A novel droop method for converter parallel operation

For the converter parallel operation, the current sharing between modules is important for the reliability of the system. Among several current sharing schemes, the droop method needs no interconnection between modules, which implies true redundancy. But the droop method has poor voltage regulation and poor current sharing characteristics. In this paper, a novel droop method is proposed for the converter parallel operation, which adaptively controls the reference voltage of each module. This greatly improves the output voltage regulation and the current sharing of the conventional droop method. The analysis of the proposed method and design procedure are provided and experimental results verify the excellent performance of the proposed method     

Improved single-phase line-interactive UPS

An improved single-phase line-interactive uninterruptible power supply (UPS) is proposed for low-power applications with low cost. The proposed UPS is comprised of two push-pull converters based on a low-voltage battery for reduced cost: one in series with the input and the other in parallel with the load. In the presence of input power, the UPS acts as an output voltage regulator and at the same time as an active filter while charging the battery. In case of loss of input power, the UPS supplies a regulated sinusoidal voltage to the load, drawing power from the battery. The series converter compensates only a small percentage of the input voltage carrying the input current and, therefore, a reduced rating is made. The parallel converter always supplies a nominal voltage and makes a seamless transition to backup mode. In the voltage determination of the parallel converter, the nominal voltage is derived using the feedback linearization concept and then a perturbed voltage is determined for the reactive power control or output voltage regulation. Experimental results obtained from a 1-kVA prototype are discussed     

一款高精度低电流的两级高电压电源调整电路

提出了一种改进的高输入电压调整电路结构,该电路结构在TSMC 0.25 μm BCD工艺平台进行验证.电路包括两个参考电压模块、两级调整电路和一个关断信号产生模块.介绍了初级电压调整和精确电压调整电路,可以产生稳定精确的输出电压,同时也提高了低输入电源电压时的输出电流能力.通过两级电源调整电路可以实现软启动功能,减小启动浪涌电压,提高启动性能.此外,关断模块产生可以可靠关闭高压模块和低压模块的两种控制信号,使得在待机模式下高压直流转换系统仅消耗极低的待机电流.该电路结构的输入电压可以在2.5～45 V宽幅范围内变化.在待机模式下,高压直流转换系统的待机电流最低仅300 nA,电源调整电路可以输出最高60 mA的负载电流.     

Robust Model-Following Control of Parallel UPS Single-Phase Inverters

This paper presents a robust control technique applied to modular uninterruptible power-supply (UPS) inverters operating in parallel. When compared to conventional proportional–integral (PI) control, the proposed technique improves the response of the output voltage to load steps and to high distorted output currents, reducing the distortion of the output voltage. Furthermore, an excellent distribution of currents between modules is achieved, resulting in fine power equalization between the inverters on stream. The crossover frequency of the different loop gains involved is moderate, so that robustness to variations of the operation point and to modeling uncertainties is achieved. A comparative study with a two-loop conventional PI control scheme is presented. Experimental results on a 1-kVA modular online UPS system confirm the viability of the proposed scheme.      

Stability and dynamic performance of current-sharing control forparalleled voltage regulator modules

The parallel operation of voltage regulator modules (VRMs) for high-end microprocessors requires a current-sharing (CS) circuit to provide a uniform load distribution among the modules. A good dynamic performance of the CS circuit is very important since the microprocessors present highly dynamic loads to the VRMs. Stability and dynamic performance of the CS control are considered. To assess these issues, a comprehensive small-signal model of the paralleled VRMs was developed and verified     

Master–Slave Current-Sharing Control of a Parallel DC–DC Converter System Over an RF Communication Interface

Using analog wireless communication, we demonstrate a master-slave load-sharing control of a parallel dc-dc buck converter system, thereby eliminating the need for physical connection to distribute the control signal among the converter modules. The current reference for the slave modules is provided by the master module using radio-frequency (RF) transmission, thereby ensuring even sharing of the load current. The effect of delay due to RF transmission on system stability and performance is analyzed, and regions of operation for a stable as well as satisfactory performance are determined. We experimentally demonstrate a satisfactory performance of the master-slave converter at 20-kHz switching frequency under steady state as well as transient conditions in the presence of a transmission delay. The proposed control concept, which can potentially attain redundancy that is achievable using a droop method, may lead to more robust and reconfigurable control implementation of distributed converters and power systems. It may also be used as a (fault-tolerant) backup for wire-based control of parallel/distributed converters.     

