Search

The pump-back test is widely adopted in the industry for characterizing and qualifying high-power converters. One popular pump-back test configuration is to tie the dc links of two converters together and circulate the current through the inductor connected between the ac terminals. In this configuration, one of the converters operates in the voltage mode to generate the virtual grid and the other converter regulates the circulating current, where hysteresis control could be applied. However, the switching characteristics of hysteresis control are unique due to the pulsewidth modulation type voltage being the virtual grid. This paper contributes to analyzing the switching frequency characteristics of the hysteresis control in this pump-back test configuration. The switching frequency tracking phenomenon is revealed and reported in this paper for the first time. Theoretical analysis, simulation validation, and experimental demonstration are provided to support the major conclusions. [ABSTRACT FROM AUTHOR]

This paper presents the work carried out by the IEEE Working Grou[ on field-measured overvoltages and their analysis for validating power system component models to be used in switching transients studies. The work uses measurement data obtained from field tests performed by Bonneville Power Administration in June 1995 where switching overvoltages were measured on one of its 230 kV lines. This paper includes a description of the switching tests and the main results derived from field measurements. Details of the switching procedures that were followed to calibrate component models used to match field-recorded waveforms can be found in the Part II paper. [ABSTRACT FROM AUTHOR]

This paper discusses the various aspects involving the research and development of smart grids. Discussed are applications, from large grid renewable integration, wide-area monitoring, protection, and control systems to microgrids. Load scheduling and power balance, communications issues, understanding customer behavior, large-area protection, and distribution control are only some aspects of the challenge of making power grids more robust and more intelligent. The potential complexity of such smart grids requires careful study and analysis before actual realization. This paper explains how such challenges are addressed using real-time simulation technologies in different laboratories around the world. [ABSTRACT FROM AUTHOR]

The energy and cooling costs of Web server farms are among their main financial expenditures. This paper explores the benefits of dynamic voltage scaling (DVS) for power management in server farms. Unlike previous work, which addressed DVS on individual servers and on load-balanced server replicas, this paper addresses DVS in multistage service pipelines. Contemporary Web server installations typically adopt a three-tier architecture in which the first tier presents a Web interface, the second executes scripts that implement business logic, and the third serves database accesses. From a user's perspective, only the end-to-end response across the entire pipeline is relevant. This paper presents a rigorous optimization methodology and an algorithm for minimizing the total energy expenditure of the multistage pipeline subject to soft end-to-end response-time constraints. A distributed power management service is designed and evaluated on a real three-tier server prototype for coordinating DVS settings in a way that minimizes global energy consumption while meeting end-to-end delay constraints. The service is shown to consume as much as 30 percent less energy compared to the default (Linux) energy saving policy. [ABSTRACT FROM AUTHOR]

This paper provides a theoretical and experimental discussion on a modular multilevel triple-star bridge-cell converter with focus on mitigating the capacitor-voltage fluctuation contained in each bridge cell. A new control method proposed in this paper is characterized by properly superimposing both common-mode voltage and circulating currents in the whole speed range. Moreover, this paper optimizes the amplitude and frequency of the superimposed common-mode voltage on the basis of theoretical analysis, thus resulting in minimizing the voltage fluctuation. The optimized control method makes both voltage fluctuation and cluster-current amplitude smaller than do the existing methods. Experimental waveforms obtained from a three-phase downscaled system using a 370-V, 15-kW, 75-Hz, six-pole, permanent-magnet synchronous motor verify good start-up performance from a standstill to the rated speed, keeping both capacitor-voltage and cluster current within acceptable levels. [ABSTRACT FROM AUTHOR]

This paper proposes a novel open-winding brushless doubly-fed reluctance generator (OW-BDFRG) with dual two-level converters in order to reduce the converter rating and switching frequency for large-scale wind turbine applications. The new converter topology is equivalent to a three-level converter directly connected to the control winding of typical BDFRG. The OW-BDFRG system with this topology structure requires lower converter rating and switching frequency, and has a more flexible control mode, better operation performance, and fault redundancy capability. For the OW-BDFRG, this paper also proposes a new control scheme combining direct power control (DPC) with sliding mode variable structure (SMVS) control to implement the power tracking. The voltage-vector switching table of DPC is redesigned according to the error signals of active and reactive powers of the power winding, as well as the sector location of control winding flux. The active and reactive powers of the OW-BDFRG can be directly decoupled and independently controlled by properly selecting the switching voltage vectors. The novelty of this paper lies in an OW-BDFRG topology driven by dual two-level converters to improve the system characteristics, and the use of SMVS control to improve the DPC accuracy and robustness to parameter variations. Finally, the effectiveness of the proposed system is verified through simulation and experimental studies. [ABSTRACT FROM AUTHOR]

Compact flicker noise models used in SPICE circuit simulators are derived from the seminal BSIM unified noise model. In this paper, we show that use of this model can give anomalous bias dependence of input referred noise. In addition, we find that the state-of-the art flicker noise models are not adequate to capture the drain bias dependence of flicker noise in short channel devices. In this paper, we address both the issues with a new compact model. [ABSTRACT FROM AUTHOR]

