Modular multi-level power conversion architectures featuring modular transformers for isolation, enhanced power-density and system reliability

Abstract

Embodiments include four iso-MMC module design candidates, each with unique features suitable for different power system requirements. These include integrated full-bridge and half-bridge DC circuits, distributed full-bridge and half-bridge DC circuits, and the presence of isolation transformers. In all of these designs, the iso-MMC design eliminates the need for a low-frequency 60 Hz transformer and bulky module capacitors, significantly reducing the size of these components. It also addresses the limitations of DC fault-tolerance in conventional MMCs by avoiding uncontrolled fault current pathways and providing galvanic isolation at the module level. The iso-MMC module designs are applicable to distributed high-voltage DC transmission systems, distributed DC systems, such as photovoltaic and battery energy storage power plants, and back-to-back AC-AC systems, such as industrial AC drives and solid-state transformers, and in many uses may offer significant improvements in power density, fault-tolerance, and scalability for high-voltage power conversion applications.

Description

Docket No. PSU24301PCT MODULAR MULTI-LEVEL POWER CONVERSION ARCHITECTURES FEATURING MODULAR TRANSFORMERS FOR ISOLATION, ENHANCED POWER-DENSITY AND SYSTEM RELIABILITY CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 63 / 549,973, entitled “MODULAR MULTI-LEVEL POWER CONVERSION ARCHITECTURES FEATURING MODULAR TRANSFORMERS FOR ISOLATION, ENHANCED POWER- DENSITY AND SYSTEM RELIABILITY,” and filed February 5, 2024, the entire contents of which are hereby incorporated by reference for all purposes. FIELD

[0002] The disclosure relates to power conversion architectures and, more specifically, to modular multi-level power conversion architectures. BACKGROUND AND SUMMARY

[0003] Power electronics converters enable power transfer from energy sources to energy loads by reconciling their differences in voltages, currents, frequency of operation and facilitating DC-DC, DC-AC or AC-AC conversion. They form an enabling technology for various applications including renewable energy integration, long distance high-voltage DC (HVDC) electric power transmission, electrified transportation systems, industrial motor drives, energy storage systems, to name a few. The building block elements of these electronic converters are semiconductor switches (such as MOSFETS, IGBTs, thyristors, etc.) and energy storage elements (such as capacitors and inductors). The switches operate at a switching frequency (^^ௌ^) and connect their associated nodes to different voltage and / or current levels leading to switched voltage and current waveforms. The energy storage devices act as smoothening elements for the switched voltages and / or currents to meet the source and load performance specifications.

[0004] For numerous case studies, for example, medium / high-voltage grid-connected applications, the ratings of the energy sources and loads go significantly beyond the ratings of the building block elements i.e. the semiconductor switches (SS) and the energy storage (ES) elements. In order to realize such power converters, several smaller power converters called modules are connected in series and / or in parallel. The Modular Multi-level Power Converter (MMC) architecture forms the most common modern-day circuit topology to realize suchDocket No. PSU24301PCT applications. An example application of such power converters is high-voltage DC (HVDC) transmission systems which interconnect two AC systems and require high-voltage high-power DC-AC power conversion. In particular, the underwater transmission cable interconnects San Francisco, CA to Pittsburg, CA and can transmit 400MW of power at a DC voltage of േ200 kV. The Trans Bay Cable project was the first HVDC system realized using the MMC architecture. While the innovation discussed in this document is inspired by the limitations in modern-day technologies of HVDC systems, many other application spaces can also benefit from this work. Other application areas include integration of photovoltaic and battery energy storage power plants to the electric grid, industrial AC drives, fast charges for electrified transportation systems, large electrolysers for hydrogen production, to name a few.

[0005] The most common implementations of medium / high-voltage power electronics systems include thyristor-based current source converters commonly referred to as Line- Commutated Converters (LCC) and IGBT / MOSFET-based voltage source converters commonly referred to as Modular Multi-level Converters (MMC), briefly discussed next.

[0006] The thyristor-based current source converter (CSC) topology is illustrated in FIG. 1. In this converter, the gate controlled thyristor devices are connected in series to meet the high-voltage requirements. These converters have been the workhorse of high-voltage DC transmission since their inception in the 1950s. With the advances in controlled semiconductor switches such as IGBTs and MOSFETs since the 1970s, the dual of CSC i.e. voltage source converters (VSC) have emerged as a viable option where the thyristor devices can be replaced with IGBTs / MOSFETs. Arguably, due to the series connection of devices only (rather than smaller standalone power converters), these converter designs are not considered truly modular. While these two converters have been the primary candidates for high voltage power conversion in the early days, innovation in circuit topologies have paved the way for modular and high performance designs with advanced power flow control capabilities.

[0007] Among the several multilevel topologies, active neutral-point clamped converter and flying capacitor converter have gained prominence. However, they face various challenges beyond three-level power conversion such as increased complexity, challenging voltage balancing algorithms and commutation issues (due to parasitic pathways leading to over- voltages during switching action). Further, commutation issues make them relatively incompatible with advanced and upcoming wide band-gap semiconductor switches which are considered to be the primary drivers of next-generation power dense and efficient converters. Thus, these topologies have limited applicability in medium to high voltage power applications.Docket No. PSU24301PCT

[0008] Modular multilevel converters (MMC) enjoy their dominance in power electronic conversion at high-voltage and power levels due to their modularity, better performance and fault tolerance, for example, due to redundancy. Built by interconnecting smaller power converter modules in unique ways, they may be the only practical option in many current and next-generation applications. FIG.2 illustrates the circuit schematic of a DC to three-phase AC MMC topology featuring half-bridge (HB) circuits as the modules.

[0009] The half-bridge MMC topology is accompanied by the following two critical limitations imposed by-design: (i) high energy storage requirements leading to voluminous capacitor requirements with limited lifetime and reliability, and (ii) presence of uncontrolled pathway during DC fault current scenarios leading to low system reliability, discussed briefly next.

