WO2020108460A1

WO2020108460A1 - Three-level power conversion system and method

Info

Publication number: WO2020108460A1
Application number: PCT/CN2019/120772
Authority: WO
Inventors: Huibin Zhu; Heping Dai
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2018-11-26
Filing date: 2019-11-26
Publication date: 2020-06-04
Also published as: EP3881422A4; CN112868172A; EP3881422A1; CN112868172B

Abstract

A system comprises a first three-level power converter having output terminals coupled to a three-level voltage bus apparatus and input terminals configured to be coupled with a solar panel array, an inverter coupled between the three-level voltage bus apparatus and an electrical grid and a second three-level power converter coupled between the three-level voltage bus apparatus and an energy storage unit.

Description

Three-Level Power Conversion System and Method

This patent application claims priority to U.S. Provisional Application No. 62/771,438, filed on November 26, 2018 entitled “Three-Level Power Conversion System and Method” which is hereby incorporated by reference herein as if reproduced in its entirety.

TECHNICAL FIELD

The present disclosure relates to a three-level power bus apparatus and method, and, in particular embodiments, to a three-level power bus apparatus and method for transferring energy in solar power conversion systems.

BACKGROUND

Renewable energy sources include solar energy, wind power, tidal wave energy and the like. A solar power conversion system may include a plurality of solar panels coupled in series or in parallel. The output of the solar panels may generate a variable dc voltage depending on a variety of factors such as time of day, location and sun tracking ability. In order to regulate the output of the solar panels, the output of the solar panels may be coupled to a dc/dc converter so as to achieve a regulated output voltage at the output of the dc/dc converter. In addition, the solar panels may be connected with a backup battery system through a battery charge control apparatus. During the day, the backup battery is charged through the output of the solar panels. When the power utility fails or the solar panels are an off-grid power system, the backup battery provides electricity to the loads coupled to the solar panels.

Since the majority of applications may be designed to run on 120 volts ac power, a solar inverter is employed to convert the variable dc output of the photovoltaic modules to a 120 volts ac power source. A plurality of multilevel inverter topologies may be employed to achieve high power as well as high efficiency conversion from solar energy to utility electricity. In particular, a high power ac output can be achieved by using a series of power semiconductor switches to convert a plurality of low voltage dc sources to a high power ac output by synthesizing a staircase voltage waveform.

Multilevel inverters may be divided into three categories, namely diode clamped multilevel inverters, flying capacitor multilevel inverters and cascaded H-bridge multilevel inverters. Furthermore, multilevel inverters may employ different pulse width modulation (PWM) techniques such as sinusoidal PWM (SPWM) , selective harmonic elimination PWM, space vector modulation and the like. Multilevel inverters are a common power topology for high and medium power applications such as utility interface for renewable power sources, flexible ac transmission systems, medium voltage motor drive systems and the like.

The diode clamped multilevel inverter is commonly referred to as a three-level neutral point clamped (NPC) inverter. A three-level NPC inverter requires two series connected capacitors coupled between the input dc buses. Each capacitor is charged to an equal potential. Furthermore, the three-level NPC inverter may comprise four switching elements and two clamping diodes. The clamping diodes help to reduce the voltage stress on the switching element to one capacitor voltage level.

An NPC inverter utilizes a staircase waveform to generate an ac output. Such a staircase waveform resembles a desired sinusoidal waveform. As a result, the output voltage of the NPC inverter may be of a low total harmonic distortion (THD) . In addition, the staircase waveform may reduce the voltage stresses. As a result, the electromagnetic compatibility (EMC) performance of the NPC inverter may be improved.

SUMMARY

These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present disclosure which provide a three-level voltage bus for improving the performance of solar power systems.

In accordance with an embodiment, an apparatus comprises a first voltage bus, a second voltage bus and a third voltage bus. The first voltage bus is connected to a plurality of three-level power conversion units. The second voltage bus is connected to the plurality of three-level power conversion units. The third voltage bus connected to the plurality of three-level power conversion units. The first voltage bus and the third voltage bus are regulated with reference to the second voltage bus through at least one three-level power conversion unit of the plurality of three-level power conversion units.

The plurality of three-level power conversion units includes a three-level boost converter, a three-level buck converter, an inverter and an inductor-inductor-capacitor (LLC) converter, and wherein output/input terminals of the three-level boost converter, the three-level buck converter, the inverter and the LLC converter are connected together through the first voltage bus, the second voltage bus and the third voltage bus. The three-level boost converter is connected between a solar panel array and the first voltage bus, the second voltage bus and the third voltage bus. The three-level buck converter is connected between an energy storage unit and the first voltage bus, the second voltage bus and the third voltage bus. The inverter is connected between an electrical grid and the first voltage bus, the second voltage bus and the third voltage bus. The LLC converter is connected between a dc load and the first voltage bus, the second voltage bus and the third voltage bus.

The energy storage unit is a battery based energy storage unit. The dc load comprises a charger configured to charge electric vehicles. In some embodiments, the inverter is a neutral point clamped inverter, and wherein a neutral point of the inverter is connected to the second voltage bus. In alternative embodiments, the inverter comprises a first neutral point clamped inverter, a second neutral point clamped inverter and a third neutral point clamped inverter. The first neutral point clamped inverter is connected to a first phase of a three-phase power system through a first wye-delta transformer. The second neutral point clamped inverter is connected to a second phase of the three-phase power system through a second wye-delta transformer. The third neutral point clamped inverter is connected to a third phase of the three-phase power system through a third wye-delta transformer. Neutral wires of the first wye-delta transformer, the second wye-delta transformer and third wye-delta transformer are connected to the second voltage bus.

In some embodiments, the plurality of three-level power conversion units includes a first three-level boost converter, a second three-level boost converter, an inverter and a three-level buck converter connected together through the first voltage bus, the second voltage bus and the third voltage bus. In alternative embodiments, the plurality of three-level power conversion units includes a first LLC converter, a second LLC converter, an inverter and a three-level boost converter connected together through the first voltage bus, the second voltage bus and the third voltage bus.

In some embodiments, the plurality of three-level power conversion units includes an LLC converter, a dual-output LLC converter, an inverter and a three-level boost converter connected together through the first voltage bus, the second voltage bus and the third voltage bus. The three-level boost converter is connected between a dual active bridge converter and the first voltage bus, the second voltage bus and the third voltage bus. A first output and a second output of the dual-output LLC converter are stacked together. A common node of the first output and the second output of the dual-output LLC converter is connected to the second voltage bus.

In accordance with another embodiment, a method comprises transferring energy from a power source to a three-level voltage bus apparatus through a first three-level power converter. The method further comprises transferring energy from the three-level voltage bus apparatus to an electrical grid through an inverter and transferring energy between the three-level voltage bus apparatus and an energy storage unit through a second three-level power converter.

The method further comprises transferring energy from the three-level voltage bus apparatus to a load through an LLC power converter, regulating voltages of the three-level voltage bus apparatus and configuring the second three-level power converter to operate as a bidirectional power converter transferring energy between the three-level voltage bus apparatus and the energy storage unit.

The three-level voltage bus apparatus comprises a first voltage bus, a second voltage bus and a third voltage bus connected to the first three-level power converter, the second three-level power converter, the inverter and the LLC power converter, and wherein the first voltage bus and the third voltage bus are regulated with reference to the second voltage bus.

