OFFSET SWITCHING INVERTERS WITH INTEGRATED FILTER AND SUCCESS CHOKE

Description

BACKGROUNDField of Invention

[0001] Embodiments of the present invention relate generally to utility scale power inverters.Discussion of Related Art

[0002] A power inverter, or inverter, is an electronic device or circuitry that converts direct current (DC) to alternating current (AC). Inverters may be used in a number of different contexts, with different DC power sources (such as lead acid batteries, photovoltaic solar panels, wind turbines, etc), and may be designed to satisfy different power demands of a system.

[0003] Utility scale solar inverters, in particular, convert variable DC output of a photovoltaic (PV) solar panel into a utility frequency AC to provide power to either a commercial electrical grid or a local, off-grid electrical network. Solar inverters are connected to a plurality of photovoltaic cells that provide DC input to the inverter. The inverter comprises at least one DC-to-AC power conversion bridge, associated filter electronics and an AC (output) module. The DC-to-AC power conversion bridge uses a plurality of electronic switches, typically insulated gate bipolar transistors (IGBTs), and diodes to convert the DC input into AC output. For grid-connected inverters providing power to an electricity grid, the AC output is filtered to provide an AC output waveform that is suitable for the grid. Furthermore, solar power inverters have special functions adapted for use with photovoltaic arrays, including maximum power point tracking and anti-islanding protection.

[0004] A sine wave inverter produces a multiple-step sinusoidal AC waveform, although in most cases the output is a choppy or rough approximation of a sine wave, rather than a smooth sine wave. As a substitute for standard AC line power, power inverter devices approximate a sine wave output because many electrical products are engineered to work best with a sine wave AC power source. Further, grid-connected inverters are designed to feed power into the electric power distribution system. They transfer synchronously with the line, and should have as little harmonic content as possible.

[0005] The output from an inverter can be single phase or three-phase. Three-phase inverters are generally used in higher power applications. A basic three-phase inverter consists of three single-phase legs each connected to one of the three load terminals. The operation of the three phase legs is coordinated so that one operates at each 120 degree point of the fundamental output waveform. Certain harmonics are eliminated and other harmonics can be removed by further processing.

[0006] As shown in FIG. 1, an LC filter comprising one or more inductor and capacitor can be used to smooth the AC waveform from a single phase inverter (as shown in FIG. 1). Such lowpass filters allow the fundamental component of the waveform to pass to the output while limiting the passage of harmonic components. LC filters may similarly be used in connection with a three phase inverter, with an LC filter applied to each output phase of the inverter.

[0007] When two or more inverters are connected in parallel, their switching times (single phase or 3-phase) can be synchronized or can be offset relative to one another in an "interleaved" configuration. Interleaving is implemented by phase-shifting the switching times of each inverter by a unique multiple of 360° / n, where n is the number of inverters. The switching of the multiple inverters is thereby staggered, and the overall switching frequency may thereby be increased.

[0008] Interleaving can result in the cancellation of higher order harmonics and a reduction in distortion. Also, the higher frequency noise reduces the size of the inverter AC output filters that are needed. Parallel interleaved three-phase inverters can provide significant cost reductions while improving system reliability and efficiency. FIG. 2 shows two 3-phase DC-AC inverters connected in parallel, with output LC filters.

[0009] Interleaved converters are sometimes magnetically coupled with a coupling inductor, and then share the same output filter. The coupling combines high frequency components (which may be interleaved) and may thereby reduce ripple. FIG. 3 shows a pair of inverters (bridges) magnetically coupled via a coupling inductor which is connected to a shared LC filter. With this arrangement the combined current from bridge 1 and bridge 2 passes through the output filter inductor. US2016 / 329705A1 as well as EP2536018A1 disclose parallel operation of power inverters e.g. by means of coupled inductors also called interphase transformers.

