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The work reported in this paper aims at developing an algorithm for system operators to solve a robust security-constrained AC optimal power flow (R-SC-ACOPF) problem, which also optimizes the settings of discrete controllers including load tap changers (LTCs) and shunt capacitor banks (SCBs). An iterative algorithm involving a master problem and a number of sub-problems is proposed. The master problem selects the optimal discrete controllers’ settings by taking into account primal SC-ACOPF constraints based on the single-component worst contingencies identified by the sub-problems. Each sub-problem is a bi-level max-min problem used to find the current worst contingency. The overall objective is to minimize the total cost, including generation and load shedding cost, while satisfying all relevant constraints. The master problem and sub-problems allow representing LTC tap positions, SCB status, and contingencies using binary variables. Two case studies are presented as applications of this novel technique. [ABSTRACT FROM AUTHOR]

The paper considers the problem of sub-synchronous resonances (SSR) in doubly fed induction generators connected to the grid through series-compensated lines. The existence of oscillations is observed on a laboratory test-bed in various conditions, with instabilities developing in the worst-case scenario. Despite the limitations of a small-scale test-bed, the ability to investigate problems and solutions in ways that would not be possible on a full-scale system is very valuable. In particular, the paper demonstrates the significant effect of the choice of dq reference frame on the severity of SSR. Stator-aligned algorithms are found to be significantly more resonant than grid-oriented algorithms, as evidenced in experiments as well as using an analysis method proposed in the paper. From this observation, an algorithm is proposed to emulate a grid-alignment using stator voltages and currents, but without measurements of the grid and series-capacitor voltages. Two control laws are then presented to regulate the active and reactive powers generated, resulting in well-damped transient responses. SSR oscillations are found to be eliminated in experime, as well as simulations of a full-scale system and using a frequency response analysis of the closed-loop system. The effectiveness of the proposed control schemes is verified through experiments for different compensation levels and varying speeds. Within the assumptions made in the design, the proposed controllers are global. [ABSTRACT FROM AUTHOR]

This paper applies the principles of distribution locational marginal pricing (DLMP) to unbalanced three-phase distribution networks. We first propose a linear model for AC optimal power flow derived through a series of approximation and reformulation techniques. Then a scenario-generation algorithm is proposed to properly model the uncertain parameters in the linear model. Through a proposed No U-Turn sampler (NUTS) based algorithm, probability density functions (PDFs) of DLMPs are calculated. These PDFs provide statistical information about the locational and temporal price risks. By means of applications to two IEEE unbalanced test networks, the numerical results show promising performance for the proposed linear model and the NUTS-based algorithm in creating PDFs of DLMPs. DLMP price densities will be increasingly useful as distribution system operators seek flexible, low risk solutions from embedded generators and aggregators of distributed energy resources. [ABSTRACT FROM AUTHOR]

This paper presents a new control strategy for enhancing the dc voltage regulation and the transient response in the multi-terminal VSC-HVDC system by ensuring swift power-sharing among converters when encountering large disturbance such as converter loss or ac fault at one of the converter stations. The quick power sharing can be realized using direct power control (DPC) since it has a merit of fast power dynamic response. On the other hand, to limit the converter current during ac fault, a new coordination strategy between DPC and vector current control (VCC) is proposed. Furthermore, a new simplified model of the MTDC system is developed to study the performance of DPC in enhancing the dc voltage dynamics response analytically. The time-domain simulation of the detailed model of the four converter stations MTDC system verify that the proposed control strategy significantly reduces the overshoot in the transient dc voltage response following an outage or ac fault. As a result, together with the droop gain selection for the maximum permissible steady-state dc voltage deviation, the proposed control strategy can help to reduce the size of the dc chopper in the MTDC grid or even avoiding it. Moreover, the comparative analysis based on time-domain simulation depicts that the proposed simplified model has the same dc voltage response compared to the detailed model of the MTDC system except for a steady-state error of 0.47% and a little bit longer settling time (1.5 ms) at transient. [ABSTRACT FROM AUTHOR]

This paper presents a two-timescale multi-objective coordinated volt/var optimization system for active distribution networks with renewable energy resources (RESs). In the hourly timescale, power loss and the number of control actions of on-load tap changers and (switchable) shunt capacitors for 24 h are minimized by coordinating on-load tap changers, (switchable) shunt capacitors, and reactive power outputs of RESs. A three-step method consisting of global search, user preference, and local search is adopted to solve the corresponding multi-objective mixed-integer nonlinear optimization problem. A rolling volt/var optimization is executed every 6 h, considering the day-ahead RES forecast errors. In the 15-min timescale, the RES reactive powers are optimized with RES uncertainties explicitly taken into consideration to minimize the pilot-bus voltage deviation under all representative scenarios. The pilot buses are those buses whose voltage magnitudes capture the network voltage profile. In this paper, a linear method is applied to quickly identify a set of pilot buses, and the impact of the identified pilot bus on the entire network voltage profile is evaluated. The IEEE 33-bus distribution network is used to evaluate the proposed two-timescale volt/var optimization formulations and methods with promising results. [ABSTRACT FROM AUTHOR]

