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 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 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 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 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 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 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 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.