Showing papers on "Voltage regulation published in 2014"
TL;DR: In this paper, a self-synchronized synchronverter is proposed to improve the performance of grid-connected inverters by removing the dedicated synchronization unit, which can automatically synchronize itself with the grid before connection and track the grid frequency after connection.
Abstract: A synchronverter is an inverter that mimics synchronous generators, which offers a mechanism for power systems to control grid-connected renewable energy and facilitates smart grid integration. Similar to other grid-connected inverters, it needs a dedicated synchronization unit, e.g., a phase-locked loop (PLL), to provide the phase, frequency, and amplitude of the grid voltage as references. In this paper, a radical step is taken to improve the synchronverter as a self-synchronized synchronverter by removing the dedicated synchronization unit. It can automatically synchronize itself with the grid before connection and track the grid frequency after connection. This considerably improves the performance, reduces the complexity, and computational burden of the controller. All the functions of the original synchronverter, such as frequency and voltage regulation, real power, and reactive power control, are maintained. Both simulation and experimental results are presented to validate the control strategy. Experimental results have shown that the proposed control strategy can improve the performance of frequency tracking by more than 65%, the performance of real power control by 83%, and the performance of reactive power control by about 70%.
793 citations
TL;DR: In this article, the relationship among compensation parameters, circuit efficiency, voltage transfer function, and conduction angle of the input current relative to the input voltage are studied, and a design and optimization method is proposed to achieve a better overall efficiency as well as good output voltage controllability.
Abstract: Inductive power transfer (IPT) is an emerging technology that may create new possibilities for wireless power charging and transfer applications. However, the rather complex control method and low efficiency remain the key obstructing factors for general deployment. In a regularly compensated IPT circuit, high efficiency and controllability of the voltage transfer function are always conflicting requirements under varying load conditions. In this paper, the relationships among compensation parameters, circuit efficiency, voltage transfer function, and conduction angle of the input current relative to the input voltage are studied. A design and optimization method is proposed to achieve a better overall efficiency as well as good output voltage controllability. An IPT system design procedure is illustrated with design curves to achieve a desirable voltage transfer ratio, optimizing between efficiency enhancement and current rating of the switches. The analysis is supported with experimental results.
404 citations
TL;DR: The cost-benefit analysis presented in this paper considers factors of BESS influence on the work stress of voltage regulation devices, load shifting and peaking power generation, as well as individual BESS cost with its lifetime estimation, and determines the cost- benefit size.
Abstract: This paper proposes an effective sizing strategy for distributed battery energy storage system (BESS) in the distribution networks under high photovoltaic (PV) penetration level. The main objective of the proposed method is to optimize the size of the distributed BESS and derive the cost-benefit analysis when the distributed BESS is applied for voltage regulation and peak load shaving. In particular, a system model that includes a physical battery model and a voltage regulation and peak load shaving oriented energy management system (EMS) is developed to apply the proposed strategy. The cost-benefit analysis presented in this paper considers factors of BESS influence on the work stress of voltage regulation devices, load shifting and peaking power generation, as well as individual BESS cost with its lifetime estimation. Based on the cost-benefit analysis, the cost-benefit size can be determined for the distributed BESS.
347 citations
TL;DR: This paper presents a distributed hierarchical control framework to ensure reliable operation of dc microgrid (MG) clusters and an adaptive droop method is proposed for this level, which determines droop coefficients according to the state-of-charge of batteries automatically.
