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Accurate load modeling is critical for credible power system stability analysis. Because of the increasing complexity in modern system load, establishing a more comprehensive load model and performing accurate parameter estimation are the two biggest challenges in composite load modeling. In this paper, an induction motor with variable frequency drive (IM-VFD) together with the traditional ZIP+IM model is utilized to describe the dynamic characteristics of the system load. We estimate the time-varying probabilistic distributions of all states and parameters to describe their stochastic characteristics, thereby representing the real-time varying features of an actual load. The Fokker-Planck operator is applied to linearize the updated load model in tensor format. We then use tensor decomposition to reduce the computational and storage costs for calculating load model parameters. A sensitivity indicator is used to analyze the sensitivities of the estimated parameter distributions for subsequent model reduction. Simulations demonstrate that the updated load model has better performance than the traditional ZIP+IM model. [ABSTRACT FROM AUTHOR]

The Virtual Power Plant (VPP) has been proposed to aggregate Distributed Generations (DGs) to act like a single power plant, thus, also has functions on the frequency and voltage support. The previous models of the VPP are static and focus on the energy trading and management. For the system transient response analysis, a dynamic VPP model must be needed. The paper proposes a reduced-order yet accurate aggregated model to represent VPP transients for the stability analysis of power systems. The goal is to provide a model that is adequate for system studies and can serve to the Transmission System Operator (TSO) to evaluate the impact of VPPs on the overall grid. The proposed model can accommodate the transient response of the most relevant controllers included in the distributed generators that compose the VPP. Using a comparison with a real-time detailed Electro-Magnetic Transients (EMT) models of the VPP confirms the validity of the proposed aggregated model. The case studies based on the IEEE 39-bus system verifies the accuracy of the proposed aggregated model on the system stability analysis. [ABSTRACT FROM AUTHOR]

This paper proposes an optimized mathematical model of the voltage-source inverter parallel-operation system (VSIPS) and proposes an improved droop control method based on this model, which can realize accurate power sharing in VSIPS. First, this paper analyzes VSIPS as a multi-input and multioutput system and proposes a precise definition of circulating current. The circulating-current model, the steady-state model, and the small-signal model are proposed subsequently based on the optimum design of wire impedance, which constitute the optimized mathematical model of VSIPS in s-domain. The circulating-current phasor model and the circulating-current power phasor model (CCPPM) are also proposed. Second, the mathematical model of traditional droop control is built and analyzed based on CCPPM, which elaborates the tradeoff of droop control between the steady-state voltage bias and the load-sharing accuracy, and proves that the $V- Q$ droop control cannot realize accurate reactive-power sharing. Third, an improved droop control method ( $\omega - P_{{\text{cir}}}$ and $V- Q_{{\text{cir}}}$ control) is proposed, which can realize the accurate active- and reactive-power sharing and eliminate the steady-state voltage bias simultaneously. Finally, simulation and experimental results are presented, which validate the proposed mathematical model of VSIPS, the analysis of droop control, and the performance of the proposed method. [ABSTRACT FROM AUTHOR]

Load modelling plays a key role in assessing peak load reduction and has attracted renewed interest recently due to increasing diversity and uncertainty in load characteristic resulted by emergence of new types of loads and more penetrations of distributed renewable generations onto electricity networks. This paper presents a methodology for the development of a static load model based load characteristic profile in medium voltage distribution networks from year-long field measurements. High-resolution load monitoring devices with the 1 s sampling rate installed at 60 primary substations in the U.K. distribution network are used to collect the data for the purpose of load modelling. Load models at these 60 substations comprising domestic, commercial/industrial, and mixed-type load demand are presented in the paper together with relevant model parameters. Field trials (which involve triggering transformer taps to initiate voltage changes) were performed throughout the year on 15 selected substations aimed at load profile validation. The developed load profiles take into account seasonal, weekly, and daily variations of P–V and Q–V characteristics. Representative 24 h (0.5 h) load matrices are developed for all 60 substations and provide an insight into the operational flexibility and network resilience to voltage variations. [ABSTRACT FROM AUTHOR]

This paper presents a distributed, robust, finite-time secondary control for both voltage and frequency restoration of an islanded microgrid with droop-controlled inverter-based distributed generators (DGs). The distributed cooperative secondary control is fully distributed (i.e., uses only the information of neighboring DGs that can communicate with one another through a sparse communication network). In contrast to existing distributed methods that require a detailed model of the system (such as line impedances, loads, other DG units parameters, and even the microgrid configuration, which are practically unknown), the proposed protocols are synthesized by considering the unmodeled dynamics, unknown disturbances, and uncertainties in their models. The other novel idea in this paper is that the consensus-based distributed controllers restore the islanded microgrid's voltage magnitudes and frequency to their reference values for all DGs within finite time, irrespective of parametric uncertainties, unmodeled dynamics, and disturbances, while providing accurate real-power sharing. Moreover, the proposed method considers the coupling between the frequency and voltage of the islanded microgrid. Unlike conventional distributed controllers, the proposed approach quickly reaches consensus and exhibits a more accurate robust performance. Finally, we verify the proposed control strategy's performance using the MATLAB/SimPowerSystems toolbox. [ABSTRACT FROM PUBLISHER]

