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As the inverter-based generation replaces the conventional synchronous generators, it may also need to fill in the missing ancillary service support. One of these ancillary services is dynamic reactive power provision and voltage control. This paper analyzes optimal strategy of reactive and active fault-current support of the inverter-based generation leading to fast voltage recovery of the system. For the purpose of the analysis, new ramping active current controller able to emulate different behavior of active current injection is proposed. By optimizing its parameters for different case studies of the system, the conclusions about optimal behavior of the inverter based generation with respect to system parameters and operating conditions are drawn. It is observed that the optimal combination of active and reactive fault-current is the most sensitive to the dynamic load component penetration levels in the system. With the increasing penetration levels, the significance of active fault-current injection increases. The results show that with higher penetration levels of dynamic load component in the heavy load areas, the ramping down of the inverter-based generation active fault-current results in slower voltage recovery of the system. Following this conclusion, a recommendation on update of current European grid codes is proposed. [ABSTRACT FROM AUTHOR]

The modern photovoltaic inverter has an active power-reactive power (Watt-VAr) control curve that is part of its management modes. The Watt-VAr curve, whose settings can be remotely modified, regulates the inverter reactive power output in function of its active power. This paper proposes the optimization of inverter segregated linear decision rules (LDRs), starting from the legacy Volt-VAr optimization in distribution networks. As opposed to classical LDRs that are interfaced with Volt-VAr optimization, the segregated LDRs are piecewise linear continuous curves that directly satisfy the reactive power capability of inverters, and their multiple segments can capture the correlation in solar power variability. The paper presents linear programming formulations for optimizing the segregated linear decision rules using stochastic optimization, robust optimization, and distributionally robust optimization. The optimization schemes are demonstrated on both meshed and radial networks, and their performance is validated via Monte-Carlo simulation. [ABSTRACT FROM AUTHOR]

In this paper five different flexibility options are analysed from a techno-economic perspective as alternatives to traditional grid expansion for a specific distribution grid in Germany. The options are: two reactive power control strategies with photovoltaic inverters (as a function of the power feed-in, or of the voltage at the connection point), one residential and two large scale battery storage applications (primary control reserve with autonomous reactive power control or self consumption maximisation strategy with autonomous reactive power control). For the pilot grid located in Southern Germany a photovoltaic expansion pathway is determined. The main goal of this work is to quantify the grid expansion actions that can be avoided by applying these five flexibility options for the assumed expansion pathway, focusing on large scale battery storages. It is shown that the five flexibility options increase the hosting capacity for PV systems, compared to a scenario without, by up to 45%. Furthermore, the results of the economic assessment indicate that the analysed flexibility options might be a viable alternative to traditional grid expansion as all of them show a cost reduction potential for the pilot region. These results could encourage DSOs to consider the integration of additional PV and battery storage systems not as a problem which triggers grid expansion, but as part of the solution reducing future grid expansion costs. [ABSTRACT FROM AUTHOR]

This paper models and analyzes a power distribution system with sources connected through inverters. The stabilizing effect of generator rotational inertia is emulated through inverter droop control logic. We model the nonlinear dynamics of the system with multiple distributed droop inverters using dynamical phasors. Linearizing the system around the steady-state, we construct closed-loop transfer functions from exogenous disturbances to system states. This allows to evaluate the load disturbance impact on the inverter power and line voltage profile. As an example, the distribution system with 150 inverters is used to illustrate the approach. [ABSTRACT FROM PUBLISHER]

The authors have proposed Flexible, Reliable and Intelligent ENergy Delivery System (FRIENDS) as a future power distribution system. In the FRIENDS, new facilities called quality control centers (QCCs), which consist of power electronics devices, distributed generators (DGs), energy storage systems (ESSs), etc., are installed between distribution substations and customers. By controlling and operating those devices in QCCs adequately, various functions can be realized. Particularly, when a power supply from the transmission network is interrupted, isolated local networks which consist of some QCCs are composed in order to realize uninterruptible power supply. In the isolated local network, the DG and ESS in QCCs are employed as backup generators. This paper proposes a novel distributed and autonomous method for balancing the supply and demand during isolated operation. This paper also investigates the effectiveness of the proposed method through some computational simulations using PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

In this paper, the Unit Commitment (UC) problem in a power network with low levels of rotational inertia is studied. Frequency-related constraints, namely the limitation on Rate-of-Change-of-Frequency (RoCoF), frequency nadir and steady-state frequency error, are derived from a uniform system frequency response model that incorporates dynamics and controls of both synchronous generators and grid-forming inverters. These constraints are then included into a stochastic UC formulation that accounts for wind power and equipment contingency uncertainties using a scenario-tree approach. In contrast to the linear RoCoF and steady-state frequency error constraints, the nadir constraint is highly nonlinear. To preserve the mixed-integer linear formulation of the stochastic UC model, we propose a computationally efficient approach that allows to recast the nadir constraint by introducing appropriate bounds on relevant decision variables of the UC model. This method is shown to be generally more accurate and computationally more efficient for medium-sized networks than a piece-wise linearization method adapted from the literature. Simulation results for a modified IEEE RTS-96 system revealed that the inclusion of inertia-related constraints significantly influences the UC decisions and increases total costs, as more synchronous machines are forced to be online to provide inertial response. [ABSTRACT FROM AUTHOR]