The first three-level power converter is a three-level boost converter comprising four switches connected in series, and wherein a middle point of the four switches of the three-level boost converter is connected to the second voltage bus. The second three-level power converter is a three-level buck converter comprising four switches connected in series, and wherein a middle point of the four switches of the three-level buck converter is connected to the second voltage bus. The inverter is a neutral point clamped inverter having a neutral point connected to the second voltage bus. The LLC power converter comprises four switches connected in series, and wherein a middle point of the four switches of the LLC power converter is connected to the second voltage bus.

In accordance with yet another embodiment, a system comprises a first three-level power converter having output terminals connected to a three-level voltage bus apparatus and input terminals configured to be coupled with a solar panel array. The system further comprises an inverter connected between the three-level voltage bus apparatus and an electrical grid, and a second three-level power converter connected between the three-level voltage bus apparatus and an energy storage unit.

The three-level voltage bus apparatus comprises a first voltage bus, a second voltage bus and a third voltage bus. The first voltage bus is connected to a positive output terminal of the first three-level power converter. The second voltage bus is connected to a neutral point of the first three-level power converter. The third voltage bus is connected to a negative output terminal of the first three-level power converter, and wherein voltages on the first voltage bus, the second voltage bus and the third voltage bus are regulated by the first three-level power converter.

The system further comprises an LLC converter connected between a dc load and the three-level voltage bus apparatus. The dc load includes a plurality of electrical vehicles. An advantage of an embodiment of the present disclosure is a three-level voltage bus connecting a plurality of three-level power conversion units so as to improve the efficiency, reliability and cost of the solar power system.

The foregoing has outlined rather broadly the features and technical advantages of the present disclosure in order that the detailed description of the disclosure that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter which form the subject of the claims of the disclosure. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present disclosure. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the disclosure as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

Figure 1 illustrates a block diagram of a three-level power conversion system in accordance with various embodiments of the present disclosure;

Figure 2 illustrates a block diagram of a first implementation of a portion of the three-level power conversion system shown in Figure 1 in accordance with various embodiments of the present disclosure;

Figure 3 illustrates a schematic diagram of the three-level power conversion system shown in Figure 2 in accordance with various embodiments of the present disclosure;

Figure 4 illustrates a block diagram of a second implementation of the portion of the three-level power conversion system shown in Figure 2 in accordance with various embodiments of the present disclosure;

Figure 5 illustrates a schematic diagram of the three-level power conversion system shown in Figure 4 in accordance with various embodiments of the present disclosure;

Figure 6 illustrates a block diagram of a first implementation of another portion of the three-level power conversion system shown in Figure 1 in accordance with various embodiments of the present disclosure;

Figure 7 illustrates a schematic diagram of the three-level power conversion system shown in Figure 6 in accordance with various embodiments of the present disclosure;

Figure 8 illustrates a block diagram of a second implementation of another portion of the three-level power conversion system shown in Figure 1 in accordance with various embodiments of the present disclosure;

Figure 9 illustrates a schematic diagram of the three-level power conversion system shown in Figure 8 in accordance with various embodiments of the present disclosure;

Figure 10 illustrates a block diagram of a third implementation of another portion of the three-level power conversion system shown in Figure 1 in accordance with various embodiments of the present disclosure;

Figure 11 illustrates a schematic diagram of the three-level power conversion system shown in Figure 10 in accordance with various embodiments of the present disclosure;

Figure 12 illustrates a block diagram of yet another portion of the three-level power conversion system shown in Figure 1 in accordance with various embodiments of the present disclosure;

Figure 13 illustrates a schematic diagram of the three-level power conversion system shown in Figure 12 in accordance with various embodiments of the present disclosure;

Figure 14 illustrates a block diagram of another three-level power conversion system in accordance with various embodiments of the present disclosure;

Figure 15 illustrates a schematic diagram of the three-level power conversion system shown in Figure 14 in accordance with various embodiments of the present disclosure;

Figure 16 illustrates a block diagram of yet another three-level power conversion system in accordance with various embodiments of the present disclosure;

Figure 17 illustrates a schematic diagram of a portion of the three-level power conversion system shown in Figure 16 in accordance with various embodiments of the present disclosure;

Figure 18 illustrates a schematic diagram of another portion of the three-level power conversion system shown in Figure 16 in accordance with various embodiments of the present disclosure;

Figure 19 illustrates a block diagram of yet another three-level power conversion system in accordance with various embodiments of the present disclosure;

Figure 20 illustrates a schematic diagram of a portion of the three-level power conversion system shown in Figure 19 in accordance with various embodiments of the present disclosure;

Figure 21 illustrates a schematic diagram of another portion of the three-level power conversion system shown in Figure 19 in accordance with various embodiments of the present disclosure; and

Figure 22 illustrates a flow chart of a method for controlling the three-level power conversion system shown in Figure 1 in accordance with various embodiments of the present disclosure.

Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the disclosure, and do not limit the scope of the disclosure.

The present disclosure will be described with respect to preferred embodiments in a specific context, namely a three-level inverter. The present disclosure may also be applied, however, to a variety of multilevel inverters including five-level inverters, seven-level inverters, nine-level inverters and the like. Hereinafter, various embodiments will be explained in detail with reference to the accompanying drawings.

Figure 1 illustrates a block diagram of a three-level power conversion system in accordance with various embodiments of the present disclosure. The three-level power conversion system 100 comprises a three-level boost converter 110, a three-level buck converter 120, an inverter 130 and an inductor-inductor-capacitor (LLC) converter 140. As shown in Figure 1, the three-level boost converter 110, the three-level buck converter 120, the inverter 130 and the LLC converter 140 are connected together through a three-level voltage bus apparatus. The three-level voltage bus apparatus comprises a plurality of voltage buses configured as an energy buffer in the three-level power conversion system 100.

As shown in Figure 1, a first voltage bus 101, a second voltage bus 102 and a third voltage bus 103 are connected between the three-level boost converter 110 and the inverter 130. Throughout the description, the first voltage bus 101 is alternatively referred to as a positive voltage bus as indicated by the letter p. The second voltage bus 102 is alternatively referred to as a midpoint voltage bus as indicated by the letter m. The third voltage bus 103 is alternatively referred to as a negative voltage bus as indicated by the letter n. The first voltage bus 101, the second voltage bus 102 and the third voltage bus 103 are part of the three-level voltage bus apparatus. The first voltage bus 101 and the third voltage bus 103 are symmetrical with reference to the second voltage bus 102.

The three-level voltage bus apparatus extends further to other power converters. As shown in Figure 1,

voltage buses

151, 152 and 153 are connected between the three-level buck converter 120 and the LLC converter 140. The

voltage buses

151, 152 and 153 are electrically connected to the

voltage buses

101, 102 and 103 respectively. In other words, the

voltage buses

101 and 151 can be collectively considered as one single entity. Likewise, the

voltage buses

102 and 152 can be collectively considered as one single entity, and the

voltage buses

103 and 153 can be collectively considered as one single entity.

