Direct-current ship power grid fault flexible ride-through method based on zero-sequence current injection

Abstract

The invention discloses a direct-current ship power grid fault flexible ride-through method based on zero-sequence current injection, which belongs to the technical field of direct-current ship power grid fault protection, and comprises the following steps: when a switch fault is detected, positioning the position of a fault switch; calculating the degree of asymmetry of the direct-current voltage according to the output voltage requirement of the inverter; the direct-current voltage asymmetry degree serves as input, the rectifier is controlled through carrier amplitude modulation and zero-sequence component injection, and asymmetric direct-current voltage is output; the method comprises the following steps of: determining locking and conducting switches when different switch faults occur, and controlling an inverter to realize generation of separated alternating-current phase voltage and optimal zero-sequence component injection through two-level carrier modulation according to asymmetric direct-current voltage so as to complete flexible fault ride-through of the frequency converter. By adopting the method, the processes of capacitor discharge, energy feedback and the like in the fault period are adjusted through the control factors, system instability and equipment reverse rotation are prevented, and continuous power supply and operation safety of a ship power grid are guaranteed.

Description

Technical Field

[0001] This invention belongs to the field of DC ship power grid fault protection technology, and in particular relates to a flexible fault ride-through method for DC ship power grids based on zero-sequence current injection. Background Technology

[0002] Currently, in the field of DC shipboard power grid fault protection, traditional technologies mainly face several key issues. On the one hand, most shipboard DC power grids typically employ a grid structure with weak energy storage (i.e., eliminating large-capacity supporting capacitors on the DC link) or rely on fast DC circuit breakers for fault isolation in order to suppress fault currents. While these methods can quickly de-energize or disconnect faults, they may also lead to power outages, reduced system reliability, and excessively high requirements for circuit breaker performance and cost. Especially when dealing with asymmetrical faults (such as single-phase grounding), traditional flexible DC control and wind power control often control the negative sequence current to zero, resulting in a lack of an effective negative sequence fault current path in the system. This may cause AC overvoltage at the converter station outlet, threatening equipment insulation and increasing costs. On the other hand, existing fault-tolerant strategies for modular multilevel converters (MMCs), such as using redundant submodules or coordinating control with mechanical circuit breakers (by generating negative level outputs to quickly reduce current), can improve reliability. However, in scenarios where redundancy is exhausted or continuous power supply is required (such as critical loads on ships), there may still be drawbacks such as uneven stress on submodules, large energy dissipation during fault clearing, slow recovery speed, and an inability to balance fault isolation and uninterrupted operation.

[0003] In DC marine power grid systems, electric propulsion frequency converters are key energy conversion devices, and their operational reliability directly affects the ship's power performance and power supply continuity. When the power switching devices of the frequency converter fail, traditional protection methods often adopt the strategy of immediately shutting down or disconnecting the faulty unit. Although this can protect the equipment from damage, it will lead to the loss of power to the propulsion system, interruption of ship operation, and may even cause safety accidents under certain operating conditions. Summary of the Invention

[0004] This invention addresses the problem that existing frequency converters typically require shutdown protection or complex hardware redundancy schemes when switching faults occur. It achieves uninterrupted operation and performance optimization under fault conditions through an innovative flexible fault ride-through control method.

[0005] To achieve the above objectives, this invention provides a flexible fault ride-through method for DC ship power grids based on zero-sequence current injection, comprising the following steps: S1. When a switch fault is detected, locate the position of the faulty switch; S2. Based on the location, calculate the required DC voltage asymmetry of the rectifier according to the inverter output voltage requirements; S3. Using the DC voltage asymmetry as input, the rectifier is controlled by carrier amplitude modulation and zero-sequence component injection to output an asymmetrical DC voltage. S4. Determine the blocking and conducting switches when different switch faults occur, and at the same time, based on the asymmetrical DC voltage, control the inverter to generate separate AC phase voltages and inject the optimal zero-sequence component through two-level carrier modulation, thereby completing the inverter's flexible fault ride-through.

[0006] Preferably, the rectifier outputs a neutral point current. It is expressed as follows: in, , It is the current flowing through the upper and lower DC capacitors. It is the neutral point input current of the inverter. It is a DC capacitor. For DC voltage asymmetry, This represents the total DC voltage.

[0007] Preferably, the neutral point current is controlled by the rectifier output. And introduce additional control factors This is used to adjust the degree of asymmetry.

[0008] Preferably, the rectifier outputs a neutral point current. The relationship between the modulated wave and the input phase current is expressed as follows: in, , and They represent a Phase modulation wave, b Phase modulation wave and c Phase modulation wave, , and They represent a Phase current, b Phase current and c Phase current.