This paper proposes an input voltage sensorless control algorithm for three-phase active boost rectifiers. Using this approach, the input ac-phase voltages can be accurately estimated from the fluctuations of other measured state variables and preceding switching state information from converter dynamics. Furthermore, the proposed control strategy reduces the input current harmonics of an ac–dc three-phase boost power factor correction (PFC) converter by injecting an additional common-mode duty ratio term to the feedback controllers’ outputs. This additional duty compensation term cancels the unwanted input harmonics, caused by the floating potential between ac source neutral and dc link negative, without requiring any access to the neutral point. A 6-kW (continuous power)/10-kW (peak power) three-phase boost PFC prototype using SiC-based semiconductor switching devices is designed and developed to validate the proposed control algorithm. The experimental results show that an input power factor of 0.999 with a conversion efficiency of 98.3%, total harmonic distortion as low as 4%, and a tightly regulated dc-link voltage with 1% ripple can be achieved. [ABSTRACT FROM PUBLISHER]

Multi-terminal High Voltage Direct Current (HVDC) using Voltage Source Converters (VSC-HVDC) is a promising technology which provides flexible control of active and reactive power and facilitates remote renewable energy integration, above all using long cables. This paper analyses an active power control strategy for multi-terminal VSC-HVDC systems tailored to enhance transient stability of hybrid AC/DC grids. The proposed strategy controls each VSC using frequency measurements of all terminals. Its performance is compared to a strategy in which each VSC is controlled using only local frequency measurements of the AC side, proving that the proposed strategy shows better performance, even taking into account reasonable communication delays. The paper also shows that the proposed strategy generally gives similar results to those obtained when each VSC is controlled using the speed of the centre of inertia (COI). The speed of the COI is a more comprehensive and richer figure than the one proposed in this paper but it is also much more complex to obtain. Simulation results with PSS/E of a test system have been used to illustrate the comparisons and the main contributions of the proposal. [ABSTRACT FROM PUBLISHER]

This paper summarizes simulation based technical feasibility and cost benefit analyses of a range of off-the-shelf electrical losses reduction intervention solutions in the Isle of Wight (IoW) 11 kV distribution network in the United Kingdom (UK), assessed over a period of 45 years. Based on presented results, the paper provides a brief commentary on the merits, drawbacks, risks and recommendations associated with each considered network intervention. Among considered network interventions, the transformer auto stop-start scheme, where one of the transformers among a pair at a primary substation would be switched depending on the substation's varying load, was found to be the optimal solution, i.e., in reducing technical losses significantly and may also have a reasonable Net Present Value (NPV) with a good rate of Return on Investment (RoI) when fully implemented. [ABSTRACT FROM PUBLISHER]

This paper presents voltage regulation and current tracking control strategies for three phase two-level grid-connected power converters. By using power-invariant Park’s transformation, an averaged mathematical model of power converters is obtained in dq synchronous reference frame. Then a novel control strategy using adaptive control and ${H}_{\infty }$ technique is proposed to regulate the dc-link output voltage as well as track a desired current reference for three-phase power rectifiers. More specifically, an efficient adaptive controller is established in the external loop for regulating dc-link output voltage in the presence of external disturbances. A set of ${H}_{\infty }$ controllers are designed in the internal loop to force the input currents track their desired values. Finally, simulation results obtained from the proposed control method are presented, analyzed, and compared with that of sliding mode control, and the superiority of the proposed control laws is verified. [ABSTRACT FROM AUTHOR]

One of the most critical faults affecting modular multilevel converter (MMC) based bipolar high-voltage direct-current (HVDC) transmission systems is the single-phase-to-ground (SPG) faults between the converter transformer and the valve. However, half-bridge (HB) and full-bridge (FB) based MMCs exhibit a different behavior following such a fault and, thus, converter protection should be addressed in a different manner for each configuration. For HB-MMCs, an SPG fault at the valve-side leads to a severe overvoltage on the submodule (SM) capacitors in the converter upper arms and to grid-side non-zero crossing currents. Although FB-MMCs only exhibit overvoltage, these are more severe than for their HB counterparts. To address this problem, this paper presents a protection strategy considering thyristor bypass branches placed in parallel with upper arms of FB-MMCs. By employing this configuration, the upper arm overvoltage in the faulted converter is mitigated and remote converters can be quickly blocked using their local protection schemes. For completeness, the effectiveness of the strategy is verified through time-domain simulations in PSCAD/ EMTDC. The studies in this paper demonstrate the effectiveness of the presented protection scheme for station internal faults occurring in FB-MMCs in bipolar HVDC systems. [ABSTRACT FROM AUTHOR]

Volt/VAr Optimization (VVO) is becoming increasingly crucial in distribution management systems with renewable sources that require setting their reactive power in coordination with legacy voltage and reactive power control devices. This paper presents a VVO method that operates in complex variables based on the Wirtinger calculus; it employs a penalty method that keeps voltage magnitudes and controller parameters within limits together with a probabilistic rounding technique for handling switched device variables. The complex variable implementation permits using the compensation technique when solving the Karush-Kuhn Tucker (KKT) system, thus achieving a significant speedup due to limiting the number of linear system factorizations. The proposed method is contrasted with a classical VVO implementation employing discrete coordinate search from the current operating point and an enhanced version that uses sensitivity information. Numerical results on distribution networks having up to 3147 nodes show that the proposed method is significantly faster than classical methods and gives operating points free of voltage magnitude violations and lower power loss. [ABSTRACT FROM AUTHOR]

