Filter device

Abstract

A filter arrangement is connected with a power converter for transferring power between a multi-phase AC voltage network and a DC voltage network. The filter arrangement comprises differential mode coils coupled between each AC terminal of an inverter bridge and a corresponding AC network phase terminal, and common mode coils coupled between each inverter bridge DC terminal and a corresponding DC network terminal. The two inductors are located in the same magnetic core structure such that each differential mode winding is wound around its own specific phase core leg, and all common mode windings are wound around their own single core leg. The winding direction of the AC side and DC side inductor windings is such that common mode currents flowing in the same direction along both inductors induce a flux in each core leg that enhances the total flux circulating in the core.

Description

TECHNICAL FIELD

[0001] The present invention relates to a filter arrangement connected to a power converter for transferring power between a direct current "DC" voltage network and a multi-phase alternating current "AC" voltage network. Furthermore, the present invention also relates to a power distribution system in which a power converter is employed in which the filter arrangement according to the present invention can be applied. BACKGROUND

[0002] Inverters are widely used in power electronic applications for conversion between DC power and AC power. The most common type of inverter, the PWM inverter, converts a DC supply voltage into an AC output voltage consisting of pulses with varying width. Here, the output voltage is formed by the so-called pulse width modulation "PWM" method, the purpose of which is to form an output voltage pulse pattern with a desired fundamental component and a minimum amount of unwanted harmonics.

[0003] The instantaneous average value of the PWM voltage pattern is not zero, as for example in a three-phase symmetrical sinusoidal voltage waveform, but varies within the range of the DC supply voltage. The non-zero average voltage value forms a voltage component common to all phases, which produces a so-called common mode current that circulates in the DC network and the AC network connected to the device and can cause harmful effects to the environment. For commonly used modulation methods, the higher the common mode voltage and the resulting common mode current, the lower the value of the inverter output AC voltage. In some applications, the voltage pulse shape is not acceptable, and therefore the output AC voltage needs to be filtered to be closer to a sinusoidal waveform. This is the case for example in many renewable energy applications, in which the generated power is fed to the public energy distribution grid.

[0004] A commonly used filter solution in PWM applications is the so-called LCL filter, which is coupled between the inverter output terminals and the power distribution network, as shown in Figure 1 The LCL filter typically comprises a capacitive filter coupled between a first inductive filter and a second inductive filter. The drawback of this filter type is the inductive elements, which are typically large and heavy, thus producing considerable power losses. Therefore, the filter requires a proper housing and an efficient cooling device. To limit the common mode current, an additional common mode coil is also needed in the filter arrangement. Overall, the LCL filter significantly increases the size and cost of the arrangement.

[0005] Another known filter solution in PWM applications comprises a series connection of an inductor and a capacitive filter arrangement, as shown in Figure 2 In a preferred embodiment of the filter, the inductor is located on the same magnetic core structure, so that each differential mode winding is wound around its own magnetic core leg and all common mode windings are wound around a common single magnetic core leg, as shown in Figure 5The disadvantage of such a filter is that the total AC current, i.e. its entire differential mode component and common mode component, has to flow along the two series coupled inductors. Especially in operating points with low AC output voltage and high output current at the same time, the common mode coil is loaded with the worst case common mode current and the worst case differential mode current at the same time. Therefore, this case has to be the basis for the common mode coil and the magnetic core leg design. Another disadvantage of such a filter is that it can be difficult to place the entire winding of the common mode coil around the same magnetic core leg. SUMMARY

[0006] It is an object of the present invention to provide a novel filter arrangement for power converters between a DC voltage network and a multiphase AC voltage network. The filter arrangement of the present invention avoids the disadvantages of the prior art, such as the size or bulky LCL filter circuit of the common mode coil under simultaneous occurrence of the worst case differential mode current component and the worst case common mode current component. The object of the present invention is achieved by what is stated in the independent claims, other preferred embodiments being disclosed in the dependent claims.

[0007] According to the present invention, the filter arrangement comprises a differential mode coil coupled between each AC terminal of the inverter bridge and the corresponding AC network phase terminal, and a common mode coil coupled between each inverter bridge DC terminal and the corresponding DC network terminal. According to the present invention, the two inductors are located in the same magnetic core structure, such that each differential mode winding is wound around its own specific phase magnetic core leg, and the entire common mode winding is advantageously wound around its own single magnetic core leg. The inductor windings are wound around their magnetic core legs such that the common mode current flowing in the same direction between the DC network and the AC network along the two inductors induces a flux in each magnetic core leg, which enhances the total flux circulating in the magnetic core.

[0008] According to an embodiment of the present invention, the DC side common mode inductor structure further comprises a differential mode inductance, such that the differential mode inductance is less than 15% of the common mode inductance.

