A novel communication strategy for decentralized control of paralleled multi-inverter systems

A new communication strategy for decentralized control of paralleled multi-inverter systems is introduced in this paper. The proposed strategy utilizes the common mode signal's circuit in the paralleled system as a channel of communication between individual inverters. Source voltage synchronization for paralleled inverters is presented to demonstrate its effectiveness. One inverter module acts as a source and a synchronized signal is sent by means of frequency modulation in this channel to all other modules via the common mode current. Each module will then receive and transform the signal to be used as a synchronized voltage command for individual inverters. The approach enables synchronization in the paralleled multi-inverter system and the method is not affected by external factors such as load change, output voltage variation, and different types of loads. The principle of operation, performance of the proposed strategy, as well as the experimental evaluation with three single-phase paralleled inverters are discussed and presented in this paper.     

A DC Voltage Monitoring and Control Method for Three-Phase Grid-Connected Wind Turbine Inverters

Space vector pulsewidth modulation (SVPWM) based three-phase voltage source inverters provide a widely used interface between electric grids and wind turbine systems. PI controllers, predictive algorithms and real-time sampling techniques are often used to overcome the shortcomings of SVPWM. These techniques depend highly on accurate measurements of inverter voltages and currents, thus making sensors the key elements in the control process. Among these sensors, the dc link voltage (Vdc) sensor is critical: if it sends out a signal with a significant error, the output current will be distorted. The Authors have developed a combination of PI and predictive methods, using them simultaneously to control a three-phase grid-connected inverter. Under this new control scheme, the PI controller is given a new task of monitoring and controlling Vdc. As a result, the output current of the inverter is of high quality, and more importantly, Vdc can be double checked for its correctness of measurements. When the Vdc sensor fails or its signals are corrupted, the Vdc PI controller will become a Vdc controller, adding an extra protective function for the reliable operation of wind turbine inverters.     

一种用于提高无线并联逆变器均流性能的控制方法

本文基于传统的无互线逆变器并联下垂法,提出一种用于提高逆变器均流性能的控制方法。同时本文考虑了连线阻抗和逆变器输出阻抗,分析了逆变器并联系统的有功环流和无功环流,本文提出的控制方法可以减小下垂法的频率和幅值的波动幅度,增加系统输出的稳定性,并提高系统的动态和静态环流抑制能力。仿真和实验结果均验证了该方案的良好性能。     

Modeling, analysis, and implementation of parallel multi-inverter systems with instantaneous average-current-sharing scheme

Parallel multi-inverter systems can be designed to have the advantages of expandable output power, improved reliability, and easy N+X redundancy operation. However, a current-sharing control scheme has to be employed to enable the inverters to share the load current equally. A multi-inverter system with instantaneous average-current-sharing scheme is presented in this paper. By introducing a disturbance source to represent all the sources that may cause current unbalances, a model of the system can be built. Some key issues are discussed based on the model, including stability of the current-sharing controller, impedance characteristics and voltage regulation. Three experimental 110 VAC/1.1 kVA inverters are built and paralleled to verify the theoretical predictions.     

信道噪声对无线远程迭代学习控制系统的影响

无线远程控制系统的主要特征是信号通过无线网络在传感器与控制器,控制器与执行器之间传输,但信号无线传输过程中受到的信道噪声干扰会严重影响系统的控制性能。针对一类控制器采用迭代学习控制方式的无线远程控制系统,首先得到系统输出误差和信道噪声在迭代域内的关系表达式,然后在此基础上就信道噪声对输出误差信号 范数平方期望收敛性能的影响进行分析。由分析可知,该期望是关于信道噪声方差和系统参数的函数,并将随迭代次数的增加收敛于一个有限的误差值。在系统参数一定的情况下,该误差值将随信道噪声方差的增大而增大,即信道噪声方差越大,其对系统跟踪性能的影响越明显。最后,通过仿真实验证明了分析结果的正确性。      

一种高性能的三相逆变电源波形控制策略

恒频恒压(CVCF)逆变电源的输出电压波形质量是衡量其性能的重要指标之一。文章采用一种重复控制和双闭环控制相结合的电压波形控制策略,以重复控制器提高逆变电源输出电压的稳态精度,减小总谐波畸变率;以双闭环控制器达到较快的动态响应速度。该方案在一台采用TMS320F240DSP控制芯片的50Hz三相PWM逆变器上得到验证。     

Zero tracking error controller for three-phase CVCF PWM inverter

A zero tracking error control scheme for three-phase CVCF PWM inverters is proposed. The proposed scheme uses repetitive controller (RC) to force output line voltages to track a sinusoidal reference signal with zero error. Minimised voltage distortion and a fast response are obtained. The validity of the proposed scheme has been verified by simulations     

An autonomous controller for improved dynamic performance of parallel-connected,single-phase inverters

A new control technique is presented for the parallel connection of distributed generation inverters. The proposed control technique is based on a modification of the power angle droop control method, and uses only locally measured feedback signals. An improvement in transient response is achieved because the real and imaginary components of the output current are used when deriving the power angle droop controller. The method achieves good active and reactive power sharing and minimises circulating current between parallel connected units. Improved transient response is obtained whilst maintaining power sharing precision or output voltage and frequency accuracy. Simulation and experimental results validate that performance is better than that attained with conventional droop-based approaches.     