The stability of power systems can largely limit the power transfer capability limit of transmission lines. The PV generator control is different from synchronous generator control. The impact of PV generator dynamics on the power transfer limit constrained by stability is not well explored in the literature. This paper focuses on improving the stability constrained power transfer capability limit of transmission lines emanating from PV generators. The PV generator is provided with dc link voltage and reactive power control and is equipped with low voltage ride through capability, voltage and frequency support functionalities. In this paper, Lyapunov function analysis based adaptive tuning laws are proposed for PV control parameters to improve the power transfer capability limit (constrained by stability) of transmission lines connecting PV generators to grid. The tuning laws are proposed for Phase Locked Loop (PLL), outer and inner control loop parameters. The proposed tuning laws have inherent robustness towards faults, changes in solar irradiation, changes in grid topology, network impedances, generation and loads in the system. The effectiveness of the proposed method is validated on a Single PV-Synchronous Machine system, modified IEEE-39 and IEEE-118 bus system. The performance of the proposed method is investigated considering various disturbances such as symmetrical and asymmetrical faults on transmission line, switching of a transmission line and change in solar irradiation of PV. Comparison with existing method shows that the proposed tuning method can achieve higher power transfer capability limit considering stability. [ABSTRACT FROM AUTHOR]

As the crucial interface of renewable energy integration, the grid-tied inverter is a complicated system with a multi-timescale feature, containing both fast dynamics and slow dynamics that determine the system stability. However, previous researches on stability analysis mainly focus on the slow dynamics without considering the impact of fast dynamics, which may bring to inaccurate stability prediction. In this paper, the impact of fast dynamics on stability is investigated for grid-tied inverter from a physical perspective. Considering the dynamics of the current control loop and network, a two degrees-of-freedom oscillator model is firstly established to extract the fast dynamics separately. The damping torque analysis is then applied to analyze the impact of the extracted fast dynamics on the damping and spring coefficients of slow dynamics. Moreover, based on the fast dynamic analysis, a novel reduced-order model is proposed with the consideration of the dominant characteristics of fast dynamics, which improves the model accuracy without increasing the complexity. Eventually, both numerical evaluations and experiments are performed to validate the proposed analysis method. [ABSTRACT FROM AUTHOR]

A phase-locked loop (PLL) is commonly used in grid-connected power converter for synchronization. A single input single output (SISO) linear model of PLL with the phase angle of the point of common coupling (PCC) voltage as input and the estimated phase angle as the output is generally used in the PLL analysis and design. However, an undesirable coupling between the magnitude and phase of the input voltage is present when a filtering stage is incorporated before the control loop, which a SISO model cannot capture, and could result in a significant disturbance in the PLL estimated phase during grid faults. This paper proposes a generalized multi-input multi-output (MIMO) linear model that captures the complete PLL dynamics during symmetrical faults for a PLL equipped with any prefilter. Furthermore, a compensation method that decouples the magnitude dynamics of the input voltage from the phase dynamics is proposed in this paper. The proposed compensation improves the PLL's phase tracking performance by ensuring that the prefiltered PLL acts only on the PCC voltage phase changes. A vital application of the proposed compensation method for PLL is during a grid fault case, wherein the converters are expected to contribute to the fault current, which requires an accurate measurement of the PCC voltage phase. The compensation method's effectiveness is demonstrated using power hardware in the loop simulation of a hardware voltage source converter interfaced to a distribution system simulated in real-time. [ABSTRACT FROM AUTHOR]

DC grid has become an important application in power transmission. However, the exist works only have the method to obtain the droop coefficient in a radial topology. Also, this droop coefficient is for V-I droop control. There is no such like method to obtain the droop coefficient for the P-V droop control. This paper proposed a new method for a complex dc grid to obtain the droop coefficients. Firstly, based on the converter control strategies, the types of converter were classified. It will help to find out which kinds of converter can be participated into the dc-side voltage control. Secondly, a virtual node was defined, and the matrix of the dc grid was reshaped to fit the new added virtual node by using the proposed simplification method. An example was shown how this method worked. After that a new method of calculating the P-V droop coefficient was proposed. Finally, a simulation model was investigated to verify if the proposed method can operate in a real system. The results showed that the simplification method can ensure the mathematical relationships of the dc system. And simulations show good performances of the droop coefficient calculation method. [ABSTRACT FROM AUTHOR]

This paper proposes a novel real-time adaptive under voltage load shedding (UVLS) method by using optimized fuzzy logic controllers to address fault-induced delayed voltage recovery (FIDVR) and short-term voltage instability (STVIS) problems. An intelligent multi-input multi-output fuzzy logic load shedding controller is developed based on a short-term voltage recovery criterion (STVRC). For the controller, the magnitude deviation and the recovery time deviation of voltage trajectory are extracted as inputs, while the load shedding amount at the current load shedding stage and the time delay of the next stage are taken as outputs. Particle swarm optimization (PSO) technique is further employed for the tuning of the controller parameters to realize the voltage recovery requirement at the lowest cost of load shedding. The proposed UVLS method is formed by a group of the fuzzy logic controllers deployed at the load buses, each monitoring its own bus voltage and shedding load in closed-loop. Responding to the severity of voltage variations, the proposed method is able to adaptively determine the location, time and amount of load shedding in real-time to make the voltage satisfy the STVRC. The effectiveness of the proposed method compared to other methods is testified by simulations on the Nordic test system. [ABSTRACT FROM AUTHOR]