This paper proposes a battery simulator (BS) based on a Z-source rectifier (ZSR), with the intention of emulating the discharge or charge characteristic of an actual lithium-polymer battery with high voltage and large capacity. The proposed BS is used for power testing for battery applications. The battery model, combined with a Shepherd model and a Thevenin model, is adopted to freely change the properties and specifications of the battery and to replicate the dynamic behavior of the battery, which is discretized to utilize the digital controller of the BS. The closed-loop voltage controller at the dc side of the ZSR is designed to emulate the rapid dynamic characteristics of the battery based on small-signal methods, considering the influence of the components of the impedance network. In this paper, battery voltage control algorithm (BVCA) is also utilized to minimize the voltage stress across switches while controlling two dc-side voltages within a wide range of output voltages. Simulation and experimental results are provided to verify the BS of the new feature and the proposed control method. [ABSTRACT FROM AUTHOR]

An improved version of power flow-controller (PFC) which is based on multi-winding transformer (MWT) is presented in this paper. The objective of this article is to control the power flow in a power-line to alleviate overload in a heavily loaded system and increase power flow of an under loaded line. The novelty of multi-winding transformer based power-flow controller is the use of unequal voltage ratio in the tap-changer windings of MWT with bi-directional switches to provide a compensating voltage required to achieve the desired power flow. This new model drastically increases the number of compensating points to make it more compact with higher flexibility. It aims to utilize the full potential of a line (bound by its thermal limits) to contribute in power flow by providing several times higher resolution with limited number of switches and reduced amount of copper and iron volume compared to existing state-of-the-art technology. The proposed MWT model has been included in Newton Raphson Load Flow (NRLF) method for power flow analysis. The feasibility of the model has been realized using a standard 5 bus test system and a modified IEEE 57 bus system. Operation of the PFC has been demonstrated using a hardware laboratory prototype for generating nearest compensating points (voltage vectors) corresponding to the obtained compensating voltages after load flow solution. The results obtained are verified by simulating the controller in MATLAB/Simulink. [ABSTRACT FROM AUTHOR]

This paper presents primary-side voltage regulated multitransformer quasi-resonant flyback converter (MTFC) for supplying isolated power switch drivers. The proposed topology offers distinct advantages over frequently used flyback converter possessing one high frequency transformer with isolated multiple outputs. Particularly, when a large number of separate dc supply units is required, then MTFC enables improved regular distribution of magnetic coupling between the common primary and the multiple secondary transformers’ windings providing a high degree of galvanic and electromagnetic isolation between multiple outputs. Primary side voltage regulation is based on the average output voltage estimation using auxiliary RDC circuit mounted across the primary windings. Operation principles of MTFC are enhanced with analytical study of cross regulation of multiple output voltages at unbalanced load conditions, indicating reduced voltage deviation of multiple outputs by applying the primary-side average voltage regulation. Experimental results of prototype 2-, 3-, and 6-transformer quasi-resonant flyback converters confirmed their cross regulation quality and application potential for independent multiple output supplies. [ABSTRACT FROM AUTHOR]

Constant power loads (CPLs) impose instability issues in DC microgrids due to their negative impedance characteristics. This paper studies the problem of voltage control design of DC microgrids with CPLs. It is assumed that the power of CPLs is uncertain and belongs to a given interval leading to an infinite number of equilibrium points of the system. We develop a polytope model for DC microgrids with uncertain CPLs. Using this model, a robust two-degree-of-freedom feedback-feedforward voltage control framework is then proposed. The voltage controller is obtained by a solution of a set of linear matrix inequalities. The voltage control design strategy for each distributed generation (DG) unit is scalable and independent of the other DGs. The effectiveness of the proposed control approach is evaluated through simulation studies in MATLAB/SimPowerSystems toolbox. [ABSTRACT FROM AUTHOR]

This paper presents a modeling approach for capturing closed-loop load behavior in distribution networks using bottom-up time-variant load models and incorporating load-flow feedback for thermal loads. Traditional load models are useful for providing insights into instantaneous changes in power, however, they do not capture the time-variant response to changes in voltage due to the effects of appliances with closed-control loops such as those which are thermostatically controlled. This paper proposes a modeling and co-simulation platform to simulate closed-loop load-modeling using state-of-the-art bottom-up load modeling techniques for the residential sector with a feedback model for time series steady-state load-flow. It incorporates time-variant load models and discrete state-space representation of thermostatically controlled appliances. The developed methodology is used in a conservation voltage reduction case study on an urban distribution network, allowing detailed assessment of the voltage response and analysis of changes in active and reactive power and network losses. [ABSTRACT FROM AUTHOR]

This paper proposes the design of optimal and robust coordinated controller of hybrid energy storage system in a naval dc microgrid (MG) application. It is able to mitigate the negative effects of the pulsed power loads and meet the practical limitations of both converters input control and state variables signals based on IEEE standards. To do this, first, the dynamic model of the dc MG, which can represent in either all-electric aircraft or shipboard power systems, is developed. Second, a novel model predictive controller (MPC) for energy storage converters is proposed such that all of the mentioned hard constraints are guaranteed. Third, a linear matrix inequality approach is used to solve the MPC conditions. Finally, to evaluate the applicability and effectiveness of the proposed approach, some experimental tests are extracted. Obtained results verify better performance of the proposed approach over other state-of-the-art control techniques. [ABSTRACT FROM AUTHOR]