3C strategy for inverters in parallel operation achieving an equalcurrent distribution

A circular chain control (3C) strategy for inverters in parallel operation is presented in the paper. In the proposed inverter system, all the modules have the same circuit configuration, and each module includes an inner current loop and an outer voltage loop control. A proportional-integral controller is adopted as the inner current loop controller to expedite the dynamic response, while an H^∞ robust controller is adopted to reach the robustness of the multimodule inverter system and to reduce possible interactive effects among inverters. With the 3C strategy, the modules are in circular chain connection and each module has an inner current loop control to track the inductor current of its previous module, achieving an equal current distribution. Simulation results of two-module and a three-module inverter systems with different kinds of loads and with modular discrepancy have demonstrated the feasibility of the proposed control scheme. Hardware measurements are also presented to verify the theoretical discussion     

Parallel processing inverter system

A novel method of instantaneous voltage and power balance control of a parallel processing inverter system is proposed. It consists of a high-speed switching PWM (pulsewidth modulated) inverter with an instantaneous current minor loop controller, a voltage major loop controller, and a power balance controller. This system realizes the following functions with only one inverter: constant AC output voltage control with reactive power control, active filtering to absorb load current harmonics, DC voltage and current control as AC-to-DC converter, and uninterruptible power supply (UPS) for stand-alone operation. This system covers a wide application range, including UPS systems, new energy systems, and active filters with voltage control functions     

An AC VRM topology for high frequency AC power distribution systems

In this paper, a series resonant converter with pulse-width modulation (PWM) control is presented as an ac voltage regulator module (VRM) for high frequency ac power distribution systems. The proposed topology has close-to-unity rated power factor, low total harmonic distortion in input current, zero voltage switching under all load conditions, low voltage stress of the active switch and high overall efficiency. Simulation and experimental results are presented to prove the performance of the proposed ac VRM converter.     

Model Predictive Control of Parallel-Connected Inverters for Uninterruptible Power Supplies

Uninterruptible power supplies (UPSs) have been used in many installations for critical loads that cannot afford power failure or surge during operation. It is often difficult to upgrade the UPS system as the load grows over time. Due to lower cost and maintenance, as well as ease of increasing system capacity, the parallel operation of modularized small-power UPS has attracted much attention in recent years. In this paper, a new scheme for parallel operation of inverters is introduced. A multiple-input–multiple-output state-space model is developed to describe the parallel-connected inverters system, and a model-predictive-control scheme suitable for paralleled inverters control is proposed. In this algorithm, the control objectives of voltage tracking and current sharing are formulated using a weighted cost function. The effectiveness and the hot-swap capability of the proposed parallel-connected inverters system have been verified with experimental results.      

Inrush current estimation and hot-swapping for safe parallel battery pack

ABSTRACT

Batteries used in electric vehicles and microgrid applications use battery modules connected in series to satisfy the voltage required for each system, and battery modules are connected in parallel to increase capacity. In this parallel connected system, in order to disconnect and reconnect a specific battery module or to reconnect a new battery module, the battery module to be newly connected should have a small state of charge (SOC) difference from the existing battery modules. In particular, when a new battery is to be connected under a load current, there is a problem that excessive inrush current may occur in a specific battery module due to the load current distributed to each module and the current due to the SOC difference. Therefore, in this paper, we propose a method of estimating the inrush current through an equivalent electrical modelling analysis for the case where a battery module is newly added in a system in which the battery modules are connected in parallel. In addition, the power management algorithm for the battery pack system with inrush current estimation is presented. The proposed method is validated through simulations and experiments of a battery pack system in which 10 battery modules of 710V and 120Ah are connected in parallel.     

