41results about How to "Reduce capacitor voltage fluctuations" patented technology

The invention provides a MMC (modular multilevel converter) sub-module capacitor voltage fluctuation suppression method with harmonic coupling injection. The method comprises the following steps: inhibition of fundamental frequency and second harmonic generation fluctuation components of bridge arm instantaneous power is taken as a target; the injection amounts of the second harmonic generation loop current and the third harmonic generation voltage are optimally designed; and finally, the calculated second harmonic generation ring current and third harmonic generation voltage are injected intothe three phases through an injection controller, so that the voltage fluctuation of a sub-module capacitor of a whole converter valve is reduced, the requirement on the capacitance value of the sub-module capacitor is reduced, the size and the cost of the sub-module capacitor are reduced, and the light weight of the modular multilevel converter is favorably realized.

The invention provides a photovoltaic grid-connected inverter low-voltage ride-through control system. On the basis of double closed-loop vector control of an inner current loop and an outer voltage loop, the system further comprises a low-voltage ride-through control module and a sub module capacitor voltage control module. When that the voltage at a grid connection point drops is detected, the low-voltage ride-through control module injects reactive current and limits active current, and the sub module capacitor voltage control module sends out a sub module capacitor voltage fluctuation range at reactive power under control of a bridge arm ring current optimization unit and / or a sub module capacitor reference voltage regulation unit. With the system, the problem that reactive power injection low-voltage ride-through is not smooth due to sub module capacitor voltage fluctuation is solved. Some inverters cannot achieve low-voltage ride-through properly. There is no need to arrange an additional power device. Low-voltage ride-through of a photovoltaic inverter system based on MMC is realized with low cost. Photovoltaic safe grid-connected operation is ensured.

The invention provides an MMC-based low-voltage ride through control method and system of a photovoltaic grid-connected inverter. On the basis of traditional inner current loop and outer voltage loop double closed-loop vector control, when a grid-connected point voltage drop is detected, a reactive current instruction value as shown in the specification is redistributed by judging the voltage drop degree to achieve reactive current injection; the capacitor voltage fluctuation range of an MMC sub-module is adjusted by optimizing bridge arm current; a new active current instruction value is introduced to limit active current; and the active current is limited, so that low-voltage ride through is achieved and safe photovoltaic grid-connected operation is ensured. Through the system and the method, capacitor voltage fluctuation is suppressed by optimizing bridge arm loop current; output reactive power is increased; and the capacitor voltage fluctuation range of the sub-module is optimized by controlling reference voltage of the sub-module, so that the problem that the capacitor voltage fluctuation of the MMC sub-module is out of the limit during reactive injection is solved; and low-voltage ride through is smoothly achieved.

The invention discloses a modular multilevel converter quasi-two-level PWM control method based on an SIC device. The method comprises the steps of determining a reference value of output voltage anda duty ratio of an output PWM modulator by current control, determining a PWM switching state time signal according to the duty ratio D of the PWM modulator, determining a circulation reference valueaccording to a PWM switching state and an energy reference value, determining a compensation voltage reference value according to a circulation actual value and the circulation reference value, and finally superposing the compensation voltage reference value on the reference voltage to obtain an output voltage modulation signal, and carrying out delay PWM modulation to obtain each bridge arm sub-module PWM control signal to realize the control of each sub-module of a modular multi-level converter. According to the method, a phenomenon of excessive output voltage is effectively prevented, the voltage fluctuation of a modular multilevel converter sub-module capacitor is reduced, requirements of energy storage of the sub-module capacitor are reduced, the volume of the modular multilevel converter is further reduced, and the cost of the modular multilevel converter is reduced.

The invention discloses a modular multilevel converter (MMC) four-quadrant frequency converter and belongs to the field of electric power and electronics, for solving the problems of capacitive voltage fluctuations existing during low-frequency operation of an MMC, quite large current impact brought by a high-frequency injection method and the like. The MMC four-quadrant frequency converter comprises two MMCs connected in a back-to-back mode, and the two MMCs are a network-side MMC and a machine-side MMC; and the power supply input end of the network-side MMC is connected with an AC power network, the DC bus output end of the network-side MMC is connected with the DC bus input end of the machine-side MMC, and the load output end of the machine-side MMC is connected with a motor type load.