[0010] US2009 / 289751A1 discloses integrated output and coupling inductors (fig 1-4) similar to those in the application used in order to connect two half-bridge outputs to a common point P1 (fig 6, 7).SUMMARY

[0011] Particularly for large scale inverter systems, AC filters required to smooth out the unacceptably rough AC power waveform of the inverters would conventionally be large and costly in order to handle the level of power and power quality required. Means to provide a power electronics system with parallel inverters in an extremely compact configuration and with efficient use of magnetic inductor material and thus drastically reducing the overall AC filter size and cost are provided in claim 1. Dependent claim 2 specifies a corresponding multiphase system.

[0012] Self-inductance cores and a coupled core of a filter assembly may be provided for three phases in a fully integrated implementation.

[0013] Still other aspects, examples, and advantages are discussed in detail below.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Various aspects of at least one example are discussed below with reference to the accompanying figures, which are not intended to be drawn to scale. The figures are included to provide illustration and a further understanding of the various aspects and examples, and are incorporated in and constitute a part of this specification, but are not intended as a definition of the limits of the invention. In the figures, identical or nearly identical components illustrated in various figures may be represented by like numerals. For purposes of clarity, not every component may be labeled in every figure. In the figures: FIG. 1 is a schematic diagram of single phase power conversion bridge with an AC filter; FIG. 2 is a schematic diagram of two three-phase power bridges connected in parallel employing output AC filters; FIG. 3 is a schematic diagram of two single phase power conversion bridges coupled via a coupling inductor with a shared output AC filter; FIG. 4 is a schematic diagram of two single phase power conversion bridges, with each bridge having an output AC filter, connected in parallel and coupled via a coupling inductor; FIG. 5 is a schematic diagram of two three phase power conversion bridges, with each phase of each bridge having an output AC filter, connected in parallel with each phase coupled via coupling inductors; FIG. 6 is a perspective view of an inductor coil in accordance with the invention; FIG. 7 is a perspective view of an example filter assembly for a single phase output of an inverter pair with connections to each inverter bridge in accordance with the invention; FIG. 8 is a perspective view of an example 3-phase inductor assembly in accordance with the invention; FIG. 9 is a top view of the example 3-phase inductor assembly of FIG. 8; FIG. 10 is a perspective view of the example 3-phase inductor assembly of FIG. 8 including example integrated cooling components; and FIG. 11 is a perspective view of the example cooling components of FIG. 10, apart from the example 3-phase inductor assembly. DETAILED DESCRIPTION

[0015] Aspects and embodiments provide inductor arrangements to couple two or more inverters in parallel, in an extremely compact configuration, with efficient use of magnetic inductor material (thereby reducing cost). Embodiments in accordance with the invention can drastically reduce the overall AC filter size and cost, and can provide a filtered AC output quality suitable for the grid. A cooling system may be mechanically integrated into the compact AC filter module for thermal management in some embodiments.

[0016] Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. Examples disclosed herein may be combined with other examples in any manner which falls into the scope of protection as defined by the appended claims.

[0017] FIG. 4 illustrates an example of two single-phase inverters 410, 420 (bridge 1 and bridge 2) connected in parallel, each inverter 410, 420 having a corresponding output inductor 412, 422 (L1 and L2), respectively providing self-inductance. The outputs are then magnetically coupled via a coupled inductor 430 (L coupled ). With this arrangement, there is half as much current passing through each of inductors L1 and L2, as there would be passing through L filter in the conventional filter arrangement of FIG. 3. Thus in the arrangement of FIG. 4 losses may be approximately halved relative to FIG. 3, for the same overall output current, reducing the amount of heat generated and providing more efficient power conversion.

[0018] Fig. 5 shows a solar inverter system 500 of similar arrangement to FIG. 4, but for two 3-phase inverters 510, 520 (bridges) connected in parallel. Corresponding phases (A, B, C) from each of the inverters 510, 520 are coupled via a coupled inductor 530. The inverters 510, 520 of solar inverter system 500 may be DC-to-AC inverters (or "power conversion bridges"), each rated for up to 1 MW (as 2MW of power cannot be handled by a single inverter). Each inverter produces a 3-phase output. The two bridges are connected in parallel and the 3-phase outputs of the 2 bridges are interleaved (180° out of phase relative to each other). The inverters 510, 520 on their own may produce an unacceptably rough AC power waveform. Therefore, an AC filter module 540, consisting of inductor and capacitor components, is used to smooth the waveform.

