On Attaining the Multiple Solutions of Selective Harmonic Elimination PWM Three-Level Waveforms Through Function Minimization

Selective harmonic elimination pulsewidth modulation techniques are some of the control methods used in voltage/current source converters. However, challenges such as the task of finding all the multiple sets of solutions of the switching angles for a given problem may be difficult to deal with. In this paper, a direct minimization of the nonlinear transcendental trigonometric Fourier functions in combination with a random search is discussed. The unipolar (three-level) waveform is used to illustrate the proposed method confirming its ability to find multiple sets of solutions, including a case where 51 angles are sought for single- and three-phase applications. A simple harmonic distortion factor is studied for each set of solutions to assess their performance against the noneliminated harmonics. The results presented both at theoretical and experimental level are in close agreement and confirm the robustness of the proposed approach.     

直流微网中混合储能系统的无互联通信网络功率分配策略

直流微网中通常采用混合储能系统作为缓冲环节,对分布式能源和负载引起的不同时间尺度功率波动进行补偿。为实现功率在能量密度型储能元件和功率密度型储能元件之间合理分配,提出无互联通信网络的分层控制策略。其中,底层控制以电压变化率作为虚构的信息载体,通过设置不同储能接口变换器输出电压关于功率的"灵敏度",确保超级电容在负载突变瞬间能够提供大部分功率;二次控制对底层控制产生的稳态误差进行补偿,以实现输出电压稳定,并保证超级电容稳态电流为零。在此控制框架下,各储能单元仅需本地信号即可实现自主协调运行,避免了互联通信网络所带来的经济性和可靠性问题。最后,实验结果验证所提方法的可行性和有效性。     

Multilevel converters for single-phase grid connected photovoltaic systems: an overview

Multilevel voltage source inverters offer several advantages compared to their conventional counterparts. By synthesising the AC output terminal voltage from several levels of DC voltages, staircase waveforms can be produced, which approach the sinusoidal waveform with low harmonic distortion, thus reducing filter requirements. The need of several sources on the DC side of the converter makes multilevel technology attractive for photovoltaic applications. This paper provides an overview on different multilevel topologies and investigates their suitability for single-phase grid connected photovoltaic systems. Several transformerless photovoltaic systems incorporating multilevel converters are compared regarding issues such as component count and stress, system power rating and the influence of the photovoltaic array earth capacitance.     

Dual ramptime nonlinear current control applied to a single phase five level cascaded inverter

This paper reports on the application of a zero average current error (ZACE) control method in single-phase five level cascaded inverter based systems. The dual ramptime method and its adaptation to the five level cascaded converter are described in detail. Recommendations on the choice of control parameters and how to improve noise immunity are included. Experimental results demonstrate the suitability of the control method, since the lower order harmonics are negligible and the ripple current frequency is kept within a narrow band.     

An optimum modulation strategy for a novel "notch" commutated 3-/spl Phi/ PWM inverter

The introduction of soft switching features in medium power static converters promises improved semiconductor utilization and higher switching frequencies. In this paper a DC-side commutated zero voltage switching (ZVS) PWM 3-/spl Phi/ VSI topology is discussed. This topology consists of a typical six-switch bridge with the addition of a simple DC bus commutating subcircuit which can provide, on demand, the necessary soft switching zero-volts interval across the inverter DC bus. The objective of this paper is to identify a specific PWM technique which, in combination with the proposed ZVS scheme, can provide optimum overall converter switching performance. Moreover, the combination of the inverter topology and the modulation strategy proposed here yields a soft switching environment at the lowest possible switching frequency. Reduction in switching losses is further enhanced by increasing the effective and reducing the actual switching frequency. Finally, this paper includes a detailed analysis and design procedure for the proposed notch commutated inverter topology. Simulation and experimental results are also presented to verify key predicted results.<>     

On considering magnetic saturation with maximum torque to currentcontrol in interior permanent magnet synchronous motor drives

