Showing papers on "AC power published in 2017"

A Survey of Distributed Optimization and Control Algorithms for Electric Power Systems

Abstract: Historically, centrally computed algorithms have been the primary means of power system optimization and control. With increasing penetrations of distributed energy resources requiring optimization and control of power systems with many controllable devices, distributed algorithms have been the subject of significant research interest. This paper surveys the literature of distributed algorithms with applications to optimization and control of power systems. In particular, this paper reviews distributed algorithms for offline solution of optimal power flow (OPF) problems as well as online algorithms for real-time solution of OPF, optimal frequency control, optimal voltage control, and optimal wide-area control problems.

Review of Active and Reactive Power Sharing Strategies in Hierarchical Controlled Microgrids

Abstract: Microgrids consist of multiple parallel-connected distributed generation (DG) units with coordinated control strategies, which are able to operate in both grid-connected and islanded modes Microgrids are attracting considerable attention since they can alleviate the stress of main transmission systems, reduce feeder losses, and improve system power quality When the islanded microgrids are concerned, it is important to maintain system stability and achieve load power sharing among the multiple parallel-connected DG units However, the poor active and reactive power sharing problems due to the influence of impedance mismatch of the DG feeders and the different ratings of the DG units are inevitable when the conventional droop control scheme is adopted Therefore, the adaptive/improved droop control, network-based control methods, and cost-based droop schemes are compared and summarized in this paper for active power sharing Moreover, nonlinear and unbalanced loads could further affect the reactive power sharing when regulating the active power, and it is difficult to share the reactive power accurately only by using the enhanced virtual impedance method Therefore, the hierarchical control strategies are utilized as supplements of the conventional droop controls and virtual impedance methods The improved hierarchical control approaches such as the algorithms based on graph theory, multi-agent system, the gain scheduling method, and predictive control have been proposed to achieve proper reactive power sharing for islanded microgrids and eliminate the effect of the communication delays on hierarchical control Finally, the future research trends on islanded microgrids are also discussed in this paper

Extended State Observer-Based Sliding-Mode Control for Three-Phase Power Converters

Abstract: This paper proposes an extended state observer (ESO) based second-order sliding-mode (SOSM) control for three-phase two-level grid-connected power converters. The proposed control technique forces the input currents to track the desired values, which can indirectly regulate the output voltage while achieving a user-defined power factor. The presented approach has two control loops. A current control loop based on an SOSM and a dc-link voltage regulation loop which consists of an ESO plus SOSM. In this work, the load connected to the dc-link capacitor is considered as an external disturbance. An ESO is used to asymptotically reject this external disturbance. Therefore, its design is considered in the control law derivation to achieve a high performance. Theoretical analysis is given to show the closed-loop behavior of the proposed controller and experimental results are presented to validate the control algorithm under a real power converter prototype.

Enhanced Virtual Synchronous Generator Control for Parallel Inverters in Microgrids

Abstract: Virtual synchronous generator (VSG) control is a promising communication-less control method in a microgrid for its inertia support feature. However, active power oscillation and improper transient active power sharing are observed when basic VSG control is applied. Moreover, the problem of reactive power sharing error, inherited from conventional droop control, should also be addressed to obtain desirable stable state performance. In this paper, an enhanced VSG control is proposed, with which oscillation damping and proper transient active power sharing are achieved by adjusting the virtual stator reactance based on state-space analyses. Furthermore, communication-less accurate reactive power sharing is achieved based on inversed voltage droop control feature ( V–Q droop control) and common ac bus voltage estimation. Simulation and experimental results verify the improvement introduced by the proposed enhanced VSG control strategy.

