Small DC-link Capacitor Research Articles

A Torque Control Method Based on I-MR Controller for IPMSM Drive With Small DC-Link Capacitor

In traditional motor drive system, large volume electrolytic capacitors are utilized to buffer energy and stabilize the dc-link voltage. However, these capacitors affect the reliability of the drives and narrow the conduction angle of diode-rectifier. As a result, the grid side current is distorted and the power factor is deteriorated. In order to settle these issues, small dc-link capacitors can replace the electrolytic capacitors. In this paper, a motor torque control method with dc-link voltage feed-forward compensation for an interior permanent magnet synchronous motor (IPMSM) drive system is proposed. Hence, an integration multi-resonant ( I-MR ) controller is established to regulate the motor torque to obtain high input power factor and low grid current harmonics. In addition, a voltage feed-forward compensation is used to diminish the error of motor torque control. The effectiveness of the proposed method is verified through simulation and experiments. Results indicate that the input power factor can reach about 99.4% and the grid current harmonics are considerably lower than the requirement of EN-61000-3-2 standard.

An Improved Sensorless Vector Control Method for IPMSM Drive with Small DC-Link Capacitors

In this paper, an improved sensorless vector control method for an interior permanent magnet synchronous motor (IPMSM) drive with small DC-link capacitors is proposed. First, a fast sliding-mode observer was applied, to enlarge the observer bandwidth. Then, an improved estimator based on a proportional integral resonant controller was designed for the estimate speed shaping, which means the actual speed can be tracked. This not only reduces the position estimate error, but also enhances the grid power factor and suppresses the input current harmonics. Simulation and experiments were conducted on a sensorless IPMSM drive system with a small film capacitor. The effectiveness of the proposed estimation method was verified by the simulation and experimental results. The estimated error was reduced and the grid current harmonics satisfy the requirements of EN61000-3-2.

Instantaneous Pulse Power Compensator for High-Density Single-Phase Inverters

In this paper, instantaneous pulse power compensator (IPPC) method is proposed to achieve power pulsation decoupling function for single-phase inverter applications. A smaller capacitor is placed in series with the traditional dc-link capacitor, and this smaller capacitor voltage is controlled using pulse currents to cancel out the dc-link voltage ripple. Unlike twice-line-frequency power decoupling method, the proposed IPPC method can compensate the pulsating power with all the orders harmonics on the dc-link line, not only the second-order component. Both modeling and simulation results show that IPPC method can achieve nearly zero voltage ripple for the dc-link capacitor since the pulse current is fully compensated. Whereas twice-line-frequency power decoupling method has limit in voltage ripple reduction, especially in small dc-link capacitance conditions. The experimental results based on a full-bridge topology decoupling circuit and a single-phase inverter application show consistent results with simulation results. A 16% reduction of the prototype size, i.e., 16% higher power density with only ∼1% efficiency penalty can be achieved by the proposed IPPC method when compared to the second-order power decoupling method.

A Novel Hybrid Control Method for Single-Phase-Input Variable Frequency Speed Control System With a Small DC-Link Capacitor

A novel hybrid control method, which consists of dual-frequency tracking control (DFTC) and virtual inertia control (VIC), is proposed for single-phase-input variable frequency speed regulation system with a small dc-link capacitor. Because of lacking a large-capacity energy storage device, the dc-link voltage is a pulsating dc, which causes a strong power coupling relationship between the rectifier and inverter side. The instantaneous power and dq -axis currents of the inverter contain both ac and dc components at the same time, which cannot be controlled well using traditional proportional-integrational (PI) controllers. Herein the DFTC that is based on a PI-resonant controller is proposed to accurately track ac and dc components of the instantaneous power and dq -axis currents respectively. With higher open-loop gain at both low frequency and the specified resonant frequency, the proposed DFTC ensures accurate tracking of the ac and dc components of power and dq -axis currents, high-power-factor, and low total harmonic distortion (THD) of input grid current. To maintain the stability of the dc-link voltage under step changes of load, a VIC is proposed. Subsequently, the input power factor and the fluctuation of dc-link voltage can be improved in the system. Simulations and experimental results are given to provide further validation of the proposed DFTC and VIC under different operating scenarios.

