A low carrier ratio and ultra-low carrier ratio three-level NPC converter neutral point voltage balance topology and control method

Abstract

The application belongs to the technical field of power electronics, and discloses a low-carrier-ratio and ultra-low-carrier-ratio three-level NPC converter midpoint voltage balance topology and control method based on hybrid bridge arm simplification, which is suitable for multi-electric aircraft high-speed motor driving and large-power electric propulsion fields such as ships and mine trucks. A three-phase hybrid bridge arm auxiliary circuit is additionally arranged in the three-phase three-level NPC circuit, the auxiliary circuit is composed of a three-level bridge arm and two two-level bridge arms, a hybrid bridge arm topology structure of one three-level bridge arm and two two-level bridge arms or two three-level bridge arms and one two-level bridge arm is adopted, and the three-level bridge arm is an I-type or T-type bridge arm. The auxiliary circuit adjusts the midpoint voltage by adopting a space vector modulation, zero sequence injection carrier modulation or model prediction control method, so that the main circuit realizes the balance control of the DC bus capacitor midpoint voltage under the condition of a low carrier ratio or an ultra-low carrier ratio. The application significantly reduces the system complexity, reduces the system volume and improves the power density.

Description

Technical Field

[0001] This invention belongs to the field of power electronics technology, specifically relating to a midpoint voltage balancing topology and control method for a low carrier ratio and ultra-low carrier ratio three-level NPC converter based on a simplified hybrid bridge arm, applicable to high-speed motor drives for multi-electric aircraft and high-power electric propulsion applications such as ships and mining trucks. Background Technology

[0002] With the development of new energy sources, multilevel topologies have gradually gained attention. Among multilevel topologies, the three-level neutral-point-clamped (NPC) topology is the most classic, mainly divided into two types: the I-type bridge arm, which consists of four switches connected in series, and the T-type clamping switch, which has the switch perpendicular to the bridge arm. This topology is widely used in medium and high voltage power electronic conversion fields due to its low output harmonic distortion rate, small voltage change rate (dv / dt), strong electromagnetic interference suppression capability, and the ability to use low-voltage power devices to reduce system costs. However, the neutral point voltage of the three-phase three-level NPC circuit has a pulsation problem, mainly at the third harmonic. When the neutral point voltage is unbalanced, it not only increases the output waveform distortion rate and causes uneven voltage distribution in power devices, but in severe cases, it can also damage power switching devices or bus capacitors.

[0003] For conventional high carrier ratio operating conditions, where the load current can be approximated as constant during the switching cycle, existing technologies have various midpoint voltage balance control methods, including space vector modulation, zero-sequence injected carrier modulation, and model predictive control, which have achieved good results in practical applications.

[0004] However, in applications such as ship electric propulsion, large mining trucks, high-speed trains, and multi-electric aircraft, systems typically need to operate at low carrier ratios (generally 10-20) or even ultra-low carrier ratios (≤10) to reduce switching losses. Under these low and ultra-low carrier ratio conditions, the load current varies significantly within a single switching cycle and can no longer be approximated as a constant value. Therefore, the traditional midpoint voltage balancing method based on the assumption of constant load current within the switching cycle is no longer applicable.

[0005] To address the aforementioned issues, existing research has proposed a main-auxiliary circuit collaborative control strategy. This strategy adds three identical three-level bridge arms to the main circuit to form an auxiliary circuit, achieving good midpoint voltage balance under low and ultra-low carrier ratio conditions. However, the introduction of the auxiliary circuit in this topology significantly increases the number of power devices, and the topology has not yet been optimized for the small-size application requirements under low and ultra-low carrier ratio conditions.

[0006] In summary, the existing technology still has the following shortcomings:

[0007] (1) Existing midpoint voltage balancing methods mostly rely on the assumption of a high carrier ratio. However, under low and ultra-low carrier ratio conditions, the control strategy based on the average midpoint current model no longer holds, which will lead to the failure of midpoint voltage balancing;

[0008] (2) In high-speed motor drive and high-power electric propulsion systems, in order to reduce switching losses, the system usually needs to operate in a low carrier ratio or even an ultra-low carrier ratio state. However, under such conditions, the classic control strategy is difficult to maintain the DC side bus capacitor voltage balance, and the midpoint voltage imbalance problem has become a key technical problem restricting the development of three-level NPC converter technology;

[0009] (3) For low carrier ratio and ultra-low carrier ratio conditions, although the existing improved three-level topology can achieve midpoint voltage balance to a certain extent, the three-phase three-level auxiliary circuit it uses requires three additional three-level bridge arms, which significantly increases the number of power devices and increases the system complexity. There is an urgent need for a simple and reliable method to maintain the midpoint voltage balance of the three-phase three-level topology under low carrier ratio and ultra-low carrier ratio. Summary of the Invention

[0010] The purpose of this invention is to propose a midpoint voltage balancing topology for three-level NPC converters suitable for low and ultra-low carrier ratios, based on a simplified hybrid bridge arm structure. This topology replaces the traditional three-phase three-level auxiliary circuit with a hybrid structure consisting of three-level and two-level bridge arms, reducing up to four clamping diodes, four power switching devices, and their corresponding four drive circuits. While ensuring midpoint voltage balance under low and ultra-low carrier ratio conditions, this topology effectively reduces system complexity and the number of power devices, thereby increasing overall power density.