[0019] To handle the level of power and power quality requirements, an AC filter would conventionally be large and costly. For example, a classical approach to this problem, even after much optimization, requires inductors that cost approximately 9% of the system cost and capacitors that are approximately 2% of the system cost. The inductor used in the classical approach also produces significant energy losses (around 4 kW), which inflates the required cooling system and adds additional cost and volume to the system.

[0020] As mentioned above, switching of the two 3-phase inverters 510, 520 in the example system 500 may be interleaved, thereby doubling the switching frequency. This essentially doubles the frequency seen by the inductors and therefore the amount of filtering required is reduced. According to the invention, for each of the two inverters 510, 520, there is a core for each AC phase that provides self-inductance 550. For each AC phase, there is also a third core that provides a coupled inductance 530 between the inverters 510, 520 (for each phase). Each of the self-inductors 550 is positioned between each inverter 510, 520 and the respective coupled inductor 530, per phase.

[0021] The AC filter module 540 thereby includes coupling between inverters and also includes coupling between phases.

[0022] As discussed above, a solar inverter system may comprise two 3-phase inverters connected in parallel, but a similar approach can be taken with more than two 3-phase inverters and / or with two or more single phase or other multi-phase inverters.

[0023] In embodiments according to the invention, the mechanical design of an AC filter module has multiple novel aspects that allow the technology to be practically and commercially realized. Overall the coupled inductor is 1 / 3 the total mass and 1 / 2 the volume of the classical inductors when designed for equivalent losses. This results in a cost reduction of the inductor components. Additional reductions in system cost can be obtained through mechanical integration of cooling, structural features, and size reduction.

[0024] In conventional inductor systems, the coils or windings are wrapped around a central core (often a straight cylindrical rod or a continuous loop or ring, doughnut). Embodiments of the present invention involve a unique winding geometry that is particularly suited for use in embodiments of an AC filter module. An example of such a winding is shown in FIG. 6.

[0025] The conductive material of an example winding 600 (e.g. copper or aluminum) may have a rectangular cross-section as shown. In various embodiments, the conductive material may be one or more strands, and may be multi-strand transpose wire in certain embodiments, e.g., to achieve additional reduction in losses. The winding 600 is shaped to form a series of concentric turns 610 in a first plane for the main inductance, then transitions to a second plane (parallel to the first) and is formed in to a series of concentric elongated turns 620. The circular opening 612 accommodates a self-inductor core and the elongated opening 622 accommodates a coupled inductor core. The coupled inductor turns 620 also contribute to the self-inductance. In certain embodiments, the winding 600 may be generally coated in an electrically insulating material, such as a plastic, except for the terminals 630.

[0026] Various embodiments of winding geometry, with respect to the example illustrated in FIG. 6, offer particular advantages. They are designed to integrate the self and coupled inductor cores, and are designed to stack in a space-efficient manner, with the main inductance turns of one winding in the same plane as the coupled inductance turns of an adjacent winding. The stacks of windings can also be packed together tightly side-by-side because of their quasi-rectangular shape. The flat / planar structure of the windings also allows good thermal contact with thermal plates, such as liquid-cooled thermal plates, which can be interposed between stacked windings, for cooling the assembly (as described in more detail below). Input and output terminals can be conveniently located at almost any desired location around the perimeter of the winding. In conventional windings, one terminal is often located inside the winding where it is less accessible.

[0027] Variations on the above winding geometry or quite different winding geometries can be used in various implementations of the present invention. For example, in some variations on the above winding geometry, the cross-section of the winding may be non-rectangular. The number of turns for the main inductance and the coupled inductance can be varied.

