Non-unity Power Factor Research Articles

A newly effective method to maximize power loss reduction in distribution networks with highly penetrated distributed generations

This paper proposes a newly chaotic maps integrated stochastic fractal search (CMSFS) for solving the optimal distributed generation placement (ODGP) problem in radial distribution networks. The objective of the problem is to minimize the network real power loss satisfying the operational constraints of distributed generations (DGs) and the network. The proposed CMSFS approach is an improvement of the standard SFS approach by integrating chaotic maps into SFS to enhance its solution quality and convergence rate. The experimental results on the IEEE 33, 69, and 118-bus networks and the power loss reduction percentages with the integration of the optimal number of non-unity power factor DG are 99.21%, 99.43% and 92.36%, respectively. Moreover, the simulation results have also shown that the proposed CMSFS can provide better solution quality than many other methods for the considered scenarios. Therefore, the proposed CMSFS can be an effective alternative approach to the ODGP problems in RDNs.

High-Efficiency Single-Phase Matrix Converter With Diverse Symmetric Bipolar Buck and Boost Operations

Single-phase matrix converters (MCs) are undergoing rapid developments due to their bipolar voltage gain (as required in dynamic voltage restorers) and step-changed frequency operation (used in high step-up ac-dc rectifiers, and to provide medium frequency isolation in traction and wind turbine converters, etc.). However, existing buck-boost MCs require high-voltage/current rating devices, and suffer from large component voltage/current stresses and ripples, significantly degrading their efficiency. This article proposes a highly efficient single-phase buck-boost MC; consisting of eight IGBTs (implemented using two full-bridge IGBT modules), second-order input and output filters, and a small value film capacitor. Efficient discrete inverting and noninverting buck and boost operations are proposed (same as that of sixteen-switch cascaded buck-boost MC) with significantly lower component voltage/current stresses and ripples. In addition, flexible inverting and noninverting buck-boost operations are proposed with independent control of buck and boost duty ratios. All of the proposed operations are free of commutation issues, compatible with reactive loads, and provide smooth input and output currents. Furthermore, all the switches have same voltage stresses, and all four switches in a full-bridge module experience the same current stresses. A comprehensive description of circuit operation is presented based on a number of proposed switch modulation strategies with a nonunity power factor load, followed by design guidelines, and comparative evaluations with existing topologies. Finally, experimental verification results are presented, using a 400-VA laboratory prototype converter.

A Two-Stage T-Type Hybrid Five-Level Transformerless Inverter for PV Applications

This article presents a two-stage T-type hybrid five-level transformerless inverter (TLI) for grid-connected photovoltaic (PV) applications. The proposed T-type hybrid five-level inverter and its level-shifted pulsewidth modulation scheme offers reduced leakage current by eliminating the high-frequency variations and sudden transitions in the voltage across PV parasitic capacitance C PV and a path for the negative current in all the modes of operation under unity and nonunity power factor conditions of the grid without degrading the waveform quality. Moreover, in this article, the proposed inverter is integrated with a traditional three-level boost converter (3LBC) for boosting the lower PV voltage to higher dc-link voltage and also to extract maximum power from the PV source. The 3LBC provides high efficiency and reduced input inductor size for the same power rating over the conventional boost converter. The simulation results of the proposed system are presented using MATLAB software, and a 500-W experimental prototype is constructed and tested in the laboratory to validate the feasibility of the proposed configuration. Finally, a comparison of the proposed inverter with other five-level TLI topologies is presented to highlight its merits.

Weighted Efficiency Enhancement for Single-Power-Conversion Microinverters Using Hybrid-Mode Modulation Strategy

This article presents a hybrid-mode modulation strategy to enhance the weighted efficiency for single-power-conversion microinverters. The proposed modulation strategy consists of two modulation schemes to achieve high efficiency under whole load condition. Under heavy load, it works with phase-shift and variable frequency controls, which decrease current stress and power losses by reducing backward power. Moreover, the variable frequency control linearizes the relation between output current and phase-shift angle, making the closed-loop controller simple and easy to implement compared with the conventional approaches. The proposed modulation also enables nonunity power factor operation. Under light load, the proposed modulation escapes from outer-mode operation, which causes high conduction and switching losses, significantly reducing the rms (root mean square) current of the transformer and extends the zero-voltage switching region. The operation principle of the proposed modulation strategy is theoretically analyzed in detail. Experimental results for a 400-W prototype are used to demonstrate the validity of the proposed modulation strategy, with up to 15% increase in efficiency under light load.

