Small DC-link Capacitor Research Articles

Decoupling of Fluctuating Power in Single-Phase Systems Through a Symmetrical Half-Bridge Circuit

Single-phase ac/dc or dc/ac systems are inherently subject to the harmonic disturbance that is caused by the well-known double-line frequency ripple power. This issue can be eased through the installation of bulky electrolytic capacitors in the dc link. Unfortunately, such passive filtering approach may inevitably lead to low power density and limited system lifetime. An alternative approach is to use active power decoupling so that the ripple power can be diverted into other energy storage devices to gain an improved system performance. Nevertheless, all existing active methods have to introduce extra energy storage elements, either inductors or film capacitors in the system to store the ripple power, and this again leads to increased component costs. In view of this, this paper presents a symmetrical half-bridge circuit which utilizes the dc-link capacitors to absorb the ripple power, and the only additional components are a pair of switches and a small filtering inductor. A design example is presented and the proposed circuit concept is also verified with simulation and experimental results. It shows that at least ten times capacitance reduction can be achieved with the proposed active power decoupling method, and both the input current and output voltage of the converter can be well regulated even when very small dc-link capacitors are employed.

Low‐frequency dc bus ripple cancellation in single phase pulse‐width modulation inverters

This study presents a topology for a single-phase pulse-width modulation (PWM) converter which achieves low-frequency ripple reduction in the dc bus even when there are grid frequency variations. A hybrid filter is introduced to absorb the low-frequency current ripple in the dc bus. The control strategy for the proposed filter does not require the measurement of the dc bus ripple current. The design criteria for selecting the filter components are also presented in this study. The effectiveness of the proposed circuit has been tested and validated experimentally. A smaller dc-link capacitor is sufficient to keep the low-frequency bus ripple to an acceptable range in the proposed topology.

Swinging Bus Operation of Inverters for Fuel Cell Applications With Small DC-Link Capacitance

For reliability reasons, the employment of small film capacitors instead of electrolytic ones is an interesting alternative for the dc-link in single-phase inverters for fuel cell applications. Due to the low capacitance that can be accomplished at an acceptable cost using this technology, there are large low-frequency voltage fluctuations (100/120 Hz and harmonics) in the dc-link caused by the double-frequency power transfer. By allowing these variations in the bus, the capacitor bank absorbs the current ripple from the inverter to avoid detrimental oscillations in the fuel cell. Traditional control strategies for inverters are usually designed to operate with nearly constant input voltage and are not able to effectively handle large (e.g., ) low-frequency input voltage fluctuations. This paper introduces the analysis of a swinging bus in the context of fuel cell standalone applications (i.e., voltage-source inverters) and proposes a nonlinear control approach to operate inverters with very large input voltage swing: the natural switching surface (NSS). Under the proposed scheme, the inverter presents excellent dynamic and steady-state characteristics, even at moderate switching frequency (e.g., 3.6 kHz). In order to illustrate the superior performance of the NSS, a comparison to a proportional-resonant (PR) controller is performed. Unlike the linear compensator, the NSS is able to reject the large bus voltage oscillations and achieve high-quality output voltage with low total harmonic distortion (THD). Simulation and experimental results are provided to illustrate the behavior of the swinging bus and to validate the NSS control scheme under the proposed demanding operating conditions.

Control of Three-Phase Inverter for AC Motor Drive With Small DC-Link Capacitor Fed by Single-Phase AC Source

If a small dc-link capacitor is used in the dc link fed by a single-phase ac source, then the dc-link voltage severely fluctuates at twice of the source frequency. To handle this fluctuation, the concept of “average voltage constraint” is proposed in this study. On the basis of this concept, a flux-weakening scheme, generating the d-axis current reference (idsr*) for the interior permanent-magnet synchronous machine, is devised. The q-axis current reference (iqsr*) is modified for the unity power factor operation in the viewpoint of an ac source without an additional sensor. The proposed scheme has been applied to the inverter-driven 1-kW compressor of an air conditioner. From the experimental results, it has been verified that the compressor operates well at the required operating condition, regardless of the severe fluctuation of dc-link voltage, because of the reduced dc-link capacitance. The frequency spectrum of the ac source current reveals that the harmonics of the source current meet the regulation of IEC 61000-3-2 Class A and that the overall power factor is above 96% without any additional circuit, such as an input filter and a power factor correction circuit.

