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This paper presents a novel seven-level (7L) voltage source converter for high-power medium-voltage applications. The proposed topology is an H-bridge connection o f two nested neutral-point clamped (NNPC) converters and is referred to as an HNNPC converter. This converter exhibits advantageous features, such as operating over a wide range o f output voltages, particularly for 10-15 kV applications, without the need to connect power semiconductors in series; high-quality output voltage; and fewer components relative to other classic seven-level topologies. A novel sinusoidal pulse width modulation technique is also developed for the proposed 7L-HNNPC converter to control flying capacitor voltages. One of the main features o f the control strategy is the independent application of control to each arm of the converter to significantly reduce the complexity of the controller. The performance of the proposed converter is studied under different operating conditions via MATLAB/Simulink simulation, and its feasibility is evaluated experimentally on a scaled-down prototype converter. [ABSTRACT FROM AUTHOR]

The nested neutral-point clamped (NNPC) converter is a four-level converter topology for medium-voltage applications with interesting properties such as operating over a wide range of voltages (2.4-7.2 KV) without the need for connecting the power semiconductor in series, high quality output voltage, and less number of components in compare to other classical four-level topologies. The control and balance of the flying capacitors (FCs) of the NNPCconverter can be done by different control techniques taking advantage of the large number of redundant switching states. This paper presents a simple single-phase modulator for the NNPC converter, which can be applied to each phase of a three-phase NNPC converter. The proposed simple technique can control and balance the FCs to their desired values. Performance of the proposed technique under different operating conditions is investigated in the MATLAB/Simulink environment. The feasibility of the proposed converter is evaluated experimentally. [ABSTRACT FROM AUTHOR]

In this paper, a new voltage source converter for medium voltage applications is presented which can operate over a wide range of voltages (2.4-7.2 kV) without the need for connecting the power semiconductor in series. The operation of the proposed converter is studied and analyzed. In order to control the proposed converter, a space-vector modulation (SVM) strategy with redundant switching states has been proposed. SVM usually has redundant switching state anyways. What is the main point we are trying to get to? These redundant switching states help to control the output voltage and balance voltages of the flying capacitors in the proposed converter. The performance of the converter under different operating conditions is investigated in MATLAB/Simulink environment. The feasibility of the proposed converter is evaluated experimentally on a 5-kVA prototype. [ABSTRACT FROM AUTHOR]

In this paper, a model-predictive control of medium voltage, grid-tied four-level diode-clamped inverter for use in high-power wind energy conversion systems is presented. The control objectives such as regulation of net dc-link voltage, reactive power generation to meet the grid operator request, dc-link capacitor voltages balancing, and switching frequency minimization are included in the cost function. The latter objective is an important requirement for high-power converters, and this has been achieved without any modifications to the software or hardware configuration. The future behavior of the grid currents and dc-link voltages is predicted for all the possible switching states using the discrete-time model of the inverter, dc-link, inductive filter, and grid. The switching state that minimizes the cost function has been chosen and applied to the inverter directly. During different operating conditions, the switching frequency is regulated between 750 and 850 Hz with the help of a lookup-table-based weighting factor, and thus, the proposed methodology appears as a promising tool to control the high-power grid-tied inverters. The results obtained through the MATLAB simulations on 4 MVA/4000 V system and dSPACE DS1103-based experiments on 5 kW/208 V prototype are in a close relationship and thus validate the proposed control strategy. [ABSTRACT FROM AUTHOR]

Abstract-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 due to its reduced number of active switches. The operating principle of the converter is elaborated; its modulation schemes are discussed. Simulated semiconductor loss analysis and comparison with the back-to-back two-level voltage source converter are presented. Finally, experimental results from a 5-kVA prototype system are provided to verify the validity of the proposed topology. [ABSTRACT FROM AUTHOR]

A multi-purpose balanced (MPB) transformer is proposed in this paper for electric railway applications. The MPB transformer is developed from impedance-matching (IM) transformers widely used in China's railway systems. The proposed MPB transformer has three main functions: 1) it provides a symmetrical two-phase voltage of 27.5 kV for railway overhead lines to power electric trains; 2) it supplies a three-phase balanced voltage of 10.5 kV for railway substations; and 3) it can accommodate a three-phase harmonic filter with improved power factor and reduced manufacturing cost. In this paper, the operating principles of the MPB transformer are elaborated, the design of the harmonic filters is discussed, and the results of computer simulations are provided. A 16-MVA MPB transformer was designed and commissioned, and real-time field measurements are provided. [ABSTRACT FROM AUTHOR]

