Safe-commutation Strategy Research Articles

New Modulation Technique for a Family of Z‐Source AC–AC Converters With a Safe Commutation Strategy

New Modulation Technique for a Family of Z‐Source AC–AC Converters With a Safe Commutation Strategy

A family of single‐phase boost AC‐AC converters based on impedance network cells with symmetric bipolar operation

SummaryIn this paper, a family of single‐phase boost AC‐AC converters based on impedance network cells with symmetric bipolar operation is proposed. The proposed converters exhibit a range of voltage gains as a result of non‐inverting and inverting boost operations, which are influenced by the input voltage and various impedance source cells. The converters leverage inherent commutation to address the commutation problem without requiring snubber circuits and safe commutation strategy. Additionally, high‐speed switching is achieved by preventing power MOSFETs' body diode conduction and related poor reverse recovery issues. The design incorporates a simple and flexible switching strategy to produce step‐change frequency at the output, using only four switches to achieve both step‐changed frequency operation and output voltage regulation. Furthermore, by reducing the total stored energy of passive components, the proposed converters effectively decrease volume, weight, and cost. The operational modes of the converters are elucidated, and their key relationships are summarized in tabular format. Finally, experimental validation through a laboratory prototype confirms the performance accuracy of the proposed converters at frequencies 25, 50, and 100 Hz.

A Direct AC–AC Switched-Capacitor Converter With Input-Series Output-Parallel and In-Phase/Out-of-Phase Capabilities

In this paper, a bidirectional transformerless multi-level direct ac-ac switched-capacitor converter (SCC) is proposed with both in-phase and out-of-phase output voltage capabilities. This converter can be used for series compensator applications, such as dynamic voltage restorers (DVRs), to compensate grid voltage sag and swell phenomena by incorporating its input-output common ground. Its input-series output-parallel capabilities reduce voltage and current stresses of the converter components in multi-level configurations, which lead to higher efficiency and lower manufacturing costs for high-power applications. Furthermore, proper utilization of an interleaving control scheme can provide lower input and output currents ripples values and smaller filter sizes. The proposed SCC operation principle and its mathematical analyses are given, in detail. Additionally, its switching patterns have been developed with a safe-commutation strategy to obtain various output voltages. Subsequently, both single- and two-level SCC prototypes are implemented as series compensators for DVR applications. Moreover, experimental results are given to generate in-phase and out-of-phase regulated output voltages. The experimental results at 2 kW show that the output voltage changes are less than 3% under the steady-state conditions, despite the large input voltage changes from 140 to 300V. The converter maximum efficiency value is equal to 95.4% at maximum output power value.

A bidirectional single‐phase Z‐source AC‐AC converter with reduced voltage stress on switches and safe commutation strategy

SummaryIn this paper, a bidirectional single‐phase Z‐source ac‐ac converter is proposed. Common features between the proposed converter and its counterparts are the capability of boost in‐phase, buck out‐of‐phase, and common ground. The output voltage gain of the proposed converter is improved compared with previous Z‐source ac‐ac converters, while the total standing voltage (TSV) of the switches has also been significantly reduced. Owing to the reduction of voltage stress of semiconductor elements, the switching device power (SDP) of the proposed converter is reduced. As a result, the cost of the converter is also reduced. A bidirectional switch is replaced instead of the output filter inductor to reduce the volume and weight of the converter. A safe commutation strategy is applied to eliminate voltage spikes on switches. Furthermore, the proposed converter can transfer power in both directions. By evaluating the converter in different operating modes, the key relationships and parametric design of the converter are presented. A laboratory prototype is made to confirm the proposed converter performance.