The invention provides a pre-charging control method of a modular multi-level converter (MMC) and a control apparatus thereof. The method includes: performing uncontrollable charging on the modular multi-level converter (MMC), and charging a sub-module capacitance series voltage of a charging loop to a charging voltage provided by an alternating current system; determining an upper bridge arm current and a lower bridge arm current of each phase unit and a capacitance voltage of each sub-module; determining the direction of the bridge arm current of the sub-module, and determining whether the bridge arm current is greater than 0; removing the sub-module if the capacitance voltage uci of the sub-module reaches a reference voltage of a stable-state operation voltage, and unlocking the next sub-module for the above operation; repeating the above operation on the sub-modules one by one until the charging of the last sub-module of a bridge arm is accomplished, and returning a marking signal for indicating the completion of charging of the bridge arm; and charging capacitors of the sub-modules of all the bridge arms to the stable-state operation voltage in sequence to accomplish the start process. The method is simple, the extension to different level numbers is facilitated, the capacitance voltage of all the sub-modules can be rapidly increased to the voltage required by stable-state operation of the converter based on uncontrollable charging.

The invention discloses a wind power generation-grid connected system based on an H-MMC and a control method thereof. The system includes a three-phase direct-drive permanent magnet wind generator andan H-MMC. The H-MMC comprises six bridge arms, which are sequentially connected from head to tail. Each bridge arm comprises N full-bridge sub-modules and a reactor, wherein the N full-bridge sub-modules are serially connected with the reactor head and tail in sequence. A three-phase winding of the three-phase direct-drive permanent magnet wind generator and the three-phase of a power frequency alternating current grid are alternately connected to the six vertices of the H-MMC. The maximum power is adopted for tracking control, grid-side current control, H-MMC capacitance and voltage balancecontrol and H-MMC bridge arm current tracking control, direct grid connection of different voltage grades can be realized, a step-up transformer is not needed, and the problem that the capacitance andvoltage of an MMC sub-module fluctuates too much when a fan is started is solved.

The invention discloses a modular multilevel converter circulating current suppression method, and relates to a flexible direct current transmission field of a power system. A hybrid particle swarm optimization algorithm is adopted to optimize PI parameters of a PI controller, a closed loop control circuit is composed of the PI controller and a controlled object, the PI controller outputs the input of the controlled object, the controlled object outputs the output of the system, the output of the system is sent to the input end of the system to form negative feedback, and is subtracted from the input of the system to generate a system error, and the error is sent to the PI controller. An objective function is constructed by the system error and the output of the PI controller, the hybridparticle swarm optimization algorithm is used for finding optimal PI parameters, the system dynamically adjusts the output and input to optimize the objective function, and when the objective functionis optimal, the parameters obtained through optimizing of the hybrid particle swarm optimization algorithm are the optimal PI parameters; the circulating current suppression method has high circulating current suppression precision, fast response and strong adaptability, can adjust the controller parameters according to the system dynamically, reduces the loss, and improves the overall steady state operation efficiency.

The invention discloses a double-frequency circulating current injection method for suppressing capacitor voltage fluctuation of an MMC sub-module under a fault. The method comprises the steps: taking the fundamental frequency and double-frequency fluctuation components of capacitor voltage of the sub-module which generates an AC single-phase fault and considers double-frequency circulation injection as suppression targets, and building an optimization target function; carrying out global optimization by taking target function minimization as a target to obtain optimal amplitude and phase of double-frequency circulating current injection; and generating a reference signal according to the optimal amplitude and phase of the double-frequency circulating current injection, and realizing the double-frequency circulating current injection in combination with a modulation algorithm. According to the method, the capacitor voltage fluctuation of the sub-module during the fault period can be effectively reduced, and the maximum voltage borne by the MMC sub-module during the fault period is reduced.

