Kunlun Chen

A High-Performance 2×27 MVA Machine Test Bench Based on Multilevel IGCT Converters

In this paper, a high-performance 22 MW/27 MVA machine test bench using pumpback topology is introduced. It proposes a cost-effective concept that works reliably even with a weak grid. A paralleling concept of two three-level IGCT converters is presented. It 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, vector control concepts for machines are presented that work reliably at 7.5 times of carrier-to-load frequency ratio, while there is a sine filter between the converter and load machines. This test bench has been experimentally validated at the customer site.

Modulation Schemes for a 30-MVA IGCT Converter Using NPC H-Bridges

This paper demonstrates the impact of modulation schemes on the power capability of a high-power converter with low pulse ratios. This integrated gate-commutated thyristor (IGCT) converter uses a five-level neutral-point-clamped H-bridge topology. It is concluded that phase-shifted carrier modulators are not attractive for such converters at low pulse ratios due to the poor total harmonic distortion (THD) performance and poor power capability. The THD performance is a key indicator for optimizing the performance of IGCT converters. Therefore, low-THD patterns such as the level-shifted carrier and optimized pulse patterns are proposed. It is explored that a smart and instantaneous pattern distribution is a key factor to ensure a robust modulator design. Compared with the standard interleaving concept, the proposed modulation schemes allow a power capability increase up to 40%. Meanwhile, the current THD is reduced from 11% to 4%-7%. The concept is implemented and experimentally validated up to 30 MVA on an industrial IGCT converter.

Test-bench for very high power variable frequency drives working under constrained grid conditions

This paper introduces a test-bench concept for high power variable frequency drives that can work reliably also with a weak and power limited grid. Experimental results are shown for a 10MW+ drive test bench with only a remote 1.25MVA feeding transformer supply. Common “pump-back” configurations circulate the test bench power via the supply grid. This produces huge distortions and potential instabilities with weak grids. Hence, we circulate the power via a virtual 50 or 60 Hz grid that is not connected to the feeding grid. Only the losses are fed from the grid directly to the dc-link of one test-bench VFD via a simple diode rectifier. Since the total system losses are typically only about 10% of the rated powers, the corresponding grid disturbances are significantly reduced. The implemented test-bench is utilizing a virtual machine but the concept can also be combined with a real motor-generator set.

Novel control strategy to improve power capability for high power converters

Power capability is one of the most important criteria for converters in high-power applications. The normal approach for converters increasing the power capability is to use devices with larger Safety Operating Area (SOA). In this paper, a novel control method is proposed to realize power capability improvement at low carrier ratios without hardware changes. It uses the carrier modulation at integer carrier ratios and optimized carrier angles to achieve the best performance in terms of the lowest peak current stress and/or thermal stress of hotspot devices. Meanwhile, for high-power semiconductor devices such as IGCT, the maximum turn-off current is related to the minimum on-state time. This paper dynamically changes the minimum on-state time according to the current requirement to adjust max turn-off current, while keeping the output power quality by injecting minimum active common mode voltage. With these controls, the power capability of high-power converters is pushed to higher level without hardware changes.

A high-frequency high-power test bench for 11MW/595Hz drives with 1.25MW grid capability

This paper presents a pumpback test bench for high-frequency high-power Variable-Frequency Drives 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 IGCT bridges with minimum switching losses. The accordingly developed pumpback test bench applies a dc power coupling concept combined with a virtual grid that can achieve 11 MW test capability with only 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 are minimized as well. The details about the virtual grid including the concept tradeoff, filter parameters and modulation schemes are presented. 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.

Control strategies for a high-performance 2×27 MVA machine test bench with multilevel IGCT converters

In this paper, control strategies of a 22 MW / 27 MVA machine test bench using pumpback topology are introduced. The current loop design at extremely low pulse ratios (e.g. 600 Hz carrier frequency / 80 Hz load frequency) is developed. The system stability and dynamic response are analyzed. The challenges for high-power machine control with a sine filter and a transformer connected between the voltage-source-inverter and the machine are listed and solutions are given. Afterwards, some specific test modes are introduced as well. Finally, the control strategy is validated with the motor test at the customer site.