Full converter-based wind power generation (FCWG, e.g., a permanent magnet synchronous generator (PMSG)), though normally considered to be decoupled from the external power grid can be actuated as an inertia source to suppress modal resonance in wind generation penetrated power systems by installing auxiliary resonance controllers (ARCs). In this paper, three possible options for ARC installation are first identified based on some derivations of the conventional control model of FCWG. The damping support mechanism of ARC is revealed, a suitable and generic configuration structure of ARC is then established, and optimal parameter tuning is conducted on the basis of this ARC configuration. The three ARC alternatives are equipped to contribute to damping by utilizing the potential energy and dynamics hidden in different inertia source components (i.e., the wind turbine rotor and DC capacitor, respectively) of FCWG. Both modal analysis and simulation results validate the effectiveness of the three proposed ARCs in suppressing modal resonance and improving system oscillation stability. Most importantly, extensive comparison investigations are carried out to fully evaluate the pros and cons of the three ARCs and thus provide constructive application guidance for system operators and wind farm owners. [ABSTRACT FROM AUTHOR]

With increasing penetration of solar PV, some distribution feeders are experiencing highly variable net-load flows and even reverse flows. To optimize distribution systems under such conditions, the scheduling of mechanical devices, such as OLTCs and capacitor banks, needs to take into account forecasted solar PV and actual grid conditions. However, these legacy switching assets are operated on a daily or hourly timescale, due to the wear and tear associated with mechanical switching, which makes them unsuitable for real-time control. Therefore, there is a natural timescale-separation between these slower mechanical assets and the responsive nature of inverter-based resources. In this paper, we present a network admissible convex formulation for holistically scheduling controllable grid assets to position voltage optimally against solar PV. An optimal hourly schedule is presented that utilizes mechanical resources to position the predicted voltages close to nominal values, while minimizing the use of inverter-based resources (i.e., DERs), making them available for control at a faster time-scale (after the uncertainty reveals itself). A convex, inner approximation of the OPF problem is adapted to a mixed-integer linear program that minimizes voltage deviations from nominal (i.e., maximizes voltage margins). The resulting OPF solution respects all the network constraints and is, hence, robust against modeling simplifications.Simulation based analysis on IEEE distribution feeders validates the approach. [ABSTRACT FROM AUTHOR]

The use of supplementary controllers for mitigating subsynchronous control interaction (SSCI) in doubly-fed induction generator based wind parks is quite promising due to their low investment costs. These SSCI damping controllers are typically designed and tested using an aggregated wind turbine (WT) model that represents the entire wind park (WP). However, no research has been reported on their implementations in a realistic WP. This paper, first presents various implementation schemes for a linear-quadratic regulator based SSCI damping controller, and discusses the corresponding practical challenges. Then, an implementation scheme that obviates the need for high rate data transfer between the WTs and the WP secondary control layer is proposed. In the proposed implementation, the SSCI damping controller receives only the WT outage information updates from the WP controller, hence it is not vulnerable to the variable communication network latency. The SSCI damping controller parameters are also modified when there is a change in WT outage information for the ultimate performance. The effectiveness of the proposed implementation scheme is confirmed with detailed electromagnetic transient simulations, considering different wind speeds at each WT and WT outages due to sudden decrease in wind speeds. [ABSTRACT FROM AUTHOR]

This paper proposes a new data-driven model-free Volt-Var control (VVC) scheme for the operation of radial power distribution systems. Unlike existing methods, the proposed VVC is neither reliant on model-based computer simulations nor limited to intuition/experience-driven rules for the operation of switchable capacitors and voltage regulators inside a feeder. Despite the model-free feature, the proposed scheme can still give optimal performances by using statistical estimations on measurement data such as the one provided by an advanced metering infrastructure system. Effectiveness and feasibility of the proposed idea is demonstrated by simulation studies on an IEEE 123 nodes test feeder. After few days of initial exploration, the proposed VVC is capable to give satisfactory results near to that of a model-based optimization technique. The promising outcome of this study suggests a novel application for emerging measurement and communication technologies in the operation of modern grids. [ABSTRACT FROM AUTHOR]

The penetration of photovoltaics (PVs) and electric vehicles (EVs) is increasing in active distribution networks (ADN), which may lead to severe voltage violation problems. This paper proposes a two-stage real-time Volt/Var control method to mitigate fast voltage violations. In the first stage, hourly on-load tap changer (OLTC) and capacitor banks (CBs) are scheduled based on the optimal power flow method. The optimization problem is formulated as a mixed-integer second-order cone programming (MISOCP) which can be effectively solved. In the second stage, a data-driven network partition method is proposed to select critical bus and assess the voltage violation risk of each control area, followed by the intra-day dispatch of EVs. Based on the partition results, reactive power of PVs and EVs is controlled in real-time to mitigate voltage violations. Considering the suddenly active power drop of PVs, a rule-based control strategy coordinating PVs and CBs is proposed to improve the reactive power reserve in ADN. The proposed approach is tested on the IEEE 33-bus and 123-bus distribution networks and simulation results verify the effectiveness both in network partition and real-time voltage control. [ABSTRACT FROM AUTHOR]

This paper presents a new mixed-integer linear programming (MILP) model to solve the multistage long-term expansion planning problem of electrical distribution systems (EDSs) considering the following alternatives: increasing the capacity of existing substations, constructing new substations, allocating capacitor banks and/or voltage regulators, constructing and/or reinforcing circuits, and modifying, if necessary, the system's topology. The aim is to minimize the investment and operation costs of the EDS over an established planning horizon. The proposed model uses a linearization technique and an approximation for transforming the original problem into an MILP model. The MILP model guarantees convergence to optimality by using existing classical optimization tools. In order to verify the efficiency of the proposed methodology, a 24-node test system was employed. [ABSTRACT FROM AUTHOR]