The proliferation of distributed energy resources (DERs), located at the Distribution System Operator (DSO) level, bring new opportunities as well as new challenges to the operations within the grid, specifically, when it comes to the interaction with the Transmission System Operator (TSO). To enable interoperability, while ensuring higher flexibility and cost-efficiency, DSOs and the TSO need to be efficiently coordinated. Difficulties behind creating such TSO-DSO coordination include the combinatorial nature of the operational planning problem involved at the transmission level as well as the nonlinearity of AC power flow within both systems. These considerations significantly increase the complexity even under the deterministic setting. In this paper, a deterministic TSO-DSO operational planning coordination problem is considered and a novel decomposition and coordination approach is developed. Within the new method, the problem is decomposed into TSO and DSO subproblems, which are efficiently coordinated by updating Lagrangian multipliers. The nonlinearities at the TSO level caused by AC power flow constraints are resolved through a dynamic linearization while guaranteeing feasibility through “ $l_1-$ proximal” terms. Numerical results based on the coordination of the 118-bus TSO system with up to 32 DSO 34-bus systems indicate that the method efficiently overcomes the computational difficulties of the problem. [ABSTRACT FROM AUTHOR]

This paper develops an efficient multi-agent deep reinforcement learning algorithm for cooperative controls in powergrids. Specifically, we consider the decentralized inverter-based secondary voltage control problem in distributed generators (DGs), which is first formulated as a cooperative multi-agent reinforcement learning (MARL) problem. We then propose a novel on-policy MARL algorithm, PowerNet, in which each agent (DG) learns a control policy based on (sub-)global reward but local states and encoded communication messages from its neighbors. Motivated by the fact that a local control from one agent has limited impact on agents distant from it, we exploit a novel spatial discount factor to reduce the effect from remote agents, to expedite the training process and improve scalability. Furthermore, a differentiable, learning-based communication protocol is employed to foster the collaborations among neighboring agents. In addition, to mitigate the effects of system uncertainty and random noise introduced during on-policy learning, we utilize an action smoothing factor to stabilize the policy execution. To facilitate training and evaluation, we develop PGSim, an efficient, high-fidelity powergrid simulation platform. Experimental results in two microgrid setups show that the developed PowerNet outperforms the conventional model-based control method, as well as several state-of-the-art MARL algorithms. The decentralized learning scheme and high sample efficiency also make it viable to large-scale power grids. [ABSTRACT FROM AUTHOR]

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 paper presents the theoretical foundation and practical implementation aspects of Fractional Order Controllers (FOCs) for power system applications. With this aim, the paper provides a comprehensive mathematical background on the stability analysis of dynamic systems with inclusion of fractional order derivatives and discusses their software implementation based on the Oustaloup's Recursive Approximation (ORA) method. Then the paper illustrates a variety of examples of ORA-based FOCs, namely, automatic generation control of synchronous machines; frequency control of a converter-interfaced energy storage system; and voltage control through a static synchronous compensator. The WSCC 9-bus test system and a realistic 1,479-bus model of the Irish transmission system are employed to test and compare the examined FOCs with their integer-order versions. [ABSTRACT FROM AUTHOR]

This paper proposes a secondary Model Predictive Control (MPC) based architecture to provide DC voltage and DC power control to MT VSC-HVDC networks interfacing wind power generation. The proposed architecture places the controller at a supervisory level, thus providing coordination amongst existing converters’ local droop controllers. Droop control is a type of decentralized DC voltage/power control. Hence, in case of secondary controller failure, droop control acts as a contingency control scheme. A simplified linear dynamic model of the MT VSC-HVDC network is utilised for the MPC's design, thus minimizing computational effort needed to compute secondary control action. Updates of droop gains and offshore and onshore “AC” variables, in the dq domain, are explicitly considered as measured input disturbances within the formulation of the MPC controller, thus, ensuring appropriate control response in order to minimise adverse impact of their variation on the overall system's performance, particularly under wind power variations. Simulation results show that MPC, whose design is based on the system's simplified linear model, is capable of delivering satisfactory performance when applied to the high fidelity non-linear full order model of a six-terminal VSC-HVDC network simulated in PSCAD. [ABSTRACT FROM AUTHOR]

This paper depicts an accident that happened in 2010 in a Brazilian chemical plant. A combination of different events, installations, and commissioning errors coupled with equipment failures involving the primary substation resulted in catastrophic failure. A single-phase failure of a 138-kV disconnect switch, during closing, initiated events and phenomena that resulted in a complete plant shutdown. This paper will walk the reader through the events, forensics analysis, and power system analysis that allowed the authors to identify what happened, how it happened, and what to do to prevent it from happening again. This paper will show how the anatomy of the problem, involving equipment failure, single-phase arcing, switching transients, overvoltage, capacitive coupling, transferred surge voltages, ungrounded system, transient overvoltage due to an arcing ground fault on an ungrounded system, insulation coordination, flashover, and lack of adequate surge protection, all of which resulted in a catastrophic failure of an arc resistant switchgear and a potentially serious arc flash incident. The research followed some steps that will be commented on here as well as the lessons learned. [ABSTRACT FROM AUTHOR]

The ever growing penetration of distributed generation (DG) in a distribution network has a profound impact on network protection and stability. Traditional protection schemes and algorithms need to be extensively investigated as more and more DGs get introduced into the network. The current version of IEEE Standard 1547 does not present a comprehensive solution for fault current detection in the presence of various kinds of DGs. Power electronic inverter-based DGs (IBDGs) are of special concern in distribution network protection as they are often incapable of providing sufficient fault current and their controllers play a principal role in the DG behavior. In this paper, the effects of voltage and current controllers for IBDGs on industrial and commercial power system protection schemes are investigated. It is shown that the inverter control mode has a direct impact on its fault current levels and duration. A simplified distribution network model with IBDG operating under voltage and current control modes was tested to verify the effects of these controllers. This paper also proposes an adaptive relaying algorithm to detect the faults in the presence of IBDGs with various types of controllers. [ABSTRACT FROM AUTHOR]