In some embodiments, the voltages on the voltages p, m and n are fully regulated by at least one converter of the three-level power conversion system 100. In some embodiments, the three-level boost converter 110 may be employed to maintain and regulate the voltages on the voltages p, m and n. In alternative embodiments, the three-level buck converter 120 may be employed to maintain and regulate the voltages on the voltages p, m and n. Furthermore, the combination of the three-level boost converter 110 and the three-level buck converter 120 may be employed to maintain and regulate the voltages on the voltages p, m and n. Alternatively, the three-level boost converter 110 and the three-level buck converter 120 may be used to regulate the voltages on the voltages p, m and n in an alternating manner.

In some embodiments, the three-level boost converter 110 is configured to be coupled to a solar panel array 150. As shown in Figure 1, the three-level boost converter 110 is connected between the solar panel array 150 and the three-level voltage bus apparatus. The output of the solar panel array 150 comprises two voltage levels. The three-level boost converter 110 is able to convert the two voltage levels into three voltage levels, namely p, m and n as shown in Figure 1. Furthermore, depending on design needs and different applications, the three-level boost converter 110 is able to regulate the voltages on the voltage buses p, m and n. Moreover, the three-level boost converter 110 may be configured as a bidirectional power converter through which a dc voltage may be established on the output terminals of the solar panel array 150 during abnormal operating conditions (e.g., a PID repair/recovery process) .

The implementation of the three-level boost converter 110 may vary depending on different applications and design needs. In some embodiments, the three-level boost converter 110 may comprise two input inductors (e.g., three-level boost converter 111 as shown in Figure 3 below) . In alternative embodiments, the three-level boost converter 110 may comprise one single input inductor (e.g., three-level boost converter 112 as shown in Figure 5 below) .

In some embodiments, the three-level buck converter 120 is configured to be coupled to an energy storage unit 160. As shown in Figure 1, the three-level buck converter 120 is connected between the energy storage unit 160 and the three-level voltage bus apparatus. In some embodiments, the three-level buck converter 120 is configured to operate as a bidirectional power converter. In an energy storage phase, the energy is transferred from the three-level voltage bus apparatus to the energy storage unit 160 through the three-level buck converter 120. The three-level buck converter 120 functions as a step-down power converter. On the other hand, in an energy releasing phase, the energy is transferred from the energy storage unit 160 to the three-level voltage bus apparatus through three-level buck converter 120. The detailed circuit configuration of the three-level buck converter 120 will be described below with respect to Figure 3.

It should be noted that during the energy releasing phase, the three-level buck converter 120 functions as a step-up power converter. Furthermore, the three-level buck converter 120 is able to regulate the voltages on the voltage buses p, m and n.

In some embodiments, the inverter 130 is configured to be coupled to a three-phase electrical grid such as an ac grid 170. As shown in Figure 1, the inverter 130 is connected between the ac grid 170 and the three-level voltage bus apparatus. Alternatively, the inverter 130 may be configured to be coupled to a single-phase ac load.

In some embodiments, the inverter 130 may be a neutral point clamped (NPC) inverter. The detailed circuit configuration of the NPC inverter will be described below with respect to Figure 9. Alternatively, the inverter 130 may be an active neutral point clamped (ANPC) inverter. The detailed circuit configuration of the ANPC inverter will be described below with respect to Figure 7.

In some embodiments, the LLC converter 140 is configured to be coupled to a dc load. As shown in Figure 1, the LLC converter 140 is connected between the load 180 and the three-level voltage bus apparatus. The LLC converter 140 converts the three voltage levels into two voltage levels, which are applied to downstream converters. The LLC converter 140 functions as a three-level dc/dc power converter. The detailed circuit configuration of the LLC converter 140 will be described below with respect to Figure 13.

It should be noted that the LLC converter described above is merely an exemplary converter and is not meant to limit the current embodiments. The power conversion unit between the load 180 and the three-level voltage bus apparatus may be implemented as a variety of power converters such as full bridge converters, half bridge converters, forward converters, flyback converters, any combinations thereof and/or the like.

In some embodiments, the energy storage unit 160 may be a battery energy storage system including a plurality of rechargeable batteries, fuel cells, any combinations thereof and the like. Alternatively, the energy storage unit 160 may be implemented as other suitable energy storage systems. For example, the energy storage unit 160 may comprise a compressing air energy storage system, a flywheel energy storage system, a pumped storage system, a supercapacitor system, any combinations thereof and the like.

In some embodiments, the load 180 may refer to downstream converters coupled to the output of the LLC converter 140. More particularly, the downstream converters may be a plurality of chargers configured to charge electric vehicles.

In operation, one of the three-level power converters of Figure 1 may measure and regulate the voltages on the voltage buses (e.g., positive voltage bus p, negative voltage bus n and/or midpoint voltage bus m) , thereby controlling the voltage balance of the three voltage buses. As a result, the inverter (e.g., inverter 130) of the three-level power conversion system 100 no longer needs to handle the voltage bus unbalance issue and/or the load unbalance issue. This leads to a cost saving. Furthermore, the regulated bus voltages help to improve the circuit performance of the three-level power conversion system 100.

Furthermore, the energy storage unit 160 can help to improve the performance of the three-level power conversion system 100. In particular, both the solar panel array 150 and the energy storage unit 160 are coupled to the same three-level voltage bus apparatus and form an integrated power conversion system. In such an integrated power conversion system, the power capacity of the solar panel array 150 and the power capacity of the energy storage unit 160 can be configured according to a predetermined dc-to-ac ratio in order to achieve the maximum solar panel energy harvesting at the lowest cost.

One advantageous feature of the three-level power conversion system shown in Figure 1 is by employing the three-level voltage bus, a three-level power converter (e.g., three-level boost converter 110 and/or three-level buck converter 120) can regulate the positive voltage bus p and the negative voltage bus n simultaneously. The regulation of the bus voltages is from an intrinsic capability without additional cost. As a result, the inverter 130 no longer needs to handle the capacitor voltage balancing issue. This configuration yields a cost saving. In addition, this configuration can simplify the PWM control scheme, thereby reducing the switching losses and improving the reliability of the three-level power conversion system 100.

Another advantage of the three-level power conversion system shown in Figure 1 is that the midpoint voltage bus m is regulated constantly. Such a regulated midpoint can be provided directly to the inverter 130 as a neutral line or a neutral point. With this neural line, the inverter 130 is able to operate under 100%three-phase unbalanced load conditions. The availability of the neutral line and the capability of operating under 100%three-phase unbalanced load conditions are advantageous features for grid-independent or micro-grid applications. This leads to a lower cost compared to conventional solutions such as a four-phase inverter as an alternative solution. Furthermore, the system configuration of Figure 1 may not require a load transformer. Instead, the three-level power conversion system 100 may use a transformer isolation barrier on the electrical grid feeding line to provide system isolation, thereby reducing the total cost of the three-level power conversion system 100.

Another advantage of the three-level power conversion system shown in Figure 1 is the system configuration shown in Figure 1 can reduce leakage current from the solar panel array 150. The solar energy industry requires that the solar panel array and the energy storage unit subsystems must be floating for the purpose of detecting ground faults and improving the safety of the solar energy system. In a floating system, it is an advantageous feature to have separate negative rails between the solar converter (e.g., three-level boost converter 110) and the energy storage converter (e.g., three-level buck converter 120) . Such a configuration helps to avoid high leakage current from the solar panel array, thereby reducing the size of the common mode filters required.