This paper introduces a screening method to determine the potential risk of interactions between a dynamic device and a generator with torsional oscillations. The proposed method introduces a factor called the radiality factor (RF) which is an indicator of the radialness of the network between the device and the generator. TheRF is calculated in the sub-synchronous frequency range and plotted against the frequency (RF curves). The RF curves can then be used for screening of torsional interactions between the generator and other dynamic devices. The critical value for the RF is determined through a large number of sensitivity studies on a test system. This method surpasses the available screening method: unit interaction factor (UIF) calculations which is only applicable for inductive networks without series compensated lines. [ABSTRACT FROM AUTHOR]

The paper proposes a framework for the selection of optimal configuration of line compensation devices, thyristor controlled series capacitors (TCSC) and conventional fixed series capacitors, for reducing the risk of subsynchronous resonance (SSR) in the network while maintaining required power transfer. The methodology developed in the paper is based on the robust risk evaluation of SSR that takes into consideration the severity of subsynchronous resonance and probability of its occurrence. The subsynchronous resonance risk index developed previously is used to assess the severity of subsynchronous resonance. The line outage model is employed to determine the probability of contingencies potentially leading to subsynchronous resonance. It is demonstrated that with relatively small participation of TCSC, even with the most basic control, in series compensation of lines, the risk of SSR can be successfully managed. [ABSTRACT FROM PUBLISHER]

This paper addresses the positive sequence modeling of a converter-interfaced synchronous generator for use in large scale grid simulations. With the increased penetration of wind and solar energy and the projected increase in energy from natural gas and hydro turbines, availability of a mathematical model representing a converter-interfaced synchronous generator in positive sequence simulations is essential. Such models are not currently available in commercial software packages. The importance of modeling the source behind the inverter due to practical limitations on the size of the dc bus capacitor is emphasized. In addition, it will be shown that there is a nominal increase in simulation time, in evaluating the transient stability of large systems using this model. [ABSTRACT FROM PUBLISHER]

This paper studies stability analysis of dc microgrids with uncertain constant power loads (CPLs). It is well known that CPLs have negative impedance effects, which may cause instability in a dc microgrid. Existing works often study the stability around a given equilibrium based on some nominal values of CPLs. However, in real applications, the equilibrium of a dc microgrid depends on the loading condition that often changes over time. Different from many previous results, this paper develops a framework that can analyze the DC microgrid stability for a given range of CPLs. The problem is formulated as a robust stability problem of a polytopic uncertain linear system. By exploiting the structure of the problem, we derive a set of sufficient conditions that can be efficiently checked by solving convex optimization problems to guarantee locally robust stability. The effectiveness and nonconservativeness of the proposed framework are demonstrated using simulation examples. [ABSTRACT FROM PUBLISHER]

Power systems are mostly run under stressed operating conditions due to rapidly increasing demand but inadequate expansion in generation and transmission capacity. Thus voltage stability has become a major concern in present day power system operation which depends on the availability and distribution of reactive power. It becomes very important to ensure the presence of sufficient amount of reactive power source in the network for secure and reliable operation of electrical power system. This paper mainly deals with short-term reactive power procurement (RPP) problem assuming an already established real power market. Contributions of this paper are three fold. First, this paper presents three new RPP methods which are formulated as optimum power flow (OPF) problems. OPFs are solved using Artificial Bee Colony (ABC) algorithm as population based stochastic solution methods are more suitable for solving reactive power optimization problem. Second, a detailed analysis of voltage stability for proposed RPP problems are performed for both normal and contingency condition to find out how these new formulations affect voltage stability. Third, proposed methods are compared with the classical RPP formulations, e.g., Minimization of L-index and loss Minimization. Finally results are reported for both test system and a large practical system. [ABSTRACT FROM PUBLISHER]

A resilient power system is capable of providing fast and efficient recovery from outages caused by natural or man-made attacks. After a major blackout, transmission, and distribution system operators (TSOs and DSOs) coordinate to restore the system in a timely and reliable manner. However, these two systems are usually operated separately without considering capabilities and limitations of each individual network, or their mutual impacts. This could prolong the restoration process and lead to unpractical outcomes. Therefore, a distributed strategy for integrated transmission and distribution systems restoration (ITDSR) has been developed in this paper. Based on the alternating direction method of multipliers (ADMM), the distributed algorithm is able to provide the coordinated restoration for both TSOs and DSOs by sharing limited information of boundary buses in an iterative procedure. The ADMM-ITDSR model includes convex ac power flow model and three-phase unbalanced branch flow model. The ITDSR strategy coordinates the operation of distributed energy resources in terms of serving high priority loads across different networks. The effectiveness and advantage of developed models and algorithms are validated and demonstrated through testing of the integrated IEEE test cases, such as 14-bus transmission with 13-node distribution systems, and 39-bus transmission with 123-node distribution systems. [ABSTRACT FROM AUTHOR]