Design of Speed Control and Reduction of Torque Ripple Factor in BLdc Motor Using Spider Based Controller

It is a very difficult process to achieve smooth drivers for the motor operating under variable speed mode. In brushless direct current motor (BLdc) when back electromotive force waveform is of trapezoidal type, the developed torque is constant in ideal conditions. However, practically, torque ripple is present in the output torque because of the physical design of the motor and its parameters. Also, the produced ripples are associated with the control and driver side of the motor. In the previous literature, the drive without a dc-link capacitor is presented but the torque ripple reduction is not effective. Hence in another work, the usage of the small capacitor is recommended and the results are improved. In this work, the quick stabilization with torque ripple reduction is presented using a bio-inspired algorithm-based technique in a BLdc motor drive. A Spider based controller is built to generate the pulsewidth modulation signals applied to the inverter and the control signal applied to the capacitor. The effect of utilizing small dc-link capacitor, on the torque ripple reduction and speed control is investigated. The performance is also compared with the case of large capacitor utilization and without a capacitor case. The proposed control strategy is verified experimentally by implementing with dsPIC30F4011 and the hardware circuit.

Direct Active and Reactive Power Control of Three-Phase Inverter for AC Motor Drives With Small DC-Link Capacitors Fed by Single-Phase Diode Rectifier

This paper proposes a power controller used for three-phase inverters with small dc-link capacitors fed by a single-phase diode rectifier. The effect of the reactive power was investigated based on a power flow analysis between a dc-link and three-phase inverter. These results enable the development of a direct active/reactive power controller in discrete time instead of adopting a classical proportional-integral (PI)-type current regulator, which directly leverages the total power flow of the system. A suitable reactive power selection method was presented to improve the overall system efficiency. The command voltage vector at each discrete time step can be simply and intuitively determined by an inverter power without requiring motor current-regulating PI gains and additional control functions. The proposed controller regulates an average motor velocity/airgap torque while simultaneously shaping a grid-side current with a high power factor and low harmonics to meet IEC61000-3-2 requirements. The proposed structure can be applied to three-phase inverters for a broad family of ac motors, such as induction and permanent magnet synchronous motors.

Smart Transformer-Based Single Phase-To-Neutral Fault Management

The smart transformer (ST), with its high control capability, offers new features for optimizing the management of the distribution grid. The ST, by controlling the voltage in each phase independently, is able to manage the operations during grid faults, in particular in the single-phase ones. The ST, when a single-phase-to-ground fault occurs, can rapidly reduce the voltage in the phase under fault. Thus, it guarantees system safety and operates with the remaining two healthy phases to avoid unnecessary interruption in the healthy lines. However, the two-phase asymmetrical operation challenges the system performances due to the high power 2nd harmonic ripple in the dc voltage, that reduces the capacitors, lifespan and the high current flowing in the neutral conductor. These issues would force the grid operator to increase the ST maintenance, in order to avoid unplanned faults in the ST hardware, due to failure of aged capacitors. This paper presents a flexible-control strategy, based on the phase-shift angle control between two healthy-phase voltages, that reduces the impact of the power 2nd harmonic oscillation and thus can delay the maintenance intervention. The proposed strategy aims to improve system performances by avoiding neutral-line current overload and attenuating ac side active power (low voltage dc-link voltage) oscillation. Depending on the grid conditions (small dc link capacitance or low ampacity of neutral cable), the voltage angle can be adapted, minimizing the impact of the two-phase operation system in the ST-fed grid. The effectiveness and feasibility of the proposed approach have been validated experimentally with a simplified microgrid setup.