[0010] Power Density Limitations

[0011] The following discussion considers one half-bridge module of FIG. 2. Since the analysis holds for each of the modules, the superscripts denoting the module number, 1 to ^^, have been ignored in the following equations. Further, the ^ or െ signs in the subscripts that denote the positive or negative arm respectively have also been dropped for simplicity.Subscript ^^ represents the AC phase where, ^^ ൌ ^^, ^^, ^^.

[0012] The current through the module capacitors (^^^ି^) can be characterized (1) using the duty ratios as expressed in (2). As a side note, duty ratio is defined as the average value of theswitch state variable ℎ (as shown in FIG. 2) over one switching period (^^ௌ^ ൌ 1 / ^^ௌ^). Toelaborate, considering a half-bridge module, when the switch state is “1", the arm is connectedto the capacitor and provides a path for the arm current. The capacitor current is zero when ℎ ൌ0 as the capacitor is bypassed. The average value of the ℎ function represents the duty ratio (d) of the switch and aids in quantifying switching variables. Hence:

[0013]

[0014] where the of the AC system can be characterized as follows:

[0015]

[0016] and the currentphases of the AC system can be characterized as follows:Docket No. PSU24301PCT

[0017]

[0018] Note thatrespectively,^^^, ^^^ represent the voltages and currents of the three-AC phases respectively, where k = A, B,C and ^^^^^represents the voltage of the module capacitor, and ^^ represents the number of modules. Substituting the ^^^and ^^^^^ି^expressions into (3):

[0019]

[0020]

[0021] It can be observedAC currents (60Hz, 120Hz) into the DC module capacitors. In other words, the module capacitors must be sized to filter the low-frequency harmonics to maintain a low ripple voltage. Hence, the DC module capacitances are voluminous for high-performance operation.

[0022] DC Fault-Tolerance Limitations

[0023] A DC short-circuit fault in HVDC systems based on conventional half-bridge MMC circuits is detrimental to the overall system. During a DC short-circuit condition, a HB-MMC circuit provides an uncontrolled pathway to fault currents. As a result, the AC grid continues to feed into the short-circuited DC node at high fault current magnitudes. FIG.3 illustrates the fault path FP highlighting the uncontrolled flow of currents from the three-phase AC system to the DC side through the diodes. Such currents are interrupted by AC side circuit breakers which break the path at the zero-crossing of the 60Hz cycle. Due to their limited response time, DC circuit breakers are necessary which are commercially available only with limited performance.

[0024] Another by-design alternative to improve the DC fault-tolerance capability and the overall reliability is to utilize full-bridge modules. These module designs allow full control over the AC and DC voltage and current during AC and DC side faults. However, they require twice the semiconductor switch rating and double the number of semiconductors in the conduction path, i.e., double switch losses. Thus, they are generally a cost-ineffective solution for HVDC applications due to higher investment and operational cost (i.e., power losses).Docket No. PSU24301PCT BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The disclosure may be better understood from reading the following description of non-limiting embodiments, with reference to the attached drawings, wherein below:

[0026] FIG. 1 illustrates a simplified circuit schematic of the state-of-the-art LCC topology;

[0027] FIG. 2 illustrates a simplified circuit schematic of the state-of-the-art MMC topology featuring half-bridge modules;

[0028] FIG. 3 illustrates a simplified circuit schematic of the state-of-the-art MMC topology during a DC short circuit fault scenario;

[0029] FIG.4 illustrates a skeleton schematic of conventional MMC converters illustrating the flow of DC and AC currents in the arms;

[0030] FIG. 5 illustrates a skeleton schematic of iso-MMC converters illustrating the distinctive features of the proposed work with the state-of-the-art: (a) presence of DC and AC circuit networks within a module, (b) three-phase interconnection between the DC circuits, (c) decoupling of the DC and AC currents within the modules, (d) presence of isolation transformers per module, to name a few;

[0031] FIG. 6 illustrates a circuit schematic of iso-MMC Candidate 1 Module Design featuring an integrated full-bridge DC circuit with a single share transformer between the DC and the three-AC phases;

[0032] FIG. 7 illustrates a circuit schematic of iso-MMC Candidate 2 Module Design featuring an integrated three-phase half-bridge DC circuit with a single share transformer between the DC and the three-AC phases;

[0033] FIG. 8 illustrates a circuit schematic of iso-MMC Candidate 3 Module Design featuring a distributed full-bridge DC circuits with three distributed transformers between the DC and each of the three-AC phases. Note the interconnected between the three DC full-bridge modules enabling reduced capacitance requirement by-design;

[0034] FIG. 9 illustrates a circuit schematic of iso-MMC Candidate 3 Module Design featuring a distributed half-bridge DC circuits with three distributed transformers between the DC and each of the three-AC phases. Note the interconnected between the three DC full-bridge modules enabling reduced capacitance requirement by-design;

[0035] FIG. 10 illustrates a skeleton architecture of DC to three-phase AC iso-MMC- based-MMC topology;

[0036] FIG. 11 illustrates a power converter architecture of DC to three-phase AC iso- MMC-based-MMC topology featuring Candidate 2 iso-MMC modules;Docket No. PSU24301PCT

[0037] FIG. 12 illustrates a power converter architecture of DC to three-phase AC iso- MMC-based-MMC topology featuring Candidate 3 iso-MMC modules;

[0038] FIG.13 illustrates a comparison of total semiconductor VA rating;

[0039] FIG. 14 illustrates comparative simulation waveforms illustrating the inherent DC fault current clearing capability of iso-MMC;

[0040] FIG.15 illustrates preliminary simulation results of the design example under study with Candidate 3 iso-MMC HVDC System;

[0041] FIG. 16 illustrates a skeleton schematic of iso-MMC converters without a three- phase interconnection;

[0042] FIG. 17 illustrates a skeleton architecture of DC to three-phase AC iso-MMC- based-MMC topology with no interconnection between the three AC phase modules;

