Capacitor Voltage Changes Research Articles

Capacitor voltage change rate based fault localization strategy for modular multilevel converters

Capacitor voltage change rate based fault localization strategy for modular multilevel converters

Development of a Bidirectional DC–DC Converter with Rapid Energy Bidirectional Transition Technology

Bidirectional DC–DC converters are key devices in the DC distribution system and the energy storage system (ESS). It is important to consider the safety of the elements in the converter for rapid conversion of the power direction. Damages may occur to the power-related components in the circuit if the direction of the inductor current or the capacitor voltage changes instantaneously. To make the power flow change smoothly and quickly, this research proposed a bidirectional DC–DC converter with rapid energy transition technology implemented in the circuit architecture. The rapid energy bidirectional transition technology added a resonance path based on the LC resonant circuit, allowing rapid energy conversion through the resonance path. Therefore, the energy in the energy storage element could be quickly converted without causing circuit surges. Analyses of the converter operating in the step-up mode, the step-down mode, and the transition operation mode are presented. The proposed circuit architecture had a high voltage-conversion ratio and a simple architecture. A prototype bidirectional DC–DC converter with a full load of 500 W, a low side voltage of 24 V, and a high side voltage of 200 V was developed to prove the concept. The feasibility of the rapid energy bidirectional transition technology was verified by the simulation results and experimental results using the prototype converter. The maximum efficiencies in the step-up mode and the step-down mode were 95.3% and 93.8% respectively. Under full-load conditions, the transient time of the energy transition from the step-up mode to the step-down mode was 17.7 μs, and the transient time of the energy transition from the step-down mode to the step-up mode was 19.3 μs.

Parallel Open-Circuit Fault Diagnosis Method of a Cascaded Full-Bridge NPC Inverter With Model Predictive Control

In this article, a parallel open-circuit fault diagnosis method (POFDM) is proposed for cascaded full-bridge neutral-point clamped inverters (CFNPCIs) employing the model predictive control. It can be inferred that the open-circuit fault will cause abnormal capacitor voltage changes in the submodule. Therefore, eight count variables are configured to correspond to the eight Insulated gate bipolar transistors (IGBTs) for each submodule. In the POFDM, the eight variables count all the abnormal changes, and the variables corresponding to the healthy switches are cleared to 0. Then, the switch whose corresponding variable is first added to the maximum is diagnosed as the fault device. Because the independent and parallel structure is adopted, the proposed POFDM can diagnose not only a single fault but also multiple faults in different submodules. And it can be easily extended to CFNPCIs with arbitrary submodules due to its parallel structure and modular software. Finally, the feasibility and validity of the POFDM are verified on the experimental prototype.

A New Modulation Scheme for a Four-Level Single Flying Capacitor Converter

This article proposes a four-level single flying capacitor converter (4L-SFCC) topology along with a new modulation technique. The proposed four-level single flying capacitor converter (4L-SFCC) topology has only one flying capacitor in each phase. This results in no redundant switching states associated with each voltage levels, and thus conventional modulation techniques cannot fulfill the capacitor voltage balancing requirement. Therefore, a new modulation technique is also proposed in this article in which the average capacitor current is set to zero during each sampling time, resulting in no net capacitor voltage change and hence voltage balancing during each sampling period. The proposed modulation scheme does not significantly increase the switching frequency and has equal switching frequency for all semiconductor switches, which is an advantage in terms of equalized switching losses. Simulation results are used to further evaluate the performance of the proposed converter. A laboratory-type experimental setup is used to validate the theoretical results.

Research on Capacitance Selection for Modular Multi-Level Converter

The capacitance of sub-module capacitor has a great impact on the cost, size, and operation of the modular multi-level converter (MMC). However, the capacitance selected by the existing methods does not always meet the requirement in practice. To find out the reasons for this improper selection, the existing selection methods are analyzed in this paper. Based on the analysis, a new capacitance-selection method is proposed. In the new method, the problems of the existing methods are solved, and the calculation of capacitor voltage is based on a proposed capacitor-voltage model whose calculation error can be smaller than 0.1%. Furthermore, the Secant Method is used in the capacitance selection to quickly calculate the required capacitance. The new capacitance-selection method can be applicable to the MMC not having a circular operating region. In addition, this paper found that the capacitor voltage and the capacitor-voltage ripple reach their maximum values when the power-factor angles of the MMC are –π/2 and π/2, respectively, and the dc component in the capacitor voltage changes according to the operating condition of the MMC. Finally, the proposed selection method and theoretical analyses are verified by both simulation and experiment.