The new IEEE 1547-2018 standard includes dynamic Volt/VAr control for photovoltaic (PV) smart inverters. While recent research has addressed the problem of optimal inverter dispatch, the interaction between inverters and the classical Volt/VAr control (VVC) regulation system needs further study. This paper proposes an approach that builds on the classical VVC solution with PVs, by computing for each inverter a rule that modulates the smart inverter reactive power in function of its real power. Two approaches are presented for computing the inverter's reactive power equation slope: the first approach is based on the robust minimization of the absolute voltage magnitude deviation via a linear program, whereas the second approach yields closed-form solutions inspired from distributionally robust chance constraints. Numerical results are presented on weakly meshed distribution networks having up to 3146 nodes; they demonstrate that the voltage violations due to intermittent real power variations are significantly reduced by the decision rules as compared to maintaining the last computed VVC set-points, even when the inverters operate at constant power factor or with default Volt/VAr settings proposed in the literature; additionally, the reactive power decision rules maintain a network loss level that is close to the average from a centralized solution. [ABSTRACT FROM AUTHOR]

Among parallel inverters in islanded microgrid, circulating current originated by output impedance mismatches and output voltage differences cannot be neglected. Conventional droop control and fixed virtual impedance control are sensitive when the load changes. In this paper, the concept of equivalent feeder is proposed to cover the mismatch of external feeder impedance. The differences of external inductor, local load, and transmission line among parallel inverters are considered. Based on equivalent feeder impedance calculation, an aggregated control strategy of improved droop control and adaptive virtual impedance control is introduced. To improve power sharing accuracy, the equivalent feeder voltage drop of each inverter is calculated and compensated to the reference voltage of droop control. Conventional virtual impedance control is defective to cope with sudden load perturbations, especially when local loads exist. In this regard, the adaptive virtual complex impedance control can adjust to the equivalent feeder impedance, which provides accurate and quick response to load changes and line impedance mismatches. Simulation results show that the proposed control strategy can reduce circulating current effectively as well as improve power sharing accuracy. [ABSTRACT FROM AUTHOR]

This paper proposes the notion of “critical cluster” in inverter based droop controlled microgrids, thus representing the neighborhood of the distributed generation units that determines the small signal stability margin of the entire system. The clustering starts from the lowest impedance electrical connection between distributed generators, termed as “critical line”. At first, the systematic study of few similar microgrids by means of the eigenvalue analysis of their small signal models reveals the correlation between the location of their dominant low frequency modes and the individual connections between neighboring inverters. Second, the sensitivity analysis of active power droop gain with respect to network parameters confirms the previous findings and shows the dominating effect of the critical line's impedance. Simulations and small signal analysis studies assess the impact of the choice of various interconnection points when two individual droop controlled islanded microgrids are connected to form a single microgrid. At the same time, simplified models on the basis of critical clusters allow the effective approximation of the stability margin corresponding to the original systems. The results reveal that different interconnection points lead to strong variations in the performance of the coupled microgrid both in terms of transient response and small signal stability. [ABSTRACT FROM PUBLISHER]

This paper considers future distribution networks featuring inverter-interfaced photovoltaic (PV) systems, and addresses the synthesis of feedback controllers that seek real- and reactive-power inverter setpoints corresponding to AC optimal power flow (OPF) solutions. The objective is to bridge the temporal gap between long-term system optimization and real-time inverter control, and enable seamless PV-owner participation without compromising system efficiency and stability. The design of the controllers is grounded on a dual \epsilon-subgradient method, while semidefinite programming relaxations are advocated to bypass the non-convexity of AC OPF formulations. Global convergence of inverter output powers is analytically established for diminishing stepsize rules for cases where: i) computational limits dictate asynchronous updates of the controller signals, and ii) inverter reference inputs may be updated at a faster rate than the power-output settling time. [ABSTRACT FROM PUBLISHER]

This paper develops an analytical model of an unbalanced radial distribution system consisting of a single-phase photovoltaic (PV), a three-phase induction machine load, a three-phase power factor correction capacitor (PFC), and a load. The analytical model is based on dynamic phasors (DP) for abc phases. The single-phase PV model includes inverter current control [proportional resonance (PR) controller], an L, or an LCL filter. The induction machine model is based on positive-, negative-, and zero-sequence components' dynamic phasors. The sequence-based induction machine model was converted to the DP-abc reference frame and interconnected with other grid components. The developed analytical model is capable of small-signal analysis and can be used to identify variety of stability and/or harmonic issues in distribution networks, e.g., instability due to weak grid. Impact of unbalance on system dynamic performance can also be investigated using this model. The analytical model is benchmarked with a high-fidelity model built in Matlab/SimPowerSystems where power electronic switching details are included. The small-signal analysis results are validated via Matlab/SimPowerSystems time-domain simulations. [ABSTRACT FROM PUBLISHER]