逆变器无差拍性能控制方案的分析与设计

分析了两种单电压环的无差拍控制方案。通过分析,比较了两者的异同,指出单纯保证输出电压无差拍控制方案的劣势。并详细分析了能够同时实现两个状态无差拍控制的单电压环控制方案。该方案利用输出电压反馈进行极点配置,实现系统对给定无差拍跟踪;利用负载电流进行前馈控制,实现系统对扰动的抑制。并针对控制策略进行了仿真,验证了方案的可行性。     

Digital repetitive learning controller for three-phase CVCF PWMinverter

In this paper, a plug-in digital repetitive leaning control scheme is proposed for three-phase constant-voltage constant-frequency (CVCF) pulsewidth modulation inverters to achieve high-quality sinusoidal output voltages. In the proposed control scheme, the repetitive controller (RC) is plugged into the stable one-sampling-ahead-preview-controlled three-phase CVCF inverter system using only two voltage sensors. The RC is designed to eliminate periodic disturbance and/or track periodic reference signal with zero tracking error, The design theory of plug-in repetitive learning controller is described systematically and the stability analysis or overall system is discussed. The merits of the controlled systems include features of minimized total harmonic distortion, robustness to parameter uncertainties, fast response, and fewer sensors. Simulation and experimental results are provided to illustrate the effectiveness of the proposed scheme     

基于Buck-Boost逆变器的离散滑模控制仿真研究

为了获得比较理想的正弦输出电压，优化逆变器的动态性能，文中基于Buck—Boost逆变器．采用了离散滑模变结构的控制策略。Buck—Boost逆变器可以获得比直流输入电源高或低的交流输出电压，文中阐述了其工作原理．并结合状态方程，推导出滑模面的存在条件、到达条件和稳定条件，然后对电路参数、控制系数以及控制算法进行了设计。仿真结果表明采用离散滑模控制的Buck—Boost逆变器对系统扰动和负载变化具有很强的鲁棒性，系统具有良好的动态响应。     

Control of distributed generation systems-Part I: Voltages and currents control

This paper discusses a digital control strategy for three-phase pulse-width modulation voltage inverters used in a single stand-alone ac distributed generation system. The proposed control strategy utilizes the perfect robust servomechanism problem control theory to allow elimination of specified unwanted voltage harmonics from the output voltages under severe nonlinear load and to achieve fast recovery performance on load transient. This technique is combined with a discrete sliding mode current controller that provides fast current limiting capability necessary under overload or short circuit conditions. The proposed control strategy has been implemented on a digital signal processor system and experimentally tested on an 80-kVA prototype unit. The results showed the effectiveness of the proposed control algorithm.     

Decoupling Control Strategy for Single Phase SPWM Parallel Inverters

A deconpling control strategy of inverter parallel system is proposed based on the equivalent output impedance of single phase voltage source SPWM （sinusoidal pulse width modulation） inverter. The active power and reactive power are calculated in terms of output voltage and current of the inverter, and sent to the other inverters in the parallel system via controller area network （CAN） bus. By calculating and decoupling the circumfluence of the active power and reactive power, the inverters can share load current via the regulation of the reference-signal phase and amplitude. Experimental results of an 110V/2kVA inverter parallel system show the feasibility of the decoupling control strategy.     

Output Impedance Design of Parallel-Connected UPS Inverters With Wireless Load-Sharing Control

This paper deals with the design of the output impedance of uninterruptible power system (UPS) inverters with parallel-connection capability. In order to avoid the need for any communication among modules, the power-sharing control loops are based on the$P/Q$droop method. Since in these systems the power-sharing accuracy is highly sensitive to the inverters output impedance, novel control loops to achieve both stable output impedance and proper power balance are proposed. In this sense, a novel wireless controller is designed by using three nested loops: 1) the inner loop is performed by using feedback linearization control techniques, providing a good quality output voltage waveform; 2) the intermediate loop enforces the output impedance of the inverter, achieving good harmonic power sharing while maintaining low output voltage total harmonic distortion; and 3) the outer loop calculates the output active and reactive powers and adjusts the output impedance value and the output voltage frequency during the load transients, obtaining excellent power sharing without deviations in either the frequency or the amplitude of the output voltage. Simulation and experimental results are reported from a parallel-connected UPS system sharing linear and nonlinear loads.     