Design of Neutral-Point Voltage Controller of a Three-Level NPC Inverter With Small DC-Link Capacitors

A neutral-point-clamped three-level inverter with small dc-link capacitors is presented in this paper. The inverter requires zero average neutral-point current for stable neutral-point voltage. The small dc-link capacitors may not maintain capacitor voltage balance, even with zero neutral-point current. This may happen due to nonlinearities present in the circuit. This requires a fast control of the neutral-point voltage. A simple carrier-based modulation strategy which allows modeling of the neutral-point voltage dynamics as a continuous function of power drawn from the inverter is proposed. This continuous model shows that the neutral-point current is proportional to the power drawn from the inverter, and it enables the use of a well-established classical control theory for the neutral-point voltage controller design. A simple proportional integral controller is designed for the neutral-point voltage control on the basis of the continuous model. The design method for optimum performance is discussed. The implementation of the proposed modulation strategy and the controller is very simple. The controller is implemented in a 7.5-kW induction machine-based drive with only 14 μF dc-link capacitors. Also, the experimental results show that fast and stable performance of the neutral-point voltage controller are achieved and thus verify the validity of the proposed control approach.

An Active Damping Technique for Small DC-Link Capacitor Based Drive System

A small dc-link capacitor based drive system shows instability when it is operated with large input line inductance at operating points with high power. This paper presents a simple, new active damping technique that can stabilize effectively the drive system at unstable operating points, offering greatly reduced input line current total harmonic distortion. The proposed method requires only a first-order, high-pass filter with a gain. Active damping voltage terms, linked directly to the dc-link voltage ripple through gain units, are injected to the drive machine for stabilizing the operating points. The stabilizing effect of the active damping terms is demonstrated for an induction machine based drive system. The effects of the added damping terms on the machine current and dc-link voltage are analyzed in detail. A design recommendation for the proposed active damping terms is given. Experimental results verifying the effectiveness of the new active damping method are presented.

Grid-Interface Bidirectional Converter for Residential DC Distribution Systems—Part One: High-Density Two-Stage Topology

With the emerging installations of multitype renewable energy sources and energy storage elements, the dc electronic distribution systems in residential buildings (dc nanogrid) are becoming an alternative future system solution, achieving a zero net-energy consumption and optimized power management. The concept of the energy control center (ECC), which interconnects the dc system to the traditional ac utility grid, is introduced, and the operation function of ECC converter suitable for dc nanogrid application is defined. This paper investigates a two-stage topology using a full bridge in series with a bidirectional synchronous rectifier dc–dc converter as a single-phase ECC for dc nanogrid, with a significant reduction of the dc-link capacitor value. The operation analysis and the design of passive components are provided. A bidirectional control system and the design process are also presented in terms of the system requirement and the small dc-link capacitor.

Distributed Active Resonance Suppression in Hybrid DC Power Systems Under Unbalanced Load Conditions

Power-electronics-based hybrid dc power systems (HDCPSs) are increasingly used in many industrial applications such as land, sea, and air vehicles. In these systems, small dc-link and LC filter capacitors are of great interest for weight saving. Usually, in HDCPSs, there are constant power loads and negative dynamic impedance of these loads may generate unstable oscillations. Besides, the risk of resonance may be increased under unbalanced load conditions. In this paper, a distributed active oscillation suppression approach is presented. It is based on the analytical study of the linearized model of the studied system around the operating point. The studied system consists of one main dc source and one storage element supplying two loads: a permanent magnet synchronous motor connected to the dc link by a voltage source inverter and a resistive load supplied by a dc/dc converter through the same dc bus. The proposed approach is particularly used to overcome the resonance under unbalanced load conditions and allows reducing dc link and LC filter capacitors for weight saving. Simulations and experimentations are carried out which confirm the validity of the proposed approach.

DC Voltage Controller for Asymmetric-Twin-Converter-Topology-Based High-Power STATCOM

A four-level static compensator integrating two 2-level converters, supplying/absorbing reactive power to/from the grid, is reported in our earlier paper. Reduced component count, simpler layout for switches, and smaller dc-link capacitor values are the attractive features of the proposed topology over the diode clamped and cascaded multilevel converters. This paper suggests further improvements in this topology. Suitable selection of the dc-link voltage values reduces distortion in the current fed to the grid. In addition, circuit topology is modified to avoid the split-capacitor dc links. This reduces the number of independent dc capacitor voltages to be controlled and eliminates the flow of third-harmonic current through the transformer. In order to improve the performance, a phase-shifted carrier-based pulsewidth modulation technique is used. A mathematical model of the system is derived, based on which a controller for the scheme is designed. The effectiveness of the scheme is verified through detailed simulation study. To confirm the viability of the scheme, experimental studies are carried out on a scaled-down laboratory prototype developed for the purpose.

