Study On Cascade H Bridge Multi-Level Converter As A Statcom

The reactive load not only increases power loss, but also deteriorates the power quality of customers seriously, so immediate reactive power compensation is a powerful means of optimizing power flow on transmission networks as well as enhancing power quality. It can also provide voltage support. Conventional reactive compensator has a long responsive time constant, bad regulative quality, huge volume and much-distorted waveform. Based on the electronic inverted technology, STATCOM has smaller volume, and can be faster and more effective to compensate the reactive power in power system. The compensated current has lower harmonic, can be controlled flexibly.Multi-level voltage source inverter (VSI) adopted in STATCOM can not only reduce the harmonic, but also need no bulky and expensive zigzag transformer needed in multiple inverters. The cascade H Bridge Multi-level inverter as the main circuit of STATCOM is adopted in the paper. Compared with diode-clamped multi-level inverter and flying-capacitor multi-level inverter, it has many dominant advantages: it uses a small number of diode and capacitors. Moreover, packaging and physical layout is very easy due to its modular structure.Several modulations commonly used are analyzed based on the following directions: output harmonic train; switching frequency; utilization efficiency of DC voltage in the paper. Their advantages and disadvantages are contrasted and summarized. Phase shift PWM (PS-PWM) has higher equivalent carrier frequency and lower harmonic and can be modulated easily after comparison.Since DC capacitors of cascade H bridge of STATCOM are separate, their voltage may not be exactly balanced during operation periods. This paper presents a control method to realize dynamic reactive compensation and keep its separate dc voltage balance at the same time. The control method uses Q -δto control reactive compensation and uses independently close-loop-control to adjust the phase of the output voltage to keep its separate dc voltage balance. The control method is easy and has high robustness during the disturbance.Power system has much-unbalanced load, which generates unbalanced load...