Nam S. Choi

A general circuit topology of multilevel inverter

A generalized circuit topology of multilevel voltage source inverters which is based on a direct extension of the three-level inverter to higher level is proposed. The circuit topologies up to five-level are presented. The proposed multilevel inverter can realize any multilevel pulsewidth modulation (PWM) scheme which leads to harmonic reduction and provides full utilization of semiconductor devices like GTOs, especially in the high power range where high voltage can be applied. The capacitor voltage balancing problem is discussed and a circuit remedy for such a problem is given.<<ETX>>

DSP based space vector PWM for three-level inverter with DC-link voltage balancing

The authors describe a novel PWM (pulse width modulation) method for a three-level inverter considering the DC link capacitor balancing problem. Each voltage vector on the space vector plane is classified in relation to the charging/discharging action of DC capacitors, and a novel modulation method is suggested based on the voltage vector selection principle. The algorithm was implemented on the Motorola DSP 56000 and tested with the 7.5 kVA prototype three-level transistor inverter. The effectiveness of the proposed PWM method was verified by the experimental results. It was shown that the proposed PWM techniques are suitable for a high-power GTO (gate turn-off thyristor) three-level inverter satisfying capacitor voltage balancing.<<ETX>>

Analysis and controller design of static VAr compensator using three-level GTO inverter

A static VAr compensator (SVC) using a three-level GTO voltage source inverter (VSI) is presented for high-voltage, high-power applications. The three-level VSI has lower harmonic components and higher DC-link voltage than the two-level VSI and thus can be operated at lower switching frequency (f/sub sw/<500 Hz) without excessive harmonic contents. From the DQ-transformed equivalent circuit of the presented SVC system, DC and AC analyses are carried out to find the steady-state and the dynamic characteristics of the system. Based on the open-loop transfer function of the system, a controller is designed to achieve fast dynamic response. The experimental results confirm the theoretical analyses and controller design.

Modeling, analysis and control of static VAr compensator using three-level inverter

A novel static VAr compensator (SVC) system using a three-level inverter is proposed for high-voltage and high-power applications. A general and simple model for the overall system is obtained using the circuit DQ-transform, and DC and AC analyses are performed to characterize the open-loop system. Using the proposed model, a novel control method which controls both the phase angle and the modulation index of the switching pattern simultaneously is suggested to provide fast response of the SVC system without using the independent voltage source. Predicted results are verified by computer simulation.<<ETX>>

A complete DC and AC analysis of three phase current source PWM rectifier using circuit D-Q transformation

The circuit D-Q transformation is used to analyze a three-phase, current-source, pulse-width modulated (PWM) rectifier. The DC operating point and AC transfer functions are completely determined. Most features of the converter are clearly interpreted. The output voltage can be controlled from zero to maximum. The system is equivalently an ideal current source in the steady state and can be described in terms of linear circuits. The input power factor can be arbitrarily controlled with a certain control range.<<ETX>>

Control of static VAr compensator (SVC) with DC voltage regulation and fast dynamics by feedforward and feedback loop

This paper proposes a new control method of static VAr compensator (SVC) with a three-level inverter to control reactive power with fast dynamics while maintaining DC side capacitor voltage constant. Firstly, using the circuit DQ-transformation, a general and simple model of SVC with a three-level inverter is obtained, and DC and AC analyses are carried out to characterize the open-loop system. Based on the transfer function matrix of the system, modulation index control for reactive power compensation and phase angle control for DC side voltage regulation are designed. By inserting feedforward path to the voltage regulation loop the modulation index is successfully decoupled from the capacitor voltage whereby fast dynamics are achieved. The experimental results confirm the theoretical analyses and control method.<<ETX>>

A high voltage large capacity dynamic VAr compensator using multilevel voltage source inverter

A multilevel pulse-width-modulation (PWM) voltage-source inverter is connected to a static volt-ampere-reactive (VAr) compensator (SVC) as a large-scale power source. The SVC can be directly connected to AC mains of 6600 V, allowing full utilization of semiconductor devices like gate turn-off thyristors (GTOs). The voltage balancing condition of the DC slide capacitors is identified based on the fundamental circuit modeling of the inverter, and a five-level PWM is used to meet the condition. Owing to the multilevel approach, a low distortion in the input currents results and filter size thus is minimized. To confirm the validity of the fundamental circuit modeling of the SVC system with a five-level inverter, the system was simulated using commercial simulation packages.<<ETX>>

Modelling and analysis of a multilevel voltage source inverter applied as a static var compensator

A multilevel PWM voltage source inverter, especially a five-level one, is introduced to obtain a static var compensator (SVC) as a large scale power source. The multilevel inverter has many advantages, such as better utilization of the switching devices, lower switching frequency at each semiconductor switch and reduced harmonics. In this paper, the SVC with five-level inverter is modelled using circuit DQ transformation and completely analysed including DC and AC characteristics. It is also pointed out that the modulation indexes depend on the values of the DC side capacitors to meet the DC side voltage balancing, Finally, through the experimental results from a 5kVA SVC, the validity of the analyses and the feasibility of the var compensation system are shown for high power applications.