Conservation voltage reduction (CVR) uses Volt-VAR optimization (VVO) methods to reduce customer power demand by controlling feeder's voltage control devices. The objective of this paper is to present a VVO approach that controls system's legacy voltage control devices and coordinates their operation with smart inverter control. An optimal power flow (OPF) formulation is proposed by developing linear and nonlinear power flow approximations for a three-phase unbalanced electric power distribution system. A bi-level VVO approach is proposed, where Level 1 optimizes the control of legacy devices and smart inverters using a linear approximate three-phase power flow. In Level 2, the control parameters for smart inverters are adjusted to obtain an optimal and feasible solution by solving the approximate nonlinear OPF model. Level 1 is modeled as a mixed integer linear program (MILP) while Level 2 as a nonlinear program with linear objective and quadratic constraints. The proposed approach is validated using 13-bus and 123-bus three-phase IEEE test feeders and a 329-bus three-phase PNNL taxonomy feeder. The results demonstrate the applicability of the framework in achieving the CVR objective. It is demonstrated that the proposed coordinated control approach help us to reduce the feeder's power demand by reducing the bus voltages; the proposed approach maintains an average feeder voltage of 0.96 p.u. A higher energy saving is reported during the minimum load conditions. The results and approximation steps are thoroughly validated using OpenDSS. [ABSTRACT FROM AUTHOR]

The ever-increasing utilization of sensitive loads in the industrial, commercial, and residential areas in distribution networks requires enhanced reliability and quality of supply. This can be achieved, thanks to self-healing features of smart grids that already include the control technologies necessary for the restoration strategy in case of a fault. In this paper, an analytical and global optimization model is proposed for the restoration problem. A novel mathematical formulation is presented for the reconfiguration problem reducing the number of required binary variables while covering more practical scenarios compared to the existing models. The considered self-healing actions besides the network reconfiguration are the nodal load-rejection, the tap setting modification of voltage regulation devices (incl. OLTCs, SVR, and CBs), and the active/reactive power dispatch of DGs. The voltage dependency of loads is also considered. Thus, the proposed optimization problem determines the most efficient restoration plan minimizing the number of de-energized nodes with the minimum number of self-healing actions. The problem is formulated as a Mixed-Integer Second Order Cone Programming (MISOCP) and solved using the Gurobi solver via the MATLAB interface YALMIP. A real 83–node distribution network is used to test and verify the presented methodology. [ABSTRACT FROM AUTHOR]

This paper proposes a decentralized control based on static var compensator (SVC) devices to improve the frequency control of high voltage transmission systems. For a realistic structure preserving power system with voltage dependent loads, we propose a sufficient condition based on a strict Lyapunov energy function, which is utilized to design a nonlinear adaptive SVC controller. This controller only requires local bus measures and parameters. Furthermore, we also present an SVC with a conventional frequency droop controller. The performance of the two considered SVC control systems are evaluated on the well known IEEE New England 39-bus 10-machine power system and compared with conventional SVC controllers. [ABSTRACT FROM AUTHOR]

In this paper, a decentralized switched current control is presented for dc microgrids. The dc microgrid is modeled considering dc–dc converters as controlled current sources instead of traditional voltage sources in droop control schemes. The proposed current control consists of switching between two current reference signals while keeping the voltage level at the loads and with proper current distribution. It is shown that the closed-loop system is stable through passivity analysis. Experimental tests are performed to illustrate the effectiveness of the proposed control strategy using a laboratory microgrid. [ABSTRACT FROM AUTHOR]

This paper addresses the distributed optimal frequency control of power systems considering a network-preserving model with nonlinear power flows and excitation voltage dynamics. Salient features of the proposed distributed control strategy are fourfold, first, nonlinearity is considered to cope with large disturbances, second, only a part of generators are controllable, third, no load measurement is required, fourth, communication connectivity is required only for the controllable generators. To this end, benefiting from the concept of “virtual load demand,” we first design the distributed controller for the controllable generators by leveraging the primal-dual decomposition technique. We then propose a method to estimate the virtual load demand of each controllable generator based on local frequencies. We derive incremental passivity conditions for the uncontrollable generators. Finally, we prove that the closed-loop system is asymptotically stable and its equilibrium attains the optimal solution to the associated economic dispatch problem. Simulations, including small and large-disturbance scenarios, are carried on the New England system, demonstrating the effectiveness of our design. [ABSTRACT FROM AUTHOR]

The five-level nested neutral-point-clamped (5L-NNPC) converter is one of the most promising topologies for medium-voltage (2.3–7.2 kV) high-power applications such as medium-voltage drives, wind energy conversion systems, and grid-connected systems. The 5L-NNPC requires a fewer number of switching devices, freewheeling and clamping diodes, and flying capacitors compared to the existing five-level multilevel converters. In the 5L-NNPC topology, each flying capacitor voltage is regulated at one-fourth of the dc-bus voltage to obtain the five-level operation. Due to the lack of redundant switching states, it is difficult to control the flying capacitor voltages by using the pulse-width-modulation-based classical control methods. This paper proposes a model-predictive current control (MPCC) approach to control the flying capacitor voltages along with the output currents of the 5L-NNPC converter. The discrete-time model of 5L-NNPC is developed to implement the MPCC scheme. The simulation and experimental studies are conducted to verify the dynamic and steady-state performance of 5L-NNPC with the MPCC scheme. The performance of the proposed MPCC approach is compared with the conventional space-vector-modulation-based voltage-balancing approach. Furthermore, the flying capacitor voltage control capability of MPCC is verified at different load power factors. [ABSTRACT FROM AUTHOR]