[0011] This invention is implemented as follows: a midpoint voltage balancing topology for a low carrier ratio and ultra-low carrier ratio three-level NPC converter, comprising:

[0012] Three-level NPC main circuit and auxiliary circuit;

[0013] The three-level NPC main circuit includes a DC bus, the midpoint of the voltage divider capacitor, and three-phase three-level bridge arms.

[0014] The auxiliary circuit is a hybrid bridge arm topology consisting of a three-level bridge arm and a two-level bridge arm;

[0015] The three-level bridge arm of the auxiliary circuit shares the DC bus and the midpoint of the voltage divider capacitor with the three-level NPC main circuit; the two-level bridge arm of the auxiliary circuit shares the DC bus with the three-level NPC main circuit.

[0016] The auxiliary circuit is used to adjust the midpoint voltage balance when the three-level NPC main circuit is operating at low carrier ratio and ultra-low carrier ratio.

[0017] Furthermore, the three-level NPC main circuit is a type I three-phase three-level NPC circuit, and the auxiliary circuit consists of one type I three-level bridge arm and two two-level bridge arms.

[0018] Furthermore, the three-level NPC main circuit is a type I three-phase three-level NPC circuit, and the auxiliary circuit consists of two type I three-level bridge arms and one two-level bridge arm.

[0019] Furthermore, the three-level NPC main circuit is a T-type three-phase three-level NPC circuit, and the auxiliary circuit consists of a T-type three-level bridge arm and two two-level bridge arms.

[0020] Furthermore, the three-level NPC main circuit is a T-type three-phase three-level NPC circuit, and the auxiliary circuit consists of two T-type three-level bridge arms and one two-level bridge arm.

[0021] Another objective of this invention is to provide a midpoint voltage balance control method for a low carrier ratio and ultra-low carrier ratio three-level NPC converter based on a simplified hybrid bridge arm, comprising the following steps:

[0022] Step 1: Set up an auxiliary circuit for the three-level NPC main circuit. The auxiliary circuit is a hybrid bridge arm topology consisting of a three-level bridge arm and a two-level bridge arm.

[0023] Step 2: Make the three-level bridge arm of the auxiliary circuit share the DC bus and the midpoint of the voltage divider capacitor with the three-level NPC main circuit, and make the two-level bridge arm of the auxiliary circuit share the DC bus with the three-level NPC main circuit.

[0024] Step 3: When the three-level NPC main circuit is operating at low carrier ratio and ultra-low carrier ratio, the auxiliary circuit is used to adjust the midpoint voltage balance.

[0025] Furthermore, the auxiliary circuit employs one of the following methods to adjust the midpoint voltage balance: space vector modulation, zero-sequence injection carrier modulation, or model predictive control.

[0026] Furthermore, the auxiliary circuit is a hybrid bridge arm topology consisting of a type I bridge arm and a two-level bridge arm; when the auxiliary circuit is composed of one type I three-level bridge arm and two two-level bridge arms, compared with the auxiliary circuit composed of three type I three-level bridge arms, it reduces 4 clamping diodes, 4 switching devices, and 4 drive circuits; when the auxiliary circuit is composed of two type I three-level bridge arms and one two-level bridge arm, compared with the auxiliary circuit composed of three type I three-level bridge arms, it reduces 2 clamping diodes, 2 switching devices, and 2 drive circuits.

[0027] Furthermore, the auxiliary circuit is a hybrid bridge arm topology consisting of a T-type bridge arm and a two-level bridge arm; when the auxiliary circuit is composed of one T-type three-level bridge arm and two two-level bridge arms, it reduces 4 switching devices and 4 drive circuits compared to the auxiliary circuit composed of three T-type three-level bridge arms; when the auxiliary circuit is composed of two T-type three-level bridge arms and one two-level bridge arm, it reduces 2 switching devices and 2 drive circuits compared to the auxiliary circuit composed of three T-type three-level bridge arms.

[0028] Furthermore, the auxiliary circuit is composed of two or more sets of hybrid bridge arm topologies connected in parallel. The hybrid bridge arm topology is one or more of the following: a three-phase hybrid bridge arm topology consisting of one type I three-level bridge arm and two two-level bridge arms; a three-phase hybrid bridge arm topology consisting of two type I three-level bridge arms and one two-level bridge arm; a three-phase hybrid bridge arm topology consisting of one type T three-level bridge arm and two two-level bridge arms; and a three-phase hybrid bridge arm topology consisting of two type T three-level bridge arms and one two-level bridge arm.

[0029] In combination with the above technical solutions and the technical problems they solve, the present invention has the following advantages and positive effects compared with the prior art:

[0030] This invention adds an auxiliary circuit to a three-phase three-level NPC circuit. Under low and ultra-low carrier ratio conditions, the output current command of the auxiliary circuit can be specifically designed for the inflow or outflow of the midpoint current, thereby completely decoupling the complex task of midpoint voltage balancing from the main circuit's control objective. Thus, the main circuit can focus on power output under low and ultra-low carrier ratio conditions, while the auxiliary circuit independently handles midpoint voltage balancing control. This clear division of labor significantly reduces the control complexity of the main circuit.