This paper presents a novel hierarchical framework for real-time, network-admissible coordination of responsive grid resources aggregated into virtual batteries (VBs). In this context, a VB represents a local aggregation of directly controlled loads, such as smart inverters, electric water heaters, and air-conditioners. The coordination is achieved by solving an optimization problem to disaggregate a feeder's desired reference trajectory into constraint-aware set-points for the VBs. Specifically, a novel, provably-tight, convex relaxation of the AC optimal power flow (OPF) problem is presented to optimally dispatch the VBs to track the feeder's desired power trajectory. In additionto the optimal VB dispatch scheme, a real-time, corrective control scheme is designed, whichis based on optimal proportional-integral (PI) control with anti-windup, to reject intra-feeder and inter-feeder disturbances that arise during operation of the power system. Simulation results conducted on a modified IEEE test system demonstrate the effectiveness of the proposed multi-layer VB coordination framework. [ABSTRACT FROM AUTHOR]

This paper proposes a multi-purpose VAr control strategy for solar PV inverters for voltage support in distribution networks. The proposed strategy can be applied under various PV power generation conditions. The inverters will normally operate in a dynamic VAr compensation mode for voltage support (including low PV and no PV periods). During mid-day when PV has surplus power, the proposed strategy will control the PV inverters to absorb VAr for voltage rise mitigation using a droop characteristic approach. During passing clouds, the strategy will mitigate voltage fluctuations by ramp-rate control of inverter VAr output. A dynamic model of the proposed PV inverter control has been developed to analyze its performance in terms of fast VAr control and voltage support under various PV generation conditions. The results of the analysis performed on an Australian distribution system show that the proposed VAr control strategy can mitigate voltage rise, and improve the voltage profile despite potential vast changes in the sun irradiation during passing cloud and also in the absence of PV output during the evening. [ABSTRACT FROM PUBLISHER]

There are more large-scale photovoltaic (PV) plants being established in rural areas due to availability of low-priced land. However, distribution grids in such areas traditionally have feeders with low X/R ratios, which makes the independent reactive power compensation method less effective on voltage regulation. Consequently, upstream step voltage regulator (SVR) may suffer from excessive tap operations with PV-induced fast voltage fluctuations. Although a battery energy storage system (BESS) can successfully smooth PV generation, frequent charge/discharge will substantially affect its cost effectiveness. In this paper, a real-time method is designed to coordinate PV inverters and BESS for voltage regulation. To keep up with fast fluctuations of PV power, this method will be executed in each 5 s control cycle. In addition, charging/discharging power of BESS is adaptively retuned by an active adjustment method in order to avoid BESS premature energy exhaustion in a long run. Finally, through a voltage margin control scheme, the upstream SVR and downstream PV inverters and BESS are coordinated for voltage regulation without any communication. This research is validated via a real-time digital simulator MatLab cosimulation platform, and it will provide valuable insights and applicable strategies to both utilities and PV owners for large-scale PV farm integration into rural networks. [ABSTRACT FROM PUBLISHER]

The increasing share of volatile and inverter-based energy sources render electric power grids increasingly susceptible to disturbances. Established Load Frequency Controls (LFC) schemes are rigid and require careful tuning, making them unsuitable for dynamically changing environments. In this paper, we present a fast and tuningless frequency control approach that tackles these shortcomings by means of modern grid monitoring and communications infrastructures in a twofold concurrent process. First, direct observation of supply and demand enables fast power balancing decoupled from the total system dynamics. Second, primary resources are actively involved in frequency restoration by systematic adjustment of their frequency reference setpoints. In contrast to the commonly used Automatic Generation Control (AGC), the proposed direct LFC does not require an integrator for frequency control in the closed loop even under partial grid observability. The approach is Lyapunov-stable for a wide range of system parameters, including ramping limits of controlled resources. A performance study against AGC has been conducted on a three-area power system in simulations as well as in a real-laboratory grid with an installed generation capacity of 110 kW. [ABSTRACT FROM PUBLISHER]

This paper presents a fast and reliable method to evaluate risk of single or multiple commutation failures for multi-infeed HVDC systems following faults occurring at the receiving-end ac system. First, a fast calculation method of multi-infeed interaction factor, describing the interactivity and mutual impact between two inverter AC buses on is proposed. It is then refined to being the AC–DC system voltage interaction factor (ADVIF) to represent the relationship in bus voltages between inverter and AC buses at the re-ceiving-end systems. Then, a critical AC–DC system voltage interaction factor (CADVIF) based on the minimum extinction angle criteria is proposed. By calculating and comparing ADVIFs and CADVIFs, which can be obtained quickly, it is possible to identify AC buses that, if a fault occurs, would cause commutation failures at one or more HVDC systems. The validity and accuracy of the proposed approach are demonstrated by comparing with simulations results using a two-infeed HVDC test system and an actual large power grid. [ABSTRACT FROM PUBLISHER]

Component mismatches and parameters drifts drastically affect stability and long-term operation of droop-controlled inverter-based microgrids. This paper analyzes and illustrates the impact of design variations and parameter drifts between angle droop controlled inverter-interfaced sources in a microgrid. It is shown that microgrid stability is very sensitive to parameter drifts, especially in frequency. A coordination control scheme that uses internode communications is proposed for improving the stability margin and ensuring the desired power sharing. Conditions for stability are derived and simulation results are presented to validate the performance of the proposal. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