The contribution of decentralized generators in the provision of ancillary services, such as voltage and frequency regulations, is currently growing in many power systems. The impact of these new regulations on islanding detection is generally investigated by calculating the nondetection zone (NDZ) of the protective relay located at generator level. This subject has already been studied in the literature, but has been generally treated by means of steady-state calculations only. As demonstrated in the present paper, the analysis yields very different results when the entire time-evolution of the system is taken into account. It is shown that the impact of the studied regulations, i.e., the P(f) and Q(U) controls, on a standard test system is to reduce the NDZ which goes against the conclusion of several previous works based on steady-state calculations. Then the generalization of this result to other loads and generators is investigated. At last, the impact of the dynamics of the grid and the dynamics of the regulations is further investigated through a parametric study which shows that, if the power regulations dynamics are slow enough compared to the voltage and frequency dynamics, then the NDZ is necessarily reduced by the addition of the new regulations. [ABSTRACT FROM AUTHOR]

The control system for stand-alone doubly fed induction generator (DFIG) is commonly based on the “sensorless” direct voltage control. This paper adopts the direct voltage control and employs negative-sequence compensation through rotor-side converter to support asymmetric “unbalanced” loads. This paper demonstrates the limitation of the conventional direct voltage control to obtain the slip angle, and the consequent limitation of the negative-sequence compensation control to support full range of asymmetric loads. To overcome these limitations, this paper proposes a new estimator of the angle of the rotor current in the synchronous reference frame. The proposed estimator requires one DFIG parameter only which is the stator inductance, which can be measured allowing using real machine parameter. This paper proposes a simple method to integrate the proposed estimator into the direct voltage control, which enables it to obtain the slip angle, and it enables the negative-sequence compensation to support full range of asymmetric loads. Experimental results for a 1.1-kW prototype DFIG are provided to validate the proposed method and to demonstrate its advantages over the conventional direct voltage control. [ABSTRACT FROM AUTHOR]

This paper presents the effectiveness of a microgrid (MG) in a distribution system. The combined heat and power (CHP) system is used to enhance the overall efficiency of the power system. Microturbines are used for CHP operation in this paper. The various methods of control of the MG are discussed. A simple low-cost model is proposed for MG based on frequency droop control for active power generation. The proposed model can be used in plug-and-play mode of operation of the MG. The model of MG is developed, and its performance is simulated in MATLAB platform. Performance of the model is studied under grid-connected and islanded conditions. [ABSTRACT FROM AUTHOR]

Detailed in this paper is a multiport power electronics interface which serves as an energy router for on-board electric and plug-in hybrid electric vehicles with inductively coupled power transfer (ICPT) and hybrid energy storage systems (HESS). The existing body of literature on HESSs lacks a unified controller and modular, flexible structure as well as integration of ICPT. In battery/ultracapacitor systems, this leads to piece-meal control of sources resulting in battery currents which are not fully decoupled from high-frequency/high-magnitude current and ultra-capacitor (UC) state of charge (SoC) not being properly controlled. A central controller is proposed in this paper which completely decouples the battery from both high-frequency and high-magnitude current, controls the SoC of the UCs, and models the SoC of the UCs in the stability analysis of the system. This system is particularly useful for online charging of HEVs in highway-type applications where ICPT pulse charging will be present. Solving the challenges of pulse charging will bring ICPT technology one step closer to widespread integration which has the potential to greatly reduce societies’ dependence on fossil fuel. Simulation and experimental results verify the feasibility of the proposed techniques. [ABSTRACT FROM AUTHOR]

This paper presents a robust input–output linearization controller as maximum power point tracking (MPPT) technique in a photovoltaic (PV) buck dc–dc converter with applications to dc microgrids, solar vehicles, or stand-alone systems. Due to the control structure proposed in this paper, the MPPT control system is able to track very fast irradiance changes. Meanwhile, the internal stability of the overall closed-loop system is guaranteed for different load scenarios. A sector condition is only required for the load current, which is satisfied for most of the current PV applications. In turn, this condition implies the robustness against oscillations in the dc bus voltage. Finally, the MPPT control system is validated through experimental results, where the closed-loop performance is evaluated under abrupt irradiance and set-point changes, parametric uncertainty, and dc bus load variations. [ABSTRACT FROM PUBLISHER]

This paper presents a converter-based permanent magnet synchronous generator (PMSG) emulator as a testbed for designing, analyzing, and testing of the generator's power electronic interface and its control system. The PMSG model is formulated in a real-time digital simulator. A voltage type ideal transformer model combined with a virtual impedance is presented as an interface algorithm. The design procedure and implementation of the virtual impedance in a simulation platform is discussed. A six-switch voltage source converter is used as a power amplifier to mimic the behavior of the PMSG supplying linear and nonlinear loads. A proportional–integral plus resonant controller is proposed as a voltage loop controller for tracking a distorted output voltage reference signal. The accuracy of the proposed emulator is investigated for the fundamental and low-order voltage harmonics. Technical challenges of the PMSG emulator are considered and proper solutions are suggested. The performance of the proposed emulator is compared with an actual PMSG. [ABSTRACT FROM AUTHOR]

Quasi-static time-series (QSTS) simulation is used to simulate the behavior of distribution systems over long periods of time (typically hours to years). The technique involves repeatedly solving the load-flow problem for a distribution system model and is useful for distributed energy resource planning. When a QSTS simulation has a small time step and a long duration, the computational burden of the simulation can be a barrier to integration into utility workflows. One way to relieve the computational burden is to simplify the system model. The segment substitution method of simplifying distribution system models introduced in this paper offers model bus reduction of up to 98% with a simplification error as low as 0.2% (0.002 p.u. voltage). In contrast to existing methods of distribution system model simplification, which rely on topological inspection and linearization, the segment substitution method uses black-box segment data and an assumed simplified topology. [ABSTRACT FROM PUBLISHER]