It should further be noted that a symmetrical PWM control scheme may be employed in the three-level power conversion system 100. The symmetrical PWM control scheme may be applied to the three-level boost converter and the three-level buck converter. As a result of having the symmetrical PWM control scheme, the three-level boost converter 110 in this system may have a lower common node voltage compared to other three-level boost power converters. The lower common node voltage helps to reduce the cost of the EMI filter.

Another advantage of the three-level power conversion system shown in Figure 1 is that the lower switch of the three-level power conversion units may function as a ground fault breaker. For example, when a ground fault is detected, some (e.g., lower switches) or all active switches of the three-level buck converter and/or the three-level boost converter may be turned off to prevent the ground fault from populating to other circuit branches. This is an important feature for a floating system.

Another benefit of connecting the three-level voltage bus apparatus to the three-level boost converter 110 and the solar panel array 150 is the freedom to generate a dc voltage on the terminals of the solar panel array, which can be used for potential induced degradation (PID) compensation of the solar panel array. Furthermore, connecting the three-level voltage bus apparatus to the three-level boost converter may generate a dc voltage on the terminals of the solar panel array during a PID repair/recovery process.

Furthermore, as an energy buffer, the three-level voltage bus apparatus may see the line frequency ripple caused by the ac load coupled to the inverter. When a neutral line is provided to the ac output, the three-level voltage bus apparatus may also see the double line frequency ripple under unbalanced load conditions. Due to these reasons and other transient response requirements, the capacitors coupled to the three-level voltage bus apparatus need to be relatively large. The energy from the energy storage unit 160 may help to reduce the bus voltage ripple. Especially, the energy from the energy storage unit may help to reduce the second line frequency harmonic during unbalanced load conditions. Since the energy storage unit may help to reduce the ripple on the voltage buses, the size of the capacitors coupled to the voltage buses can be reduced.

The three-level voltage bus apparatus may be formed by a plurality of laminated bus layers. The midpoint layer may be placed between the positive voltage bus layer and the negative voltage bus layer. Such a bus layer configuration helps to reduce the actual insulation voltage stress, thereby improving the insulation stress margin as well as the system reliability.

Figure 2 illustrates a block diagram of a first implementation of a portion of the three-level power conversion system shown in Figure 1 in accordance with various embodiments of the present disclosure. A three-level boost converter 111 and a three-level buck converter 120 are placed on opposite sides of the three-level voltage bus apparatus. The three-level voltage bus apparatus includes a positive voltage bus p, a midpoint voltage bus m and a negative voltage bus n. The three-level boost converter 111 is employed to convert a two-level voltage generated by the solar panel array 150 into a three-level voltage. The three-level buck converter 120 is employed to convert the three-level voltage into a two-level voltage applied to the energy storage unit 160.

Figure 3 illustrates a schematic diagram of the three-level power conversion system shown in Figure 2 in accordance with various embodiments of the present disclosure. The three-level boost converter 111 includes a plurality of switches, two input inductors, two input capacitors and two output capacitors.

As shown in Figure 3, a first input capacitor CIN1 and a second input capacitor CIN2 are connected in series between two output terminals of the solar panel array (shown in Figure 2) . In some embodiment, the first input capacitor CIN1 and the second input capacitor CIN2 have the same capacitance. As a result, the voltage applied to the input capacitors is divided evenly across each capacitor. More particularly, the first input capacitor CIN1 has an output voltage VIN/2 with reference to the common node of capacitors CIN1 and CIN2. Likewise, the second input capacitor CIN2 has an output voltage –VIN/2 with reference to the common node of capacitors CIN1 and CIN2. The common node of capacitors CIN1 and CIN2 is connected to the midpoint voltage bus m according to some embodiments. The common node of capacitors CIN1 and CIN2 may be alternatively referred to as a neutral point of the three-level power conversion system throughout the description.

It should be noted that while Figure 1 illustrates the three-level boost converter 111 with two input capacitors (e.g., the first capacitor C1 and the second capacitor C2) , the three-level boost converter 111 could accommodate any number of input capacitors. The number of input capacitors illustrated herein is limited solely for the purpose of clearly illustrating the inventive aspects of the various embodiments. The present disclosure is not limited to any specific number of input capacitors. One of ordinary skill in the art would recognize many variations, alternatives, and modifications.

The three-level boost converter 111 comprises switches S11, S12, S13 and S14 connected in series between the positive voltage bus p and the negative voltage bus n. As shown in Figure 3, a first inductor L1 is connected to a common node of the switches S11 and S12. A second inductor L2 is connected to a common node of the switches S13 and S14. The common node of the switches S12 and S13 is connected to the common node of capacitors CIN1 and CIN2. A first output capacitor C1 is connected between the positive voltage bus p and the midpoint voltage bus m. A second output capacitor C2 is connected between the midpoint voltage bus m and the negative voltage bus n.

The first input capacitor CIN1, the first inductor L1, switches S11-S12 and the first output capacitor C1 form a first boost converter. The first boost converter is employed to regulate the voltage between the positive voltage bus p and the midpoint voltage bus m. The second input capacitor CIN2, the second inductor L2, switches S13-S14 and the second output capacitor C2 form a second boost converter. The second boost converter is employed to regulate the voltage between the midpoint voltage bus m and the negative voltage bus n. The combination of the first boost converter and the second boost converter functions as a three-level boost converter.

The three-level buck converter 120 includes a plurality of switches, one inductor, two input capacitors and one output capacitor. As shown in Figure 3, the three-level buck converter 120 comprises switches S15, S16, S17 and S18 connected in series between the positive voltage bus p and the negative voltage bus n. An output inductor L3 is connected to a common node of the switches S15 and S16. In should be noted that, depending on different applications and design needs, the output inductor L3 may be split into two separate inductors. For example, a first output inductor is connected to the common node of switches S15 and S16. A second output inductor is connected to the common node of switches S17 and S18.

A first input capacitor C3 is connected between the positive voltage bus p and the midpoint voltage bus m. A second input capacitor C4 is connected between the midpoint voltage bus m and the negative voltage bus n. The common node of the switches S16 and S17 is connected to the common node of capacitors C3 and C4.

The input capacitors C3-C4, the output inductor L3, switches S15-S18 and the output capacitor Co form a three-level buck converter. In particular, the three-level buck converter is configured to receive three voltage levels p, m and n and generate a two-level voltage applied to the battery energy storage system.

In accordance with an embodiment, the switches (e.g., switches S11-S18) may be an insulated gate bipolar transistor (IGBT) device. Alternatively, the switching element can be any controllable switches such as metal oxide semiconductor field-effect transistor (MOSFET) devices, integrated gate commutated thyristor (IGCT) devices, gate turn-off thyristor (GTO) devices, silicon controlled rectifier (SCR) devices, junction gate field-effect transistor (JFET) devices, MOS controlled thyristor (MCT) devices and the like.

It should be noted that when switches S11-S18 are implemented by MOSFET devices, the body diodes of switches S11-S18 can be used to provide a freewheeling channel. On the other hand, when switches S11-S18 are implemented by IGBT devices, a separate freewheeling diode is required to be connected in parallel with its corresponding switch.