An isolated high‐gain impedance source‐based single‐phase AC–AC converter with safe commutation and variable output frequency

SummaryConventional AC–AC converters have some critical disadvantages, such as inappropriate input and output power quality features, employing snubber circuits to suppress the voltage stress on high‐frequency switches without a safe commutation method that increases the converter size and cost, confined voltage gain, and using huge passive components. This paper proposes an isolated high‐gain single‐phase AC–AC converter with variable output frequency, extended voltage gain, and a safe commutation strategy. Ten semiconductors are only employed for the proposed topology that only two switches and two diodes are operated with high‐frequency PWM signals. This new topology has noteworthy specifications like continuous input and output current, voltage isolation, extended inverting and non‐inverting buck‐boost modes, and frequency regulation. The high voltage gain has been achieved with small values for passive components in the impedance source section, reducing the volume and cost of the converter. The impedance source‐based section of the converter comprises one input inductor, two coupled inductors, and two capacitors in each converter leg. A safe commutation strategy has been designed and applied to the converter to mitigate the voltage and current stresses without extra snubber circuits. The number of semiconductors that simultaneously conduct in each state of the converter has been reduced. So, the conduction loss of the converter diminished. The achieved efficiency equals 95.5% and 93.5% in step‐down and step‐up modes, respectively, validated by an accurate power loss model and experimental analysis. An experimental prototype has been designed to confirm the performance of the suggested converter. Results confirm the theory and operation of the proposed converter.

A Class of Bidirectional Single-Phase Z-Source AC–AC Converter With Continuous Input Current and Reduced Component Count

In this paper, a class of single-phase Z-source ac-ac converters is proposed. Share ground and continuous input current are the common features of the proposed converters. The proposed converters have unique features that differentiate them from their counterparts. These features include the buck operation in both in-phase and out-of-phase mode, being bidirectional, the minimum component counts, the ability to boost without duty cycle limitation, and reducing total power ratting of switches is known as switching device power (SDP) parameter. Reducing SDP leads to a reduction in the voltage and current rating of the semiconductors and consequently to a reduction in converter' cost. A safe commutation strategy is applied to eliminate voltage and current spikes on the switches without snubber circuits. The experimental results verify the performance of the laboratory prototype of the proposed converters.

A High-Frequency Isolated Four-Switch Single-Phase Quasi-Z-Source AC–AC Converter With Inherent Commutation and Step-Changed Frequency Operation

In this article, a high-frequency isolated single-phase quasi-Z-source ac–ac converter is proposed. This converter can generate a broad range of output voltage while also allowing the output frequency to be changed. The configuration is divided into two similar subcircuits in a way that only one unidirectional switch exists in each subcircuit, and complementary switches are removed. Therefore, the proposed converter has inherent commutation without the necessity to apply any delays or, safe commutation strategies. Two switches on the output side are responsible for changing the frequency. So, voltage regulation and changeable frequency are obtained with only four unidirectional switches and a simple switching strategy. Electrical isolation is also acquired via employing a compact-sized high-frequency transformer. All the aforementioned features result in the proposed converter's small size, compact structure, low cost, high reliability, and high power density. A comprehensive analysis of the circuit operation principles is discussed. Then, a 100 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> /50 Hz laboratory prototype is implemented to indicate the accuracy of theoretical results.

A high‐frequency transformer‐based buck‐boost AC‐AC converter with high efficiency and wide range conversion ratio for DVR application

AbstractThis paper proposes an isolated single‐phase impedance‐source ac‐ac converter with a wide voltage conversion ratio and a safe commutation strategy. The new topology has noteworthy features like continuous input current, galvanic isolation, and non‐inverting and inverting buck‐boost modes with a wide voltage coverage. Also, only one switch operates with high‐frequency PWM signals, reducing the semiconductor switching loss. The galvanic isolation has been achieved by employing a high‐frequency transformer (HFT), eliminating low‐frequency transformers (LFT) from the topology. A safe commutation strategy has been designed and applied to the converter to mitigate the voltage and current spikes on power switches, which eliminates additional snubber circuits. A prototype has been designed to validate the suggested converter performance in stand‐alone and DVR applications. The peak obtained efficiency equals 96.1% and 94.9% in inverting and non‐inverting step‐down and step‐up modes, respectively, validated by an accurate power loss model and experimental results. Experimental results confirm the theory and operation of the proposed converter.