This paper introduces a new and general frame-of-reference for dynamic solutions of multi-terminal VSC-HVDC systems using the Newton-Raphson method. Three VSC dynamic models are derived to conform to each pairing AC sub-network—the slack converter whose aim is to control its DC voltage, the scheduled-power converter which injects a scheduled amount of power and the passive converter which is connected to an AC network with no frequency control equipment. Each VSC unit makes provisions for the phase reactor, AC filter, DC capacitor, DC smoothing inductor and LTC transformer. The VSC itself is a positive-sequence lumped-type model whose core elements are a phase-shifting transformer and an equivalent shunt susceptance which account for the phase-shifting and scaling nature of the PWM control. In turn, the DC side of each pairing VSC unit is linked to a DC system of an arbitrary configuration. All this enables the assembly of any number of VSCs, giving rise to a comprehensive formulation of multi-terminal VSC-HVDC systems. The prowess of the proposed multi-terminal dynamic model is demonstrated by carrying out a comparison against the widely-used EMT-type package Simulink, using a three terminal VSC-HVDC system, with very good results. Furthermore, a six-terminal VSC-HVDC system forming a DC ring is used to show the applicability of the proposed unified approach when solving multi-terminal VSC-HVDC links for system-wide dynamic studies. [ABSTRACT FROM PUBLISHER]

This paper applies a known stochastic approximation method to solve a two-phase optimal power dispatch problem under load uncertainty using a novel realistic model for random loads. The stochastic power dispatch problem is solved with the Robbins–Monro method applied with the Kiefer–Wolfowitz procedure with random directions. The constraints of this optimization problem have been investigated and considered by truncated algorithms. Two numerical examples are presented for illustration of the significant improvement obtained with the proposed method. In the first example, the optimizable cost of the system can be reduced by 1.6% under 2.68% load variation. In the second example, the results show that the possibility of voltage violations is reduced from 49.5% to 0.01%. All the improvements are compared with deterministic optimal solutions validated with scenario-based Monte Carlo simulations. The method is 60 times faster than scenario-based Monte Carlo for similar accuracy for the IEEE 30-bus test system. [ABSTRACT FROM PUBLISHER]

The discrete coordinate-descent algorithm is a practical approach that is currently used in centralized Volt/VAr Control (VVC) implementations, mainly due to its good performance and speed for real-time applications. Its viability is however challenged by the increasing number of distributed generation that contribute to the VVC solution, in addition to the conventional transformer taps and switched capacitors. This paper presents the exact computation of sensitivity factors that speed up the discrete coordinate-descent implementation, by significantly reducing the number of forward/backward substitutions in the current injection power flow method; the speed up is achieved without affecting the control setting quality of the original implementation. The optimality of the discrete coordinate-descent solutions is investigated by computing the gaps relative to mixed-integer linear programming set-points, derived from a polyhedral reformulation of the VVC problem. The sensitivity-based discrete coordinate-descent algorithm is tested starting from two initial points, the default one given by the current control set-points, and a continuous solution obtained from a linear approximation of the VVC problem. Numerical results on networks with up to 3145 nodes show that the sensitivity-based approach significantly improves the runtime of the discrete coordinate-descent algorithm, and that the linear programming initialization leads to VVC solutions with gaps relative to the mixed-integer set-points that are less than 0.5%. [ABSTRACT FROM PUBLISHER]

This paper proposes a novel variable-inertia emulation control (VIEC) scheme that enables voltage-source-converter based high-voltage DC (VSC-HVDC) transmission systems to flexibly support AC grid frequency stability like synchronous generators. The VIEC scheme allows us to extract the energy from the augmented DC capacitance for inertia emulation by controlling the DC voltage without affecting the stability of the AC system connected on the remote side. In particular, with the proposed VIEC scheme, the inertia time constant can be flexibly emulated and its value can be automatically adjusted according to the rate of change of grid frequency as well as grid frequency deviations. This leads to more effective inertial support across different timescales. Modal analysis is carried out to investigate the impacts of inertia and DC capacitance, and to obtain the optimal control parameters. The effectiveness and advantages of the proposed VIEC scheme are demonstrated on an IEEE benchmark system in the presence of faults and load changes. [ABSTRACT FROM AUTHOR]

This paper introduces significant performance enhancements to the ac filterless LCC HVDC by including fixed series capacitors at the primary side of a converter transformer. In terms of technical performance, 1) the amount of active power that can be transmitted is increased by more than 60% compared with ac filterless LCC HVDC, especially under severe unbalanced fault such as single-phase fault (most common fault in power systems); 2) the required voltage level of the controllable capacitor for Commutation Failure (CF) elimination is reduced by more than 70%, which leads to considerable reductions of the associated costs and losses. In terms of economic performance, due to the reduction of the required voltage from controllable capacitors (hence, the number of power electronic devices), the cost of the proposed converter station is lower than that of the ac filterless LCC HVDC. Theoretical analysis is presented to illustrate the performance enhancements and select the size of the series capacitor. Simulation results for various kinds of faults and cost analysis are presented to validate the technical and economic performances of the proposed method. Comparisons are made with ac filterless LCC HVDC. Finally, various practical issues and possible solutions are discussed. [ABSTRACT FROM AUTHOR]

In this paper, a two-stage optimization framework is proposed for optimal placement and centralized real-time control of low-voltage static var compensators (LV-SVCs) in unbalanced distribution circuits to achieve feeder level benefits namely voltage regulation and energy loss minimization. The number, locations, and the optimal real-time reactive power injections from the LV-SVCs are determined from a proposed three-phase unbalanced ac optimal power flow (ACOPF). The ACOPF is formulated as a multi-objective optimization with operating constraints written in rectangular coordinates. The resulting nonlinear nonconvex problem is solved using the predictor-corrector primal-dual interior point method. The proposed approach is scalable for application to large distribution circuits, treats the constraints of physical space limitations effectively, and can take advantage of communication capabilities of LV-SVCs for centralized real-time control. The benefits of the proposed real-time control of LV-SVCs as compared to their operation with local autonomous voltage-based distributed control is demonstrated. The results show that, the proposed approach addresses the LV-SVC placement and control problem effectively to minimize energy losses while maintaining the voltage regulation. [ABSTRACT FROM AUTHOR]