This paper proposes a simplified small-signal model for output voltage control of a single-phase asymmetrical cascaded H-bridge multilevel inverter (ACHMI). The ACHMI is an n-series connected H-bridge converter, each one with a unique value at the dc link and usually scaled at 1:2:6:… or 1:3:9:…. By assuming that the small-signal variation component is equal in all n converter terminal ports, a simplified small-signal model is obtained. This assumption is carefully described and justified. To verify the veracity of the proposed model, two distinct control strategies are applied. One is a single-loop control scheme based on a modified proportional-integral (PI) controller. The other one is a double-loop control scheme based on a PI controller with feedforward action of the load current. Both controllers are tuned based on the dynamic behavior of the proposed model. Since the designed controllers based on the simplified model make the ACHMI output voltage to follow the reference without steady-state error, the proposed simplified model truly represents the inverter. Experimental results show the efficacy of the simplified model of the ACHMI through the two mentioned control strategies as well as the ACHMI installed in a microgrid. [ABSTRACT FROM PUBLISHER]

With the increasing importance of battery energy storage systems (BESS) in microgrids, accurate modeling plays a key role in understanding their behavior. This paper investigates and compares the performance of BESS models with different depths of detail. Specifically, several models are examined: an average model represented by voltage sources; an ideal dc source behind a voltage source converter; a back-to-back buck/boost and bidirectional three-phase converter, with all models sharing the same control system and parameters; and two additional proposed models where the switches are replaced by dependent sources to help analyze the differences observed in the performance of the models. All these models are developed in PSCAD and their performances are simulated and compared considering various issues such as voltage and frequency stability and total harmonic distortion in a benchmark test microgrid. It is shown through simulation results and eigenvalue studies that the proposed models can exhibit a different performance, especially when the system is heavily loaded, highlighting the need for more accurate modeling under certain microgrid conditions. [ABSTRACT FROM AUTHOR]

The resilience and reliability of modern power systems are threatened by increasingly severe weather events and cyber-physical security events. An effective restoration methodology is desired to optimally integrate emerging smart grid technologies and pave the way for developing self-healing smart grids. In this paper, a sequential service restoration (SSR) framework is proposed to generate restoration solutions for distribution systems and microgrids in the event of large-scale power outages. The restoration solution contains a sequence of control actions that properly coordinate switches, distributed generators, and switchable loads to form multiple isolated microgrids. The SSR can be applied for three-phase unbalanced distribution systems and microgrids and can adapt to various operation conditions. Mathematical models are introduced for three-phase unbalanced power flow, voltage regulators, transformers, and loads. The SSR problem is formulated as a mixed-integer linear programming model, and its effectiveness is evaluated via the modified IEEE 123 node test feeder. [ABSTRACT FROM PUBLISHER]

Conservation voltage reduction (CVR) plays an important role in energy savings and shaving peak demand. This paper quantifies energy savings for refrigeration loads (RLs) with experimentally validated air conditioner and refrigerator models. It is demonstrated that peak demand reduction and energy savings exist simultaneously during the implementation of CVR. A 1666-bus real meshed network with residential, small, and large commercial customers is simulated for the calculation of energy savings. Different working patterns: weekday and weekend; weather conditions: hottest and average summer days during peak load of the year; and load types: mixed load (ZIP and RLs) and ZIP load are studied. In addition, the IEEE 8500-node radial system with 100% residential customers is also used to show the energy savings of CVR. It is concluded that the economic benefits of CVR are larger for meshed networks than for radial systems. It is also shown experimentally and by simulation that CVR produces significant savings for both utilities and customers. [ABSTRACT FROM PUBLISHER]

In emerging distribution systems with a proliferation of distributed energy resources (DERs) and flexible demand assets, operation characters of the unbalance network and voltage regulation devices need to be accurately addressed for ensuring the secure and economic operation. This paper focuses on the modeling and solution approach of AC optimal power flow (ACOPF) for unbalanced distribution systems with DERs and voltage regulation transformers (VRT). The ACOPF problem is formulated as a chordal relaxation based semidefinite programming (SDP) model, and a tighter convexification model of VRTs is proposed for mitigating solution inexactness. Analytical conditions are presented and proved to determine whether global optimal solution to the original ACOPF problem can be retrieved from solutions of the chordal relaxation based SDP model. Numerical studies on modified IEEE 34-bus and 8500-node systems show that the proposed approach presents a better computational performance as compared to rank relaxation based SDP approaches and general nonlinear solvers. [ABSTRACT FROM PUBLISHER]

In this paper, we propose a novel coordinated control framework to handle negative voltage impacts caused by wind power in a weak subtransmission system. The designed controller addresses the problem of optimal coordination of controls with different response time and control characteristics. In our control design, the traditional voltage regulators (TVRs) such as on-load tap changers and capacitors are primarily used to regulate voltages in the network. If the control actions of TVRs cannot meet the control requirements, an event-triggered load-side aggregate controller will be activated immediately to help regulate voltages. The control schedules of TVRs are calculated through a multitime-step optimization-based model-predictive control method and are updated with the help of wind power predictions. The load-side controller is realized by controlling active power consumptions of load aggregators through an optimal distributed algorithm that relies on local information exchanges between neighboring load aggregators. The effectiveness of the proposed control scheme is tested through a case study on a modified IEEE 30-bus test system with high penetration of wind power. [ABSTRACT FROM PUBLISHER]