This is the second part of a two-part paper on a new adaptive coordinated voltage control (ACVC) strategy. The overall basic scheme and the online control for prepared faults have been presented in Part I. In this paper, learning control is explored to accumulate knowledge, so that the online control performances can be improved for unknown emergencies. For unknown situations where no past experiences can be exploited, a learning scheme is used by which the control knowledge can be acquired gradually. The learnt knowledge goes into the database and is improved the next time a related situation happens. After full knowledge is acquired, the unknown fault becomes a prepared fault with prepared knowledge. With the learning scheme, control for any emergency can be realized in a rapid and effective response. The learning process is demonstrated by providing control for an unknown emergency in the New England 39-bus power system. The ACVC performance for a sequence of randomly generated fault and load event emergencies is presented at the end of this paper. [ABSTRACT FROM PUBLISHER]

The unbalanced nature of distribution systems requires a multiphase load-flow solution capable of handling arbitrary network topologies and providing accurate results. The need for detailed analysis of secondary grid systems found in dense urban areas and the modeling of distribution networks including the subtransmission level requires using highly efficient and large-scale system-capable methods. In this paper, three different load-flow solution algorithms are presented using the modified-augmented-nodal-analysis formulation. The load-flow solution algorithms are compared for the IEEE 8500-node distribution test feeder using a proposed regulator tap control strategy. [ABSTRACT FROM PUBLISHER]

Modern electronic systems are performing more and more safety-critical tasks, whereby a robust communication is crucial. To protect those communication channels, error detection and correction codes (EDCCs) are widely used. The very construction of an EDCC has an impact on the resiliency against electromagnetic interference. Within this paper, the effectiveness of several EDCCs is assessed and compared. It is found that the effectiveness depends on the ratio between the channel's bit rate and the frequency of the incoming disturbance. Specific focus is being given to incorrect data perceived as correct: false negatives. Each of the considered EDCCs has an increased vulnerability for false negatives at bit rates that have a specific ratio to the disturbance frequency. The main cause behind this is the repetitive or alternating bit patterns within the codes themselves. Finally, foundations are laid to develop codes that are as robust as possible against harsh electromagnetic disturbances. [ABSTRACT FROM AUTHOR]

Based on the knowledge accumulated off-line and feature extractions, a novel data-based learning and control method is proposed for the long-term voltage stability problem in this paper. All the spatial-temporal data is considered and the features of different emergency events are extracted by principle component analysis which can reduce the dimension and reveal the significant internal structure of the data. An artificial neural network is used to build a classifier to reinforce the relationship directly between the system dynamics and optimal control actions. With the prepared control knowledge, it is faster to find an optimal control action online with a good system performance. Simulation results on the 6-bus system, New England 39-bus system and Iceland 189-bus system are given to show the potential of this method for on-line control. [ABSTRACT FROM AUTHOR]

Voltage imbalances are one of the most severe challenges in electrical networks, which negatively affect their loads and other connected equipment. This paper proposes a voltage support control strategy to mitigate the voltage imbalance in inverter-based low voltage distribution networks. The control scheme is derived taking in mind the following control objectives: a) to increase the positive sequence voltage as much as possible, b) to decrease the negative sequence voltage as much as possible, c) to inject the power generated by the primary source, and d) to minimize the output current of the inverter. The innovative contribution of the proposed solution is based on the design of a control algorithm that meets the aforementioned objectives without resorting to communications with other grid components. The theoretical results are experimentally validated by selected tests on a laboratory setup with X/R ratio close to one. [ABSTRACT FROM AUTHOR]

This paper presents a novel distributed voltage control method for multi-terminal direct current (MTDC) systems based on the average consensus algorithm. The proposed method combines the idea of secondary integral voltage control and regulator synchronization problem. It realizes adjustable tradeoff between two conflicting control objectives, that is, voltage regulation and power sharing. The proposed control algorithm only needs a low-bandwidth communication between two adjacent voltage source converters (VSCs) and local information to work effectively. Moreover, regarding the system dynamic response, a parameter tuning method for the proposed controller is provided to achieve the fastest convergence of the distributed control algorithm. This paper also develops a small-signal dynamic model for VSC-MTDC systems with distributed controllers to evaluate the system dynamic performance. The proposed method is validated by time-domain simulation of a 4-terminal VSC-MTDC system. The simulation results show that flexible tradeoff between voltage regulation and power sharing can be achieved under necessary constraints on the DC bus voltages and power sharing ratio. [ABSTRACT FROM AUTHOR]