Conservation voltage reduction (CVR) has been traditionally applied adopting moderate settings at primary substations and when distributed generation was uncommon. However, as new infrastructure is deployed across European-style medium voltage (MV) and low voltage (LV) networks, driven by increasing photovoltaic (PV) penetration levels, the opportunity arises to develop more advanced CVR schemes. This paper proposes a centralized, three-phase AC optimal power flow (OPF)-based CVR scheme that, using monitoring, on-load tap changers and capacitors across MV and LV, actively manages voltages to minimize energy consumption, even with high PV penetration, while considering MV–LV constraints. To tackle scalability issues brought by discrete variables, a two-stage approach is proposed to solve the OPF as a non-linear programming problem (relaxing integer variables). A process that continuously checks customer voltages is adopted to trigger the optimization only when needed. Moreover, CVR benefits are not only quantified at a network level but also for customers, providing useful insights to policy makers. The proposed control is assessed using a realistic, unbalanced U.K. residential MV–LV network (2,400+ customers) with high PV penetration, and 1-min resolution time-varying profiles and load models. Results demonstrate that the proposed control effectively coordinates voltage regulation in MV and LV levels throughout the day, minimizing energy imports for all customers. [ABSTRACT FROM AUTHOR]

DC faults are critical events for the multi-terminal HVdc protection systems, where knowing the critical fault time is fundamental in the protections design. Existing literature analyzes the critical fault time with simulations or with complex analysis, requiring large computational effort and they are not robust to parameters variations. Furthermore, standard approaches rely on simplified modular multilevel converter (MMC) models neglecting the influence of converter control loops dynamic. As result, the prediction of the node voltages and especially the dc currents become increasingly inaccurate as bigger is the distance from the fault. This paper introduces a fault current estimation method in MMC-based multi-terminal dc (MTdc) grids, which improves the current differential equation based methodologies including the dynamics of power and dc voltage controllers. The proposed method gives the possibility of analyzing the fault current under different grid conditions (e.g., fault resistance and limiting inductor value). Furthermore, this method is extended to bipolar MTdc systems in case of pole-to-pole and pole-to-ground faults, and it is validated by means of simulations in PSCAD environment and standard CIGRE MTdc grid. [ABSTRACT FROM AUTHOR]

Smart inverters provide additional control capability to help optimize the operation of distribution systems. This paper proposes a framework for exact optimal active and reactive power dispatch of distributed photovoltaic (PV) generation, switched capacitors, and voltage regulators in large multi-phase unbalanced distribution systems. The objectives of the optimal dispatch are minimization of the energy loss, PV active power curtailment, and operations of capacitors and voltage regulators, in addition to elimination of voltage violations and reverse power flow. The optimization problem is formulated in rectangular coordinates as a nonlinear, nonconvex problem. Effective computational strategies are proposed to allow the application of predictor-corrector primal-dual interior point method to solve optimization problems in real-time with a large number of constraints and variables, including discrete variables corresponding to switched capacitors and voltage regulators. The accuracy of the numerical solution and the ability to implement the proposed framework are validated using the unbalanced multi-phase IEEE 34-bus and EPRI 2,998-bus distribution systems with 15-minute load and PV data. The results show a significant loss reduction and elimination of both voltage violations and reverse power flow. [ABSTRACT FROM AUTHOR]

Excitation boosters (EBs) are aimed to improve fault ride through capability of synchronous generators equipped with bus fed static excitation systems by means of adding a supplementary voltage or current source to the rectifier. However, this paper shows that EBs governed by local on–off controls may result in adverse effects in case of remote faults from generators that may occur in multi-machine systems. To sort this problem out, a novel EB Wide Area Control System (WACS) that modulates the EB voltage using the generator speed deviation with respect to the speed of the center of inertia (COI) is proposed. The speed of the COI is obtained using a Wide Area Measurement System (WAMS), whereas the control laws are deduced from the Lyapunov stability theory. Results show that transient stability of multi-machine systems can be enhanced using EB WACS. [ABSTRACT FROM AUTHOR]

This paper proposes a multi-timescale volt/var optimization for the optimal dispatch of battery energy storage system in smart distribution grids. It aims to coordinate the substation on-load tap changer operation on slow-timescale (hourly basis) with the photovoltaic inverters and battery storage operations on fast-timescale (15 min basis). This coordination is achieved by using two-stage stochastic programming and implemented via model predictive control. The power loss and energy purchase cost are reduced while maintaining voltages within limits. The forecasting uncertainties are modeled by generating a large number of random scenarios and then subsequently reducing scenario numbers to establish a tradeoff between computational burden and accuracy of the solution. The mixed-integer second-order cone program (MISOCP) is formulated with reduced scenarios, which achieves global optimum. Simulation results demonstrate the effectiveness of proposed MISOCP model in keeping the voltages within limits under forecasting uncertainties. [ABSTRACT FROM AUTHOR]