The proliferation of inverter-based generation and advanced sensing, controls, and communication infrastructure have facilitated the accelerated deployment of microgrids. A coordinated network of microgrids can maintain reliable power delivery to critical facilities during extreme events. Low-inertia offered by the power-electronics interfaced energy resources, however, can present significant challenges to ensuring stable operation of the microgrids. In this work, distributed small-signal stability conditions for inverter-based microgrids are developed that involve the droop-controller parameters and the network parameters (e.g. line impedances, loads). The distributed closed-form parametric stability conditions derived in this paper can be verified in a computationally efficient manner, facilitating the reliable design and operations of networks of microgrids. Dynamic phasor models have been used to capture the effects of electromagnetic transients. Numerical results are presented, along with PSCAD simulations, to validate the analytical stability conditions. Effects of design choices, such as the conductor types, and inverter sizes, on the small-signal stability of inverter-based microgrids are investigated to identify interpretable stable/unstable region estimates. [ABSTRACT FROM AUTHOR]

This paper details a modeling procedure that incorporates composite loads in stand-alone microgrids in which, because of the low system inertia provided by inverter-interfaced generation units, the grid dynamics is not neglected. The paper introduces a methodology based on 1) separately treating the plants (RL grid elements) from reference frames and control systems; and 2) establishing a vector valued function to methodologically describe all plants in a similar way. Induction motors equations are rearranged to be integrated within the model, giving as a result a highly structured, compact system model. Next, bifurcation theory is adapted to the problem to show that composite loads are a need in the microgrid modeling if more realistic results about oscillations and mainly about load margin are pursued. Thanks to the modeling procedure, this is proven by means of a series of analyses conducted in a microgrid of considerable larger dimensions than those presented to date in the literature. [ABSTRACT FROM AUTHOR]

This paper shows that the current an inverter interfaced distributed generator (IIDG) contributes to a fault varies considerably, due mainly to fast response of its controller. This paper proposes a method to extend the conventional fault analysis methods so that IIDG contribution can be estimated in the fault analysis. The proposed method gives rms profiles of the fault currents of interest (IIDG contribution and the fault currents the protective device will see). Test results, based on a prototype feeder, show that the proposed approach can estimate the fault current's contributions under both balanced and unbalanced fault conditions. [ABSTRACT FROM AUTHOR]

This paper presents a novel hybrid converter configuration for conventional Line-Commutated Converter (LCC) HVDC technology aiming to eliminate commutation failures under serious faults. Dynamic series insertion of capacitors during commutation is utilized to increase the effective commutation voltage. The operating principles are presented followed by detailed mathematical analysis for both zero impedance single-phase and three-phase faults in order to select the required capacitor size and its voltage level. The performance of the proposed method is validated by simulation results in Real Time Digital Simulator (RTDS) and the results show that the proposed converter configuration is able to eliminate commutation failures under both fault cases. Consequently partial power transferring capability during single-phase fault and fast fault recovery from three-phase fault can be achieved. Further simulation results show that the harmonic content of inverter AC voltage and current are not significantly increased and the voltage stress of the thyristor valve is comparable to that of the original benchmark system. [ABSTRACT FROM PUBLISHER]

Control of inverter-based microgrids (IBMGs) with power sharing capability is a challenging task due to specific nature of microgrid instabilities. Specifically, droop-controlled microgrids prone to become unstable when the $X/R$ ratio is near unity and the lines are short. Virtual impedance is thought to be one of the promising tools to prevent this type of instability allowing for a broad range of droop coefficients for efficient power sharing between inverters. However, proper sizing of virtual impedance for particular microgrid configuration is still an open question. In this paper, a systematic method for determining the size of virtual impedance in a plug-and-play fashion is proposed for enhancing the stability of the droop modes. Based on the Lyapunov method, concise and simple stability certificates are derived to provide flexible choices of the droop gains for different inverter ratings. The implementation of the virtual impedance with the proposed approach for stability enhancement is verified by both numerical simulation through MATLAB/Simulink and experiment via a small-scale prototype for different scenarios. [ABSTRACT FROM AUTHOR]

This paper develops a tractable formulation for optimal placement and sizing of inverter-based renewable systems in multi-phase distribution networks. The goal of the formulation is to minimize the cost of inverter installation, average power import, and average distributed generation curtailment. Three-phase and single-phase inverter models are presented that preserve the underlying mappings between renewable uncertainty to power injection. The uncertainty of distributed generators (DGs) and loads are characterized by a finite set of scenarios. Linear multi-phase power flow approximations are used in conjunction with scenario reduction techniques to arrive at a tractable two-stage stochastic formulation for optimal DG placement and sizing. First-stage decisions are locations for DG deployment and capacity sizes, and second-stage decisions include DG real power curtailment, reactive power support, as well as feeder voltage profile. The resulting formulation is a mixed-integer second-order cone program and can be solved efficiently either by existing optimization solvers or by relaxing the binary variables to the [0,1] interval. Simulation studies on standard multi-phase IEEE test feeders promise that optimal stochastic planning of DGs reduces costs during validation, compared to a scheme where uncertainty is only represented by its average value. [ABSTRACT FROM AUTHOR]