DC-Link Voltage Stabilization Using d-Axis Current Injection for SPMSM Driver with Small DC-Link Capacitance

Motor drivers employ film capacitors instead of the conventional electrolytic capacitors to achieve better reliability and smaller volume. However, these drivers have an instability problem because of a resonance between the small dc-link capacitance and grid side line inductance. In order to solve the problem, this paper proposes an active damping method using d-axis current injection. Since the injected current acts as damping impedance, the proposed method suppress dc-link voltage fluctuations. The relationship between the injected current and the virtual damping impedance is mathematically analyzed. Moreover, the proposed method is able to reduce the torque ripple caused by the injected current in spite of system loss increase. The increased loss also can be minimized by characteristics of the motor. The feasibility of the proposed method is proved by several experimental results.

Design and Control of Small DC-Link Capacitor-Based Three-Level Inverter with Neutral-Point Voltage Balancing

This paper presents a method to improve the quality of input-output currents in a three-level neutral-point clamped (NPC) inverter with small direct current-link (DC-link) capacitor systems. The inverter systems with the small DC-link capacitors have several advantages in terms of cost, volume, life-time, and reliability when compared to inverters that use large DC-link capacitors. However, there are problems with respect to the deterioration of the input current quality and a severe ripple of neutral-point voltage (NPV), which can cause an aggravated output current. To mitigate these issues, an additional circuit is applied for the input current shaping and a compensation algorithm is applied to reduce the ripple voltage of NPV. The effectiveness of the proposed design and control method is verified with various simulation and experimental results.

Over‐current protection method for PMSM VSI with small DC‐link capacitor

In this study, an over-current protection method for permanent magnet synchronous motor (PMSM) voltage source inverter (VSI) employing small DC-link capacitor is proposed. Since VSI with small DC-link capacitor has become popular for its long lifetime and compactness. However, the small DC-link capacitor brings the risk of excessive voltage in the DC-link capacitor when the traditional `all-turn-off' over-current protection is utilised. In order to avoid the damage for VSI caused by excessive pumping-up voltage, based on the analysis for the `all-turn-off' method, the proposed method could eliminate reversal current flowing into the DC-link capacitor through switching logic. Consequently, the electro-magnetic energy stored in PMSM windings is converted into mechanical energy and pumping-up voltage is cancelled. Detailed analysis of the proposed method is presented. The effectiveness of the proposed method is verified by simulation and experiments.

Reducing the dc-Link Capacitance: A Bridgeless PFC Boost Rectifier That Reduces the Second-Order Power Ripple at the dc Output

This Article Proposes a Novel single-phase bridgeles power factor correction (PFC) boost rectifier with a small dc-link capacitor that can reduce the second-order power ripple at the dc output side. An auxiliary circuit was added to decouple the active power at the dc link, which forces the voltage ripple to divert to the auxiliary capacitor. Therefore, the dc-link capacitance can be reduced significantly. As a result, the bulky electrolytic capacitor can be replaced with a small film capacitor. The experimental results verified the validation of the proposed method.

Modeling and Control of Three-Phase Grid-Connected Power Supply With a Small DC-Link Capacitor for Electrolyzers

In this paper, a new feed forward dc-link voltage compensation method is proposed. The target applications are the converters built with diode bridge frontends, small capacitance in the dc-link which feeds a dc/dc converter used for electrolysis. The compensation method can be tuned in a way to minimize the grid side total harmonic distortion (THD) and partial weighted harmonic distortion (PWHD): thus, the converter can comply with the standards with less filtering at the grid side. In order to design the controllers and perform stability analysis, average model has been derived. However, the performance of the proposed control method is a trade-off between the size of the line filter, the value of the line current THD and PWHD, and the amplitude of the load current ripple. The theoretical work has been validated through experiments on a 5 kW dc power supply used for an electrolyzer. In the experiments, the THD of the grid current can be reduced from 30.5% to 29.5% (without filter and including compensation) and 28.5% (with filter and compensation), and the PWHD from 50% to 38.9% by applying the proposed method and also keeping the load current peak-to-peak ripple under 10% of its nominal value.