This paper considers the co-operation of distributed generators (DGs), battery energy storage systems (BESSs) and voltage regulating devices for integrated peak shaving and voltage regulation in distribution grids through a co-optimization framework, which aims to minimize the operational costs while fulfilling the operational constraints of network and devices. To account for the uncertainties of load demand and generation, we then convert the co-optimization model into a two-stage stochastic program where state-of-charge (SoC) trajectories of BESSs and operation of voltage regulating devices are optimized at the first stage for day-ahead scheduling, while the reactive powers of DGs and BESSs are left at the second stage for potential intra-day scheduling to handle short-term voltage issues. The proposed co-optimization scheme is validated on the IEEE 37-node test feeder and compared with other practices. [ABSTRACT FROM AUTHOR]

The focus of this paper is on analyzing the impact of conservation voltage reduction in the presence of active devices such as solar photovoltaic (PV) and developing controls that leverage these distributed energy resources. An event-driven predictive approach for real-time volt/volt-ampere reactive (VAR) optimization, along with a local two-level adaptive volt/VAR droop-based control algorithm for advanced distribution management systems, is introduced. The methodology covers aggregated and autonomous controls under different timescale operations, including the impact and effect of unpredicted events such as cloud transients on PV power production. Besides, the control schemes include the uncertainties in PV power generation and load power demand. The proposed methodology is validated in a real-time framework using the real-time digital simulator platform through co-simulation with models based on Python and OpenDSS (Open Distribution System Simulator). The developed methodology is tested on the modified IEEE 123-feeder test system. The results reveal that the proposed methodology works well in the presence of high penetrations of PV power, produces significant energy savings, and mitigates over-/undervoltage problems. [ABSTRACT FROM AUTHOR]

A centralized reactive power compensation system is proposed for low voltage (LV) distribution networks. It can be connected with any bus which needs reactive power. The current industry practice is to locally install reactive power compensation system to maintain the local bus voltage and power factor. By centralizing capacitor banks together, it can help to maintain bus voltages and power factors as well as reduce the power cable losses. Besides, the centralized reactive power system can be easily expanded to meet any future load increase. A reasonably sized centralized reactive power compensation system will be capable of meeting the requirements of the network and the optimization algorithm proposed in this paper can help to find this optimal size by minimizing the expected total cost (ETCH). Different load situations and their respective probabilities are also considered in the proposed algorithm. The concept of the centralized reactive power compensation system is applied to a local shipyard power system to verify its effectiveness. The results show that an optimally sized centralized reactive power system exists and is capable of maintaining bus voltages as well as reducing the power losses in the distribution network. A significant power loss reduction can be obtained at the optimal capacity of the centralized reactive power compensation system in the case study. [ABSTRACT FROM PUBLISHER]

This paper proposes a novel method for topology detection in distribution networks called the Time-series signature verification method for topology detection (TSV-Top). The TSV-Top analyzes data from phasor measurement units (PMU or $\mu$ PMU) installed on power distribution feeders. The TSV-Top relies on the fact that measurement data time series from power systems contain similar trends when network topology changes occur, and basically performs the projection of actual voltage phasorial patterns onto a library of signals associated with possible topology transictions of a given distribution network. The proposed algorithm is validated using a 33-bus testbed and the IEEE 123-bus feeder, and then compared with another state-of-the-art algorithm using the IEEE 37-bus feeder. [ABSTRACT FROM AUTHOR]

In this paper, a day-ahead market-clearing model for smart distribution systems is proposed. Various types of distributed energy resources (DERs), such as distributed energy storage, distributed generators, microgrids, and load aggregators, can bid into the day-ahead distribution-level electricity market. Considering system Volt/VAR control, network reconfiguration, and interactions with the wholesale market, an optimization model is built to clear the day-ahead market, through which the distribution locational marginal pricing (DLMPs) for both active power and reactive power are determined. Through derivations of the Lagrangian function and sensitivity factors, DLMPs are decomposed to five components (i.e., marginal costs for active power, reactive power, congestion, voltage support, and loss), which provide price signals to motivate DERs to contribute to congestion management and voltage support. Finally, case studies demonstrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The multi-objective reactive power coordination control model was presented in this paper to overcome the drawback that reactive power compensation devices lack in coordination and have high active power loss. In this model, the minimum bus voltage deviation and total power loss of reactive power compensation devices are taken as objective functions, by bringing the lower voltage reactors and capacitors into the static var compensator control system and considering the reactive power compensation devices interaction between different transformer substations. Due to the different sensitivity of control variables in control model and lack of the local search ability, control variables are divided into sensitive variables and non-sensitive variables. Then, the improved non-dominated sorting genetic algorithm II (NSGA-II) with secondary search ability is used to search the Pareto optimal solution set. Two substations in large power grid with strong voltage coupling are coordinated in different load level. The results obtained by proposed NSGA-II algorithm can provide a variety of optimal control strategies. Compared to the conventional NSGA-II algorithm and normal boundary intersection method, the improved NSGA-II algorithm has better convergence curve and distribution of the Pareto solution sets. [ABSTRACT FROM PUBLISHER]

Online power distribution system time series analyses are typically divided into uniform intervals (e.g., market-driven) ranging from minutes to hours. This paper presents a constraint-driven approach to refining an initial time window structure at critical times within an injection forecast. Implicit temporal load capability (ITLC), which is formalized in this paper, uses forecasted nodal injection characteristics to identify critical times when control actions are necessary in order to maintain feasibility. ITLC and control selection algorithms are presented. Quasi-static time series and ITLC simulation results are presented using a multiphase, 2556-node distribution test circuit. With ITLC, a subset of the initial time windows are subdivided at the analytically-selected critical times. This approach is more efficient than using a large number of short time windows that span the entire forecast horizon. [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]