Individual microgrids have proven their ability to provide uninterrupted power to critical end-use loads during severe events. Building on the performance of individual microgrids during extreme events, there has been an increasing interest in the operations of network microgrids. By networking microgrids during extreme events, it is possible to share resources, increase the duration for which they can operate islanded, improve efficiency, and increase the resiliency of critical end-use loads. While there are benefits to networking the operations of resiliency-based microgrids, the switching operations that they require introduce transients, which can result in a loss of dynamic stability. The issue of dynamic stability is especially acute in microgrids with high penetrations of inverter-connected generation, and correspondingly low system inertia. While the low inertia of these microgrids can be increased by over-sizing the rotating generators, the increased capital and operating costs can become a barrier to deployment. This paper will present a method of augmenting primary frequency controls to support the switching transients necessary for the operation of networked microgrids, without the need to over-size rotating machines. [ABSTRACT FROM AUTHOR]

The islanding detection is one of the most important issues for grid-connection of the distributed generation (DG) systems. Several recent researches have developed two types of islanding detection methods (IDMs) with high-performance. One is the power and voltage (P-V) characteristic of load based IDM, and the other is the inverter's switching frequency based IDM. Although the above IDMs have no non-detection zone (NDZ) theoretically, their usefulness mainly depends on the control performance of the inverter in practice. In other words, the inverter must achieve the decoupled real and reactive control and successful operation of phase-locked loop (PLL). To deal with these problems, this paper describes the design of inverter controller to improve both system stability and islanding detection performance with the NDZ. The proposed controller uses the magnitude and phase of measured voltage directly instead of using the direct and quadrature axes voltages and currents. Additionally, it does not need an instant phase of point of common coupling (PCC) voltage, which is required in the conventional controller, and therefore makes it possible for the PLL independent control. Several case studies are carried out to verify that the proposed controller has better performance than the conventional controller. [ABSTRACT FROM PUBLISHER]

This paper introduces an adaptive voltage and frequency control method for inverter-based distributed generations (DGs) in a multi-microgrid (MMG) structure using distributed cooperative control and adaptive neural networks (ANN). First, model-based controllers are designed using the Lyapunov theory and dynamics of the inverter-based DGs. ANNs are then utilized to approximate these dynamics, resulting in an intelligent controller, which does not require a priori information about DG parameters. Also, the proposed controllers do not require the use of voltage and current proportional-integral controllers normally found in the literature. The effectiveness of the proposed controllers are verified through simulations under different scenarios on an MMG test system. Using Lyapunov analysis, it is proved that the tracking error and the neural network weights are uniformly ultimately bounded, which results in achieving superior dynamic voltage and frequency regulation. [ABSTRACT FROM AUTHOR]

Recently, transformerless (TL) inverters are being extensively used in small-scale photovoltaic (PV) systems due to their compact-size, lighter-weight, lower-cost and higher-efficiency compared to their counterpart with transformer. However, the growing penetration of these small-scale based PV systems and low-inertia induction motor (IM) loads in low-voltage distribution networks (DNs) make the grid more vulnerable to short-term voltage stability (STVS). Hence, this paper thoroughly examines the STVS of DN with high-penetration of TL-PV units, and provides countermeasures by TL-PV systems to mitigate any short-term voltage instability. The detailed dynamic model of the TL inverter is developed first. Next, three control strategies: (1) constant peak current (CPC), (2) constant active current (CAC), and (3) constant active power (CAP) with low voltage ride through (LVRT) and dynamic voltage support (DVS) capabilities are proposed to improve STVS. The impacts of different level of PV penetrations and LVRT capability of TL-PV inverters on the STVS are explored. Moreover, countermeasures, such as LVRT with CAC and CAP controls and DVS by the TL-PV systems are designed and implemented. Several case studies are carried out on an IEEE 4 bus system first, and later extended to IEEE 13 node test feeder. The results show that DVS can further improve STVS in residential DNs. In addition, CAP and CAC control strategies can speed-up the postdisturbance voltage recovery compared with the conventional CPC control. [ABSTRACT FROM AUTHOR]

Collective control of distributed energy resources (DER)—such as photovoltaic (PV) inverters or battery storage—have the potential to provide regulation services to the bulk electric grid. While optimal power flow techniques may be used to coordinate DER for this purpose, these approaches typically rely on accurate network models and a large number of system measurements. In this paper, we consider an approach that alleviates these modeling and measurement requirements. Here, we consider a two-dimensional adaptive control scheme known as extremum seeking, or ES, to perform optimization without knowledge of a model of the distribution network. We apply this scheme to enable simultaneous feeder head active power and voltage magnitude reference tracking, as well as feeder voltage regulation. From the perspective of the transmission grid, this approach essentially transforms the distribution feeder into a controllable (P,V) bus. Simulation results confirm the ability of the approach to track substation real power and voltage reference signals while maintaining distribution system voltages within acceptable tolerances. [ABSTRACT FROM PUBLISHER]