Coordinated Control Method of Voltage and Reactive Power for Active Distribution Networks Based on Soft Open Point

Abstract: The increasing penetration of distributed generators (DGs) exacerbates the risk of voltage violations in active distribution networks (ADNs). The conventional voltage regulation devices limited by the physical constraints are difficult to meet the requirement of real-time voltage and VAR control (VVC) with high precision when DGs fluctuate frequently. However, soft open point (SOP), a flexible power electronic device, can be used as the continuous reactive power source to realize the fast voltage regulation. Considering the cooperation of SOP and multiple regulation devices, this paper proposes a coordinated VVC method based on SOP for ADNs. First, a time-series model of coordinated VVC is developed to minimize operation costs and eliminate voltage violations of ADNs. Then, by applying the linearization and conic relaxation, the original nonconvex mixed-integer nonlinear optimization model is converted into a mixed-integer second-order cone programming model which can be efficiently solved to meet the requirement of voltage regulation rapidity. Case studies are carried out on the IEEE 33-node system and IEEE 123-node system to illustrate the effectiveness of the proposed method.

Distributed Adaptive Virtual Impedance Control for Accurate Reactive Power Sharing Based on Consensus Control in Microgrids

Abstract: To achieve accurate reactive power sharing regardless of the effects of mismatched line impedance, this paper proposes a reactive power sharing method that employs both consensus control and adaptive virtual impedance control for islanded microgrids. The consensus control is used to find the reactive power mismatch among distributed generation (DG) units. The reactive power mismatch term is fed to a proportional integral controller to generate the virtual impedance correction. With fully distributed regulation of the DG virtual impedance, the load reactive power will be shared accurately among DGs and the circulating line currents among DGs are effectively suppressed. The control strategy does not require the knowledge of the line impedances. Also, the average voltage restoration based on a dynamic consensus control is proposed to recover the decreased output voltage of each DG due to the droop action and the added virtual impedance. Simulation results show the effectiveness of the proposed method in achieving load reactive power sharing and the voltage restoration.

Distributed Secondary Voltage and Frequency Control for Islanded Microgrids With Uncertain Communication Links

Abstract: This paper presents a robust distributed secondary control (DSC) scheme for inverter-based microgrids (MGs) in a distribution sparse network with uncertain communication links. By using the iterative learning mechanics, two discrete-time DSC controllers are designed, which enable all the distributed energy resources (DERs) in an MG to achieve the voltage/frequency restoration and active power sharing accuracy, respectively. In special, the secondary control inputs are merely updated at the end of each round of iteration, and thus, each DER only needs to share information with its neighbors intermittently in a low-bandwidth communication manner. This way, the communication costs are greatly reduced, and some sufficient conditions on the system stability and robustness to the uncertainties are also derived by using the tools of Lyapunov stability theory, algebraic graph theory, and matrix inequality theory. The proposed controllers are implemented on local DERs, and thus, no central controller is required. Moreover, the desired control objective can also be guaranteed even if all DERs are subject to internal uncertainties and external noises including initial voltage and/or frequency resetting errors and measurement disturbances, which then improves the system reliability and robustness. The effectiveness of the proposed DSC scheme is verified by the simulation of an islanded MG in MATLAB/SimPowerSystems.

A State-Independent Linear Power Flow Model With Accurate Estimation of Voltage Magnitude

Abstract: Linearized power flow models are of great interest in power system studies such as contingency analyses and reliability assessments, especially for large-scale systems. One of the most popular models—the classical DC power flow model—is widely used and praised for its state independence, robustness, and computational efficiency. Despite its advantages, however, the DC power flow model fails to consider reactive power or bus voltage magnitude. This paper closes this gap by proposing a decoupled linearized power flow (DLPF) model with respect to voltage magnitude and phase angle. The model is state independent but is distinguished by its high accuracy in voltage magnitude. Moreover, this paper presents an in-depth analysis of the DLPF model with the purpose of accelerating its computation speed, leading to the fast DLPF (FDLPF) model. The approximation that is applied to obtain the FDLPF model from the DLPF model is justified by a theoretical derivation and numerical tests. The proposed methods are provably accurate and robust for several cases, including radial distribution systems, meshed large-scale transmission systems and ill-conditioned systems. Finally, expressions for sensitivity with regard to MW flow and bus voltage are provided as a potential application.