Abstract: This paper presents a distributed hierarchical control framework to ensure reliable operation of dc microgrid (MG) clusters. In this hierarchy, primary control is used to regulate the common bus voltage inside each MG locally. An adaptive droop method is proposed for this level, which determines droop coefficients according to the state-of-charge (SOC) of batteries automatically. A small-signal model is developed to investigate effects of the system parameters, constant power loads, as well as line impedance between the MGs on stability of these systems. In the secondary level, a distributed consensus-based voltage regulator is introduced to eliminate the average voltage deviation over the MGs. This distributed averaging method allows the power flow control between the MGs to be achieved at the same time, as it can be accomplished only at the cost of having voltage deviation inside the system. Another distributed policy is employed then to regulate the power flow among the MGs according to their local SOCs. The proposed distributed controllers on each MG communicate with only the neighbor MGs through a communication infrastructure. Finally, the developed small-signal model is expanded for MG clusters with all the proposed control loops. The effectiveness of the proposed hierarchical scheme is verified through detailed hardware-in-the-loop simulations.
332 citations
TL;DR: This paper presents a practical dc microgrid developed in the Water and Energy Research Laboratory (WERL) in the Nanyang University of Technology, Singapore using a novel hierarchical control technique.
Abstract: DC microgrids are becoming popular in low-voltage distribution systems due to the better compatibility with photovoltaic panels, electric vehicles, and DC loads. This paper presents a practical dc microgrid developed in the Water and Energy Research Laboratory (WERL) in the Nanyang University of Technology, Singapore. The coordination control among multiple dc sources and energy storages is implemented using a novel hierarchical control technique. The bus voltage essentially acts as an indicator of supply-demand balance. A wireless control is implemented for the reliable operation of the grid. A reasonable compromise between the maximum power harvest and effective battery management is further enhanced using the coordination control based on a central energy management system. The feasibility and effectiveness of the proposed control strategies have been tested by a DC microgrid in WERL.
324 citations
TL;DR: A novel high step-up converter, which is suitable for renewable energy system, is proposed in this paper, and the configuration of the proposed converter not only reduces the current stress but also constrains the input current ripple, which decreases the conduction losses and lengthens the lifetime of the input source.
Abstract: A novel high step-up converter, which is suitable for renewable energy system, is proposed in this paper. Through a voltage multiplier module composed of switched capacitors and coupled inductors, a conventional interleaved boost converter obtains high step-up gain without operating at extreme duty ratio. The configuration of the proposed converter not only reduces the current stress but also constrains the input current ripple, which decreases the conduction losses and lengthens the lifetime of the input source. In addition, due to the lossless passive clamp performance, leakage energy is recycled to the output terminal. Hence, large voltage spikes across the main switches are alleviated, and the efficiency is improved. Even the low voltage stress makes the low-voltage-rated MOSFETs be adopted for reductions of conduction losses and cost. Finally, the prototype circuit with 40-V input voltage, 380-V output, and 1000-W output power is operated to verify its performance. The highest efficiency is 97.1%.
281 citations
TL;DR: In this paper, a high-voltage solid-state transformer (SST) lab prototype is presented as the active grid interface in smart grid architecture, where the designs of the key components of the system, including both power stage and controller platform, are presented.
Abstract: Solid-state transformer (SST) has been regarded as one of the most important emerging technologies for traction system and smart grid application. This paper presents the system design and performance demonstration of a high-voltage SST lab prototype that works as the active grid interface in smart grid architecture. Specifically, the designs of the key components of the system, including both power stage and controller platform, are presented. In addition, the advanced control system is developed to achieve high-performance operation. Furthermore, integration issues of SST with dc microgrid are presented. Lastly, tests under different scenarios are conducted to verify the following advanced features of the presented SST technology: 1) VAR compensation; 2) voltage regulation; 3) source voltage sag operation; and 4) microgrid integration.
272 citations
TL;DR: A combined system of a thyristor-controlled reactor (TCR) and a shunt hybrid power filter (SHPF) for harmonic and reactive power compensation is proposed and the simulation and experimental results are found to be quite satisfactory to mitigate harmonic distortions and reactivePower compensation.
