Rodrigo Huerta

Mitigation of noneliminated harmonics of SHEPWM three-level multipulse three-phase active front end converters with low switching frequency for meeting standard IEEE-519-92

Three-level NPC active-front-end three-phase (3LAFE) converters can be applied to high-power medium voltage applications, with 4-quadrant unity-power-factor operation, allowing a broad range for power conversion systems, such as decentralized energy distribution and utilization. Switching losses and network interaction limit the operation of 3LAFE high-power converters, especially when GTO-converters are used. That is why, low switching frequency and reduced harmonic distortion are the main goals when using optimum pattern PWM where selective harmonic elimination modulation method (SHEPWM) is used. This paper introduces a simple method to choose a good operating point for two 3LAFE converters in a 12-pulse configuration based on the behavior of noneliminated harmonics by SHE method. A complete characterization is given as a base of SHE and the optimization criteria is fully explained.

Resonances in a high-power active-front-end rectifier system

This paper presents the application of high-power three-level active-front-end rectifiers to regenerate energy in a downhill conveyor system. The selective harmonic elimination method is used to eliminate harmonics 11 and 13, working with very low switching frequency, where six-pulse harmonic orders 6k/spl plusmn/1 are eliminated by the delta-wye connection of the transformer. In this way, the input current at the mains is highly sinusoidal with small harmonics starting at frequencies of order 23 and 25. Resonances have been detected, originated mainly by the capacitances of feeding cables and noneliminated harmonics, which produce high-voltage distortion. Theoretical and field measurements present the problem and the solution by using a specially designed high-pass power filter.

High-power regenerative converter for ore transportation under failure conditions

The use of three-level neutral-point-clamped (3L-NPC) voltage source inverters for drive applications in the megawatt range is becoming a standard solution. In combination with the same converter circuit in rectifier operation as active front end (AFE), four-quadrant operation is also available. An existing plant, which has been in operation with eight converters for several years, has shown some failures, which leads to considerations of failure mechanisms and failure rates. All failures were on the AFE. A first failure of a power semiconductor leads to subsequent breakdowns of others. Some failures are analyzed and failure conditions are described. Some failure reasons can be excluded and a probable reason is pointed out. Finally, a recommended solution is described and its result after its implementation is discussed.

A mitigation method for non-eliminated harmonics of SHEPWM three-level multipulse three-phase active front end converter

Three-level NPC active-front-end (3LAFE) converters can be applied for high-power medium voltage applications, with 4-quadrant unity-power factor operation, allowing for decentralized energy distribution and utilization, hence making full regenerative systems possible. Switching losses and network interaction limit the operation of 3LAFE high-power converters, especially in the case of GTO-inverters. That is why obtaining a low switching frequency and reduced harmonic distortion are the main goals when using optimum pattern PWM with a selective harmonic elimination method (SHEPWM). For 12-pulse configurations, the lowest switching frequency can be achieved using a 3-angle scheme for SHEPWM where the 11th and 13th harmonics can be eliminated by the correct firing angle application. The remaining non-eliminated harmonics of orders 23, 25, 35 and 37 play a main role on defining the voltage distortion injected into the power network, where the resulting harmonic current injection depends on the series impedance between the network and 3LAFE. This paper presents the experimental study concerning the regenerative unity-power factor operation of a 3LAFE converter, highlighting the possible reduction of non-eliminated harmonics by the introduction of slight variations of modulation index and DC-link voltage regarding a selective harmonic distortion factor (SHD). This work opens new possibilities for avoiding or reducing harmonic filters, especially by operating within weak networks and stringent conditions to be studied in further projects.