The invention discloses a three-phase four-wire system general electric energy quality controller and a control method thereof. A circuit topology structure of the electric energy quality controller comprises a power supply side circuit, and A, B and C three-phase power grid power supply ends Vsa, Vsb and Vsc are connected with the power supply side circuit; the power supply side circuit is connected with a load side circuit through a common bridge arm, the load side circuit is connected with series coupling transformers Ta, Tb and Tc to form a series compensation device, and the output of thecommon bridge arm is connected with a power supply neutral line N and neutral lines of converter sides of the series coupling transformers Ta, Tb and Tc through a filter inductor Lf7 at the same time; a three-phase compensation voltage is output to compensate a three-phase power grid voltage such that a load voltage is a three-phase constant, balanced and symmetrical sine voltage. A single energystorage capacitor structure is adopted, the capacitor capacity is small, the capacitor voltage fluctuation is low, and the loss is small; few power electronic devices are adopted, and the formed universal power quality controller is simple in circuit structure, low in manufacturing cost and high in efficiency; and the control method is simple and easy to implement.

The invention discloses a frequency-doubled circulation injection method for suppressing MMC bridge arm power fluctuation under a fault, and the method comprises the steps: taking a fundamental frequency and a frequency-doubled fluctuation component of bridge arm instantaneous power when an AC single-phase fault occurs as suppression targets, and building an optimization target function; carrying out global optimization by taking target function minimization as a target, and obtaining the optimal amplitude and phase of frequency-doubled circulation injection; and generating a reference signal according to the optimal amplitude and phase of the frequency-doubled circulation injection, and realizing the frequency-doubled circulation injection in combination with a modulation algorithm. According to the method, the capacitor voltage fluctuation of the sub-module during the fault period can be effectively reduced, and the maximum voltage borne by the MMC sub-module during the fault period is reduced.

The present invention relates to a flexible direct current transmission system overhead line submodule packet sorting modulation strategy, belonging to the electric power technology field. Aiming at the high-voltage and large-capacity modularization multi-level converter modulation and submodule inputting and removing problems, a packet sorting modulation method for reducing the submodule capacitor voltage sorting operation amount is provided. An optimal capacitor voltage sorting operation amount is taken as an object, and the optimal packet quantity of a converter bridge arm submodule is determined; a submodule capacitor voltage matrix after packet is established, and the intra-group vertical sorting and inter-group transverse sorting of the submodule capacitor voltage matrix after packet are performed to form a submodule matrix taking submodule capacitor voltage as an order; and in each control period, according to bridge arm current parameters, submodule input and removing control are performed according to a direction of increasing or decreasing the submodule voltage. The submodule capacitor voltage deviation control is introduced to avoid too big capacitor voltage deviation and reduce the power device switch frequency.

The invention relates to a capacitor voltage fluctuation suppression method of a modular multi-level converter under a low frequency and a low voltage. Under unbalanced control, an upper bridge arm and a lower bridge arm take turns; and one bridge arm mainly outputs an alternating-current voltage and an alternating current and the other bridge arm mainly outputs a direct-current voltage instead of an alternating current. On the basis of alternate switching, the module capacitor voltage difference of the three-phase upper and lower bridge arms can be suppressed well. With the method, a problem that the module capacitor voltage fluctuation is too large at the low frequency band when the modular multi-level converter is used for driving the motor can be solved; and thus a negative effect on a load due to a high-frequency injection method can be eliminated. According to the method, changing of a hardware circuit is not required; the low-frequency-band working performance of the modular multi-level converter can be improved substantially; the application range in the industry is wide; and practicability is high.

The invention provides a lightweight method and device for a hybrid modular multilevel converter. When the hybrid modular multilevel converter is stable, the third harmonic voltage correction amount is determined. The three-phase circulation suppression voltage correction is determined. The modulation wave voltage of the hybrid modular multilevel converter is determined, and the voltage fluctuation of a sub-module capacitor is reduced. The lightweight effect is obvious, and the hybrid modular multilevel converter can stably operate. According to the technical scheme provided by the invention, the third harmonic voltage correction is introduced, which solves the problem that voltage negative drop of a full bridge sub-module occurs when the modulation ratio increases; the hybrid MMC can pass a direct current fault without blocking; consideration are given to the characteristics of economy and control flexibility; the degree of the voltage fluctuation of the sub-module capacitor and the cost of the sub-module capacitor are greatly reduced on the premise of constant direct current bus voltage and transmission power; and the modular multilevel converter is lightweight.