[0009] Preferably, the modulated wave is the original wave generated by dual closed-loop control.

[0010] Preferably, adjustment is made by inserting a proportional-integral controller. ,Will It is injected into the three-phase modulated wave to control the degree of asymmetry.

[0011] Preferably, in order to fully utilize the asymmetrical voltage and avoid overmodulation, the modulated wave is shifted to the middle of the carrier wave by injecting a DC zero-sequence component. The injected DC zero-sequence component is calculated as follows: in, This represents the injected DC zero-sequence component.

[0012] The technical effects of this invention are as follows: by controlling the rectifier output asymmetrical DC voltage through carrier amplitude modulation and zero-sequence component injection, a negative sequence current path is actively provided, the fault current amplitude is limited, the reliance on hardware redundancy and circuit breakers is reduced, and the system is guaranteed to operate stably under fault conditions. Attached Figure Description

[0013] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings: Figure 1 This is a flowchart of the DC ship power grid fault flexible ride-through method based on zero-sequence current injection in an embodiment of the present invention; Figure 2 This is a schematic diagram of optimized rectifier modulation with zero-sequence component injection in an embodiment of the present invention, wherein (a) is x When <0, (b) is x >0; Figure 3 This is a schematic diagram of closed-loop control of DC voltage asymmetry in an embodiment of the present invention; Figure 4 The switch in the embodiment of the present invention S u1 A schematic diagram of phase-tolerance control for open-circuit faults; Figure 5 This is a schematic diagram of the phase-to-fault-tolerant control of optimal zero-sequence current injection in an embodiment of the present invention, (a) is x When <0, (b) is x >0. Detailed Implementation

[0014] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0015] Example After the fault occurs, the DC-side capacitor of the propulsion motor converter first discharges rapidly, causing the DC bus voltage to drop rapidly. Subsequently, the motor speed decreases, and the current consumption decreases. If the voltage cannot recover, the speed will continue to drop to zero. Afterward, the motor may begin to reverse under the influence of load torque (such as propeller hydraulic torque) and feed energy back to the grid, further exacerbating system instability. Based on an in-depth analysis of the transient process of this converter fault, such as... Figure 1 As shown, this embodiment provides a flexible fault ride-through method for DC ship power grids based on zero-sequence current injection, including the following steps: S1. When a switch fault is detected, locate the faulty switch, specifically as follows: In actual operation, when an open-circuit fault occurs, the instantaneous current of the faulty phase will drop to around 0 A, accompanied by ripple oscillation. The reconstructed real-time current amplitude reflects the fault characteristics and is a current amplitude near 0 A. Therefore, a relatively small current amplitude should be selected as the diagnostic threshold. Outside the angle removal interval, under normal operating conditions, the real-time current amplitude obtained by reconstructing the three-phase current is theoretically much greater than 10% of the rated current amplitude. However, under fault conditions, the real-time current amplitude drops to around 0 A. Therefore, setting the diagnostic threshold range to 10% of the normal current amplitude can distinguish between fault and normal states, ensuring the accuracy and speed of open-circuit fault detection. After setting the threshold range for the real-time current amplitude, when the absolute value of the current amplitude of a certain phase is less than the threshold, the current amplitude of that phase is considered abnormal. Combining the abnormal real-time current amplitude of the faulty phase with the range of current phase angles when the current amplitude is below the threshold, the location of the fault is comprehensively determined.

[0016] S2. Based on the location of the fault switch, calculate the required DC voltage asymmetry of the rectifier according to the inverter output voltage requirements.

[0017] When the upper switch fails, the reference voltage point will be changed from the neutral port. O Converted to negative port N Accordingly, when the downstream switch malfunctions, the positive terminal will be... P As a neutral point. DC voltage asymmetry. x The sign of the voltage is determined by the voltage asymmetry controlled by the rectifier, and the control method of the asymmetrical voltage is determined by the position of the fault switch.

[0018] like Figure 4 As shown, the inverter can only use a switch. S u2 , S u3 and S u4 To achieve u Phase voltage synthesis. uThe only DC voltage that can be used is the DC voltage at the lower port. U dcL .therefore, U dcL The requirements for SPWM synthesis must be met: in, v s It is the maximum amplitude of the AC voltage.

[0019] Combined and DC voltage asymmetry The requirements are: in, Indicates as Figure 4 The DC voltage at the upper port of the inverter shown is... Indicates as Figure 4 The DC voltage at the lower port of the inverter is shown. Indicates as Figure 4 The inverter AC port output line voltage fundamental amplitude is shown.