Abstract: This paper proposes a combined system of a thyristor-controlled reactor (TCR) and a shunt hybrid power filter (SHPF) for harmonic and reactive power compensation. The SHPF is the combination of a small-rating active power filter (APF) and a fifth-harmonic-tuned LC passive filter. The tuned passive filter and the TCR form a shunt passive filter (SPF) to compensate reactive power. The small-rating APF is used to improve the filtering characteristics of SPF and to suppress the possibility of resonance between the SPF and line inductances. A proportional-integral controller was used, and a triggering alpha was extracted using a lookup table to control the TCR. A nonlinear control of APF was developed for current tracking and voltage regulation. The latter is based on a decoupled control strategy, which considers that the controlled system may be divided into an inner fast loop and an outer slow one. Thus, an exact linearization control was applied to the inner loop, and a nonlinear feedback control law was used for the outer voltage loop. Integral compensators were added in both current and voltage loops in order to eliminate the steady-state errors due to system parameter uncertainty. The simulation and experimental results are found to be quite satisfactory to mitigate harmonic distortions and reactive power compensation.
263 citations
TL;DR: In this article, a systematic method for determining the active and reactive power set points for PV inverters in residential systems is proposed, with the objective of optimizing the operation of the distribution feeder and ensuring voltage regulation.
Abstract: Low-voltage distribution feeders were designed to sustain unidirectional power flows to residential neighborhoods. The increased penetration of roof-top photovoltaic (PV) systems has highlighted pressing needs to address power quality and reliability concerns, especially when PV generation exceeds the household demand. A systematic method for determining the active- and reactive-power set points for PV inverters in residential systems is proposed in this paper, with the objective of optimizing the operation of the distribution feeder and ensuring voltage regulation. Binary PV-inverter selection variables and nonlinear power-flow relations render the optimal inverter dispatch problem nonconvex and NP-hard. Nevertheless, sparsity-promoting regularization approaches and semidefinite relaxation techniques are leveraged to obtain a computationally feasible convex reformulation. The merits of the proposed approach are demonstrated using real-world PV-generation and load-profile data for an illustrative low-voltage residential distribution system.
256 citations
252 citations
TL;DR: In this paper, the problem posed by complex, nonlinear controllers for power system load flows employing multi-terminal voltage source converter (VSC) HVDC systems is addressed.
Abstract: This paper addresses the problem posed by complex, nonlinear controllers for power system load flows employing multi-terminal voltage source converter (VSC) HVDC systems. More realistic dc grid control strategies can thus be carefully considered in power flow analysis of ac/dc grids. Power flow methods for multi-terminal VSC-HVDC (MTDC) systems are analyzed for different types of dc voltage control techniques and the weaknesses of present methods are addressed. As distributed voltage control is likely to be adopted by practical dc grids, a new generalized algorithm is proposed to solve the power flow problems with various nonlinear voltage droops, and the method to incorporate this algorithm with ac power flow models is also developed. With five sets of voltage characteristics implemented, the proposed scheme is applied to a five-terminal test system and shows satisfactory performance. For a range of wind power variations and converter outages, post-contingency behaviors of the system under the five control scenarios are examined. The impact of these controls on the power flow solutions is assessed.
TL;DR: In this article, an optimal reactive power coordination strategy based on the load and irradiance forecast is proposed to minimize the number of tap operations so as not to reduce the operating life of the tap control mechanism and avoid runaway.
Abstract: The uptake of variable megawatts from photovoltaics (PV) challenges distribution system operation. The primary problem is significant voltage rise in the feeder that forces existing voltage control devices such as on-load tap-changers and line voltage regulators to operate continuously. The consequence is the deterioration of the operating life of the voltage control mechanism. Also, conventional non-coordinated reactive power control can result in the operation of the line regulator at its control limit (runaway condition). This paper proposes an optimal reactive power coordination strategy based on the load and irradiance forecast. The objective is to minimize the number of tap operations so as not to reduce the operating life of the tap control mechanism and avoid runaway. The proposed objective is achieved by coordinating various reactive power control options in the distribution network while satisfying constraints such as maximum power point tracking of PV and voltage limits of the feeder. The option of voltage support from PV plant is also considered. The problem is formulated as constrained optimization and solved through the interior point technique. The effectiveness of the approach is demonstrated in a realistic distribution network model.