The invention discloses a modular multilevel converter quasi-two-level PWM control method based on an SIC device. The method comprises the steps of determining a reference value of output voltage anda duty ratio of an output PWM modulator by current control, determining a PWM switching state time signal according to the duty ratio D of the PWM modulator, determining a circulation reference valueaccording to a PWM switching state and an energy reference value, determining a compensation voltage reference value according to a circulation actual value and the circulation reference value, and finally superposing the compensation voltage reference value on the reference voltage to obtain an output voltage modulation signal, and carrying out delay PWM modulation to obtain each bridge arm sub-module PWM control signal to realize the control of each sub-module of a modular multi-level converter. According to the method, a phenomenon of excessive output voltage is effectively prevented, the voltage fluctuation of a modular multilevel converter sub-module capacitor is reduced, requirements of energy storage of the sub-module capacitor are reduced, the volume of the modular multilevel converter is further reduced, and the cost of the modular multilevel converter is reduced.

The invention relates to an MMC converter modulation method and system capable of shutting direct current fault current. A switching state of a modular multilevel converter bridge arm submodule is determined according to an input modulation signal; steady-state voltage of a DC-DC converter is determined according to an equivalent circuit model of the DC-DC converter constructed by the bridge arm submodule capacitor voltage in the switching state; the number of the submodules input by a lower bridge arm under the steady-state voltage is calculated; according to the number of the input submodules, a DC-DC converter transfer function is defined, and the switching sequence is determined according to a preset manner to adjust the capacitor voltage of the modular multilevel converter bridge armsubmodule to be in a voltage stabilization state. The scheme can effectively provide safe, stable and reliable control protection for the DC-DC converter capable of shutting the direct current fault current.

An embodiment of the invention discloses a parameter design and control method for a hybrid modular multilevel converter, which increases a modulation ratio of the hybrid modular multilevel converter to an optimal operation point through the coordination and cooperation of third harmonic voltage injection and a full-bridge sub-module negative level utilization method. The parameter design and control method comprises the steps of firstly, increasing the modulation ratio of the hybrid modular multilevel converter to the maximum extent through adopting a third harmonic voltage injection method, and secondly, increasing the modulation ratio to the optimal point through configuration of the cascade number of full-bridge sub-modules in bridge arms. When the hybrid modular multilevel converter outputs rated active power, the modulation ratio operates at an optimal operating point, so that a fundamental component of bridge arm power fluctuation can be changed into zero, the capacitance voltage fluctuation of the sub-module is greatly reduced, the capacitance use amount of the hybrid modular multilevel converter is greatly reduced, the number of required full-bridge sub-modules is reduced to the maximum extent, and the total cost and size of the device are reduced.

The invention provides a novel modular multi-level converter topology, and relates to a modular multi-level converter. The novel modular multi-level converter topology aims to solve the problems that an existing traditional modular multi-level converter is high in capacitor voltage fluctuation amplitude, high in capacitor capacity, low in power density and large in required module number. According to the modular multi-level converter, a connecting mode of an upper power sub-module and a lower power sub-module of the traditional modular multi-level converter is modified, a middle power module is added, the advantages of the traditional modular multi-level converter are kept, and partial defects of a traditional topology are overcome. When the same number of levels is output, the number of modules required by a novel topology is less, and therefore the cost is lowered easily. When load is the same, the capacitor voltage of the novel topology is less in fluctuation, the capacity of a capacitor of the modular multi-level converter is reduced easily, the cost is lowered, and losses are reduced. The novel modular multi-level converter topology is suitable for high-voltage high-power electric energy conversion occasions.

The invention provides a derating operation method and device for a modular multilevel converter. The method comprises the steps that the number of submodules input by the upper bridge arm of the modular multilevel converter is acquired; and if the number of the submodules input by the upper bridge arm of the modular multilevel converter is less than N / 2 or greater than N / 2, whether the number ofthe submodules input by the upper bridge arm is in an increasing phase or a decreasing phase is judged, and corresponding bridge arm reactor switching control is carried out. Derating operation of themodular multi-level converter is realized, and the derating effect is remarkable. According to the invention, the process is simple and free of complicated operation; the requirement on the controller of the modular multilevel converter is low; the investment cost is low; only a few thyristors need to be added; compared with a CCSC, the method can reduce the fundamental frequency and double frequency components of the capacitor voltage waveform of the submodules to a greater extent; the capacitor voltage fluctuation of the submodules is reduced; and the method has certain significance for further reducing the weight of the submodules of the converter.