[0043] FIG. 18 illustrates a circuit schematic of iso-MMC Candidate 3 Module Design featuring a distributed full-bridge DC circuits with three distributed transformers between the DC and each of the three-AC phases but not interconnected between the three AC modules;

[0044] FIG. 19 illustrates a circuit schematic of iso-MMC Candidate 3 Module Design with additional annotations for the derivation of the modulation strategy;

[0045] FIG. 20 illustrates the details of a modulation strategy for iso-MMC DC-AC converter system to confirm its feasibility;

[0046] FIG.21 illustrates a skeleton architecture of DC to three-phase AC iso-DDC-based single-star topology;

[0047] FIG.22 illustrates a skeleton architecture of DC to three-phase AC iso-DDC-based- single-star topology with no interconnection between the three AC phase modules;

[0048] FIG.23 illustrates a power converter architecture of a distributed DC to three-phase AC iso-based single-star topology featuring Candidate 3 iso-modules;

[0049] FIG. 24 illustrates a skeleton power converter architecture of a back-to-back connected three-phase AC to three-phase AC iso-based topology with three-phase AC integration. The DC link capacitor ^^ௗ^is optional or could be realized using several lower voltage capacitors in series;

[0050] FIG. 25 illustrates a skeleton power converter architecture of a back-to-back connected three-phase AC to three-phase AC iso-based topology without three-phase AC interconnection. The DC link capacitor ^^ௗ^is optional or could be realized using several lower voltage capacitors in series;Docket No. PSU24301PCT

[0051] FIG. 26 illustrates a skeleton architecture of DC to three-phase AC iso-MMC- based-MMC topology in single-star configuration with three-phase interconnection and a single DC bus;

[0052] FIG. 27 illustrates a skeleton architecture of DC to three-phase AC iso-MMC- based-MMC topology in single-star configuration without three-phase interconnection and a single DC bus; and

[0053] FIG.28 illustrates a multi-pole iso-MMC without single point of failure. DETAILED DESCRIPTION

[0054] Power electronics converters are crucial for transferring power from energy sources to loads, accommodating differences in voltage, current, and frequency. They are used in various applications such as renewable energy integration, HVDC transmission, and industrial motor drives. MMCs are a common topology for these applications due to their modularity and fault tolerance. The proposed iso-MMC architectures aim to enhance power density, scale to ultra-high DC voltage limits, and improve fault-tolerance. The iso-MMC design includes isolation transformers in every module, which differentiates it from conventional MMCs. The design features three-phase AC integration, eliminating the need for bulky capacitors and allowing for higher power density. It also decouples DC currents from AC currents, enabling higher DC voltages without increasing semiconductor ratings.

[0055] The present disclosure describes circuit design innovations suitable for modular power converter systems typically most relevant to medium / high voltage applications. The primary advantages are as follows: (i) increased power density due to three-phase AC integrated designs that eliminate low-frequency harmonic currents leading to high capacitance value requirements, (ii) presence of modular isolation transformers that replace a single line- frequency 60Hz transformers which are bulky and have supply-chain issues, (iii) increased fault-tolerance leading to high reliability due to absence of uncontrolled diode pathways and presence of galvanic isolation at each module level, (iv) decoupling of DC and AC currents in the modules reducing the semiconductor switch design requirements particularly for cases when the voltage transfer ratio is farther than 1. Preliminary simulation results confirm the analytical results and prove the feasibility. The iso-MMC modules can be utilized to form HVDC or distributed DC-AC converter systems. Further, note that any of the proposed DC- AC circuit designs can be cascaded together to form AC / AC converters to connect two AC systems operating at different voltages and / or frequencies.

[0056] iso-MMC ArchitecturesDocket No. PSU24301PCT

[0057] Embodiments of the disclosed technology are directed to a power conversion architecture for HVDC Systems targeting (i) high converter power density and reduced substation size & cost, (ii) scaling to ultra-high DC voltage limits permitting longer distances for transmission without penalty in semiconductor switch ratings, and (iii) higher fault- tolerance capability. The proposed family of circuit design architectures are titled as iso-MMC to highlight the contrasting feature i.e. the presence of isolation transformers in every module. While iso-MMC retains the skeleton structure of conventional half-bridge (HB) MMCs, it has distinct differentiating features such as (i) three-phase AC integration between the modules which eliminates the need for bulky capacitors due to absence of low frequency circulating currents and permits high power density, (ii) presence of isolation transformers in every module which can enable improved power density and fault-tolerance due to the distributed isolation of the DC and AC grid, (iii) the decoupling of DC currents from AC currents enabling higher DC voltages without penalty in semiconductor ratings. The next few figures illustrate the several converter design variants that belong to the family of iso-MMC converters.

[0058] 3-Phase Integrated Designs

[0059] In order to draw attention to the distinctions between the skeleton circuit networks of conventional MMC and iso-MMC circuit designs, simple block diagram circuit figures are used, also illustrated in FIGs.4 and 5, respectively. The Phase A, Phase B, and Phase C circuits are each identified in FIGs.4 and 5.

[0060] As discussed previously, the arms in conventional MMCs feature both DC and AC currents. Further, there are no interconnections between the three AC phases or intermediate isolation transformers. These features are in contrast to iso-MMC designs, also shown in FIG. 5. One common feature in all iso-MMC designs, which is in contrast to conventional MMC designs, is the presence of a DC-side circuits in addition to the three-phase AC module circuits. The DC circuit segments have been highlighted in purple traces throughout the document. The presence of module transformers are highlighted in the green boxes. The interconnections of the DC side circuits and the AC side circuits in purple and gray traces respectively highlight the potential to decouple the DC and AC side currents (in stark contrast to conventional MMCs).