A Trans-Inverse Magnetic Coupling Single-Phase AC-AC Converter

This paper introduces a new single-phase AC-AC converter based on an impedance source circuit. Like the existing single-phase impedance source AC-AC converters, it has the buck-boost ability and direct ac conversion. The input and output voltage possesses the same ground, and the phase angle is maintained and reversed smoothly. The presented converter utilizes a coupled transformer which allows the designer to exploit the transformer’s turns ratio as a variable to attain the desired output voltage. Additionally, the used transformer provides an option to obtain higher voltage gain by decreasing the turns ratio. Hence, smaller size of the coupled inductors is required for the higher voltage cases. To eliminate the switching voltage and current spikes on the power switches, a safe commutation strategy is used instead of utilizing snubber circuits. Furthermore, the input current is continuous and sinusoidal with low harmonics thanks to embedding the input inductor in series with the input source. Additionally, a dynamic voltage restorer is presented based on the proposed converter to compensate the voltage sag and swell faults. Simulation results are provided to evaluate the theoretical analysis. Finally, a laboratory prototype has been fabricated to demonstrate the validation of the presented converter.

A single‐phase q‐source AC‐AC converter with continuous input current, safe commutation feature, and reduced bidirectional switch counts

AbstractThis paper proposes a single‐phase q‐source AC‐AC converter with a wide and flexible voltage gain. The converter only incorporates two bidirectional power switches. This new topology has many bold features like continuous input current, non‐inverting boost, and inverting buck modes. The high voltage gain is achieved by utilizing a coupled inductor in the converter. The converter gain is increased by the turns ratio of the coupled inductor proportionately. The proposed q‐source comprises one input inductor, one coupled inductor, and one capacitor, which are less than other competitors. In order to reduce the voltage and current stresses on power semiconductors, a safe commutation strategy for the converter is also introduced that eliminates snubber circuits. The input and output filters are no longer required due to the proposed converter specifications. So the volume and cost of the converter are decreased significantly. The achieved efficiency equals 94.5% due to utilizing the proposed optimized topology and switching strategy. In this paper, the operating principle of the converter is presented in detail. An experimental prototype has been designed to examine the performance of the suggested converter. Results confirm the theory and operation of the proposed topology.

Magnetically Coupled Single-Phase AC-AC Converter With Reduced Number of Passive Components

An AC-AC direct single-phase converter based on an impedance network is presented in this manuscript. It possesses all privileges of similar impedance source AC-AC converters such as buck-boost ability, maintaining or reversing the phase angle and sharing the same ground between input and output voltage. Furthermore, a magnetic coupling is exploited to provide high voltage gain by adjusting its turns ratio along with the duty cycle. A safe commutation strategy is implemented to avoid current and voltage spikes across switches needless to utilize snubber circuits. The presented converter offers continuous input current, and ac to ac conversion is done directly without using dc storage, making it appropriate for dynamic voltage restorer to compensate voltage sags and voltage swells. In addition, LC input and output filters are eliminated thanks to impedance network structure. In this regard, the presented topology offers good features in size and cost by reducing passive components compared to similar structures. Also, the comparative investigation shows that the proposed converter benefits from superior operational ranges among similar well-known topologies for the same conditions. Finally, theoretical analyses and operation modes of the proposed converter are discussed and testified by both simulation and experimental results.

A safe commutation strategy based on LRCD snubber circuit for five‐level active neutral point clamped inverter

Spike voltage generated during the turn-off transient is one of the key threats to IGBT. When the traditional RCD snubber is applied in the five-level active neutral point clamped (5L-ANPC) inverter to clamp the spike voltage, a transient large current caused by the charging of the snubber capacitor will increase power losses and current stresses of IGBTs. This study proposes a LRCD snubber for the 5L-ANPC inverter. It can effectively suppress the spike voltages and the transient large currents of IGBTs, thus reducing the power loss. For the 5L-ANPC inverter, the traditional commutation method also poses a threat to IGBTs. Near the zero-crossing point of the output voltage, the traditional commutation method will cause the IGBTs that operate at the carrier frequency to withstand three times of the rated voltage. This study proposes a safe commutation strategy based on the LRCD snubber. When the output voltage passes through zero-crossing point, the proposed commutation strategy uses four transition switching states to avoid the breakdown of IGBTs induced by overvoltage. It can ensure that all the IGBTs operate in the safe operating area (SOA). Experimental results are presented to verify the validity of the proposed method.

