Control of circulating current in two parallel three-phase boost rectifiers

One unique feature in parallel three-phase converters is a potential zero-sequence circulating current. To avoid the circulating current, most present technology uses an isolation approach, such as transformers or separate power supplies. This paper proposes a parallel system where individual converters connect both AC and DC sides directly without additional passive components to reduce size and cost of the overall parallel system. In this case, the control of the circulating current becomes an important objective in the converter design. This paper: (1) develops an averaged model of the parallel converters based on a phase-leg averaging technique; (2) a zero-sequence model is then developed to predict the dynamics of the zero-sequence current; (3) based on the zero-sequence model, this paper introduces a new control variable, which is associated with space-vector modulation; (4) a strong zero-sequence current control loop is designed to suppress the circulating current; and (5) simulation and experimental results validate the developed model and the proposed control scheme.     

Interleaved PWM with discontinuous space-vector modulation

This paper describes the effect of interleaved discontinuous space-vector modulation (SVM) in paralleled three-phase systems using three-phase pulsewidth modulation (PWM) rectifiers as an example. At the discontinuous point of the SVM, the phase shift between the switching signals of the paralleled modules generates a zero-sequence excitation to the system. Because the conventional control in a balanced three-phase system with only dq channels cannot reject this disturbance, a beat-frequency circulating current will develop on the zero axis. Based on this observation, a SVM without using zero vectors is used to eliminate the cause of pure zero-sequence current for parallel operation. Using this SVM, the circulating current is observable in dq channels. It can be suppressed dynamically by strong current loops of power-factor-correction (PFC) circuits. The concept is verified experimentally on a breadboard system     

Harmonic and reactive power compensation based on the generalizedinstantaneous reactive power theory for three-phase four-wire systems

This paper presents harmonic and reactive power compensation based on a generalized theory of instantaneous reactive power for three-phase power systems. This new theory gives a generalized definition of instantaneous reactive power, which is valid for sinusoidal or nonsinusoidal and balanced or unbalanced three-phase power systems with or without zero-sequence currents and/or voltages. The properties and physical meanings of the newly defined instantaneous reactive power are discussed in detail. A harmonic and reactive power compensator based on the new theory for a three-phase harmonic-distorted power system with zero-sequence components in the load current and/or source voltage is then used as an example to show harmonic and reactive power measurement and compensation using the new theory. Simulation and experimental results are presented     

Modeling and Coordinate Control of Circulating Currents in Parallel Three-Phase Boost Rectifiers

In this paper, definition of the circulating currents of multiphase paralleled converters is first presented, and the circulating-current-generating mechanism is clearly explained. Thus, based on this definition, an averaged model of the circulating current is proposed. It is seen from this model that the circulating current consists of not only the zero-sequence but also the nonzero-sequence components. The governing differential equation also shows explicitly the relation between the circulating currents and the affecting factors such as different pulse width-modulation strategies. With this understanding, a simple coordinate control is then presented to reduce the circulating current. The phenomenon of the intrinsic circulating current is also explained. Furthermore, a prototype system is constructed, and the proposed control is implemented using TMS320F2812 DSP. Both simulation and experimental results verify the validity of the proposed theory and control     

Analysis of power losses for instantaneous compensation of three-phase four-wire systems

This paper deals with the effect that the instantaneous compensation in three-phase four-wire systems, including or not the compensation of the neutral current, has on the supply line power losses. Thus, for three-phase circuits, the instantaneous compensation criterion has been established based on the instantaneous power theory. According to the instantaneous value concept the noninstantaneous power current is reduced, without altering the instantaneous active power. Two approaches are marked in this paper for instantaneous compensation: the first one is for eliminating the total noninstantaneous power current but the neutral current can still flow. The second one for eliminating the modified noninstantaneous power current, thus the neutral current component is compensated. It demonstrates that, in common situations of medium and low relative-values of the zero-sequence voltage, the total losses (line and neutral losses) obtained with the second approach are lower than those obtained with the first approach. The same results are obtained when a criterion based on the average value concept is used. Simulated and experimental results are obtained to confirm the theoretical properties and to show the compensator performance.     

Comparisons Between the p--q and p--q--r Theories in Three-Phase Four-Wire Systems

This paper presents a comparative analysis between results from applications of the p-q and the p-q-r theories in shunt active power filters for three-phase four-wire systems, discussing aspects related to the influence of the system voltage in the control methods that calculate the compensating currents. It is shown that in some cases, a preprocessing of the system voltage is required if the goal is to achieve sinusoidal compensated currents. On the other hand, when the goal is to compensate zero-sequence current, the need of energy storage elements in the active filter is discussed. In this case, if zero-sequence components are present simultaneously in the system voltage and load current, they produce zero-sequence power flow, and the control methods based on both theories must contain additional calculations to allow the elimination of energy storage elements in the active filter. A control strategy based on the p-q theory is proposed to eliminate the neutral current without the need of energy storage elements, with the advantage of avoiding the extra transformation from alphabeta0 to pqr coordinates that is needed in the p-q-r theory. Simulation results are presented for the purpose of comparing the performance of both control methods.     

