A three-state switching cell based frequency multiplier string pv power converter

Abstract

A three-state switching cell based frequency multiplying string PV power converter apparatus. The power converter apparatus includes a plurality of cascaded three-state switching cells arranged in two or more power units configured to receive power from two or more variable DC power sources, such as string photovoltaic power sources, and produce a multilevel terminal voltage. The apparatus generates a reduced amplitude ripple and multiplies the effective switching frequency in the reactive components by multiples of two, four, or more times the fundamental switching frequency of the three-state switching cells. Frequency multiplication and multilevel voltage generation is aided by cascading two three-state switching cells sharing a single transformer. The reduced voltage and current ripple amplitude and increased effective frequency allow reduced component size resulting in an efficient power converter apparatus with high power density

Description

[0001] A THREE-STATE SWITCHING CELL BASED FREQUENCY MULTIPLIER STRING PV POWER CONVERTER

[0002] TECHNICAL FIELD

[0003] The aspects of the disclosed embodiments relate generally to power conversion apparatus and more particularly to power converters used in photovoltaic inverter applications.

[0004] BACKGROUND

[0005] Two stage photovoltaic (PV) inverters having a DC-DC converter stage followed by a DC-AC inverter stage are commonly used for feeding solar power into the grid. Multilevel converter topologies such as T-type, flying capacitor, neutral point clamped, cascaded multilevel converters, and modular multilevel converters have been widely adopted for use to provide DC-AC power conversion in PV applications due to the many advantages they provide.

[0006] Despite their many advantages, developing a multi-level converter topology that can achieve high efficiency and high-power density remains a challenge. Bulky reactive components utilized for filtering impede development of high efficiency and high-power density converters. Reactive component size may be reduced by increasing the switching frequency of the converter. However, this leads to increased switching losses, lower efficiency, and increased heat.

[0007] To avoid the increased switching losses created by increasing the switching frequency, converters have been developed that can multiply the effective switching frequency in reactive components used for filtering. Unfortunately, most of these topologies can only double the switching frequency and often have increased component count, complex design and control requirements, and increased voltage levels which adversely affect power density, and efficiency.

[0008] Thus, there is a need for improved power converters which are suitable for PV DC-AC inverter applications, that reduce the size and cost of reactive components, and have better overall converter efficiency. Accordingly, it would be desirable to provide methods and apparatus that addresses at least some of the problems described above.

[0009] BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In the following detailed portion of the present disclosure, the invention will be explained in more detail with reference to the example embodiments shown in the drawings, in which like references indicate like elements and: Figure 1 illustrates a diagram of an exemplary three-state switching cell based single phase power converter apparatus incorporating aspects of the disclosed embodiments;

[0011] Figure 2 illustrates schematic diagrams showing switching states of an exemplary three-state switching cell incorporating aspects of the disclosed embodiments;

[0012] Figure 3 illustrates graphs showing exemplary operating waveforms of a three-state switching cell based single phase power inverter apparatus incorporating aspects of the disclosed embodiments;

[0013] Figure 4 illustrates graphs showing exemplary operating waveforms of a three-state switching cell based single phase power inverter apparatus incorporating aspects of the disclosed embodiments;

[0014] Figure 5 illustrates an exemplary three phase power converter apparatus incorporating aspects of the disclosed embodiments;

[0015] Figure 6 illustrates graphs showing exemplary operating waveforms of a three-phase power converter apparatus incorporating aspects of the disclosed embodiments;

[0016] Figure 7 illustrates graphs showing exemplary operating waveforms of a three-phase power converter apparatus incorporating aspects of the disclosed embodiments.

[0017] Figure 8 illustrates a schematic diagram showing an exemplary additional cascaded power unit incorporating aspects of the disclosed embodiments

[0018] DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENTS

[0019] Figure 1 illustrates a diagram of an exemplary three-state switching cell (TSSC) based power converter apparatus 100 incorporating aspects of the disclosed embodiments. The exemplary apparatus 100 is directed to an improved DC-AC power converter employing cascaded power units to achieve a multilevel voltage at the output terminals and frequency multiplication in the reactive components. The TSSC based frequency multiplier topologies disclosed herein provide bidirectional multilevel power conversion appropriate for use in string photovoltaic (PV) inverter applications that overcome many of the drawbacks of conventional power converters.

[0020] As is illustrated in Figure 1, in one embodiment, the three-state switching cell based power converter apparatus 100 comprises a first three-state switching cell 160 with a first switching leg 102, a second switching leg 104, and a first transformer T1. The first switching leg 102 and the second switching leg 104 are coupled in parallel with a first power source VD 1. A first winding 120 of the first transformer T1 is coupled between a midpoint 124 of the first switching leg 102 and an intermediate voltage (vi). A second winding 122 of the first transformer T1 is coupled between a midpoint 126 of the second switching leg 104 and the intermediate voltage (vi).

[0021] The three-state switching cell based power converter apparatus 100 also comprises a second three-state switching cell 162 with a third switching leg 106, a fourth switching leg 108, and a second transformer T2. The third switching leg 106 and the fourth switching leg 108 are coupled in parallel with the first power source VD 1.