[0031] This invention addresses the structural limitations of existing three-phase three-level auxiliary circuits operating under low and ultra-low carrier ratio conditions by innovatively proposing a hybrid topology consisting of a three-level bridge arm and two-level bridge arms. Taking an auxiliary circuit employing one type-I three-level bridge arm and two two-level bridge arms as an example, this topology can reduce the number of clamping diodes, switching devices, and drive circuits by up to four. Furthermore, the two-level bridge arms in this hybrid topology can utilize a mature two-level drive scheme, further reducing system complexity. This, in turn, improves the overall power density, safety, and operational reliability of the system.

[0032] The hybrid bridge arm topology proposed in this invention includes two basic configurations: one three-level bridge arm plus two two-level bridge arms, and two three-level bridge arms plus one two-level bridge arm. The three-level bridge arms can be either I-type or T-type structures. These various topology combinations offer high flexibility, allowing for the selection of the optimal adaptable structure based on actual application requirements such as different voltage levels, power levels, cost control, and volume constraints. This demonstrates good engineering adaptability and promotional value.

[0033] This invention defines the auxiliary circuit as a hybrid bridge-arm topology consisting of three-level and two-level bridge arms. It further defines the three-level bridge arms as sharing a DC bus and the midpoint of the voltage divider capacitor with the main circuit, and the two-level bridge arms as sharing a DC bus with the main circuit. This structure does not simply reduce the number of auxiliary bridge arms, but rather simplifies the bridge arms that do not directly perform the midpoint connection function to two-level bridge arms while retaining the direct midpoint adjustment channel. It is generally understood that reducing three-level bridge arms may weaken the midpoint voltage regulation capability. However, this solution forms a midpoint current regulation path by connecting the midpoint with three-level bridge arms, and forms a three-phase current channel on the auxiliary side with the cooperation of two-level bridge arms. This allows the auxiliary circuit to still participate in the charging and discharging balance of the voltage divider capacitor at low and ultra-low carrier ratios. The resulting effect is that, under conditions where the modulation freedom of the main circuit is limited, the auxiliary circuit can independently undertake the midpoint voltage regulation task, while reducing the complexity of the auxiliary topology. This achieves the technical effect of retaining midpoint regulation capability and simplifying the bridge arm structure, an effect that cannot be expected simply by reducing the number of components.

[0034] Midpoint voltage balancing in existing three-level NPC converters typically relies on the modulation strategy of the main circuit itself or a complete three-level auxiliary structure. These approaches are either limited by the switching state selection space under low and ultra-low carrier ratios, or require relatively complete three-phase three-level auxiliary bridge arms to support midpoint regulation, resulting in high structural complexity. This invention takes a different approach: without changing the basic output structure of the three-phase three-level NPC main circuit, it configures a hybrid bridge arm auxiliary circuit on the DC bus side, limiting the three-level bridge arm to share the midpoint with the voltage divider capacitor, and limiting the two-level bridge arm to share only the DC bus. This connection reorganizes the energy path between the auxiliary circuit and midpoint voltage regulation, concentrating the midpoint balancing function on the three-level bridge arm with midpoint connection capability, while the two-level bridge arm cooperates to form the auxiliary current path. This approach differs from simply adding auxiliary circuits and from relying solely on redundant vector regulation of the main circuit. Even those skilled in the art who recognize the fluctuations in midpoint voltage under low carrier ratios will find it difficult to directly derive a solution that replaces the complete three-level auxiliary bridge arm with a hybrid bridge arm while still maintaining midpoint regulation functionality. Therefore, this invention possesses substantial features and represents a significant advancement. Attached Figure Description

[0035] Figure 1 This is a diagram of the midpoint voltage balance topology of a three-level NPC converter with low carrier ratio and ultra-low carrier ratio provided in an embodiment of the present invention.

[0036] Figure 2 The main circuit provided in this embodiment of the invention is a type I three-phase three-level NPC circuit, and the auxiliary circuit is a circuit diagram in which a type I three-level bridge arm and two two-level bridge arms constitute a hybrid bridge arm.

[0037] Figure 3The main circuit provided in this embodiment of the invention is a type I three-phase three-level NPC circuit, and the auxiliary circuit is a circuit diagram in which two type I three-level bridge arms and one two-level bridge arm constitute a hybrid bridge arm.

[0038] Figure 4 The main circuit provided in this embodiment of the invention is a T-type three-phase three-level NPC circuit, and the auxiliary circuit is a circuit diagram in which a hybrid bridge arm is composed of one T-type three-level bridge arm and two two-level bridge arms.

[0039] Figure 5 The main circuit provided in this embodiment of the invention is a T-type three-phase three-level NPC circuit, and the auxiliary circuit is a circuit diagram in which two T-type three-level bridge arms and one two-level bridge arm constitute a hybrid bridge arm.

[0040] Figure 6 The circuit diagram is a current type I main-auxiliary topology provided by existing technology. The main circuit is a type I three-phase three-level NPC circuit, and the auxiliary circuit is a complete type I three-level circuit diagram composed of three type I three-level bridge arms.

[0041] Figure 7 The circuit diagram is a conventional T-type main and auxiliary circuit diagram provided by existing technology. The main circuit is a T-type three-phase three-level NPC circuit, and the auxiliary circuit is a complete T-type three-level circuit diagram composed of three T-type three-level bridge arms.

