A Very Simple Control Strategy for Power Factor Correctors Driving High-Brightness LEDs

This paper presents a new control strategy for power factor correctors (PFCs) that are used to drive high-brightness LEDs. This control strategy is extremely simple and is based on the use of a conventional peak-current-mode controller with a suitable selection of the compensation ramp waveform. Neither an analog multiplier nor an input voltage sensor is needed to achieve quasi-sinusoidal line waveforms at nominal conditions and full load. If the converter belongs to the flyback family (flyback, buck–boost, SEPIC, Cuk and Zeta), the line waveform appears notably distorted if the compensation function is a linear ramp, but becomes almost sinusoidal if the linear ramp is substituted by a properly chosen exponential function. The line waveform is slightly distorted when the load varies or when the converter works under either overvoltage or undervoltage conditions. However, the waveform maintains a very high power factor (PF) even under these conditions. Moreover, the line current is cycle-by-cycle-controlled due to the peak-current-mode control, and hence, the input-current feedback loop is extremely fast, thereby allowing this type of control to be used with high-frequency lines (above 400 Hz).      

A digital power factor correction (PFC) control strategy optimized for DSP

A predictive algorithm for digital control power factor correction (PFC) is presented in this paper. Based on this algorithm, all of the duty cycles required to achieve unity power factor in one half line period are calculated in advance by digital signal processors (DSP). A boost converter controlled by these precalculated duty cycles can achieve sinusoidal current waveform. One main advantage is that the digital control PFC implementation based on this control strategy can operate at a high switching frequency which is not directly dependent on the processing speed of DSP. Input voltage feed-forward compensation makes the output voltage insensitive to the input voltage variation and guarantees sinusoidal input current even if the input voltage is distorted. A prototype of boost PFC controlled by a DSP evaluation board was set up to implement the proposed predictive control strategy. Both the simulation and experimental results show that the proposed predictive strategy for PFC achieves near unity power factor.     

Instantaneous Reactive Power Theory Applied to Active Power Filter Compensation: Different Approaches, Assessment, and Experimental Results

In this paper, the five main formulations of the instantaneous reactive power theory have been chosen to study nonlinear load compensation. They are p-q original theory, d-q transformation, modified or cross-product formulation, p-q-r reference frame, and vectorial theory. The obtention of the compensation current according to each formulation has been established. Next, the behavior of an active power filter (APF) that is implemented with those different control algorithms has been studied. On one hand, a simulation platform with control, APF, and load has been built to test them. Results obtained in an unbalanced and nonsinusoidal three-phase four-wire system have been compared by means of the most adequate indexes. On the other hand, the APF control strategies have been implemented in an experimental platform constituted by a 20-kVA power inverter and a 400-MHz digital signal processing controller board. The final analysis shows that, in general, the five theories present a different behavior, which depends on supply voltage, with respect to distortion. However, all of them widely decrease the waveform distortion. Moreover, a more general compensation objective is possible. It obtains balanced and sinusoidal source current in any conditions of the supply voltage.     

Improved Direct Power Control of Grid-Connected DC/AC Converters

This paper proposes new direct power control (DPC) strategies for three-phase dc/ac converters with improved dynamic response and steady-state performance. As with an electrical machine, source and converter flux, which equal the integration of the respective source and converter voltage, are used to define active and reactive power flow. Optimization of the lookup table used in conventional DPC is outlined first so as to improve power control and reduce current distortion. Then, constant switching frequency DPC is developed where the required converter voltage vector within a fixed half switching period is calculated directly from the active and reactive power errors. Detailed angle compensation due to the finite sampling frequency and the use of an integral controller to further improve the power control accuracy are described. Both simulation and experimental results are used to compare conventional DPC and vector control, and to demonstrate the effectiveness and robustness of the proposed control strategies during active and reactive power steps, and line inductance variations.      

Analytical evaluation of harmonic distortion in PWM AC drives using the notion of stator flux ripple

This paper presents a method to evaluate harmonic distortion due to space vector-based pulse-width modulation (PWM) strategies for ac drives. The proposed method is general enough to deal with division of zero vector time as well as division of active vector time within a subcycle. The method is based on the notion of stator flux ripple, which is a measure of line current ripple. Expressions for RMS ripple over a subcycle are derived for six switching sequences in terms of magnitude and angle of the reference vector, and subcycle duration. The sequences considered include those involving division of active vector time within a subcycle. Further, analytical closed form expressions are derived for the total RMS harmonic distortion factor corresponding to six space vector-based synchronized PWM strategies, proposed recently, for high power drives. The square of the distortion factor turns out to be a quadratic polynomial in modulation index (M), and the coefficients differ with PWM strategies and pulse numbers. These expressions are validated through Fourier analysis as well as experimental measurements. The concept of stator flux ripple provides insight into current ripple as well as torque ripple corresponding to different sequences and strategies.     