Multi-Timescale Coordinated Voltage/Var Control of High Renewable-Penetrated Distribution Systems

Abstract: This paper proposes a multi-timescale coordinated stochastic voltage/var control method for high renewable-penetrated distribution networks. It aims to utilize multiple devices to counteract uncertain voltage fluctuation and deviation. In the hourly timescale (first stage), capacitor banks and transformer tap changers are scheduled before stochastic renewable output and load variations are realized. In the 15-min timescale (second stage), inverters that interface the renewable energy resources provide var support to supplement the first-stage decision after uncertainty is observed. The coordination is model as a two-stage stochastic programming problem with scenario reduction. It is then converted to a deterministic mixed-integer quadratic programming equivalence model and solved by commercial solvers combined. Compared with existing methods, the proposed volt/var control can achieve lower expected energy loss and can sustain a secure voltage level under random load demand and renewable power injection. The proposed method is verified on the IEEE 33-bus distribution network and compared with existing practices.

Real-Time Optimal Power Flow

Abstract: Future power networks are expected to incorporate a large number of distributed energy resources, which introduce randomness and fluctuations as well as fast control capabilities But traditional optimal power flow methods are only appropriate for applications that operate on a slow timescale In this paper, we build on recent work to develop a real-time algorithm for AC optimal power flow, based on quasi-Newton methods The algorithm uses second-order information to provide suboptimal solutions on a fast timescale, and can be shown to track the optimal power flow solution when the estimated second-order information is sufficiently accurate We also give a specific implementation based on L-BFGS-B method, and show by simulation that the proposed algorithm has good performance and is computationally efficient

Control Strategy for Three-Phase Grid-Connected PV Inverters Enabling Current Limitation Under Unbalanced Faults

Abstract: Power quality and voltage control are among the most important aspects of the grid-connected power converter operation under faults. Nonsinusoidal current may be injected during unbalanced voltage sag, and active or/and reactive power may include double frequency content. This paper introduces a novel control strategy to mitigate the double grid frequency oscillations in the active power and dc-link voltage of the two-stage three-phase grid-connected photovoltaic (PV) inverters during unbalanced faults. With the proposed control method, PV inverter injects sinusoidal currents under unbalanced grid faults. In addition, an efficient and easy-to-implement current limitation method is introduced, which can effectively limit the injected currents to the rated value during faults. In this case, the fault-ride-through operation is ensured, and it will not trigger the overcurrent protection. A non-maximum power point tracking (non-MPPT) operation mode is proposed for the dc–dc converter. The mode is enabled under severe faults when the converter cannot handle the maximum PV power. Finally, experimental validation is provided by implementing a method in an experimental setup, including a 2 kW PV inverter.

Model Predictive Control for Shunt Active Filters With Fixed Switching Frequency

Abstract: This paper presents a modification to the classical Model Predictive Control (MPC) algorithm and its application to active power filters. The proposed control is able to retain all the advantages of a finite control set MPC while improving the generated waveforms harmonic spectrum. In fact, a modulation algorithm, based on the cost function ratio for different output vectors, is inherently included in the MPC. The cost function-based modulator is introduced and its effectiveness on reducing the current ripple is demonstrated. The presented solution provides an effective and straightforward single loop controller, maintaining an excellent dynamic performance despite the modulated output and it is self-synchronizing with the grid. This promising method is applied to the control of a shunt active filter for harmonic content reduction through a reactive power compensation methodology. Significant results obtained by experimental testing are reported and commented, showing that MPC is a viable control solution for active filtering systems.

