CH714079A2 - Method and converter for potential-free electrical energy transmission. - Google Patents

Abstract

With a method according to the invention and / or a converter for potential-free electrical energy transmission between a primary-side AC voltage system (ua, ub, uc) with a mains frequency and a secondary-side DC voltage system (u dc) parasitic switching-frequency voltage components generated by the bridge branches of a primary-side bridge circuit (4), ie a switching frequency push-pull voltage system and / or a switching frequency common-mode voltage system, for the potential-free energy transfer from the primary side (2, 3, 4, 5) to the secondary side (6, 7, 8) used. In this case, the power transfer between primary and secondary side (6, 7, 8) similar to a dual active bridge can be adjusted by appropriate phase shift of the switching frequency clocking of both sides.

Description

Description For the rapid charging of batteries of electric vehicles, there are currently typically two-stage converter systems, i. E. a network-side three-phase pulse rectifier system with a regulated output voltage and a downstream, from the voltage generated by the three-phase pulse rectifier system - also referred to as intermediate circuit voltage - powered floating DC / DC converter used. The pulse rectifier stage has i.A. Boost converter characteristic, i. the mains phase voltages are fed to three ballast inductors, and their respective second terminals are connected to the inputs of phase bridge arms, which are arranged between a positive and a negative DC bus voltage rail and together form a self-commutated three-phase bridge circuit. A phase bridge branch is thereby in the simplest case with Zweilevelcharakteristik, i. formed by a series circuit of two electronic switches, such as transistors with antiparallel freewheeling diodes, wherein the circuit point between the two switches, respectively the transistors represents the bridge branch input. In order to avoid a short circuit of the intermediate circuit voltage, only one of the two transistors is switched through in each case, so that in the end the same function as with a changeover switch, the possibility of connecting the input to the positive or the negative intermediate circuit voltage rail is given, i. With reference to an imaginary center point of the intermediate circuit voltage, a positive or a negative voltage can be connected to the bridge branch input (two-level characteristic). The individual switches and thus the bridge branches acting as switches are controlled with pulse width modulated square wave signals with a switching frequency.

In order to achieve the required to ensure low system perturbations typically required sinusoidal power, a power-frequency pulse width modulated voltage is generated at the input of each phase bridge branch such that the occurring over the associated ballast inductance difference of the associated sinusoidal network phase voltage and the power frequency contained in the pulse width modulated voltage fundamental Sinusoidal mains current with an amplitude causes such that the DC / DC converter removed from the DC link power replenished and the level of the DC link voltage is maintained at a predetermined setpoint. In order to avoid a load on the grid with reactive power, the grid phase currents for power supply from the grid are set in phase with the associated grid phase voltages (ohmic grid behavior) and for power feedback in the grid with a phase shift of 180 °.

In addition to the ultimately current-forming line frequency fundamental oscillations (hereinafter referred to as mains frequency push-pull voltage system), the voltage system formed from the three pulse width modulated phase bridge branch input voltages typically also includes • optionally a triple mains frequency common mode voltage system for maximizing the driveability of the self-commutated three-phase bridge circuit, • a switching frequency local (to one switching period referenced) and thus also global (based on a grid period) mean-free differential mode voltage system and • a switching frequency local and global mean-free common-mode voltage system.

Each of these voltage systems is formed by three phase voltage components. Since the grid neutral is typically not connected to any point of the voltage link, i. the three-phase pulse rectifier system as described above is only at the three power phase terminals, is caused by the three power frequency and by the switching frequency common-mode voltage system no current flow. Only the switching frequency push-pull voltage system can (in addition to the mains frequency push-pull voltage system) act as a current-forming device, with the network representing a purely sinusoidal mains voltage and negligible internal impedance for switching frequency push-pull current components. Accordingly, the switched-frequency push-pull voltage system causes a switching-frequency push-pull current which is limited solely by the ballast inductances and finds expression in a switching-frequency fluctuation or a switching-frequency ripple of the sinusoidal mains currents.

For the potential-separated DC / DC converter downstream of the pulse rectifier system, full-bridge circuits in the form of a dual active bridge converter, ie primary and secondary circuits according to the prior art, are provided in the area of higher power. a primary side fed from the intermediate circuit voltage full bridge and a secondary side, located at the DC output voltage full bridge and arranged between two full bridges transformer whose primary winding ends are placed at the outputs of the primary-side full bridge and the secondary winding ends to the input of the secondary full bridge used. Here as well as elsewhere, the terms "input" and "output" refer to a power flow from the primary side to the secondary side. But it is also a power flow in the opposite direction feasible.

