Output Current Feedback Research Articles

Grid Impedance Detection Based On Complex Coefficient Filter and Full-order Capacitor Current Observer for Three-phase Grid-connected Inverters

This article proposes a new grid impedance detection method incorporating the complex coefficient filter (CCF) with full-order capacitor current observer for a T-type three-level grid-connected inverter controlled by the inverter output current feedback. Compared with conventional CCF impedance detection algorithms, the proposed method reduces the number of current sensors and detects the grid impedance accurately. First, based on the sampled inverter output current and grid-connected voltage signals, the grid-connected current is estimated. Then, the CCF method is used to extract harmonics from the grid-connected current and voltage signals to calculate the grid impedance. Finally, the correctness of the full-order capacitor current observer is verified by simulation and the feasibility and effectiveness of the proposed algorithm are verified experimentally based on a laboratory prototype.

A novel high-performance two poles and two zeros digital compensation control strategy for electric vehicle lithium battery charging systems

This study proposes a 1 kW power converter of the lithium battery charging system for electric vehicles. Its proposed architecture comprises an interleaved boost power factor corrector (PFC) converter and a full-bridge phase-shift (FBPS) converter with a new two poles and two zeros (TPTZ) digital compensation function. Unlike the traditional one pole and one zero compensation control, the proposed TPTZ compensation control considers both the output voltage and the output current feedback, effectively controlling the gain bandwidth fc and phase margin PM and improving system stability and performance. The interleaved boost PFC converter can adjust the power factor close to 1.0. On the other hand, the FBPS converter adopts an average switching model and the duty cycle loss factor to derive the small-signal model of the full-bridge phase-shift converter. The model can obtain the transfer function of the output voltage and current to the duty cycle. The corresponding digital compensation control loop was designed and applied to the converters using the derived transfer function. Further, the charge system's stability with MATLAB simulation was verified. Finally, the proposed architecture with the proposed TPTZ control method was compared with the traditional battery charger architecture, in which the PFC converter was connected with an FBPS converter in series. The study found that the proposed battery charging system performs better than the traditional system under the test conditions (25 %, 50 %, and 100 % load). In addition, the proposed TPTZ method has 95 % and 96 % efficiency when charging with constant voltage mode and constant current mode at 100 % load, respectively. Furthermore, this study proposes a battery charging system with a TPTZ digital compensation function that not only improves system performance but also reduces circuit size, suitable for charging lithium batteries in electric vehicles.

An Improved Dynamic Control and Resonance Damping for Voltage Source Inverters in Microgrids Through a Novel Active Damping Technique

Microgrids have the potential to provide customers with improved power quality and reliability. However, there are technical challenges related to the operation and control of microgrids and the distributed energy source (DERs) that constitute them. Resonance caused by the low-pass filters connected to the voltage source inverters of the DERs is one of those issues in the operation of microgrids. This paper proposes a novel technique of Active Damping Algorithm (ADA) for resonance damping and improving the dynamic stability of a microgrid composed of multiple converter-interfaced DERs. The paper considers two cases; one is where the resistor and the filter capacitor are connected in parallel, and the other is where they are connected in series. The proposed active damping method is based on output current feedback with automatically calculated and adjusted damping coefficients to suppress resonance. The proposed method was verified through simulation and experimental tests carried out using PSCAD/EMTDS and an operational microgrid in Beijing, China. The results from both the experiment and simulation analysis showed that the devised method was able to achieve the intended target; where the filter resonance is effectively surpassed and the voltage and frequency dynamic responses are quite stable and within the acceptable range during large power changes in the microgrid system.

Modular Circulating Current and Second Harmonic Current Suppression Strategy by Virtual Impedance for DC Solid-State Transformer

This article proposes a circulating current and second harmonic current (SHC) suppression method by introducing virtual impedance into the circulating current and output current feedback control loops of the dc solid-state transformer (DCSST). The virtual impedance is employed to adjust impedance in the circulating current feedback control loop to decrease the circulating current when the transmission efficiency of each dual active bridge is different. And a SHC suppression method, which introduces the virtual impedance containing a bandpass filter, is adopted to reduce the SHC and improve the dynamic performance of the DCSST. Moreover, a relatively independent modular control method, which is conducive to the modular expansion of the DCSST, is proposed. Compared with traditional DCSST control methods, the proposed method can simultaneously realize the output current sharing and effectively reduce the SHC with modular control. Finally, a DCSST prototype is built and the results of the experiment verify the validity and effectiveness of the proposed control strategy and solutions.

