Neutral point potential balance control method for direct current side in NPC three-level structure

Abstract

The invention relates to a neutral point potential balance control method for a direct current side in an NPC three-level structure. A neutral point clamping type three-level inverter circuit is adopted in a photovoltaic grid-connected inverter, so that neutral point potential imbalance is prone to being caused; a software control algorithm, namely, a control method conducting hysteresis comparison on a neutral point cannot not completely eliminate low frequency fluctuation of neutral point potential; a control algorithm for injecting zero sequence voltage is relatively complex. The neutral point potential balance control method for the direct current side in the NPC three-level structure is characterized in that the difference value of one half of busbar voltage on the direct current side of a photovoltaic inverter and neutral-point voltage, the current value which needs to be injected into the neutral point in a present switching period is obtained by a PI regulator, the acting time of redundant small vectors in each sector in space vector pulse width modulation is used as a modulation method, and the allocation proportion of the redundant small vectors in one switching period is computed. The neutral point potential balance control method for the direct current side in the NPC three-level structure eliminates direct current bias and low frequency fluctuation of the neutral-point potential of a diode clamping type three-level inverter on the existing hardware condition, and neutral-point balance is achieved.

Description

Technical field [0001] The present invention involves a control method of a diode clamp type (NPC) three -level inverter circuit DC -side point potential fluctuation method, which specifically involves the control method of the DC midpoint potential balance in the DC side of the NPC three -level structure. Background technique [0002] Many photovoltaic grid -connected inverters use mid -point pliers three -level three -level inverter circuits, and the input side uses capacitor voltage, which can add one to the output of the inverter The level makes the pwm waveform output closer to the sine waveform of the voltage of the three -phase grid voltage than the traditional two levels.Because there is current flow in the mid -point point of the DC side, it is easy to cause the unbalance of the mid -point potential and bring a very unfavorable impact on the performance and safety of the inverter. Therefore, it is necessary to properly control the imbalance of the mid -point potential.A...

AI Technical Summary

Problems solved by technology

Due to the inflow and outflow of current at the midpoint of the DC side, it is easy to cause the imbalance of the midpoint potential, which will have a very adverse impact on the performance and safety of the inverter. Therefore, the midpoint potential imbalance must be eliminated through appropriate control.

At present, the research on software control algorithms is relatively extensive. Some control the midpoint potential to fluctuate within a certain range by performing hysteresis comparison on the midpoint. This method is simple and easy to implement, but it does not completely eliminate the low-frequency fluctuation of the midpoint potential.

Some studies have also shown that the neutral point current can be controlled to zero by injecting zero-sequence voltage to achieve the balance of the mid-point potential. This method can completely eliminate the low-frequency fluctuation of the mid-point potential and achieve better control effects. The voltage control algorithm is relatively complex and not easy to apply in engineering, and the calculation of zero-sequence voltage will be inaccurate near the zero-crossing point of each phase modulation voltage

Images