[0042] Figure 8 The main circuit provided in Embodiment 1 of the present invention is a type I three-phase three-level NPC circuit, and the auxiliary circuit is a waveform of the midpoint voltage balance effect of a hybrid bridge arm composed of one type I three-level bridge arm and two two-level bridge arms.

[0043] Figure 9 The main circuit provided in Embodiment 2 of the present invention is a type I three-phase three-level NPC circuit, and the auxiliary circuit is a waveform of the midpoint voltage balance effect of a hybrid bridge arm composed of two type I three-level bridge arms and one two-level bridge arm.

[0044] Figure 10 The main circuit provided in Embodiment 3 of the present invention is a T-type three-phase three-level NPC circuit, and the auxiliary circuit is a waveform of the midpoint voltage balance effect of a hybrid bridge arm composed of one T-type three-level bridge arm and two two-level bridge arms.

[0045] Figure 11 The main circuit provided in Embodiment 4 of the present invention is a T-type three-phase three-level NPC circuit, and the auxiliary circuit is a waveform of the midpoint voltage balance effect of a hybrid bridge arm composed of two T-type three-level bridge arms and one two-level bridge arm.

[0046] Figure 12The main circuit provided in Comparative Example 1 of this invention is a type I three-phase three-level NPC circuit, and the auxiliary circuit is a waveform showing the midpoint voltage balance effect of a complete type I three-level circuit composed of three type I three-level bridge arms.

[0047] Figure 13 The main circuit provided in Comparative Example 2 of this invention is a T-type three-phase three-level NPC circuit, and the auxiliary circuit is a waveform showing the midpoint voltage balance effect of a complete T-type three-level circuit composed of three T-type three-level bridge arms. Detailed Implementation

[0048] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and do not constitute any limitation on the invention.

[0049] Combination Figures 1 to 5 As can be seen, this embodiment of the invention adds a hybrid bridge arm auxiliary circuit to the three-phase three-level NPC main circuit. This auxiliary circuit participates in the regulation of the DC bus midpoint voltage, thereby achieving midpoint voltage balance under low carrier ratio and ultra-low carrier ratio conditions. The main circuit still provides three-phase output to the AC motor. On the DC side, voltage dividing capacitors C1 and C2 form three points: P, O, and N, where point O is the midpoint. dc1 u dc2 These represent the voltages of the upper and lower capacitors, respectively. During low and ultra-low carrier ratio operation, the main circuit switching frequency is low, and the load current cannot be approximated as a constant value within the switching cycle. Relying on the main circuit's own modulation strategy, the sampled value differs significantly from the actual value, and the switching state and modulation freedom available for adjusting the midpoint current are limited, easily leading to voltage imbalance between C1 and C2. This application does not simply add a complete three-phase three-level auxiliary converter, but simplifies the auxiliary circuit into a hybrid bridge arm structure combining three-level and two-level bridge arms. The three-level bridge arm shares the DC bus and the midpoint of the voltage divider capacitor with the main circuit, while the two-level bridge arm only shares the DC bus with the main circuit. This reduces the number of three-level devices in the auxiliary bridge arm while retaining the midpoint adjustment channel.

[0050] Figure 2 An auxiliary circuit for a type I hybrid bridge arm, consisting of a type I three-level bridge arm and two two-level bridge arms, is presented. Figure 3 An auxiliary circuit consisting of two type I three-level bridge arms and one two-level bridge arm is given; Figure 4 , Figure 5 For the corresponding T-type main circuit, hybrid auxiliary topologies are adopted, namely one T-type three-level bridge arm plus two two-level bridge arms, and two T-type three-level bridge arms plus one two-level bridge arm. In each diagram, the auxiliary circuit is connected in parallel with the main circuit to the same DC bus, and the auxiliary side output current i a_s i b_s ic_s A controllable current path is formed via an auxiliary branch. When u is detected... dc1 with u dc2 When a deviation exists, the controller can change the switching state of the auxiliary bridge arm through space vector modulation, zero-sequence injection carrier modulation, or model predictive control, so that the auxiliary circuit flows into or out of the midpoint O with the corresponding current, thereby changing the charging and discharging relationship of C1 and C2 and making the midpoint potential tend to be balanced.

[0051] This invention addresses the difficulty of adjusting the midpoint voltage in low-carrier-ratio and ultra-low-carrier-ratio three-level NPC converters. Instead of relying on conventional adjustments using redundant vectors within the main circuit or employing a complete three-phase three-level auxiliary circuit, it proposes a simplified auxiliary topology composed of a hybrid of three-level and two-level bridge arms. This structure retains the three-level adjustment capability directly coupled to the midpoint O while reducing the complexity of the auxiliary circuit by converting some bridge arms to two levels. Furthermore, it can be adapted to both type I and type T three-level NPC main circuits, forming... Figures 1 to 5 The diagram shows several feasible structures. Therefore, this scheme is specifically designed around midpoint voltage balance in both topology and controlled object, exhibiting clear targeting and technical characteristics that distinguish it from conventional three-level NPC circuits.