Critical-Conduction-Mode-Based Soft-Switching Modulation for Three-Phase PV Inverters With Reactive Power Transfer Capability

In this article, an improved critical-conduction-mode-based soft-switching modulation technique is proposed for three-phase photovoltaic (PV) inverter applications under not only the unity power factor operating condition, but also nonunity power factor operating condition in order to reduce switching loss. With the proposed improvements, for the typical power factor range from the 0.8 lagging condition to the 0.8 leading condition in PV applications, zero-voltage-switch turn- on or valley drain–source voltage turn- on is achieved, which is especially beneficial for silicon carbide (SiC)-based systems at a high switching frequency operation to achieve not only high power density but also high efficiency. The proposed modulation technique is digitally implemented with one low-cost microcontroller. The capability of reactive power transfer and benefits of the improved modulation for achieving soft switching under nonunity power factor conditions are experimentally verified on a SiC-based three-phase bidirectional ac–dc converter prototype. The tested peak efficiency is 98.9% under the unity power factor operation, and above 98.0% within the aforementioned typical power factor range in PV applications, at above 300 kHz switching frequency operation.

Two-Layer Pulsewidth Modulation Strategy for Common-Mode Voltage and Current Harmonic Distortion Reduction in Vienna Rectifier

Vienna rectifier, which is a nongenerative-boost-type rectifier, is widely used in industrial applications, such as wind turbine systems. However, the Vienna rectifier will face the challenge of reducing common-mode voltage (CMV), maintaining the sinusoidal input currents, and balancing neutral-point (NP) voltage in practice. As these issues are mutually coupled, the conventional space vector modulation (SVM) and various types of enhanced schemes cannot solve these problems properly, especially under nonunity power factor operation. To overcome these limitations, a two-layer pulsewidth modulation scheme through vector selection is developed. In the first layer of the proposed strategy, the NP voltage oscillations are mitigated by adjusting the switching sequence and dwell time of large, medium, small, and zero vectors with reduced CMV as the same in large, medium, and zero vector modulation. In the second layer of the proposed strategy, the switching states are reselected, and the dwell times are real time adjusted to eliminate the distortions in input currents. Through the aforementioned two-layer control method, CMV reduction, input current distortion elimination, and NP voltage balancing in Vienna rectifier can be realized simultaneously. The correctness and effectiveness of the proposed SVM method are verified by the experiment.

Optimal allocation of distributed generation for planning master–slave controlled microgrids

This study proposes a novel problem formulation for a planning distributed generation (DG) allocation for microgrids, using the master–slave approach. In the previous planning studies, all DGs have the same operating mode (e.g. operate at unity power factor). For master–slave controlled microgrid, DGs have two possible operating modes: master (non-unity power factor operation) and slave (unity power factor operation). For planning a master–slave controlled microgrid, in addition to DG siting, the optimal DG operating mode is determined by including a new set of constraints in the planning problem. Thus, the proposed formulation is capable of determining the optimal location of the master and slave DGs with the main objective of minimizing the microgrid's energy losses. The proposed model is formulated as a mixed-integer non-linear programming problem; incorporated into an optimal power flow framework and tested on the IEEE 38-bus systems considering a variable load profile. In addition to this, sensitivity analysis is carried for case studies with different load types and reactive power injection by the slave DGs in the system (e.g. operate at fixed non-unity power factor). The proposed approach can serve as an efficient tool for utility operators for planning microgrids.