[0022] The three-state switching cell based power converter apparatus 100 also comprises a third three-state switching cell 164 with a fifth switching 110 leg and a sixth switching leg 112. The fifth switching 110 leg and the sixth switching leg 112 are coupled in parallel with a next power source VD2, A first winding 136 of the second transformer T2 is coupled between a midpoint 130 of the fourth switching leg 108 and a midpoint 132 of the fifth switching leg 110. A second winding 138 of the second transformer T2 is coupled between a midpoint 128 of the third switching leg 106 and a midpoint 134 of the sixth switching leg 112.

[0023] The three-state switching cell based power converter apparatus 100 also comprises a fourth three-state switching cell 166 with a seventh switching leg 114, an eighth switching leg 116 and a third transformer T3. The seventh switching leg 114 and the eighth switching leg 116 are coupled in parallel with a next power source VD2. A first winding 144 of the third transformer T3 is coupled between a midpoint 140 of the seventh switching leg 114 and the intermediate voltage (vi). A second winding 146 of the third transformer T3 is coupled between a midpoint 142 of the eighth switching leg 116 and the intermediate voltage (vi).

[0024] A filter inductor (Lf) is connected in series between the intermediate voltage (vi) and a point of common coupling PCC and a filter capacitor (Cf) is connected in parallel with the point of common coupling PCC. The second power source VD2 and the next power source comprise the same power source VD2.

[0025] In one embodiment, the effective switching frequency is multiplied by a factor, such as eight, times the fundamental switching frequency of the TSSCs. Voltage and current ripple in the reactive components Lf, Cfare reduced proportionally to the increased effective frequency produced by the frequency multiplication features of the apparatus 100. Increased effective frequency and reduced ripple leads to significantly reduced size and volume of the reactive components Lf, Cf thereby providing highly efficient and high-power density converter topologies.

[0026] Generally, DC-AC inverters used in PV applications strive to: generate more levels in inverter voltage which results is a near sinusoidal and low total harmonic distortion (THD) inverter output voltage; minimize harmonics in grid current; reduce filtering requirements; enable use of low voltage switching devices (having more voltage levels reduces the voltage handled by each device); reduce the rate of voltage change (dv / dt) of the switching devices; have smaller common mode voltage and reduced leakage current, thereby allowing a smaller electro-magnetic interference (EMI) fdter; and enable fault- tolerant operation using redundant switching states and suitable control schemes. As will be discussed further below, the exemplary apparatus 100, and related topologies employing more than two cascaded power units 148, 150, address at least some the above goals.

[0027] The exemplary apparatus 100 is configured to receive a first DC power VDI and a second DC power VD2, such as the power delivered by a string PV source, and produce an AC output power vo, ioat a point of common connection PCC. For off-grid operation a suitable load Romay be connected to the PCC. For grid G connected operation, the load Romay be disconnected and a grid G may be connected to the PCC through a grid inductor Lgusing switches S„i, S„2, such as relays.

[0028] Power conversion in the exemplary apparatus 100 is achieved through the use of four coupled TSSCs 160, 162, 164, 166 configured to provide frequency multiplication in the reactive components and generate multilevel output voltage. Each TSSC includes two switching legs where, each switching leg 102, 104, 106, 108, 110, 112, 114, 116 comprises a first switching device Si, S3, 85, 87, 89, 811, S13, S15 connected in series with a second switching device 82,84,8 ,88,810, 812, 814, 816 and forming a midpoint 124, 126, 128, 130, 132, 134, 140, 142 between the first switching device and the second switching device, and wherein each switching device Si, S2, S3, S4, Ss, S , S7, Ss, S9, S10, Su, S12, S13, S14, S15, Sn comprises one of a silicon-carbide metal-oxide-semiconductor field-effect transistor (SIC MOSFET), an insulated gate bipolar transistor (IGBT), and a gallium -nitride metal-oxide-semiconductor field-effect transistor (GAN MOSFET).

[0029] As used herein, the term “switching leg” refers to a pair of series connected switching devices forming a circuit node, referred to herein as a midpoint, between the two switching devices. For example, the switching leg 102, includes two series connected switching devices Si, S2 forming a midpoint 124 between the two switching devices Si, S2.

[0030] In the illustrated embodiment, the exemplary apparatus 100 includes a first three-state switching cell 160 comprising a first switching leg 102, a second switching leg 104, and a first transformer Ti. The first switching leg 102 and the second switching leg 104 are coupled in parallel with a first power source VDI. A first winding 120 of the first transformer Ti is coupled between a midpoint 124 of the first switching leg 102 and an intermediate voltage v,, and a second winding 122 of the first transformer Ti is coupled between a midpoint 126 of the second switching leg 104 and the intermediate voltage v,. The intermediate voltage v, is generated at the output terminals 156, 158 of the cascaded power units 148 150 and may also be referred to as a terminal voltage. A second three-state switching cell 162 includes a third switching leg 106, a fourth switching leg 108, and a second transformer T2, where the third switching leg 106 and the fourth switching leg 108 are coupled in parallel with the first power source VDI.