Since the dual active bridge converter has no output inductance and no input inductance, the reverse voltage load of the power transistors used in the bridge arms directly through the DC link or. the output voltage, i. determined directly by the circuit (circuit constraint). Furthermore, the control of the power transfer from the primary to the secondary side is simply possible via the phase shift of the clocking of the primary and secondary full bridge. Both bridges generate in the simplest case, a symmetrical switching frequency rectangular AC voltage. Due to the phase shift of the two square-wave AC voltages, relatively high voltage differences to the secondary voltage occur after the rising and falling edge of the primary AC voltage (the primary voltage can be converted to the secondary side taking into account the turns ratio and a transformer equivalent circuit diagram with leakage inductance concentrated on the secondary side), which come to lie on the stray inductance of the transformer, and cause a current build-up or current reduction, which ultimately results in the primary and secondary winding, a trapezoidal AC resulting in a corresponding power transfer from the primary to the secondary side. A reduction / increase of the phase shift leads to a longer / shorter increase and decrease of the current and thus to a higher / lower amplitude of the primary or secondary side trapezoidal current, whereby the controllability of the power transfer on the phase shift is clearly understandable. It should also be emphasized that the circuit is implicitly bidirectional, i. By phase advance of the Sekundärrechteckwechselspannung against the Primärrechteckwechselspannung also a power feedback from the secondary side into the DC link and from there through the bidirectional three-phase rectifier rectifier system in the three-phase network can be done.

However, the technical advantages of the overall arrangement formed by the pulse rectifier system and the DC / DC converter is offset by a relatively high implementation effort (high number of power transistors and magnetic components). Furthermore, the achievable energy conversion efficiency is potentially limited by the two-tier nature of the overall arrangement.

A one-stage energy conversion can be achieved with the use of matrix converter circuits, in which case, however, due to the necessary for the realization four-quadrant switch (bipolar blocking capability and separate controllability of both current flow directions) a complex converter structure, and complex control (complex, of the size ratios of the mains phase voltages or the flow directions of the input phase currents dependent Kommutierungssequenzen the four-quadrant switch) are to be taken into account. Furthermore, matrix converters, due to the counter-series connection of two unipolar power transistors with antiparallel freewheeling diode required for the realization of four-quadrant switches, still have relatively high conduction losses, regardless of the single stage.

The object of the invention is therefore to provide a method and a converter for potential-free electrical energy transmission, which has at least one of the following properties: simple circuit structure or a relatively low semiconductor number, a defined by circuit constraint reverse voltage load of the semiconductor, a small number of magnetic components and a small number of semiconductors in the current flow path, as well as easy controllability of the power flow.

The object is achieved by a method and a converter for potential-free electrical energy transmission according to the claims.

In the method for potential-free electrical energy transmission between a primary-side AC voltage system (ua, Ub, uc) with a network frequency and a secondary-side DC system (udc), the following steps are performed: • by driving a primary-side bridge circuit at the inputs of phase bridge branches this primary side Bridge circuit is a system of voltage of phase bridge branch input voltages formed, • this voltage system comprises at least the following components: o a mains frequency push-pull voltage system, o a switching frequency push-pull voltage system, o a switching frequency common-mode voltage system.

In this case, switching-frequency differential current components are formed on the primary side with the switching frequency push-pull voltage system and / or switching frequency common-mode current components flowing through the primary-side bridge circuit 4, wherein these switching frequency push-pull current components and / or common mode components are transformed by at least one transformer to a secondary side , and used for transmitting electrical power to the secondary side.

The converter, which may be designed to carry out the method for floating electrical energy transmission, comprises: • power phase terminals for connection to an AC system, which are each connected via Vorschaltinduktivitäten to inputs of phase bridge branches of a primary-side bridge circuit, • wherein a positive and a negative voltage rail of the primary-side bridge circuit are connected to a primary-side DC voltage buffer arrangement.

In this case, a circuit with high-pass characteristic for receiving common mode currents and / or push-pull currents from the inputs of the phase bridge arms is arranged, said circuit having a high-pass property at least one primary winding of a transformer, and at least one secondary winding of the transformer forms a series circuit with a blocking capacitor 6, and wherein • either only one such series connection is present, and this series circuit is connected at a first end to a first phase input of a secondary-side bridge circuit and at a second end to a second phase input of the secondary-side bridge circuit 8b, or two or more such series circuits are present, and these series circuits are each connected at first ends to phase inputs of a secondary-side bridge circuit and are connected at second ends to a common secondary-side star point.

The secondary-side star point can also be called star point of the blocking capacitors, wherein the order of the blocking capacitors and secondary windings in the series circuits is interchangeable.

The circuit with high-pass characteristic is designed to perform switching-frequency currents during operation of the converter, and to attenuate line-frequency currents, ideally to block. The circuit with high-pass property leads through the primary-side bridge circuit and the primary-side DC buffer arrangement and said at least one primary winding. The circuit with high-pass characteristic can have further elements, which are different depending on the embodiment. Depending on the embodiment, it may be designed to carry only switching-frequency common-mode currents, only switching-frequency differential mode currents or both types of currents.

With the method and / or the converter, it is possible, during operation of a single- or multi-phase AC voltage system, in particular a three-phase rectifier system, generated by the bridge branches parasitic switching frequency voltage components, i. the switching-frequency push-pull voltage system and / or the switching-frequency common-mode voltage system, for the potential-free energy transfer from the primary to the secondary side.

In this case, the magnetic circuit of the ballast inductance can also be used for the magnetic coupling of the input or primary side and the output or secondary side circuit part. Furthermore, the power transfer between primary and secondary side similar to a dual active bridge can be adjusted by appropriate phase shift of the switching frequency clocking of both sides.

It is thus exchanged during operation, electrical power between the primary-side AC voltage system and a primary-side DC buffer arrangement medium line frequency voltages and currents, and there will be electrical power between the primary-side DC buffer arrangement and the secondary side via the at least one transformer by means of switching frequency voltages and Exchanged currents.