TL;DR: In this paper, the authors discuss the extension of electromechanical stability models of voltage source converter high voltage direct current (VSC HVDC) to multi-terminal (MTDC) systems.
Abstract: This paper discusses the extension of electromechanical stability models of voltage source converter high voltage direct current (VSC HVDC) to multi-terminal (MTDC) systems. The paper introduces a control model with a cascaded DC voltage control at every converter that allows a two-terminal VSC HVDC system to cope with converter outages. When extended to an MTDC system, the model naturally evolves into a master-slave set-up with converters taking over the DC voltage control in case the DC voltage controlling converter fails. It is shown that the model can be used to include a voltage droop control to share the power imbalance after a contingency in the DC system amongst the converters in the system. Finally, the paper discusses two possible model reductions, in line with the assumptions made in transient stability modeling. The control algorithms and VSC HVDC systems have been implemented using both MatDyn, an open source MATLAB transient stability program, as well as the commercial power system simulation package EUROSTAG.
TL;DR: A distributed two-layer control structure for ac microgrids that regulates the active and reactive powers of CCVSIs and is verified on a microgrid test system and IEEE 34 test feeder.
Abstract: This paper proposes a distributed two-layer control structure for ac microgrids. Inverter-based distributed generators (DGs) can operate either as voltage-controlled voltage source inverters (VCVSI) or current-controlled voltage source inverters (CCVSI). VCVSIs provide the voltage and frequency support, whereas CCVSIs regulate the generated active and reactive powers. The proposed control structure has two main layers. The first layer deals with the voltage and frequency control of VCVSIs. The second layer regulates the active and reactive powers of CCVSIs. These controllers are implemented through two communication networks with one-way communication links and are fully distributed; each DG only requires its own information and the information of its neighbors on the communication network graph. The proposed control framework is verified on a microgrid test system and IEEE 34 test feeder.
TL;DR: The proposed active-network converter with switched-capacitor technique can achieve high voltage gain without extremely high duty ratio and the voltage stress of the active switches and output diodes is low.
Abstract: The voltage gain of traditional boost converter is limited due to the high current ripple, high voltage stress across active switch and diode, and low efficiency associated with large duty ratio operation. High voltage gain is required in applications, such as the renewable energy power systems with low input voltage. A high step-up voltage gain active-network converter with switched-capacitor technique is proposed in this paper. The proposed converter can achieve high voltage gain without extremely high duty ratio. In addition, the voltage stress of the active switches and output diodes is low. Therefore, low voltage components can be adopted to reduce the conduction loss and cost. The operating principle and steady-state analysis are discussed in detail. A prototype with 20-40-V input voltage, 200-V output voltage, and 200-W output power has been established in the laboratory. Experimental results are given to verify the analysis and advantages of the proposed converter.
TL;DR: In this article, an optimized distributed control approach based on DN sensitivity analysis and on decentralized reactive/active power regulation capable of maintaining voltage levels within regulatory limits and to offer ancillary services to the distribution network, such as voltage regulation, is proposed.
Abstract: The increasing penetration of distributed generation (DG) power plants into distribution networks (DNs) causes various issues concerning, e.g., stability, protection equipment, and voltage regulation. Thus, the necessity to develop proper control techniques to allow power delivery to customers in compliance with power quality and reliability standards (PQR) has become a relevant issue in recent years. This paper proposes an optimized distributed control approach based on DN sensitivity analysis and on decentralized reactive/active power regulation capable of maintaining voltage levels within regulatory limits and to offer ancillary services to the DN, such as voltage regulation. At the same time, it tries to minimize DN active power losses and the reactive power exchanged with the DN by the DG units. The validation of the proposed control technique has been conducted through a several number of simulations on a real MV Italian distribution system.