[0020] Similarly, when an open-circuit fault occurs in the downstream switch, the DC voltage at the downstream port is isolated from the AC load. The asymmetry has the following requirements: Table 1 shows the switch status when different switch faults occur. The internal switch of the healthy phase is locked, while the internal switch of the faulty phase remains closed to enable two-level inverter operation.

[0021] Table 1 Phase-Tolerant Fault Control

[0022] S3, will The DC voltage asymmetry is used as input, and the rectifier is controlled by carrier amplitude modulation and zero-sequence component injection to output an asymmetrical DC voltage, specifically: Optimized modulation of rectifiers with zero-sequence component injection, such as Figure 2 As shown. Based on the "volt-second balance" principle, the vector action time in modulation is determined by the intersection point between the modulating wave and the carrier wave. The modulating wave of phase y. m y ( y = a , b , c ) and in y "The photo produces" P "Upcarrier in state" c P Compare; my With low carrier c N Compared to each y "The photo produces" N "State. When the DC voltage asymmetry is..." x In such cases, the amplitude of the modulated wave needs to be adjusted to change the vector action time in order to satisfy the "volt-second balance" principle. The amplitudes of the upper and lower carrier waves need to be proportional to the DC voltages at the upper and lower ports.

[0023] in, c Pmax yes c P The maximum amplitude of the upcarrier, c Nmin yes c N The minimum amplitude of the downloaded wave, This represents the total DC voltage.

[0024] Therefore, in order to obtain the true length of stay, c P The amplitude should be determined by the coefficient. Adjustment, c N The amplitude should be determined by factors. Adjust. If , If it is less than 1, then make c P The amplitude decreased, while The amplitude will then be less than -1, making c N The amplitude increased. m 0 is the zero-sequence component injected to actively control DC voltage asymmetry, refer to... Figure 2 .

[0025] Rectifier output neutral point current It can be represented as follows: in, , It is the current flowing through the upper and lower DC capacitors. It is the neutral point input current of the inverter. It is a DC capacitor. This refers to the DC voltage asymmetry (i.e., the DC side voltage imbalance). This represents the total DC voltage.

[0026] By controlling the neutral point current output of the rectifier And inject additional zero-order components This is used to adjust the degree of asymmetry. Current The relationship between the modulated wave and the input phase current can be expressed as follows: in, , and They represent a Phase modulation wave, b Phase modulation wave and c Phase modulation wave, , and They represent a Phase current, b Phase current and c Phase current.

[0027] From the above equation, we can show that the current It can be adjusted To increase or decrease.

[0028] Adjust by inserting a proportional-integral (PI) controller. ,Will It is injected into a three-phase modulated wave to control the degree of asymmetry; the modulated wave is the original wave generated by the dual closed-loop control. For example... Figure 3 As shown, The DC voltage asymmetry obtained in step S2 is the product of the input signals, and the output of the square module is the product of the input signals.

[0029] S4. Determine the blocking and conducting switches when different switching faults occur. Simultaneously, based on the asymmetrical DC voltage, control the inverter to achieve the generation of separated AC phase voltages and the injection of the optimal DC zero-sequence component through two-level carrier modulation, completing the inverter's flexible fault ride-through. Specifically: when S u1 When a fault occurs, the upper DC voltage is isolated from the AC load. Healthy phases V and W perform power balancing because the current in phase U comes only from ports O and N, not port P. Figure 4 Showing when S u1 Phase-by-phase FTC control in case of a fault. Internal switch S v2 S v3 S w2 and S w3 The circuit is blocked, and phases v and w are controlled by an external switch S. v1 S v4 S w1 and S w4SPWM modulation is used for generation. Therefore, most of the current in phases v and w comes from port P, avoiding the direct use of the neutral port O. Faulty phase u uses switch S. u2 S u4 To generate phase u voltage, while switching S u3 Continuous conduction.

[0030] Unlike three-level rectified modulation, the T-type inverter operates using a two-level inverter. When the upper switch fails, the reference voltage point will shift from the neutral port. O Converted to negative port N Accordingly, when a fault occurs in the downstream switch, the positive terminal will be... P As a neutral point. DC voltage asymmetry. x The sign of the voltage is determined by the voltage asymmetry controlled by the rectifier. The control method for the asymmetrical voltage is determined by the position of the fault switch.