[0052] In the control method of this invention, when the three-level NPC main circuit operates under low carrier ratio and ultra-low carrier ratio conditions, the main circuit's own switching frequency is low, and the load current cannot be approximated as a constant value within the switching cycle. This results in a significant difference between the sampled value and the actual value of the main circuit controller, and the switching state and modulation freedom available for adjusting the midpoint current are limited. Furthermore, the DC-side voltage divider capacitor is prone to voltage imbalance between the upper and lower capacitors. Therefore, an auxiliary circuit is added outside the main circuit to form an independent midpoint voltage adjustment channel. For the type I bridge arm auxiliary topology, the auxiliary circuit can be composed of one type I three-level bridge arm and two two-level bridge arms, or two type I three-level bridge arms and one two-level bridge arm. For the type T bridge arm auxiliary topology, the auxiliary circuit can be composed of one type T three-level bridge arm and two two-level bridge arms, or two type T three-level bridge arms and one two-level bridge arm. In this circuit, the three-level bridge arm shares the DC bus and the midpoint of the voltage divider capacitor with the main circuit, enabling it to directly participate in midpoint current regulation. The two-level bridge arm shares the DC bus with the main circuit, working in conjunction with the three-level bridge arm to form a three-phase auxiliary output. Thus, without altering the basic three-phase three-level NPC structure of the main circuit, the auxiliary circuit adjusts the switching state of the auxiliary bridge arm using methods such as space vector modulation, zero-sequence injection carrier modulation, or model predictive control. This allows the auxiliary circuit to participate in the charging and discharging balance of the upper and lower voltage divider capacitors according to the direction of the midpoint voltage deviation, thereby suppressing midpoint voltage offset.

[0053] This invention utilizes a simplified structure of hybrid bridge arms in the auxiliary circuit and its integration with the midpoint balance control objective. Traditionally, a complete three-phase three-level auxiliary circuit requires three-level bridge arms for each of the three auxiliary arms, resulting in a large number of components and drivers. This method does not simply stack complete auxiliary three-level conversion units; instead, it retains one or two three-level bridge arms capable of connecting to the midpoint of the voltage divider capacitors, while simplifying the remaining arms to two-level bridge arms. For a type I auxiliary circuit, combining one type I bridge arm with two two-level bridge arms reduces the number of clamping diodes, switching devices, and driving circuits by 4 compared to three type I bridge arms; combining two type I bridge arms with one two-level bridge arm reduces the number of clamping diodes, switching devices, and driving circuits by 2. For a type T auxiliary circuit, combining one type T bridge arm with two two-level bridge arms reduces the number of switching devices and driving circuits by 4 compared to three type T bridge arms; combining two type T bridge arms with one two-level bridge arm reduces the number of switching devices and driving circuits by 2. It can be seen that this method reduces the device size and driving complexity of the auxiliary topology while maintaining the adjustable midpoint voltage function of the auxiliary circuit. It also allows two or more sets of hybrid bridge arm topologies to be used in parallel, and is compatible with type I or type T three-phase three-level NPC main circuits. It has the technical characteristics of simplification of structure and control function.

[0054] This invention provides a midpoint voltage balancing topology for a low carrier ratio and ultra-low carrier ratio three-level NPC converter based on a simplified hybrid bridge arm, which is illustrated below through specific embodiments.

[0055] The present invention adopts the following four preferred technical solutions:

[0056] 1. The main circuit is a type I three-phase three-level NPC circuit, and the auxiliary circuit consists of one type I three-level bridge arm and two two-level bridge arms.

[0057] Main circuit: Type I three-phase three-level NPC circuit, each phase bridge arm consists of four switching transistors (S... x1 -S x4 ) and two clamping diodes (D x1 and D x2 It is composed of a main power output under low carrier ratio and ultra-low carrier ratio conditions.

[0058] Auxiliary circuit: A hybrid bridge topology consisting of one type I three-level bridge arm and two two-level bridge arms. The type I bridge arm shares the DC bus and the midpoint of the voltage divider capacitor with the main circuit; the two-level bridge arms only share the DC bus with the main circuit.

[0059] Features: The main circuit operates at low and ultra-low carrier ratios, undertaking the main power output; the auxiliary circuit operates at high carrier ratios, responsible for balancing the midpoint voltage of the capacitor. Compared with the auxiliary circuit composed of three type I three-level NPC bridge arms (12 switching transistors + 6 clamping diodes), this scheme reduces 4 clamping diodes, 4 switching transistors and their corresponding 4 drive circuits, significantly simplifying the structure.

[0060] 2. The main circuit is a type I three-phase three-level NPC circuit, and the auxiliary circuit consists of two type I three-level bridge arms and one two-level bridge arm.

[0061] Main circuit: Type I three-phase three-level NPC circuit, each phase bridge arm consists of four switching transistors (S... x1 -S x4 ) and two clamping diodes (D x1 and D x2 It is composed of ) and serves as the main power output under low carrier ratio and ultra-low carrier ratio conditions.

[0062] Auxiliary circuit: A hybrid bridge topology consisting of two type I three-level bridge arms and one two-level bridge arm. The type I bridge arms share the DC bus and the midpoint of the voltage divider capacitor with the main circuit; the two-level bridge arm only shares the DC bus with the main circuit.

[0063] Features: The main circuit operates at low and ultra-low carrier ratios, and is responsible for the main power output; the auxiliary circuit operates at high carrier ratios and is responsible for balancing the midpoint voltage of the capacitor. Compared with the auxiliary circuit composed of three type I three-level NPC bridge arms (12 switching transistors + 6 clamping diodes), this scheme reduces 2 clamping diodes, 2 switching transistors and their corresponding 2 drive circuits.