Comprehensive sharing control strategy for input‐series output‐parallel connected modular DC–DC converters

The systems consisting of multiple DC–DC converters with input-series output-parallel (ISOP) connection are widely used in high-power applications. In the ISOP connection, the total input voltage and output current should be divided equally between the converters. However, in order to achieve this purpose, a proper control scheme should be used. This study proposes two control schemes: (i) output current feedback (OCF) and (ii) input voltage feedback (IVF). Based on the power balance, in the IVF scheme, the total output current, and in the OCF scheme, the total input voltage are divided automatically between the converters, and there is no need to use an extra control loop. The IVF and the OCF control schemes consist of two control loops: a common output voltage regulator (OVR) loop and individual input voltage sharing loops in the IVF scheme or individual output current sharing loops in the OCF scheme. The proposed control scheme can effectively improve the stability and dynamic characteristics of system. Finally, both the results of the simulation and laboratory prototype of an ISOP system consisting two forward converters have verified the performance of the IVF and OCF control schemes.

An adaptive voltage‐regulation control strategy of an electric spring based on output current feedback

With the increasing penetration of distributed generation (DG) in power systems, the voltage instability problem of power systems is receiving a great deal of attention. As new power electronics equipment, an electric spring (ES) can effectively restrain the bus voltage fluctuation caused by DG, and in addition achieve the new control paradigm of load demand following power generation. In this article, an adaptive voltage‐regulation control strategy of ES is proposed, in which the proportional integral (PI) postgain changes dynamically according to the real‐time output current of the ES. The simulation results show that, when the proposed control strategy is applied, bus voltage can still be stabilized effectively by the ES under the condition that noncritical loads change. The proposed control strategy brings the adaptive voltage‐regulation capability of the ES, and therefore, it can be in series with multiple noncritical loads reliably and safely, and accordingly the costs can be greatly reduced. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Introducing Self-Oscillating Technique for a Soft-Switched LED Driver

In this paper, a self-oscillating soft-switched converter for light-emitting-diode (LED) driver application is presented. In the proposed converter, the self-oscillation conditions are provided with a low number of additional components, and this converter provides a zero-current switching (ZCS) condition for instant switch turn- OFF . This converter does not need additional output current feedback because the output current changes only 10% when the output voltage changes by 33%. The self-oscillating part of this converter applies three low-power BJT and guarantees a soft-switching condition. The proposed self-oscillating control circuit can be implemented with a low cost since it does not need any IC and power supply for the control circuit. The proposed self-oscillating circuit detects the proper moment to turn the switch OFF under the ZCS condition. In order to justify the validity of theoretical analysis, the results of the implemented prototype for driving an LED string are presented. The measured power factor of the prototype shows that, although the input current of the converter is not sinusoidal, this converter can meet the IEC61000-3-2 standard at an operating power of less than 25 W, in addition to having a simple structure and achieving the robust ZCS condition.

Resistance capacitive inverter allocation method based on fast reactive support based on Microgrid

In the low voltage microgrid multi inverters parallel system, the sudden change of load will lead to voltage fluctuation in the microgrid, and the fast reactive power support capability of the inverter is a necessary way to maintain voltage stability. By introducing resistance capacitance virtual complex impedance into inverter output current feedback, a fast reactive power support RC inverter inverter and its parallel power allocation method are proposed in this paper. The equivalent output impedance of the inverter is designed into resistive and capacitive, can be realized in the micro grid support fast reactive power public connection point, so as to keep the system voltage stability, and can inhibit the resonant inverter output impedance and the impedance between, to further reduce the voltage distortion. The effectiveness of the control method is verified by simulation and experiment.

Passivity-based active stabilization for DC microgrid applications

In this paper, an improved active stabilization strategy of the interface converters in microgrid applications is proposed on the basis of the passivity-based stability criterion (PBSC). As a critical part of AC and DC hybrid microgrids, the DC microgrid is taken as an example. In particular, a stabilization method with a proportional-integral (PI) controller and firstorder high-pass filter (HPF) is proposed to meet the passivity requirements of the overall control diagram with respect to the output voltage. Meanwhile, an output current feedback control loop is introduced to ensure the output impedance passivity. Moreover, a small-signal model of the parallel interface converter system is established to comprehensively study the influence of control parameters on the passivity of the converters. Based on the active stabilization method proposed in this study, by manipulating the control diagram of each interface converter, the passivity and stability of the DC microgrids with variable configuration can be guaranteed. Therefore, a generic and simplified design approach is realized. A simulation model with three interface converters is implemented in MATLAB/Simulink, and the effectiveness of the proposed passivity-based active stabilization algorithm is verified by using this simulation model.