[0061] FIG.6 illustrates the circuit schematic of Candidate 1 module design. This module features an integrated DC full-bridge circuit (shown in purple rectangular box). The midpoints of the full-bridges connect to an isolation transformer which is shared between the DC and AC modules. The AC modules comprise of two pairs of back-to-back connected semiconductor devices. The inductor and capacitor elements filter out the switching waveforms derived fromDocket No. PSU24301PCTthe switching action. The inductors (^^^, ^^^, ^^^) could simply be the parasitic leakageinductance of the transformer or could be introduced as additional elements to meet the filtering design specifications. Further, note that while MOSFET symbols are utilized to represent the presence of switches, any other power semiconductor switching device technology can be utilized including IGBTs, HEMTs, etc.. The switching devices could be silicon-based or wide- bandgap-based (such as silicon carbide or gallium nitride) or ultra-wide-bandgap-based devices currently under research. Lastly, the back-to-back devices permitting bidirectional voltage blocking capability and bidirectional current carrying capability (i.e. four-quadrant devices) could be replaced by a single device when such devices are widely available off-the-shelf in the future. The back-to-back switches could be replaced with a single switch with unidirectional voltage blocking capability by maintaining a DC bias across the AC valve arms.

[0062] FIG.7 illustrates the circuit schematic of Candidate 2 module design. This module features an integrated DC half-bridge circuit per-phase (shown in purple rectangular box). The midpoints of the half-bridges connect to an isolation transformer which is shared between the DC and AC modules. Due to the presence of three primary windings and a half-bridge design, the primary side features a star-connected configuration. Similar to Candidate 1, the AC modules comprise of two pairs of back-to-back connected semiconductor devices. The inductor and capacitor elements filter out the switching waveforms derived from the switching action. The back-to-back switches could be replaced with a single switch with unidirectional voltage blocking capability by maintaining a DC bias across the AC valve arms.

[0063] FIG.8 illustrates the circuit schematic of Candidate 3 module design. This module features a distributed DC full-bridge circuit per-phase (shown in three purple rectangular boxes for one phase each). The midpoints of the full-bridges connect to their own isolation transformer. Note that the isolation transformer is now not shared between the three AC phases. Similar to Candidate 1, the AC modules comprise of two pairs of back-to-back connected semiconductor devices. The inductor and capacitor elements filter out the switching waveforms derived from the switching action. While in the case of Candidate 1&2, the DC circuit is shared between the three AC phases, in the case of Candidate 3, separate DC circuits exist. However, note that the DC circuits are connected to each other to avoid low-frequency harmonics due to three-phase power balance. The back-to-back switches could be replaced with a single switch with unidirectional voltage blocking capability by maintaining a DC bias across the AC valve arms.

[0064] FIG.9 illustrates the circuit schematic of Candidate 4 module design. This module features a distributed DC half-bridge circuit per-phase (shown in three purple rectangular boxesDocket No. PSU24301PCT for one phase each). The midpoints of the half-bridges connect to their own isolation transformer. The return path forms a star connected configuration. Similar to Candidate 3, note that the isolation transformer is now not shared between the three AC phases. Similar to all the candidates, the AC modules comprise of two pairs of back-to-back connected semiconductor devices. The inductor and capacitor elements filter out the switching waveforms derived from the switching action. Again, note that the DC circuits are connected to each other to avoid low- frequency harmonics due to three-phase power balance. The back-to-back switches could be replaced with a single switch with unidirectional voltage blocking capability by maintaining a DC bias across the AC valve arms.

[0065] While four variants of iso-MMC modules have been presented, the application design specifications could determine the best suitable candidate for specific cases. For example, Candidate 3 would best suited for very-high-power systems due to the highest level of modularity. Similarly, Candidate 1 is expected to be most suitable for lower power designs due to reduced number of switches requiring smaller auxiliary circuits such as gate drivers.

[0066] FIGs. 6-9 lay emphasis on the module design. The interconnection of several of these modules in series to realize an HVDC system is showcased in FIG.10. Each of the fourcandidates comprise (i) three “^^ ^" ports that connect to the “^^ െ" AC ports of the ^^^ െ 1^௧^module, (ii) three “^^ െ" ports that connect to the “^^ ^" AC ports of the ^^^ ^ 1^௧^ module, (iii)one “^^^^ ^" port that connect to the “^^^^ െ" DC port of the ^^^ െ 1^௧^ module, and (iv) one“^^^^ െ" port that connect to the “^^^^ ^" DC port of the ^^^ ^ 1^௧^ module. Here, ^^ ൌ ^^, ^^, ^^.To reiterate, note the structural similarity between the conventional HB module-based MMC circuits and FIG. 10 with the following two exceptions: (i) the three AC phase modules are now integration i.e. they are interconnected with each other, (ii) iso-MMC modules comprise of circuit elements associated with the DC side resulting in two additional ports in the module design, and (iii) the DC and AC current pathways are now decoupled.

[0067] FIG. 11 illustrates the power converter architecture of DC to three-phase AC iso- MMC-based-MMC topology featuring Candidate 2 iso-MMC modules. FIG.12 illustrates the power converter architecture of DC to three-phase AC iso-MMC-based-MMC topology featuring Candidate 3 iso-MMC modules. Note that similar circuits can be drawn out for the remaining candidates.

[0068] The proposed circuit designs that use any of the candidates (FIGs.6-9) as modules to form modular multi-level power converter permit innovation impact over the conventional designs in the following three areas, which are permitted by the inherent circuit design.Docket No. PSU24301PCT

[0069] Design Feature 1: High Power Density by eliminating the 60Hz Transformer & Bulky Module Capacitors

[0070] The iso-MMC circuit concept will inherently improve the power density of MMC systems in two ways:

[0071]

[0072] hile still providing galvanic isolation between the DC and AC terminals by modular isolation transformers. Note that the frequency, ^^, is inversely proportional to the area product of the transformer. Hence, high-frequency modular isolation transformers operating at a few kilo- Hertz will inherently be significantly smaller than a 60Hz transformer. Note that certain applications may drive the use of lower frequency operation and hence, lower frequency transformer designs which are also permissible.