[0031] A third three-state switching cell 164 includes a fifth switching 110 leg, a sixth switching leg 112, and a second transformer T2, where the fifth switching 110 leg and the sixth switching leg 112 are coupled in parallel with a second power source VD2. A first winding 136 of the second transformer T2 is coupled between a midpoint 130 of the fourth switching leg 108 and a midpoint 132 of the fifth switching leg 110, and a second winding 138 of the second transformer T2 is coupled between a midpoint 128 of the third switching leg 106 and a midpoint 134 of the sixth switching leg 112. Coupling the second TSSC 162 with the third TSSC 164 through a shared transformer T2 provides a cascading configuration within the apparatus 100 that creates an interdependence between the two power units 148, 150.

[0032] A fourth three-state switching cell 166 includes a seventh switching leg 114, an eighth switching leg 116, and a third transformer T3, where the seventh switching leg 114 and the eighth switching leg 116 are coupled in parallel with the second power source VD2. A first winding 144 of the third transformer T3 is coupled between a midpoint 140 of the seventh switching leg 114 and the intermediate voltage v,, and a second winding 146 of the third transformer T3 is coupled between a midpoint 142 of the eighth switching leg 116 and the intermediate voltage v,.

[0033] A controller 152 is configured to generate switching signals 154 configured to operate the four TSSCs 160, 162, 164, 166. Any suitable digital controller configured to generate switching signals capable of efficiently driving the switching devices Si, S2, S3, S4, Ss, Sg, S7, S8, S9, S10, Sn, S , Sn, Sn, Sis, S / 6may be advantageously employed as the controller 152. In certain embodiments, it may be advantageous to configure the controller 152 to employ pulse width modulation (PWM) when generating the switching signals 154. When desired the PWM may include a sinusoidal pulse width modulation (SPWM).

[0034] In the embodiment illustrated in Figure 1, the switching devices Si, S2, S3, S , S5, Sg, S7, S8, S9, S10, Sn, Si2, S1 , Sn, Sis, Sn are illustrated as metal oxide semiconductor field effect transistors (MOSFET). However, any suitable switching device, capable of switching the desired power at the desired frequencies, may be advantageously employed as any of the switching devices Si, S2, S , S4, Ss, Sg, S7, S8, S9, S10, Sn, Si2, S1 , S14, Sis, Sig without straying from the spirit and scope of the present disclosure.

[0035] It is instructive to view the apparatus 100 as having two cascaded power units 148, 150. The first power unit 148 receives power from a first source VDI and produces a first end 156 of the intermediate voltage Vi, while the second power unit 150 receives power from a second power source VD2 and produces the second end 158 of the intermediate voltage v,. Cascading of the first 148 and second 150 power units is accomplished through the second transformer T2 which is shared by the second TSSC 162 and the third TSSC 164. It is this coupling of two TSSCs through a shared transformer that contributes to the frequency multiplication and multilevel voltage generation features of the exemplary apparatus 100.

[0036] As will be described in more detail below, the intermediate voltage v, generated by the two cascaded power units 148, 150 is a sinusoidal AC voltage that includes nine voltage levels, and exposes the reactive elements Lf, Cf to a frequency that is eight times the switching frequency fsof the TSSCs. To remove the high frequency components from the intermediate voltage v,, an output fdter, formed by a series connected inductor Lf and a parallel connected capacitor Cf is included in the apparatus 100. The output filter is formed by a filter inductor Lf connected in series between one end 156 of the intermediate voltage Vi and a point of common coupling PCC, and a filter capacitor Cf connected across the output voltage voand in parallel with the point of common coupling PCC.

[0037] As an aid to understanding, the present disclosure describes power flow in one direction from a DC source to an AC output power. This described power flow is appropriate for use in PV inverter applications. It should be understood that the presently disclosed power converter apparatus and topologies, such as the apparatus 100, are not so limited and may, when desired, be operated to transfer power in either direction, from the sources VDI, VD2 to the PCC or from the PCC to the sources VDI, VD2, by controlling the switching signals 154 used to operate the four TSSCs 160, 162, 164, 166.

[0038] Maximum power point tracking (MPPT) is a technique used with variable power sources, such as PV solar based systems, and wind turbine based systems.. MPPT seeks to optimize the system as load characteristic change to maximize efficiency of the power transfer. The flexible control schemes afforded by the exemplary apparatus 100 make the disclosed embodiments ideally suited for use in string PV inverter applications that employ distributed MPPT to improve power extraction from the source PV panels VDI, VD2. Also, the multilevel output produced by the exemplary apparatus 100 yields very low total harmonic distortion (THD) in both the output voltage and output current.