[0064] 3. The main circuit is a T-type three-phase three-level NPC circuit, and the auxiliary circuit consists of one T-type three-level bridge arm and two two-level bridge arms.

[0065] Main circuit: T-type three-phase three-level NPC circuit, each phase consisting of two bridge arm switching transistors (S... x1 ,S x4 ) and two reverse series clamping switches (S x2 ,S x3 It is composed of a main power output under low carrier ratio and ultra-low carrier ratio conditions.

[0066] Auxiliary circuit: A hybrid bridge topology consisting of one T-type three-level bridge arm and two two-level bridge arms. The T-type bridge arm shares the DC bus and the midpoint of the voltage divider capacitor with the main circuit; the two-level bridge arms only share the DC bus with the main circuit.

[0067] Features: The main circuit operates at low and ultra-low carrier ratios and is responsible for the main power output; the auxiliary circuit operates at high carrier ratios and is responsible for balancing the midpoint voltage of the capacitor. Compared with the auxiliary circuit composed of three T-type three-level NPC bridge arms (12 switching transistors), this scheme reduces 4 switching transistors and their corresponding 4-way drive circuits.

[0068] 4. The main circuit is a T-type three-phase three-level NPC circuit, and the auxiliary circuit consists of two T-type three-level bridge arms and one two-level bridge arm.

[0069] Main circuit: T-type three-phase three-level NPC circuit, each phase consisting of two bridge arm switching transistors (S... x1 ,S x4 ) and two reverse series clamping switches (S x2 ,S x3 It is composed of ) and serves as the main power output under low carrier ratio and ultra-low carrier ratio conditions.

[0070] Auxiliary circuit: A hybrid bridge topology consisting of two T-type three-level bridge arms and one two-level bridge arm. The T-type bridge arms share the DC bus and the midpoint of the voltage divider capacitor with the main circuit; the two-level bridge arm only shares the DC bus with the main circuit.

[0071] Features: The main circuit operates at low and ultra-low carrier ratios and is responsible for the main power output; the auxiliary circuit operates at high carrier ratios and is responsible for balancing the midpoint voltage of the capacitor. Compared with the auxiliary circuit composed of three T-type three-level NPC bridge arms (12 switching transistors), this scheme reduces 2 switching transistors and their corresponding 2 drive circuits.

[0072] Example 1

[0073] This embodiment provides a technical solution under low carrier ratio and ultra-low carrier ratio conditions. The solution adopts a type I three-phase three-level main circuit and is supplemented by an auxiliary circuit consisting of a type I three-level bridge arm and two two-level bridge arms forming a hybrid bridge arm.

[0074] Main circuit: Type I three-phase three-level NPC circuit.

[0075] Auxiliary circuit: Phase A uses a type I three-level bridge arm, sharing the DC bus and the midpoint of the voltage divider capacitor with the main circuit; Phases B and C use traditional two-level bridge arms, sharing only the DC bus with the main circuit. Figure 2 As shown in the diagram. This auxiliary circuit uses a total of 2 clamping diodes and 8 switching devices.

[0076] Parameters: The main circuit does not include midpoint balancing control, its fundamental frequency is 500Hz, its carrier frequency is 1500Hz, and its carrier ratio is 3 (which is an ultra-low carrier ratio); the auxiliary circuit adopts a control strategy with midpoint voltage balancing, its fundamental frequency is 50Hz, its carrier frequency is 10kHz, and its carrier ratio is 200 (which is a high carrier ratio).

[0077] Verification results: Under the action of the auxiliary circuit, the adjustment time for the midpoint voltage to recover from the initial deviation to the equilibrium state is 1.49ms, the steady-state midpoint voltage ripple is 6.30V, and the voltage ripple accounts for 1.17% of the total voltage. Figure 8 As shown.

[0078] Example 2

[0079] This embodiment provides a technical solution under low carrier ratio and ultra-low carrier ratio conditions. The solution adopts a type I three-phase three-level main circuit and is supplemented by an auxiliary circuit consisting of two type I three-level bridge arms and one two-level bridge arm to form a hybrid bridge arm.

[0080] Main circuit: Type I three-phase three-level NPC circuit.

[0081] Auxiliary circuit: Phases A and B use a type I three-level bridge arm, sharing the DC bus and the midpoint of the voltage divider capacitor with the main circuit. Phase C uses a traditional two-level bridge arm, sharing only the DC bus with the main circuit. Figure 3 As shown in the diagram. This auxiliary circuit uses a total of 4 clamping diodes and 10 switching devices.

[0082] Parameters: exactly the same as in Example 1.

[0083] Verification results: The settling time for the midpoint voltage to recover from the initial deviation to the equilibrium state was 1.51 ms; the steady-state midpoint voltage ripple was 6.15 V; and the voltage ripple accounted for 1.14% of the total voltage. Figure 9 As shown.

[0084] Example 3

[0085] This embodiment provides a technical solution under low carrier ratio and ultra-low carrier ratio conditions. The solution adopts a T-type three-phase three-level main circuit and is supplemented by an auxiliary circuit consisting of a T-type three-level bridge arm and two two-level bridge arms forming a hybrid bridge arm.

[0086] Main circuit: T-type three-phase three-level NPC circuit.