As shown in Figure 3, diodes D11, D12, D13, D14, D15, D16, D17 and D18 are required to provide reverse conducting paths for the three-level power conversion system. In other words, diodes D11-D18 are anti-parallel diodes. In some embodiments, diodes D11-D18 are co-packaged with their respective IGBT devices. In alternative embodiments, didoes D11-D18 are placed outside their respective IGBT devices.

It should further be noted that while Figure 3 shows each bidirectional switch is formed by diodes and IGBT devices connected in an anti-parallel arrangement, one of ordinary skill in the art would recognize many variations, alternatives and modifications. For example, the bidirectional switch may be implemented by some new semiconductor switches such as anti-paralleled reverse blocking IGBTs arrangement.

In operation, both the three-level boost converter 111 and the three-level buck converter 120 can be configured as a bidirectional power converter. For example, the three-level boost converter 111 may function as a three-level buck converter to establish a dc voltage on the terminals of the solar panel array if necessary. The three-level buck converter 120 may function as a three-level boost converter to transfer power from the battery tank to the three-level voltage bus apparatus.

It should be noted that throughout the description, the three-level boost converter and the three-level buck converter can be configured as a bidirectional power converter. In other words, depending on the direction of the power flow, the three-level boost converter may function as a three- level buck converter. Likewise, the three-level buck converter may function as a three-level boost converter.

In operation, a symmetrical PWM control scheme may be applied to the three-level power conversion system shown in Figure 3 when the battery coupled to the three-level buck converter is floating. In other words, the grounding requirement is not necessary for the battery terminals. On the other hand, an asymmetrical PWM control scheme may be applied to the three-level power conversion system shown in Figure 3 when the battery coupled to the three-level buck converter is not floating. In other words, the grounding requirement is necessary for the battery terminals.

Figure 4 illustrates a block diagram of a second implementation of the portion of the three-level power conversion system shown in Figure 2 in accordance with various embodiments of the present disclosure. The block diagram shown in Figure 4 is similar to that shown in Figure 2 except that the three-level boost converter is implemented differently. A three-level boost converter 112 is used to replace the three-level boost converter 111 shown in Figure 2. The detailed schematic diagram of the three-level boost converter 112 will be discussed below with respect to Figure 5.

Figure 5 illustrates a schematic diagram of the three-level power conversion system shown in Figure 4 in accordance with various embodiments of the present disclosure. The three-level boost converter 112 is similar to the three-level boost converter 111 shown in Figure 3 except that the two input inductors of the three-level boost converter 111 has been combined together as a single inductor. As shown in Figure 5, an input capacitor CIN is connected between a first terminal of the input inductor L1 and a common node of the switches S13-S14. A second terminal of the input inductor L1 is connected to a common node of the switches S11-S12. The operating principle of the three-level boost converter 112 is similar to that of the three-level boost converter 111, and hence is not discussed again herein to avoid repetition.

Figure 6 illustrates a block diagram of a first implementation of another portion of the three-level power conversion system shown in Figure 1 in accordance with various embodiments of the present disclosure. In some embodiments, the three-level boost converter 110 shown in Figure 1 is implemented as a three-level boost converter 112. The inverter 130 shown in Figure 1 is implemented as an ANPC inverter 131. The electrical grid 170 is replaced by a single phase ac load 171. The three-level boost converter 112 and the ANPC inverter 131 are connected together through the three-level voltage bus apparatus. The three-level voltage bus apparatus includes a positive voltage bus p, a midpoint voltage bus m and a negative voltage bus n. The three-level boost converter 112 is employed to convert a two-level voltage generated by the solar panel array 150 into a three-level voltage. The ANPC inverter 131 is employed to convert the three-level voltage into a multilevel voltage applied to the ac load 171.

Figure 7 illustrates a schematic diagram of the three-level power conversion system shown in Figure 6 in accordance with various embodiments of the present disclosure. The three-level boost converter 112 has been discussed in detail above with respect to Figures 3, and hence is not discussed again herein.

The ANPC inverter 131 comprises six switches and an output inductor LA. As shown in Figure 7, switches S21, S22, S23 and S24 are connected in series between the positive voltage bus p and the negative voltage bus n. Switches S25 and S26 are connected in series between a common node of switches S21-S22 and a common node of switches S23-S24. The common node of switches S25 and S26 is connected to the midpoint voltage bus m. The common node of the switches S22 and S23 is connected to the output inductor LA. The operating principle of the ANPC inverter is well known, and hence is not discussed herein to avoid repetition.

Figure 8 illustrates a block diagram of a second implementation of another portion of the three-level power conversion system shown in Figure 1 in accordance with various embodiments of the present disclosure. The block diagram shown in Figure 8 is similar to that shown in Figure 6 except that the inverter is implemented as an NPC inverter 132. The schematic diagram of the NPC inverter 132 will be described in detail below with respect to Figure 9.

Figure 9 illustrates a schematic diagram of the three-level power conversion system shown in Figure 8 in accordance with various embodiments of the present disclosure. The schematic diagram of Figure 9 is similar to that of Figure 7 except that the switches S25 and S26 have been replaced by two diodes D25 and D26. The operating principle of the NPC inverter is well known, and hence is not discussed herein to avoid repetition.

Figure 10 illustrates a block diagram of a third implementation of another portion of the three-level power conversion system shown in Figure 1 in accordance with various embodiments of the present disclosure. The block diagram shown in Figure 10 is similar to that shown in Figure 6 except that the inverter is implemented as a three-phase ANPC inverter 130. The load is a three-phase electrical grid. The schematic diagram of the three-phase ANPC inverter 130 will be described in detail below with respect to Figure 11.

Figure 11 illustrates a schematic diagram of the three-level power conversion system shown in Figure 10 in accordance with various embodiments of the present disclosure. The three-level power conversion system comprises the three-level boost converter 112 and the three-phase ANPC inverter 130. As shown in Figure 11, the three-level boost converter 112 and the three-phase ANPC inverter 130 are connected together by the three-level voltage bus apparatus. The three-level boost converter 112 has been discussed in detail above with respect to Figures 3, and hence is not discussed again herein.

The three-phase ANPC inverter 130 includes a first phase inverter 220, a second phase inverter 230 and a third phase inverter 240. The first phase inverter 220 comprises switches S21, S22, S23 and S24 connected in series between the positive voltage bus p and the negative voltage bus n. The first phase inverter 220 further comprises switches S25 and S26 connected in series between the common node of S21-S22 and the common node of S23-S24. The common node of S25-S26 is connected to the midpoint voltage bus m. A first output filter is placed between the common node of S22-S23 and a first wye-delta transformer 125. The first output filter is formed by an inductor LA and a capacitor C20. The capacitor C20 is connected in parallel with a wye side of the first wye-delta transformer 125. The delta side of the first wye-delta transformer 125 is connected to a first phase (phase A) of the three-phase electrical grid (shown in Figure 10) .

The second phase inverter 230 comprises switches S31, S32, S33 and S34 connected in series between the positive voltage bus p and the negative voltage bus n. The second phase inverter 230 further comprises switches S35 and S36 connected in series between the common node of S31-S32 and the common node of S33-S34. The common node of S35-S36 is connected to the midpoint voltage bus m as shown in Figure 11. A second output filter is placed between the common node of S32-S33 and a second wye-delta transformer 135. The second output filter is formed by an inductor LB and a capacitor C30. The capacitor C30 is connected in parallel with a wye side of the second wye-delta transformer 135. The delta side of the second wye-delta transformer 135 is connected to a second phase (phase B) of the three-phase electrical grid.