Distributed Reactive Power Sharing Control for Microgrids With Event-Triggered Communication

Abstract: Due to the inherently distributed and heterogeneous nature of the microgrids, distributed control can be a promising approach to improve the stability, reliability, and scalability of the microgrids compared with centralized control strategies. This paper studies the distributed reactive power sharing problem for a microgrid with connected ac inverters. Under the standard decoupling approximation for bus angle differences, the reactive power flow of each inverter is dependent on the voltage amplitudes of its neighboring inverters connected by electrical power lines. Using the Lyapunov approach, a novel distributed voltage controller with nonlinear state feedback is proposed for reactive power sharing of the microgrid. It is proved that the inverters can achieve accurate reactive power sharing under the proposed controller if the communication network of inverters is connected. Then, by introducing the sampling and holding scheme, we extend the proposed controller to the nonlinear state feedback control with event-triggered communication among inverters. The new event-triggered control approach can dramatically reduce the amount of communication of the microgrid, and significantly relax the requirement for precise real-time information transmission among the inverters. Both the proposed controllers are validated by simulations on a group of inverters with time-varying loads.

Review of harmonic analysis, modeling and mitigation techniques

Abstract: Power quality problems are manifested in voltage, current or frequency deviations causing malfunction of sensitive equipment. Integration of inverter connected PV and wind power plants, and rampant rise in nonlinear loads have led to harmonic problem in power system. Nonlinear loads and switched devices energized by sinusoidal sources or linear loads and switched devices with non-sinusoidal sources, produce harmonics in distribution system. Academic harmonic analysis study consists of modeling nonlinear loads to develop Norton and Thevenin equivalent circuits of devices for integration into harmonic analysis software. Experimental researchers often use harmonic analyzers to measure the harmonics in real systems to evaluate suitable mitigation alternatives. The distortive power losses force utilities to increase apparent power to maintain reliable and uniform power supply. Harmonic analyzers use data acquisition hardware and inbuilt software algorithms to perform onsite measurements. Harmonic analyzers help find true power factor, total harmonic distortions, reactive and distortive power losses. Use of shunt capacitance at unity power factor worsens the situation instead of supplying distortive power compensation. Active power factor correction techniques, using smart algorithm to cancel the distortive power, have been reviewed for further research. Nonlinear physics of harmonic phenomenon is described to explore its applications. Harmonic mitigation technologies have been compared, current state of the art technology reviewed and demonstrated by designing a harmonic filter. Measurement of harmonics, waveform distortions, and true power factor (TPF) of single and three phase electronic loads is carried out to test their compliance to harmonic standard limits. Energy conservation concept requires reduction of harmonics in distribution networks. This study found 60±10% reduction in power factor and more than 2% increase in line losses due to widespread use of nonlinear loads. Utility apparent power demand increases due to consumers’ inadvertent violation of IEC Standard 61000-3-2 and IEEE Standard 519–1992.

Fast-Frequency Response Provided by DFIG-Wind Turbines and its Impact on the Grid

Abstract: This paper presents a methodology for the analysis of frequency dynamics in large-scale power systems with high level of wind energy penetration by means of a simplified model for DFIG-based wind turbines. In addition, a virtual inertia controller version of the optimized power point tracking (OPPT) method is implemented for this kind of wind turbines, where the maximum power point tracking curve is shifted to drive variations in the active power injection as a function of both the grid frequency deviation and its time derivative. The proposed methodology integrates the model in a primary frequency control scheme to analyze the interaction with the rest of the plants in the power system. It is also proven that, under real wind conditions, the proposed version of the OPPT method allows us to smooth the wind power injected into the grid, thereby reducing frequency fluctuations.

