Novel Voltage Balancing Techniques for Modular Multilevel PV Inverters

Abstract

Modular multilevel converters (MMC) are widely used in medium and high voltage transmission systems and in STATCOM applications for improving the power quality. In this paper, a modified configuration of MMC is considered to integrate a MW PV plant with the high voltage grid more likely at the transmission side of the power system. A string of PV panels is connected across each of the submodules (SMs) of the modular multilevel PV inverter (MMPI). In addition, two novel voltage-balancing techniques have been proposed to regulate the SM capacitor voltages of the MMPI at different maximum power point voltages. The first technique utilizes the concept of capacitor sorting based on their voltages. The proposed algorithm generates the switching states based on the voltages of the other SM capacitors and hence reduces the ripple in the capacitor voltage. In the second technique, a novel and simplified logic function based voltage balancing technique is developed. This technique is computationally efficient, reduces the ripple in capacitor voltage, eliminates the requirement of a look-up table and can be easily extended to an N-level MMPI. A three-level and seven-level MMPI is simulated in MATLAB/SIMULINK. The simulated MMPI is tested under two scenarios for different irradiance and temperature conditions and various waveforms are showcased to ensure the efficacy of the proposed techniques.