This paper proposes a multi-input muti-output control strategy for dispatchable power electronic based distributed energy resources (DERs) in islanded mode operation of microgrids. The controller is designed based on the $dq$ model of the DER unit. Cascade, feedforward, and internal model control principle, incorporating the theory of integral control and repetitive controller, are used. The proposed controller regulates the voltage at the output of DERs under any load (balanced, unbalanced, and harmonic) variation and/or fault condition. The stability of the closed-loop system has been investigated. Enhanced phase-locked loop (EPLL) is used to determine the system variables in the $dq$ frame under unbalanced (and balanced) operating conditions. The structure of the EPLL is further modified to regulate the microgrid frequency and a stability analysis of proposed frequency control loop is presented. The performance of the proposed controller is verified through test case studies, carried out on an autonomous microgrid with single and multiple DER units. [ABSTRACT FROM PUBLISHER]

LLC resonant converter is a nonlinear system, limiting the use of typical linear control methods. This paper proposed a new nonlinear control strategy, using load feedback linearization for an LLC resonant converter. Compared with the conventional PI controllers, the proposed feedback linearized control strategy can achieve better performance with elimination of the nonlinear characteristics. The LLC resonant converter's dynamic model is built based on fundamental harmonic approximation using extended describing function. By assuming the dynamics of resonant network is much faster than the output voltage and controller, the LLC resonant converter's model is simplified from seven-order state equations to two-order ones. Then, the feedback linearized control strategy is presented. A double loop PI controller is designed to regulate the modulation voltage. The switching frequency can be calculated as a function of the load, input voltage, and modulation voltage. Finally, a 200 W laboratory prototype is built to verify the proposed control scheme. The settling time of the LLC resonant converter is reduced from 38.8 to 20.4 ms under the positive load step using the proposed controller. Experimental results prove the superiority of the proposed feedback linearized controller over the conventional PI controller. [ABSTRACT FROM PUBLISHER]

The aim of this paper is to derive a general AC/DC power flow model with Voltage Source Converter Multi-Terminal High-Voltage Direct Current Systems (VSC MTDCs). The equations of AC, DC grids, and VSCs are formulated in augmented rectangular coordinates. The proposed model is composed of two nodal equations and two power constraints for each AC bus, as well as one nodal equation and one power constraint for each DC bus. In this model, the VSC equations are included in the AC and DC grid model—its power balance equation and one of the control equations are regarded as the power constraints of the AC bus connected to it, whereas the other control equation is regarded as the power constraint of the DC bus connected to it. Therefore, the number of equations of the proposed model is determined only by that of AC and DC buses, and the model is systematically well organized; the variety of VSC control strategies and AC/DC linking configurations does not influence the overall structure. The proposed approach enables to solve the load flow of the most general AC/MTDC, consisting of multiple AC and DC grids connected by VSCs, and is suitable for control strategies including the droop control and any new ones in the future. The model also leads to higher computational efficiency. By demonstrations on AC/DC power systems with several VSCs, this method is proved to be effective, flexible, and efficient. [ABSTRACT FROM AUTHOR]

This paper proposes a frequency-modulation control scheme for a dc/dc current-source parallel-resonant converter with two possible configurations. The basic configuration comprises an external voltage loop, an internal current loop, and a frequency modulator: the voltage loop is responsible for regulating the output voltage, the current loop makes the system controllable and limits the input current, and the modulator provides robustness against variations in resonant component values. The enhanced configuration introduces the output inductor current as a feed-forward term and clearly improves the transient response to fast load changes. The theoretical design of these control schemes is performed systematically by first deriving their small-signal models and second using Bode diagram analysis. The actual performance of the proposed control schemes is experimentally validated by testing on a laboratory prototype. [ABSTRACT FROM PUBLISHER]

Exploring high-performance controller for buck converter is challenging since it can be easily affected by converter model accuracy. In this paper, a novel nonaveraged current discrete-time (NCD) model is proposed, in which inductor current is expressed as time-varying equations during switch-on and switch-off states. It achieves higher accuracy than the conventional averaged model at high-frequency range, thus can be used to optimize high-speed controller design. Based on the NCD model, a multiloop minimum switching cycle (MMSC) control strategy, composed of output feedback (OF), line feed forward (LFF), and reference feed forward (RFF) loops, is proposed and tuned for buck converter operating in continuous conduction mode. Mutual influences among three loops are considered and eliminated by specifically designed LFF and RFF compensations, which adapt the OF compensation. With consideration of sampling and calculation delays, relationship between transient switching cycles and geometric center of controller poles is discovered from a calculated output voltage error series. Furthermore, theoretical minimum switching cycles are calculated by moving the center inside the unit cycle of complex plane, which ensures system stability. Moreover, load/line transient response and reference tracking time are simultaneously optimized to the minimum switching cycles. Effectiveness of the controller is proved by converter closed-loop pole/zero plots, transient response simulations, and experiments. [ABSTRACT FROM AUTHOR]

Voltage regulation by means of coordinated voltage control is one of the challenging aspects in an active distribution system operation. Integration of distributed generation (DG), which can also be operated in Volt/VAr control mode, may introduce adverse effects including control interactions, operational conflicts, steady-state voltage variations, and oscillations. Therefore, examining and analyzing the phenomenon (both steady state and dynamic) of the control interactions among multiple Volt/VAr support DG units and voltage regulating devices such as tap changers and capacitor banks would be essential for effective voltage control in active distribution systems. In this paper, the interactions among DG units and voltage regulating devices are identified using their simultaneous and nonsimultaneous responses through time-domain simulations. For this task, an analytical technique is proposed and small signal modeling studies have been conducted. The proposed methodology could be beneficial to distribution network planners and operators to ensure seamless network operation from voltage control perspective with increasing penetration of DG units. [ABSTRACT FROM PUBLISHER]