Practical Control Implementation of a Three- to Single-Phase Online UPS

A high-performance three- to single-phase online uninterruptible power supply (UPS) is proposed. The proposed UPS is composed of a rectifier, a battery charger/discharger, and an inverter. The rectifier has the capability of power-factor correction and regulates a dc-link voltage. When the rectifier becomes unavailable or when the current required by the load exceeds the output rating of the rectifier, the charger/discharger supplies the power demanded by the load to a dc-link capacitor. The inverter provides a regulated sinusoidal output voltage and limits an output current under an impulsive load. New control algorithms of the rectifier, the charger/discharger, and the inverter are proposed. The proposed algorithms of the rectifier and the charger/discharger improve dynamic performance at step load change. To improve the transient response of the output voltage at outage of an input source, a mode change method of the charger/discharger is also proposed. Additionally, the proposed current-limit algorithm of the inverter can be implemented without additional hardware, and it increases the reliability of the UPS.      

Active input-voltage and load-current sharing in input-series and output-parallel connected modular DC-DC converters using dynamic input-voltage reference scheme

This paper explores a new configuration for modular DC/DC converters, namely, series connection at the input, and parallel connection at the output, such that the converters share the input voltage and load current equally. This is an important step toward realizing a truly modular power system architecture, where low-power, low-voltage, building block modules can be connected in any series/parallel combination at input or at output, to realize any given system specifications. A three-loop control scheme, consisting of a common output voltage loop, individual inner current loops, and individual input voltage loops, is proposed to achieve input voltage and load current sharing. The output voltage loop provides the basic reference for inner current loops, which is modified by the respective input voltage loops. The average of converter input voltages, which is dynamically varying, is chosen as the reference for input voltage loops. This choice of reference eliminates interaction among different control loops. The input-series and output-parallel (ISOP) configuration is analyzed using the incremental negative resistance model of DC/DC converters. Based on the analysis, design methods for input voltage controller are developed. Analysis and proposed design methods are verified through simulation, and experimentally, on an ISOP system consisting of two forward converters.     

基于参数自整定模糊PID在线式UPS DC母线稳压控制

针对在线式UPS的动态稳定性能的改善和整流模块的数字化控制的问题,以TI公司TMS320F2808为控制器,提出了模糊参数自整定PID方法,并在UPS中实现了DC母线稳压控制。文中主要利用F2808资源实现模糊PID稳压算法并产生SCR触发脉冲,并通过实验观察到,突加满载时,电压跌幅小,满载突卸时,母线电压振荡小。最后,样机动态测试验证了模糊PID应用于在线式UPS DC母线稳压控制的有效性和可行性。     

IGBT芯片参数对瞬态均流特性的影响

IGBT模块一般采用多芯片并联的方式进行封装,但由于模块参数、驱动控制等差异,模块内部存在电流分布不均衡的问题。相比于稳态电流分布,瞬态电流分布的影响因素众多,是研究的热点。针对IGBT的芯片参数开展了模块内部各支路瞬态电流分布特性的研究。通过建立IGBT芯片模型及芯片并联的瞬态电路分析模型,计算单一芯片参数与多种芯片参数作用下IGBT模块的瞬态电流分布,提出新的适用于芯片支路瞬态均流分析的评价指标,得到了IGBT芯片参数对并联瞬态均流影响的规律。研究结果表明阈值电压、跨导和栅极电阻是对瞬态均流影响最大的3个芯片参数,且需要关注阈值电压与跨导的共同影响。研究结果对IGBT的芯片筛选及并联后的瞬态均流计算具有指导意义。     

一种新型UPS无互联线并联下垂特性控制

传统的UPS无互联线并联下垂特性控制是采用频率下垂和幅度下垂来调节输出电压。这种控制方法虽然是有效的，但是它容易导致系统正反馈。文中提出了一种基于神经网络解耦控制的下垂特性控制方案。这种新的控制方法能有效地克服传统下垂特性控制的局限性。由仿真结果得，该方案下逆变电源均分负载电流的效果好，而且系统动态响应性能高。     