Bridgeless Cuk-Derived Single Power Conversion Inverter With Reactive-Power Capability

This paper presents a bridgeless Cuk-derived single power conversion inverter with reduced number of power components, which has reactive-power capability. The secondary diode of the conventional unfolding-type Cuk inverter is integrated into one of the secondary-side switches in the bridgeless Cuk-derived configuration, which reduces the number of active-power components. A modulation technique is developed to provide a current path during reverse power flow; this technique enables operations under unity and nonunity power factors. A repetitive controller coupled with a linear feedback controller and a feed-forward controller are used to achieve accurate output current tracking under the harsh control environment. Driving the diode-integrated switch in synchronous rectification mode provides higher power conversion efficiency than that under diode-based secondary side rectification. Circuit operation and design guidelines of the bridgeless Cuk inverter are presented in detail. A 500-VA prototype inverter was fabricated to demonstrate the desirable performance of the bridgeless Cuk-derived inverter.

A generalized switching function-based discontinuous space vector modulation technique for unbalanced two-phase three-leg inverters

This paper presents a discontinuous space vector modulation technique for unbalanced two-phase three-leg inverters. This technique is based on the shift-angle and generalized modulation algorithm obtained for generating the unbalanced two-phase output voltage. Furthermore, the discontinuous switching sequence intends to improve the commutations of power switching devices in each inverter leg that achieves a minimum number of switching state changes in one sampling cycle. Therefore, the switch commutations can be reduced by one-third in one main period. The step- by-step procedure of the modulation algorithm for easy implementation in a digital control platform is discussed. The performance of the developed modulation technique is verified through both simulation and experimental results in a nonunity power factor balanced two-phase load and asymmetrical two-phase induction motor drive.

Coordinated Electric Vehicle Charging With Reactive Power Support to Distribution Grids

We develop hierarchical coordination frameworks to optimally manage active and reactive power dispatch of number of spatially distributed electric vehicles (EVs) incorporating distribution grid level constraints. The frameworks consist of detailed mathematical models, which can benefit the operation of both entities involved, i.e., the grid operations and EV charging. The first model comprises of a comprehensive optimal power flow model at the distribution grid level, while the second model represents detailed optimal EV charging with reactive power support to the grid. We demonstrate benefits of coordinated dispatch of active and reactive power from EVs using a 33-node distribution feeder with large number of EVs (more than 5000). Case studies demonstrate that, in constrained distribution grids, coordinated charging reduces the average cost of EV charging if the charging takes place at nonunity power factor mode compared to unity power factor. Similarly, the results also demonstrate that distribution grids can accommodate charging of increased number of EVs, if EV charging takes place at nonunity power factor mode compared to unity power factor.

Analysis to Input Current Zero Crossing Distortion of Bridgeless Rectifier Operating under Different Power Factors

The principles and operating characteristics of bridgeless rectifiers under different power factors are discussed with emphasis on analyzing the input current distortion. Firstly, two driving modes are analyzed and compared. Based on the results of comparison it is concluded that the complementary drive mode is a better choice in terms of reducing the current distortion when bridgeless rectifier operates on non-unity power factor. Then, the mechanism causing input current zero-crossing distortion is analyzed. The input current during the distortion is expressed by the piecewise function when a bridgeless rectifier operates under complementary drive mode. Based on the piecewise function, the harmonic analysis is performed. Besides, the relationships between the input current Total Harmonic Distortion (THD) and the filtering inductance, the input current amplitude and the power factor angle are also investigated, which is useful when designing bridgeless rectifiers and selecting the corresponding parameters. Finally, the accuracy of the theoretical analysis is verified through the simulation and experiment.

Dual-Comparison One-Cycle Control for Single-Phase Bidirectional Power Converters

Dual-comparison one-cycle control for the bidirectional power flow in single-phase grid-connected converters is proposed in this paper. Advantages of dual-comparison one-cycle control when compared to conventional one-cycle control is that it does not give any steady-state dc offset, no light-load instability, and implements unipolar pulsewidth modulation without having to sense the grid voltage. Addition of a fictitious current term, which is generated from the gating signal of one of the active devices, using a bandpass filter, enables the bidirectional power flow. The quality factor of the bandpass filter is shown to impact the total harmonic distortion in current, fundamental displacement angle, and steady-state dc offset. The optimal value of a quality factor, obtained with aid of simulations, gives a superior performance. The current drawn or injected into the grid is seen to lag the grid voltage. The reason for a nonunity displacement power factor is analyzed and a compensation to achieve a unity displacement power factor is also proposed in this paper. Detailed simulation and experimental studies are carried out to validate the proposed control and the high performance that can be obtained.