[0009] According to an embodiment of the present invention, the filter arrangement further comprises a capacitive coupling between the DC network terminals and the AC network terminals. An advantageous filter arrangement comprises a first capacitor bank comprising a capacitor between each DC network terminal and the star point, and a second capacitor bank comprising a capacitor between each AC network terminal and the star point.

[0010] According to an embodiment of the present invention, the star point of the capacitive filter arrangement is coupled to ground via a capacitor.

[0011] According to the present invention, the capacitor C 43 -C 49 and C 33 -C 39The network from the AC network is coupled to the DC network in order to filter common mode currents.

[0012] According to an embodiment of the present application, the filter device of the present application is employed in a system, wherein the connected AC network has at least 2 phases and the connected DC network has at least 2 poles.

[0013] According to an embodiment of the present application, a power converter provided with the filter device of the present application is employed to transfer power between a DC network and a multi-phase AC network in a system, wherein the power supplied to the DC network is supplied by a battery, a rectified AC network or a renewable source such as a solar panel or a wind turbine.

[0014] According to an embodiment of the present application, a power converter provided with the filter device of the present application is employed to transfer power between a DC network and a multi-phase AC network in a system, wherein the power supplied to the AC network is supplied by an AC generator or by an AC network.

[0015] According to an embodiment of the present application, a power converter provided with the filter device of the present application is employed to transfer power between a DC network and a multi-phase AC network in a system, wherein the DC network is loaded by a battery charger or by an AC motor via an inverter.

[0016] According to an embodiment of the present application, a power converter provided with the filter device of the present application is employed to transfer power between a DC network and a multi-phase AC network in a system, wherein the AC network is loaded by an AC network or by an AC motor.

[0017] According to an embodiment of the present application, the filter device of the present application is up to 30% more efficient than prior art such as LCL filter and DE 102008026870 Al of Siemens, when the modulation index is 0,4.(100) BRIEF DESCRIPTION OF DRAWINGS

[0018] The present application will be explained in more detail using examples with reference to the attached drawings, in which

[0019] Figure 1 A filter device of the prior art is shown;

[0020] Figure 2 A filter device of the prior art is shown;

[0021] Figure 3 A filter device according to the present application is shown;

[0022] Figure 4 A filter device according to the present application is shown;

[0023] Figure 5shows an inductor structure according to the present invention;

[0024] Figure 6 shows the magnetic flux flow in the inductor core according to the present invention;

[0025] Figure 7 shows a power distribution system according to the present invention;

[0026] Figure 8 A graph showing power loss of the filter device of the present invention compared with the prior art is shown. DETAILED DESCRIPTION

[0027] Figure 1 A simplified main circuit diagram of a prior art power converter 10 is shown, which is used for supplying power to a multi-phase AC voltage network U AC12 and DC voltage network U DC11 The simplified main circuit diagram is shown as a single line diagram. In the converter device, the DC voltage U DC11 Through the inverter unit INU 11 Inverted into three-phase AC voltage U AC11 Output voltage U AC11 It consists of pulses, the height of which is the inverter unit INU 11 The DC voltage of the intermediate circuit. Due to its high content of harmful harmonics due to its pulse-like voltage shape, this voltage cannot usually be connected to the public distribution network. Therefore, such a connection must be made through a heavy filter that removes most of the harmful harmonics. Usually, the filter, e.g. Figure 1 LCL 11 , by the first differential mode inductor L 11 , the second differential mode inductor L 12 In order to limit the common mode current supplied to the AC network, an additional common mode inductor L is required in the filter device. 13 .

[0028] Figure 2 The main circuit diagram of another prior art power converter 20 is shown. The power converter is used in a circuit having DC+ and DC- poles and a storage capacitor C 21 Power is transferred between the filtered DC voltage network and a three-phase AC voltage network having phases U, V, and W. The inverter bridge B is shown in simplified form. 20 It is obvious to those skilled in the art that the inverter bridge generates a three-phase output AC voltage having a U2 phase, a V2 phase, and a W2 phase. Due to the pulse shape, a three-phase differential mode coil L is used, which is connected in series between the inverter AC terminals U2, V2, and W2 and the corresponding network terminals U, V, and W. 21 and 3-phase common mode coil L22 filter means for filtering the output voltage to a more sinusoidal waveform, the filter means comprising a first capacitor bank C 23 , C 24 between each DC voltage network pole DC+, DC- and the star point N2 26 -C 28 and a second capacitor bank C 29 -C between each AC voltage network phase U, V, W and the star point N2