TL;DR: In this article, the voltage sensitivity matrix and quasi-static analysis were used to develop a coordinated active power dependent (APD) characteristic for photovoltaic (PV) systems along a feeder.
Abstract: High penetrations of photovoltaic (PV) systems in distribution grids have brought about new challenges such as reverse power flow and voltage rise. One of the proposed remedies for voltage rise is reactive power contribution by PV systems. Recent German Grid Codes (GGC) introduce an active power dependent (APD) standard characteristic curve, ${\rm Q}({\rm P})$ , for inverter-coupled distributed generators. This study utilizes the voltage sensitivity matrix and quasi-static analysis in order to locally and systematically develop a coordinated ${\rm Q}({\rm P})$ characteristic for each PV system along a feeder. The main aim of this paper is to evaluate the technical performance of different aspects of proposed ${\rm Q}({\rm P})$ characteristics. In fact, the proposed method is a systematic approach to set parameters in the GGC ${\rm Q}({\rm P})$ characteristic. In the proposed APD method the reactive power is determined based on the local feed-in active power of each PV system. However, the local voltage is also indirectly taken into account. Therefore, this method regulates the voltage in order to keep it under the upper steady-state voltage limit. Moreover, several variants of the proposed method are considered and implemented in a simple grid and a complex utility grid. The results demonstrate the voltage-regulation advantages of the proposed method in contrast to the GGC standard characteristic.
TL;DR: In this article, a voltage source in series with the dc bus line is connected to compensate the ripple voltage on the dc-link capacitor, so as to make the output have a near zero ripple voltage.
Abstract: A technique for reduction of the dc-link capacitance in a capacitor-supported system is presented. The concept is based on connecting a voltage source in series with the dc bus line to compensate the ripple voltage on the dc-link capacitor, so as to make the output have a near zero ripple voltage. Since the voltage compensator processes small ripple voltage on the dc link and reactive power only, it can be implemented with low-voltage devices. The overall required energy storage of the dc-link, formed by a reduced value of dc-link capacitor and the voltage compensator, is reduced, allowing the replacement of popularly used electrolytic capacitors with alternatives of longer lifetime, like power film capacitors, or extending the system lifetime even if there is a significant reduction in the capacitance of electrolytic capacitors due to the aging effect. Comprehensive analysis on the static and dynamic characteristics of the system, and hold-up time requirement will be discussed. The proposed technique is exemplified on an ac-dc-dc power conversion system. Theoretical predictions are favorably verified by experimental results.
TL;DR: Two coordinated voltage control algorithms suitable for usage in distribution networks including several distributed energy resources are proposed and studied and the network effects and costs are compared using statistical distribution network planning and also practical implementation issues are discussed.
Abstract: Connecting distributed generation (DG) to weak distribution networks can often cause voltage rise problems. Traditionally, these voltage rise problems have been mitigated by passive methods such as reinforcing the network. This can, however, lead to high connection costs of DG. The connection costs can in many cases be lowered if active voltage control methods are used instead of the passive approach. In this paper, two coordinated voltage control algorithms suitable for usage in distribution networks including several distributed energy resources are proposed and studied. The first algorithm uses control rules to determine its control actions and the second algorithm utilizes optimization. The operation of the implemented algorithms is, at first, studied using time domain simulations. Thereafter, the network effects and costs of both algorithms are compared using statistical distribution network planning and also practical implementation issues are discussed.
TL;DR: In this article, the authors proposed a dc circuit breaker for voltage source converter (VSC) based high-voltage dc transmission (HVDC) systems, which consists of a number of semiconductor devices in series.
