Partial Power Converter Research Articles

A Two-Switch Forward Partial Power Converter for Step-Up/Down String PV Systems

This letter proposes a novel step-up/down series-connected partial power converter (S-PPC) for string photovoltaic (PV) applications. The proposed bidirectional two-switch forward (BFwd) dc–dc S-PPC performs partial power processing, reducing component stresses and losses. The PV active power is unidirectional, but the BFwd alone operates with bidirectional active power flow; with bidirectional current at the input and bipolar voltage at the output, characterizing two distinct modes of operation. To validate the analysis, a 950-W system prototype of the novel BFwd S-PPC was implemented experimentally for a realistic PV application example, where the transformer turns ratio was designed to decrease the nonactive power processed, resulting in a high power factor ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ &gt;\text{0.75}$</tex-math></inline-formula> ). The prototype reached a rated/peak system efficiency of 98.68%/99.48%. Compared with previously published step-up/down S-PPCs, the BFwd S-PPC employs fewer active switches (and gate drivers), which reduces its complexity and cost; but moreover, increases its performance. In fact, the proposed BFwd S-PPC presents higher efficiencies than most step-up/down S-PPCs in the literature.

A Proposed Controllable Crowbar for a Brushless Doubly-Fed Reluctance Generator, a Grid-Integrated Wind Turbine

Brushless doubly fed reluctance generators (BDFRGs) are hopeful generators for using inside variable speed wind turbines (VSWTs), as these generators introduce a promising economical value because of their lower manufacturing and maintenance costs besides their higher reliability. For integrating WT generators, global networks codes require enabling these generators to stay connected under grid disturbances. The behavior of the BDFRG is strongly affected by grid disturbances, due to the small rating of the used partial power converters, as these converters cannot withstand high faults currents which leads to quick tripping of BDFRG. VSWTs can be safeguarded against faults using the crowbar. Usually, the conventual crowbar is shunt connected across the converter to protect it, but this configuration leads to absorbing reactive power with huge amounts from the grid, leading for more voltage decaying and more power system stability deterioration. This study proposes a simpler self-controllable crowbar to enhance the ability of the BDFRG to remain in service under faults. The operation technique of the proposed crowbar is compared to other crowbar operation techniques, the effectiveness of the proposed system would be analyzed. Through the simulation results and behavior analysis, the proposed crowbar technique demonstrates a decent improvement in the conduct of the studied system under faults.

Investigation of a converter fault for a DFIG wind turbine and analysis of the resulting gearbox component loads

Abstract Doubly fed induction generators are widely used in wind turbines in the field. The main advantage is the significantly lower cost of the partial scale power converter compared to the full scale power converter. Converter faults in wind turbines lead to significant generator torque excitations that lead to dynamic loads in the drivetrain. Dynamic loads and changing rotational speeds can lead to gearbox damages. The generator torque excitations have the highest influence on the high speed shaft torque due to the coupling to the generator. Gearbox damages occur mainly on the components of the high speed shaft. Thus, in this paper the influence of a converter fault in a wind turbine with doubly fed induction generator on the high speed shaft component damages is investigated. It is shown that the converter fault can induce dynamic torque excitations with a maximum increase to around 2.5 times rated torque. Due to the resulting dynamic gearbox loading the safety against scuffing in the high speed gear stage decreases by maximum 19 percent. The smearing risk in the high speed shaft bearings increases to around 2 times the value during rated power production.

Demystifying Non-Isolated DC–DC Topologies on Partial Power Processing Architectures

This paper discusses the possibility of achieving partial power processing with non-isolated DC–DC topologies. To this end, partial power converter architectures are seen as an interesting solution for reducing the power processed by the converter. We observed via simulations that single-inductor non-isolated topologies cannot achieve partial power processing since the obtained current and voltage waveforms were the same as those found in a full-power converter. However, when using double inductor non-isolated topologies, reduced current and improved efficiencies were achieved. In order to confirm the results obtained from the simulations, single- and double-inductor topologies were tested experimentally. Finally, it was concluded that a double-inductor non-isolated topology can improve its performance by using partial power processing.