[0031] In a three-phase balanced system with a star-connected load, if the three-phase modulating waves increase or decrease simultaneously, for example, by injecting zero-sequence components, no zero-sequence current will be generated, and the output current will not be distorted and will remain sinusoidal. Therefore, even if a zero-sequence component is injected into the modulating wave, no DC component will appear in the three-phase current. To fully utilize the asymmetrical voltage and avoid overmodulation, the modulating wave should be shifted to the middle of the carrier wave by injecting a DC zero-sequence component. The optimal DC zero-sequence component to be injected can be calculated as follows: in, This represents the optimal DC zero-sequence component injected.

[0032] Inverter carrier reconfiguration, such as Figure 5 As shown, the red triangular wave represents the carrier adjustment of the faulty phase, the black triangular wave represents the healthy phase carrier that has not been adjusted, and the blue reference sine wave is adjusted to the middle of the faulty phase carrier.

[0033] This application's method significantly improves system reliability and continuity. By accurately locating the faulty switch and reconstructing the modulation strategy, it avoids system-wide shutdown due to a single switch failure, meeting the stringent power supply continuity requirements of critical loads such as ships. It optimizes output performance under fault conditions by actively controlling the rectifier side to generate asymmetrical DC voltage through optimal zero-sequence component injection and carrier amplitude modulation. This allows the inverter to synthesize a balanced three-phase AC voltage even after a fault, maximizing output waveform quality and load capacity, and overcoming the performance degradation (such as voltage drop and waveform distortion) of traditional fault-tolerant solutions. It also offers superior economy and versatility. This method eliminates the need for redundant power hardware or complex additional circuits, reducing cost and system complexity. Its control-based implementation makes it easy to apply and port to inverter platforms of different power levels. By generating asymmetrical DC voltage through carrier amplitude modulation on the rectifier side, combined with specific blocking / conduction strategies and two-level modulation on the inverter side for the faulty switch, a complete "detection-calculation-control" fault-tolerant system solution that does not rely on hardware redundancy is ultimately formed.

[0034] This invention employs the aforementioned flexible ride-through method for DC ship power grid faults based on zero-sequence current injection. Upon detecting a switch fault, the modulation strategy is reconstructed in real time, and zero-sequence current injection and DC voltage asymmetry control are actively utilized to achieve uninterrupted operation (i.e., "flexible ride-through") under fault conditions, ensuring continuous and reliable power supply to the ship's propulsion system.

[0035] The above description is merely a preferred embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for flexible ride-through of DC ship power system faults based on zero sequence current injection, characterized in that, The method comprises the following steps: S1, when detecting a switch fault, locating the position of the faulty switch; S2, based on the position, calculating the required DC voltage asymmetry of the rectifier according to the inverter output voltage demand; S3, taking the DC voltage asymmetry as input, controlling the rectifier through carrier amplitude modulation and zero sequence component injection to output an asymmetric DC voltage; S4, determining the blocking and conducting switches when different switch faults occur, and controlling the inverter to generate separated AC phase voltages and optimal zero sequence component injection through two-level carrier modulation according to the asymmetric DC voltage, to complete the flexible fault ride-through of the frequency converter.

2. The zero sequence current injection based flexible ride through of DC shipboard power system fault method according to claim 1, characterized in that, The rectifier outputs a neutral point current is represented as follows: wherein, , is the current flowing through the upper and lower DC capacitors, is the neutral point input current of the inverter, is the DC capacitor, is the DC voltage unbalance, is the total DC voltage.

3. The zero sequence current injection based flexible ride through of DC shipboard power system faults method according to claim 2, characterized in that, By controlling the neutral point current in the rectifier output and introducing an additional control factor to adjust the degree of asymmetry.

4. The zero sequence current injection based flexible ride through of DC shipboard power system faults method according to claim 3, characterized in that, The rectifier outputs a neutral point current The relationship between the modulation wave and the input phase current is represented as follows: wherein , and represent a phase modulated waves, b phase modulated waves and c phase modulated waves, , and represent a phase currents, b phase currents and c phase currents.

5. The zero sequence current injection based flexible ride through of DC shipboard power system faults method according to claim 4, characterized in that, The modulation wave is an original wave generated by double closed-loop control.

6. The zero sequence current injection based flexible ride through of DC shipboard power system faults method according to claim 5, characterized in that, Adjusting by inserting a proportional-integral controller , the is injected into the three-phase modulation wave to control the degree of asymmetry.

7. The zero sequence current injection based flexible ride through of DC ship power system faults method according to claim 6, characterized in that, In order to fully utilize the asymmetric voltage and avoid overmodulation, the modulation wave is shifted to the middle of the carrier by injecting a DC zero sequence component, and the optimal DC zero sequence component is calculated as follows: wherein, represents the injected optimal direct current zero sequence component.

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