Abstract: This paper proposes a dc circuit breaker for voltage source converter (VSC) based high-voltage dc transmission (HVDC) systems. Technical challenges for applying dc circuit breakers are to increase blocking voltage and to suppress surge voltage at the current clearing. The proposed dc circuit breaker is a solid-state breaker which consists of a number of semiconductor devices in series. It maintains equal voltage balancing among the devices to apply it to high-voltage applications. Moreover, the surge voltage across the circuit breaker is reduced by employing a freewheeling diode. Considering a system rated at 300 MW in power and 250 kV in dc voltage, the conduction loss of the proposed circuit breaker is estimated to be 0.045% of the rated power. The value is smaller than the power loss of the VSCs. A downscaled HVDC system rated at 10 kW in power and 360 V in dc voltage was built and tested. A series of experimental results demonstrates the dc fault clearing and rapid restoration of power transmission.
TL;DR: In this paper, a power management strategy for PV/battery hybrid systems in islanded micro-grids is proposed, which enables the photovoltaic (PV)/battery unit to operate as a voltage source that employs an adaptive droop control to share the load with other sources while charging the battery.
Abstract: In this paper, a power management strategy for PV/battery hybrid systems in islanded microgrids is proposed. The control strategy enables the photovoltaic (PV)/battery unit to operate as a voltage source that employs an adaptive droop control to share the load with other sources while charging the battery. Also, the PV/battery unit can track and supply the maximum PV power to the microgrid as long as there is sufficient load. Otherwise, the hybrid unit will autonomously follow the changing load while storing the excess energy in the battery. The control strategy is designed to modify the PV operating point to match the load autonomously whenever the available PV power is higher than the load and the battery is fully charged. In addition, the battery can provide the operational functions that a separate storage unit may provide in an islanded microgrid, such as regulating voltage and frequency, and supplying deficit power in the microgrid. This is achieved by utilizing multi-loop control and multi-segment adaptive droop control without relying on communications or a state machine. Small-signal models of the proposed control loops are developed to investigate system stability. The system performance is validated using experimental results from a 3-KVA prototype microgrid.
TL;DR: In this article, a dc-link voltage stabilization algorithm using an active damping is proposed so that the dclink voltage can be stabilized with reduced DC-link capacitors in the motor drive system.
Abstract: In conventional motor drive systems using pulsewidth modulation (PWM) inverters, large electrolytic capacitors are used for stabilization of the dc-link voltage. Since the electrolytic capacitors are bulky and reduce reliability of the system due to short lifetime, there have been many efforts to eliminate or reduce the electrolytic capacitors in the motor drive system. However, the PWM inverter with reduced dc-link capacitor has a problem that the dc-link voltage is less stable compared to the conventional inverter because the capability of storing energy is also reduced. In this paper, a dc-link voltage stabilization algorithm using an active damping is proposed so that the dc-link voltage can be stabilized with reduced dc-link capacitor. To achieve load-/source-independent stabilization, a source state estimator which estimates both source voltage and current is also proposed. The fluctuation of the dc-link voltage due to a step load change can be also suppressed under the tolerance range using the estimated source current. The effectiveness of the proposed methods is evaluated by experimental results.
TL;DR: An efficient autonomous power conditioning circuit, with the function of impedance matching, energy storage and voltage regulation, is presented, finding that the efficiency of the energy storage is greatly improved and up to 64.95%.
Abstract: This paper presents a micro-electro-mechanical system (MEMS) piezoelectric power generator array for vibration energy harvesting. A complete design flow of the vibration-based energy harvester using the finite element method (FEM) is proposed. The modal analysis is selected to calculate the resonant frequency of the harvester, and harmonic analysis is performed to investigate the influence of the geometric parameters on the output voltage. Based on simulation results, a MEMS Pb(Zr,Ti)O3 (PZT) cantilever array with an integrated large Si proof mass is designed and fabricated to improve output voltage and power. Test results show that the fabricated generator, with five cantilever beams (with unit dimensions of about 3 × 2.4 × 0.05 mm3) and an individual integrated Si mass dimension of about 8 × 12.4 × 0.5 mm3, produces a output power of 66.75 μW, or a power density of 5.19 μW∙mm-3∙g-2 with an optimal resistive load of 220 kΩ from 5 m/s2 vibration acceleration at its resonant frequency of 234.5 Hz. In view of high internal impedance characteristic of the PZT generator, an efficient autonomous power conditioning circuit, with the function of impedance matching, energy storage and voltage regulation, is then presented, finding that the efficiency of the energy storage is greatly improved and up to 64.95%. The proposed self-supplied energy generator with power conditioning circuit could provide a very promising complete power supply solution for wireless sensor node loads.
