Said Farouk Said El-Barbari

A Comprehensive Study of Neutral-Point Self-Balancing Effect in Neutral-Point-Clamped Three-Level Inverters

Neutral point (NP) balancing problems in neutral-point-clamped (NPC) three-level inverters have been widely discussed in the past. Most of the previous researches have focused on the active NP control algorithms based on the common-mode duty cycle injection or the differential-mode harmonic current injection. Various active control methods have been proven to work well in real applications. On the other hand, it is well known that sometimes the NP is self-balanced, even without any active NP control. However, due to the uncertainty and lack of the theoretical explanations, active NP controllers are typically preferred for real applications, and the self-balancing characteristic of the NPC topology did not draw much attention in the past. Our paper covers this gap: the mechanisms of self-balancing characteristic are explored systematically. With the help of a precise mathematical model, readers can easily evaluate the self-balancing capability of their own design. In addition, this paper discusses some methods to enhance the self-balancing capability of an existing system. It is shown here that the self-balancing effect depends strongly on the switching frequency. Especially, for high-power insulated gate bipolar transistor/integrated gate-commutated thyristor/gate turn-OFF thyristors (IGBT/IGCT/GTO) applications, the effect is so strong at all power factors that this “do-nothing” control is sufficient and no other active NP controls are needed.

Internal Fault Detection and Isolation for Paralleled Voltage Source Converters

This paper presented a systematic analysis of the protection methods for paralleled VSCs system especially with common DC bus from four types of internal faults, including short or open circuit faults of single switch or diode. The potential damages of such faults relating to circulating current were first explained. After that internal faults detection and isolation schemes based on de-saturation and circulating currents were shown in detail. The study confirmed that if the system can be shut down for a short period, SCRs can be used to recovery the system from internal faults with one redundant VSC. Otherwise, the topology with isolated DC buses for each VSC or other fast DC current breakers should be used. Finally, experimental results using a 2+1 VSCs system validated the analysis and protection schemes.

Improved asymmetric space vector modulation for voltage source converters with low carrier ratio

This paper presents an improved asymmetric space vector modulation (ASVM) for two level voltage source converters (VSCs) when the switching frequency is only 9 times of line frequency. By adding two pulses in each line cycle when the fundamental voltage crosses zero, the total harmonic distortion (THD) of output current can be reduced significantly with very limited penalty. The applications of improved ASVM in a single VSC or two interleaved VSCs systems are shown separately. With optimization, the ac current THD can be reduced to as low as 50% for single VSC and even lower to 20% for interleaved VSCs systems. Such THD reduction has close relationship with modulation index and interleaving angle. In addition, improved ASVM can also reduce the amplitude of circulating current which mainly determined the size of inter-phase inductors. Finally, the weights of total inductors needed to meet the same THD requirement are compared to demonstrate the benefits of improved ASVM when different PWM schemes are used. The analysis results are verified by experiments on a demo system.

Cascaded H-bridge control for PV application

In this paper, application of a cascaded H-bridge (CHB) converter to connect strings of solar panels to a single-phase medium voltage AC grid is discussed. With solar insolation levels differing from one string to another, it is challenging to control a CHB converter in order to minimize the variations in the dc link voltages of each of the converter modules and hence the device voltage stress levels. To minimize the device voltage stresses, in this paper, a new Maximum Power Point Tracking (MPPT) method is proposed. The proposed method equalizes the power drawn from each of the solar strings connected to the CHB across a large geographical area (e.g. along a railway line) and avoids excess device stresses. The proposed MPPT method is verified using simulation results.