This paper emphasizes the importance of including unbalanced distribution systems for voltage stability studies in power systems. The paper aims to: discuss the various simulation methods for power system analysis; highlight the need for modeling unbalanced distribution system for accurate load margin assessment; demonstrate the influence of net-load unbalance (NLU) on voltage stability margin (VSM). We also share a T&D co-simulation interface with commercial power system solvers. The distribution system is evolving rapidly with high proliferation of distributed energy resources (DERs); these are not guaranteed to proliferate in a balanced manner and uncertainty resulting due to these DERs is well acknowledged. These uncertainties cannot be captured or visualized without representing the distribution system in detail along with the transmission system. We show the impact of proliferation of DERs in various 3-phase proportions on voltage stability margin through T&D co-simulation. We also study the impact of volt/VAR control on voltage stability margin. This analysis is only possible by representing the distribution system in detail through T&D co-simulation. Higher percentage of net-load unbalance (NLU) in distribution system aggravates the voltage stability margin of the distribution system, which can further negatively influence the overall voltage stability margin of the system. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive study on current loop auto-tuning for self-commissioning of sensorless interior permanent magnet synchronous machine (IPMSM) drives. Well-tuned current controllers are essential for both high-performance operation and self-commissioning procedure. However, current controller design parameters, i.e., $L_{d}$ and $L_{q}$ , are not known at the beginning of the self-commissioning procedure where their estimation also has unique challenges. For example, an initial rotor position to determine the actual d- and q-axes is not known. Also, feedback controllers cannot be used in the estimation because they are not tuned yet. As a solution, this paper proposes a method to estimate the spatial inductance map by spatially scanning the motor via sinusoidal voltage injection in a controlled manner. Then it uses the estimated inductance values to tune the current controllers. The proposed method identifies $L_{d}$ and $L_{q}$ values using open-loop voltage injection without needing actual d- and q-axis positions. The whole procedure takes around a second. Practical considerations such as an automatic selection of the injection voltage, and digital control and dead-time effects are carefully addressed. The findings are experimentally verified on a 3-phase IPMSM drive. Furthermore, its applicability to different motor types is also demonstrated. [ABSTRACT FROM AUTHOR]

The possibility of operating distributed generation (DG) in islanded mode brings several benefits but still faces some problems, especially related to the transition from a grid-connected to an islanded operating condition. When connected to the main grid, DG units operate in PQ mode. During this transition, their controls must be changed to regulate frequency and voltage. The delay for islanding detection, associated with abrupt changes in the control references, may result in large and detrimental transients. This paper proposes a new control structure for a distributed synchronous generator (DSG) to allow a smooth transition from the grid-connected to the islanded operation mode. The proposed control structure uses a compensation loop that was conceived based on the main ideas of the anti-windup bumpless transfer (AWBT) theory (although it is not, strictly speaking, an AWBT controller). The results show that the proposed structure ensures smaller excursions of the DSG variables during the transition and, therefore, the possibility of a successful formation of the islanded system is extended to a wider range of operating conditions. Moreover, with the proposed control, the success of this transition does not depend on the islanding detection time, which is the main contribution of this paper. [ABSTRACT FROM AUTHOR]

Series-connected-based offshore wind farms can eliminate bulky central offshore converters, transformers, and platforms. In a series-connected-based offshore wind farm, the dc voltage of the converters should be changed within a large range to control the dc-link voltage (grid side) and maximize wind power acquisition (generator side). In this paper, the full-bridge-based modular multilevel converter (FB-MMC) is proposed as grid- and generator-side converters. The dc voltage of the FB-MMC can vary in a large range because the full-bridge submodule (FBSM) is used. The control method of the FB-MMC in a series-connected-based offshore wind farm is different from those of traditional ones. The dc current control and capacitor voltage balance control are proposed in this paper, with which the FB-MMC can operate normally in the series-connected offshore wind farm. Moreover, the capacitor ripple voltage in the generator-side FB-MMC is studied. The result shows that the capacitor in the FBSM can be designed in rated power and frequency, and it will be feasible when the power and frequency decrease. Simulation and experiment results verify the feasibility of the proposed configuration and the corresponding control algorithm. [ABSTRACT FROM AUTHOR]

Symmetrical ac-side current control is widely used for grid-connected modular multilevel converters (MMCs) under grid fault conditions. However, it is revealed that this control method can lead to unbalanced arm currents and consequently asymmetrical electrical stresses and nonuniform temperature distributions. This paper aims to analyze the mechanism of the unbalanced arm currents and propose an effective method to solve this problem. First, the electrical quantities inside MMCs are analyzed under unbalanced grid fault conditions. The analytical expressions of the unbalanced arm currents are derived considering different fault severities and operating conditions to reveal the “side-effects” of the symmetrical ac-side current control. After that, a zero-sequence ac voltage injection method is proposed. The injected voltage is directly derived from negative-sequence ac current controller and ac-side current with pure mathematical manipulations, which can be easily obtained without extra controllers. The effect of the injected voltage on the original modulation index is very limited according to an accurate quantitative analyzation. Moreover, the proposed method does not affect the already achieved symmetrical ac-side current control and the circulating current suppression. The effectiveness and performance of the proposed analysis and control method are verified by both simulation and experimental results. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) can be configured to perform ac/ac conversion, which makes them suitable as railway power supplies. In this paper, a hierarchical control scheme for ac/ac MMCs for railway power supplies is devised and evaluated, considering the requirements and the operating conditions specific to this application. Furthermore, admittance models of the ac/ac MMC are developed, showing how the suggested hierarchical control scheme affects the three-phase and the single-phase side admittances of the converter. These models allow for analyzing the stability of the interconnected system using the impedance-based stability criterion and the passivity-based stability assessment. Finally, the findings presented in this paper are validated experimentally, using a down-scaled MMC. [ABSTRACT FROM AUTHOR]

This paper proposes an insulated two-stage ac–dc converter to be applied in unidirectional power flow systems. The two-stage structure is the combination of a Hexverter and a series-type 12 pulse rectifier. The key features of the proposed structure are inherited from the modular multilevel converter concept such as modularity, scalability, and expandability. Besides the topology, this article presents a complete control strategy based on the output (dc) voltage regulation and Hexverter's stored energy. In particular, an unusual method to regulate the Hexverter's branch energy unbalances is proposed. All currents in the Hexverter are controlled which increases the robustness for operation in harsh environments such as subsea systems. Mathematical analysis of the control loops is carried out to develop the necessary control-oriented models for each control loop. The converter and the control strategy are verified through experiments in a 24 cells Hexverter and a current-fed series-type 12-pulse rectifier. [ABSTRACT FROM AUTHOR]

