# US 11,909,214 B2 · Topology of series-connected MMC with a small number of modules

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- Number: US 11,909,214 B2
- Document ID: US11909214B2
- Status: Granted
- Kind: B2
- Application number: 17/640,603
- Filed: 2021-05-07
- Published: 2023-01-19
- Granted: 2024-02-20
- Earliest claimed: 2021-03-08
- Assignees: Southeast University
- Applicants: Southeast University
- Inventors: Wu Chen; Liangcai Shu; Haozhe Jin; Xiaokun He
- Web page: https://patentgrep.com/patent/US11909214B2

## CPC

- H02J 3/38: Arrangements for feeding a single network from two or more generators or sources in parallel; Arrangements for feeding already energised networks from additional generators or sources in parallel
- H02J 3/36: Arrangements for transfer of electric power between AC networks via high-voltage DC [HVDC] links; Arrangements for transfer of electric power between generators and networks via HVDC links
- H02M 1/0074: {Plural converter units whose inputs are connected in series}
- H02M 1/0095: {Hybrid converter topologies, e.g. NPC mixed with flying capacitor, thyristor converter mixed with MMC or charge pump mixed with buck}
- H02M 7/4835: {comprising two or more cells, each including a switchable capacitor, the capacitors having a nominal charge voltage which corresponds to a given fraction of the input voltage, and the capacitors being selectively connected in series to determine the instantaneous output voltage}
- H02M 7/487: Neutral point clamped inverters
- H02M 7/5395: by pulse-width modulation
- Y02E 60/60: Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]
- H02J: ELECTRIC POWER NETWORKS; CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02M: APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF (transformers H01F; dynamo-electric converters H02K47/00; controlling transformers, reactors or choke coils, control or regulation of electric motors, generators or dynamo-electric converters H02P)
- Y02E: REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION

## Abstract

The present disclosure relates to the field of electric power system, and more particularly to a topology of a series-connected MMC with a small number of modules, where the topology is composed of a three-phase bridge circuit, half-bridge valve strings, a three-phase filter inductor, and a three-phase grid frequency transformer. The topology of a series-connected MMC with a small number of modules in the present disclosure needs only two half-bridge valve strings, thus greatly reducing the number of the submodules as compared with the conventional MMC structure. When achieving the same high DC voltage output, the present disclosure can improve the power density of the MMC, realize stable three-phase AC output voltage, and further achieve balance of capacitor voltages in the two half-bridge valve strings. Compared to the conventional MMC topology, the topology in the present disclosure can reduce the number of submodules by nearly ⅔, and has a greater AC-DC voltage transfer ratio, thus reducing the cost of the MMC converter, reducing the device size, and improving the power density.

## Claims

### Claim 1

A topology of a series-connected MMC converter with a small number of modules, wherein the topology is composed of a three-phase bridge circuit, half-bridge valve strings, a three-phase filter inductor, and a three-phase grid frequency transformer;
the three-phase bridge circuit is formed by series connection of a phase-A bridge circuit, a phase-B bridge circuit, and a phase-C bridge circuit, any of which is formed by combination of a high-voltage DC capacitor, a first high-voltage switch string, and a second high-voltage switch string;
and the first high-voltage switch string and the second high-voltage switch string are connected in series and then connected in parallel to the high-voltage DC capacitor;
the half-bridge valve string is formed by series connection of a set of half-bridge submodules, full-bridge submodules, or half-bridge/full-bridge hybrid submodules; and
the other terminals of the three-phase filter inductor are respectively connected to primary winding terminals of the three-phase grid frequency transformer, and secondary winding terminals of the three-phase grid frequency transformer form a high-voltage AC port; and a connection manner of the three-phase grid frequency transformer is one of Y/Y, Δ/Y, Δ/Δ, and Y/Δ structures.

### Claim 2 (depends on 1)

The topology of a series-connected MMC converter with a small number of modules according to claim 1, wherein the bridge circuit of any phase is a three-level structure.

### Claim 3 (depends on 1)

The topology of a series-connected MMC converter with a small number of modules according to claim 1, wherein the lower terminal of the first high-voltage switch string and the upper terminal of the second high-voltage switch string together form an AC output midpoint of a bridge circuit of the corresponding phase; and the capacitor positive terminal of the phase-A bridge circuit and the capacitor negative terminal of the phase-C bridge circuit form a high-voltage DC port.

### Claim 4 (depends on 1)

The topology of a series-connected MMC converter with a small number of modules according to claim 1, wherein the high-voltage switch string in the bridge circuit of each phase is composed of a set of series-connected semiconductor switches or a set of series-connected half-bridge modules, wherein any half-bridge module consists of two semiconductor switches and one equalizing capacitor.

### Claim 5 (depends on 1)

The topology of a series-connected MMC converter with a small number of modules according to claim 1, wherein the AC output midpoint of the phase-A bridge circuit is connected to the positive terminal of a half-bridge valve string 1, and the negative terminal of the half-bridge valve string 1 is connected to the phase-A filter inductor;
the AC output midpoint of the phase-C bridge circuit is connected to the negative terminal of a half-bridge valve string 2, and the positive terminal of the half-bridge valve string 2 is connected to the phase-C filter inductor; and
the AC output midpoint of the phase-B bridge circuit is directly connected to the phase-B filter inductor.

### Claim 6 (depends on 1)

The topology of a series-connected MMC converter with a small number of modules according to claim 1, wherein switches in the three-phase bridge circuit are all controlled with duty ratio of 50%, and drive signals of first and second switches in the bridge circuit of each phase are opposite.

### Claim 7 (depends on 1)

The topology of a series-connected MMC converter with a small number of modules according to claim 1, wherein in the phase-A bridge circuit, a first switch drive signal vSA1 and a phase-A modulation wave vA are identical in phase;
in the phase-B bridge circuit, a first switch drive signal vSB1 and a phase-B modulation wave vB are identical in phase; and
in the phase-C bridge circuit, a first switch drive signal vSC1 and a phase-C modulation wave vC are identical in phase.

### Claim 8 (depends on 7)

The topology of a series-connected MMC converter with a small number of modules according to claim 7, wherein a modulation wave signal of the first half-bridge valve string 1 connected to the midpoint of the phase-A bridge circuit is (vSA1−vSB1)×(⅔×VM)/(N×VSM)−(vA−vB), VM being the voltage across the high-voltage DC port, N being the number of submodules in a half-bridge valve string, and VSM being the capacitor voltage of the submodules in the half-bridge valve string; and
a modulation wave signal of the second half-bridge valve string 2 connected to the midpoint of the phase-C bridge circuit is (vSB1−vSC1)×(⅔×VM)/(N×VSM)−(vB−vC), where three-phase modulation waves vA, vB, vC can be obtained by closed-loop control on the capacitor voltage of the valve strings or control over the AC-side voltages or currents.