Flux-Weakening Strategy of an Induction Machine Driven by an Electrolytic-Capacitor-Less Inverter

This paper presents a novel flux-weakening strategy for an induction machine driven by an electrolytic-capacitor-less inverter. In the electrolytic-capacitor-less inverter, the dc-link voltage is fluctuating at six times the frequency of the input three-phase source due to its small dc-link capacitance. Hence, the decoupling of the fluctuation and maximum utilization of the dc-link voltage is a major issue in the electrolytic-capacitor-less inverter. In this paper, the cost function is set to increase the voltage utilization of the inverter for the flux-weakening operation of an induction machine. With the proposed flux-weakening strategy, the operating speed of the induction machine is extended above the base speed without any stability problems. The experimental results show the effectiveness of the proposed strategy.

Nonlinear control techniques of a controllable rectifier/inverter-motor drive system with a small dc-link capacitor

The necessity of the converters compactness in many applications imposes the reduction of their different components size when it is possible. In this paper, a control method allowing the use of a small size dc-link capacitor for the cascade of voltage controlled-rectifier/inverter-motor drive system is proposed. This is achieved by adding the power balance equation in the system’s model and the application of an exact input/output feedback linearization technique in a way that the rectifier controller compensates any sudden change in the inverter load, which is here an induction motor. Since the exact input/output feedback linearization technique is sensitive to the uncertainties over system parameters, a robust control strategy based on sliding mode controller is proposed. By this approach, the dc-link voltage becomes almost insensitive to the load variations. As a result, the level of the dc-link voltage could be stabilized with a small size dc-link capacitor. Without any considerations of the RMS current stress on this dc-link capacitor, a calculation method of a minimum value of this capacitor based on its storage energy is proposed. All the investigations are shown by computer simulations and the performance of controlled system is verified by experimentation results.

Application of SMC With I/O Feedback Linearization to the Control of the Cascade Controlled-Rectifier/Inverter-Motor Drive System With Small dc-Link Capacitor

The necessity of the compactness of the converters in many applications imposes the reduction of the size of their different components when it is possible. In this paper, a control method allowing the use of a small size dc-link capacitor ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> ) for the cascade of voltage controlled-rectifier/inverter-motor drive system is proposed. This is achieved by adding the power balance equation in the system's model and the application of an exact I/O feedback linearization technique in a way that the rectifier controller compensates any sudden change in the inverter load, which is here an induction motor. Since the exact I/O feedback linearization technique is sensitive to the uncertainties over system parameters, a robust control strategy based on sliding mode controller (SMC) is proposed. By this approach, the dc-link voltage ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> ) becomes almost insensitive to the load variations. As a result, the level of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> could be stabilized with a small <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> . Without any considerations of the RMS current stress on the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> , a calculation method of a minimum dc-link capacitor <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> (min) based on its storage energy is proposed. All the investigations are shown by computer simulations and the performance of controlled system is verified by experimentation results.

Induction Motor Drives Equipped With Diode Rectifier and Small DC-Link Capacitance

This paper deals with sensorless vector-controlled induction motor drives that are fed by a frequency converter that is equipped with a diode front-end rectifier. A small dc-link capacitance is used, which makes it possible to replace the electrolytic capacitors with film capacitors. The natural frequency of the dc link is chosen to be considerably higher than six times the mains frequency but lower than the switching frequency. A recently proposed sensorless controller can be exploited; only minor modifications for small capacitances are needed. The simulation and experimental results of a 2.2-kW drive that is equipped with a capacitance of only 24 demonstrate operation in a wide speed range.