A Full-Bridge Partial-Power Processing Converter for Three-Stage Small Wind Turbine Systems

&#x0D; In order to process the energy generated by small wind turbines (SWT) in grid-connected systems, three-stage configurations (rectifier, dc-dc converter, and inverter) have been suitable due to low cost of three-phase diode rectifiers, facility to perform the maximum power point tracking in dc-dc converters, and power decoupling between the grid and wind generator. As drawback, the three-stage solutions can present higher losses in relation to two-stage systems due to the additional converter. To reach a better efficiency, partial-power converters (PPC) can be used, in which only a part of the power generated by the SWT is processed by the converter. In this method, topology, power/voltage levels, and the operating range characteristics can impact the power handled by the converter. Since the experimental analysis of PPC applied to SWT systems remains to be investigated in the literature, this paper analyzes the Full-Bridge with Zero-Voltage Switching operating as PPC in SWT systems connected to the single-phase grid. In order to evaluate the performance of the proposed structure, experimental results are verified for a 1.5 kW SWT, in which the FullBridge PPC processes only 70% of the generated power. In relation to the full-power processing, the partial-power processing has reduced the losses in 35.9%.&#x0D;

Recent development and future trends of resonance in doubly fed induction generator system under weak grid

In recent years, the wind energy conversion system faces significant challenges for stable and secure operations, especially in the weak grid due to high penetration of wind energy into the grid. Doubly fed induction generator (DFIG) is one of the most popular wind turbine generators due to its cost-effective variable speed and low-cost partial size power converter. Impedance interaction between series or parallel compensated weak grid and the DFIG system causes sub-synchronous resonance (SSR) and high-frequency resonance (HFR). A comprehensive review of the state-of-the-art SSR and HFR in the DFIG system under a weak grid is presented. First, based on features, such as supplementary hardware, linear and non-linear control strategies, the resonance damping techniques are classified into different types. Then, the advantages and disadvantages of SSR analysis and damping techniques techniques are put forward. Moreover, the latest development of state-of-the-art SSR and HFR analysis and damping techniques are supplemented. Finally, the contemporary and future trends of the SSR and HFR techniques are discussed.

High-Performance Buck-Boost Partial Power Quasi-Z-Source Series Resonance Converter

Power converters are essential components of renewable energy systems. The converter efficiency decides the overall system efficiency. In a full power converter, its components process the whole input power, even if it is not mandatory. Partial power processing (PPP) is an exciting concept for improving overall system efficiency and reducing system cost and size. In PPP, the converter processes the amount of power needed to be processed. This work proposes a PPP based on the high-performance Quasi-Z-Source Series Resonance converter (QZSSRC). The proposed partial power converter belongs to the series input parallel output (SIPO) architecture. Previous publications in this area mainly consider converters with step-down voltage regulation, like the phase-shifted full-bridge converter. They underperform in this application due to operating at the maximum power when bucking the voltage the most. This paper studies two other types of dc-dc converters, buck-boost and boost, in SIPO partial power converters. Theoretical operation principles are corroborated with full experimental results given and analyzed. A 300 W prototype of the QZSSRC is used to convert the power of up to 2 kW in a PPP system with overall system efficiency values of over 99%.

A Review of Series-Connected Partial Power Converters for DC–DC Applications

This article presents a review of series-connected partial power converters (S-PPCs) for dc–dc applications, which allows carrying out the partial power processing (PPP), whose main goal is to achieve a reduction of the power processed by the converters. An analysis of the S-PPCs’ characteristics, topologies, and applications concerning the active and nonactive power processing is presented. The power processing factor (PPF) is then defined, which refers to the active power and depends exclusively on the voltage regulation range. The so-called Fryze power factor (PF) is used to evaluate the nonactive power processed, which depends on the topology voltage and current waveforms. Due to the lack of research around step-up/-down S-PPCs, this article presents the restrictions and requirements for the design of this type of S-PPCs. Finally, it is demonstrated that the turns ratio of magnetic devices can be optimized to reduce the nonactive power and improve the converter Fryze PF, ensuring PPP. In order to validate the analyses, two 2200-W prototypes were built and evaluated for a photovoltaic (PV) application example. Experimental results show that the reduction of both the active and nonactive power processed by the S-PPCs results in lower component ratings and higher efficiencies.