[0087] Auxiliary circuit: Phase A uses a T-type three-level bridge arm, sharing the DC bus and the midpoint of the voltage divider capacitor with the main circuit. Phases B and C use traditional two-level bridge arms, sharing only the DC bus with the main circuit. Figure 4 As shown in the diagram. This auxiliary circuit uses a total of 8 switching devices.

[0088] Circuit parameters: exactly the same as in Example 1.

[0089] Verification results: The settling time for the midpoint voltage to recover from the initial deviation to the equilibrium state was 1.49 ms; the steady-state midpoint voltage ripple was 6.30 V; and the voltage ripple accounted for 1.17% of the total voltage. Figure 10 As shown.

[0090] Example 4

[0091] This embodiment provides a technical solution under low carrier ratio and ultra-low carrier ratio conditions. The solution adopts a T-type three-phase three-level main circuit and is supplemented by an auxiliary circuit consisting of two T-type three-level bridge arms and one two-level bridge arm forming a hybrid bridge arm.

[0092] Main circuit: T-type three-phase three-level NPC circuit.

[0093] Auxiliary circuit: Phases A and B use a T-type three-level bridge arm, sharing the DC bus and the midpoint of the voltage divider capacitor with the main circuit. Phase C uses a traditional two-level bridge arm, sharing only the DC bus with the main circuit. Figure 5 As shown in the figure. This auxiliary circuit uses a total of 10 switching devices.

[0094] Circuit parameters: exactly the same as in Example 1.

[0095] Verification results: The settling time for the midpoint voltage to recover from the initial deviation to the equilibrium state was 1.50 ms; the steady-state midpoint voltage ripple was 5.92 V; and the voltage ripple accounted for 1.10% of the total voltage. Figure 11 As shown.

[0096] Comparative Example 1

[0097] To compare the effects of this invention, Comparative Example 1 shows a complete Type I auxiliary circuit scheme under low and ultra-low carrier ratio conditions. This scheme is not a technical solution claimed in this invention. This scheme uses a Type I three-phase three-level main circuit, supplemented by a Type I three-phase three-level bridge arm auxiliary circuit composed of three Type I three-level bridge arms.

[0098] Main circuit: Type I three-phase three-level NPC circuit.

[0099] Auxiliary circuit: Type I three-phase three-level NPC circuit, each bridge arm shares the DC bus and the midpoint of the voltage divider capacitor with the main circuit, such as... Figure 6 As shown in the diagram, this auxiliary circuit uses a total of 6 clamping diodes and 12 switching devices.

[0100] Circuit parameters: exactly the same as in Example 1.

[0101] Verification results: The settling time for the midpoint voltage to recover from the initial deviation to the equilibrium state was 2.44 ms; the steady-state midpoint voltage ripple was 5.08 V; and the voltage ripple accounted for 0.94% of the total voltage. Figure 12 As shown.

[0102] Comparative Example 2

[0103] To compare the effects of this invention, Comparative Example 2 demonstrates a complete T-type auxiliary circuit scheme under low and ultra-low carrier ratio conditions. This scheme is not a technical solution claimed in this invention. This scheme employs a T-type three-phase three-level main circuit, supplemented by a T-type three-phase three-level bridge arm auxiliary circuit composed of three T-type three-level bridge arms.

[0104] Main circuit: T-type three-phase three-level NPC circuit.

[0105] Auxiliary circuit: T-type three-phase three-level NPC circuit, each bridge arm shares the DC bus and the midpoint of the voltage divider capacitor with the main circuit, such as... Figure 7 As shown in the diagram. This auxiliary circuit uses a total of 12 switching devices.

[0106] Circuit parameters: exactly the same as in Example 1.

[0107] Verification results: The settling time for the midpoint voltage to recover from the initial deviation to the equilibrium state was 2.34 ms; the steady-state midpoint voltage ripple was 5.24 V; and the voltage ripple accounted for 0.97% of the total voltage. Figure 13 As shown.

[0108] Comparative analysis of examples:

[0109] Under the above conditions, the number of components, dynamic response time of midpoint voltage, and steady-state ripple of the four auxiliary circuit topologies are compared, and the results are shown in Table 1.

[0110] Table 1 Comparison of auxiliary circuit parameters and results in Examples 1 to 4

[0111]

[0112] The settling time of Examples 1 to 4 is around 1.50ms, which is better than the 2.44ms and 2.34ms of Comparative Examples 1 and 2, respectively. The ripple of the comparative examples is better than that of the examples, with Comparative Example 1 having the smallest ripple at 5.08V, accounting for 0.94% of the total voltage. Among Examples 1 to 4, compared to Comparative Example 1, which has an auxiliary circuit with three I-arms, six clamping diodes, 12 switching devices, and 12 switch drivers, Example 3 uses the fewest components, with only eight switching devices and eight switch drivers. Example 4 has the best steady-state ripple at 5.92V, accounting for 1.10% of the total voltage, and uses a moderate number of components. Overall, as the number of three-level bridge arms in the auxiliary circuit decreases, the number of components and system complexity decrease significantly, and the dynamic response time is around 1.50ms, while the steady-state ripple increases slightly from 5.92V to 6.30V.

