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A model predictive power factor control (MPPFC) scheme is proposed for high-power medium-voltage (MV) current source rectifiers (CSRs). Facing two major issues on input power factor (PF) and LC resonance, the proposed MPPFC uses a novel line reactive power reference estimator to achieve unitary input PF or maximum achievable input PF for a CSR at various operating states, and involves active damping function into itself to mitigate LC resonance at line side. An online capacitance estimation method is associated with the proposed line reactive power reference estimator to guarantee more accurate input PF control, and a novel damping current calculation method, which avoids the use of band-stop filter, is also designed for active damping purpose. In comparison with traditional linear control, the proposed MPPFC not only realizes input PF regulation, but also achieves better steady-state performance on meeting the grid code requirements. In contrast to traditional model predictive control (MPC) for a CSR, the proposed MPPFC achieve more accurate reactive power control. Moreover, it overcomes the challenge on the realization of active damping based on MPC, which extends the application of MPC into high-power MV CSR. Simulation on a high-power CSR (1 MVA/4160 V/139 A) and experiments on a low-power prototype (5 kVA/208 V/13.9 A) are both conducted to verify the effectiveness of MPPFC. [ABSTRACT FROM PUBLISHER]

Current-source converters (CSCs) used in high-power (megawatt level) medium-voltage (MV) (2.3–6.6 kV) drives require three-phase capacitors to assist the commutation of switching devices and to filter out harmonics of the converter output PWM current. Different types of modulation schemes with different harmonic performance lead to different capacitor selections. Conventional space vector modulation (SVM) featuring high dynamic performance cannot be used in practice even with the help of active damping, which can effectively damp the possible LC resonance introduced by conventional SVM. Recently, a natural-sampling SVM (NS-SVM) is proposed for MV CSC drives. It features superior low-order harmonics and has been highly considered for MV CSCs in practice. This study, therefore, focuses on two issues. One is why conventional SVM cannot be used in practice even with the help of active damping. On this basis, the second is capacitor minimization based on NS-SVM. Finally, experimental verification is provided. [ABSTRACT FROM PUBLISHER]

This paper presents a model predictive switching pattern control (MPSPC) for a current-source converter (CSC), which achieves superb low-order harmonics elimination performance in steady state and improved transient responses. Based on a proposed space-vector-based selective harmonic elimination (SHE) method and prediction of load current at the next sampling instant, MPSPC prefers to following a precalculated SHE-pulse width modulation (PWM) pattern in steady state, and governing the CSC through a model predictive control (MPC) approach during transients. In comparison with existing schemes, the advantages of MPSPC are threefold: First, quantization error, introduced by a constant sampling frequency in MPC and degrading steady-state low-order harmonic elimination, is mitigated in the proposed scheme. Second, there is no weighting factor in the cost function, as used in existing schemes. Finally, MPSPC is totally realized based on one-step prediction, which simplifies the structure of the scheme. Both simulation and experimental results verify the steady state and dynamic performance of MPSPC with different SHE-PWM patterns. [ABSTRACT FROM AUTHOR]

The device switching frequency of current-source converters (CSCs) in high-power medium-voltage (MV) drive applications is normally around 500 Hz. Conventional space vector modulation (SVM) features fast dynamic response but its output contains low-order harmonics with high magnitudes. Recently, a natural-sampling-based SVM (NS-SVM) with superior low-order harmonics performance has been proposed for MV CSCs. This study further investigates the low-order harmonics performance of different space vector sequences for MV CSCs based on NS-SVM. Dwell times equations based on NS-SVM for each space vector sequence with minimized switching frequency are analyzed and derived; comparisons of different vector sequences in terms of low-order harmonics and load/grid-side total demand distortion performance are thoroughly conducted. Among all the investigated sequences, sequence 2 (SQ2) is the best one; its linear two-step dwell times equations are provided for online implementation. Simulations and experiments are provided to verify its performance. [ABSTRACT FROM AUTHOR]

The device switching frequency of current source converters (CSCs) in high-power medium-voltage (MV) drive applications is normally around 500 Hz. Conventional space vector modulation (SVM) features fast dynamic response but its output contains low-order harmonics with high magnitudes. In this paper, a natural-sampling-based SVM (NS-SVM) scheme is proposed to substantially suppress the low-order harmonics in practical CSC-based drives. This study proposes an equivalent discretization of NS-SVM wherein natural sampling pulse width modulation performance can be achieved with regular sampling frequency (same as a conventional SVM) instead of a high or even infinite sampling frequency. Equations for natural-sampling-based dwell time calculations for each selected vector are developed. The corresponding simplified but precise enough equations are derived on the basis of the Newton–Raphson algorithm to achieve fast online digital implementation. The proposed NS-SVM features superior low-order harmonic performance and inherits the high dynamic response and low sampling frequency of conventional SVM. Simulations and experiments are provided to verify the proposed NS-SVM scheme. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to develop a model and propose control strategies to mitigate dc-link current ripple for current-source grid-connected converter (CSGCC) under unbalanced grid conditions. Based on the model of instant active power under unbalanced grid conditions, this paper proposes the optimized negative-sequence current references for eliminating the double-frequency oscillations on active power at ac side of CSGCC. Both the single CSGCC and the paralleled CSGCCs are considered in this paper. In order to track the asymmetric current references more accurately while maintaining LC resonance damping, the hybrid current controller is proposed by combining the closed-loop fundamental current controller under double synchronous frames and the closed-loop high-bandwidth harmonic current controller under stationary frame. The design of the hybrid current controller is analyzed. Finally, the experimental results are shown to verify that the control strategies can mitigate the dc-link current ripple effectively, and also provide good LC resonance damping performance for grid currents. [ABSTRACT FROM PUBLISHER]