[0028] FIG. 7 shows an embodiment of an AC filter assembly 700 for single-phase output of an inverter pair with connections to each inverter bridge. The assembly comprises 16 windings similar to those shown in FIG. 6 (with circular openings to accommodate the cores), stacked in two side-by-side stacks of 8. Self-inductance turns of the 8 windings connected to bridge 1 are wound around self-inductance core 710. Similarly, self-inductance turns of the 8 windings connected to bridge 2 are wound around self-inductance core 720. The coupled inductor turns of all 16 windings are wound around the coupler core 730.

[0029] The physically interleaved windings stacked on each core are positioned to cancel what could otherwise be massive losses in the coupler core. There are multiple windings in parallel surrounding the various cores, which tends to reduce or minimize current crowding that can occur due to proximity of the windings to the magnetic material (cores) and other conductors (windings).

[0030] For a pair of inverters with 3-phase (interleaved) output, three separate mechanical assemblies like that shown in FIG. 7 may be used. In certain embodiments, however, the windings and inductors for all three phases of two or more inverters may be integrated into a single, compact assembly. An example of such an assembly is illustrated in perspective view in FIG. 8. A top view of the same example assembly is illustrated in FIG. 9.

[0031] In the example of FIG. 8, the 1st and 5th limbs of each core are optional, depending on application, and / or the yoke of one or more of the cores may be removed, e.g., in an air core design. For example, a yokeless design for the self-inductor cores is enabled due to the core material magnetic properties and the arrangement of the cores. Such can reduce the core material mass and cost significantly.

[0032] The self-inductor cores and coupler cores can be made of any suitable magnetic material. In some embodiments the self-inductor cores are powdered iron or powdered iron alloys, and the coupler cores are pillars or rods made of an amorphous material, with the perimeter constructed from Cold Rolled Grain Oriented laminated steel. Such a composition and construction can improve power losses and dissipation in the magnetic material.

[0033] FIG. 10 shows an example AC filter module for a pair of interleaved 3-phase inverters comprising the assembly of FIG. 8 integrated with a liquid cooling system rack (shown individually in FIG. 11 below). For example, aluminum thermal plates with internal flow channels may be interposed between the phase A / phase B windings and between the phase B / phase C windings, and above the phase A windings and below the phase C windings. Liquid coolant supplied via a main coolant inlet manifold and coolant lines, may be circulated through the plates, in parallel, to cool the electrical and magnetic components, and then directed via corresponding outlet lines and an outlet manifold to an air-cooled heat exchanger. A pair of solid aluminum heat spreader plates may be included to help dissipate heat generated deep inside the module, near the coupler core, for example.

[0034] As mentioned above, in the assemblies shown in FIGS. 8-10, windings are tightly packed close together, reducing the size of the cores and the amount of magnetic material that is needed, and thereby reducing losses. Magnetic material in the pairs of phases may be shared in some embodiments. Embodiments of an inductor assembly, such as the example assembly shown in FIGS. 8-10, including a compact arrangement of inductor windings for three phases (e.g., as compared to three separate assemblies like that assembly shown in FIG. 7), may provide improved compensation and cancellation of noise between coils of the assembly.

[0035] In some embodiments, an assembly or module as shown in FIGS. 8-10 may be potted, e.g., in high temperature, thermally conductive, electrically insulating material.

[0036] FIG. 11 illustrates the example cooling system components of the module shown in FIG. 10. The example cooling system includes liquid coolant that is distributed through coolant lines to various thermal plates to remove heat generated from the inductor coils and electrical components.

[0037] Embodiments of the present invention provide a number of advantages, including the reduction of an AC filter size and cost, through use of a compact configuration, with efficient use of magnetic inductor and conductor materials. Examples and embodiments of AC filter / inductor assemblies described herein have the effect of providing an increased power density. In addition, they may provide reduced losses due to lower current from the self-inductors being "upstream" of the coupled inductor, and reduced losses due to the interleaved physical arrangement of the windings on the coupler cores. The reduced losses result in less heat generation, and reduced requirement for cooling. The design of the cores and windings provide for simple assembly, supporting manufacturing feasibility. Further, the design allows for the use of liquid cooling of a filter / inductor, which is generally more cost-efficient than air-cooling, and allows for greater control or optimization of the degree of thermal transfer.

