Control of a Cascade STATCOM With Star Configuration Under Unbalanced Conditions

A control scheme for star-connected cascade static synchronous compensators (STATCOMs) operating under unbalanced conditions is proposed. The STATCOM is assumed to be connected to an equivalent three-phase star-connected power supply. By selecting the line-to-neutral voltages of the equivalent power supply, zero average active power in each phase can be obtained under unbalanced compensation currents or unbalanced supply voltages. Furthermore, to implement a separate control for the three-phase dc-link voltages, the average active power in each phase can also be adjusted to a target value determined by the dc-link voltage control loop. Then, by forcing the converter neutral voltage to be equal to the counterpart of the equivalent power supply, the STATCOM can be decoupled into three single-phase systems and the line-to-neutral voltage of the equivalent power supply can be used as the input voltage to the corresponding phase leg. Accordingly, reference current tracking and dc-link voltage maintaining can be simultaneously achieved under unbalanced conditions. The valid operating range of the star-connected cascade STATCOM under unbalanced conditions is also analyzed. The proposed control scheme has been tested using the power systems computer-aided design/electromagnetic transient in dc system (EMTDC) simulation results and the experimental results based on a 30-kVAr cascade STATCOM laboratory prototype are proposed.      

Experimental evaluation of direct torque-controlled 3-phase induction motor under inverter faults

ABSTRACT

In this paper, analysis and design of fault-tolerant converter topology for direct torque-controlled (DTC) induction motor (IM) drive suitable for low, medium and high power applications is proposed. The proposed converter topology can restore normal operation of the drive after the occurrence of open-circuit or short-circuit of power switches in the inverter. It consists of a current-controlled three-level boost converter (TLBC) to boost the dc-link voltage at input terminals of an inverter during post-fault, balance the voltages at dc-link capacitors and retains all the advantages of the conventional IM drive. Simulation and experimental results are presented for pre- and post-fault operation. The results are compared with conventional fault-tolerant DTC of the drive to highlight the merits of proposed converter.     

Efficient Modulation Technique for a Four-Leg Fault-Tolerant Neutral-Point-Clamped Inverter

This paper presents a new low-loss modulation technique for the hybrid three-level four-leg converter. The total losses of the converter are reduced by about 18% on average compared to the standard three-leg neutral-point-clamped converter. Furthermore, the low-frequency voltage oscillation in the neutral point is completely cancelled, and the maximum benefit of the dc-link voltage is obtained. All these facts, together with the fault-tolerant ability due to the fourth leg, make this topology very interesting for applications such as wind generation, in which it is important to maximize efficiency and reliability. Some experimental results confirm the good performance of the proposed modulation technique.     

An Improved Control Strategy of Limiting the DC-Link Voltage Fluctuation for a Doubly Fed Induction Wind Generator

The paper presents to develop a new control strategy of limiting the dc-link voltage fluctuation for a back-to-back pulsewidth modulation converter in a doubly fed induction generator (DFIG) for wind turbine systems. The reasons of dc-link voltage fluctuation are analyzed. An improved control strategy with the instantaneous rotor power feedback is proposed to limit the fluctuation range of the dc-link voltage. An experimental rig is set up to valid the proposed strategy, and the dynamic performances of the DFIG are compared with the traditional control method under a constant grid voltage. Furthermore, the capabilities of keeping the dc-link voltage stable are also compared in the ride-through control of DFIG during a three-phase grid fault, by using a developed 2 MW DFIG wind power system model. Both the experimental and simulation results have shown that the proposed control strategy is more effective, and the fluctuation of the dc-link voltage may be successfully limited in a small range under a constant grid voltage and a non-serious grid voltage dip.     

TRIAC dimmable ballast with power equalization

A two-stage, two-wire TRIAC dimmable electronic ballast for fluorescent lamps is presented in this paper. It is constructed by using a flyback converter as the input power factor corrector to supply a half-bridge series-resonant parallel-loaded inverter to ballast the lamp. The flyback converter is operated in discontinuous conduction mode so that the filtered input current profile is the same as the TRIAC-controlled voltage waveform. The switches in the inverter are switched at a constant frequency slightly higher than the resonant frequency of the resonant tank. Based on the constant average input current characteristics of the inverter, the dimming operation is simply achieved by pulsewidth modulation control of the magnitude of the flyback converter output voltage. No synchronization network is required between the input and output stages. In addition, a linear power equalization scheme is developed so that the dc-link voltage (and hence the lamp power) is in a linear relationship with the firing angle of the TRIAC. The average output voltage of the dimmer controls the equalized flyback converter output voltage. Modeling, analysis, and design of the ballast will be described. A prototype was implemented to verify the experimental measurements with the theoretical predictions.     