[0039] In the exemplary apparatus 100 the first power unit 148 receives power from a first source VDI, and the second power unit 150 receives power from a second source VD2. Due to the cascading nature of the disclosed topology, the exemplary apparatus 100 may be scaled up to consume any desired number of two or more power sources by including an additional cascaded power unit for each additional power source. As will be described further below, additional cascaded power units may be added between the first power unit 148 and the second power unit 150 at the point marked by numeral 168.

[0040] The number of voltage levels, effective switching frequency, and peak to peak inductor voltage of a three-state switching cell based power converter apparatus, such as the exemplary apparatus 100, may be calculated based on characteristics of the herein disclosed converter topology. The number of voltage levels generated in the intermediate voltage v, is given by equation 1 : NL= (4 * NJ + 1 = (4 * + 1 ,

[0041] (1) where NL is the number of voltage levels in the intermediate voltage v,, N is the number of input sources, and Nuis the number of cascaded power units. The effective switching frequency to which the fdter components are exposed is given by equation (2): fe = (4 * ) = (4 * Nu),(2)where feis the effective switching frequency, and N and Nuare as given above. Peak-to-peak inductor voltage ripple is given by equation (3):

[0042] _ VDl or VD2 > VD1 or VD2

[0043] VP~P ~ Ni ~ Nu (3) where vp.pis the peak-to-peak voltage ripple across the fdter inductor Lf. Peak-to-peak inductor voltage and effective switching frequency are the key design parameters used for selection of the fdter inductor Lf. Higher frequency and lower peak-to-peak voltage reduce the size of the fdter inductor Lf, thereby increasing power density and converter efficiency.

[0044] A conventional TSSC includes two switching legs and one autotransformer. This would indicate the exemplary apparatus 100, which includes four TSSCs 160, 162, 164, 166, would include four autotransformers, one for each TSSC. However, in the exemplary topology incorporated into the apparatus 100, the power units 148, 150 are cascaded by coupling two TSSCs 162, 164 through a single shared transformer T2. It can be shown that each of the coupled TSSCs 162, 164 retain their basic characteristics, i.e. the three voltage levels, of a TSSC even though the auto transformer is replaced with a single shared transformer T2.

[0045] Figure 2 illustrates schematic diagrams 202, 204, 206, 208 showing switching states of an exemplary TSSC incorporating aspects of the disclosed embodiments. Each TSSC includes two switching legs 210, 212 formed by four switching devices Si, S2, S3, S4. A transformer T is configured as an autotransformer with a first end of each transformer winding 214, 216 coupled to a respective one midpoint 218, 220 of the switching legs 210, 212, and a second end of each transformer winding 222, 224 coupled to a load 226. A transformer configured in this fashion may be referred to as an autotransformer. When a TSSC transitions through its switching states 202, 204, 206, 208 it produces three level voltages VTO at its output terminals: VD, VD / 2, and 0, where VD represents the input voltage. As used herein, a switch or switching device is referred to as ‘on’ or ‘turned on’ when it is conducting current, and is referred to as ‘off or ‘turned off when it is not conducting current. In the TSSCs 202, 204, 206, 208 illustrated in Figure 2, switching devices that are turned off are shown with grey shaded lines and switching devices that are turned on are shown with black lines.

[0046] In the first switching state 202, switching devices Si and S3 are on, and switching devices S2and S4 are off, resulting in an output voltage VTO across the load 226 equal to the input voltage VD. In the next switching state 204, switching devices S2and S3 are on and switching devices Si and S4 are off, resulting in an output voltage VTO across the load 226 equal to half of the input voltage VTO = VD / . In a third switching state 206, switching devices Si and S4 are on and switching devices S2and S3 are off, resulting in an output voltage VTO across the load 226 equal to half of the input voltage VTO = VD / . In the final switching state 208, switching devices S2 and S4 are on and switching devices Si and S3 are off, resulting in an output voltage VTO across the load 226 equal zero volts VTO = 0.

[0047] As described above, the topology employed in the exemplary apparatus 100 achieves multilevel intermediate voltage v, and frequency multiplication in the reactive components Lf, Cf through the use of four TSSCs configured as two cascaded power units 148, 150. Each TSSC 160, 162, 164, 166 produces three voltage levels (VD, 0.5VD, 0). When operated with phase shifted switching, the two power units 148, 150 produce nine output voltage levels (2VD, I.5VD, VD, 0.5VD, 0, -2VD, -1.5VD, - VD, -0.5VD). The intermediate voltage v, produced by the exemplary apparatus 100 can be derived based on the terminal voltages of each TSSC 160, 162, 164, 166 as shown in equation 4:

[0048] Vi=(Vroi + ^7’03)—(VTO2 + ^TO ), where VT01. VT02, VT03, and VT04represent the terminal voltages generated by each of the four TSSCs 160, 162, 164, 166 respectively.

