Richard S. Zhang

Three-dimensional space vector modulation for four-leg voltage-source converters

Four-leg voltage-source converters can effectively provide the neutral connection in three-phase four-wire systems. They can be used in inverter, rectifier, and active filter applications to handle the neutral current caused by the unbalanced and/or nonlinear load or unbalanced source. In this paper, three-dimensional (3-D) space vector modulation (SVM) schemes are proposed for controlling the four-leg voltage-source converters. Important issues for 3-D SVM, such as definition of 3-D vectors, identification of adjacent switching vectors in the 3-D space, and switching vector sequencing schemes and comparisons are addressed. The proposed 3-D SVM is a superset of the traditional two-dimensional (2-D) SVM, and thus it inherits all the merits of the traditional 2-D SVM. A 100 kW 5 kHz four-leg inverter and a 20 kHz four-leg rectifier prototypes are built and controlled by the proposed 3-D SVM. Experimental results are presented to validate the effectiveness of the 3-D SVM.

A new family of active antiislanding schemes based on DQ implementation for grid-connected inverters

Unintentional islanding protection of distributed generation is a key function for standards compliance. For those distributed generations that use an inverter as grid interface, the function can be implemented as part of the inverter control. Existing antiislanding schemes used in inverters have power quality and nondetection zone issues. This paper proposes a new family of schemes that have negligible power quality degradation and no nondetection zone. Design guidelines based on frequency-domain analysis is also provided. Finally, both simulation and experimental results validate the proposed schemes.

Modular High-Power Shunt-Interleaved Drive System: A Realization up to 35 MW for Oil & Gas Applications

This paper describes the experimental results of a medium-voltage modular 35-MW drive for oil and gas applications realized by interconnecting voltage-source three-level converters equally sharing the motor current. Drives rated for tens of megawatts are increasingly needed as torque helpers for gas turbines in the oil and gas industry, and one envisions that they will replace them completely in the future. These drives must be very reliable, often capable of reversible power flow and high power quality when connected to weak grids in remote areas. The pronounced torsional eigenmodes of large compressors and gas turbines demand for very low torque harmonic content.

New high power, high performance power converter systems

A new, high performance, low cost power converter system architecture is proposed. The system consists of a main converter and a multifunctional load conditioner. The main converter deals with most of the power flow running at a low switching frequency. The load conditioner is designed at a much lower power level running at a high-switching frequency. The load conditioner can (1) act like a current source and inject harmonic currents required by the load; (2) act like an active resistor to provide damping to the main converter; and (3) for three-phase inverters, decouple the coupling sources in the main inverter model in the rotating coordinates to make the control loop design for the main inverter much easier. The concept has been proved by simulation and experimental results on a 150 kW high performance three-phase utility power supply prototype. The proposed system configuration can be used in high power DC-DC converters, inverters, PFC and UPS applications.

Four-legged three-phase PFC rectifier with fault tolerant capability

A four-legged three-phase PFC rectifier is proposed to handle unbalanced source voltage and to provide fault tolerant operation. The four-legged PFC rectifier is controlled by a three-dimensional space vector modulation (3D SVM) scheme, and runs in the overmodulation region when the source is unbalanced and/or a leg failure occurs. Modeling and control of the four-legged PFC rectifier are presented. Under unbalanced source, operation of the four-legged rectifier with three control strategies-equal current, equal resistance and constant power are analyzed. Two rectifier failure modes are studied: (1) entire leg fails; (2) active switches fail with the anti-parallel diodes still functional. It turns out that in both failure modes, the PFC rectifier can have an equal or higher than 2/3 of the rated output power level. The operation of the four-legged rectifier with 3D SVM is demonstrated by experimental results. The simulation results show that the proposed four-legged rectifier is advantageous over directly coupled three single-phase PFC rectifier and three-legged three-phase PFC rectifier.

AC load conditioner and DC bus conditioner for a DC distribution power system

A PEBB (power electronics building block) based power converter system is described to provide a secondary utility bus in a DC distributed power system on a ship. The power converter system consists of a main inverter and a multi-functional load conditioner to achieve low voltage distortion under unbalanced and/or nonlinear loads. A four-legged three-phase power converter topology is used for both the main inverter and the load conditioner. Modeling and control design aspects of the power converter system are addressed with emphasis on the impacts of different load conditions. Experimental results show that the utility sub-system has a high performance under different load conditions. However, when an unbalanced load or nonlinear load is connected to the secondary utility bus, the whole utility sub-system behaves like a nonlinear load to the DC distribution bus by drawing a low frequency ripple current. The nonlinear loading effect has been identified as one of the system level control issues which impacts greatly on the stability of the DC distribution bus in a large signal sense. Two approaches are suggested to eliminate the nonlinear loading effect and to improve the stability of the DC distribution bus. One is to use the load conditioner with a modified control reference, another is to use a DC bus conditioner, which is an extension of the AC load conditioner concept by performing active filter and damping functions. Simulation results prove that both approaches are effective. Tradeoffs and system design considerations are discussed.

Electric Systems For High Power Compressor Trains In Oil And Gas Applications - System Design, Validation Approach, And Performances.

This paper presents the development process for a new variable frequency drive (VFD) system for large size compressor trains for natural gas and liquefied natural gas (LNG) applications. The paper covers the multiple steps of the product development process, from identification of applications requirements, definition of design 61 ELECTRIC SYSTEMS FOR HIGH POWER COMPRESSOR TRAINS IN OILAND GASAPPLICATIONS— SYSTEM DESIGN,VALIDATIONAPPROACH,AND PERFORMANCE

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.