TL;DR: In this article, a reactive power control is proposed to regulate the maximum and minimum phase voltages at the point of common coupling within the limits established in grid codes for continuous operation, which can help to mitigate the adverse effects of these perturbations by injecting the reactive power during the sag and the postfault operation.
Abstract: Grid faults are one of the most severe problems for network operation. Distributed generation power plants can help to mitigate the adverse effects of these perturbations by injecting the reactive power during the sag and the postfault operation. Thus, the risk of cascade disconnection and voltage collapse can be reduced. The proposed reactive power control is intended to regulate the maximum and minimum phase voltages at the point of common coupling within the limits established in grid codes for continuous operation. In balanced three-phase voltage sags, the control increases the voltage in each phase above the lower regulated limit by injecting the positive sequence reactive power. In unbalanced voltage sags, positive and negative sequence reactive powers are combined to flexibly raise and equalize the phase voltages; the maximum phase voltage is regulated below the upper limit and the minimum phase voltage just above the lower limit. The proposed control strategy is tested by considering a distant grid fault and a large grid impedance. Selected experimental results are reported in order to validate the behavior of the control scheme.
Book•
06 Jan 2014
TL;DR: In this paper, the authors present an overview of the main components used in distribution system design, including lines, Transformers, and Underground Cables, as well as the per-unit system.
Abstract: Preface Acknowledgments Author Distribution System Planning and Automation Load Characteristics Application of Distribution Transformers Design of Subtransmission Lines and Distribution Substations Design Considerations of Primary Systems Design Considerations of Secondary Systems Voltage-Drop and Power-Loss Calculations Application of Capacitors to Distribution Systems Distribution System Voltage Regulation Distribution System Protection Distribution System Reliability Electric Power Quality Distributed Generation and Renewable Energy Energy Storage Systems for Electric Power Utility Systems Concept of Smart Grid and Its Applications Appendix A: Impedance Tables for Lines, Transformers, and Underground Cables Appendix B: Graphic Symbols Used in Distribution System Design Appendix C: Standard Device Numbers Used in Protection Systems Appendix D: The Per-Unit System Appendix E: Glossary for Distribution System Terminology Notation Answers to Selected Problems Index
TL;DR: In this paper, a two-stage approach for solving the optimal voltage regulation problem in unbalanced radial distribution system in the presence of photovoltaic (PV) generation is presented.
Abstract: This paper presents a two-stage approach for solving the optimal voltage regulation problem in unbalanced radial distribution system in the presence of photovoltaic (PV) generation. The on-load tap changer (OLTC) and static VAr compensator (SVC) have been considered as the voltage control devices in this work. The formulated voltage control problem is a mixed-integer nonlinear programming problem which remains unsolved even after 8 h due to its computational burden. However, the proposed two-stage approach can solve this problem in less than 10 min. The feasibility of the proposed approach has been demonstrated on a modified IEEE 123-bus radial distribution system.
TL;DR: A new real power capping method is proposed in this paper to prevent overvoltages by adaptively setting the power caps for PV inverters in real time.
Abstract: For distribution networks with a high penetration of photovoltaic (PV) systems, overvoltage is a common and major issue that needs be addressed to not only assure reliable and secure system operation, but also to fully utilize PV generation capacity. A new real power capping method is proposed in this paper to prevent overvoltages by adaptively setting the power caps for PV inverters in real time. The proposed method can maintain voltage profiles below a preset upper limit while maximizing the PV generation and fairly distributing the real power curtailments among all the PV systems in the network. As a result, each of the PV systems in the network has equal opportunity to generate electricity and shares the responsibility of voltage regulation. The method does not require global information and can be implemented either under a centralized supervisory control scheme or in a distributed way via consensus control. Both steady state and dynamic simulation studies under various scenarios have been carried out on a 33-bus distribution system and the IEEE 13-bus test feeder to verify the effectiveness of the method.