[0029] Figure 3 A main circuit diagram of a power converter 30 for transferring power between a DC voltage network and a 3-phase AC voltage network is shown. An inverter bridge B 30 is similar to the inverter bridge of the power converter 20. An energy storage filter capacitor C 31 is coupled to the DC terminals B3+, B3- of the bridge B 30 . According to the invention, the filter means in the power converter 30 comprises a common mode coil L32 on the DC voltage network side of the inverter bridge, i.e. one winding on both lines between the inverter terminals B3+, B3- and the corresponding DC network poles DC+, DC-, respectively. In addition, the filter means is similar to the filter means of the power converter 20, comprising a 3-phase differential mode coil L 31 between the inverter AC terminals U3, V3, W3 and the corresponding AC network phases U, V, W. The solid line adjacent to the inductor symbol indicates the magnetic core leg of the inductor coil. The capacitive filter in this exemplary embodiment of the invention comprises a first capacitor bank C 33 , C 34 between each DC voltage network pole DC+, DC- and the star point N3 36 -C 38 and a second capacitor bank C 39 -C between each AC voltage network phase U, V, W and the star point N3

[0030] As is known, at constant output current, the lower the inverter DC input power, the lower the AC output frequency. Thus, the DC input current is also low at low output frequency, which brings a significant benefit when using the filter means of the invention due to low power losses in the common mode inductor. In the prior art, all inductors are placed on the AC side of the inverter, the inductor current and the current proportional losses are independent of the output frequency.

[0031] The novel idea is not to set a limit to the number of AC network phases, e.g. although a 3-phase network is used in the examples herein, a single phase AC supply is possible. The number of DC network poles can also be greater than 2. Figure 4 An example of a main circuit diagram of a power converter 40 is shown, where the inverter bridge B 40 A 3-level output AC voltage pattern can be formed from a bipolar DC voltage source. Since these types of inverter bridge types, such as NPC, TNPC, ANPC etc. are familiar to the person skilled in the art, the bridge is only presented in a simplified form here. According to the invention, the filter arrangement comprises a common mode coil L 42 i.e. one winding between each inverter DC terminal B4+, B 4N , B4- and the corresponding DC network poles DC+, N, DC-. In addition, the filter arrangement is similar to the filter arrangement of the power converter 30, comprising energy storage filter capacitors C 41 , C 42 , a first filter capacitor bank C43... C 45 between each DC network pole and the star point N4, a second filter capacitor bank C46... C 48 between each AC network phase and the star point N4, and a capacitor C 49 for grounding the star point N4.

[0032] Figure 5 The main structure of an advantageous exemplary embodiment of an inductor according to the invention is shown. The magnetic core of the inductor comprises 4 core legs 51-54 connected by a magnetic yoke 55, 56. A 3-phase differential mode coil L 31 is formed by windings around the core legs 52... 54. A common mode coil L 32 is formed by windings around the single core leg 51. The small black dots next to each winding indicate the direction of rotation of the winding around the core leg, and the markings next to the winding ends indicate the connections of the inductor to Figure 3 other components of the converter circuit in

[0033] Figure 6 The magnetic flux flow in the magnetic core of Figure 5 is shown. In this figure, the hatched areas represent the magnetic core, and the core legs are marked by the numbers 51-54, as in Figure 5

[0034] The upper part of the figure shows the magnetic flux flow in the case where there is no common mode current component in the power converter circuit. In this case, the currents of the two windings around the core leg 51 are equal but opposite (i.e. Figure 3 ​The current from DC+ to B3+ is equal to the current from B3- to DC-), so the magnetic flux Φ they generate in the core 51 is dmD are also equal and cancel each other out. This means that in this case the common mode coil has no effect on the operation of the converter. The current in each differential mode coil winding around the core legs 52-54 generates its own magnetic flux. Normally, the sum of the output currents is zero, so the flux Φ in the core legs 52-54 is dmA The sum is also zero, and none of these fluxes circulate through the core leg 51 (ie, despite the shared mechanical structure, the common mode coil and the differential mode coil have no influence on each other).

[0035] The lower part of the figure shows the flux flow in the power converter circuit with common mode current (common mode current = current flowing in the same direction between the connected DC network and AC network in all main circuit lines, e.g. Figure 3 In this case, the currents in the two windings around the core 51 are in the same direction, so the magnetic flux Φ they generate is cmD The directions of the inductor windings are also the same and reinforce each other. The same common-mode current also flows through the differential-mode coil L31 phase, and because the inductor windings rotate around their core legs, the circulating magnetic flux ΦcmA in the core legs 52-54 is further enhanced. Therefore, in the common-mode state, all windings enhance the circulating magnetic flux, which means that all windings constitute an impedance to the common-mode current, which is beneficial in reducing the common-mode current.