[0073] (ii) The iso-MMC eliminates the need of bulky module capacitors otherwise necessary in conventional half / full-bridge MMC designs due to circulating currents imposed by the circuit design. In contrast, iso-MMC features a three-phase integrated design. Because the average module capacitor current over one switching frequency cycle is zero, iso-MMC module capacitors will be significantly smaller than HB module capacitors, by a factor of .ratings

[0076] Theoretically, conventional MMCs are capable of reaching high voltage and power ratings due to their modularity. However, the penalty in semiconductor switch ratings (VA or voltage ൈ current) increases roughly linearly as a function of the voltage transfer ratio, ^^, defined as the ratio between the DC voltage and the three-phase AC voltage. This trend isDocket No. PSU24301PCT summarized in FIG. 13 and is, more fundamentally, a result of both DC and AC current circulation in the half-bridge modules. Similar trends are observed in other classical topologies where the penalty rises as ^^ is farther away from 1. In contrast, the iso-MMC circuit concept overcomes this dependence by exploiting the turns ratio, ^^, of the high-frequency transformers. More fundamentally, as noted in FIG.12, the separation of the DC and AC currents in the dcvt and acvt respectively, due to the modular transformers, permit this design feature. Ideally, the VA can be maintained at a constant value but a multi-objective optimization methodology, which includes the transformer design variables, will be adopted for overall system sizing. In summary, while iso-MMC features multiple stages of power conversion, which is in contrast to a conventional half-bridge module design, the iso-MMC semiconductor ratings are significantly lower for the high DC voltages. Further optimization is achievable in iso-MMCs with advancements in development of four-quadrant devices when two devices in the acvt can be replaced by a single device.

[0077] Design Feature 3: High Fault-Tolerance Capability due to absence of uncontrolled DC fault current pathways and galvanic isolation at each module

[0078] The lack of fault-tolerance to DC short-circuit faults in half-bridge MMC converters motivates the use of either full-bridge modules, which increase cost and reduce efficiency, or HVDC circuit breakers with limited performance in response times and voltage capabilities. In contrast, the iso-MMC circuit design does not feature any uncontrolled paths. FIG. 14 illustrates preliminary simulation results confirming that the AC grid doesn’t continuously feed a short-circuit fault at the DC ports.

[0079] FIG. 15 illustrates circuit simulation results to confirm the feasibility of iso-MMC HVDC systems for Candidate 3 (FIGs.8, 12). The top window waveforms present the DC and AC side voltages. The next window shows the DC and AC side currents. The middle window shows the capacitor voltage of one module. The next window shows the capacitor current averaged over one switching cycle. Note that the capacitor current is zero confirming the analytical work that concludes elimination of bulky capacitance requirements. Lastly, the switch blocking voltage of the DC and AC valve switches are shown.

[0080] 1-Phase Designs

[0081] In the conventional MMC circuit designs, the three AC phases are not interconnected to each other. As noted above, this is the fundamental reason for high capacitance requirement due to presence of low-frequency harmonic currents in the modules. The proposed iso-MMC designs feature an interconnection between the three AC phase modules to overcome this drawback due to power balance. However, this design feature has aDocket No. PSU24301PCT trade-off - the interconnection drives proximity of the modules which poses a voltage isolation risk. Each of the modules have to be rated to withstand the high DC voltage. Hence, the design specifications can drive the need for disintegration of the three-phase connection. FIGs.16 and 17 present the skeleton design for such cases. FIG.18 illustrates the circuit schematic with iso- MMC Candidate 3. Note the absence of interconnection between the three AC phases. Similar design is feasible with Candidate 4. These designs are still expected to be more efficient, power dense (due to the presence of high-frequency transformers), and fault tolerant as compared to conventional MMC designs.

[0082] An Example Modulation Strategy

[0083] In order to realize the desired power conversion operation, various modulation strategies can be developed with emphasis on optimization of semiconductor switch ratings, capacitor minimization, voltage transfer ratio, total harmonic distortion, losses, and so on. Further, these modulation strategies could be application-specific. This section proposes a modulation strategy to confirm the feasibility of the iso-MMC concept under discussion. Various other modulation strategies can be derived for the various iso-MMC candidates and is not the primary focus of this innovation. This section also presents preliminary quantitative analysis for comparison between iso-MMC and state-of-the-art.

[0084] The iso-MMC candidate from FIG.8 is considered for the analysis due to its highest degree of modularity in comparison to other candidates. Similar extensions are possible for other candidates. FIG.19 illustrates Candidate 3 with additional annotations featuring various voltages, currents, and switching functions critical to derive the modulation strategy. FIG. 20 presents the switching functions of the semiconductor switches corresponding to Phase A along for selected voltage waveforms for the purposes of illustration.

[0085] Observing the Phase A DC valve, the circuit comprises of two half-bridges. The first half-bridge switches are labeled as ^^^^and ^^′^^. The midpoint of the half-bridge connects to the positive terminal of the transformer. The second half-bridge switches are labeled as ^^^ଶand ^^′^ଶ. The midpoint of the second half-bridge connects to the negative terminal of the transformer. Their corresponding switching functions are denoted with the variable ℎ, whereℎ ൌ 1 denotes that the switch is “on" and ℎ ൌ 0 denotes that the switch is “off". The isolationtransformer comprises of a single primary winding and a secondary winding with midpoint access with the turns ratio of 1:t:t.

[0086] Observing the Phase A AC valve, the circuit comprises of four switches. Switches ^^^ଷ^and ^^^ଷ^are connected back-to-back and provide the bidirectional blocking voltage andDocket No. PSU24301PCT bidirectional current carrying capability for AC operation. These switches connect between the positive and the midpoint terminals of the secondary winding. Similarly, switches ^^^ସ^and ^^^ସ^are connected back-to-back and provide the bidirectional blocking voltage and bidirectional current carrying capability for AC operation. These switches connect between the negative and the midpoint terminals of the secondary winding. Again, their corresponding switching functions are denoted with the variable ℎ. For the purposes of the discussed modulation approach, the switching functions of switches ^^^ଷ^and ^^^ଷ^are synchronized and denoted as ℎ^ଷ. Similarly, the switching functions of switches ^^^ସ^and ^^^ସ^are synchronized and denoted as ℎ^ସ. Commutation issues can drive distinct switching functions for the series connected switches and has not been considered for feasibility studies.