Sensorless control of switched reluctance drive with small DC-link capacitor for high-speed blowers

This paper presents a design concept of a sensorless control for high-speed SRM drives with small DC-link capacitor. In a cost-sensitive application, the expensive large DC-link capacitor should be replaced with a smaller DC-link capacitor for cost reduction. Using a small DC-link capacitor, the DC-link voltage extremely fluctuates during operation. The fluctuating DC-link voltage deteriorates the control stability and performance of the sensorless control. In the literature, the previous work has shown that the workable speed range of the existing sensorless control concept for high speed has been degraded to a smaller range when operating with fluctuating DC-link voltage. The proposed design concept deals with the challenge of improving the control stability of the sensorless control to operate in a wider operating speed range up to 75% (8000–35,000 rpm) under the fluctuating DC-link voltage. The control scheme is developed based on two basis sensorless control techniques, inactive phase method and current gradient method for covering the entire speed range. The proposed solutions work based on the appropriate selection of the detected signals according to the state of DC-link voltage. At the end, the functionality of the proposed technique is validated by experimental results.

Resonance Reduction for AC Drives With Small Capacitance in the DC Link

Pulse-width modulated ac drives equipped with a small dc-link capacitor are becoming an attractive solution for electric drive applications with moderate requirements for shaft dynamic performance. However, when these drives are fed from a weak grid, a resonance between the line side impedance and the dc-link capacitor appears. Due to this resonance, the total harmonic distortion and the partially weighted harmonic distortion of the line currents are increased, which may raise compatibility problems with the ac line harmonic standards. This paper proposes a novel dc-link voltage compensation method, which can reduce the amplitude of the aforementioned oscillation, while the large dc-link voltage variation, caused by the rectification, can be eliminated from the motor current. The method has been experimentally validated and the results show excellent performance.

Single-Phase Grid-Connected Motor Drive System With DC-Link Shunt Compensator and Small DC-Link Capacitor

The single-phase diode rectifier system with small dc-link capacitor shows wide diode conduction time and it improves the grid current harmonics. By shaping the output power, the system meets the grid current harmonics regulation without any power factor corrector or grid filter inductor. However, the system has torque ripple and suffers efficiency degradation due to the insufficient dc-link voltage. To solve this problem, this paper proposes the dc-link shunt compensator (DSC) for small dc-link capacitor systems. DSC is located on dc-node in parallel and operates as voltage source, improving the system performances. This circuit helps the grid current-shaping control during grid-connection time, and reduces the flux-weakening current by supplying the energy to the motor during grid-disconnection time. This paper presents a power control method and the design guideline of DSC. The feasibility of DSC is verified by simulation and experimental results.

Discontinuous Grid Current Control of Motor Drive System With Single-Phase Diode Rectifier and Small DC-Link Capacitor

In this paper, a control method for motor drive system without electrolytic capacitor is proposed. Since dc-link capacitor of this motor drive is about 1% of the conventional one, it can satisfy the grid current regulation of IEC61000-3-2 without bulky filter circuits by directly controlling the electric output power of the inverter. However, dc-link voltage of this drive fluctuates as the rectified grid voltage. It causes higher motor current, especially at the low dc-link voltage area, and severely harms the drive efficiency. To solve this problem, proposed method keeps the dc-link voltage higher than certain limit while controlling the output power. It can improve the efficiency of the drive by lowering the d-axis current at low dc-link voltage area, with satisfying the regulation with small grid current harmonics. The proposed method includes the motor current reference generation for the limited dc-link voltage and selection of the limitation value. The improved performance of the proposed method was verified by the experimental results.

Harmonic Emissions of Three-Phase Diode Rectifiers in Distribution Networks

Harmonic emissions have been changed in distribution networks, with respect to frequency range and magnitude, due to the penetration of modern power electronics systems. Two new frequency ranges 2–9 and 9–150 kHz have been identified as new disturbing frequency ranges affecting distribution networks. This paper presents the effects of grid-connected three-phase systems with different front-end topologies: conventional, small dc-link capacitor, and electronic inductor. A power converter with a small dc-link capacitor can create a resonant frequency with the line impedance below and above 1 kHz depending on the grid configurations. The resonant effects depend on many factors, such as load power levels, filter types, and the number of parallel drives. These issues can affect the grid current harmonics and power quality of the distribution networks. Analyses and simulations have been carried out for three different topologies and the results have been verified by experimental test at system level. Current harmonic emissions have been considered for 0–2, 2–9, and 9–150 kHz frequency ranges.