For medium-voltage high-power drives fed by current-source converters (CSCs), a transformerless configuration has benefits in both cost and volume. Removing the isolation transformer necessitates a common-mode choke to undertake the major portion of the drive's common-mode voltage (CMV) stress, which would otherwise cause premature failure of the motor insulation system. On the other hand, as device switching frequency in high-power CSCs is normally limited to a few hundred hertz, improving harmonic performance via modulation has always been a challenge. Selective harmonic elimination (SHE) is a preferred modulation scheme in such a system owing to its ability to eliminate several unwanted low-order harmonic currents. Alternatively, conventional space vector modulation (SVM) provides continuous modulation index adjustment capability, but its output current contains low-order harmonics with high magnitudes that may arouse harmful resonances in the system. Aiming at reducing CMV and improving harmonic performance at the same time, this paper proposes a new SVM-based modulation strategy for high-power CSCs using synthesized reference trajectory on the hexagon in the αβ plane. Along with the proposed strategy, two methods of reference trajectory optimization (RTO) are investigated. By introducing a coefficient to the duty-cycle function for RTO, the first method can remove an extra low-order harmonic component, or minimize the weighted total harmonic distortion. The second method, with a different approach of RTO, eliminates the unwanted low-order harmonics by combining the proposed SVM with the SHE. The proposed concepts are verified by both simulation and experimental results. [ABSTRACT FROM AUTHOR]

This paper presents a generalized multilevel direct power control (ML-DPC) scheme for grid-connected multilevel power converters. The proposed method extends the original DPC operating principle by considering only the closest subset of two-level voltage vectors to the present switching state. The implementation of this principle requires the power derivatives for feedback, which can present numerical problems when applied experimentally, mainly due to high measurement noise sensitivity. Therefore, a derivative estimator is proposed based on the converter-grid model in the synchronous reference frame. In addition, a virtual flux observer is developed to achieve synchronization and improve robustness in the presence of grid voltage harmonics. The proposed method is applicable to any multilevel converter topology of any number of levels. In this paper, simulations and experimental results are presented for a seven-level cascaded H-bridge converter. [ABSTRACT FROM AUTHOR]

Half-cycle symmetry selective-harmonic-elimination pulsewidth-modulation (SHE-PWM) problems may have a large number of valid solutions, which are beneficial to the optimization design. This paper proposes a novel generalized formulation of half-cycle symmetry SHE-PWM problems for multilevel inverters. The advantages of the proposed formulation include simplicity in format, flexibility in PWMwaveforms, and a broad solution space. A method to obtain initial values for the SHE-PWM equations according to the reference modulation index M and the initial phase angle of output fundamental voltage is proposed and investigated thoroughly. Take a five-level inverter with M = 0.75 as an example; six sets of solutions for two typical initial phase angles of 0°? and 90°? are presented. Simulations and experiments are carried out. It is demonstrated that the experimental results agree well with simulated ones, which proves the validity and practicability of the new method proposed. [ABSTRACT FROM AUTHOR]

The device switching frequency of current source converters (CSCs) in high-power medium-voltage applications is usually several hundred Hz. Selective harmonic elimination (SHE) has been the dominant modulation scheme because of its capability to eliminate unwanted low-order harmonics at low switching frequencies. Conventional space vector modulation (SVM), as another CSC modulation scheme, provides variable modulation index control but its output contains low-order harmonics with high magnitudes. In this paper, a multisampling SVM (MS-SYM) is proposed to substantially suppress the low-order harmonics in practical CSC-based drives. Investigation of the switching frequency of the proposed modulation method is carried out and methods to reduce additional switchings are developed. The proposed MSSVM method exhibits a superior low-order harmonic performance comparable to that of SHE and provides the same fast and flexible control capability as the conventional SVM. Simulation and experimental results are provided to verify the proposed methods. [ABSTRACT FROM AUTHOR]

This paper presents an integrated ac choke that incorporates the common-mode (CM) suppression function into a three-phase differential-mode (DM) inductor. The proposed choke, together with other filter components, provides a transformerless solution to issues caused by the switching CM voltage in neutral-connected power converter systems. The required high CM-to-DM inductance ratio is generated by different magnetic paths in the novel structure. An approach based on magnetic circuit calculation can be applied to the weight-minimized design in which a reasonably small core loss confirms the choke's heat dissipation capability. The existence and uniqueness of the optimization are proven by a design example and analysis. Comparison to separate DM and CM inductors demonstrates the benefits of integration: the iron and copper material savings, the volume and weight reduction, and overall system efficiency improvement. The finite element method is used to tune the design parameters with which a prototype is implemented. The simulation and experimental results of the prototype in a voltage source converter verify the feasibility and effectiveness of the integrated choke. [ABSTRACT FROM AUTHOR]

This paper proposes an input power factor control strategy for a current-source drive with active front-end. The proposed strategy is realized without modification of the drive's pulsewidth modulation and speed control schemes through the collaborative use of two methods: a modulation index regulation method and a flux adjustment method. Specifically, the modulation index regulation method functions to directly control the dc-link current and voltage, and it can effectively correct a leading input power factor as long as the space vector modulation is used for the inverter. On the other hand, the flux adjustment method can improve the power factor when the power factor is lagging or when the selective harmonic elimination modulation is used for the inverter at high motor speeds. When implemented together, the two methods will complement each other's functionalities and improve the overall compensation performance. Experimental results are obtained from a 600-hp and an 1100-hp drive system. [ABSTRACT FROM AUTHOR]