Due to the inductor–capacitor (LC) filter, a pulsewidth-modulation (PWM) current-source converter may experience LC resonance. The resonance can be excited by system harmonics either from the PWM process or from the grid voltage distortions and hence results in the line current distorted. In this paper, feedback controls using different variables in the LC filter are first investigated and comprehensively analyzed. It is shown that the inductor-voltage feedback or the capacitor-voltage feedback can damp the current oscillations by increasing the damping ratio, while the inductor-current feedback or the capacitor-current feedback is able to suppress low-order harmonics by raising the resonance frequency. On this basis, a group of combined variable feedback control methods is presented by combining the feedbacks of the inductor/capacitor current and the inductor/capacitor voltage. Thus, not only the current oscillations can be damped, but also the low-order harmonics can be suppressed. Therefore, the line current waveform is improved, and the system performance can be flexibly optimized. A practical design method for feedback gains is also presented in this paper. Finally, the simulation and experimental results verify the feasibility and validity of the proposition. [ABSTRACT FROM PUBLISHER]

Multimodular matrix converters (MMMCs) built with three- to single-phase modules have emerged as a candidate for high-power applications. In order to reach higher voltage and power levels, constituent modules of an MMMC are fed by isolated secondary windings of a phase-shifting transformer, while their outputs are connected in series to supply the load. This paper presents a new modulation strategy for the MMMC. Unlike previously proposed strategies that aim to simultaneously fabricate sinusoidal output voltage and input current, the new method only focuses on generating multilevel output voltage waveforms. Nonetheless, because power balance among modules on a same output phase is guaranteed, and owing to the harmonic elimination capability of the phase-shifting transformer, sinusoidal input current with good quality can still be achieved. Compared with previous methods, the proposed strategy cannot adjust the input power factor but is simpler, more straightforward, and associated with lower switching losses. Its use makes possible the employment of an indirect module structure that significantly reduces the total number of switches and, thereby, the overall semiconductor cost and eliminates the need for complex device commutations, which is essential in direct matrix converter modules. The presented concepts are verified by simulation and experimental results. [ABSTRACT FROM PUBLISHER]

In this paper, a current-source-converter (CSC)-based high-power high-speed (HPHS) permanent-magnet synchronous motor drive is proposed for high-speed compressor applications. The most important feature of the proposed CSC-fed HPHS drive is that the switching frequency is limited to 420 Hz at 200-Hz fundamental frequency. In the drive system, the whole operation range is divided into three regions, namely, the high-, medium-, and low-speed regions, where different modulation strategies and control schemes are used with the requirement of switching frequency and output filter. In particular, a two-pulse space vector modulation with dynamic capacitor voltage control is proposed for the high-speed region to achieve better dynamic and harmonic performance. Both simulation and experiments verify the proposed control strategy. [ABSTRACT FROM PUBLISHER]

An improved direct voltage control (DVC) strategy is proposed for the control of terminal voltage and frequency of a stand-alone wind-driven self-excited induction generator with variable loads. The DVC strategy, including a proportional-integral (PI) regulator, lead–lag corrector, and a feed-forward compensator, is designed based on the system transfer function matrix. The PI regulator can eliminate the steady-state tracking errors of the terminal voltage. The lead–lag corrector is employed to enlarge the phase stability margin of the dominant loops while the feed-forward compensator is adopted to mitigate the voltage harmonics resulting from the cross-coupling dynamics. The procedures for capacity design of the excitation capacitors, the voltage source inverter, and the consumer load are also presented. The simulation and experimental results verify that the proposed strategy has a fast dynamic response and can effectively control the generated voltage with low harmonic distortions under different linear or nonlinear loads. [ABSTRACT FROM AUTHOR]

In this paper, a hybrid pulsewidth modulation (PWM) scheme is proposed to suppress the LC resonance for the high-power current-source inverter (CSI) fed drives with low switching frequency. In CSI drive systems, the selective harmonic elimination (SHE) is selected due to the better performance of harmonics, but the LC resonance cannot be effectively damped due to the lack of control flexibility of SHE. In this paper, SHE is only used for the steady-state operation. The modulation scheme is then switched to space vector modulation with dynamic capacitor voltage control when the transient event occurs. The smoothed transitions between different modulation schemes are proposed. The proposed hybrid method effectively solves the resonant issues in CSI drives, while maintaining the power quality. Both simulation and experiments verify the performance of the proposed hybrid PWM scheme. [ABSTRACT FROM AUTHOR]