Proper modeling of inverter-based microgrids is crucial for accurate assessment of stability boundaries. It has been recently realized that the stability conditions for such microgrids are significantly different from those known for large-scale power systems. In particular, the network dynamics, despite its fast nature, appears to have major influence on stability of slower modes. While detailed models are available, they are both computationally expensive and not transparent enough to provide an insight into the instability mechanisms and factors. In this paper, a computationally efficient and accurate reduced-order model is proposed for modeling inverter-based microgrids. The developed model has a structure similar to quasi-stationary model and at the same time properly accounts for the effects of network dynamics. The main factors affecting microgrid stability are analyzed using the developed reduced-order model and shown to be unique for microgrids, having no direct analogy in large-scale power systems. Particularly, it has been discovered that the stability limits for the conventional droop-based system are determined by the ratio of inverter rating to network capacity, leading to a smaller stability region for microgrids with shorter lines. Finally, the results are verified with different models based on both frequency and time domain analyses. [ABSTRACT FROM PUBLISHER]

For a droop-controlled micro-grid, it is essential to design the droop coefficients for the inverter controllers in a rational way since they have great influences on the operation characteristics of the system. In this paper, a novel approach based on matrix perturbation theory is proposed for the coordinated optimization of the droop coefficients in inverter controllers. Rigorous perturbation analysis is applied to the droop coefficients to identify their influence on the system state matrix. Furthermore, the increments of the eigensolutions are obtained based on the matrix perturbation theory and are then utilized in the iterative parameter-optimization process. An eigenvalue-based objective function is proposed, aimed at ensuring the stability of the system, enhancing the damping characteristics, and maintaining the stability margin for a wide range of operating conditions. Finally, the effectiveness and robustness of the proposed approach are tested via a low-voltage micro-grid prototype. [ABSTRACT FROM PUBLISHER]

The paper presents an integrated microgrid laboratory system with a flexible and reliable multimicrogrid structure; it contains multiple distributed generation systems and energy storage systems and integrates with a diesel generator that serves as a back-up power source and flywheel energy storage for fast balancing to provide uninterruptible power-supply services in cooperation with the diesel generator. The microgrid system, by adopting the master–slave control strategy, can be transited flexibly between grid-connected and islanded modes and can be disconnected from the utility when a fault occurs or the power quality falls below specified standards. The developed bi-directional inverter which is applied in the system plays an important role. The small microgrids of this system are intended to operate separately or in the form of one large microgrid with a certain switch status. Furthermore, experiments on control, protection, and other technologies have been carried out. The results show that the operation conditions meet the related IEEE Standard 1547 and power quality requirements. The integrated microgrid laboratory system is able to operate stably and reliably under different conditions, including mode transition and fault events. [ABSTRACT FROM PUBLISHER]

This paper details a procedure based on bifurcation theory to evaluate the impact that droops and primary reserve scheduling have on the microgrid stability. The methodology is based on finding the worst primary reserve share-that is, the share closest to instability-that can be found after rescheduling the droops of selected generating units that support frequency (and voltage) regulation. The solution-which consists of a measure of the distance to instability in a given direction-is found in a multi-parameter space endowed with coordinates corresponding to the droop coefficients. Two stages are proposed to achieve the solution. First, an investigation of the distance to bifurcation is computed in a one-dimensional parameter space in a defined search direction. Then the direction of this search is updated by calculating the normal vector at the found bifurcation point. The procedure is iteratively repeated until the closest bifurcation is found. The proposed approach is analyzed in a 69-bus and 11-generation unit isolated microgrid. It is shown through the analysis of some scenarios how the distances and normal vectors provide valuable insight on the correct scheduling from the stability point of view, giving advice on how the primary reserve should be more reliably scheduled. [ABSTRACT FROM AUTHOR]

Commutation failure, which is a very frequent dynamic event in HVDC inverters, can deteriorate the availability of HVDC links and thus affect the performance of the power system. Most commutation failures are caused by voltage reduction due to ac system faults. In this paper, efforts are made to lower the effect of commutation failure on the power system, and the remedy is implemented in a fuzzy controller. In order to achieve control flexibility, the output of the fuzzy controller is made appropriate by setting a self-adjusting proportional factor. The validity and effectiveness of the fuzzy controller is verified by simulation which shows that the fuzzy controller can decrease commutation failure frequency induced by ac system faults. [ABSTRACT FROM AUTHOR]

Distribution grids currently lack comprehensive real-time metering. Nevertheless, grid operators require precise knowledge of loads and renewable generation to accomplish any feeder optimization task. At the same time, new grid technologies, such as solar photovoltaics and energy storage units, are interfaced via inverters with advanced sensing and actuation capabilities. In this context, this two-part work puts forth the idea of engaging power electronics to probe an electric grid and record its voltage response at actuated and metered buses, to infer nonmetered loads. Probing can be accomplished by commanding inverters to momentarily perturb their power injections. Multiple probing actions can be induced within a few tens of seconds. In Part I, load inference via grid probing is formulated as an implicit nonlinear system identification task, which is shown to be topologically observable under certain conditions. The conditions can be readily checked upon solving a max-flow problem on a bipartite graph derived from the feeder topology and the placement of probed and nonmetered buses. The analysis holds for single- and multiphase grids, radial or meshed, and applies to phasor or magnitude-only voltage data. Using probing to learn nonconstant-power loads is also analyzed as a special case. [ABSTRACT FROM AUTHOR]