Optimal Allocation of Distributed Generation Using Hybrid Grey Wolf Optimizer

Abstract: Optimal allocation of distributed generation units is essential to ensure power loss minimization, while meeting the real and reactive power demands in a distribution network. This paper proposes a solution to this non-convex, discrete problem by using the hybrid grey wolf optimizer, a new metaheuristic algorithm. This algorithm is applied to IEEE 33-, IEEE 69-, and Indian 85-bus radial distribution systems to minimize the power loss. The results show that there is a considerable reduction in the power loss and an enhancement of the voltage profile of the buses across the network. Comparisons show that the proposed method outperforms all other metaheuristic methods, and matches the best results by other methods, including exhaustive search, suggesting that the solution obtained is a global optimum. Furthermore, unlike for most other metaheuristic methods, this is achieved with no tuning of the algorithm on the part of the user, except for the specification of the population size.

Centralized Control Architecture for Coordination of Distributed Renewable Generation and Energy Storage in Islanded AC Microgrids

Abstract: The coordinated operation of distributed energy resources such as storage and generation units and also loads is required for the reliable operation of an islanded microgrid. Since in islanded microgrids the storage units are commonly responsible for regulating the voltage amplitude and frequency in the local power system, the coordination should consider safe operating limits for the stored energy, which prevents fast degradation or damage to the storage units. This paper proposes a centralized control architecture, applicable for local area power systems such as a small-scale microgrid. The centralized architecture is based on three supervisory control tasks which consider: active power curtailment of generation for avoiding overcharge of the storage units, load shedding actions for preventing deep discharge of the storage units, and equalization of the state of charge (SoC) among distributed storage systems for avoiding uneven degradation. The proposed equalization method has proved to be effective for equalizing the SoC of distributed energy storage systems and for ensuring uniform charge/discharge ratios regardless of differences in the capacity of the storage units. Additionally, the strategy is complemented with an optimal scheduling of load connection, which minimizes the connection and disconnection cycles of the loads within a time horizon of 24 h. The proposed architecture is verified experimentally in a lab-scale prototype of a microgrid, which has real communication between the microgrid and the central controller.

Rapid Active Power Control of Photovoltaic Systems for Grid Frequency Support

Abstract: As deployment of power electronic coupled generation such as photovoltaic (PV) systems increases, grid operators have shown increasing interest in calling on inverter-coupled generation to help mitigate frequency contingency events by rapidly surging active power into the grid. When responding to contingency events, the faster the active power is provided, the more effective it may be for arresting the frequency event. This paper proposes a predictive PV inverter control method for very fast and accurate control of active power. This rapid active power control (RAPC) method will increase the effectiveness of various higher-level controls designed to mitigate grid frequency contingency events, including fast power-frequency droop, inertia emulation, and fast frequency response, without the need for energy storage. The RAPC method, coupled with a maximum power point estimation method, is implemented in a prototype PV inverter connected to a PV array. The prototype inverter’s response to various frequency events is experimentally confirmed to be fast (beginning within 2 line cycles and completing within 4.5 line cycles of a severe test event) and accurate (below 2% steady-state error).

A Smart Power Meter to Monitor Energy Flow in Smart Grids: The Role of Advanced Sensing and IoT in the Electric Grid of the Future

Abstract: This paper aims to describe the role of advanced sensing systems in the electric grid of the future. In detail, the project, development, and experimental validation of a smart power meter are described in the following. The authors provide an outline of the potentialities of the sensing systems and IoT to monitor efficiently the energy flow among nodes of an electric network. The described power meter uses the metrics proposed in the IEEE Standard 1459–2010 to analyze and process voltage and current signals. Information concerning the power consumption and power quality could allow the power grid to route efficiently the energy by means of more suitable decision criteria. The new scenario has changed the way to exchange energy in the grid. Now, energy flow must be able to change its direction according to needs. Energy cannot be now routed by considering just only the criterion based on the simple shortening of transmission path. So, even energy coming from a far node should be preferred, if it has higher quality standards. In this view, the proposed smart power meter intends to support the smart power grid to monitor electricity among different nodes in an efficient and effective way.