In embodiments, the push-pull current components and / or common mode current components transformed to the secondary side are passed through an output buffer arrangement by means of a secondary-side bridge circuit.

In embodiments, the high-pass circuit has at least one star circuit of feedback capacitors respectively connected at first terminals to the mains phase terminals and connected to second terminals at a common primary-side neutral point, and the high-pass circuit further comprises the ballast inductances Each of the ballast inductances forms a primary winding. Together with a respective associated secondary winding, this each forms a phase transformer. The primary-side star point can also be called star point of the feedback capacitors.

In embodiments, the high pass circuit comprises a star connection of series circuits of separate primary windings and feedback capacitors, these series circuits each being connected at first ends to the inputs of the phase bridge branches and connected at second ends to a common primary side neutral point.

In embodiments, the common primary-side neutral point is connected to a center of the primary-side DC buffer arrangement, or to the positive, or to the negative voltage rail of the primary-side bridge circuit.

In embodiments, the common primary-side neutral point is connected via a primary winding of a common-mode voltage transformer to a center of the primary-side DC buffering arrangement, or to the positive, or to the negative voltage rail of the primary-side bridge circuit.

In embodiments, the common primary-side neutral point is connected via a series capacitor to a center of the primary-side DC buffer arrangement, or to the positive, or to the negative voltage rail of the primary-side bridge circuit.

In embodiments, the converter has a three-phase common-mode inductance lying in series with the ballast inductances, in particular with a fourth winding on a magnet core of this three-phase common-mode inductance, this fourth winding forming the secondary winding.

In embodiments, the converter comprises two or more of the series circuits of secondary windings each having a blocking capacitor, these series circuits being connected at second ends to a common secondary star point, and this star point, optionally via a series capacitor, to a midpoint of a secondary side Output buffer arrangement is connected, or to the positive, or to the negative voltage rail of the secondary-side bridge circuit.

In all embodiments, it is understood that a star circuit of feedback capacitors (primary side) respectively blocking capacitors (secondary side), which is connected at its neutral point with a positive or negative voltage rail of the associated bridge circuit, possibly via an inductance and / or capacitance, is equivalent to two star circuits, one of which is connected at its neutral point with the positive and the other at its neutral point with the negative voltage rail.

Other embodiments and their operation are described below in connection with a three-phase network, the statements apply analogously but also for networks with one or more than three phases.

In one embodiment of the inventive device may be arranged on the input side, a primary-side three-phase bridge circuit as a three-phase rectifier, which forms at the DC output, hereinafter referred to as a buffer voltage, capacitively supported DC voltage to which, however, no (secondary side) consumer is connected. As mentioned above, an explicit DC / DC converter or explicit transformer is not arranged for the realization of the potential separation of primary and secondary side, but the potential separation and magnetic coupling is by secondary windings on the, an air gap of the magnetic circuit having Vorschaltinduktivitäten the phases ( Phase secondary windings, hereinafter also called secondary windings) realized - the windings of Vorschaltinduktivitäten are in this sense as phase primary windings (hereinafter also called primary windings) - and the first end of each secondary winding led to the phase inputs of a secondary-side three-phase bridge circuit, at the output to forming DC output voltage occurs. The respective second ends of the secondary windings are connected to the input terminals of a star connection of secondary-side blocking capacitors whose neutral point is connected to the center of the capacitively-supported DC output voltage; Alternatively, the same function as a connection of the neutral point with the positive or negative terminal of the output voltage, or the arrangement of two condenser star circuits with connection of a neutral point with the positive and connection of the other neutral point with the negative terminal of the output voltage possible. Furthermore, return capacitors are arranged in star-type branching off from the mains phase terminals on the primary side, the star point of these capacitors being connected to the center of the buffer voltage; Alternatively, the connection of the star point to the positive or the negative pole of the buffer voltage, or the arrangement of two capacitor star circuits with connection of one star point to the positive and connection of the other star point to the negative pole of the buffer voltage is functionally identical possible.

For the description of the function of the device is assumed in the first step that the transistors of the secondary-side three-phase bridge circuit are not clocked and the output DC voltage has a high value such that the anti-parallel to the transistors of the secondary-side three-phase bridge diodes regardless of possibly over the phase secondary windings occurring voltages in the off state or the secondary windings remain de-energized.

The setting of sinusoidal network currents is then the same as for a conventional three-phase pulse rectifier by pulse width modulation of the primary-side three-phase bridge circuit such that a mains frequency push-pull voltage system is formed, which leads in connection with the mains voltage to power frequency sinusoidal network phase currents and thus to a corresponding power transfer to the buffer voltage. In addition to the sinusoidal profile, the current in the ballast inductors has a switching-frequency ripple, which is caused on the one hand by the switching-frequency push-pull voltage system and closes across the (ideal, that is to say impedance-free) network. On the other hand, the switching-frequency common-mode voltage system also has a current-forming effect, since the feedback capacitors produce a connection which is low-impedance for switching-frequency changes to the DC side of the primary-side three-phase bridge circuit. The ballast inductors therefore have a further switching-frequency fin component, which, however, closes via the return capacitors and not via the network. In the following, the entire switching-frequency inductance occurring over a ballast inductance is referred to briefly as a switching-frequency primary winding voltage. If the ballast inductors provided with a secondary winding are seen as phase transformers, the superimposition of the three current components described above due to the secondary windings assumed to be without current forms the magnetization phase currents of the phase transformers.