[0036] Figure 7 An example of a power distribution system is shown, in which a power converter PC employing a filter device according to the present invention can be applied. This figure illustrates various alternatives at a conceptual level; however, these alternatives do not necessarily all occur simultaneously in a real system. In this exemplary system, power supplied to the system can be supplied to the DC voltage network by wind turbines WT via power converter PC1, by solar panels SP via power converter PC2, by distribution grid DG1 via rectifier REC, or by batteries B via power converter PC3. Power can also be supplied to the system from the AC side, either directly from distribution grid DG2 or via transformer T, or by AC machine M2 operating as a generator.

[0037] In this exemplary system, power from the system can be supplied to motor M1 via inverter INU through the DC voltage network, or to the battery via battery charger PC3. Power from the system can also be supplied directly or via transformer T to AC-side loads such as motor M2, or to the AC voltage grid.

[0038] It is worth noting that if many inverter units with local energy storage capacitors are connected to a common DC voltage network, the current in the DC voltage link can increase due to harmful resonance phenomena caused by the separate capacitors and the stray inductance of the DC link between them. Therefore, additional inductance in the DC link can be beneficial in order to lower the resonance frequency and damp it. According to the present invention, the common mode inductance can contain a small amount of differential mode inductance, advantageously less than 15% of the common mode inductance, in order to meet this need.

[0039] The specific examples provided in the above description are not exhaustive, nor should they be interpreted as limiting the scope and / or applicability of the appended claims, unless explicitly stated otherwise. The features cited in the appended claims are freely combinable with each other, unless explicitly stated otherwise. The verbs "comprise" and "include" are used in this document as open limitations that neither exclude nor require the existence of unrecited features. Furthermore, it should be understood that the use of "a" or "an", i.e. a singular form, throughout this document does not exclude a plurality.

Claims

1. A filter device (10, 20, 30, 40) connected to a power converter for transmitting power between an alternating current (AC) voltage network and a direct current (DC) network, the power converter comprising an inverter bridge having AC terminals and DC terminals, and comprising - The first inductor (L 11 、L 21 、L 31 、L 41 ), comprising a differential mode coil between each inverter bridge AC terminal and the corresponding AC network connection, - The first capacitor bank (C 11 、C 21 -C 24 、C 31 -C 34 、C 41 -C 44 ), including capacitors between each power converter DC network connection and the star point (N2, N3, N4), - Second capacitor bank (C 26 -C 28 、C 36 -C 38 、C 46 -C 48 ), comprising a capacitor between each power converter AC network connection and the star point (N2, N3, N4), characterized in that, The filter device further comprises a second inductor (L 32 、L 42 ), the second inductor comprises a common mode coil between each DC terminal of the inverter bridge and a corresponding DC network connection, wherein the first inductor and the second inductor are magnetically coupled via a common magnetic core structure (LS), wherein the capacitors of the first capacitor bank are positioned upstream of the second inductor, and - a capacitor for grounding the star point, Wherein, the second inductor further includes a differential mode inductor, such that the differential mode inductor is less than 15% of the common mode inductor, and The first inductor wire and the second inductor wire are wound around the magnetic core structure so that all common mode coil windings are wound around a common magnetic core leg, each differential mode coil wire is wound around its own magnetic core leg, and the ends of the magnetic core legs are connected to each other through a magnetic core yoke component.

2. The filter device according to claim 1, wherein The direction of rotation of each winding about its core leg is such that common mode current flowing in the same direction along the first and second inductors induces a flux in each core leg that enhances the total flux circulating in the core.

3. A power distribution system, using a power converter of the filter device according to any one of claims 1 to 2 to transmit power between an AC voltage network and a DC voltage network, characterized in that: The AC voltage network has at least 2 phases and the DC voltage network has at least 2 poles.

4. A power distribution system, wherein the power converter used in the filter device according to any one of claims 1 to 2 transmits power between an AC voltage network and a DC voltage network, characterized in that: The power supplied to the DC network is supplied by batteries, a rectified AC network or a renewable source.

5. The power distribution system of claim 4, wherein the renewable source is a solar panel or a wind turbine.

6. A power distribution system, wherein the power converter used in the filter device according to any one of claims 1 to 2 transmits power between an AC voltage network and a DC voltage network, characterized in that: The power supplied to the AC network is supplied by an AC generator or by the AC network.

7. A power distribution system, wherein the power converter used in the filter device according to any one of claims 1 to 2 transmits power between an AC voltage network and a DC voltage network, characterized in that: The DC network is loaded by a battery charger or by an AC motor via an inverter.

8. A power distribution system, wherein the power converter used in the filter device according to any one of claims 1 to 2 transmits power between an AC voltage network and a DC voltage network, characterized in that: The AC network is loaded by an AC network or by an AC electric motor.

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