Model Predictive Control of a Voltage-Source Inverter With Seamless Transition Between Islanded and Grid-Connected Operations

Abstract: Inverter-based distributed generation (DG) system is becoming an attractive solution for high penetration of renewable energy sources to the main grid. DG system should be able to supply power to the local loads whenever necessary even in case of utility power outage. Thus, the inverters in DG systems are expected to operate in both grid-connected and islanded mode, where they are acting as a current source for the ac grid and a voltage-source for the load, respectively. Transition between modes of operation is nontrivial and can cause deviations in voltage and current, because of mismatch in frequency, phase, and amplitude between the inverter output voltage and the grid voltage. Thus, it is necessary to have seamless transition between grid-connected and islanded mode. This paper presents a new control strategy with seamless transfer characteristics for a grid-connected voltage-source inverter using model predictive control (MPC) framework. The main objectives of the proposed predictive controller are: 1) decoupled power control in grid-connected mode, which enables the proposed power electronics interface to provide ancillary services such as reactive power compensation; 2) load voltage control in islanded mode; and 3) seamless transition between modes of operation through proposed synchronization and phase adjustment algorithm. The proposed controller features simplicity to implement since only one cost function should be minimized for all modes of operation, and hence no ambiguity in the control algorithm that could cause mode transition problems. An autotuning strategy for weight factors in MPC cost function is proposed to simplify the weight factor tuning strategy. The stability analysis of the proposed controller is provided. Simulation and experimental results validate the expected performance and effectiveness of the proposed control strategy.

Higher Order Compensation for Inductive-Power-Transfer Converters With Constant-Voltage or Constant-Current Output Combating Transformer Parameter Constraints

Abstract: Compensation is crucial for improving performance of inductive-power-transfer (IPT) converters. With proper compensation at some specific frequencies, an IPT converter can achieve load-independent constant output voltage or current, near zero reactive power, and soft switching of power switches simultaneously, resulting in simplified control circuitry, reduced component ratings, and improved power conversion efficiency. However, constant output voltage or current depends significantly on parameters of the transformer, which is often space constrained, making the converter design hard to optimize. To free the design from the constraints imposed by the transformer parameters, this paper proposes a family of higher order compensation circuits for IPT converters that achieves any desired constant-voltage or constant-current (CC) output with near zero reactive power and soft switching. Detailed derivation of the compensation method is given for the desired transfer function not constrained by transformer parameters. Prototypes of CC IPT configurations based on a single transformer are constructed to verify the analysis with three different output specifications.

A Novel Nine-Level Inverter Employing One Voltage Source and Reduced Components as High-Frequency AC Power Source

Abstract: Increasing demands for power supplies have contributed to the population of high-frequency ac (HFAC) power distribution system (PDS), and in order to increase the power capacity, multilevel inverters (MLIs) frequently serving as the high-frequency (HF) source-stage have obtained a prominent development. Existing MLIs commonly use more than one voltage source or a great number of power devices to enlarge the level numbers, and HF modulation (HFM) methods are usually adopted to decrease the total harmonic distortion (THD). All of these have increased the complexity and decreased the efficiency for the conversion from dc to HF ac. In this paper, a nine-level inverter employing only one input source and fewer components is proposed for HFAC PDS. It makes full use of the conversion of series and parallel connections of one voltage source and two capacitors to realize nine output levels, thus lower THD can be obtained without HFM methods. The voltage stress on power devices is relatively relieved, which has broadened its range of applications as well. Moreover, the proposed nine-level inverter is equipped with the inherent self-voltage balancing ability, thus the modulation algorithm gets simplified. The circuit structure, modulation method, capacitor calculation, loss analysis, and performance comparisons are presented in this paper, and all the superior performances of the proposed nine-level inverter are verified by simulation and experimental prototypes with rated output power of 200 W. The accordance of theoretical analysis, simulation, and experimental results confirms the feasibility of proposed nine-level inverter.

