The invention relates to a low frequency ring current suppression method of a modularized multilevel converter under carrier stacked pulse width modulation. According to the low frequency ring currentsuppression method, on the premise that the output voltage level of a phase bridge arm is strictly equal to the level of a target waveform, a pulse distribution mode of a fixed route of an original state machine is improved reasonably, an appropriate redundant state is used adequately at a level moment affecting a ring current, changes of the ring current are always limited within the reasonablerange, the magnitude of low frequency ring current caused by capacitor voltage fluctuation of unit modules is suppressed effectively, and switch current stress of the unit modules is lowered. Meanwhile, according to the low frequency ring current suppression method, on the premise of not generating narrow pulses, uniformity and minimality of switching motions of the unit modules are maintained tothe maximum extent, and balance of capacitor voltage of the unit modules is not affected. In addition, according to the low frequency ring current suppression method, limiting frequency bands are appropriate to make sure that loss increasing of the unit modules is not caused by the frequent switching motions of the unit modules.

Low frequency ring current suppression method of modularized multilevel converter under carrier stacked pulse width modulation

The invention provides a circulating current suppression method and system for a modular multilevel converter. The method comprises the following steps of acquiring an output voltage reference value vs* of any phase of the modular multilevel converter; obtaining a common-mode voltage reference value vcm* of the phase of the modular multilevel converter during zero-sequence current control; calculating a compensation term delta vcm of the common-mode voltage reference by utilizing a compensation algorithm; respectively calculating a compensated modulation coefficient nuc of an upper bridge armand a compensated modulation coefficient nlc of a lower bridge arm by utilizing the output voltage reference value vs *, the common-mode voltage reference value vcm *, the compensation term delta vcmof the common-mode voltage reference and the direct-current voltage Vdc; and obtaining the number of submodules of the upper and lower bridge arms according to the compensated modulation coefficientsnuc and nlc of the upper and lower bridge arms. According to the present invention, the circulating current of each phase of the modular multilevel converter is suppressed based on the capacitor voltage feedforward, the frequency and phase sequence information of the circulating current is not needed, and the circulating current suppression method and system is particularly suitable for the occasions of imbalance of an alternating voltage system, high harmonic content or fundamental frequency change.

Circulating current suppression method and system for modular multilevel converter

The invention relates to a marine DC power grid system without capacitor on the DC side. The system comprises an AC generator, an AC circuit breaker, a diode or thyristor rectifier circuit, a fuse anda DC/DC and DC/AC converter based on a modular multilevel converter. The AC generator is connected to a common DC power grid through the AC circuit breaker and the diode or a thyristor rectifier circuit. The common DC power grid has no capacitor equipment. The common DC power grid is connected with the corresponding load through the fuse and the DC/DC and DC/AC converters based on the modular multilevel converters. DC/DC and DC/AC conversion is performed through the modular multilevel converter to drive different types of loads. The system is simple in structure and high in reliability.

Marine DC power grid system without capacitor on DC side

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.

Capacitor voltage fluctuation suppression method of modular multi-level converter under low frequency and low voltage

The invention relates to a low-voltage ship DC power grid structure based on energy weak storage, and a control method. A DC power grid structure of a DC circuit breaker with the rapid turn-off capability is replaced by a common circuit breaker. The structure is mainly characterized in that a high-capacity capacitor with a voltage supporting effect is cancelled from a DC link, and a DC power gridis in an energy weak storage state, and can achieve the power loss protection rapidly when a fault occurs. The power grid structure is that the controllable rectification is carried out through a front-end converter with the DC short-circuit fault ride-through capacity, and the voltage fluctuation of a DC power grid is restrained through an interface converter with the high switching frequency ofa supercapacitor. By the adoption of the power grid structure, the structure can achieve the automatic current limiting when a DC side short-circuit fault occurs, does not need a special DC circuit breaker, thereby simplifying a circuit, and facilitating the popularization of the ship DC power grid.

Low-voltage ship DC power grid structure based on energy weak storage, and control method

The invention relates to an active damping method of an LCL type grid-connected inverter. The method comprises the following steps: collecting inverter side current based on a high-speed ADC; acquiring or estimating grid-connected side current, acquiring a direct-current bus capacitor voltage, sending the acquired inverter side current, grid-connected side current and direct-current bus capacitorvoltage into an FPGA, calculating an active damping control voltage in real time by adopting the FPGA, and carrying out PWM to realize active damping control. According to the method, a resonance problem caused by active damping digital control delay can be suppressed, and the control complexity is reduced.

Active damping method of LCL type grid-connected inverter

The invention relates to a series voltage compensation capacitor-free filter type rectifying circuit and a control method thereof. The circuit comprises an uncontrolled rectifying circuit and a voltage regulating circuit which are connected in series, and the voltage regulating circuit outputs harmonic components in output voltage of the uncontrolled rectifying circuit so as to eliminate output voltage harmonics of the uncontrolled rectifying circuit. According to the invention, because the voltage regulating circuit adopts the H-bridge module containing the independent capacitor, in order tostabilize the capacitor voltage, a method for adding the offset into the harmonic voltage output by the H-bridge module is provided; the capacitor voltage of the module is ensured to be in a normal working range through closed-loop adjustment of the offset; the invention further provides specific control steps of the voltage regulating circuit; through the circuit and the control strategy of the invention, the stability of the direct current side voltage under uncontrolled rectification is realized; the adverse effect caused by capacitor discharge in a short-circuit fault is avoided due to thefact that the direct current side does not need capacitor filtering; the structure of the rectifying circuit is effectively reduced; and the circuit is especially suitable for medium-voltage rectification occasions.

Series voltage compensation capacitor-free filter type rectifying circuit and control method thereof

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