Experiment and Research of Energy Depositing Rate of Exploding Wires Under Different Initial Capacitor Voltages for Thin Film Production

Thin film produced by exploding wires is affected by many factors; one of such factors is the energy depositing rate. The research in this paper focused on measuring the voltage and current of an exploding copper wire that was 15 cm long and 0.3 mm in diameter. The time-dependent wire resistance and the energy deposited into the wire were deduced from the measured voltage and current, and the maximum wire resistance was observed at the time when part of the wire mass was heated up to the boiling point, which was called critical time and critical point, respectively. From the experiment, the following statements could be surmised: The maximum wire resistance increases from about 224.4 to 266.1 mΩ, and the critical time reduces from about 8.8 to 7.2 μs when the initial capacitor voltage changes from 10 to 20 kV, as well as the fact that the mean energy depositing rate varies from about 13.3 to 27.6 J/μs before the critical point. Also, thin copper film is prepared by this method under initial capacitor voltages of 10,15, and 20 kV, respectively. The SEM micrographs of the film samples are also presented and analyzed. This result demonstrates that a faster energy depositing rate can help to heat up the wire mass more uniformly and more wire mass would evaporate before the critical time to create a greater wire resistance at the critical point, and the film surface is more uniform and has less void fraction.

A Fully Soft Switched Two Quadrant Bidirectional Soft Switching Converter for Ultra Capacitor Interface Circuits

This paper describes a two quadrant bidirectional soft switching converter for ultra capacitor interface circuits. The total efficiency of the energy storage system in terms of size and cost can be increased by a combination of batteries and ultra capacitors. The required system energy is provided by a battery, while an ultra capacitor is used at high load power pulses. The ultra capacitor voltage changes during charge and discharge modes, therefore an interface circuit is required between the ultra capacitor and the battery. This interface circuit must have good efficiency while providing bidirectional power conversion to capture energy from regenerative braking, downhill driving and the protecting ultra capacitor from immediate discharge. In this paper a fully soft switched two quadrant bidirectional soft switching converter for ultra capacitor interface circuits is introduced and the elements of the converter are reduced considerably. In this paper, zero voltage transient (ZVT) and zero current transient (ZCT) techniques are applied to increase efficiency. The proposed converter acts as a ZCT Buck to charge the ultra capacitor. On the other hand, it acts as a ZVT Boost to discharge the ultra capacitor. A laboratory prototype converter is designed and realized for hybrid vehicle applications. The experimental results presented confirm the theoretical and simulation results.

A Current Distortion Compensation Scheme for Four-Switch Inverters

The four-switch inverter, having a lower number of insulated gate bipolar transistors(IGBTs), has been studied for the possibility of reducing the inverter cost. But it has a limited performance in the low-frequency region, because the balance among the phase currents collapses due to the fluctuation of the center tap voltage of the DC-link capacitors. This problem could be solved if the DC-link capacitance is infinitely large, but it is a costly solution. In this paper, this problem is looked at from the perspective of source impedance and the voltage variation caused by the current flow through the capacitors. The source impedance of the center tap is large compared with other normal IGBT arms. This causes an asymmetry among the three voltage sources, resulting in phase current distortion and unbalance. Second, the capacitor voltage change caused by current flow is another source of current distortion and unbalance. The voltage errors are derived, and based on them, a compensation method is proposed. Effectiveness of the proposed method is demonstrated by the simulation and experimental results.

A single-switch AC/DC converter with voltage regulated storage capacitor

A single-stage power-factor-corrected AC/DC converter (SSPFC) usually causes high voltage stress on the intermediate storage capacitor, due to the lack of control of this voltage. The storage capacitor voltage varies largely with line voltage, and load current and is usually higher than the peak line voltage. This paper presents a new single-switch SSPFC based on a flyback topology for which the storage capacitor voltage is loosely regulated by the output voltage. Without using extra power switches to increase the control dimension, the proposed converter uses a flyback converter with dual-output transformer to achieve the control purpose. The range of storage capacitor voltage change against the change of input voltage and load current is significantly reduced. Moreover, the maximum storage capacitor voltage can stay below the peak line voltage at high line condition. Experimental results verifying the operation of the proposed SSPFC are also reported.