AC line voltage harmonics compensator with excessive current control

We investigate an instantaneous common terminal voltage-controlled harmonics compensator constructed by a shunt active filter with an appropriate series inductance including the line impedance. This compensator can reduce or compensate both the ac line voltage distortion derived from the downstream utility source voltage harmonics and the upstream current harmonics by nonlinear loads at the same time. The control system can be easily constructed without directly detecting the common terminal voltage to be compensated. Therefore, the main circuit configuration and the control system are simple. The harmonics compensation level and the compensation current can be easily adjusted by changing the feedback gain for the sensing inductance voltage drop. In this paper, we describe the basic principle of the control method, the modified control method, the circuit construction by the pulsewidth-modulation-controlled shunt active filter and the control system of the compensator. Then, we show some operating waveforms for the cases of the downstream voltage distortion and the upstream harmonics current from the nonlinear loads by simulation analysis and experiments to verify the feasibility.     

Shunt active power filter synthesizing resistive loads

The paper discusses the use of a shunt active power filter to compensate for the line current distortion and to improve the power factor. The advantages of the resistive load synthesis over the sinusoidal current synthesis when the filter is used in a system where the voltage is not perfectly sinusoidal are presented. The control circuit is based on analogic multipliers, and the currents follow the same waveforms of the respective line voltages. Experimental results of connecting a three-phase active power filter to a nonsinusoidal grid are presented     

Single-phase high power factor pre-regulator with three-level modulation scheme

A control scheme for the single-phase three-level pulse-width modulation active rectifier is proposed. A hysteresis current control scheme is used to draw the sinusoidal line current in phase with the mains voltage. The line current command is derived from a voltage controller and a phase-locked loop circuit. The blocking voltage of each power device is clamped to half of the DC-link voltage in the proposed active rectifier. In order to generate the three-level voltage pattern on the DC side of the active rectifier, the region detector of the line voltage, capacitor voltage compensator and hysteresis current comparator are employed in the adopted control algorithm to achieve high input power factor and low current distortion. To investigate the proposed control algorithm, the adopted rectifier is simulated and experimental tests from a laboratory prototype undertaken.     

基于瞬时无功理论的不对称负荷SVC装置研究

针对电力系统日益增多的三相不对称负荷所造成的电网功率因数低、电压波形畸变等问题,本文以平衡化原理与瞬时无功理论为理论支撑,提出一种静止无功补偿装置(Static Var Compensator)补偿导纳新算法。该算法通过对负荷电流有功分量和无功分量的解耦,利用提取出的无功分量计算理想补偿导纳,相比传统算法检测更为简便,且不受谐波分量影响。在Matlab/Simulink环境下建立TCR+FC型SVC模型,通过对三相不对称负荷的仿真,证明了所提算法的正确与可行,也验证了SVC装置对平衡三相不对称网络的有效性。     

Operation of the LCC-type parallel resonant converter as a lowharmonic rectifier

A single-phase high-frequency transformer isolated single-stage AC-to-DC controlled rectifier with low line current harmonic distortion using a variable-frequency controlled LCC-type (or series-parallel) resonant power converter (SPRC) is presented. A simple analysis and design procedure is used for designing the converter for low line current harmonic distortion and high power factor operation. The converter performance characteristics have been verified with SPICE3 simulations (without active control) and experimental prototype SPRC (rated at 150 W, with and without active control) for variation in load as well as line voltage. When operated with active current shaping, this converter operates in zero-voltage-switching mode for the complete range, maintaining power factor close to unity with low line current distortion and low peak current compared to the parallel resonant converter     

Discrete Frequency Tuning Active Filter for Power System Harmonics

Severe voltage distortion, due to power system harmonic resonance, has been reported in recent years. This issue becomes more significant in high penetration of a photovoltaic (PV) network. A conventional voltage detection active filter operates as similar conductance for all harmonic frequencies to resolve this problem whether in a fixed conductance command or in an automatic gain adjustment control. However, its filtering capability is impeded by the mismatch between the active filter and the radial line, and the voltage distortion may still be significant. This paper proposes a discrete frequency tuning active filter to suppress power system harmonics. The active filter operates as variable conductance for each individual harmonic frequency. Each harmonic conductance is dynamically adjusted according to the corresponding harmonic voltage distortion of the active filter installation point in response to increase or decrease of nonlinear loads, or variation of resonant frequency in the power system. The mismatching problem between the feeder impedance and the active filter can be avoided effectively. Therefore, harmonic voltage distortion can be maintained at an allowable level throughout the feeder with lower peak current and lower rms current of the active filter, and loads installed at various locations of the power system receive more uniform voltage waveform.     