Are now applied to the bridge arms of the secondary side bridge circuit with a defined time delay or phase shift the same control commands as the corresponding bridge arms of the primary-side three-phase bridge circuit, in addition to the phase magnetization switching frequency, • the Windungszahlverhältnis the Phasenprimär- and phase secondary windings, • the ratio of output voltage and buffer voltage, and • the scattering coefficients of the phase transformers form certain currents in the phase secondary windings (secondary winding phase currents), which also result in corresponding flux-compensating current components in the phase primary windings (power transfer primary winding phase currents) which add to the phase magnetization currents and ultimately one Effect power transfer from the primary side to the secondary side.

In embodiments, the primary-side bridge circuit and the secondary-side bridge circuit (which may be single-phase or multi-phase) are driven with switching signals of the same switching frequency.

A control of a power transfer from the primary to the secondary side can be realized by varying a phase shift between the switching signals of the primary-side bridge circuit and the switching signals of the secondary-side bridge circuit.

The frequency-balanced push-pull phase voltages contained in the pulse width modulated secondary phase voltages generated by the bridge arms of the secondary-side three-phase bridge circuit are taken over by the blocking capacitors, which in the first approximation represent an interruption for mains-frequency operations and prevent the formation of mains-frequency balanced mode phase currents. Also, depending on the operating mode in the pulse width modulated secondary phase voltages contained triple mains frequency common mode voltage system (which can be provided as on the primary side for maximizing the linear Aussteuerbereiches the secondary-side three-phase bridge circuit) comes to the common mode voltages acting in parallel blocking capacitors, which ideally no triple mains frequency common mode current occurs against the midpoint of the output voltage. Therefore, only the sum of the secondary-side switching-frequency push-pull and switching-frequency common-mode voltage system - referred to below as a switching-frequency secondary winding voltage system (which is formed from three switching-frequency secondary winding phase voltages), which is a phase-shifted copy of the primary-side primary phase windings in terms of the local relative on-times, is obtained via the phase secondary windings switching frequency push-pull and the switching frequency common-mode voltage system, hereinafter referred to as switching frequency primary winding phase voltage system (formed from the three primary winding phase voltages) represents. In each phase, the switching frequency primary winding and secondary winding phase voltage is by subtracting the pulse width modulated phase voltage formed by the associated bridge branch and the sum of low frequency differential and low frequency common mode phase voltage component (the three phase combination of the sum of both voltage components is hereinafter referred to as low frequency primary or secondary side voltage system has three primary or secondary low-frequency phase voltages) and therefore has, for example omitting a triple mains frequency common mode phase voltage component (primary and secondary side) on a positive and a negative purely sinusoidal mains frequency envelope and locally shows the mean zero. The same applies essentially to the power transfer primary winding phase currents and the secondary winding phase currents, since their local duty cycle is approximately equal to each of the associated switching frequency primary winding phase voltage and associated secondary winding phase voltage, and the currents also do not have a low frequency component, i. have a local mean equal to zero. Overall, each power transfer primary winding phase current is therefore considered to be proportional to the associated switching-frequency primary winding phase voltage.

Since due to the connection of the neutral point of the Rückführkondensatorsternschaltung with the center of the buffer voltage and the connection of the star point of Abblockkondensatorsternschaltung with the midpoint of the output voltage, the phases primary and secondary side with respect to the training schaltfrequenter streams are decoupled (with an open star point, the current sum to zero forced and thus gives a linear dependence of the phase currents), each phase can be analyzed separately for the analysis of the power transfer via the phase transformers.

Now, if a section in the vicinity of the amplitude of the primary side formed mains frequency Gegentaktprimärphasenspannung considered and an amplitude almost equal to half the buffer voltage assumed (high modulation), the output of the associated bridge branch of the primary-side three-phase bridge circuit is relatively long with the positive terminal of the buffer voltage and remain connected to the negative terminal of the buffer voltage only briefly. Accordingly, the switching frequency primary winding phase voltage then has very small voltage values for a large part of a clock period and a power transfer primary winding phase current of small amplitude is formed and ultimately only a small power supply to the associated secondary phase circuit takes place.

Completely different conditions exist in the vicinity of the zero crossing of a primary-side low-frequency phase voltage, since then the voltage formed at the associated bridge branch input (symmetrical square-wave voltage with an amplitude equal to half the buffer voltage) has only a switching-frequency component which occurs as a primary winding phase voltage and a high ( maximum) switching frequency power transfer to the associated phase secondary winding causes.

Overall, so that the power transfer from the primary to the secondary side, especially in the vicinity of the zero crossings of a mains phase voltage or generally the low-frequency primary phase voltage, whereas the maximum power transfer from the network into the buffer voltage in the vicinity of the maxima of a mains phase voltage occurs. In this respect, there is advantageously a very good utilization of the semiconductors of the primary-side three-phase bridge circuit. In the interaction of all three phases, the fluctuations of the phase-related phase powers with a doubled mains frequency are balanced out and, as a result, a temporally quasi-constant power is supplied from the network to the buffer voltage. The three-phase compensation is also given for the power transfer of the three phases from the primary to the secondary side, which is covered by the buffer voltage, so advantageous for the buffer voltage capacitive supporting electrical memory (when operating the device on a balanced three-phase network) no significant Requirements for compensation of low-frequency power swings and thus a higher capacity value.

