Phase-shift spwm modulation method of variable carrier variable modulation wave based on cascaded energy storage system

Abstract

The invention discloses a variable carrier wave variable modulation wave phase shift SPWM modulation method based on a cascade energy storage system. The method comprises steps that S1, communication between a main control machine and each power unit is carried out to acquire a capacitor and cell state of each power unit; S2, an output power of each power unit during present modulation is calculated by the main control machine according to the capacitor and cell state of each power unit; S3, a waveform signal s outputted by a cascade multi-level inverter is divided into s1-sn by the main control machine according to the output power of each power unit during present modulation, a DC voltage of each power unit, a maximum cell SOC and a cell derating ratio, s1-sn comprise fundamental waves and multiple harmonics of the s, and the time-domain superposition result of the s1-sn is equal to the s; S4, time domain shift of the carrier wave v is carried out by the main control machine to acquire N carrier waves v1-vn, the start time of the vi is T / N*(i-1), and the i is not smaller than 1 and is not greater than N; and S5, vi and si modulation is respectively carried out under the control action of the main control machine to acquire N modulation waves, and the N modulation waves are inputted to the N power units.

Description

technical field [0001] The present invention relates to the technical field of photovoltaic power generation, in particular to a carrier-variable modulation wave phase-shift SPWM modulation method based on a cascaded energy storage system. Background technique [0002] Photovoltaic power generation systems are characterized by uncertainty and intermittency. It is necessary to install a photovoltaic energy storage system to stabilize the output power of the system, thereby reducing the impact on the grid and sensitive loads. The photovoltaic energy storage system can supply photovoltaic power generation to households and store excess electricity in batteries. When the batteries are full, they will be automatically input into the grid to facilitate deployment to other users. [0003] Existing multi-unit modulation algorithm for series photovoltaic energy storage system (Reference: [1] Han Fei. Multi-level inverter and its application research in photovoltaic grid-connected sys...

AI Technical Summary

Problems solved by technology

[0006] 1) Cannot be adjusted for unit DC voltage changes

Since this modulation method is based on equal voltage modulation, it cannot adjust the output waveform distortion caused by DC voltage fluctuations and hardware problems according to voltage changes.

[0007] 2) The higher harmonics are relatively large and cannot be eliminated

Since the output waveform is superimposed by multiple two levels, and there is no RCR or RC filter at the back end, although the harmonic full load can be lower than 5%, it is still different from the normal power grid

[0008] 3) Since all units are controlled together, it is impossible to achieve separate control of harmonics and current for each unit, reducing internal current flow

[0011] 1) Some units have high switching frequency, some units have no switch, and the heat distribution is uneven

[0012] 2) The harmonics are relatively large

Since the harmonic elimination PWM method can only eliminate low-order harmonics or high-order even-numbered harmonics, it cannot eliminate high-order odd-order harmonics, so there are still more high-order odd-order harmonics

[0013] 3) Because the upper and lower edges of each carrier are connected, it is impossible to calculate the next one

Unable to calculate by unit to achieve perfect compensation

[0014] 4) When the modulation ratio decreases, some units will have no PWM voltage output, resulting in inconsistent output power between units, and the equivalent switching frequency of the output voltage will decrease, which will increase the harmonic content of the output voltage

[0015] It can be seen that the above two methods have the situation that when the bus voltage of each unit is unbalanced due to hardware and software, it cannot be automatically adjusted. At this time, the effect of outputting multi-level will be much worse; Offset the low-order harmonics, resulting in the fact that the grid voltage and current high-order harmonics are a bit large and cannot be compensated

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