Neutral-point-clamped inverter with improved voltage waveform andcontrol range

A hysteresis controller-based PWM method for neutral point clamped (NPC) inverters is proposed. The closed-loop system uses the two-axis components of the transformed line-to-line voltage vectors as the control variables. In its linear control range, the PWM method includes two operating modes, namely, low and high references. At high references' mode, the output voltage is generated by switching among four vectors instead of three to improve the waveform generation. The proposed system has a linear control range 15.5% wider than conventional phase control methods. Furthermore, the control range can be easily expanded beyond the higher linear controllable range up to the square waveforms without suddenly increasing the waveform distortion. Therefore, the fundamental component of the output voltage can be smoothly and continuously controlled from zero up to square waveforms. The control circuit includes a hysteresis controller that is capable of stabilizing the neutral point potential variations within fixed limits during steady and transient states. The control method also offers high performance and fast response. Computer simulations and experimental results on a laboratory-tested motor drive are given to validate the proposed control scheme     

A voltage control strategy for current-regulated PWM inverters

Alternative voltage control strategies for current-regulated PWM inverters are analyzed, including previously established feedforward and feedforward/feedback controllers and a newly proposed decoupling feedback control strategy. The steady-state and dynamic characteristics of each of these control methods are illustrated and compared for a selected inverter design. It is shown that the feedforward controller exhibits steady-state error and an undesirable overshoot of the output voltages during startup. The addition of a feedback loop eliminates the steady-state error and reduces the overshoot; however, the natural response is underdamped regardless of the choice of feedback gains. A decoupling feedback control strategy that eliminates the disadvantages of the feedforward and feedforward/feedback controllers is described. Using the decoupling feedback controller, it is possible to eliminate the steady-state error and place the closed-loop poles wherever desired. Moreover, if the closed-loop poles are selected appropriately, it is possible to eliminate the overshoot of the output voltages during startup transients     

Neural-network-based maximum-power-point tracking of coupled-inductor interleaved-boost-converter-supplied PV system using fuzzy controller

The photovoltaic (PV) generator exhibits a nonlinear V-I characteristic and its maximum power (MP) point varies with solar insolation. In this paper, a feedforward MP-point tracking scheme is developed for the coupled-inductor interleaved-boost-converter-fed PV system using a fuzzy controller. The proposed converter has lower switch current stress and improved efficiency over the noncoupled converter system. For a given solar insolation, the tracking algorithm changes the duty ratio of the converter such that the solar cell array voltage equals the voltage corresponding to the MP point. This is done by the feedforward loop, which generates an error signal by comparing the instantaneous array voltage and reference voltage corresponding to the MP point. Depending on the error and change of error signals, the fuzzy controller generates a control signal for the pulsewidth-modulation generator which in turn adjusts the duty ratio of the converter. The reference voltage corresponding to the MP point for the feedforward loop is obtained by an offline trained neural network. Experimental data are used for offline training of the neural network, which employs a backpropagation algorithm. The proposed peak power tracking effectiveness is demonstrated through simulation and experimental results. Tracking performance of the proposed controller is also compared with the conventional proportional-plus-integral-controller-based system. These studies reveal that the fuzzy controller results in better tracking performance.     

Novel space vector based current controllers for PWM-inverters

Two novel, simple control strategies for current-controlled pulse-width modulated (PWM) transistor inverters are presented. Both methods are based on the three-level hysteresis comparators which select appropriate inverter output voltage vectors via a switching electrically programmable read-only memory (EPROM) table. The first controller works with current components represented in a stationary coordinate system (AC components) and the second with components represented in a rotated (field-oriented) coordinate system (DC components). The theoretical principles of these methods are discussed. The results of a comparative study, which illustrates the performance of the proposed controller in comparison to the most popular scheme (based on three independent two-level hysteresis comparators), are presented     

Operation of a Medium-Voltage Drive Under Faulty Conditions

This paper presents a method for operating cascaded multilevel inverters when one or more power cells are damaged. The method is based on the use of additional switches in the power circuit to bypass the faulty cell. To control the cells, the angle of phase shifting in the carrier signals is modified according to the number of operating cells, to minimize the load voltage distortion, when the inverter operates in failure mode. The reference signals of the pulsewidth-modulation modulators are also modified to increase the output voltage. Simulation and experimental results show the effectiveness of this method, which significantly increases the reliability of the drive.