High-Frequency-Link-Based Grid-Tied PV System With Small DC-Link Capacitor and Low-Frequency Ripple-Free Maximum Power Point Tracking

This paper proposes a grid-tied photovoltaic (PV) system consisting of modular current-fed dual-active-bridge (CF-DAB) dc–dc converter with cascaded multilevel inverter. The proposed converter allows a small dc-link capacitor in the three-phase wye-connected PV system; therefore, the system reliability can be improved by replacing electrolytic capacitors with film capacitors. The low-frequency ripple-free maximum power point tracking (MPPT) is also realized in the proposed converter. First of all, to minimize the influence resulting from reduced capacitance, a dc-link voltage synchronizing control is developed. Then, a detailed design of power mitigation control based on CF-DAB dynamic model is presented to prevent the large low-frequency voltage variation propagating from the dc-link to PV side. Finally, a novel variable step-size MPPT algorithm is proposed to ensure not only high MPPT efficiency, but also fast maximum power extraction under rapid irradiation change. A downscaled 5-kW PV converter module with a small dc-link capacitor was built in the laboratory with the proposed control and MPPT algorithm, and experimental results are given to validate the converter performance.

Hybrid SHM–SHE Modulation Technique for a Four-Leg NPC Inverter With DC Capacitor Self-Voltage Balancing

In this paper, selective harmonic mitigation (SHM) and selective harmonic elimination (SHE) modulation techniques have been combined to produce appropriate pulses for a four-leg three-level neutral point clamped (NPC) inverter. In the proposed hybrid method, the SHM technique is applied to phase legs in order to mitigate non-triplen harmonics (fifth, seventh, . . .), and the fourth leg is controlled by the SHE principle to eliminate important low-order triplen harmonics (third, ninth, . . .) completely. In this context, it is demonstrated that using SHM instead of an SHE modulator for phase legs leads to a reduced vast range of harmonic orders in the NPC output voltage, significantly. Moreover, it is proved that the presented hybrid switching technique has the ability of dc-bus capacitor voltage balancing, inherently. Furthermore, analyses are performed to calculate the dc-bus capacitances accurately to show 5% voltage ripple acceptably. Simulation and experimental results validate a good dynamic performance of the proposed hybrid modulation technique in firing the NPC inverter switches to generate low-harmonic voltage waveforms supplying balanced and unbalanced loads while balancing the dc capacitor voltages in a four-wire network with small dc-link capacitors.

Direct Power Control of a Three-Phase Inverter for Grid Input Current Shaping of a Single-Phase Diode Rectifier With a Small DC-Link Capacitor

This paper describes motor drive system fed by a single-phase diode rectifier without power factor correction (PFC) circuit or the input filter. The system considered in this paper consists of a single-phase diode rectifier, a three-phase inverter, and a small dc-link capacitor. Since dc-link capacitance of the proposed system is few microfarads, the shape of the grid input current is directly affected by electrical output power of the inverter. Using this aspect, two goals of the drive system, controlling torque of the motor and suppressing the grid input current harmonics, can be simultaneously achieved by controlling the output power of the inverter. The proposed method includes the motor current reference generation and the direct output power regulation by modifying the output voltage reference. With the proposed method, the harmonic components of the grid input current can be reduced under the limits of the regulation IEC61000-3-2, lower than those in the conventional method. Also, the cost and size of the inverter system can be significantly reduced by removing electrolytic dc-link capacitor and input filter from the system. The performance of the proposed shaping method was validated by the experimental results using the motor drive system with a 5μF film capacitor at dc link.