Voltage instability occurs when a power system is unable to meet the reactive power demand, and is typically corrected by switching on additional reactive power devices such as capacitor banks. Real-time monitoring and communication technologies can potentially improve voltage stability by enabling the rapid detection of low voltages and the implementation of corrective actions. These corrective actions, however, will only be effective in restoring stability if they are chosen in a timely, scalable manner. In this paper, we propose a submodular optimization approach for designing a control strategy that prevents voltage instability. Our key insight is that the voltage deviation from the desired level is a supermodular function of the set of reactive power injections that are employed, leading to computationally efficient control algorithms for stabilization with provable optimality guarantees. This submodular control framework is tested on the IEEE 300-bus transmission system. [ABSTRACT FROM PUBLISHER]

This paper proposes a generic pole-to-pole short-circuit fault current calculation method for dc grids. The calculation procedure begins from the simplified RLC equivalent model of a single modular multilevel converter, and then the prefault matrices and faulted matrices are established and modified to calculate the dc fault currents of all the branches. The proposed approaches are validated by comparing with the electromagnetic transient (EMT) simulation results on PSCAD/EMTDC. Besides, two case studies showed that the calculation method can be easily used to evaluate the severity of a dc fault. Moreover, the calculation can be applied to select the parameters of a fault current limiter (to match the circuit breaker capacity. The main contributions of the proposed numerical calculation method are: 1) The proposed method is accurate and much more time efficient than the EMT simulations; 2) the proposed method can handle all kinds of dc grid networks including the ring, radial, and meshed topologies; and 3) the proposed method is applicable to dc grid with multiple dc voltage level areas connected with dc/dc converters. [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]

With the development of a new smart load device, the electric spring (ES), the operation of distribution systems with high renewable penetration becomes more flexible. The ESs can be installed at noncritical loads for grid support. This paper proposes a two-level voltage management scheme to optimize the voltage profiles of the network, especially at chosen critical buses. In the upper level, the tap positions of load tap changer and capacitor banks switching are optimized to prevent the voltages along the feeder from being out of limits. The model predictive control technique is applied to handle the uncertainties in renewable energy and demand. In the lower level, the responsibility of maintaining the voltages of the critical buses is shared among the ES in a distributed way via consensus control which is suitable for systems with limited communication and calculation capabilities. The proposed management scheme is verified on a modified IEEE 15-bus distribution network. The results show that different voltage regulation devices can work together to maintain the voltage of critical buses by sharing the responsibility in the proposed scheme. [ABSTRACT FROM PUBLISHER]

This paper investigates two novel control strategies that enable system inertia supports by permanent magnet synchronous generator (PMSG) wind turbines during transient events. The first strategy seeks to provide inertia support to the system through simultaneous utilization of dc-link capacitor energy, and wind turbine (WT) rotor kinetic energy (KE). The second strategy supports system inertia through orderly exerting dc-link capacitor energy of WT and then WT rotor KE via a cascading control scheme. Both strategies can effectively provide system inertia support by fully utilizing WT's own potentials, while the second strategy distinguishes itself by minimizing its impacts on wind energy harvesting. Case studies of one synchronous generator connected with a PMSG-based WT considering sudden load variations have been studied to validate and compare the two proposed strategies on providing rapid inertia response for the system. [ABSTRACT FROM AUTHOR]

It is well known that traditional line-commutated converter (LCC) based high voltage direct current (HVDC) system is not able to control its reactive power and terminal AC voltages. This paper investigates the reactive power and AC voltage control at the inverter side of the LCC HVDC system with controllable capacitors. The system's ability of operating under negative extinction angle is utilized to achieve a wide range of reactive power control and, in particular, the ability of exporting reactive power. In connection with the inverter AC terminal voltage or reactive power control, among different control possibilities at the rectifier side, active power control is desirable since large variations of active power transfer is very unfavorable. Detailed theoretical analysis is carried out first to show the reactive power controllability, and the capacitor voltage level is selected based on the desired control range. In addition, a new extinction angle measurement approach is proposed for negative extinction angle measurements. The effectiveness of the reactive power/voltage control capability for the proposed system is validated through simulation results using Real-Time Digital Simulator (RTDS). To verify the effectiveness of the reactive power and voltage control, CCC HVDC and LCC HVDC with SVC are also set up in RTDS, and simulation comparisons are made. Furthermore, contribution to AC voltage control in power system using the proposed method is demonstrated through simulation results of the modified two-area four-machine AC power system. [ABSTRACT FROM AUTHOR]

Accurate representation of system loads is critical for performing power system dynamic studies. As of date, in positive sequence bulk power system dynamic studies, the WECC composite load model is the state-of-the-art model for representing the aggregated behavior of loads at the substation bus. Although used widely, this model is continuously undergoing improvements to ensure that a reasonable representation of different load components can be obtained. One such refinement is the development of a model for representing the aggregated response of VFDs. This paper proposed a generic positive sequence model used to capture the aggregated response of a large number of VFDs connected at a load bus for bulk system dynamic studies. The model was developed and validated using laboratory test measurements of response different VFDs to voltage sags. [ABSTRACT FROM AUTHOR]