[0113] Therefore, in low-complexity and small-volume application scenarios, the auxiliary circuit proposed in this invention, consisting of one type I or type T three-level bridge arm and two two-level bridge arms, or consisting of two type I or type T three-level bridge arms and one two-level bridge arm, has significant engineering advantages over the existing three-level bridge arm auxiliary circuits.

[0114] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made by those skilled in the art within the spirit and principles of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A midpoint voltage balancing topology for a low carrier ratio and ultra-low carrier ratio three-level NPC converter, characterized in that, Includes a three-level NPC main circuit and auxiliary circuit; The three-level NPC main circuit includes a DC bus, the midpoint of the voltage divider capacitor, and three-phase three-level bridge arms. The auxiliary circuit is a hybrid bridge arm topology consisting of a three-level bridge arm and a two-level bridge arm; The three-level bridge arm of the auxiliary circuit shares the DC bus and the midpoint of the voltage divider capacitor with the three-level NPC main circuit; the two-level bridge arm of the auxiliary circuit shares the DC bus with the three-level NPC main circuit. The auxiliary circuit is used to adjust the midpoint voltage balance when the three-level NPC main circuit is operating at low carrier ratio and ultra-low carrier ratio.

2. The midpoint voltage balancing topology of the low carrier ratio and ultra-low carrier ratio three-level NPC converter according to claim 1, characterized in that, The three-level NPC main circuit is a type I three-phase three-level NPC circuit, and the auxiliary circuit consists of one type I three-level bridge arm and two two-level bridge arms.

3. The midpoint voltage balancing topology of the low carrier ratio and ultra-low carrier ratio three-level NPC converter according to claim 1, characterized in that, The three-level NPC main circuit is a type I three-phase three-level NPC circuit, and the auxiliary circuit consists of two type I three-level bridge arms and one two-level bridge arm.

4. The midpoint voltage balancing topology of the low carrier ratio and ultra-low carrier ratio three-level NPC converter according to claim 1, characterized in that, The three-level NPC main circuit is a T-type three-phase three-level NPC circuit, and the auxiliary circuit consists of a T-type three-level bridge arm and two two-level bridge arms.

5. The midpoint voltage balancing topology of the low carrier ratio and ultra-low carrier ratio three-level NPC converter according to claim 1, characterized in that, The three-level NPC main circuit is a T-type three-phase three-level NPC circuit, and the auxiliary circuit consists of two T-type three-level bridge arms and one two-level bridge arm.

6. A method for controlling the midpoint voltage balance of a low carrier ratio and ultra-low carrier ratio three-level NPC converter based on a hybrid bridge arm simplified topology, implementing the midpoint voltage balance topology of the low carrier ratio and ultra-low carrier ratio three-level NPC converter as described in any one of claims 1-6, characterized in that, Includes the following steps: Step 1: Set up an auxiliary circuit for the three-level NPC main circuit. The auxiliary circuit is a hybrid bridge arm topology consisting of a three-level bridge arm and a two-level bridge arm. Step 2: Make the three-level bridge arm of the auxiliary circuit share the DC bus and the midpoint of the voltage divider capacitor with the three-level NPC main circuit, and make the two-level bridge arm of the auxiliary circuit share the DC bus with the three-level NPC main circuit. Step 3: When the three-level NPC main circuit is operating at low carrier ratio and ultra-low carrier ratio, the auxiliary circuit is used to adjust the midpoint voltage balance.

7. The method according to claim 6, characterized in that, The auxiliary circuit uses one of the following methods to adjust the midpoint voltage balance: space vector modulation, zero-sequence injection carrier modulation, or model predictive control.

8. The method according to claim 6, characterized in that, The auxiliary circuit is a hybrid bridge arm topology consisting of a type I bridge arm and a two-level bridge arm. When the auxiliary circuit is composed of one type I three-level bridge arm and two two-level bridge arms, it reduces the number of clamping diodes, switching devices, and driving circuits by 4 compared to the auxiliary circuit composed of three type I three-level bridge arms. When the auxiliary circuit is composed of two type I three-level bridge arms and one two-level bridge arm, it reduces the number of clamping diodes, switching devices, and driving circuits by 2 compared to the auxiliary circuit composed of three type I three-level bridge arms.

9. The method according to claim 6, characterized in that, The auxiliary circuit is a hybrid bridge arm topology consisting of a T-type bridge arm and a two-level bridge arm. When the auxiliary circuit is composed of one T-type three-level bridge arm and two two-level bridge arms, it reduces 4 switching devices and 4 drive circuits compared to the auxiliary circuit composed of three T-type three-level bridge arms. When the auxiliary circuit is composed of two T-type three-level bridge arms and one two-level bridge arm, it reduces 2 switching devices and 2 drive circuits compared to the auxiliary circuit composed of three T-type three-level bridge arms.

10. The method according to claim 6, characterized in that, The auxiliary circuit is composed of two or more sets of hybrid bridge arm topologies connected in parallel. The hybrid bridge arm topology is one or more of the following: a three-phase hybrid bridge arm topology consisting of one type I three-level bridge arm and two two-level bridge arms; a three-phase hybrid bridge arm topology consisting of two type I three-level bridge arms and one two-level bridge arm; a three-phase hybrid bridge arm topology consisting of one type T three-level bridge arm and two two-level bridge arms; and a three-phase hybrid bridge arm topology consisting of two type T three-level bridge arms and one two-level bridge arm.

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