Single‐phase AC‐AC Z‐source converter based on asymmetrical gamma structure with continuous input current and safe commutation strategy

Single‐phase AC‐AC Z‐source converter based on asymmetrical gamma structure with continuous input current and safe commutation strategy

A Class of Single-Phase Z-Source AC–AC Converters With Magnetic Coupling and Safe-Commutation Strategy

This article proposes a family of single-phase transformer based Z-source ac-ac converters (ZSACs). The proposed converters inherit all the benefits associated with the classical single-phase ZSAC, which are providing the buck-boost operation, maintaining or reversing the phase angle, and improving reliability. The proposed converters also have the improved input profile, continuous input current, and shared ground between input and output. Moreover, they use the turns ratio of transformers to produce the required voltage gain, which can be tuned by either increasing or decreasing the turns ratio of transformer depending on the configuration of Z-source structure. A safe-commutation strategy is also presented to achieve the soft commutation, which eliminates the voltage and current spikes without any additional snubber circuits. The features of proposed converters are comprehensively discussed with the detailed theoretical analysis. Thereafter, its performances are evaluated by both simulation and experimental tests.

Novel High-Efficiency Frequency-Variable Buck–Boost AC–AC Converter With Safe-Commutation and Continuous Current

In this article, a novel high-efficiency single-phase frequency-variable buck–boost ac–ac converter is proposed. AC input voltage can be bucked and boosted with step-changed frequency operation. In the topology, only six switches and total ten semiconductors are utilized, which is at the lowest level among existing works. Only two of the switches are operating at high frequency and only one switch is switched on and off in each switching period, Therefore, the proposed converter has less power loss and features high efficiency. With simple and flexible control strategy, the converter is immune of commutation problem and thus no additional safe-commutation strategy or snubber circuit is needed. Power density of proposed converter is high considering the volume metric of the energy storage components. Moreover, the input current of proposed ac–ac converter is continuous. Therefore bulky input filter required by other existing works is totally removed, which further improve the power density. Operation principles and circuit analysis is provided. To verify the performance, a 200-W laboratory prototype is constructed and experiments are conducted in operation of buck and boost mode at step-changed output frequency of 30, 60, and 120 Hz.

Compensating Voltage Sag in Distribution Systems Using Single Phase Quasi-Z-Source AC/AC Converter

The utilization of a single-phase Quasi-Z-source AC/AC converter for voltage sag mitigation is introduced in this paper. The suggested topology can compensate voltage sag up to 50%, which is an advantage in comparison with traditional structure, as most voltage sags are not more than 50%. The suggested topology has superior characteristics compared to the conventional topologies and has the following specifications: using the same ground for sharing output and input voltages; functioning in the continuous current mode; employing direct AC/AC converter; applying safe-commutation scheme; the ability to expand of the output voltage range increasing reliability and reducing the harmonics in the output voltage. Because of using safe commutation strategy, as shown in results, no ripple and spike exist in the output voltage. The hardware prototype of the offered structure has magnificently been developed to obtain the experimental results. The experimental results were obtained based on the resistive and inductive loads for a few different sags in the input voltage. The results presented showed good performance despite the simplicity in the control scheme. Moreover, they confirm the good performance of the proposed topology.

Single-Phase Safe-Commutation Trans-Z-Source AC–AC Converter With Continuous Input Current

This paper extends the trans-Z-source concept to ac–ac power conversion and proposes a novel single-phase trans-Z-source ac–ac converter. The proposed converter retains the advantages of its existing Z-source/quasi-Z-source counterparts in which the output voltage can be boosted and in-phase with the input voltage or bucked/boosted and out-of-phase with the input voltage. Meanwhile, a coupled inductor is introduced into the proposed converter, and the turns ratio of the coupled inductor become another control variable beside the duty cycle to obtain extreme voltage gain. Importantly, the input voltage and output voltage share the same ground, and the input current could be continuous, which is a benefit for reducing input current harmonics. Most important of all, the safe-commutation strategy for the proposed converter is deep developed, and analytic expression between the switches state and loop circuit voltage/current condition is derived out. Hence, the current/voltage ripple can be avoided without a loss snubber circuit, which benefits a reduced cost and enhanced reliability. The performance analysis is discussed in consideration of parasitic resistance, and system parameter design is optimized based on theoretic calculation. A laboratory prototype is fabricated, and experiments are performed to verify the validation of the proposed converter.