In principle, it is sufficient if the switching frequency at least five times, in particular at least ten times, in particular at least fifteen times higher than the mains frequency. In other words, fundamental oscillations of the switching-frequency currents, which contribute to the power transmission through the transformer (s), have a frequency at least five times, in particular at least ten times, in particular at least fifteen times higher than the fundamental frequency of the mains frequency of the single- or multi-phase AC voltage system. It is thus possible to separate the switching-frequency current components in individual line sections of the converter from the mains-frequency current components and to use them for the isolated power transmission.

In typical applications, the line frequency is 50 Hz, and the switching frequency is more than 500 Hz or more than 1 kHz.

Control As mentioned above, the power flow between the primary side and the secondary side can be adjusted by time shifting (phase shifting) of basically the same timing of the primary-side and secondary-side three-phase bridge circuits. In the simplest case, a constant phase shift is selected over an entire network period, whereby a reversal of the power flow direction is possible by setting a phase advance instead of a phase lag.

Alternatively, the phase shift of the individual phases may also be influenced separately and varied over time with the goal of optimization by, for example, maximizing the efficiency of the power transmission or maximizing the utilization or minimizing the stress on individual components over the network period. Another possible goal of optimization is to maximize the operating range with soft, i. voltage or currentless switching of the power semiconductor is.

It should be noted that for generating the drive commands of the bridge arms of the primary-side three-phase bridge circuit in addition to a conventional undershoot method with triangular carrier signal and in the simplest case purely sinusoidal modulation functions, a space vector modulation application may apply, this also the sections clamping of a bridge branch and exclusive timing of the two remaining Phases, as well as the deliberate generation of a high switching frequency push-pull voltage system (and a correspondingly reduced common-mode voltage system) or the generation of a high switching-frequency common-mode voltage system (and a correspondingly reduced push-pull voltage system).

In addition to the above-mentioned purely resistive power behavior of the Dreiphasenpulsgleichrichterteiles the device, an operation with a general phase position of the mains current is possible. The system can thus also be used to cover the reactive power of the network or possibly for harmonic filtering of the network, without the switching frequency power transfer from the primary to the secondary side would be significantly affected.

Furthermore, it should be noted that the buffer voltage can be used directly for the supply of a consumer, in which case only the difference of the power received from the network and the power consumed by the consumer is transmitted to the secondary side.

Finally, it should be noted that a single-phase version of the device is possible, in which case only one Vorschaltinduktivität to be arranged and provided with a secondary winding and the buffer voltage for the partial compensation of the double mains power draw from the network can be used by the power extracted from the buffer voltage and transferred to the secondary side in a switching manner is maintained at as constant a value as possible by corresponding variation of the phase shift of the timing of the primary and secondary full bridges (or simplified half bridges). Possible Variants of the Circuit Structure As already mentioned above, the star point of the primary-side feedback capacitors can be connected functionally equal to the positive or the negative pole of the buffer voltage instead of the center of the buffer voltage, or two feedback capacitor star circuits can be connected with a positive star and connecting the other neutral point to the negative pole of the buffer voltage.

The same is also possible on the secondary side, where the star point of the blocking capacitors can be connected functionally equal to the positive or negative terminal of the output voltage instead of the midpoint of the output voltage, or two Abblockkondensatorsternschaltungen with connection of a neutral point with the positive and connection of the other neutral point can be provided with the negative terminal of the output voltage.

Furthermore, in order to achieve a further degree of freedom for the sizing of the system with respect to the contribution of the switching frequency common mode voltage system and the switching frequency push-pull system to the power transfer from the primary to the secondary side, in the junction of the neutral point of the feedback capacitors with a point of the buffer voltage Series capacitor inserted; such a circuit expansion is also possible on the secondary side, in which case a series capacitor is to be placed in the connection of the star point of the blocking capacitor star circuit with the output voltage.

Moreover, it is possible to perform the connection of the capacitor star point with the DC side of the associated three-phase bridge circuit only on the primary side or only on the secondary side. If e.g. the star point connection provided only on the primary side, the buffer voltage has no common mode voltage to ground or a low interference emission via parasitic coupling capacitances to earth.

However, then the switching frequency common mode voltage system can not be used for the power transfer from the primary to the secondary side, and a limitation of the switching frequency common mode currents through the feedback capacitors can be made by a three-phase common mode inductance in series with the Vorschaltinduktivität.

In addition to the above description of the embodiment of the primary and / or secondary-side three-phase bridge circuits with two-point characteristic, embodiments with multipoint characteristic are also possible, whereby, however, the voltage component available for the switching-frequency potential separation is reduced. Nevertheless, this embodiment is advantageous for operation of the device at a widely varying mains voltage or wide-ranging output voltage, since then a low-voltage / output voltage two-point clocking and high line voltage / output voltage can be done a three-point clocking of the respective circuit part.

