interferences with grid or load resonances. Reference list of symbols

1 Converter

2 Control system

3 Transformer

4 LC-Filter

5 LC-Filter

6 Grid

7 DC-Link capacity C _D c

8 LC-Filter

9 Load

10 Voltage controller

1 1 Current controller

12 PWM Modulator

13 Reactive power controller

14 Observer

15 Integrated control unit with model-logic

16 First control loop

7 Second control loop

18 Voltage controller with model-logic

19 Current controller with model-logic

20 DC-Link reference signal U _{DC r} ef

21 Current reference signal l _ref

23 Transformer voltage U _s

24 Load current l _L

25 Converter current Inverter

26 DC-link voltage U _D c

27 Reactive power reference signal l _L - _re f