[0038] In various embodiments, windings, arrangements, assemblies, and modules in accord with aspects of those illustrated in FIGS. 4-10 may be beneficially applied to provide electrical filtering to any of numerous power converter applications, including those of solar inverters as described herein, but also of DC-to-DC converters, AC-to-DC converters, and other DC-to-AC converters for applications other than solar. Such arrangements may provide compact and efficient filtering to remove high frequency components from an electrical waveform at inputs and / or outputs of various power converters. Such arrangements may also be beneficially adapted to differing scale of power conversion equipment than those discussed herein. For example, power factor correction (PFC) equipment, uninterruptible power supply (UPS) equipment, and the like.

[0039] Having described above several aspects of at least one embodiment, it is to be appreciated various alterations, and modifications will readily occur to those skilled in the art. The scope of protection is defined by the appended claims.

Claims

1. A power electronics system, comprising: a first inverter (410) configured to receive direct current, DC power from a power source; a second inverter (420) configured to receive DC power from the power source; a first inductor coil winding (600) including a first series of concentric circular turns (610) in a first plane and a first series of concentric elongated turns (620) in a second plane parallel to the first plane, the first series of concentric circular turns forming a first circular opening (612) and the first series of concentric elongated turns forming a first elongated opening (622), such that the first circular opening overlaps in part the first elongated opening, the first circular opening accommodating a first self-inductance core (710) and the first elongated opening accommodating a coupled core (730) and the first self-inductance core (710), a second inductor coil winding (600) including a second series of concentric circular turns (610) in the second plane and a second series of concentric elongated turns (620) in the first plane, the second series of concentric circular turns forming a second circular opening (612) and the second series of concentric elongated turns forming a second elongated opening (622), such that the second circular opening overlaps in part the second elongated opening, the second circular opening accommodating a second self-inductance core (720) and the second elongated opening accommodating the coupled core (730) and the second self-inductance core (720); wherein the first series of concentric circular turns forms a first output inductor (412, L1A, L1B, L1C) connected in series to an alternating current, AC output of the first inverter; wherein the second series of concentric circular turns forms a second output inductor (422, L2A, L2B, L2C) connected in series to an AC output of the second inverter; wherein the first series of concentric elongated turns and the second series of concentric elongated turns together form a coupling inductor (430, 530, LAcoupled, LBcoupled, LCcoupled) configured to receive current from the first output inductor (412, L1A, L1B, L1C) and the second output inductor (422, L2A, L2B, L2C), the coupling inductor having an output; an output capacitor (Cf, 540) having a first end coupled to the output of the coupling inductor; and an AC power output to provide current from the coupling inductor.

2. The power electronics system of claim 1, wherein the first inverter (410, 510) is a multiphase inverter; wherein the second inverter (420, 520) is a multiphase inverter; wherein the first output inductor is one of a first plurality of output inductors (550, L1A, L1B, L1C), each of the first plurality of output inductors connected in series to an output phase of the first multi-phase inverter; wherein the second output inductor is one of a second plurality of output inductors (550, L2A, L2B, L2C), each of the second plurality of output inductors connected in series to an output phase of the second multi-phase inverter; wherein the coupling inductor is one of a plurality of coupling inductors (530), each of the plurality of coupling inductors configured to receive current from a respective output inductor of the first plurality of output inductors (550, L1A, L1B, L1C) and a respective output inductor of the second plurality of output inductors (550, L2A, L2B, L2C); wherein the output capacitor is one of a plurality of output capacitors (Cf, 540), each of the output capacitors having a first end coupled to the output of a corresponding coupling inductor (530, LAcoupled, LBcoupled, LCcoupled); and wherein the AC power output is a multi-phase alternating current, AC power output to provide current from the plurality of coupling inductors (530).