This paper presents a new method for analysis of frequency synchronization of lossless power networks whose sources are frequency-droop controlled inverters. Unlike most existing approaches, our method focuses on the interaction between inverters and the network to provide new physical insight into how those interactions create frequency synchronization. We introduce two structure-preserving models of such a network (one bus-oriented and one line-oriented), and show that frequency synchronization corresponds to convergence to an equilibrium of these models. We derive a necessary condition for existence of such equilibria, and determine a simple test for their local stability. Finally, we introduce a sufficient condition for frequency synchronization of such networks, and show that it consists of a set of local criteria which can each be determined from local measurements. [ABSTRACT FROM PUBLISHER]

A major technical obstacle for rooftop photovoltaics (PV) integration into existing distribution systems is the voltage rise due to the reverse power flow from the distributed PV sources. This paper describes the implementation of a voltage control loop within PV inverters that maintains the voltage within acceptable bounds by absorbing or supplying reactive power. In principle, this can be considered to be a form of distributed Volt/VAr control, which is conventionally performed by coordinated control of capacitor banks and transformer tap changers. Comprehensive simulation studies on detailed feeder models are used to demonstrate that the proposed control scheme will mitigate voltage rises. [ABSTRACT FROM PUBLISHER]

This paper describes a power management method in a feeder with distributed generations (DGs) and energy storage devices. To ensure stability, reliability, and safety of the system, the proposed power management method deals with the DGs in two groups, which are utility and independent power producer (IPP). When the feeder is islanded from grid, the utility-owned DG receives fault signal from the protection relay. Thereafter, it is allowed to operate itself when the feeder is islanded because it is directly controlled by the utility. In contrast, the IPP-owned DG should be stopped to avoid several problems, which can be caused by the islanding operation. Then, the proposed power management method operates the above DGs to maximize the efficiency with no serious impact to grid. To do so, the inverter controller of DG is designed to implement it in a part of practical distribution system, which is located at the Do-gok area in Seoul, Korea. Several case studies are carried out to verify the effectiveness of proposed method. [ABSTRACT FROM PUBLISHER]

In this paper, an algorithm is presented, suitable for simulating the dynamic behavior of LV Microgrids both under grid connected and autonomous operation. The algorithm follows the stability approach, focusing on low-frequency dynamics, and adjusts the standard methodology so that the dynamic analysis of the system can be carried out, even in the absence of a synchronous machine when all the sources are interfaced to the network with inverters. Proper network representation allows for the modeling of all the characteristic unbalances of the LV network. The capability of the algorithm to simulate the operating modes of a Microgrid is demonstrated by representative study cases. [ABSTRACT FROM AUTHOR]

We consider the decentralized control of radial distribution systems with controllable photovoltaic inverters and energy storage resources. For such systems, we investigate the problem of designing fully decentralized controllers that minimize the expected cost of balancing demand, while guaranteeing the satisfaction of individual resource and distribution system voltage constraints. Employing a linear approximation of the branch flow model, we formulate this problem as the design of a decentralized disturbance-feedback controller that minimizes the expected value of a convex quadratic cost function, subject to robust convex quadratic constraints on the system state and input. As such problems are, in general, computationally intractable, we derive a tractable inner approximation to this decentralized control problem, which enables the efficient computation of an affine control policy via the solution of a finite-dimensional conic program. As affine policies are, in general, suboptimal for the family of systems considered, we provide an efficient method to bound their suboptimality via the optimal solution of another finite-dimensional conic program. A case study of a 12 kV radial distribution system demonstrates that decentralized affine controllers can perform close to optimal. [ABSTRACT FROM PUBLISHER]

Power electronic converters for integrating renewable energy resources into power systems can be divided into grid-forming and grid-following inverters. They possess certain similarities, but several important differences, which means that the relationship between them is quite subtle and sometimes obscure. In this article, a new perspective based on duality is proposed to create new insights. It successfully unifies the grid interfacing and synchronization characteristics of the two inverter types in a symmetric, elegant, and technology-neutral form. Analysis shows that the grid-forming and grid-following inverters are duals of each other in several ways including a) synchronization controllers: frequency droop control and phase-locked loop (PLL); b) grid-interfacing characteristics: current-following voltage-forming and voltage-following current-forming; c) swing characteristics: current-angle swing and voltage-angle swing; d) inner-loop controllers: output impedance shaping and output admittance shaping; and e) grid strength compatibility: strong-grid instability and weak-grid instability. The swing equations are also derived in dual form, which reveal the dynamic interaction between the grid strength, the synchronization controllers, and the inner-loop controllers. Insights are generated into cases of poor stability in both small-signal and transient/large-signal. The theoretical analysis and simulation results are used to illustrate cases for single-inverter systems, two-inverter systems, and multi-inverter networks. [ABSTRACT FROM AUTHOR]

Describes an integrated control and protection scheme for the grid-connected forced-commutated inverter. Components of the control scheme; Concept applied to the system; Application of the switching capability of power semiconductors to the inverter.