Application of a three-level NPC inverter as a three-phase four-wire power quality compensator by generalized 3DSVM

A two-level four-leg inverter has been developed for the three-phase four-wire power quality compensators. When it is applied to medium and large capacity compensators, the voltage stress across each switch is so high that the corresponding dv/dt causes large electromagnetic interference. The multilevel voltage source inverter topologies are good substitutes, since they can reduce voltage stress and improves output harmonic contents. The existing three-level neutral point clamped (NPC) inverter in three-phase three-wire systems can be used in three-phase four-wire systems also, because the split dc capacitors provide a neutral connection. This paper presents a comparison study between the three-level four-leg NPC inverter and the three-level NPC inverter. A fast and generalized applicable three-dimensional space vector modulation (3DSVM) is proposed for controlling a three-level NPC inverter in a three-phase four-wire system. The zero-sequence component of each vector is considered in order to implement the neutral current compensation. Both simulation and experimental results are given to show the effectiveness of the proposed 3DSVM control strategy. Comparisons between the 3DSVM and the 3-D hysteresis control strategy are also achieved.     

Reference current estimation under distorted line voltage for control of shunt active power filters

Shunt active power filters (APF) are used in power systems for the compensation of harmonic currents generated for non linear loads. A new digital reference current estimation method for control of APF using a Kalman digital algorithm is presented. Its capability of prediction avoids the effects of computational lags derived from the digital signal processing. The characteristics of the proposed technique are: the harmonic current compensation in a global or a selective way, the fast dynamical response and its independence from disturbances in the line voltage waveform. Simulation and experimental results under distorted supply voltages demonstrate the usefulness of the presented technique to improve the filtering performance.     

基于DSP的D-STATCOM硬件实现及控制算法

针对电网无功功率实现动态无功补偿的问题,采用模块化的设计方法,设计出一种基于DSP的D-STATCOM电路。根据瞬时无功功率理论计算参考电流和补偿电流,并采用直接电流控制方法产生补偿电流。建立了整个补偿过程的数学模型,分析了D-STATCOM中电流控制算法、电压控制算法以及IGBT晶闸管通断时间控制算法。结果表明:D-STATCOM完成无功补偿,达到抑制谐波的目的。     

基于零序电流分布的自耦变压器电流保护分析

文中基于自耦变压器零序电流的分布特点,探讨了自耦变压器零序电流保护的构成及整定原则,重点分析了自耦变压器零序差动保护。     

Reference voltage calculation method based on zero-sequence component optimisation for a regional compensation DVR

This paper describes the design of a dynamic voltage restorer (DVR) that can simultaneously protect several sensitive loads from voltage sags in a region of an MV distribution network. A novel reference voltage calculation method based on zero-sequence voltage optimisation is proposed for this DVR to optimise cost-effectiveness in compensation of voltage sags with different characteristics in an ungrounded neutral system. Based on a detailed analysis of the characteristics of voltage sags caused by different types of faults and the effect of the wiring mode of the transformer on these characteristics, the optimisation target of the reference voltage calculation is presented with several constraints. The reference voltages under all types of voltage sags are calculated by optimising the zero-sequence component, which can reduce the degree of swell in the phase-to-ground voltage after compensation to the maximum extent and can improve the symmetry degree of the output voltages of the DVR, thereby effectively increasing the compensation ability. The validity and effectiveness of the proposed method are verified by simulation and experimental results.     

A novel active power filter based on the least compensation currentcontrol method

A new least compensation current technique to detect the harmonic and reactive current for active power filters is presented in this paper. With the technique, a novel active power filter based on the least compensated current control is proposed, in which the measurement of harmonic and reactive current and the generation of compensated current are completed in the same closed-loop. Compared with the existing control methods of active power filter, the proposed method has simpler structure, quicker dynamic, higher reliability and better compensation performance. Simulating and experimental results are provided that verified theoretical predictions and demonstrate the effectiveness of the proposed control method and circuit configuration     

自适应零序电流速断保护中实时计算系统参数算法仿真与研究

针对传统零序电流速断保护[1]的灵敏度受系统运行方式变化和接地短路类型影响的局限性,提出了实时检测系统参数来实现自适应式零序电流速断保护。文中以计算系统等值零序内阻抗为例,并通过仿真研究证实了方法的有效性和可行性。     