In embodiments, there is a functional separation of the ballast inductors and the phase transformers. Then, as for a conventional three-phase pulse rectifier system, there are to be arranged ballast inductors and further branching from the input terminals of the bridge branches of the primary-side three-phase bridge circuit to provide separate primary windings of phase transformers and to pass the second ends of these phase primary windings via primary side blocking capacitors to a neutral point which is connected to a point of the buffer voltage. Like the secondary-side blocking capacitors, the primary-side blocking capacitors then take on low-frequency voltage components and suppress low-frequency currents through the primary windings of the phase transformers, which could lead to premagnetization or saturation of the transformer cores; Thus, only switching-frequency voltage components come to lie over the primary windings of the phase transformers. Advantageously, then no air gap of the magnetic circuits of the phase transformers provided, whereby no air gap scattering field occurs and the high-frequency losses of the phase transformers are reduced.

With a relatively high buffer voltage or a relatively high output voltage, only the switching-frequency common-mode voltage system can be used for the power transfer from the primary to the secondary side. This is possible because the secondary windings of the phase transformers are connected in series and fed via a blocking capacitor to the input of a secondary-side single-phase bridge circuit, at the output of which the output voltage to be formed occurs. Accordingly, a reduction of the implementation effort is then given. The timing of the secondary-side single-phase bridge circuit takes place in the simplest case such that at its input a symmetrical AC voltage with one of the primary-side three-phase bridge circuit same switching frequency is formed, the power transfer from the primary to the secondary side adjusted by the phase shift of the secondary clock compared to the primary side clocking can be. Furthermore, in the sense of minimizing current components not contributing to the power transfer, modulation of the duty cycle of the secondary full bridge is possible such that for a locally positive value of the low-frequency common mode voltage the width of the positive pulse and for a locally negative value of the low-frequency common mode voltage the width of the negative Pulse of the input voltage of the secondary side full bridge is increased.

As an alternative to the series connection of the secondary windings, the switching frequency common mode voltage can also be tapped by means of a fourth winding on the magnetic core of a three-phase common mode inductor connected in series with the ballast inductors, the ends of these in the sense of power transfer to be seen as a secondary winding winding back to the input of a secondary side Single phase bridge circuit are performed, the control of which is carried out as described above.

Another alternative is the arrangement of an explicit common mode voltage transformer whose primary winding is placed between the neutral point of the feedback capacitors and the midpoint of the buffer voltage (or the positive or negative pole of the buffer voltage). The Vorschaltinduktivitäten then remain without secondary winding. The secondary winding of the common-mode voltage transformer is then passed through the same as for the above-described series connection of the secondary windings of the phase transformers via a blocking capacitor to the input of a single-phase bridge circuit.

Alternatively, only the switching frequency push-pull voltage system can be used for the power transfer to the secondary side. This can be advantageous given high modulation of the primary-side pulse rectifier part of the device. For this purpose, the neutral point of the primary-side feedback capacitors can be easily detached from the center or from the positive or negative pole of the buffer voltage and remain isolated. By the switching frequency common mode voltage system then no current or current load of the components can be done. A similar circuit modification is then advantageous to make secondary side, i. the star point of the blocking capacitors should be disconnected from the center point or from the positive or the negative terminal of the output voltage and left isolated. The control of the device is the same as for connecting the capacitor star points with buffer voltage or output voltage.

The invention will be exemplified by means of possible embodiments and outgoing of the following figures. The pictures show:

Fig. 1: Isolated two-stage pulse rectifier according to the prior art.

Fig. 2: Structure of the power part of an embodiment of a system according to the invention.

Fig. 3: Control circuit for power unit shown in Fig. 2 with superimposed output voltage control.

FIG. 4 shows an equivalent circuit diagram relating to the secondary side of a phase to the power section shown in FIG. 2.

5 shows characteristic time courses of the system according to FIG. 2 within a network period.

6 shows a local excerpt of the time profiles of transformer voltages and winding currents for one phase from the circuit according to FIG. 2.

Fig. 7: variant with a transformer which uses the switching frequency common-mode voltage system for power transmission.

Fig. 8: variant with series-connected secondary windings of the transformers, whereby only the schaltfre-frequency common-mode voltage system is used for power transmission.

Fig. 9: variant of the invention with a three-phase common mode choke with fourth winding which serves as a secondary winding whereby the switching frequency common mode voltage system is used for power transmission.

Fig. 10: variant of the invention with separate Vorschaltinduktivitäten and 3hasenprimärwicklungen the primary-side pulse rectifier system.

Basically, the same parts are provided with the same reference numerals in the figures.

Fig. 1 shows an insulated two-stage pulse rectifier according to the prior art. Switches S1 to S6 together with La, Lb and Lc form a pulse rectifier system which generates an intermediate circuit voltage uZk. Switches S7 to S10 form a primary-side and switches S11 to S14 form a secondary-side full bridge of a dual active bridge converter.

Fig. 2 shows a structure of the power part of an embodiment of a system according to the invention. The switches S1 to S6 form a primary-side three-phase bridge circuit 4 which can be operated as a three-phase pulse rectifier, and the switches S7 to S12 form a secondary-side three-phase bridge circuit 8.