[0049] Each TSSC is capable of producing three voltage levels (VD, 0.5VD, 0). By configuring the controller 152 to employing phased switching of each TSSC, nine different voltage levels may be generated in the intermediate voltage v, by the apparatus 100. Replacing each TSSC terminal voltage VT01. VT02, VT03, and VT04in equation 3 with its three possible output values and computing the permutations, yields nine voltage levels as shown in equation 4: v0= {(VD, 0.5Ffl, 0 ) + (VD, 0.5VD, 0 )} - {(VD, 0.5Ffl, 0 ) + (VD, 0.5VD, 0 )}

[0050] = (2VD, 1.5VD, VD, 0.5VD, 0, -0.57D, -VD, -1.5VD, -2VD) For example, when 2VD is required at the intermediate voltage v,, TSSCs 160 and 164 will be operated to produce their maximum output voltage (VD), and TSSCs 162 and 166 will be operated to produce zero (0) volts. Then in accordance with equation 3, the intermediate voltage v,, of the cascaded power units 148, 150, will equal 2VD.

[0051] Figure 3 illustrates graphs 300 showing exemplary operating waveforms of a three-state switching cell based single phase power inverter apparatus incorporating aspects of the disclosed embodiments. The exemplary operating waveforms 300 illustrate the frequency multiplication and multilevel output voltage generation capabilities of a DC-AC power converter such as the exemplary three-state switching cell based power inverter apparatus 100 described above and with reference to Figure 1. In the graphs 300, time is depicted along a horizontal axis 302 increasing to the right, and signal magnitude (volts or amperes) is depicted along vertical axes 304 increasing upwards.

[0052] Those skilled in the art will readily recognize that the exemplary apparatus 300 may be beneficially operated in a variety of power conversion scenarios including bi-directional operation. However, as an aid to understanding, the operating waveforms shown in graphs 300 correspond one exemplary operating scenario where the exemplary apparatus 100 receives a three hundred volt (300v) DC input power at each source VDI, VD2, and produces a five hundred volt (500v) fifty hertz (50Hz) AC output voltage, with a converter switching frequency of ten kilohertz (10kHz).

[0053] Graph 306 shows the 10kHz carrier signal used to generate switching signals for the four TSSCs 160, 162, 164, 166. Voltages VL across the filter inductor Lf is shown in graph 308 and illustrates the frequency multiplication characteristics of the apparatus 100. The multilevel intermediate voltage v, is shown in graph 310, and the output current i„ in amperes and output voltage voin volts delivered to a load connected to the PCC, are shown in graphs 312 and 314 respectively.

[0054] As may be observed in graph 310, the exemplary converter apparatus 100 is producing nine voltage levels at the intermediate voltage v, as indicated by equation 3. A perfectly sinusoidal output voltage voand output current i„ are produced as shown in graphs 312 and 314 respectively.

[0055] Figure 4 illustrates graphs 400 showing exemplary operating waveforms of a three-state switching cell based single phase power inverter apparatus incorporating aspects of the disclosed embodiments. The graphs 400 illustrate the time slice 316 shown in the graphs 300 with an expanded time scale to better show the frequency multiplication features of the exemplary apparatus 100. In the graphs 400, time is depicted along a horizontal axis 402 increasing to the right and signal magnitude (volts or amperes) is depicted along the vertical axes 404 increasing upwards. Graph 406 shows the 10kHz carrier signal used to generate switching signals for the four TSSCs. Voltage VL across the filter inductor Lf is shown in graph 408 with the corresponding intermediate voltage Vi shown in graph 410. Graphs 412 and 414 show the corresponding output current i„ in amperes and output voltage voin volts.

[0056] The carrier signal 406 has a fundamental switching frequency, referred to herein as the switching frequency fs, of ten kilohertz (10kHz). Equation 2 predicts that the effective frequency applied to reactive components of a converter apparatus 100 having two cascaded power units is eight times the switching frequency, or eighty kilohertz (80kHz) in the illustrated example. Graph 408 shows the inductor is charging and discharging eight times in each switching cycle 408 providing an inductor operating frequency of eighty kilohertz (80kHz), which, as predicted by equation 2, is eight times the switching frequency of 10kHz.

[0057] Due to the multilevel voltage generation, the inductor ripple becomes smaller as shown by equation 3. The peak-to-peak ripple as given by equation 3 is the source voltage divided by the number of sources, which in the illustrated example is 300volts / 2sources=150v. The expected inductor ripple can be observed in the inductor voltage VL illustrated in graph 408.

[0058] Figure 5 illustrates an exemplary three phase power converter apparatus 500 incorporating aspects of the disclosed embodiments. The apparatus 500 of the disclosed embodiments incorporates three TSSC based frequency multiplying power converters 100a, 100b, 100c configured to produce three phase power having multilevel intermediate voltages vja, vn,. v,cand frequency multiplied ripple voltages on the reactive components Lfa, Lib, LfC, Cfa, Cfb, Cfc. The exemplary apparatus 500 is suitable for operation as an inverter in PV power systems where the apparatus 500 receives power from string PV sources and delivers three-phase AC power to a grid.

[0059] The exemplary three phase power converter apparatus 500 includes: a first power converter apparatus 100a configured to receive power from a first power source 502 and a second power source 504, and produce a first intermediate voltage vja; a second power converter apparatus 100b configured to receive power from a third power source 506 and a fourth power source 508, and produce a second intermediate voltage Vit, and a third power converter apparatus 100c configured to receive power from a fifth power source 510 and a sixth power source 512, and produce a third intermediate voltage vje.