The well-known and widely applied Thévenin theorem is applied to the operational analysis of ac transmission circuits. The approach is to model the transmission circuit \mathcalN as an $n$-port. The calculation of the Thévenin equivalent impedance is found by applying an $n$-vector of currents to \mathcalN and observing the behavior of the terminal voltages of \mathcalN. The Thévenin equivalent voltage is an $n$-vector and this is simply the open-circuit voltage of \mathcalN . The equivalent circuit is applied to conveniently study the transmission circuit performance for a range of different operating conditions. The treatment of controlled sources in the model is described. The application of Thévenin's theorem to a class of multiport circuits has been described before, and the application to three-phase circuits has been suggested; however, the contribution in this paper is the treatment of transformers and modeling mutual coupling between phases in transmission components, and the application to general polyphase circuits. [ABSTRACT FROM AUTHOR]

In this paper, the performance of the dual stator-winding induction generator (DWIG) variable frequency ac (VFAC) generating system with unbalanced loads is analyzed, and an asymmetrical operation analysis method based on a three-port load network model is proposed. This method has four key steps: (1) deriving the three-port load network model for three-phase four-line unbalanced loads; (2) establishing the positive sequence, negative sequence, and zero sequence equivalent circuits for the DWIG; (3) combining the above model and circuits together to obtain the system whole equivalent model; and (4) writing the corresponding equations and solving them. The experimental results and the calculation results are almost consistent, which proves that the proposed method is correct and valid, and the DWIG VFAC generating system with unbalanced loads has a good performance. [ABSTRACT FROM PUBLISHER]

Power flow analysis is one of the tools that is required in most of the distribution system studies. An important characteristic of distribution systems is the load unbalance in the phases and a three-phase power flow analysis is needed. In this paper, a three-phase linear power flow (3LPF) formulation is derived based on the fact that in a typical distribution system, voltage angles and magnitudes vary within relatively narrow boundaries. The accuracy of the proposed 3LPF is verified using several test cases. Potential applications of the proposed method are in distribution systems state estimation and volt-VAR optimization. [ABSTRACT FROM PUBLISHER]

This paper proposes a unified power flow algorithm based on the Newton–Raphson (NR) method to obtain a feasible solution for hybrid AC/DC grids incorporating voltage source converters (VSCs). The power flow model of hybrid AC/DC grids is composed of an AC grid model, a VSC station model, and a DC grid model. Detailed converter losses are involved in VSC station modeling. In the DC grid model, a unified dc control expression is applied to adapt to different control methods. To obtain sparse Jacobian matrix, the mismatch equations and unknown variables are classified into three categories corresponding to AC, VSC, and DC models. Then the proposed algorithm achieved quadratic convergence with the NR method. Once the solution is infeasible (variable out-of-limit) or diverging, the constrained load flow (CLF) method is applied automatically to achieve a feasible solution by revising the references of ac/dc grids. Simulations were carried out for the hybrid AC/DC test system composed of two AC grids and two DC grids. The results under different DC voltage control modes show that the proposed algorithm is able to obtain a feasible solution with high accuracy and convergence speed. [ABSTRACT FROM PUBLISHER]

This paper proposes a quasi-steady-state modeling approach for an approximation of long-term frequency dynamics in power systems. A specific phenomenon of concern is an onset of frequency swings during load/generation imbalance scenarios. The effects of system voltage characteristics, system inertia, and, more importantly, damping controllers are explained and quantified using the described quasi-steady-state models. Application of the methodology to a 14-generator benchmark system demonstrates that described models are suitable for simulation of different disturbance scenarios that can trigger frequency instability. Moreover, linearization of the proposed models can provide convenient means for impact assessment and coordinated design of damping controllers. To demonstrate this, coordinated tuning of multiband power system stabilizers to improve frequency dynamics has been performed and validated through nonlinear simulations using a commercial transient stability software. [ABSTRACT FROM PUBLISHER]

This paper presents an universal active power filter for harmonic and reactive power compensation with uninterrupted power supplies (UPS) features. The configurations does not use transformer in the series part. Transformerless modern UPS systems have been rapidly replacing the old technology due to their performance and size attributes. Reducing the numbers of passive elements and/or switches in active power filters and UPS topologies not only reduces the cost of the whole system but also provides some advantages, such as great compactness, smaller weight, and higher reliability. However, the cost reduction requires the use of more complex control strategies. The model of the proposed system is derived and it is observed that the system can be reconfigurable to operate with four or three leg depending on the issue. A complete control system, including the pulsewidth modulation techniques, is developed and a comparison between the proposed filter and the standard one is done, as well. Simulated and experimental results validate the theoretical considerations. [ABSTRACT FROM AUTHOR]

In this paper, a new procedure based on a backward/forward sweep (BFS) algorithm for solving power flows in weakly meshed dc traction networks is presented. The proposed technique is able to consider the trains as nonlinear and nonsmooth (nondifferentiable) voltage-dependent loads or generators. This feature permits the inclusion of the trains’ overcurrent protection and the squeeze control. With the use of the mentioned controls, the conventional power flow problem becomes a voltage constrained power flow problem, and the interaction between the trains and the network can be accurately modeled. However, the train control induces a highly nonsmooth voltage-dependent load characteristic, causing convergence problems in most of the derivative-based algorithms. The proposed algorithm is faster, more robust, and more stable than the derivative-based ones. In addition, the authors present all of the formulation in a compact matrix-based form by means of the graph theory application and the node incidence matrix. [ABSTRACT FROM PUBLISHER]