Problems in the Classic Frequency Shift Islanding Detection Methods Applied to Energy Storage Converters and a Coping Strategy

This paper first derives a usable formula for frequency shift islanding detection methods on the basis of the parallel R , L , C load and the conclusions in current literature: the angle by which the total output current of the distributed resources (DR) units leads the point of common coupling (PCC) voltage must be conducted to have the same shifting direction as the load admittance angle during the variation of the frequency. According to the formula and multi-DR operation, the scenarios in which the classic frequency shift methods are applied to energy storage converters are analyzed. The results indicate that the setting of the angle by which the energy storage converter current leads the PCC voltage may need to be modified when running state changes. It results in the problems that the classic methods are not applicable for non-unity power factor (non-UPF) control and have to distinguish between generation mode and consumption mode for UPF control. On account of the problems, a coping strategy, i.e., an improved method, is proposed. The analyses indicate that the improved method is applicable in every state. The last simulations and experiments confirm the preceding conclusions.

An Isolated High-Frequency Link Microinverter Operated with Secondary-Side Modulation for Efficiency Improvement

This paper discusses the operation of a single-stage, isolated, high-frequency ac-link-based single-phase dc–ac converter, suitable for photovoltaic microinverter applications, controlled using phase modulation of the secondary-side cycloconverter devices. A detailed analysis is presented explaining the impact of prominent circuit nonidealities such as device capacitance and transformer leakage inductance on circuit behavior. The proposed extended-dead-time based modulation scheme helps in achieving zero-voltage-switching of the primary devices over the entire line cycle of the ac output and also offers the benefit of reduced transformer rms current due to absence of circulating current in the zero state. A flyback-based regenerative clamp circuit is used for mitigating voltage spikes on the secondary devices arising due to leakage inductance induced oscillations. A generic commutation strategy is also discussed, which makes operation with nonunity power factor loads also possible. Experimental results on a 260-W laboratory prototype under different load conditions are presented to illustrate the discussed principles and highlight the performance improvements.

Comprehensive Power Losses Model for Electronic Power Transformer

The electronic power transformer (EPT) has higher power losses than the conventional transformer. However, the EPT can correct the power factor, compensate the unbalanced current, and reduce the line power losses in the distribution network. Therefore, the higher power losses of the EPT and the consequent reduced power losses in the distribution network require a comprehensive consideration when comparing the power losses of the EPT and conventional transformer. In this paper, a comprehensive power losses analysis model for the EPT in distribution networks is proposed. By analyzing the EPT self-losses and considering the impact of the nonunity power factor and the three-phase unbalanced current, the overall power losses in the distribution network when using the EPT to replace the conventional transformer is analyzed, and the conditions in which the application of the EPT can cause less power losses are obtained. Based on this, the sensitivity analysis for the EPT comprehensive power losses model is carried out by comparing the value of each parameter variation impact on the EPT losses model. In the case study, the validity of the comprehensive power losses model is verified.

Neutral Point Clamped MOSFET Inverter With Full-Bridge Configuration for Nonisolated Grid-Tied Photovoltaic System

Existing nonisolated full-bridge neutral point clamped (NPC) inverters for a single-phase grid-tied photovoltaic (PV) system have limitations such as shoot-through and low European Union (EU) efficiency. In order to address these limitations, an NPC superjunction MOSFET nonisolated inverter with full-bridge configuration (NIIFBC) is proposed in this paper. This inverter reduces the possibility of shoot-through, thereby improving the reliability of the grid-tied PV system. It controls the grid current by energizing two coupled inductors individually during positive and negative half-grid cycles. This obviates the possibility of reverse recovery loss in switches due to their body diodes. Furthermore, two external silicon carbide diodes of a clamping branch cause negligible reverse recovery loss in switches besides a constant common-mode voltage. Therefore, the main claims of NIIFBC are low leakage current, high EU efficiency, and reliability. A generalized leakage current model for the proposed inverter is developed. In order to validate the veracity of the model and the claims of NIIFBC, a 1-kW prototype is designed and developed. The experimental results of NIIFBC validate the claims made by the authors. Its performance comparison with the existing nonisolated full-bridge inverters is given. Furthermore, a variant circuit of NIIFBC operating at nonunity power factor is proposed.