[0087] As a review, in a full-bridge converter, when both of the top switches (in this case, ^^^^and ^^^ଶ) are “on" or “off", the output voltage (in this case the primary-side transformer voltage, ^^^ି^) is zero. When the first half-bridge top switch is “on" and the second half-bridge top switch is “off", the output voltage is ^^^^^. Lastly, when the first half-bridge top switch is “off" and the second half-bridge top switch is “on", the output voltage is െ^^^^^.

[0088] Let us consider FIG.20 (left) for illustration purposes which considers an instant of operation when the ac output voltage is positive. In the proposed modulation strategy, the switching function of the first half-bridge, ℎ^^, is considered as the reference signal. The second half-bridge switching function, ℎ^ଶ, is delayed by duty ratio, ^^^as denoted in FIG.20. Considering the full-bridge converter operation, the primary-side transformer voltage, ^^^ି^, can be derived and is illustrated in the third window. The secondary-side transformer voltage follows ^^^ି^except that it is scaled by the transformer turns-ratio. Finally, the AC valve switches, ^^^ଷ^and ^^^ଷ^, are utilized to rectify the transformer AC voltage to a positive value, as shown in the plot of the unfiltered voltage ^^௨ି^. The ^^^, ^^^elements filter out of the switched waveform, also shown in the bottom window. This concept can be quantified using equations as characterized next. Note that the width of the duty ratio, ^^^, determines the magnitude of the output voltage, ^^^ି^. The duty ratio of the ℎ^ଷand ℎ^ସswitching functions are 50% where ℎ^ଷis “on" during the positive 60Hz cycle and ℎ^ସis “on" during the negative 60Hz cycle.

[0089] Using the principles of state-space averaging, the module’s output voltage can be quantified as a function of the duty ratio and the effective voltage, as noted in (6):

[0090] Docket No. PSU24301PCT

[0091] The above equation can be rearranged to arrive at the duty ratio expression (7):

[0092]

[0093] Several of topologyfor an HVDC system. voltages and the DC and the ac voltage is noted in (8) and (9), respectively.

[0094]

[0095] and

[0096]

[0097] where, nnumber of series connected iso-The duty ratio expression that relates the system DC voltage to the AC voltage is now finally derived, as shown in (10):

[0098]

[0099] Similarthat for a negative output voltage, the AC valve switches, ^^^ସ^and ^^^ସ^, are utilized to rectify the transformer AC voltage to a negative value. This operation is illustrated in FIG. 20 (middle). The entire converter operation for several 60Hz cycle is shown in FIG. 20 (right). This modulation strategy has also been adopted to arrive at the simulation results of FIG.15 as well as the quantified comparative results discussed next.

[0100] Preliminary Comparative Study: iso-MMC versus HB-MMC

[0101] In order to quantify the expected qualitative benefits in terms of semiconductor switch rating and energy storage requirements, preliminary analytical calculations and simulation verification studies have been conducted. Table 1 compares the iso-MMC Candidate 3 with half-bridge module based HVDC converter systems for a design example.

[0102] Referring to FIG.13 for comparison of total semiconductor voltage ൈ current (VA) rating, the design example’s voltage transfer ratio, ^^, is ଷଷ^^^ ଷଷ^^^^ଶ^ൌ 5.77. Note that theanalytical results from the chart match with the simulation results noted in the table. ComparingDocket No. PSU24301PCT the capacitance energy storage, the per-unit values are related by the ratio of line-frequency and switching frequency, also concluded in Design Feature 1 section above.

[0103]

[0104] thefollowing = = = Power frequency = 60Hz, Switching Frequency = 5kHz, n=60.

[0105] Table 2 qualitatively compares the iso-MMC Candidate 3 with (FIG. 12) and without (FIG. 18) three-phase integration with half-bridge module based HVDC converter systems for a design example.

[0106] Referring to FIG.13 for comparison of total semiconductor voltage ൈ current (VA) rating, iso-MMC designs are expected to have improved switching ratings (and hence, efficiency) for HVDC specifications with high voltage transfer ratio. In terms of power density,3-phase integrated (3^^ െ ^^) designs are expected to be more power dense due to capacitor sizereduction and use of high-frequency isolation transformers. Similarly, while the 1-phase (1^^) designs are expected to be more power dense but only due to the use of high-frequency isolation transformers. In general, iso-MMC designs are expected to be more fault tolerance due to absence of uncontrolled fault current pathways during a DC short circuit fault and due to the presence of galvanically isolated transformers at each module level. These qualitative results have been quantified in Table 3.

[0107]

[0108] Table 2: Qualitative Comparison and Generic Conclusions.Docket No. PSU24301PCT

[0109]

[0110] with the following parameters: DC Voltage = 330kV, AC Phase Voltage = 33kV, Power Rating = 500MVA, Power frequency = 60Hz, Switching Frequency = 5kHz, n=60.

[0111] iso-MMC for Distributed DC (DDC) Systems

[0112] As noted previously, while the innovation discussed in this document is inspired by the limitations in modern-day technologies of HVDC systems, many other application spaces can also benefit from this work. Other application areas include integration of photovoltaic and battery energy storage power plants to the electric grid, industrial AC drives, fast charges for electrified transportation systems, large electrolysers for hydrogen production, to name a few. FIGs.21 and 22 illustrate the skeleton schematics for three-phase integrated and single-phase designs.

[0113] Note that in contrast to the HVDC skeleton structure (FIGs. 10 and 17), DDC systems comprise of a single-star network. Further, the DC ports of each of the modules comprise of DC energy sources or loads. As an example, in a photovoltaic power plant, the DCsources ^^^^^^1 െ ^^^^^^^^ could be photovoltaic panels. In a battery energy storage power plant,the DC sources ^^^^^^1 െ ^^^^^^^^ could be batteries. A hybrid photovoltaic+battery energy storagepower plant would comprise of a combination of photovoltaic panels and batteries.