Effects of voltage-waveform distortion in TCR-type compensators

The effect of system voltage distortion on the performance of a thyristor controlled reactor (TCR) type of compensator is examined. Toward this end, the real and imaginary values of harmonic current through the TCR, and the optimal gating angle that controls it, are calculated with a view toward minimizing the RMS value of the supply current and to achieving an optimum power factor (PF). The precision of the formulae obtained, assuming an equal conduction angle of the thyristors in the two half periods, is compared with the results achieved using two different methods for simulating the TCR, one by means of a Pascal program and the other with PSPICE. The optimum PF values, calculated by analytical methods that use the conventional formulation (which assumes that the voltage waveform is sinusoidal), are compared with those obtained using the formulation developed here     

基于智能整流技术的电网电流谐波补偿方法研究

目前用于电网电流谐波补偿的电器设备,主要以PWM整流器为主。基于PWM整流器的电源产品只能被动地减小自身向电网输出的谐波电流,而对电网中业已存在的电流谐波污染束手无策。为了解决电网中电流谐波污染以及相关联的电压波形失真问题,采用基于SRM(智能整流模块)技术对电网电流谐波进行补偿。仿真结果表明,基于SRM的电力电子装置在从电网吸取电流并在向负载供电的同时,还能对电网电压的波形进行补偿,使电网电压波形接近正弦波形。     

A novel PWM scheme for single-phase three-levelpower-factor-correction circuit

This paper presents a control scheme for a single-phase AC-to-DC power converter with three-level pulsewidth modulation. A single-phase power-factor-correction circuit is proposed to improve the power quality. The hysteresis current control technique for a diode bridge, with two power switches is adopted to achieve a high power factor and low harmonic distortion. A control scheme is presented where the line current is driven to follow the reference sinusoidal current which is derived from the DC-link voltage regulator, the capacitor voltage balance compensator and the output power estimator. The blocking voltage of each power device is clamped to half of the DC-link voltage. The high power factor and low current total harmonic distortion are verified by computer simulations and hardware tests     

Control of power factor correcting boost converter withoutinstantaneous measurement of input current

This paper proposes a new control method for the constant-frequency control of power factor correcting boost power converter using a sinewave template modulated PWM signal which eliminates the need for instantaneous measurement of the line current for the switching control of the boost converter. The control strategy is based on the notion that the line current can be forced to trace a deterministic waveform such as a sinusoid by considering the implicit model of the sinewave in the boost converter controller structure. The modulating sinewave template is generated using the line voltage, the boost converter output voltage and the load current. The paper provides the analysis and the design of the controller and presents simulation and implementation results to demonstrate its effectiveness     

A simple input current wave-shaping technique for three-phase diode and SCR converters

Three-phase converters using diode or silicon-controlled rectifier (SCR) are widely employed to convert the commercial AC supply to DC. Such converters inject harmonics into the power supply system and thereby distort supply system voltage waveform. A simple input current wave-shape improvement technique using a shunt-connected harmonic current compensator is presented in this work, intended to reduce the total harmonic distortion (THD) of input current of three-phase diode and SCR phase-controlled rectifiers operating with inductive loads, by matching them to the specific converter as a combined package. The compensator proposed here comprises of a three-limb voltage source converter using insulated-gate bipolar transistor, working on instantaneous current and voltage measurements of the compensator only and not of the load. The technique uses a simple feedforward control for AC source current harmonic compensation of rectifiers without monitoring the AC line currents, i.e. use of online computation. The proposed system is simulated and tested on a laboratory prototype. The measured input current THD values without additional line filters are found to be below 8.3%, which is within acceptable limits, proving that the new technique is capable of compensating predetermined current harmonics of diode or SCRs.     

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.     

风力发电系统变流器的直接功率控制策略

以永磁直驱型风力发电系统为研究对象,针对其变流器结构和控制策略进行了研究。通过选择最优双PWM“背靠背”变流拓扑结构,并采用直接功率控制策略进一步提高了风力发电系统的并网性能。建立了输出功率为10 kW的并网系统仿真模型,验证控制策略的正确性。结果表明,基于直接功率控制策略的“背靠背”变流拓扑具有结构合理、控制策略新颖的优点,在保证直流侧电压稳定的同时,电网电流谐波畸变率低、波形良好,能够实现单位功率因数并网,满足并网要求。     

改进型p-q法在并联有源滤波器中的应用原理与仿真

针对实际电网电压波形存在畸变的问题,本文提出一种新的检测方法,避免由于电网电压不平衡所造成的检测误差,使检测结果更为准确,补偿效果更为理想。本文在介绍传统两种检测方法原理的基础上,提出改进策略,对改进的方法原理进行介绍,在电网电压畸变或仍对称或不对称两种情况下进行仿真比较,最终得出改进后方法的正确性与准确性。     

Integrated power factor compensator without load current measurement

A simple strategy and low cost control for the switching mode rectifier to work simultaneously as a power factor corrector and an active power filter (APF) to reduce current harmonics drawn from the nonlinear load are analysed and presented in this paper. The principal component of the control circuit is an Intel 80196MC microcontroller that performs the dc bus voltage and line current control. The sliding mode control is used in the current loop to achieve fast line current dynamics. The source currents only are measured in the proposed control scheme instead of both the source and load currents needed in the conventional control approach. A simple proportional-integral control is adopted in the voltage loop to achieve slow dc bus dynamics. The proposed control strategy can achieve a high power factor and low current harmonics. No dedicated APF is needed in the proposed control strategy. To demonstrate the effectiveness of the integrated power factor compensator for elimination of reactive power and current harmonics, software simulation and hardware tests are performed.