Zero-voltage-zero-current switching in high-output-voltagefull-bridge PWM converters using the interwinding capacitance

A novel zero-voltage and zero-current-switching (ZVZCS) full-bridge (FB) pulsewidth-modulated (PWM) power converter is proposed. The new converter uses the interwinding capacitance and a small primary-side inductor to achieve a zero-current-zero-voltage turn off and a zero-current turn on of the passive-to-active leg transistors. The turn off of the active-to-passive leg transistors is with zero voltage, and the turn on is with zero voltage and zero current across them. The ringing caused by the parasitic interwinding capacitance and by the reverse recovery of the rectifiers is reduced. The new converter is attractive for high-output-voltage applications (600-1000 V), where the interwinding capacitance is sufficiently dominant. In addition, switches such as insulated gate bipolar transistors (IGBTs) and MCTs can be used at higher frequencies which is particularly desirable for high-power application (above 2 kW). The experimental results obtained from an IGBT-based 62.5-kHz DC/DC power converter with a rated output voltage of 600 V and a nominal power of 1.2 kW are presented     

A digitally controlled switch mode power supply based on matrix converter

High power telecommunication power supply systems consist of a three-phase switch mode rectifier followed by a dc/dc converter to supply loads at -48 V dc. These rectifiers draw significant harmonic currents from the utility, resulting in poor input power factor with high total harmonic distortion (THD). In this paper, a digitally controlled three-phase switch mode power supply based on a matrix converter is proposed for telecommunication applications. In the proposed approach, the matrix converter directly converts the low frequency (50/60Hz, three-phase) input to a high frequency (10/20kHz, one-phase) ac output without a dc-link. The output of the matrix converter is then processed via a high frequency isolation transformer to produce -48V dc. Digital control of the system ensures that the output voltage is regulated and the input currents are of high quality under varying load conditions. Due to the absence of dc-link electrolytic capacitors, power density of the proposed rectifier is expected to be higher. Analysis, design example and experimental results are presented from a three-phase 208-V, 1.5-kW laboratory prototype converter.     

Feed-Forward Space Vector Modulation for Single-Phase Multilevel Cascaded Converters With Any DC Voltage Ratio

Modulation techniques for multilevel converters can create distorted output voltages and currents if the dc-link voltages are unbalanced. This situation can be avoided if the instantaneous dc voltage error is not taken into account in the modulation process. This paper proposes a feed-forward space vector modulation method for a single-phase multilevel cascade converter. Using this modulation technique, the modulated output voltage of the power converter always generates the reference determined by the controller, even in worst case voltage unbalance conditions. In addition, the possibility of optimizing the dc voltage ratio between the H-bridges of the power converter is introduced. Experimental results from a 5-kVA prototype are presented in order to validate the proposed modulation technique.      

New cascaded control system for current-source rectifiers

A control method for current-source rectifiers (CSRs), which realizes substantially sinusoidal line currents, unity displacement power factor, and a dc-link current control with excellent dynamic properties is presented. CSRs are usually operated by pulsewidth-modulation (PWM) or space-vector-modulation techniques. However, due to the mains LC filter resonant circuits when using these modulation methods the system stability has to be investigated, resulting in restrictions on the minimum PWM frequency and the minimum size of the LC filter. Furthermore most known dc-link current control loops use dc-link inductors of considerable size. This limits the dynamic performance and, therefore, reduces the attainable efficiency of CSRs. To overcome these problems, a new cascaded dc-link current control system for CSRs is presented. Its inner capacitor voltage controller is based on a time-discrete modulation method, which realizes a fundamentally stable control of the mains LC filter resonant circuits, avoiding the mentioned restrictions. The system controlled by the superimposed dc-link current controller is linearized by a new approach, allowing excellent dynamic performance and, therefore, a comparatively small dc-link inductor to be used. The paper includes guidelines on how to design the mains filter components and the dc-link inductor. The feasibility of the presented cascaded controller is confirmed by measurements taken on a 60-kVA model current-source converter and different loads.     