[0060] One or more of the first power converter apparatus 100a, the second power converter apparatus 100b, and the third power converter apparatus 100c include a TSSC based power converter apparatus such as the exemplary power converter apparatus 100 described above and with respect to Figure 1. Any of the above-described embodiments of the exemplary power converter apparatus 100 may be advantageously employed as the power converter apparatuses 100a, 100b, 100c included in the three-phase power converter apparatus 500.

[0061] In embodiments where the exemplary apparatus 500 is being operated as an inverter, it may be beneficial to include filtering on the AC side or output of each phase to remove unwanted higher frequencies and reduce THD from the intermediate voltage. Filtering of the output voltage may be achieved by coupling a filter inductor in series between the intermediate voltage and the load. In one embodiment, the first intermediate voltage vja, the second intermediate voltage vu>, and the third intermediate voltage vJCare each coupled to one phase of a three-phase load 516 through a respective one of the first filter inductor Lfa, the second filter inductor Lfb, and a third filter inductor LfC.

[0062] When desired, additional filtering may be provided by coupling a filter capacitor in parallel with each phase of the three-phase load 516. In certain embodiments, this additional filtering may be achieve using three star connected filter capacitors 514 coupled to the first filter inductor Lfa, the second filter inductor Lfb, and a third filter inductor LfC.

[0063] As described above, the exemplary apparatus 100, which is employed in each phase of the exemplary apparatus 500, may be beneficially operated by driving each TSSC based on pulse width modulation techniques. When included in a three-phase power conversion apparatus, such as the exemplary three- phase power conversion apparatus 500, the pulse width modulation may be configured as one of a sinusoidal pulse width modulation, a space vector pulse width modulation, and discontinuous pulse width modulation. The control flexibility provided by using pulse width modulation techniques to drive each TSSC based power converter 100a, 100b, 100c, allows each phase of the converter to be optimized as the operating point of the sources 502, 504, 506, 508, 510, 512 varies. This control flexibility and optimization is advantageous in systems that may benefit from MPPT.

[0064] Figure 6 illustrates graphs 600 showing exemplary operating waveforms of a three-phase power converter apparatus incorporating aspects of the disclosed embodiments. The exemplary operating waveforms 600 illustrate the frequency multiplication and multilevel output voltage generation capabilities of a three phase DC-AC power converter such as the exemplary three-phase power converter apparatus 500 described above and with reference to Figure 5.

[0065] The graphs 600, time is depicted along a horizontal axis 602 increasing to the right and signal magnitude (volts or amperes) is depicted along the vertical axes 604 increasing upwards. Waveforms shown in graphs 614, 616, and 618 include operating waveforms for each of the three phases 100a, 100b, and 100c and are marked with corresponding subscripts a, b, c respectively. While the exemplary apparatus 500 may be beneficially operated in a variety of power conversion scenarios, as an aid to understanding, the operating waveforms shown in graphs 600 correspond one possible scenario where the exemplary apparatus 500 receives three hundred volt (300v) DC input power at each power source 502, 504, 506, 508, 510, 512, and produces a five hundred volt (500v) fifty hertz (50Hz) three phase AC output power, with a converter switching frequency of ten kilohertz (10kHz).

[0066] Graph 606 shows a 10kHz carrier signal used to generate switching signals for the three power converter apparatuses 100a, 100b, 100c. Voltages V „. V / ,. and vcacross the three filter inductors La, Lb, and Lccan be seen in graphs 612, 610, 608 respectively. The multilevel intermediate voltages vja, Vit, v,care shown in graph 614 and illustrate the multilevel intermediate voltages generated by the exemplary apparatus 500. Graphs 616 and 618 show the output current loa-> lob-, loc in amperes and output voltage Voa, Vob, Voc in volts delivered to the three phase load 516. It should be noted that each of the power converter apparatuses 100a, 100b, 100c includes a power converter apparatus, such as the exemplary apparatus 100 described above, therefore the operating waveforms produced by each phase a, b, c, have similar characteristics as the operating waveforms described above and with respect to Figures 3 and 4.

[0067] Figure 7 illustrates graphs 700 showing exemplary operating waveforms of a three-phase power converter apparatus, such as the exemplary apparatus 500, incorporating aspects of the disclosed embodiments. The graphs 700 illustrate a time slice 626 shown in the graphs 600 with an expanded time scale to better show the frequency multiplication features of the exemplary apparatus 500. In the graphs 700, time is depicted along a horizontal axis 702 increasing to the right and signal magnitude (volts or amperes) is depicted along the vertical axes 704 increasing upwards. Waveforms shown in graphs 714, 716, and 718 include operating waveforms for each of the three phases 100a, 100b, and 100c and are marked with corresponding subscripts a, b, c respectively.