The influence of magnetic saturation on maximum torque to current controlled interior permanent magnet synchronous motor drives is discussed in this paper. A maximum torque to current condition that takes into account magnetic saturation and determines the optimal d-axis current is derived. For the implementation of the proposed controller, an experimental procedure is used to adjust its parameters, therefore, the knowledge of the exact model is not required. Selected experimental results are presented to verify the theoretical considerations and to confirm the high performance of the suggested controller     

Slow sampling on-line harmonics/interharmonics estimation technique for smart meters

This paper presents a modified gradient search (MGS) technique to estimate harmonics/interharmonics of power system voltages and currents. The proposed technique, which is applied to spectral estimation, consumes very low processing power and needs a few samples per cycle for real-time implementation in smart meters. The computational burden of the proposed technique is lower than the discrete Fourier transform (DFT) and fast Fourier transform (FFT) and can be implemented recursively on digital signal processors (DSPs). In addition, the effects of slow sampling on the accuracy and estimation latency have been investigated. Performance of the proposed method is evaluated by simulations in MATLAB-Simulink and confirmed by experimental results.     

A new digital zero average current error control algorithm for inverters

Inverter performance is typically measured against its ability to produce a high fidelity output sinusoidal waveform with narrow band switching frequency and good transient response. For this purpose, pulse-width modulation techniques are used to generate the required control signals for the inverter semiconductor switches. Current control voltage-source inverters can be realized using one of the recently proposed zero average current error (ZACE) techniques based on analogue technology. A new and improved algorithm that belongs to the family of ZACE methods suitable for digital technology is proposed in this paper. Advantages include a more robust algorithm with quicker computation allowing fast response and higher switching frequency from a digital signal processor. The effectiveness of the new current control algorithm is demonstrated through simulations first. Experimental results taken from a low power laboratory prototype are also presented to verify the theoretical considerations.     

A market‐oriented wind power dispatch strategy using adaptive price thresholds and battery energy storage

In this paper, an adaptive dispatch strategy is presented to maximize the revenue for grid‐tied wind power plant coupled with a battery energy storage system (BESS). The proposed idea is mainly based on time‐varying market‐price thresholds, which are varied according to the proposed algorithm in an adaptive manner. The variable nature of wind power and market price signals leads to the idea of storing energy at low price periods and consequently selling it at high prices. In fact, the wind farm operators can take advantage of the price variability to earn additional income and to maximize the operational profit based on the choice of best price thresholds at each instant of time. This research study proposes an efficient strategy for intermittent power dispatch along with the optimal operation of a BESS in the presence of physical limits and constraints. The strategy is tested and validated with different BESSs, and the percentage improvement of income is calculated. The simulation results, based on actual wind farm and market‐price data, depict the proficiency of the proposed methodology over standard linear programming methods.     

A Five-Level Symmetrically Defined Selective Harmonic Elimination PWM Strategy: Analysis and Experimental Validation

A five-level symmetrically defined multilevel selective harmonic elimination pulsewidth modulation (MSHE-PWM) strategy is reported in this paper. It is mathematically expressed using Fourier-based equations on a line-to-neutral basis. An equal number of switching transitions when compared against the well-known multicarrier phase-shifted sinusoidal PWM (MPS-SPWM) technique is investigated. For this paper, it is assumed that the four triangular carriers of the MPS-SPWM method have nine per unit frequency resulting in seventeen switching transitions for every quarter period. For the proposed MSHE-PWM method, this allows control of sixteen harmonics and the fundamental. It is confirmed that the proposed MSHE-PWM offers significantly higher converter bandwidth in the standard range of the modulation indices. Moreover, when the bandwidth is reduced to be equal with the one offered with the MPS-PWM, the modulation index can be increased resulting in a higher gain and at a reduced switching frequency overall. Selected solutions for the switching transitions are presented and verified experimentally in order to confirm the effectiveness of the proposed technique.