A New Class of Single-Phase High-Frequency Isolated Z-Source AC–AC Converters With Reduced Passive Components

In this paper, a class of single-phase Z-source (ZS) ac–ac converters is proposed with high-frequency transformer (HFT) isolation. The proposed HFT isolated (HFTI) ZS ac–ac converters possess all the features of their nonisolated counterparts, such as providing wide range of buck-boost output voltage with reversing or maintaining the phase angle, suppressing the in-rush and harmonic currents, and improved reliability. In addition, the proposed converters incorporate HFT for electrical isolation and safety, and therefore can save an external bulky line frequency transformer, for applications such as dynamic voltage restorers, etc. The proposed HFTI ZS converters are obtained from conventional (nonisolated) ZS ac–ac converters by adding only one extra bidirectional switch, and replacing two inductors with an HFT, thus saving one magnetic core. The switching signals for buck and boost modes are presented with safe-commutation strategy to remove the switch voltage spikes. A quasi-ZS-based HFTI ac–ac is used to discuss the operation principle and circuit analysis of the proposed class of HFTI ZS ac–ac converters. Various ZS-based HFTI proposed ac–ac converters are also presented thereafter. Moreover, a laboratory prototype of the proposed converter is constructed and experiments are conducted to produce output voltage of 110 Vrms / 60 Hz, which verify the operation of the proposed converters.

A Single-Phase Buck Matrix Converter With High-Frequency Transformer Isolation and Reduced Switch Count

In this paper, a new type of matrix converter (MC) also called a single-phase high-frequency transformer isolated (HFTI) buck MC is proposed. The proposed converter can provide step-down operation of the input voltage with various types of output voltages such as in-phase and out-of-phase output voltages, rectified (or positive) output voltage, and output voltage with step-changed frequency. By incorporating HFT isolation, the proposed MC saves an extra bulky line frequency transformer, which is required for the conventional MCs to provide electrical isolation and safety, when used in application such as dynamic voltage restorers, etc. Two different circuit variations of the proposed HFTI MC are presented with and without continuous output currents, with the latter having less passive components. The safe-commutation strategy is also employed for the proposed HFTI MC to provide current path for the inductor during dead-time, which avoids switch voltage spikes without adding any snubber circuits. The operation principle and circuit analysis of the proposed MC are presented, and switching strategies are also developed to obtain various output voltages. Moreover, a prototype of the proposed MC is fabricated, and experiments are performed to produce in-phase/out-of-phase and rectified output voltages, and output voltage with step-changed frequency.

A Family of High-Frequency Isolated Single-Phase Z-Source AC–AC Converters With Safe-Commutation Strategy

This paper extends the high-frequency transformer isolation concept to the Z-source (ZS) ac–ac converters and introduces a new family of high-frequency transformer isolated (HFTI) ZS ac–ac converters. The proposed HFTI-ZS converters retain all the benefits of their existing nonisolated counterparts, such as providing a larger range of output voltage with buck–boost functionality, reversing or maintaining the phase angle, reducing the in-rush and harmonic currents, and improving reliability. In addition to these benefits, the high-frequency (20 kHz) transformer (HFT) in the proposed ZS ac–ac converters provides the electrical isolation and safety with high power density as it eliminates the need for bulky and heavy line frequency (50 or 60 Hz) transformer for galvanic isolation, in applications such as dynamic voltage restorers, etc. The dc-blocking capacitor added in series with the HFT results in only ac voltage applied across its windings, which avoids its saturation. Various ZS-based HFTI ac–ac converters are proposed in this paper, and to verify their operation and advantages, example of quasi-ZS (qZS)-based isolated ac–ac converter is considered in detail. The commutation strategy is also developed to achieve the safe commutation, which avoids the current and voltage spikes without using any RC snubber. A 200-W laboratory prototype of HFTI-qZS ac–ac converter is fabricated and experiments are performed to validate the advantages of the proposed ac–ac converters.