[0068] Graph 706 shows the 10kHz carrier signal used to generate switching signals for the three power converter apparatuses 100a, 100b, 100c. Voltages vi.a. VU,. and vcacross the three filter inductors La, Lb, and Lccan be seen in graphs 712, 710, 808 respectively. The multilevel intermediate voltages vja, Vib, ViCare shown in graph 714 and illustrate the multilevel intermediate voltages generated by each of the phases of the exemplary apparatus 500. Graphs 716 and 718 show the output current ioa, iob, ioc in amperes and output voltage voa, „b,ocin volts delivered to the three phase load 516.

[0069] The carrier signal 706 has a fundamental switching frequency, or switching frequency fs, often kilohertz (10kHz). Equation 2 predicts that the effective switching frequency of each converter apparatus 100a, 100b, 100c having two cascaded power units is eight times the switching frequency, which will be eighty kilohertz (80kHz) in the illustrated example. Graphs 708, 710, and 712 show each filter inductor La, Lb, and Lcis charging and discharging eight times in each switching cycle 720 providing an inductor operating frequency of eighty kilohertz (80kHz), which, as predicted by equation 2, is eight times the switching frequency of 10kHz.

[0070] As discussed above, the multilevel voltage generated by each power converter 100a, 100b, 100c, reduces the inductor voltage ripple as shown by equation 3. The expected inductor voltage ripple, with reduced amplitude and increased frequency, can be observed in the inductor voltages V «. VI ,. VCillustrated in graph 708.

[0071] Figure 8 illustrates a schematic diagram showing an exemplary additional cascaded power unit 800 incorporating aspects of the disclosed embodiments. The exemplary additional power unit 800 is appropriate for adding additional power sources VDA to a three-state switching cell based power converter apparatus, such as the exemplary apparatus 100 described above and with respect to Figure 1. As discussed above, the exemplary apparatus 100 may be scaled up to consume any desired number of two or more power sources by including additional cascaded power units, such as the exemplary cascaded power unit 800, between the first power unit 148 and the second power unit 150 at the point 168.

[0072] The exemplary additional cascaded power unit 800 receives power from an additional power source VDA, and includes a first additional TSSCs 804 and a second additional TSSC 806 coupled in parallel with an additional power source VDA. The midpoints 812, 814 of the first additional TSSC 804 are coupled to a shared transformer within a prior power unit 816, and the midpoints 808,810 of the second additional TSSC 806 are coupled through a shared transformer TA to the next power unit 818.

[0073] For example, consider adding an additional power unit, such as the exemplary additional power unit 800, to the exemplary apparatus 100 at the point 168, for a total of three power sources. When this is done, the first power unit 148 becomes the prior power unit 816, and midpoints 812, 814 of the first additional TSSC 804 are coupled to the shared transformer T2 in the first power unit 148. The second power unit 150 becomes the next power unit 818 and midpoints 808, 810 of the second additional power unit 806 are coupled through the additional transformer to midpoints 132, 134 of the third TSSC 164. Any desired number of additional power units 800 may be added to the exemplary apparatus 100 by inserting an additional power unit 800 between a prior power unit and a next power unit as described above.

[0074] Inclusion of additional power units, such as the exemplary additional power unit 800 increases the number of voltage levels in the intermediate voltage v, and increases the effective frequency applied to the reactive components in the output filter Lf, Cf. Thus, while there have been shown, described, and pointed out, fundamental novel features of the invention as applied to the exemplary embodiments thereof, it will be understood that various omissions, substitutions and changes in the form and details of devices and methods illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit and scope of the presently disclosed invention. Further, it is expressly intended that all combinations of those elements, which perform substantially the same function in substantially the same way to achieve the same results, are within the scope of the invention. Moreover, it should be recognized that structures and / or elements shown and / or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.

Claims

CLAIMS1. A three-state switching cell based power converter apparatus (100) comprising: a first three-state switching cell (160) comprising a first switching leg (102), a second switching leg (104), and a first transformer (Ti), wherein the first switching leg (102) and the second switching leg (104) are coupled in parallel with a first power source (VDI), and wherein a first winding (120) of the first transformer (Ti) is coupled between a midpoint (124) of the first switching leg (102) and an intermediate voltage (v,), and a second winding (122) of the first transformer (Ti) is coupled between a midpoint (126) of the second switching leg (104) and the intermediate voltage (v,); a second three-state switching cell (162) comprising a third switching leg (106), a fourth switching leg (108), and a second transformer (T2), wherein the third switching leg (106) and the fourth switching leg (108) are coupled in parallel with the first power source (VDI); a third three-state switching cell (164) comprising a fifth switching (110) leg, and a sixth switching leg (112), wherein the fifth switching (110) leg and the sixth switching leg (112) are coupled in parallel with a next power source (VD2), and wherein a first winding (136) of the second transformer (T2) is coupled between a midpoint (130) of the fourth switching leg (108) and a midpoint (132) of the fifth switching leg (110), and a second winding (138) of the second transformer (T2) is coupled between a midpoint (128) of the third switching leg (106) and a midpoint (134) of the sixth switching leg (112); a fourth three-state switching cell (166) comprising a seventh switching leg (114), an eighth switching leg (116), and a third transformer (T3), wherein the seventh switching leg (114) and the eighth switching leg (116) are coupled in parallel with a next power source (VD2), and wherein a first winding (144) of the third transformer (T3) is coupled between a midpoint (140) of the seventh switching leg (114) and the intermediate voltage (v,), and a second winding (146) of the third transformer (T3) is coupled between a midpoint (142) of the eighth switching leg (116) and the intermediate voltage (v,); a filter inductor (Lf) connected in series between the intermediate voltage (v,) and a point of common coupling (PCC); and a filter capacitor (Cf) connected in parallel with the point of common coupling (PCC), wherein the second power source (VD2) and the next power source comprise the same power source (VD2).