Current Harmonics From Single-Phase Grid-Connected Inverters—Examination and Suppression

Environmental conditions and operational modes may significantly impact the distortion level of the injected current from single-phase grid-connected inverter systems, such as photovoltaic (PV) inverters, which may operate in cloudy days with a maximum power point tracking, in a nonunity power factor, or in the low-voltage ride-through mode with reactive current injection. In this paper, the mechanism of the harmonic current injection from grid-connected single-phase inverter systems is thus explored, and the analysis is conducted on single-phase PV systems. In particular, the analysis is focused on the impacts of the power factor and the feed-in grid current level on the quality of the feed-in grid current from single-phase inverters. As a consequence, an internal model principle-based high-performance current control solution is tailor-made and developed for single-phase grid-connected systems to produce high-quality currents in different operation conditions, where a design procedure is also provided. The developed current controller in this paper can achieve a minimum steady-state error while maintaining a relatively fast transient response, and also being feasible in other single-phase applications as a promising harmonic mitigation solution. Experiments on single-phase grid-connected systems have verified the correctness of the relevant analysis and also the effectiveness of the tailor-made control solution in terms of good harmonic mitigation.

Dynamic Modeling of Electric Springs

The use of “Electric Springs” is a novel way of distributed voltage control while simultaneously achieving effective demand-side management through modulation of noncritical loads in response to the fluctuations in intermittent renewable energy sources (e.g., wind). The proof-of-concept has been successfully demonstrated on a simple 10-kVA test system hardware. However, to show the effectiveness of such electric springs when installed in large numbers across the power system, there is a need to develop simple and yet accurate simulation models for these electric springs which can be incorporated in large-scale power system simulation studies. This paper describes the dynamic simulation approach for electric springs which is appropriate for voltage and frequency control studies at the power system level. The proposed model is validated by comparing the simulation results against the experimental results. Close similarity between the simulation and experimental results gave us the confidence to use this electric spring model for investigating the effectiveness of their collective operation when distributed in large number across a power system. Effectiveness of an electric spring under unity and non-unity load power factors and different proportions of critical and noncritical loads is also demonstrated.

Simultaneous distributed generation and capacitor placement and sizing in radial distribution system considering reactive power market

In this paper, Teaching-Learning-Based Optimization (TLBO) algorithm is used to determine the location and size of capacitor banks and Distributed Generation (DG) units simultaneously, in order to maximize the Benefit in regards to Cost (BRC). The benefits which are considered for DG units and capacitor banks are based on economic aspects. Therefore, for the DG units, the benefits are loss reduction, active power selling in the active power market, and energy not supplied reduction. On the other side, capacitor placement is performed to decrease the active power loss of networks, and reactive power selling (i.e., not purchasing reactive power as ancillary service from reactive power market). The DG units and capacitor banks are owned by distribution utility; consequently, their operation is controlled by the utility. The costs of DG units and capacitor banks include investment, operation, and maintenance costs. In addition, the cost of reactive power purchased for DGs is taken into account for non-unity power factor DG's (i.e., induction generator). The maximization process of BRC is performed using TLBO algorithm which has been selected from two alternative algorithms, i.e., particle swarm optimization and shuffled frog leaping algorithm. The study looked at a 69-bus radial distribution system. The results were reviewed, concluding that simultaneous DG and capacitor placement, considering the reactive power market, is a profitable way for loss reduction. Moreover, the strong performance of TLBO is shown.

Range of the Linear Modulation in Matrix Converters

This paper is focused on the improvement of the control range of matrix converters operating with nonunity input power factor. The analysis leads to the determination in closed form of a modulation strategy that allows increasing the maximum output voltage compared to the traditional strategies under the same operating conditions. The improvement of the control range is made possible by the information provided by the measurement of load currents, which is usually available in electric drives. The feasibility of the new strategy is verified by computer simulations and experimental tests.