The third phase inverter 240 comprises switches S41, S42, S43 and S44 connected in series between the positive voltage bus p and the negative voltage bus n. The third phase inverter 240 further comprises switches S45 and S46 connected in series between the common node of S41-S42 and the common node of S43-S44. The common node of S45-S46 is connected to the midpoint voltage bus m as shown in Figure 11. A third output filter is placed between the common node of S42-S43 and a third wye-delta transformer 145. The third output filter is formed by an inductor LC and a capacitor C40. The capacitor C40 is connected in parallel with a wye side of the third wye-delta transformer 145. The delta side of the third wye-delta transformer 145 is connected to a third phase (phase C) of the three-phase electrical grid.

In operation, the three-level power conversion system shown in Figure 11 is able to regulate the three-phase under various conditions (e.g., unbalanced and nonlinear load conditions) . Furthermore, the three-level power conversion system shown in Figure 11 is compatible of driving a single-phase ac transformer for a modular three-phase system under a balance control mechanism.

In some embodiments, when neutral grounding is required, the three-level power conversion system shown in Figure 11 provides a neutral point ready for earth grounding. When a three-phase unbalanced load occurs, the energy storage unit (not shown but illustrated in Figure 1) coupled to the three-level power conversion system shown in Figure 11 can be used to actively compensate the second harmonic on the three-level voltage bus apparatus, thereby reducing the capacity requirement of the capacitors coupled to the three-level voltage bus apparatus.

Figure 12 illustrates a block diagram of yet another portion of the three-level power conversion system shown in Figure 1 in accordance with various embodiments of the present disclosure. A three-level boost converter 112 and an LLC converter 140 are placed on opposite sides of a three-level voltage bus apparatus. The three-level voltage bus apparatus includes a positive voltage bus p, a midpoint voltage bus m and a negative voltage bus n. The three-level boost converter 112 is employed to convert a two-level voltage generated by the solar panel array 150 into a three-level voltage. The three-level boost converter 110 is able to regulate the voltages on the voltage buses p, m and n. The LLC converter 140 is employed to convert the three-level voltage into a two-level voltage applied to the load 180. The LLC converter 140 is a three-level LLC resonant converter. The load 180 may comprise a plurality of chargers configured to charge a plurality of electrical vehicles.

Figure 13 illustrates a schematic diagram of the three-level power conversion system shown in Figure 12 in accordance with various embodiments of the present disclosure. The three-level power conversion system shown in Figure 13 comprises the three-level boost converter 112 and the LLC converter 140. As shown in Figure 13, the three-level boost converter 112 and the LLC converter 140 are connected together by the three-level voltage bus apparatus. The three-level boost converter 112 has been discussed in detail above with respect to Figures 3, and hence is not discussed again herein.

The LLC converter 140 comprises a switch network 161, a resonant tank 162, a transformer 163, a rectifier 164 and an output filter 165. As shown in Figure 13, the switch network 161, the resonant tank 162, the transformer 163, the rectifier 164 and the output filter 165 are coupled to each other and connected in cascade between the input capacitors C3, C4 and the load RL.

The switch network 161 comprises switches S1, S2, S3 and S4 connected in series between the positive voltage bus p and the negative voltage bus n. The common node of switches S2 and S3 is connected to the midpoint voltage bus m as shown in Figure 13. The common node of switches S1 and S2 is connected to a first terminal of the transformer 163 through the resonant tank 162. The common node of switches S3 and S4 is connected to a second terminal of the transformer 163.

The resonant tank 162 may be implemented in a variety of ways. For example, the main resonant tank comprises a series resonant inductor Lr, a parallel resonant inductor Lm and a series resonant capacitor Cr.

The series resonant inductor and the parallel resonant inductor may be implemented as external inductors. A person skilled in the art will recognize that there may be many variation, alternatives and modifications. For example, the series resonant inductor may be implemented as a leakage inductance of the transformer 163.

In sum, the resonant tank 162 includes three key resonant elements, namely the series resonant inductor, the series resonant capacitor and the parallel resonant inductor. Such a configuration is commonly referred to as an LLC resonant converter. According to the operating principle of LLC resonant converters, at a switching frequency approximately equal to the resonant frequency of the resonant tank 162, the resonant tank 162 helps to achieve zero voltage switching for the primary side switching elements and zero current switching for the secondary side switching elements.

The LLC converter 140 may further comprise a transformer 163, a rectifier 164 and an output filter 165. The transformer 163 provides electrical isolation between the primary side and the secondary side of the LLC converter 140. In accordance with an embodiment, the transformer 163 may be formed of two transformer windings, namely a primary transformer winding NP and a secondary transformer winding NS as shown in Figure 13. Alternatively, the transformer 163 may have a center tapped secondary so as to have three transformer windings including a primary transformer winding, a first secondary transformer winding and a second secondary transformer winding.

It should be noted that the transformers described above and throughout the description are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, the transformer 163 may further comprise a variety of bias windings and gate drive auxiliary windings.

The rectifier 164 converts an alternating polarity waveform received from the output of the transformer 163 to a single polarity waveform. When the transformer 163 is of a center tapped secondary, the rectifier 164 may be formed of a pair of switching elements such as n-type metal oxide semiconductor (NMOS) transistors. Alternatively, the rectifier 164 may be formed of a pair of diodes. On the other hand, when the transformer 163 is of a single secondary winding, the rectifier 164 may be a full-wave rectifier coupled to the single secondary winding of the transformer 163.

Furthermore, the rectifier 164 may be formed by other types of controllable devices such as metal oxide semiconductor field effect transistor (MOSFET) devices, bipolar junction transistor (BJT) devices, super junction transistor (SJT) devices, insulated gate bipolar transistor (IGBT) devices, gallium nitride (GaN) based power devices and/or the like. The detailed operation and structure of the rectifier 114 are well known in the art, and hence are not discussed herein.

The output filter 165 is used to attenuate the switching ripple of the LLC converter 140. According to the operation principles of isolated dc/dc converters, the output filter 165 may be an L-C filter formed by an inductor and a plurality of capacitors. One person skilled in the art will recognize that some isolated dc/dc converter topologies such as forward converters may require an L-C filter. On the other hand, some isolated dc/dc converter topologies such as LLC resonant converters may include an output filter formed by a capacitor. One person skilled in the art will further recognize that different output filter configurations apply to different power converter topologies as appropriate. The configuration variations of the output filter 165 are within various embodiments of the present disclosure.

Figure 14 illustrates a block diagram of another three-level power conversion system in accordance with various embodiments of the present disclosure. The three-level power conversion system 1400 is similar to the three-level power conversion system 100 shown in Figure 1 except that the LLC converter 140 of Figure 1 has been replaced by a three-level boost converter 112. As shown in Figure 14, the three-level boost converter 112 is connected between a power source 155 and the three-level voltage bus apparatus. In some embodiments, the power source 155 is a solar panel array. In alternative embodiments, the power source 155 may be implemented as an energy storage unit such as a battery tank and the like.