Current control for induction motor drives using random PWM

Current control in voltage-source inverters with random pulsewidth modulation (RPWM) is investigated. The random modulation is introduced to alleviate the undesirable acoustic, vibration, and EMI effects in inverter-fed AC drive systems. A novel RPWM digital technique with dithering of the switching frequency and compensation of the processing time is described. Design of the current control loop is discussed. Results of investigation of an experimental drive system are presented, proving the feasibility of the proposed solutions     

Analysis of the eddy-current induced artifacts and the temporal compensation in nuclear magnetic resonance imaging

Reduction of eddy currents by a temporal compensation of the input current waveform to the gradient coil is studied with an analytic solution. The technique is the inverse filtering of the eddy-current affected field response, which is calculated from the diffusion equation. The limitation of the temporal compensation due to the spatially variant eddy currents is also investigated for whole-body diagnostic imaging systems and small-bore nuclear magnetic resonance (NMR) microscopy systems. Within a limited imaging volume of less than 60% of the gradient coil diameter, most of eddy-current problems can be solved by the technique.     

A novel principle of magnetic flux compensation and its application in power systems

A novel principle of magnetic flux compensation of a linear transformer is proposed. The current of the primary winding is detected and multiplied by a compensation coefficient to get a reference current, which is tracked by a voltage source inverter to produce a compensation current. This compensation current is injected inversely in phase into the secondary winding. According to the law of superposition and the transformer's voltage equations, the main magnetic flux in the transformer and the equivalent impedance of the primary winding correlate linearly with the current compensation coefficient, and this equivalent impedance varies linearly with the current compensation coefficient. This new principle can be widely applied in power systems, such as series hybrid active power filter, fault current limiter, continuously controllable reactor, arc suppression coil and power system load flow control, and many FACTS devices can be implemented in terms of the new principle. A current control scheme is analysed. The theory is validated by experimental results, and some new devices based on the principle are discussed.     

Prechirp technique as a linear dispersion compensation forultrahigh-speed long-span intensity modulation directed detectionoptical communication systems

Prechirp technique, as a linear dispersion compensation for intensity modulation direct detection (IM-DD) optical transmission systems, has been investigated theoretically and experimentally. This technique is based on a predistortion technique in an optical transmitter. Implementation to the ordinary IM-DD optical transmitter, which uses an external intensity modulator, is easily realized merely by adding a small injection current modulation to a semiconductor laser diode, allowable optical transmission fiber dispersion will be more than doubled with this technique, modified prechirp technique, which utilizes a time division superimposing prechirped bit streams, has also been investigated to achieve greater dispersion compensation capability     

三相四线制系统新型中线滤波装置研究

三相四线制电路中由于非线性负载的大量使用,使得中线上累加了大量的零序谐波电流,给电网的安全运行带来隐患。为此本文提出了一种新的中线零序谐波治理方案,即采用Zigzag变压器与有源电力滤波器（Active Power Filter,APF）相结合的方法。Zigzag变压器因其接线简单、性价比高而在中线谐波治理方面倍受青睐。两者的结合使得目标装置具有较高的性价比和广阔的市场应用前景。经仿真验证,此方案滤波效果在三相负载对称和不对称时均较好,是解决目前日益严重的中线零序谐波污染问题的一种切实可行的方案。     

Common-Mode Ripple Current Estimator for Parallel Three-Phase Inverters

For the three-phase parallel voltage source inverter systems with common dc link, several control methods were developed to suppress the common-mode circulating current. The common-mode ripple current is the main disturbance source of the common-mode circulating current control system that forms part of the parallel discontinuous pulsewidth modulation (PWM) inverter system. In this paper, a real-time analysis method for the common-mode ripple voltage of the three-phase discontinuous PWM inverter is proposed, whereby the amplitude of the common-mode ripple current can be estimated correctly, the hysteresis width of the circulating current controller can be adjusted to be as small as possible, and consequently, the rms circulating current is minimized as well. The simulation and experimental results are presented to confirm the theoretical analysis method of the common-mode ripple current and the performance of the proposed common-mode ripple current estimator.      

基于传统功率理论谐波抑制和无功补偿方法

基于传统功率理论，提出了一种谐波抑制和无功补偿方法，根据有功功率可求等效线性负载，进一步求得该时刻补偿电流值。应用于有源电力滤波器控制中，可有效地抑制谐波电流和补偿无功功率，减小了电流信号检测中偶然误差对补偿效果的影响。这种方法可以应用在单相、三相电力系统的滤波电路中。仿真证明了这种方法的有效性。