Inductor Current Measurement Strategy for High-Precision Output Current Control

Recently, high brightness white light-emitted diodes (LEDs) have become the best lighting sources. A primary-side regulator (PSR) is widely used for the LED lighting application. However, the PSR often experiences the drawback of output current inaccuracy which will shorten the LED’s life-time. The objective of this paper is to propose the inductor current measurement strategy (ICMS) which can be adopted for different circuit topologies to achieve high-precision output current control under no output current feedback condition. The proposed ICMS senses the slope of inductor current to obtain the correct peak and valley values of inductor current. With the correct value and conduction time of the inductor current, the proposed ICMS can achieve high-precision output current regulation. The operation principle and mathematic derivations of the proposed ICMS are developed thoroughly. Two ICMS-based prototypes with different circuit topology for LED lighting application are built and tested. Computer simulations and experimental results are presented to confirm the theoretical results.

Novel SQUID Current Sensors With High Linearity at High Frequencies

We have developed novel SQUID current sensors consisting of a sensitive 16-SQUID input stage followed by an output stage of 40, 160, or 640 low-inductance SQUIDs. A highly gradiometric design of the individual SQUIDs with ap 5 mum maximum linewidth makes the devices insensitive to external magnetic noise and allows cool-down and operation in the Earth magnetic field. Output current feedback (OCF) is used to increase the linear signal range. When driving a terminated 50 Omega coaxial cable, the sensor with 160 output SQUIDs exhibits a signal range of one flux quantum Phi0 at a power dissipation of about 100 nW. At 10 MHz and 4.2 K, a total flux noise level of 0.34 muPhi0/radic(Hz) is achieved corresponding to a dynamic range of plusmn 1.5 times 106 radic(Hz). At higher frequencies, interaction of preamplifier voltage noise with the output cable impedance increases the sensor noise (a general problem in high-speed SQUID systems). The small-signal bandwidth is >200 MHz. A very high slew rate of > 50 Phi0/mus is achieved allowing one to use the full signal range at frequencies up to 16 MHz with a total harmonic distortion below ap1%.

Suppression of Line Voltage Related Distortion in Current Controlled Grid Connected Inverters

The influence of selected control strategies on the level of low-order current harmonic distortion generated by an inverter connected to a distorted grid is investigated through a combination of theoretical and experimental studies. A detailed theoretical analysis, based on the concept of harmonic impedance, establishes the suitability of inductor current feedback versus output current feedback with respect to inverter power quality. Experimental results, obtained from a purpose-built 500-W, three-level, half-bridge inverter with an L-C-L output filter, verify the efficacy of inductor current as the feedback variable, yielding an output current total harmonic distortion (THD) some 29% lower than that achieved using output current feedback. A feed-forward grid voltage disturbance rejection scheme is proposed as a means to further reduce the level of low-order current harmonic distortion. Results obtained from an inverter with inductor current feedback and optimized feed-forward disturbance rejection show a THD of just 3% at full-load, representing an improvement of some 53% on the same inverter with output current feedback and no feed-forward compensation. Significant improvements in THD were also achieved across the entire load range. It is concluded that the use of inductor current feedback and feed-forward voltage disturbance rejection represent cost-effect mechanisms for achieving improved output current quality.

Three-Phase Step-Up/Down Current-Source Rectifier with Sinusoidal Supply Currents.

This paper presents three-phase current-source rectifier, which is based on the cascade combination of buck-boost and buck DC-DC converters and has the step-up and step-down characteristics of output current, whereas the conventional current-source rectifiers have no step-up characteristics. The introduction of pulsewidth modulation makes it possible to obtain the widely controllable output current and to sinusoidally waveshape the supply currents with a near unity power factor. Three insulated-gate bipolar transistors (IGBTs) provide the necessary control over the currents for unidirectional power flow. To countract the oscillatory transient operations with single-loop output current feedback, the tested system includes the minor loop for the current in flyback inductor. The controllability of the output current is examined for the variation in its reference. The experimental results and state-space simulations for steady-state and transient conditions confirm the relevance of the presented configuration as an useful rectifier with near unity power factor sinusoidal supply currents and satisfactory output current regulation.