Figure 15 illustrates a schematic diagram of the three-level power conversion system shown in Figure 14 in accordance with various embodiments of the present disclosure. The three-level voltage bus apparatus includes a positive voltage bus p, a midpoint voltage bus m and a negative voltage bus n. As shown in Figure 15, the three-level boost converter 111 is connected between a solar panel array and the three-level voltage bus apparatus. The three-level buck converter 120 is connected between a battery tank and the three-level voltage bus apparatus. The three-level boost converter 112 is connected between a power source and the three-level voltage bus apparatus. The power source may be implemented as a solar panel array or a battery tank as shown in Figure 15. The ANPC inverter 131 is connected between a load and the three-level voltage bus apparatus. The power conversion units shown in Figure 15 have described above, and hence are not discussed again herein.

Figure 16 illustrates a block diagram of yet another three-level power conversion system in accordance with various embodiments of the present disclosure. The three-level power conversion system 1600 is similar to the three-level power conversion system 100 shown in Figure 1 except that the solar panel array of Figure 1 has been replaced by a power source 155. Furthermore, an LLC converter 140 is connected between the power source 155 and the three-level voltage bus apparatus, and a three-level boost converter 112 is connected between the energy storage unit 160 and the three-level voltage bus apparatus. In some embodiments, the power source 155 is a power supply connected to the electrical grid. In alternative embodiments, the power source 155 may be implemented as an energy storage unit such as a battery tank and the like.

Figure 17 illustrates a schematic diagram of a portion of the three-level power conversion system shown in Figure 16 in accordance with various embodiments of the present disclosure. The three-level voltage bus apparatus includes a positive voltage bus p, a midpoint voltage bus m and a negative voltage bus n. The portion of the three-level power conversion system shown in Figure 17 includes the LLC converter 140 and the ANPC inverter 131. As shown in Figure 17, the LLC converter 140 is connected between the power source and the three-level voltage bus apparatus. The ANPC inverter 131 is connected between the three-level voltage bus apparatus and a load.

Figure 18 illustrates a schematic diagram of another portion of the three-level power conversion system shown in Figure 16 in accordance with various embodiments of the present disclosure. The three-level voltage bus apparatus includes a positive voltage bus p, a midpoint voltage bus m and a negative voltage bus n. The portion of the three-level power conversion system shown in Figure 18 includes the three-level boost converter 112 and the LLC converter 140. As shown in Figure 18, the three-level boost converter 112 is connected between the energy storage unit (shown in Figure 16) and the three-level voltage bus apparatus. The LLC converter 140 is connected between the three-level voltage bus apparatus and a load RL. In some embodiments, the load comprises a plurality of chargers configured to charge a plurality of electric vehicles.

Figure 19 illustrates a block diagram of yet another three-level power conversion system in accordance with various embodiments of the present disclosure. The three-level power conversion system 1900 is similar to the three-level power conversion system 100 shown in Figure 1 except that the three-level boost converter of Figure 1 has been replaced by a dual-output LLC converter 141. Furthermore, the three-level buck converter of Figure 1 has been replaced by a dual active bridge (DAB) converter 190 and a three-level boost converter 113 connected in cascade.

Figure 20 illustrates a schematic diagram of a portion of the three-level power conversion system shown in Figure 19 in accordance with various embodiments of the present disclosure. The three-level voltage bus apparatus includes a positive voltage bus p, a midpoint voltage bus m and a negative voltage bus n. The portion of the three-level power conversion system shown in Figure 20 includes the dual-output LLC converter 141 and the ANPC inverter 131. As shown in Figure 20, the dual-output LLC converter 141 is connected between the solar panel array and the three-level voltage bus apparatus. The ANPC inverter 131 is connected between the three-level voltage bus apparatus and a load. The three-level voltage bus apparatus functions as an energy buffer between the dual-output LLC converter 141 and the ANPC inverter 131.

The dual-output LLC converter 141 includes a primary side circuit and a secondary side circuit. The primary side circuit of the dual-output LLC converter 141 is similar to the primary side circuit of the LLC converter 140 shown in Figure 13, and hence is not discussed again to avoid repetition. The secondary side circuit includes a first secondary winding NS1, a second secondary winding NS2, a first rectifier 146, a second rectifier 148, a first output capacitor Co1 and a second output capacitor Co2. The first rectifier 146 comprises diodes D51, D61, D71 and D81. The second rectifier 148 comprises diodes D52, D62, D72 and D82 as shown in Figure 20.

As shown in Figure 20, the first secondary winding NS1, the first rectifier 146 and the first output capacitor Co1 form a first output of the dual-output LLC converter 141. The second secondary winding NS2, the second rectifier 148 and the second output capacitor Co2 form a second output of the dual-output LLC converter 141. The first output and the second output are stacked together and further coupled to the three-level voltage bus apparatus. The common node of the first output and the second output is connected to the midpoint voltage bus m.

Figure 21 illustrates a schematic diagram of another portion of the three-level power conversion system shown in Figure 19 in accordance with various embodiments of the present disclosure. The three-level voltage bus apparatus includes a positive voltage bus p, a midpoint voltage bus m and a negative voltage bus n. The portion of the three-level power conversion system shown in Figure 21 includes the DAB converter 190, the three-level boost converter 113 and the LLC converter 140. These three converters are connected in cascade between the input capacitors CIN1, CIN2 and the load RL.

As shown in Figure 21, the DAB converter 190 is connected between the energy storage unit (battery tank shown in Figure 19) and the three-level boost converter 113. The three-level boost converter 113 is connected to three-level voltage bus apparatus. The LLC converter 140 is connected between the three-level voltage bus apparatus and the load RL. In some embodiments, the load comprises a plurality of chargers configured to charge a plurality of electrical vehicles.

The DAB converter 190 is similar to the LLC converter 140 except that the DAB converter 190 does not comprise a resonant capacitor. The operating principle of the DAB converter is well known, and hence is not discussed herein.

The three-level boost converter 113 is similar to the three-level boost converter 112 described above except that the input inductor is from the transformer T1. More particularly, the resonant inductor Lr of the DAB converter 190 functions as the input inductor of the three-level boost converter 112.

Figure 22 illustrates a flow chart of a method for controlling the three-level power conversion system shown in Figure 1 in accordance with various embodiments of the present disclosure. This flowchart shown in Figure 22 is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, various steps illustrated in Figure 22 may be added, removed, replaced, rearranged and repeated.

At step 2202, a plurality of power conversion units is coupled to a three-level voltage bus apparatus. The three-level voltage bus apparatus includes a positive voltage bus p, a midpoint voltage bus m and a negative voltage bus n. The power conversion units include a three-level boost converter, a three-level buck converter, an inverter and an LLC converter and the like.

At step 2204, the energy of a power source is transferred to the three-level voltage bus apparatus through a three-level power converter. In some embodiments, the power source is a solar panel array. The three-level power converter is a three-level boost converter. In some embodiments, the three-level power converter is able to detect the voltages on the buses p, m, n, and regulate the voltages based upon the detected voltages.

At step 2206, the energy of the three-level voltage bus apparatus is transferred to an electrical grid through an inverter. In some embodiments, the inverter is an NPC inverter. Alternatively, the inverter is an ANPC inverter. Furthermore, the electrical grid may be a single phase grid. The inverter is implemented as a single phase inverter. Alternatively, the electrical grid may be a three-phase grid. The inverter is implemented as a three-phase inverter.