The invention relates to an optimum design method for an extra-high voltage multi-level flexible direct-current power transmission operation interval, and belongs to the field of high-voltage direct-current power transmission converter control strategies. The method comprises the following steps: obtaining upper and lower bridge arm currents of a converter through current transformer measurement, and averaging to obtain a current error generated by bridge arm alternating ring current; integrating the current error to obtain ring current voltage drop generated by the bridge arm alternating ring current; calculating direct-current voltage, internal phase electromotive force of the converter and the ring current voltage drop to obtain upper and lower bridge arm voltage reference values, and participating modulation. The method has the advantages that internal components of the converter do not need to be modified, and system parameters do not need to be changed, so that the original performances of the converter are guaranteed; the power operation interval of the converter is optimized on the basis of original equipment, and the original construction occupied area and relatively low investment cost are guaranteed. The power operation interval of the converter is optimized, and simultaneously the capacitor voltage fluctuation of sub-modules is reduced, so that the bridge arm ring current is inhibited.

The invention discloses a marine medium-voltage grid structure base on a multi-terminal DC-DC converter, comprising a power generator, a propulsion motor and a low-voltage direct-current grid. CascadeH-bridge three-phase converter I is connected with the power generator; cascade H-bridge three-phase converter II is connected with the propulsion motor; the low-voltage direct-current grid is connected with low-voltage equipment. The power generator, the propulsion motor and the low-voltage direct-current grid are connected through a triple-terminal DC-DC converter; three terminals of the triple-terminal DC-DC converter have identical structure, all the three terminals are of half-bridge structure and are connected to a high-frequency transformer via a connection inductor. The marine medium-voltage grid structure base on the multi-terminal DC-DC converter provided herein allows energy to be transmitted from a high-capacitance high-voltage module to a low-voltage module, so that three-phase capacitance and voltage balance is achieved.

The invention discloses a 66kV collection access-based flexible direct-current offshore converter station, a system and a control method, and the flexible direct-current offshore converter station is characterized in that the offshore converter station comprises a connection transformer, a flexible direct-current converter valve and a bridge arm reactor, wherein the primary side of the connection transformer is connected with a bus bar of an offshore wind plant; the primary side is connected with the input end of the flexible direct current converter valve, the output end of the flexible direct current converter valve is connected with the first end of the bridge arm reactor, and the second end of the reactor is connected with the direct current submarine cable. And the investment and maintenance cost of an offshore booster station is saved. By increasing control strategies, increasing module voltage, reducing the total number of converter valve modules, reducing capacitance value, improving transmission capacity of equipment and reducing size and weight of an offshore wind power plant, the problems of land occupation, large load, high cost and the like of an offshore converter station can be effectively solved, technical cost of engineering implementation is remarkably reduced, and the method is suitable for large-scale popularization and application. The method has better economical efficiency and is of great significance to large-scale development and utilization of offshore wind power.

The invention discloses an MMC sub-module voltage fluctuation suppression method for injecting common-mode voltage and circulating current, and the method comprises the steps: carrying out the information collection of an upper bridge arm and a lower bridge arm, and obtaining an upper arm current, a lower arm current, an upper arm voltage and a lower arm voltage; performing common-mode injection system control to obtain three-phase output voltage and three-phase output current; performing common-mode injection calculation to obtain a common-mode current reference value; performing circulation control according to the three-phase output current to obtain a three-phase circulation simulation reference current value; and according to the common-mode current reference value, the direct-current bus voltage feedback value and the three-phase circulating current simulated reference current value, performing current comparison calculation, obtaining a current-sharing reference value, promoting PWM to generate a trigger pulse, driving an MMC bridge arm power device, and completing voltage fluctuation suppression. According to the method, the problem of large bridge arm capacitance fluctuation is solved, the circulating current reference value is calculated by injecting the common-mode voltage and combining the typical half-bridge MMC / DC-AC topology, MMC bridge arm circulating current is effectively restrained, sub-module capacitance voltage fluctuation is reduced, and the safe and reliable operation capacity of a system is enhanced.