Control of Single-Phase-to-Three-Phase AC/DC/AC PWM Converters for Induction Motor Drives

This paper proposes a novel control scheme of single-phase-to-three-phase pulsewidth-modulation (PWM) converters for low-power three-phase induction motor drives, where a single-phase half-bridge PWM rectifier and a two-leg inverter are used. With this converter topology, the number of switching devices is reduced to six from ten in the case of full-bridge rectifier and three-leg inverter systems. In addition, the source voltage sensor is eliminated with a state observer, which controls the deviation between the model current and the system current to be zero. A simple scalar voltage modulation method is used for a two-leg inverter, and a new technique to eliminate the effect of the dc-link voltage ripple on the inverter output current is proposed. Although the converter topology itself is of lower cost than the conventional one, it retains the same functions such as sinusoidal input current, unity power factor, dc-link voltage control, bidirectional power flow, and variable-voltage and variable-frequency output voltage. The experimental results for the V/f control of 3-hp induction motor drives controlled by a digital signal processor TMS320C31 chip have verified the effectiveness of the proposed scheme     

Control of the single-phase three-leg AC/AC converter

This paper investigates the control of a single-phase three-leg ac/ac reversible converter in which a leg is shared by both the grid and the load side. Pulsewidth-modulation (PWM) techniques based on scalar and vector approaches are developed, introducing the concept of local and general apportioning factor and, also, a complete equivalence between scalar and space vector PWM. A hysteresis current controller capable of taking into account the shared leg is developed and a zero current error linear controller is presented. Furthermore, a control strategy to obtain maximum utilization of the dc-bus voltage is proposed. In addition, several relevant characteristics of the converter are addressed, such as voltage rating, harmonic distortion, shared leg and capacitor currents, and power rating. The converter is compared to four-leg and two-leg converters. Experimental results are presented.     

一种具有能量回馈功能的级联型高压变频器的仿真研究

本文采用成熟的三相PWM整流技术,通过改造级联型高压变频器单个功率单元的整流部分,对直流母线电容电压进行闭环控制来稳定功率单元直流母线电容上的电压,实现能量回馈。这种方法不仅解决了一般的级联型高压变频器能量不能回馈,单个功率单元直流母线电容电压不稳的问题,而且还能使网侧的功率因数为1,使级联型高压变频器成为真正的绿色变频器。     

A Comprehensive Study of a Hybrid Modulation Technique for the Neutral-Point-Clamped Converter

This paper presents a hybrid modulation technique for the three-level neutral-point-clamped converter. A modulation strategy, based on two modulation signals per phase, was presented previously. This strategy completely removes the low-frequency voltage oscillations that appear at the neutral point (NP) in some operation conditions. However, it also has a major drawback: it significantly increases the switching losses of the converter. The proposal in this paper combines such a modulation strategy with sinusoidal pulsewidth modulation (SPWM). The main characteristic of this hybrid modulation is the reduction in switching losses at the cost of some low-frequency voltage oscillations at the NP. The amplitude of these oscillations can be controlled by varying the combination of the two strategies. The performance of the hybrid modulation is analyzed and compared with the original strategies. Power losses and oscillation amplitudes on the dc-link capacitors are evaluated. Experimental results show how the hybrid modulation performs by balancing the dc-link capacitors.      

The three-level ZVS-PWM DC-to-DC converter

A novel high-frequency DC-to-DC power converter for high voltage and high power is introduced which features zero voltage switching (ZVS), operation at constant frequency, regulation by pulse width modulation (PWM), and low RMS current stress upon power switches. Its greatest attribute, in comparison with the full-bridge (FB-ZVS-PWM) converter, is that the voltage across the switches is half of the input voltage, This property is achieved due to the use of a three-level leg in place of the conventional two-switch leg. Operation, analysis, design procedure and example, and simulation are presented. A prototype operating at 100 kHz, rated at 600 V input voltage, and 1.5 kW output power and 25 A output current has been fabricated and successfully tested in the laboratory. The measured efficiency at full load was 93%     