At step 2208, the energy of the three-level voltage bus apparatus is transferred to a load through an LLC converter. In some embodiments, the load comprises a plurality of chargers configured to charge a plurality of electric vehicles. The LLC converter is a three-level LLC resonant converter.

At step 2208, the energy is transferred between the three-level voltage bus apparatus and an energy storage unit through a three-level buck converter. In some embodiments, the three-level buck converter is a bidirectional converter. During an energy storage phase, the energy is transferred from the three-level voltage bus apparatus to the energy storage unit. During an energy releasing phase, the energy is transferred from the energy storage unit to the three-level voltage bus apparatus. During the energy releasing phase, the three-level buck converter is able to detect the voltages on the buses p, m, n, and regulate the voltages based upon the detected voltages.

Although embodiments of the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims.

For example, in one embodiment a system is disclosed that includes a first three-level power converter means having output terminals connected to a three-level voltage bus apparatus and input terminals configured to be coupled with a solar panel array, an inverter means connected between the three-level voltage bus apparatus and an electrical grid, and a second three-level power converter means connected between the three-level voltage bus apparatus and an energy storage unit. The use of the term connected is intended to include where the various means are both directly and indirectly connected.

Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. The specification and drawings are, accordingly, to be regarded simply as an illustration of the disclosure as defined by the appended claims, and are contemplated to cover any and all modifications, variations, combinations or equivalents that fall within the scope of the present disclosure.

Claims

An apparatus comprising:

a first voltage bus coupled to a plurality of three-level power conversion units;

a second voltage bus coupled to the plurality of three-level power conversion units; and

a third voltage bus coupled to the plurality of three-level power conversion units, wherein the first voltage bus and the third voltage bus are regulated with reference to the second voltage bus through at least one three-level power conversion unit of the plurality of three-level power conversion units.
The apparatus of claim 1, wherein:

the plurality of three-level power conversion units includes a three-level boost converter, a three-level buck converter, an inverter and an inductor-inductor-capacitor (LLC) converter, and wherein output/input terminals of the three-level boost converter, the three-level buck converter, the inverter and the LLC converter are coupled together through the first voltage bus, the second voltage bus and the third voltage bus.
The apparatus of claim 2, wherein:

the three-level boost converter is coupled between a solar panel array and the first voltage bus, the second voltage bus and the third voltage bus;

the three-level buck converter is coupled between an energy storage unit and the first voltage bus, the second voltage bus and the third voltage bus;

the inverter is coupled between an electrical grid and the first voltage bus, the second voltage bus and the third voltage bus; and

the LLC converter is coupled between a dc load and the first voltage bus, the second voltage bus and the third voltage bus.
The apparatus of claim 3, wherein:

the energy storage unit is a battery based energy storage unit; and

the dc load comprises a charger configured to charge electric vehicles.
The apparatus of claims 3 or 4, wherein:

the inverter is a neutral point clamped inverter, and wherein a neutral point of the inverter is coupled to the second voltage bus.
The apparatus of claims 3 or 4, wherein:

the inverter comprises a first neutral point clamped inverter, a second neutral point clamped inverter and a third neutral point clamped inverter, and wherein:

the first neutral point clamped inverter is coupled to a first phase of a three-phase power system through a first wye-delta transformer;

the second neutral point clamped inverter is coupled to a second phase of the three- phase power system through a second wye-delta transformer; and

the third neutral point clamped inverter is coupled to a third phase of the three-phase power system through a third wye-delta transformer.
The apparatus of claim 6, wherein:

neutral wires of the first wye-delta transformer, the second wye-delta transformer and third wye-delta transformer are coupled to the second voltage bus.
The apparatus of claim 1, wherein:

the plurality of three-level power conversion units includes a first three-level boost converter, a second three-level boost converter, an inverter and a three-level buck converter coupled together through the first voltage bus, the second voltage bus and the third voltage bus.
The apparatus of claim 1, wherein:

the plurality of three-level power conversion units includes a first LLC converter, a second LLC converter, an inverter and a three-level boost converter coupled together through the first voltage bus, the second voltage bus and the third voltage bus.
The apparatus of claim 1, wherein:

the plurality of three-level power conversion units includes an LLC converter, a dual-output LLC converter, an inverter and a three-level boost converter coupled together through the first voltage bus, the second voltage bus and the third voltage bus.
The apparatus of claim 10, wherein:

the three-level boost converter is coupled between a dual active bridge converter and the first voltage bus, the second voltage bus and the third voltage bus; and

a first output and a second output of the dual-output LLC converter are stacked together, and wherein a common node of the first output and the second output of the dual-output LLC converter is coupled to the second voltage bus.
A method comprising:

transferring energy from a power source to a three-level voltage bus apparatus through a first three-level power converter;

transferring energy from the three-level voltage bus apparatus to an electrical grid through an inverter; and

transferring energy between the three-level voltage bus apparatus and an energy storage unit through a second three-level power converter.
The method of claim 12, further comprising:

transferring energy from the three-level voltage bus apparatus to a load through an LLC power converter; and

regulating voltages of the three-level voltage bus apparatus.
The method of claims 12 or 13, wherein:

the three-level voltage bus apparatus comprises a first voltage bus, a second voltage bus and a third voltage bus coupled to the first three-level power converter, the second three-level power converter, the inverter and the LLC power converter, and wherein the first voltage bus and the third voltage bus are regulated with reference to the second voltage bus.
The method of claims 13 or 14, wherein:

the first three-level power converter is a three-level boost converter comprising four switches coupled in series, and wherein a middle point of the four switches of the three-level boost converter is coupled to the second voltage bus;

the second three-level power converter is a three-level buck converter comprising four switches coupled in series, and wherein a middle point of the four switches of the three-level buck converter is coupled to the second voltage bus;

the inverter is a neutral point clamped inverter having a neutral point coupled to the second voltage bus; and

the LLC power converter comprises four switches coupled in series, and wherein a middle point of the four switches of the LLC power converter is coupled to the second voltage bus.
The method of claims 12 or 13, further comprising:

configuring the second three-level power converter to operate as a bidirectional power converter transferring energy between the three-level voltage bus apparatus and the energy storage unit.
A system comprising:

a first three-level power converter having output terminals coupled to a three-level voltage bus apparatus and input terminals configured to be coupled with a solar panel array;

an inverter coupled between the three-level voltage bus apparatus and an electrical grid; and

a second three-level power converter coupled between the three-level voltage bus apparatus and an energy storage unit.
The system of claim 17, wherein the three-level voltage bus apparatus comprises:

a first voltage bus coupled to a positive output terminal of the first three-level power converter;

a second voltage bus coupled to a neutral point of the first three-level power converter; and

a third voltage bus coupled to a negative output terminal of the first three-level power converter, and wherein voltages on the first voltage bus, the second voltage bus and the third voltage bus are regulated by the first three-level power converter.
The system of claims 17 or 18, further comprising:

an LLC converter coupled between a dc load and the three-level voltage bus apparatus.
The system of claim 19, wherein:

the dc load includes a plurality of electric vehicles.