Demanding higher power, higher power quality, lower switching loss, and eliminating interface transformers in industrial applications caused significant attention toward multilevel converters. This paper presents current control of a seven-level topology for medium-voltage high-power applications. This topology has fewer active switches and components, and less control complexity in comparison to the other existing classic and advanced seven-level topologies. A comprehensive review has been done on seven-level topologies to demonstrate the advantages of this topology in this paper. A control method based on model predictive control (MPC) is developed to balance the flying capacitors of this topology at their desired values and control the output currents without the need of pulsewidth modulation blocks, and proportional-integral (PI) controllers, which is the advantage of MPC in power electronics applications. In order to evaluate the performance of the seven-level topology and the developed control method effectiveness, experimental results are shown for different conditions, which demonstrate the feasibility of the seven-level topology and the developed control technique. [ABSTRACT FROM AUTHOR]

This paper proposes a new control scheme to eliminate the 3rd harmonic in the output currents of grid-following inverters under unbalanced grid conditions. Unbalanced grids adversely affect the performance of grid-following inverters due to the oscillations appearing on the DC-link voltage with a frequency twice the line frequency. The paper is based on instantaneous active reactive control (IARC) technique due to its advantages over other existing methods. However, the presence of severe asymmetrical 3rd harmonic distortions in the inverter output currents is the main challenge with IARC method, which impairs the power quality requirements. This paper enhances the IARC scheme by proposing a 3rd harmonic elimination approach by adopting a current reference generation method using the symmetric sequence components concept. Furthermore, the proposed scheme complies with the grid code requirements by injecting the requested reactive current. Finally, the proposed approach is evaluated by theoretical and simulation analysis and validated experimentally using a hardware setup. [ABSTRACT FROM AUTHOR]

With the proliferation of distributed generation in distribution networks, quickly and smoothly converting to a state of regional autonomy in the case of distribution network failure is one of the main challenges for the stable operation of a multimicrogrid (MMG). To address the aforementioned problem, this paper proposes coordinated control of the passive transition from grid-connected to islanded operation for a three/single-phase hybrid MMG. Under the premise of fully considering the constraints on the tie-line transfer power undertaken by the master power supply (MPS), the following two scenarios regarding safety and emergency are formulated: 1) when the output power of the MPS is in the safe range, the proposed strategy coordinates the energy storage (ES) in each submicrogrid (SMG) to achieve power balance; 2) when the output power is in the emergency range, this paper proposes a three-phase balance method based on a bubble sort to realize the combination optimization of the single-phase source-load-storage (SLS) and a fast power regulation method based on implicit enumeration for the SLS to reduce the tie-line transfer power, which could prevent failure of the mode transition from power overload. The test results verify the effectiveness and feasibility of the proposed coordination strategy. [ABSTRACT FROM AUTHOR]

The main technical challenge of the medium-voltage motor drives with modular multilevel converter (MMC) is the serious low-frequency fluctuation on the capacitor voltages, and the existing suppression methods inevitably inject high-frequency circulating currents in MMC. However, the output currents are evidently distorted by the large injected components, while little research has been done to study and solve this problem. This paper analyzes and proves the essential interactions between the output current distortion and the injected components, and the specific dominant output harmonic groups are derived and summarized, revealing the nature of the problem, and it can be taken as a favorable reference for the controller designing. Based on the derivations, this paper proposes a novel suppression method for the greatly improved harmonic characteristics of the output currents during the low-frequency operation. The output harmonics can be accurately measured by the proposed control and effectively suppressed for much lower current total harmonic distortion, benefiting for the superior output control performances with improved efficiency. The simulation results on PSCAD/EMTDC and experimental verifications on a 2.2 kW MMC-based motor drive prototype prove the validity of the presented harmonic analysis and suppression method. [ABSTRACT FROM AUTHOR]

With the increasing penetration of the wind power, the stability issues of the weak AC grid-connected doubly fed induction generator (DFIG)-based wind turbines during low-voltage ride through (LVRT) cannot be neglected. In order to explore the instability mechanism of DFIG system during weak grid fault, the small signal state-space model is established in this paper. The results of the modal analysis show that the dominant unstable poles are mainly impacted by the phase-locked loop (PLL), rotor current control loop, and terminal voltage during the fault, where the PLL is the dominant factor. Furthermore, the impact of each factor on the system dynamic stability is comprehensively evaluated, which indicates that the controller bandwidth under normal grid condition is no longer applicable to the fault condition due to the interaction between the controller and grid. Then, the optimal current proportion which can significantly improve the system stability is deduced. Finally, based on the analysis, this paper proposes the improved LVRT control schemes from two aspects of either injecting active current or decreasing PLL bandwidth to enhance the small signal stability of the system. The effectiveness of the proposed LVRT control strategies is validated by the simulation and experiments. [ABSTRACT FROM AUTHOR]

In this paper, we propose a distributed voltage control in power distribution networks through reactive power compensation. The proposed control can operate in a distributed fashion where each bus makes its decision based on local voltage measurements and communication with neighboring buses, always satisfy the reactive power capacity constraint, drive the voltage magnitude into an acceptable range, and minimize an operational cost. We also perform various numerical case studies to demonstrate the effectiveness and robustness of the controller using the nonlinear power flow model. [ABSTRACT FROM AUTHOR]