High performance series-parallel type on-line UPS

An improved single-phase uninterruptible power supply (UPS) is proposed that is composed of a series converter and a parallel converter. The series converter regulates the output voltage, and then the parallel converter provides reactive current compensation and battery charging. A fast detection technique of the line voltage is proposed, which has almost zero transition time from the line power mode to the power failure mode. Furthermore, a current controller of the parallel converter for unity power factor is suggested and it is derived using the feedback linearization technique. A current limit technique in the power failure mode is proposed to protect the parallel converter without a system trip under any impulsive load. All control algorithms are implemented in software with a single-chip microcontroller. Experimental results obtained under a 3?kVA prototype show good transient and steady-state performance such as almost negligible transition time, 97% power efficiency and 99% power factor.     

Characterization and performance comparison of ripple-based control for voltage regulator modules

Three recently developed control methods for voltage regulator modules, namely, V/sup 2/ control, enhanced V/sup 2/ control, and enhanced V/sup 2/ control without output voltage dynamic feedback, are analyzed and compared in this paper. All three methods utilize the output voltage switching ripple for pulse-width modulation (PWM), hence, are collectively referred to as ripple-based control. A general modeling method based on the Krylov-Bogoliubov-Mitropolsky ripple estimation technique is applied to develop averaged models for single-channel as well as multichannel buck converters employing each of the control methods. Unlike existing models that are limited to small-signal operation, the proposed models are valid for large-signal operation and are capable of predicting subharmonic instability without including any sample-and-hold block as used in previous models. The paper also shows that adding parallel, high-quality ceramic capacitors at the output, which are ignored in previous models, can lead to pulse skipping and ripple instability, and a solution based on proper selection of the ceramic capacitors and/or ramp compensation at the PWM is presented. The models are further applied to analyze and compare the performance of the three control methods in terms of ripple stability, effective load current feedforward gain, and output impedance.     

Analysis and design of a multiple feedback loop control strategyfor single-phase voltage-source UPS inverters

This paper presents the analysis and design of a multiple feedback loop control scheme for single-phase voltage-source uninterruptible power supply (UPS) inverters with an L-C filter. The control scheme is based on sensing the current in the capacitor of the load filter and using it in an inner feedback loop. An outer voltage feedback loop is also incorporated to ensure that the load voltage is sinusoidal and well regulated. A general state-space averaged model of the UPS system is first derived and used to establish the steady-steady quiescent point. A linearized small signal dynamic model is then developed from the system general model using perturbation and small-signal approximation. The linearized system model is employed to examine the incremental dynamics of the power circuit and select appropriate feedback variables for stable operation of the closed-loop UPS system. Experimental verification of a laboratory model of the UPS system under the proposed closed-loop operation is provided for both linear and nonlinear loads. It is shown that the control scheme offers improved performance measures over existing schemes, It is simple to implement and capable of producing nearly perfect sinusoidal load voltage waveform at moderate switching frequency and reasonable size of filter parameters. Furthermore, the scheme has excellent dynamic response and high voltage utilization of the DC source     

Combination voltage-controlled and current-controlled PWM invertersfor UPS parallel operation

In this paper, a scheme of combination of voltage-controlled and current-controlled PWM inverters for parallel operation of a single-phase uninterruptible power supply (UPS) is proposed. The voltage-controlled PWM inverter (VCPI) unit as a master is developed to keep a constant sinusoidal wave output voltage. The current-controlled PWM inverter (CCPI) units are operated as slave controlled to track the distributive current. The power distribution center (PDC) performs the function of distributing the output current of each active unit. In this proposed scheme of parallel operation, each of the units can be designed as nearly independent, and the CCPI units do not need a PLL circuit for synchronization. As a result, the parallel operation of UPS is easy to implement and to expand system capacity. For the purpose of illustration, the system, including three single-phase units which operate in parallel, is analyzed and experimental results are given