[0114] FIG. 23 illustrates the circuit schematic where the iso-MMC Candidate 3 is populated in FIG. 21. Several other variants can be derived by combining the iso-MMC candidates with three-phase integrated and single-phase designs.

[0115] iso-MMC for Back-To-Back AC-AC Systems

[0116] The above discussion focuses on DC to three-phase AC power converters. The proposed innovations in circuit designs are also applicable to back-to-back connected AC-DC- AC power conversion applications such as industrial AC drives and solid-state transformers, to name a few. FIGs. 24 and 25 illustrate the block diagram schematic of AC-DC-AC converters for three-phase integrated and single-phase designs. Any of the iso-MMCDocket No. PSU24301PCT candidates (FIGs.6-9) can be populated as modules for such applicable cases for power transfer between the ABC and PQR AC systems.

[0117] FIG. 26 illustrates a skeleton architecture of the DC to three-phase AC iso-MMC- based-MMC topology in single-star configuration with three-phase interconnection and a single DC bus. In this example, it will be noted that any of the four iso-MMC module candidates can be used as the iso-MMC module.

[0118] FIG. 27 illustrates a skeleton architecture of the DC to three-phase AC iso-MMC- based-MMC topology in single-star configuration with three-phase interconnection and a single DC bus. In this example, it will be noted that any of the four iso-MMC module candidates can be used as the iso-MMC module.

[0119] Fault tolerance of HVDC substations is a major barrier for scaling up rated power, e.g., symmetric monopole MMC-HVDC are limited by maximum power system contingency limits (e.g., 1.5GW in WECC). To eliminate line frequency transformers and DC cables as single point of failures, bipolar HVDC configurations with two-line frequency transformers are required that further increase weight, size, and cost of substations. In contrast, as shown in FIG. 28, iso-MMC eliminates line frequency transformers as single points of failure and readily scales beyond MMC-HVDC power capacity limits by combining modular high-frequency transformers with additional DC conductors for power capacity and fault tolerance. EXAMPLES

[0120] In an example, a modular multi-level power converter (MMC) includes: an integrated direct current (DC) module shared between multiple alternating current (AC) phases; a plurality of AC modules, each corresponding to one of the AC phases; and a plurality of isolation transformers electrically coupled between the DC module and each of the plurality of AC modules.

[0121] In certain examples, the DC module is a full-bridge circuit. In other examples, the DC module is a half-bridge circuit.

[0122] In certain examples, each of the plurality of AC modules includes two pairs of back- to-back connected semiconductor devices.

[0123] In certain examples, the two pairs of back-to-back connected semiconductor devices permit bidirectional voltage blocking capability and bidirectional current carrying capability.

[0124] In certain examples, the back-to-back switches could be replaced with a single switch with unidirectional voltage blocking capability by maintaining a DC bias across the AC valve arms.Docket No. PSU24301PCT

[0125] In certain examples, each of the plurality of AC modules includes switching devices.

[0126] In certain examples, the switching devices are MOSFETs, IGBTs, or HEMTs.

[0127] In certain examples, the switching devices are silicon-based or wide-bandgap-based or ultra-wide-bandgap-based devices.

[0128] In certain examples, the switching devices use silicon carbide or gallium nitride semiconductor materials.

[0129] In certain examples, each of the plurality of AC modules includes an inductor element and a capacitor element configured to filter out switching waveforms derived from switching action.

[0130] In certain examples, each inductor element is parasitic leakage inductance of the isolation transformer.

[0131] In certain examples, each inductor element is introduced as an additional element to meet filtering design specifications.

[0132] In an example, a modular multi-level power converter (MMC) includes: an integrated direct current (DC) module shared between multiple alternating current (AC) phases, the DC module having a plurality of integrated DC half-bridge circuits; a plurality of AC modules, each corresponding to one of the AC phases; and a plurality of isolation transformers shared between the DC and AC modules, wherein midpoints of the half-bridge circuits electrically connect to the plurality of isolation transformers.

[0133] In certain examples, each of the plurality of AC modules includes two pairs of back- to-back connected semiconductor devices.

[0134] In certain examples, the back-to-back switches could be replaced with a single switch with unidirectional voltage blocking capability by maintaining a DC bias across the AC valve arms.

[0135] In certain examples, each of the plurality of AC modules includes an inductor element and a capacitor element configured to filter out switching waveforms derived from switching action.

[0136] In an example, a modular multi-level power converter (MMC) includes: an integrated direct current (DC) module having an integrated DC full-bridge circuit per-phase; a plurality of alternating current (AC) modules; and a plurality of isolation transformers shared between the DC and AC modules, wherein midpoints of the full-bridge circuits electrically connect to a corresponding dedicated one of the plurality of isolation transformers.Docket No. PSU24301PCT

[0137] In certain examples, each of the plurality of AC modules includes two pairs of back- to-back connected semiconductor devices.

[0138] In certain examples, the back-to-back switches could be replaced with a single switch with unidirectional voltage blocking capability by maintaining a DC bias across the AC valve arms.

[0139] In certain examples, each of the plurality of AC modules includes an inductor element and a capacitor element configured to filter out switching waveforms derived from switching action.

[0140] In certain examples, the full-bridge circuits are connected to each other to avoid low-frequency harmonics due to three-phase power balance.

[0141] In an example, a modular multi-level power converter (MMC) includes: an integrated direct current (DC) module having a distributed DC half-bridge circuit per-phase; a plurality of alternating current (AC) modules; and a plurality of isolation transformers shared between the DC and AC modules, wherein midpoints of the half-bridge circuits electrically connect to a corresponding dedicated one of the plurality of isolation transformers.

[0142] In certain examples, each of the plurality of AC modules includes two pairs of back- to-back connected semiconductor devices.

[0143] In certain examples, the back-to-back switches could be replaced with a single switch with unidirectional voltage blocking capability by maintaining a DC bias across the AC valve arms.