In this letter, a systematic static voltage stability index for evaluating system loading status in distribution networks is deduced from complex power flow models. The proposed index facilitates impact analysis on the integration of inverter-based distributed generators under different control modes, which may affect the static voltage stability. The case study, including various scenarios, is performed to validate the accuracy of the theoretical results. [ABSTRACT FROM AUTHOR]

An ordinary-differential-equation (ODE) enabled reachable dynamics analysis approach is devised to provably enclose all possible dynamic trajectories of networked microgrids (NMs) under both uncertain renewable power injections and intermittent large disturbance events. As a formal verification tool for the NM dynamics, its new contributions are threefold: 1) An ODE-enabled NMs model is established with a thorough formulation of the hierarchical control of DERs as well as the network transients; 2) A hybrid automaton method is established to empower the reachability analysis of dynamic transitions in NMs caused by arbitrary large disturbances; 3) A zonotope bundle technique is introduced in the reachable set calculation to capture the fast transients and strong nonlinearity upon the occurrence of disturbances, which allows for the reliable formal verification with superior precision and convergence performance. Extensive case studies are performed to demonstrate the effectiveness of the new approach in formally verifying the dynamical performance of disturbed NMs equipped with hierarchical inverter control. [ABSTRACT FROM AUTHOR]

Due to delays in network reinforcement, many utilities adopt System Integrity Protection Scheme (SIPS) as the short-term last defense to prevent system instability after critical disturbances. One of the challenges in the design of a SIPS is to handle the effects of intermittent renewable generation. Penetration level, geographic distribution, forecast errors, inverters control modes, and synchronous generation displacement are among the sources of effects of renewables that affect the reliable operation of an SIPS. To adapt to variations in system conditions, a new proposition of Adaptable System Integrity Protection Scheme (ASIPS) utilizing offline, online, and edge computing resources is proposed in this article. It aims to maintain reliable operation and reduce control costs under the presence of output variations of renewables. Possible impacts of renewables to the SIPS operations are demonstrated. Based on continuous system state monitoring using Phasor Measurement Units, the proposed response-based ASIPS would trigger optimal controls determined online by using a linearized formulation, or due to time criticality, activate quasi-optimal controls periodically updated and stored in look-up tables using offline computing power. Simulation results demonstrate the effectiveness of the proposed ASIPS in preventing system instability by using reduced control efforts and costs according to real-time operation conditions. [ABSTRACT FROM AUTHOR]

In this article, a simple control scheme based on an adaptive $Q-V$ droop coefficient is proposed to improve the reactive power sharing amongst sources in an islanded microgrid. All the sources present in the network independently calculate and apply an adaptive $Q-V$ droop coefficient after every step change in load, thereby achieving system-wide improved reactive power sharing. The core computation of the proposed method is based on the transient energy delivered by the respective inverter after every load change. The proposed control does not require communication; neither depends on the knowledge of feeder impedances nor requires measurements from distant nodes. Also, it can work satisfactorily irrespective of the location of the load change. Further, the strategy is also immune to any load change happening during the compensation process. A significant improvement in reactive power sharing has been achieved by using the proposed scheme without compromising the voltage regulation, and without causing significant power transients. The effectiveness of the proposed scheme has been validated by performing detailed simulation studies. Its viability is confirmed by carrying out experimental studies on a laboratory prototype. [ABSTRACT FROM AUTHOR]

In this letter, a phase reformulation-based sliding framework is proposed for microgrid to adequately decouple control interacts cross the quadrature axes. This is implemented by formulating another virtual coordinate that rotates with the synchronism frequency, while the original coordinate rotates with the reference frequency. The deviation during transients between coordinates is thus to be eventually mitigated by the intersected controller. The case study in PSCAD/EMTDC shows that, with the sliding framework, frequency and voltage fluctuations can be dramatically reduced. [ABSTRACT FROM AUTHOR]

The responses of an inverter-based distributed generator (IBDG) to abnormal voltage and frequency are different from those of a conventional generator because of the inverter's operating modes. In particular, although the momentary cessation (MC) mode is essential to protecting the distribution system, the MC mode can affect the power system transient stability by temporarily ceasing significant amounts of IBDG generation. To mitigate these negative effects, this study analyzed the impacts of MC capability on the power system transient stability. First, the transient stability was analyzed in relation to MC capability using the single-machine equivalent (SIME) method. A critical MC operating point (CMCOP) was then proposed to assess the severity of a transient impact and to secure the transient stability under the contingency conditions. A case study based on the Korean power system validated the feasibility and effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This study presents a general decentralized control scheme for cascaded H-bridge inverter-based static compensators in medium-/high-voltage power network. The H-bridge modules are controlled in a decentralized manner without a central controller and each module makes decisions based on individual local controller. The local controller includes two parts: first, a Q-ω boost control is introduced to share reactive power equally; and second, a scalable P-V control is established for balancing dc-link voltage of inverters. The proposed scheme can achieve frequency synchronization autonomously, and thus has advantages of improved reliability, scalability, and decreased costs. Moreover, an improved decentralized control is also proposed to adapt the grid-side dynamics when contingencies will occur. The system stability conditions are derived, and the feasibility of the proposed method is verified by simulation results. [ABSTRACT FROM AUTHOR]