Starting from a network star point 1, mains phase voltages ua, ub, uc are applied to mains phase terminals a, b, c. Of these lead each Vorschaltinduktivitäten 3, which here also have the function of phase primary windings 3 of transformers Ta, Tb, Tc, on inputs a ', b', c 'of phase bridge branches of the primary-side three-phase bridge circuit 4. Currents through these phase primary windings are iTa> p , iTbiP, iTCiP. To a positive and a negative voltage rail of the primary-side three-phase bridge circuit 4, a primary-side DC buffer arrangement 5 is connected, to which a buffer voltage uZk is applied. The buffer arrangement 5 can be realized by one or more capacitors. It can also be realized by other voltage-influencing elements. To the power phase terminals a, b, c, a star connection of feedback capacitors 2 is connected. Of the

Star point of the star connection of feedback capacitors 2 can be connected in accordance with embodiments with the center of the buffer voltage. An optionally arranged in this connection series capacitor is shown in dashed lines.

Phase secondary windings 7 of the transformers Ta, Tb, Tc are each guided at a first end to phase inputs of the secondary-side three-phase bridge circuit 8. At a second end they are placed on connections of a star connection secondary blocking capacitors 6. To a positive and a negative voltage rail of the secondary-side three-phase bridge circuit 8 is connected a secondary-side output buffer arrangement, here in the form of an output capacitor arrangement 9, to which a DC output voltage udc is applied. The star point of the star connection of blocking capacitors 6 can be connected according to embodiments with the center of the DC output voltage. An optionally arranged in this connection series capacitor is shown in dashed lines.

FIG. 3 shows a control circuit for the power section shown in FIG. 2 with superimposed output voltage control Gdc, control of the buffer voltage GZk and subordinate control Gd, Gq of the input currents in a rotating coordinate system with instantaneous angle φ.

The measured mains voltages and network currents are transformed into a rotating d / q coordinate system, wherein the instantaneous angle φ of the d-axis is determined by a phase-locked loop such that the q-component of the transformed mains voltage has an average value of zero.

The measured buffer capacitor voltage uZk is compared with a predetermined desired value and fed to a controller Gzk resulting in a desired value for the d-portion of the input current results with which the measured d-part of the input current is compared and fed to a controller Gd. The q-component of the measured input current is compared with a reference value and fed to a regulator Gq. To the output signals of the regulators Gd and Gq, the respective voltage drops at the ballast inductances and the mains voltage, in the sense of feedforward control, are added and divided by the measured buffer capacitor voltage uZk, resulting in duty cycles dd and dq which, by back transformation into a non-rotating coordinate system, provide the duty ratios , db, dc of the three half-bridges of the two pulse equalizer systems.

The measured output voltage udc is compared with a predetermined setpoint value and the difference fed to a controller Gdc wherein the output signal 9 of the controller is supplied to an oscillator which generates two triangular shaped by the angle θ carrier signals tp and ts for the primary side and the secondary side in each case with the duty ratios da, db, dc are blended whereby pulse width modulated drive signals for the primary and secondary half bridges result which each have the same pulse widths on the primary side and secondary side, but within a pulse period are shifted by the angle θ.

Fig. 4 shows a secondary side equivalent circuit diagram of a phase wherein the switching frequency push-pull and common-mode voltage systems of the primary and secondary side are replaced by equivalent voltage sources and the phase transformer Ta, Tb, Tc is replaced by a magnetizing inductance Lm and a leakage inductance Ls ,

FIG. 5 shows characteristic time courses of the system according to FIG. 2 within a network period; FIG. Mains phase voltages ua, ub, uc, mains phase currents ia, ib, ic, secondary side power transfer phase current iTa.s and its local mean, local mean of the transmitted via the transformer Ta instantaneous power pya and the output voltage udc.

6 shows a local excerpt of the time profiles of the primary and secondary transformer voltages UTa, p, uTa, s and of the resulting primary and secondary side winding currents iTa, P, ha, s for one phase from the circuit according to FIG. 2 ,

Fig. 7 shows an embodiment with separate pulse rectifier inductances La, Lb, Lc and a single common mode voltage transformer Tcn which uses the switching frequency common mode voltage supply system with its primary winding connected between the neutral point of the flyback capacitors 2 and the midpoint of the buffer voltage uZk. Its secondary winding is connected in series with a blocking capacitor 6 to the two phase inputs of a secondary-side single-phase bridge circuit 8b with switches S7 to S10.

For terminology, it should be noted that a single-phase bridge circuit is so designated because it is connected to a phase on the AC side. By contrast, this one phase is connected to two terminals or two bridge branches of the bridge circuit. The word part "on" therefore refers to the number of connected phases and not to the number of bridge branches.

Fig. 8 shows an embodiment with series-connected secondary windings of the transformers Ta, Tb, Tc which are also connected in series with a blocking capacitor 6. Thereby, only the switching frequency common-mode voltage system for power transmission from the primary side to a secondary-side single-phase bridge circuit 8b (here a full bridge) constituted by the switches S7 to S10 becomes.

Fig. 9 shows an embodiment with a common mode choke Tcn with four windings with three winding between the inductors La, Lb, Lc and the feedback capacitors of the pulse rectifier system are connected and the remaining winding in series with a blocking capacitor to a secondary side single-phase bridge circuit (here a Full bridge), formed from S7 to S10, is switched. The star point of the feedback capacitors is connected to the positive, the negative, or a mid-point voltage rail of the primary-side DC buffer arrangement.