[0144] In certain examples, each of the plurality of AC modules includes an inductor element and a capacitor element configured to filter out switching waveforms derived from switching action.

[0145] In certain examples, the half-bridge circuits are connected to each other to avoid low-frequency harmonics due to three-phase power balance.

[0146] The description of embodiments has been presented for purposes of illustration and description. Suitable modifications and variations to the embodiments may be performed in light of the above description or may be acquired from practicing the methods. The methods may be performed by executing stored instructions with one or more logic devices (e.g., processors) in combination with one or more hardware elements, such as storage devices, memory, hardware network interfaces / antennas, switches, actuators, clock circuits, and so on. The described methods and associated actions may also be performed in various orders in addition to the order described in this application, in parallel, and / or simultaneously. The described systems are exemplary in nature, and may include additional elements and / or omitDocket No. PSU24301PCT elements. The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various systems and configurations, and other features, functions, and / or properties disclosed.

[0147] As used herein, the terms “system” or “module” or “modulator” may include a hardware and / or software system that operates to perform one or more functions. For example, a module or system may include a computer processor, controller, or other logic-based device that performs operations based on instructions stored on a tangible and non-transitory computer readable storage medium, such as a computer memory. Alternatively, a module or system may include a hard-wired device that performs operations based on hard-wired logic of the device. Various modules or units shown in the attached figures may represent the hardware that operates based on software or hardwired instructions, the software that directs hardware to perform the operations, or a combination thereof.

[0148] The foregoing described aspects depict different components contained within, or connected with different other components. It is to be understood that such depicted architectures are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively "associated" such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as "associated with" each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being "operably connected," or "operably coupled," to each other to achieve the desired functionality.

[0149] As used in this application, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural of said elements or steps, unless such exclusion is stated. Furthermore, references to “one embodiment” or “one example” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. The terms “first,” “second,” “third,” and so on are used merely as labels, and are not intended to impose numerical requirements or a particular positional order on their objects. The following claims particularly point out subject matter from the above disclosure that is regarded as novel and non-obvious.

Claims

Docket No. PSU24301PCT CLAIMS:

1. A modular multi-level power converter (MMC), comprising: an integrated direct current (DC) module shared between multiple alternating current (AC) phases; a plurality of AC modules, each corresponding to one of the AC phases; and a plurality of isolation transformers electrically coupled between the DC module and each of the plurality of AC modules.

2. The MMC of claim 1, wherein the DC module is a full-bridge circuit.

3. The MMC of claim 1, wherein each of the plurality of AC modules includes two pairs of back-to-back connected semiconductor devices.

4. The MMC of claim 3, wherein the two pairs of back-to-back connected semiconductor devices permit bidirectional voltage blocking capability and bidirectional current carrying capability.

5. The MMC of claim 4, wherein the back-to-back switches are replaced with a single switch with unidirectional voltage blocking capability by maintaining a DC bias across AC valve arms.

6. The MMC of claim 1, wherein each of the plurality of AC modules includes switching devices.

7. The MMC of claim 6, wherein the switching devices are MOSFETs, IGBTs, or HEMTs.

8. The MMC of claim 6, wherein the switching devices are silicon-based or wide- bandgap-based or ultra-wide-bandgap-based devices.

9. The MMC of claim 1, wherein each of the plurality of AC modules includes an inductor element and a capacitor element configured to filter out switching waveforms derived from switching action.Docket No. PSU24301PCT 10. The MMC of claim 9, wherein each inductor element is parasitic leakage inductance of the isolation transformer.

11. The MMC of claim 9, wherein each inductor element is introduced as an additional element to meet filtering design specifications.

12. A modular multi-level power converter (MMC), comprising: an integrated direct current (DC) module shared between multiple alternating current (AC) phases, the DC module having a plurality of integrated DC half-bridge circuits; a plurality of AC modules, each corresponding to one of the AC phases; and a plurality of isolation transformers shared between the DC and AC modules, wherein midpoints of the half-bridge circuits electrically connect to the plurality of isolation transformers.

13. The MMC of claim 12, wherein each of the plurality of AC modules includes two pairs of back-to-back connected semiconductor devices.

14. The MMC of claim 13, wherein the back-to-back switches are replaced with a single switch with unidirectional voltage blocking capability by maintaining a DC bias across AC valve arms.

15. The MMC of claim 12, wherein each of the plurality of AC modules includes an inductor element and a capacitor element configured to filter out switching waveforms derived from switching action.

16. A modular multi-level power converter (MMC), comprising: an integrated direct current (DC) module having an integrated DC full-bridge circuit per-phase; a plurality of alternating current (AC) modules; and a plurality of isolation transformers shared between the DC and AC modules, wherein midpoints of the full-bridge circuits electrically connect to a corresponding dedicated one of the plurality of isolation transformers.Docket No. PSU24301PCT 17. The MMC of claim 16, wherein each of the plurality of AC modules includes two pairs of back-to-back connected semiconductor devices.

18. The MMC of claim 16, wherein each of the plurality of AC modules includes an inductor element and a capacitor element configured to filter out switching waveforms derived from switching action.

19. The MMC of claim 16, wherein the full-bridge circuits are connected to each other to avoid low-frequency harmonics due to three-phase power balance.

20. A modular multi-level power converter (MMC), comprising: an integrated direct current (DC) module having a distributed DC half-bridge circuit per-phase; a plurality of alternating current (AC) modules; and a plurality of isolation transformers shared between the DC and AC modules, wherein midpoints of the half-bridge circuits electrically connect to a corresponding dedicated one of the plurality of isolation transformers.

21. The MMC of claim 19, wherein each of the plurality of AC modules includes two pairs of back-to-back connected semiconductor devices.

22. The MMC of claim 19, wherein each of the plurality of AC modules includes an inductor element and a capacitor element configured to filter out switching waveforms derived from switching action.

23. The MMC of claim 19, wherein the half-bridge circuits are connected to each other to avoid low-frequency harmonics due to three-phase power balance.

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