Analysis and design of a novel, single-stage, three-phase AC/DC step-down converter with electrical isolation

A novel, single-stage, three-phase AC/DC step-down converter with electrical isolation has been presented. The front semi-stage, which is a three-phase AC/DC buck-boost converter, is operated in discontinuous conduction mode (DCM) to achieve unity power factor, pure sinusoidal input current and low total harmonic distortion of input current. The rear semi-stage, which is a DC/DC forward converter, is operated in continuous conduction mode at heavy load and is operated in DCM at light load for step-down voltage conversion. Also, the proposed converter has low voltage stress across the DC-link capacitor. In addition, the sixth times line frequency ripple component of DC-link voltage can be eliminated. Thus, smaller DC-link capacitor is required. Furthermore, the steady-state analysis of voltage gain and boundary-operating condition are discussed. The selections of inductors and capacitors are presented. Finally, simulated and experimental results are supplied to verify the theoretical analysis.

A Theoretical minimum DC-link capacitance in PWM converter–inverter systems

In the PWM converter-inverter system, if the converter current control is fast enough to match the inverter power at all instants, then no current flows through the DC-link capacitor, making the DC-link voltage constant, even with a very small DC-link capacitance. A matching control method making the capacitor current zero suggests the possibility of using a tiny DC-link capacitor. However, in practice, a certain amount of capacitance is required, since the matching control itself is not perfect and there are one-step delays in the PWM. In this work, the authors investigate a theoretical low bound of the DC-link capacitance, considering an extreme case in which the DC-link voltage rises to a maximum value in a single PWM period. Utilising the optimisation technique, the theoretical minimum capacitance is found to be the one that limits the voltage rise below a certain threshold value. Analytical calculation results are compared with MATLAB simulation results.

DC-Link Stabilization and Voltage Sag Ride-Through of Inverter Drives

Previous results concerning instability of the dc link in inverter drives fed from a dc grid or via a rectifier are extended. It is shown that rectifier-inverter drives equipped with small (film) dc-link capacitors may need active stabilization. The impact of limited bandwidth and switching frequency in the inverter-motor current control loop is considered, and recommendations for selection of the dc-link capacitor, the switching frequency, and the dc-link stabilization control law in relation to each other are given. This control law is incorporated in a field-weakening (to enhance voltage sag ride-through) current controller for which design recommendations are presented

Predictive current control of distribution static compensator for reactive power compensation

The paper describes modelling and current control techniques of a distribution static compensator (D-STATCOM). Current control is based on the predictive and the space vector pulse-width modulation (PWM) scheme. The predictive current-controlled PWM D-STATCOM can maintain its performance with reactive power compensation and a fixed switching frequency. By using the space vector control, low ripple and offset in the current and the voltage as well as fast dynamic responses are achieved using a small DC link capacitance.

A feedback linearizing control scheme for a PWM converter-inverter having a very small DC-link capacitor

This paper addresses a control method of reducing the size of the DC-link capacitors of a converter-inverter system. The main idea is to utilize the inverter operation status in the current control of the converter. Specifically, the information on the load power is incorporated in synthesizing the converter current control input so that a proper DC voltage level is maintained. We describe the dynamics of load current and apply feedback linearization theory to obtain an input output linearized system. Theoretically, this control strategy is effective in regulating the DC voltage level, even though the DC-link capacitor is arbitrarily small and load varies abruptly. The superior performance is demonstrated through simulation and experiment. An experiment was performed with a 9-kW pulsewidth modulation converter-vector inverter system having a 75-/spl mu/F DC-link capacitor.

Performance of a single-stage UPS system for single-phase trapezoidal-shaped AC-voltage supplies

A high-performance single-stage UPS system for single-phase AC power supply systems is presented. The topology generates a trapezoidal-shaped load voltage that is synchronized with the AC supply; therefore, a small DC-link capacitor is required. This feature leads to a high-input power factor (0.94 at nominal voltage), and due to the single-stage structure, high efficiency (93% at nominal voltage) is also achieved. Moreover, in both cases the supply current spectrum presents low-order harmonics, all of which satisfy IEC 555. The high performance is validated using both resistive (linear) and diode RC (nonlinear) loads. The dynamic performance of the UPS system is also analyzed under different transient conditions. Specifically, the supply to battery, battery to supply transition, load transient, and supply/battery-voltage transients are performed. They show a transfer time of 500 /spl mu/s (from AC supply to battery) and overvoltages of at most 15%. The dynamic and static evaluations are performed on a 1 kVA UPS system. The design guidelines for the key components and control parameters are also included.

Half Bridge PWM Converter with a Small DC Link Capacitor.

Half Bridge PWM Converter with a Small DC Link Capacitor.