TL;DR: In this article, a phase-locked loop (PLL)-based seamless transfer control method between grid-connected and islanding modes in a three-phase gridconnected inverter is proposed.
Abstract: This paper proposes a phase-locked loop (PLL)-based seamless transfer control method between grid-connected and islanding modes in a three-phase grid-connected inverter. The PLL is used to synchronize the phase of the load voltage to a grid voltage in grid-connected operation, and to generate an angle with the desired frequency in islanding operation. The stability of both the grid current loop for grid-connected operation and the load voltage control loop for islanding operation is analyzed. The phase and magnitude of the load voltage are successively matched to the grid voltage for a seamless transfer from islanding to grid-connected operation. When grid voltage sag occurs, an operating sequence including a PLL operation is suggested in order to transfer smoothly to islanding operation and to provide a stable and seamless voltage to a sensitive load under the voltage sag condition. The simulation and experimental results are carried out to verify the effectiveness of the proposed algorithm.
TL;DR: In this paper, the built-in transformer voltage multiplier cell is inserted into each phase of the conventional interleaved boost converter to provide additional control freedom for the voltage gain extension without extreme duty cycle.
Abstract: In this paper, the built-in transformer voltage multiplier cell is inserted into each phase of the conventional interleaved boost converter to provide additional control freedom for the voltage gain extension without extreme duty cycle. The voltage multiplier cell is only composed of the built-in transformer windings, diodes and small capacitors. And additional active switches are not required to simplify the circuit configuration. Furthermore, the switch voltage stress and the diode peak current are also minimized due to the built-in transformer voltage multiplier cells to improve the conversion efficiency. Moreover, there is no reverse-recovery problem for the clamp diodes and the reverse-recovery current for the regenerative and output diodes are controlled by the leakage inductance of the built-in transformer to reduce the relative losses. In addition, the switch turn-off voltage spikes are suppressed effectively by the ingenious and inherent passive clamp scheme and zero current switch (ZCS) turn-on is realized for the switches, which can enhance the power device reliability. Finally, a 40 V-input 380 V-output 1 kW prototype is built to demonstrate the clear advantages of the proposed converter.
TL;DR: In this article, a novel isolated topology was proposed to meet the high efficiency over a wide input voltage range requirement for maximum power point tracking in PV power conditioning systems. But the performance of this topology is limited due to the low currents, zero-voltage switching (ZVS) and low-current switching of the primary side devices, and direct power transfer to the load for the majority of switching cycle.
Abstract: Modular photovoltaic (PV) power conditioning systems (PCSs) require a high-efficiency dc-dc converter stage capable of regulation over a wide input voltage range for maximum power point tracking. In order to mitigate ground leakage currents and to be able to use a high-efficiency, nonisolated grid-tied inverter, it is also desirable for this microconverter to provide galvanic isolation between the PV module and the inverter. This paper presents a novel, isolated topology that is able to meet the high efficiency over a wide input voltage range requirement. This topology yields high efficiency through low circulating currents, zero-voltage switching (ZVS) and low-current switching of the primary side devices, ZCS of the output diodes, and direct power transfer to the load for the majority of switching cycle. This topology is also able to provide voltage regulation through basic fixed-frequency pulsewidth modulated (PWM) control. These features are able to be achieved with the simple addition of a secondary-side bidirectional ac switch to the isolated series resonant converter. Detailed analysis of the operation of this converter is discussed along with a detailed design procedure. Experimental results of a 300-W prototype are given. The prototype reached a peak power stage efficiency of 98.3% and a California Energy Commission (CEC) weighted power stage efficiency of 98.0% at the nominal input voltage.