Partial Power Processing Based Converter for Electric Vehicle Fast Charging Stations

This paper focuses on the design of a charging unit for an electric vehicle fast charging station. With this purpose, in first place, different solutions that exist for fast charging stations are described through a brief introduction. Then, partial power processing architectures are introduced and proposed as attractive strategies to improve the performance of this type of applications. Furthermore, through a series of simulations, it is observed that partial power processing based converters obtain reduced processed power ratio and efficiency results compared to conventional full power converters. So, with the aim of verifying the conclusions obtained through the simulations, two downscaled prototypes are assembled and tested. Finally, it is concluded that, in case galvanic isolation is not required for the charging unit converter, partial power converters are smaller and more efficient alternatives than conventional full power converters.

A High Step-Up Partial Power Processing DC/DC T-Source Converter for UPS Application

In this paper, a new modified structure of a DC/DC T-source converter is proposed. Since the proposed converter provides high voltage gain, it is suitable for photovoltaic integration into uninterruptible power supply (UPS) systems. The proposed structure employs partial power processing technique to increase the output voltage as well as efficiency without requiring new hardware. Partial power converters (PPCs) process only a fraction of flowing power while the remaining power directly flows through output. This generally causes an improvement in efficiency and output voltage. A total of two structures are presented: conventional partial power T-source converters and improved partial power T-source converters. The key advantage of the improved partial power converter is a higher voltage gain. Furthermore, it reduces the voltage and the current stresses on switches and diodes. The steady-state operation principles are described for both converters and the governed rules and equations are derived. The PPCs and full power converter are compared in terms of efficiency, voltage gain, voltage stress, and current stress of converter elements. The converter performance is evaluated through experimental and simulation studies. The presented results show good consistency with the theoretical analysis.

Design and implementation of a 22 kW full-bridge push–pull series partial power converter for stationary battery energy storage system with battery charger

A wide variety of AC/DC power converter topologies have been developed in order to improve the system efficiency, input power factor and system redundancy for stationary battery energy storage systems. Due to the nature of high-power batteries, there is a big voltage difference between battery terminals from the end of discharge to the high charge value. To prevent unregulated battery voltages from harming the system loads, several techniques are used in the industry. A well-known old technique named as diode dropper is simple but suffers from low efficiency. Using a DC-DC converter is more advantageous, although it increases the cost. In this paper, the use of partial power processing converters which attract interest these days has been proposed as an alternative. The proposed full bridge/push-pull series connected partial power converter has a slight modification compared to the classical one presented in the literature. A system with 22 kW power rating was designed and tested. In order to compare the results, a two-switch buck-boost converter was also designed and tested for the same conditions. The results show that the proposed converter is superior to both the two-switch buck-boost converter and other topologies in terms of efficiency and response speed. Efficiencies of 97%–99% have been attained with the proposed converter.

Review of Architectures Based on Partial Power Processing for DC-DC Applications

This paper presents a review of advanced architectures based on the partial power processing concept, whose main objective is to achieve a reduction of the power processed by the converter. If the power processed by the converter is decreased, the power losses generated by the power converter are reduced, obtaining lower sized converters and higher system efficiencies. Through the review 3 different partial power processing strategies are distinguished: Differential Power Converters, Partial Power Converters and Mixed strategies. Each strategy is subdivided into smaller groups that entail different architectures with their own advantages and disadvantages. Also, due to the lack of agreement that exists in the sources around the naming of the different architectures, this paper seeks to stablish a nomenclature that avoids confusion when indexing this type of architectures. Regarding Partial Power Converters an extensive application oriented description is also developed. Finally, the main conclusions obtained through the review are presented.