2. The apparatus (100) according to claim 1 wherein, each switching leg (102, 104, 106, 108, 110, 112, 114, 116) comprises a first switching device (A / , A,?, A , A- , A / / , A / ,?, A / ) connected in series with a second switching device (A2, A\ AV,, A\ A / «, AA, AA, A / ,) forming the midpoint (124, 126, 128, 130, 132, 134, 136, 138) between the first switching device and the second switching device, and wherein each switching device (Si, S2, S3, Si, 5, Sg, S?, Ss, S9, S10, Sn, S12, S13, Su, S15,l) comprises one of asilicon-carbide metal-oxide-semiconductor field-effect transistor, an insulated gate bipolar transistor, and a gallium-nitride metal-oxide-semiconductor field-effect transistor.

3. The apparatus (100) according to any one of the preceding claims further comprising an additional power unit (800), wherein the additional power unit (800) comprises a first additional three state switching cell (804) and a second additional three state switching cell (806) coupled in parallel with an additional power source (VDA), and wherein a first and a second midpoint (812, 814) of the first additional three state switching cell (804) are coupled to a shared transformer (T2) in a prior power unit (148), and a first and a second midpoint 808, 810 of the second additional three state switching cell (806) are coupled through an additional transformer (TA) to the next power unit 150.

4. The apparatus (100) according any one of the preceding claims wherein, a first input capacitor (Cm) is coupled in parallel with the first power source (VDI) and a second input capacitor (CD2) is coupled in parallel with the second power source (VD2).

5. The apparatus (100) according any one of the preceding claims wherein, the apparatus (100) further comprises a controller (154) configured to operate the first three-state switching cell (160), the second three-state switching cell (162), the third three-state switching cell (164), and the fourth three-state switching cell (166) based on a pulse width modulation and phase shifted switching to produce a sinusoidal intermediate voltage (v,) comprising nine voltage levels (310), and a voltage across the filter inductor (408) comprising a ripple frequency ( )) eight times of a switching frequency (406).

6. The apparatus (100) according to any one of the preceding claims wherein, the pulse width modulation comprises a sinusoidal pulse width modulation.

7. The apparatus (100) according any one of the preceding claims wherein, the first power source (VDI) and the second power source (V^) each comprise one or more photo-voltaic panels.

8. The apparatus (100) according any one of the preceding claims wherein, the point of common connection is coupled to a power grid.

9. A three phase power converter apparatus (500), the apparatus (500) comprising: a first power converter apparatus (100a) configured to receive power from a first power source (502) and a second power source (504), and produce a first intermediate voltage (vja); a second power converter apparatus (100b) configured to receive power from a third power source (506) and a fourth power source (508), and produce a second intermediate voltage (v ) a third power converter apparatus (100c) configured to receive power from a fifth power source (510) and a sixth power source (512), and produce a third intermediate voltage (vJC),wherein one or more of the first power converter apparatus (100a), the second power converter apparatus (100b), and the third power converter apparatus (100c) comprise a three-state switching cell based power converter apparatus (100) according to any one of the preceding claims.

10. The apparatus 500 according to the preceding claim wherein, the first intermediate voltage (vja), the second intermediate voltage (v»), and the third intermediate voltage (vJC) are each coupled to a three- phase load ( 16) through a respective one of the first filter inductor (Lfa), the second filter inductor (Lfb), and a third filter inductor (LfC).

11. The apparatus (500) according to the preceding claims 8 or 9 wherein, three star connected filter capacitors (514) are coupled to the first filter inductor (Lfa), the second filter inductor (Lfb), and a third filter inductor (LfC).

12. The apparatus (500) according to any one of the preceding claims 8 through 10 wherein, one or more of the first power source (502), the second power source (504), the third power source (506), the fourth power source (508), the fifth power source (510), and the sixth power source (512) comprise a photovoltaic panel.

13. The apparatus (500) according to any one of the preceding claims 8 through 11 wherein, the three- phase load (516) comprises a power grid.

14. The apparatus (500) according to any one of the preceding claims wherein, each power converter apparatus (100a, 100b, 100c) is operated with pulse width modulation, and the pulse width modulation comprises one of a sinusoidal pulse width modulation, a space vector pulse width modulation, and discontinuous pulse width modulation.

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