D. Xu

A Medium Voltage AC Drive with Parallel Current Source Inverters For High Power Applications

A power circuit topology suitable for large medium voltage AC drive applications is presented. The line side harmonics requirements are met by using an 18 pulse phase-controlled rectifier. The output horsepower of the drive is increased by connecting two inverters in parallel. The low switching frequency used in the inverters makes this approach especially attractive in very high power applications. A new control scheme is proposed to ensure proper current sharing between the two output inverters even when the power switching devices and the two bridges are severely asymmetric. The start up issues and the control response to transient conditions are analyzed and the boundaries of the controllability of the system are discussed. The proposed system is experimentally verified by a 208 V, 5 hp laboratory set up

A Novel Three-phase Three-leg AC/AC Converter Using Nine IGBTs

This paper proposes a novel three-phase nine-switch AC/AC converter topology. This converter features sinusoidal inputs and outputs, unity input power factor, and more importantly, low manufacturing cost. The operating principle of the converter is elaborated, its modulation schemes are discussed, and experimental results from a 5 kVA prototype system are provided.

Multilevel Current Source Inverters with Phase Shifted Trapezoidal PWM

A 7-level current source inverter (CSI) is proposed for high-power applications. The inverter topology consists of three bridge current source inverters whose outputs are connected in parallel for a large induction motor. The multilevel inverter is fed by an 18-pulse SCR rectifier with three isolated outputs for the reduction of supply-side current THD and the elimination of possible circulating current among the inverters. A phase-shifted trapezoidal PWM (TPWM) scheme is proposed for the inverter to generate multiple current levels. Optimal phase angles are investigated for the reduction of inverter current THD and weighted THD (WTHD). Similar to multilevel voltage source inverters, the multilevel CSI can produce motor-friendly current and voltage waveforms with limited switching frequencies, which is a desirable feature for large motor drives

Space Vector Modulation for High Power Five Level Current Source Inverters

A 5-level current source inverter (CSI) is proposed in this paper for high-power applications. The converter topology consists of two full-bridge current source inverters whose outputs are connected in parallel for a large induction motor. The two inverters are fed by two isolation transformers to the elimination of possible circulating currents. A space vector modulation scheme is developed for the inverters to generate the desirable output current. The inverter current harmonics are investigated at different amplitude modulation index and fundamental frequencies. The developed scheme for multilevel CSI can produce adjustable output current with limited switching frequencies, which are very desirable features for high dynamic performance large motor drives.

Novel Nine-Switch PWM Rectifier-Inverter Topology for Three-Phase UPS Applications

Abstract A novel three-phase PWM rectifier-inverter topology (Fig. 2) for UPS applications is proposed in this paper. The topology uses only nine IGBT devices for AC/AC conversion through a quasi d.c. link circuit. This converter topology features sinusoidal inputs and outputs, unity input power factor, and more importantly, low manufacturing cost. The operating principle of the converter is elaborated and a dedicated space vector modulation scheme is presented. The performance of the proposed converter is verified by experiments on a 5 kVA prototyping UPS system.

Cost effective method for DFIG fault ride-through during symmetrical voltage dip

This paper presents a cost-effective scheme to improve the fault ride through capability of wind turbines with doubly fed induction generators. As an alternative to using crowbar, the proposed scheme includes the stator side series-connected braking resistors, dc-link chopper and coordinated control strategy of the whole system. The proposed scheme enable the rotor converter always be connected to the system such that the DFIG will not lose controllability during fault and can almost immediately generate reactive power to support the grid after the beginning of the fault. The series-connected braking resistors can help the transient decay very fast and, at the same time, dissipate the energy of the wind turbine to reduce the rotor speed deviation during fault. Dynamic behavior of DFIG-based wind turbines during grid faults is discussed based on theoretical analysis and simulation results. It is shown that the proposed scheme can not only help the whole system ride through the fault but also reduce the stress on turbine mechanical systems.

Space Vector Modulation for High-Power Three-Level NPC Rectifiers Without Even Order Harmonics

Space vector modulation (SVM) is a preferred PWM scheme for multilevel converters mainly due to its flexibility, easy digital implementation and good harmonic profile. However, the conventional SVM scheme normally produces even-order voltage/current harmonics at the rectifier inputs. The even-order harmonic currents are strictly regulated by the harmonic guidelines such as the IEEE Standards 519-1992 in North America. In this paper, the mechanism of even-order harmonic generation is analyzed, and a new SVM scheme that has the ability to eliminate these harmonics is proposed for three-level neutral point clamped (NPC) rectifiers. The harmonic profile of both schemes is investigated by computer simulation. The effectiveness of the proposed scheme is experimentally verified

Progressive natural balance of neutral-point voltage of three-level NPC inverter with a modified SVM scheme

It is well known that the neutral point voltage of three-level NPC (neutral point clamped) inverter tends to deviate during operation. The voltage deviation can cause uneven voltage distribution among switching devices and additional harmonic distortion. The problem is normally mitigated by a closed loop control of the neutral point voltage. In this paper, the problem is fully analyzed from mathematical point of view. It is proven mathematically that the neutral point voltage can be naturally balanced from initial unbalanced neutral-point voltage when the inverter output PWM (pulse width modulation) sequences do not contain even-order harmonics, non-zero load resistance and different three phase switching states. The experimental results are provided to verify the theoretical analysis

An Effective Method to Suppress Resonance in Input LC Filter of a PWM Current-Source Rectifier

The PWM current-source rectifier (CSR) is becoming a preferred choice to provide a DC current source for DC loads or current source fed drives. However the control of the resonance of the input LC filter is one of the main challenges. To dampen the LC resonance with minimum control and implementation efforts, a two-step Posicast controller for modulation signal shaping is first examined, which can be easily inserted into the existing CSR control loop. The parameters of the two-step Posicast controller are fixed by the lightly damped plant, which makes it unsuitable at high power levels due to the inaccurate compensation associated with the low switching frequency. A three-step compensator is then designed in this work. By selecting its parameters according to the existing control algorithm, the three-step compensator is very effective for the low switching frequency CSR with precise resonance compensation and easy implementation. Detailed design and analysis are presented. Simulation and experimental results are provided to verify the effectiveness of the proposed damping approach

Overview of offshore wind farm configurations

Offshore wind energy has been attracting great attention. Compared with onshore wind power systems, offshore wind power applications present significantly greater economic challenges mainly due to the required bulky and costly offshore substation. To lower the cost of offshore wind power systems, various configurations are proposed in both industry and academia. The present work investigates existing offshore wind farm configurations.