An Approach Towards Extreme Fast Charging Station Power Delivery for Electric Vehicles with Partial Power Processing

This article proposes an approach for realizing the power delivery scheme for an extreme fast charging (XFC) station that is meant to simultaneously charge multiple electric vehicles (EVs). A cascaded H-bridge converter is utilized to directly interface with the medium voltage grid while dual-active-bridge based soft-switched solid-state transformers are used to achieve galvanic isolation. The proposed approach eliminates redundant power conversion by making use of partial power rated dc–dc converters to charge the individual EVs. Partial power processing enables independent charging control over each EV, while processing only a fraction of the total battery charging power. Practical implementation schemes for the partial power charger unit are analyzed. A phase-shifted full-bridge converter-based charger is proposed. Design and control considerations for enabling multiple charging points are elucidated. Experimental results from a down-scaled laboratory test-bed are provided to validate the control aspects, functionality, and effectiveness of the proposed XFC station power delivery scheme. With a down-scaled partial power converter that is rated to handle only 27% of the battery power, an efficiency improvement of 0.6% at full-load and 1.6% at 50% load is demonstrated.

A Systematic Approach to Extract State-Space Averaged Equations and Small-Signal Model of Partial-Power Converters

An isolated full power (FP) converter can be easily transformed into a partial-power (PP) converter by changing the connections of source and load to the converter. This is known as PP processing technique which can improve voltage gain, power rating, and efficiency of the system. This paper discusses the dynamic behavior of the PP converter. A general procedure is proposed to find the dynamic equations and ac small-signal relations for input-to-output voltage and duty ratio-to-output voltage transfer functions of the PP converter. It requires only the dynamic equations of the FP converter. Therefore, circuit analysis is not necessary. The presented methodology is applicable to the converters with any number of switching states. In order to validate the proposed method, it is applied to PP quasi-Z-source dc–dc converter. Several experimental/simulation tests are performed, and the results are compared to the modeling results. Frequency responses of $G_{\mathrm {vd}}$ and $G_{\mathrm {vg}}$ are obtained by simulations, while experiments verify the theoretical predictions.

Analysis of Partial Power DC–DC Converters for Two-Stage Photovoltaic Systems

Two-stage photovoltaic (PV) configurations have become increasingly popular due to the decoupling between the inverter dc-link voltage and the PV voltage, adding flexibility to extend the maximum power point tracking range. However, the additional dc–dc converter increases the power converter losses. The concept of partial power converters (PPCs), which reduce the amount of power handled by the dc stage, can mitigate this effect. However, the type of topology, its power and voltage rating, efficiency, and an operating range can vary significantly depending on the function (boosting or reducing voltage) and type of PV application and scale (micro-, string-, or multi-sting inverter). This paper analyzes the possible configuration of connections of PPC depending on the application and scale of the PV system and introduces a new buck-type PPC. Three solutions for practical PV systems are further elaborated, including experimental validation. Results show that the PPC concept greatly improves the overall PV system efficiency with the added benefit that the dc–dc stage power ratings achieved are only a fraction of the PV system, reducing size and cost of the power converter without affecting the system performance.

Evaluation of Power Processing in Series-Connected Partial-Power Converters

This paper proposes an analytical methodology to evaluate the power processed by dc–dc converters operating as series voltage regulators, which provide an alternative to increase efficiency in photovoltaic systems. Via the analysis of both active and nonactive power processing, the proposed methodology allows to clearly distinguish among circuit topologies as truly partial-power processing (PPP) or just partial active power processing topologies. When an isolated dc–dc topology is connected in series as a voltage regulator, the overall processed power can be reduced, which reduces power losses and improves efficiency. Conversely, this paper also demonstrates that some series-regulator topologies may not actually reduce the proportion of nonactive processed power. To demonstrate the applications of the proposed methodology and to emphasize its significance, two well-known series-connected voltage regulators (flyback and full-bridge phase shift) and a full-power regulator (boost) were evaluated. The results indicate that the full-bridge series regulator can perform a true PPP, whereas the flyback series-regulator processes the same amount of power as that processed by conventional nonisolated boost converters and cannot be considered a partial-power topology. This study finding contradicts assertions in the literature that this topology achieves high-efficiency dc–dc conversion through PPP. To confirm the theoretical analysis, experimental results from three 750-W prototypes are presented alongside its simulations.