The present invention discloses a strategy and system of direct current active charging under passive starting. The critical problem of current direct current active charging is realizing rapid and accurate response of submodule capacitor voltage. The technical scheme of the invention comprises: an active terminal enters alternating current charging, direct current voltage is V1, the number of direct current active charging submodules of a passive terminal is V1 / VSMR, and the VSMR is the rated voltage of submodules; the active terminal performs voltage reduction unlocking, the direct current voltage is V2, the bridge arm current and the submodule voltage VSM of the passive terminal are detected, if the bridge arm current is larger than I1 and the submodule voltage VSM is larger than VSMmin, the number of the direct current active charging submodules of the passive terminal is V2 / VSMR, or else, the number of the direct current active charging submodules of the passive terminal remains unchanged; the active terminal performs total-voltage unlocking, the direct current voltage is V3, the submodule voltage VSM is detected, if VSM>VSMmax, the number of the direct current active charging submodules of the passive terminal is V3 / VSMR, or else, the number of the direct current active charging submodules remains unchanged. The strategy and system of direct current active charging under passive starting realizes rapid and accurate response of submodule capacitor voltage.

The invention provides an NPC three-level virtual synchronous machine bridge arm fault tolerance model prediction control method which is used for solving the technical problem that after an NPC type three-level VSG bridge arm breaks down, a control strategy fails. The method comprises the following steps: firstly, reconstructing an NPC three-level VSG structure into a VSG bridge arm fault-tolerant structure, and establishing a prediction current model under a fault condition and a voltage vector of the VSG bridge arm fault-tolerant structure; secondly, acquiring the direct current side capacitor voltage at the current moment by using a sensor, and predicting the direct current capacitor voltage difference of the model at the next moment according to the direct current side capacitor voltage at the current moment; and finally, constructing a cost function with a delay compensation function, and selecting a voltage vector corresponding to the minimum value of the cost function as an optimal voltage vector to act on the next moment, thereby realizing VSG bridge arm fault-tolerant operation. After a switch device breaks down, the NPC three-level VSG can continuously run in a fault-tolerant mode, the effectiveness of the method is verified, and the running reliability of the VSG is improved.

The invention discloses a modular submarine power supply capacitor voltage ripple suppression method and system and a controller, and the method comprises the steps: taking each phase of bridge arm of a modular submarine power supply as a target object, and combining the equivalent capacitance values of an upper bridge arm and a lower bridge arm according to the principle that the upper bridge arm and the lower bridge arm are in parallel connection with the AC side current for shunting; and extracting a circulating current instruction value used for suppressing a capacitor voltage ripple component, obtaining modulation signals of the upper bridge arm and the lower bridge arm, and finally determining that the upper bridge arm and the lower bridge arm are put into sub-modules so as to realize ripple suppression of the capacitor voltage on the premise of meeting the requirements of direct-current side voltage input and alternating-current side voltage output. The problems that capacitor voltage ripples reduce the internal energy stability of the modular submarine power supply and increase the harmonic distortion rate of the output voltage on the alternating current side of the power supply are solved.

The invention discloses a H-MMC-based wind power generation-grid-connected system and a control method thereof. The system includes a three-phase direct drive permanent magnet wind generator and an H-MMC; the H-MMC includes six bridge arms, each The bridge arms are connected end to end in sequence; each of the bridge arms includes N full-bridge sub-modules and a reactor, and the N full-bridge sub-modules are connected in series end-to-end with the reactor in turn; the three-phase direct drive permanent The three-phase winding of the magnetic wind generator and the three-phase of the power frequency AC grid are alternately connected to the six vertices of the H-MMC. This system adopts maximum power tracking control, grid side current control, H‑MMC capacitor voltage balance control, H‑MMC bridge arm current tracking control, which can realize direct grid connection at different voltage levels, saves the step-up transformer, and solves the problem of fan The problem that the capacitor voltage of the MMC sub-module fluctuates too much during startup.