High-Performance Transformerless Online UPS

In this paper, a high-performance single-phase transformerless online uninterruptible power supply (UPS) is proposed. The proposed UPS is composed of a four-leg-type converter, which operates as a rectifier, a battery charger/discharger, and an inverter. The rectifier has the capability of power-factor collection and regulates a constant dc-link voltage. The battery charger/discharger eliminates the need for the transformer and the increase of the number of battery and supplies the power demanded by the load to the dc-link capacitor in the event of the input-power failure or abrupt decrease of the input voltage. The inverter provides a regulated sinusoidal output voltage to the load and limits the output current under an impulsive load. The control of the dc-link voltage enhances the transient response of the output voltage and the utilization of the input power. By utilizing the battery charger/discharger, the overall efficiency of the system is improved, and the size, weight, and cost of the system are significantly reduced. Experimental results obtained with a 3-kVA prototype show a normal efficiency of over 95.6% and an input power factor of over 99.7%.      

Multiphase Bidirectional Flyback Converter Topology for Hybrid Electric Vehicles

For hybrid electric vehicles, the batteries and the drive dc link may be at different voltages. The batteries are at low voltage to obtain higher volumetric efficiencies, and the dc link is at higher voltage to have higher efficiency on the motor side. Therefore, a power interface between the batteries and the drive's dc link is essential. This power interface should handle power flow from battery to motor, motor to battery, external genset to battery, and grid to battery. This paper proposes a multi-power-port topology which is capable of handling multiple power sources and still maintains simplicity and features like obtaining high gain, wide load variations, lower output-current ripple, and capability of parallel-battery energy due to the modular structure. The scheme incorporates a transformer winding technique which drastically reduces the leakage inductance of the coupled inductor. The development and testing of a bidirectional flyback dc-dc converter for hybrid electric vehicle is described in this paper. Simple hysteresis voltage control is used for dc-link voltage regulation. The experimental results are presented to show the working of the proposed converter.     

Multicarrier PWM With DC-Link Ripple Feedforward Compensation for Multilevel Inverters

Like most power converter topologies, multilevel inverters are controlled with modulation techniques that are conceptually based on nonlinear waveform synthesis assuming constant dc-link voltages. However, real applications have load and supply dependent dc-links that usually present important low frequency ripple, which is also modulated and transmitted to the load, generating undesirable low frequency voltage and current distortion. This paper introduces a simple but effective dc-link ripple feedforward strategy into traditional carrier-based modulation techniques. The dc-link ripples are measured and used to modify the carriers or the reference directly in the modulation stage. Simulation and experimental results show the accuracy of the proposed method, eliminating low order harmonics in the load current.     

A novel single-stage half-bridge AC-DC converter with high powerfactor

A new single-stage AC-DC power converter based on a half-bridge converter suitable for low-power applications is proposed. The proposed converter offers high power factor and direct conversion from the line voltage to an isolated DC output voltage. High power factor is achieved by adding a resonant circuit between the rectifying diodes and half-bridge leg. For soft switching, a half-bridge series-loaded resonant converter is adopted as a DC-DC converter part. A prototype is built and tested to show the validity of the proposed converter     

Three-phase power quality compensator under the unbalanced sources and nonlinear loads

A three-phase voltage-source inverter for a power quality compensator under the unbalanced mains and nonlinear loads is proposed to provide balanced three-phase source current and improve input power factor. The proposed converter is based on the conventional three-phase voltage-source inverter with three additional ac power switches to achieve three-level pulsewidth modulation. The voltage stress of three ac power switches is clamped to half the dc-link voltage. The balanced reference mains currents are estimated using the dc-bus voltage and load currents. A proportional-integral voltage controller is used in the outer loop to compensate the switching losses of the voltage-source inverter. To perform the integrated power quality compensation, a hysteresis current control scheme is adopted to track the balanced line current command in phase with mains voltage. Three voltage levels are generated on the ac terminal of the proposed inverter. Computer simulation and experimental results are provided to verify the effectiveness of the proposed control scheme.     

基于矩阵变换器的电力电子变压器

矩阵式变换器是一种新型的交交直接变换的电源变换器,可以实现交流电诸参数(相数、相位、幅值、频率)的变换。和传统的变换器相比,它具有不需要中间直流储能环节;功率双相流动:具有优良的输入电流波形和输出电压波形;可自由控制的功率因数等优点。采用矩阵变换器的电力电子变压器省略了繁重的储能电容,省去直流环节的电容,可减小温度变化对系统的影响。因此基于变换矩阵的电力电子变压器更适合大功率系统,并且适合于在未来高温的电力变换场合。