Compared with the conventional two-level dual-active-bridge (DAB) dc–dc converter, the three-level DAB converter could operate with lower voltage rated switching devices and provide more control degrees of freedom, which enhances the control flexibility. This paper proposes a modulation strategy with five control degrees of freedom. In the previous work with multiple control degrees, the optimization strategies for efficiency or inductor current minimization are investigated; however, these strategies all need detailed mode analysis and complex mathematical optimization tools to obtain the optimal duty cycles. In this paper, an optimization strategy for the root-mean-square value of the inductor current is proposed. This method extracts the fundamental and third-harmonic components from the high-frequency voltages on both sides and adjusts the amplitude of the bridge voltages by the vector diagram to obtain the optimal duty cycles. The inductor current can be reduced significantly at light loads with the proposed strategy, so the efficiency of the converter can be improved. Moreover, the optimization process does not need the detailed operation analysis of the converter, nor does it require the complex mathematical optimization tools. Finally, the performance of the three-level DAB and the optimization strategy are verified by the experimental results. [ABSTRACT FROM AUTHOR]

This paper improves the position sensorless drive by sensing actual machine phase voltages. A high-bandwidth phase voltage measurement is developed for pulsewidth modulation (PWM) voltage inverters. On the basis, the actual phase voltage is obtained based on the digital integration of PWM voltage using the capture modulator in existing drive microcontrollers (MCU's). Comparing to existing phase voltage measurement, no separated A/D converter and communication hardware are required because PWM pulses are directly measured using MCUs. However, for standard machines without neutral points, only line-to-line ac PWM voltages can be measured for the phase voltage reconstruction. Since the capture based on transistor-transistor logic (TTL) logics receives only digital signals, a preprocess circuit to convert ac PWM line voltages to equivalent digital signals is proposed. This paper clearly explains the voltage sensing hardware using MCU capture modulator. According to experimental results, a 150-MHz sampling rate for phase voltage measurement is achieved based on the proposed capture-based voltage measurement. Although the physical limitation of back electromotive force estimation still appears, the proposed phase voltage measurement substantially enhances the sensorless drive performance at low speed. [ABSTRACT FROM AUTHOR]

This paper deals with an ac–ac voltage regulator/compensator based on a hybrid transformer with a matrix converter realizing electrical coupling in the hybrid transformer unit. The parameters of electrical energy, such as voltage amplitude, are very important, particularly from the viewpoint of the final consumer and sensitive loads connected to the grid. Furthermore, a hybrid transformer using a matrix converter can be used for power-flow control and for transient stability improvement in ac transmission systems. Active power flow and voltage magnitude in transmission lines can vary by using a conventional mechanical or thyristor-controlled regulator. However, the control parameters can be executed only in steps depending on the taps available in the electromagnetic transformer. An approach for obtaining continuous control of the voltage magnitude and phase shift using a conventional transformer with two windings and a pulsewidth-modulated matrix converter is presented in this paper. The operating principles and steady-state properties are discussed. Detailed computer simulation results are presented in this paper. Furthermore, experimental test results obtained from a three-phase 3 kVA laboratory model are also presented. [ABSTRACT FROM AUTHOR]

In order to strengthen the onshore transmission network in many parts of the world, voltage-source current–high-voltage direct current (VSC–HVDC) will increasingly be utilized. The effect of the operating point of a VSC-HVDC link and the control strategies employed can substantially affect the electromechanical oscillatory behavior of the ac network as well as the dc-side dynamics. In order for the full, flexible capability of VSC-HVDC to be exploited, further study of the effects of these controllers and their interactions with ac system responses is necessary. This paper addresses this gap. Both modal analysis and transient stability analysis are used to highlight tradeoffs between candidate VSC-HVDC power controllers and to study the electromechanical performance of the integrated ac/dc model. Tests are carried out on both a generic two-area model and a large-scale realistic network with detailed ac generator and HVDC models. [ABSTRACT FROM PUBLISHER]

Distribution system state estimation (DSSE) tools are required to estimate the real operating conditions of power distribution networks. The accuracy of the quantities estimated through DSSE plays a key role in the effectiveness of the management and control functions. This paper presents a comprehensive mathematical analysis aimed at highlighting the most important factors affecting the accuracy of the voltage profile obtained by means of weighted least squares estimators. The proposed analysis shows the way in which the uncertainty components of the voltage estimations depend on the measurement system available on the field. In particular, this paper considers synchrophasor measurements provided by phasor measurement units and hybrid measurement systems also presenting conventional devices. Tests performed on a 95-bus network prove the validity of the theoretical analysis and show the importance of the results also from a meter placement perspective, emphasizing the requirements needed to achieve specific accuracy targets. [ABSTRACT FROM AUTHOR]

Overall electric energy conservation and improvement in electrical energy efficiency has been a topic of interest to the electric power industry for long time, since 40% of total primary energy used is in the form of electricity. There are many facets to the energy conservation and/or energy efficiency techniques. Energy conservation by voltage reduction popularly called “CVR” has recently gained some momentum, in particular, to reduce the peak demand. This is also an integral part of the discussion in the future smarter grid technology. This paper addresses the pros and cons of the CVR application including a thorough literature search, practical applications, and a quantifiable measure of its actual amount of energy saving from utilities and the cost associated with it. It is expected that this paper will serve as a foundation to all practicing engineers addressing the issue of CVR. [ABSTRACT FROM AUTHOR]