Due to exponential (Moores law) scaling of advanced CMOS technologies, the challenges associated with delivering power to performance and mobile computing systems are outpacing the capabilities of conventional voltage regulator (VR) topologies. To continue to scale throughput at constant power density, the level of parallelism in microprocessor architectures is expected to increase substantially. In this paper, we present a power conversion topology to provide independent multicore regulation in the 0.8–1.4 V range from a 12-V dc bus. The topology uses a multistage ladder converter to manage power delivery to digital circuits stacked in vertical voltage domains. This approach has several advantages with regard to systems efficiency as it allows a more moderate conversion ratio of the main dc–dc converter. Moreover, the parallel converter only needs to process a fraction of the power of each core as the current can be “recycled” by adjacent cores in the stack. We develop a dynamical model for a multiple-input, multiple-output control scheme that uses a simple integral-control law, augmented with fast voltage- and current-mode feedforward. Measurement results of a discrete prototype verify the control scheme and demonstrate the potential advantages in system efficiency but also emphasize the remaining challenges in meeting stringent VR dynamic response requirements. [ABSTRACT FROM PUBLISHER]

The plug-in electric vehicle (PEV) is a new atypical load in power systems. In future, PEV load will play a significant role in the distribution grids. This integrated load into the power grid may overload the system components, increase power losses, and affect the voltage profile in the distribution systems. Currently, the constant power load model is most commonly used for the modeling of the electric vehicle (EV) load that considers the EV loads as constant power elements without considering the voltage dependence of the EV charging system. EV load demand cannot be considered as a constant power due to the fact that modeling as a constant power load will not provide accurate information about the behavior of the charging system during the charging process. In this paper, an accurate model representing the realistic behavior of EV load is developed which is based on the ZIP load model with the ZIP parameters established through the realistic EV load data. The proposed model can be used to analyze the true behavior of the EV charger integrated to an electricity grid and determine the impacts of EV charging load on the grid. A realistic charging system was used to test and capture the EV load behavior and extract the coefficients of the EV ZIP load model, which have been verified using computer simulations and laboratory experiments. Additionally, a comparative study between the proposed ZIP load model and the constant power load model was carried out, and the results were verified with the practical EV load data. The results confirm that EV represented using constant power load will not provide the true reflection of the EV load behavior and the EV impacts on the power grid. [ABSTRACT FROM AUTHOR]

The complexity and strong nonlinearity of the model of a self-excited induction generator hinder the systematic design of a voltage regulation system. Using a special reference frame aligned with the stator voltage vector, the paper succeeds in developing a control-oriented linearized model that relates small deviations of the capacitance, load admittance, and angular velocity, to corresponding deviations of the voltage amplitude. Transfer functions are also computed based on the linear model. A stability analysis predicts rapidly decaying oscillatory transients combined with a primary component with slower exponential decay. Simulated transient responses of the full and linearized models demonstrate the validity of the approximation and are in good agreement with experiments. [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]

A new discrete-time state feedback controller is presented, which allows high-bandwidth voltage control of a buck converter for any load condition, whether it operates in discontinuous conduction mode (DCM), continuous conduction mode (CCM), or at the boundary of these regions. This makes the buck converter applicable for a wide range of applications. For the control design process, two large-signal models, which represent the buck converter's discrete time dynamics in CCM and DCM, are developed. A simple proportional-integral regulator is used for the voltage control of the converter. The operation mode is detected and the voltage controller is connected in cascade to a current controller in CCM or to a nonlinear state feedback decoupling structure in DCM. In this paper, the modeling and design of the proposed control topology are introduced and its performance is demonstrated in simulation and experiment. [ABSTRACT FROM PUBLISHER]

Conservation voltage reduction (CVR) and distributed-generation (DG) integration are popular strategies implemented by utilities to improve energy efficiency. This paper investigates the interactions between CVR and DG placement to minimize load consumption in distribution networks, while keeping the lowest voltage level within the predefined range. The optimal placement of DG units is formulated as a stochastic optimization problem considering the uncertainty of DG outputs and load consumptions. A sample average approximation algorithm-based technique is developed to solve the formulated problem effectively. A multiple replications procedure is developed to test the stability of the solution and calculate the confidence interval of the gap between the candidate solution and optimal solution. The proposed method has been applied to the IEEE 37-bus distribution test system with different scenarios. The numerical results indicate that the implementations of CVR and DG, if combined, can achieve significant energy savings. [ABSTRACT FROM AUTHOR]

This paper presents a finite control set model predictive strategy and its application to the load voltage control of two-level four-leg inverters. The proposed approach uses the novel discrete-time model of the inverter and output $LC$ filter in order to predict the variables to be controlled. These predictions are carried out for the 16 switching states of the inverter and are evaluated using a cost function. The switching state that forces the load voltages to be closest to their respective references is chosen and applied to the inverter. The behavior of the predictive controller has been investigated, and the changes to both inductive and capacitive filter parameters have been considered. In order to improve the reliability of the fourth leg as well as the overall inverter efficiency, a solution is proposed, which combines hardware and software reconfigurations. The feasibility of the proposed method is verified through simulation and experimental results considering single-/three-phase, balanced/unbalanced, and linear/nonlinear loads. [ABSTRACT FROM PUBLISHER]