Step-Down Partial Power DC-DC Converters for Two-Stage Photovoltaic String Inverters

Photovoltaic (PV) systems composed by two energy conversion stages are attractive from an operation point of view. This is because the maximum power point tracking (MPPT) range is extended, due to the voltage decoupling between the PV system and the dc-link. Nevertheless, the additional dc-dc conversion stage increases the volume, cost and power converter losses. Therefore, central inverters based on a single-stage converter, have been a mainstream solution to interface large-scale PV arrays composed of several strings connected in parallel made by the series connections of PV modules. The concept of partial power converters (PPC), previously reported as a voltage step-up stage, has not addressed in depth for all types of PV applications. In this work, a PPC performing voltage step-down operation is proposed and analyzed. This concept is interesting from the industry point of view, since with the new isolation standards of PV modules are reaching 1500 V, increasing both the size of the string and dc-link voltage for single-stage inverters. Since grid connection remains typically at 690 V, larger strings impose more demanding operation for single-stage central inverters (required to operate at lower modulation indexes and demand higher blocking voltage devices), making the proposed step-down PPC an attractive solution. Theoretical analysis and an experimental test-bench was built in order to validate the PPC concept, the control performance and the improvement of the conversion efficiency. The experimental results corroborate the benefits of using a PPC, in terms of increasing the system efficiency by reducing the processed power of the converter, while not affecting the system performance.

Series-Connected Partial-Power Converters Applied to PV Systems: A Design Approach Based on Step-Up/Down Voltage Regulation Range

This paper proposes a novel approach to reduce the power processed by series-connected partial-power converters (S-PPC) applied to string/multistring level maximum power point tracking photovoltaic systems. A design procedure based on the definition of the voltage regulation range is presented. It introduces an additional degree of freedom that allows a proper design of the converter in order to reduce not only the active power but also the nonactive power, reducing losses and increasing power density. By means of the proposed approach, it is also demonstrated that by replacing the conventional step-up partial-power converter by a step-up/down partial-power converter, the active and nonactive power can be further reduced, allowing to better explore the benefits of the partial-power concept. In order to validate the proposed approach, two 750-W prototypes of full-bridge S-PPC and full-bridge/push–pull S-PPC were implemented and experimentally evaluated. The step-up/down prototype presented a reduction of 46.9% in nonactive power and 23.4% of its volume, resulting in a higher efficiency and power density in comparison to the voltage step-up prototype counterpart.

Starting and Braking of a Large Variable Speed Hydrogenerating Unit Subjected to Converter and Sensor Faults

Doubly fed induction machine (DFIM) is increasingly being preferred in large rated variable speed pumped storage power plants (PSPP), because it provides better energy efficiency due to optimization of pump/turbine efficiency. Increased efficiency results from the flexibility of operating speed at different hydraulic heads. In doubly fed configuration, partial rated power converters feed the rotor circuit. Parallel-connected back-to-back voltage-source converter on rotor side is responsible for starting, braking (dynamic/regenerative), and speed (real power) control of the unit. This paper presents smooth starting and regenerative braking, and assesses performance under power converter and sensor faults of a 250-MW DFIM pump turbine unit to be commissioned in Tehri PSPP, India. Survivability of the unit during faults is analyzed based on current, speed, and dc-link voltage. Further, this paper investigates fault-tolerant operation of 250-MW DFIM unit at open-circuit fault in converters. A 2.2-kW DFIM is tested in the laboratory to corroborate the simulation.

Step-Up Partial Power DC-DC Converters for Two-Stage PV Systems with Interleaved Current Performance

This work presents a partial power converter allowing us to obtain, with a single DC-DC converter, the same feature as the classical interleaved operation of two converters. More precisely, the proposed topology performs similarly as the input-parallel output-series (IPOS) configuration reducing the current ripple at the input of the system and dividing the individual converters power rating, compared to a single converter. The proposed topology consists of a partial DC-DC converter processing only a fraction of the total power, thus allowing high efficiency. Experimental results are provided to validate the proposed converter topology with a Flyback-based 100 W test bench with a transformer turns ratio n 1 = n 2 . Experimental results show high performances reducing the input current ripple around 30 % , further increasing the conversion efficiency.