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Recently, the transformerless design of medium-voltage drives is an emerging design concept of variable-speed drives (VSD). Here, no dedicated feed transformer is used. The VSD is directly connected to the on-site facility transformer, where multiple other loads are connected as well. The benefits of such a transformerless VSD are reduced total volume, weight, losses, and system costs. However, without a dedicated feed transformer, the converter-generated common-mode (CM) voltage may generate harmful bearing currents that could yield serious bearing failures within a short time. For this reason, the suppression of bearing currents is a fundamental design aspect of VSD systems. In this publication, a comprehensive bearing current study highlights the two dominating bearing current mechanisms in transformerless VSDs. Both mechanisms are caused by the converter-generated CM voltage applied to the machine terminals. Their physical understanding is used to define thresholds that ensure a sufficient mitigation of bearing currents. The newly introduced CM voltage limits allow the development of a generalized CM filter design for transformerless VSDs. Accordingly, a suitable filter grounding concept including a CM filter design methodology for transformerless high-power medium-voltage converters is presented for the first time. Here, both the CM voltage injection to the grid and the CM voltage applied to the machine terminals is considered. At the machine-side, the novel design reference to prevent bearing currents is applied. By this, the reliable transformerless operation of general purpose drives and retrofit applications is feasible. [ABSTRACT FROM PUBLISHER]

This paper presents a pumpback test bench for high-frequency high-power variable-frequency drives (VFDs) with 11 MW power rating and up to 595 Hz load frequency. A voltage combiner concept is applied to allow a high-frequency output from the two three-level neutral-point-clamped integrated gate-commutated thyristor bridges with minimum switching losses. The accordingly developed pumpback test bench applies a dc power coupling concept combined with a virtual grid concept that can achieve 11 MW test capability with only a 1.25 MW facility transformer. By applying a virtual machine concept, the pumpback test can be conducted without the need of high-frequency machines, while the circulating power losses in the pumpback loop are minimized as well. The virtual grid including the concept tradeoff, filter parameters, and modulation schemes is presented in detail. Moreover, design and installation guidelines for high-frequency components, especially the cables, are presented. The high-frequency high-power VFD and its pumpback test bench are both successfully built and experimentally validated at their rated operation points. [ABSTRACT FROM AUTHOR]

In this paper, the impact of dc-link voltage ripples on the machine-side spectrum is investigated. The analysis is targeted for a high-power converter using neutral-point-clamped (NPC) H-bridge topology. Due to the high amplitude of second harmonic and additional f0/{2} subharmonic that is introduced by a special modulation scheme, their impacts on the machine are at question. In this work, these questions are addressed via mathematical modeling followed by simulations and experimental validations. The conclusions are that the second harmonic in the dc-link leads to a small amount of amplitude distortion of the command voltage, while the f0/{2} subharmonic generates small amount of second and fourth even-order harmonics. These voltage ripples do not harm the machine and will not reduce the output voltage capability of the converter. [ABSTRACT FROM PUBLISHER]

In this paper, a high-performance 22 MW/27 MVA test bench using a pumpback topology is introduced for testing electrical machines. It proposes a cost-effective concept that works reliably even with a weak grid. A paralleling concept of two three-level integrated gate-commutated thyristor (IGCT) converters is presented. I generates a five-level pulse pattern with low harmonic distortion at 600-Hz carrier frequency and 80-Hz load frequency. The selected magnetic device minimizes the circulating current, which ensures the full power utilization of IGCT converters. Moreover, the control concept works reliably at 7.5 times of the carrier-to-load frequency ratio while there is a sine filter and a variable-frequency transformer between the converter and load machines. This test bench has been experimentally validated at the customer site. [ABSTRACT FROM PUBLISHER]

The design of modulation schemes at low pulse ratios is a known topic. However, most of previous work has focused on optimized pulse patterns, e.g., selective harmonic elimination. For industrial applications, standard carrier modulators are still highly attractive in many cases. Unfortunately, few papers presented insightful studies on carrier modulators at extremely low pulse ratios. This paper covers this gap. At extremely low pulse ratios, several special phenomena of carrier modulators are introduced for the first time. For example, a significant performance variation effect can be observed when carrier modulators are used. Moreover, the thermal unbalance of the hotspot devices in the converter may lead to significant power capability degradation of the converter unit. In this paper, the mechanisms behind this are explored and explained in detail for the first time. A quantitative comparison between different carrier-based modulation schemes is presented. Based on that, several design instruction for modulators at low pulse ratios are summarized at the end. [ABSTRACT FROM AUTHOR]

Neutral-point (NP) balancing issues in the neutral-point-clamped three-level converters have been widely discussed in the last decades. Numbers of prior NP control algorithms have been proven working well in terms of NP potential balancing. However, it seems that the impact of these algorithms on the ac side did not draw much attention in the past, e.g., some algorithms inject additional common mode voltages with considerable amplitude that may saturate the duty cycles or inject low-order harmonic currents into the ac load. As a consequence, compared with the original system without NP controls, the total harmonic distortion (THD) performance at the ac side becomes worse, i.e., the THD standards are violated and a redesign of the system (e.g., ac filter) may be essential. Therefore, particularly for industrial applications, low distortion and high margins for duty cycles are typically important criteria for designing an NP controller. In this paper, a novel NP controller with full power-factor (PF) range and low distortion is proposed, which has the minimum common voltage injection at no unbalance or slight unbalance conditions. On the other hand, at heavy unbalance conditions, the controller has the maximum balancing ability at any PFs. Its superior performance is summarized via a comparison between different NP controls in this paper. [ABSTRACT FROM AUTHOR]