Fig. 10 shows an embodiment of the invention with a primary-side pulse rectifier system consisting of the switches S1 to S6 and the Vorschaltinduktivitäten La, Lb, Lc wherein the inputs of the phase bridge branches further, the first terminals three transformers Ta, Tb, Tc are connected and whose second terminals are in turn connected to the first terminals of three blocking capacitors. The second terminals of the blocking capacitors are connected together and connected to the center, or negative or positive reference rail of the buffer voltage of the pulse rectifier system.

Claims (13)

claims 1. A method for potential-free electrical energy transmission between a primary-side AC voltage system (ua, ub, uc) with a mains frequency and a secondary-side DC system (udc), in which • by driving a primary-side bridge circuit (4) at inputs (a1, b ', c) o a mains frequency push-pull voltage system, o a switching-frequency push-pull voltage system, o a switching-frequency common-mode voltage system, characterized in that • switching-frequency on the primary side with the switching-frequency push-pull voltage system Push-pull current components and / or switching frequency common-mode current components are formed with the switching-frequency common-mode voltage system, which by the primary-side bridge circuit (4) f let, and • these switching frequency Gegentaktstromkomponenten and / or common mode components are transformed by at least one transformer to a secondary side (6, 7, 8), and used for transmitting electrical power to the secondary side (6, 7, 8). 2. Method according to claim 1, wherein • the push-pull current components and / or common-mode current components transformed on the secondary side (6, 7, 8) are passed through an output buffer arrangement (9) by means of a secondary-side bridge circuit (8, 8b). 3. The method according to claim 2, wherein the primary-side bridge circuit (4) and the secondary-side bridge circuit (8, 8b) are driven with switching signals of the same switching frequency. 4. The method according to claim 3, wherein a control of a power transfer from the primary to the secondary side by varying a phase shift between the switching signals of the primary-side bridge circuit (4) and the switching signals of the secondary-side bridge circuit (8). 5. The method according to any one of the preceding claims, in which electrical power between the primary-side AC voltage system and a primary-side DC buffer arrangement (5) is replaced by mains frequency voltages and currents, and electrical power between the primary-side DC buffer arrangement (5) and the secondary side over the at least one transformer is replaced by switching frequency voltages and currents. 6. converter, preferably for carrying out the method according to one of claims 1 to 5, for the potential-free electrical energy transmission, comprising • mains phase terminals (a, b, c) for connection to an AC system, each via Vorschaltinduktivitäten (3) to inputs (a1, b ', c) are connected by phase bridge branches of a primary-side bridge circuit (4), wherein a positive and a negative voltage rail of the primary-side bridge circuit (4) to a primary-side DC buffer arrangement (5) are connected, characterized in that a circuit with high-pass characteristic for receiving common-mode currents and / or balanced currents, from the inputs (a1, b ', c) of the phase bridge arms, said high-pass circuit having at least one primary winding of a transformer, and at least one secondary winding of the transformer connecting in series with a blocking capacitor or (6), and, • either only one such series connection is present, and this series circuit is connected at a first end to a first phase input of a secondary-side bridge circuit (8b) and at a second end to a second phase input of the secondary-side bridge circuit (8b) , • or two or more such series circuits, and these series circuits are each connected at first ends to phase inputs of a secondary-side bridge circuit (8) and are connected at second ends to a common secondary-side star point. 7. A converter according to claim 6, wherein the circuit with high-pass characteristic at least one star circuit of feedback capacitors (2), which are respectively connected to first terminals to the mains phase terminals (a, b, c) and connected to second terminals at a common primary-side neutral point , and the high pass circuit further comprises the ballast inductances (3), and each of the ballast inductances (3) forms a primary winding. 8. A converter according to claim 6, wherein the circuit with high-pass characteristic comprises a star connection of series circuits of separate primary windings and feedback capacitors (2), said series circuits are connected at first ends to the inputs (a1, b ', c) of the phase bridge branches and to second ends are connected to a common primary-side neutral point. 9. Converter according to claim 7 or 8, wherein the common primary-side neutral point is connected to a center of the primary-side DC buffer arrangement (5), or to the positive, or to the negative voltage rail of the primary-side bridge circuit (4). 10. A converter according to claim 7 or 8, wherein the common primary-side neutral point is connected via a primary winding of a common-mode voltage transformer to a center of the primary-side DC buffer arrangement (5), or to the positive, or to the negative voltage rail of the primary-side bridge circuit (4). 11. A converter according to claim 7 or 8, wherein the common primary-side neutral point is connected via a series capacitor to a center of the primary-side DC buffer arrangement (5), or to the positive, or to the negative voltage rail of the primary-side bridge circuit (4). 12. A converter according to one of claims 6 to 11, comprising a three-phase common mode inductance lying in series with the ballast inductances, in particular with a fourth winding on a magnetic core of this three-phase common mode inductance, this fourth winding forming a secondary winding. 13. A converter according to one of claims 6 to 12, comprising two or more of the series circuits of secondary windings, each with a blocking capacitor (6), said series circuits are connected at second ends to a common secondary side star point, and wherein this neutral point, optionally via a series capacitor is connected to a center of a secondary-side output buffer arrangement (9), or to the positive, or to the negative voltage rail of the secondary-side bridge circuit (8).