A Single-Phase Transformerless Inverter With Charge Pump Circuit Concept for Grid-Tied PV Applications

Abstract: This paper proposes a new single-phase transformerless photovoltaic (PV) inverter for grid-tied PV systems. The topology is derived from the concept of a charge pump circuit in order to eliminate the leakage current. It is composed of four power switches, two diodes, two capacitors, and an LCL output filter. The neutral of the grid is directly connected to the negative polarity of the PV panel that creates a constant common mode voltage and zero leakage current. The charge pump circuit generates the negative output voltage of the proposed inverter during the negative cycle. A proportional resonant control strategy is used to control the injected current. The main benefits of the proposed inverter are: 1) the neutral of the grid is directly connected to the negative terminal of the PV panel, so the leakage current is eliminated; 2) its compact size; 3) low cost; 4) the used dc voltage of the proposed inverter is the same as the full-bridge inverter (unlike neutral point clamped (NPC), active NPC, and half-bridge inverters); 5) flexible grounding configuration; 6) capability of reactive power flow; and 7) high efficiency. A complete description of the operating principle and analysis of the proposed inverter are presented. Experimental results are presented to confirm both the theoretical analysis and the concept of the proposed inverter. The obtained results clearly validate the performance of the proposed inverter and its practical application in grid-tied PV systems.

Methods and strategies for overvoltage prevention in low voltage distribution systems with PV

Abstract: The rapid development of photovoltaic (PV) systems in electrical grids brings new challenges in the control and operation of power systems. A considerable share of already installed PV units is small-scale units, usually connected to low-voltage (LV) distribution systems that were not designed to handle a high share of PV power. This study provides an in-depth review of methods and strategies proposed to prevent overvoltage in LV grids with PV and discusses the effectiveness, advantages, and disadvantages of them in detail. On the basis of the mathematical framework presented in this study, the overvoltage caused by high PV penetration is described, solutions to facilitate higher PV penetration are classified, and their effectiveness, advantages, and disadvantages are illustrated. The investigated solutions include the grid reinforcement, electrical energy storage application, reactive power absorption by PV inverters, application of active medium-voltage to LV transformers, active power curtailment, and demand response. Coordination between voltage control units by localised, distributed, and centralised voltage control methods is compared using the voltage sensitivity analysis. On the basis of the analysis, a combination of overvoltage prevention methods and coordination between voltage control units can provide an efficient solution to increase the PV hosting capacity of LV grids.

Power Flow Analysis for Low-Voltage AC and DC Microgrids Considering Droop Control and Virtual Impedance

Abstract: In the low-voltage (LV) ac microgrids (MGs), with a relatively high R/X ratio, virtual impedance is usually adopted to improve the performance of droop control applied to distributed generators (DGs). At the same time, LV dc MG using virtual impedance as droop control is emerging without adequate power flow studies. In this paper, power flow analyses for both ac and dc MGs are formulated and implemented. The mathematical models for both types of MGs considering the concept of virtual impedance are used to be in conformity with the practical control of the DGs. As a result, calculation accuracy is improved for both ac and dc MG power flow analyses, comparing with previous methods without considering virtual impedance. Case studies are conducted to verify the proposed power flow analyses in terms of convergence and accuracy. Investigation of the impact to the system of internal control parameters adopted by DGs is also conducted by using proposed method.

Flexible Power Regulation and Current-Limited Control of the Grid-Connected Inverter Under Unbalanced Grid Voltage Faults

Abstract: The grid-connected inverters may experience excessive current stress in case of unbalanced grid voltage fault ride through (FRT), which significantly affects the reliability of the power supply system. In order to solve the problem, the inherent mechanisms of the excessive current phenomenon with the conventional FRT solutions are discussed. The quantitative analysis of three-phase current peak values is conducted and a novel current-limited control strategy is proposed to achieve the flexible active and reactive power regulation and successful FRT in a safe current operation area with the aim of improving the system reliability under grid faults. Finally, the simulation and experiments of traditional and proposed FRT solutions are carried out. The results verify the effectiveness of the proposed method.