Low-frequency high-voltage ac transmission scheme has recently been proposed as an alternative approach for bulk power transmission. This paper proposes a multi-period optimal power flow (OPF) for a multi-frequency HVac transmission system that interconnects both conventional 50/60-Hz and low-frequency grids using back-to-back converters with a centralized control scheme. The OPF objective is to minimize system losses by determining the optimal dispatch for generators, shunt capacitors, and converters. The OPF constraints include the operational constraints of all HVac grid and converter stations. The resulting mixed-integer nonlinear programing problem is solved using a proposed framework based on the predictor-corrector primal-dual interior-point method. The proposed OPF formulation and solution approach are verified using a multi-frequency HVac transmission system that is modified from the IEEE 57-bus system. The results with the optimal dispatch from the proposed method during a simulated day show a significant loss reduction and an improved voltage regulation compared to those when an arbitrary dispatch is chosen. [ABSTRACT FROM AUTHOR]

Optimal power flow (OPF) is a fundamental problem in power system operating and planning. The need for distributed solutions is rapidly growing in distribution systems with radial networks, discrete equipments and many distributed generation units. Typically the model is difficult to solve due to the highly non-convexity and even non-continuity of the arising OPF problem. In this paper, we address these challenges by developing a penalty dual decomposition (PDD) based algorithm to obtain distributed solutions. The proposed PDD based algorithm mainly consists of two loops: in the outer loop we update the dual variables and the penalty parameter according to the constraint violation, while in the inner loop we divide the primal variables into several blocks and employ the block successive upper-bound minimization (BSUM) method to iteratively optimize each block variables with fixed penalty parameter and dual variables. Each subproblem in the proposed PDD based algorithm can be solved either in closed-form or by the bisection method and every limit point generated by the proposed algorithm is guaranteed to be a stationary point of the OPF problem. Simulation results, and comparison with centralized PDD and concave-convex procedure (CCCP) based algorithms are presented to validate the effectiveness of the proposed algorithm. [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]

In the distribution management system (DMS), an essential application for the energy efficiency improvement is loss reduction. Loss reduction often involves the control of reactive power (var) resources to optimize the var flow in the network. Traditionally, most available var resources in the distribution network have been switchable shunt capacitor banks. Today distributed energy resources (DER) are becoming another significant var resource due to their increasing penetration, which brings new opportunities as well as challenges for loss reduction. This paper proposes an advanced loss reduction approach to achieve the optimal control coordination among multiple capacitors and DERs. The proposed approach and solution are developed on the basis of the detailed multi-phase distribution network modeling and the state-of-the-art optimization technology. This work also investigates the impact of the DER var control on the loss reduction improvement and voltage violation correction. The effectiveness of the proposed approach is demonstrated on practical utility distribution circuits with varying degree of unbalance and model complexity. The performance of the solution method proves the capability of online solution speed for large and complex distribution systems. [ABSTRACT FROM PUBLISHER]

The modular multilevel converter (MMC) provides promising development for high-voltage direct current (HVDC) applications, including multiterminal HVDC (MTDC) and renewable energy integration. This paper, considering an offshore wind farm (OWF) integrated MMC MTDC system, investigates its start-up process with three main developments: 1) it further develops the mathematical model of MTDC with active networks and proposes a hierarchical start-up control scheme; 2) for the terminal which connects the OWF, it proposes a reduced dc voltage control scheme of mitigating the current surges with deblocking the converter at zero voltage difference on submodules (SMs) and proposes an overall sequential start-up control scheme for the offshore integrated MTDC; and 3) it analyzes and compares different start-up control schemes. To evaluate the proposed sequential start-up control scheme, an offshore MMC HVDC system is established on the RTDS. The simulation results verify effectiveness of the proposed scheme on the MMC MTDC system with two control paradigms, i.e., master–slave control and droop control, respectively. In comparison with different start-up control schemes, the superiority of the mitigation of voltage spikes and current surges are shown using the proposed scheme with less complexity and easier implementation. [ABSTRACT FROM PUBLISHER]

Motivated by several recent incidents, this paper investigates the impact of wind farm spatial distribution and shunt compensation levels on triggering system-side sub-synchronous oscillations (SSO) in wind-integrated power systems. Quantitative analysis on system eigenvalue sensitivities with respect to wind farm and transmission line parameters as well as system operating conditions is performed systematically to understand the multi-wind-farm-induced SSO. Coupling relationship for wind turbines with identical and different parameter settings on the studied SSO modes is investigated. Electro-magnetic simulations are performed on a multi-machine wind farm model derived from a practical wind-integrated power system, in order to verify the proposed study. [ABSTRACT FROM AUTHOR]

The hybrid energy storage system (HESS) plays an essential role in the shipboard medium-voltage dc (MVdc) system to provide backup power, buffer large load change, as well as improve power quality. In this paper, a novel virtual resistor and capacitor droop (VRCD) control is first proposed for HESS including battery, supercapacitor, and flywheel. It enables the generator and multiple types of energy storages (ESs) with different characteristics to respond to multiple-frequency unbalanced power instead of simple high-frequency and low-frequency power sharing by the conventional droop control method. A detailed design procedure for the VRCD control is presented. Also, the secondary state of charge recovery control is proposed to maintain high energy reserve of ESs for further power demand. Simulations are conducted to demonstrate the effectiveness of the control scheme proposed in a shipboard MVdc under particular loads fluctuation, such as the propulsion motor and the pulsed load. [ABSTRACT FROM AUTHOR]