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In the conventional sliding mode control, the four-leg distribution static compensator (DSTATCOM) currents are controlled based on a current error in each phase. However, the four currents in a four-wire system can not be independently controlled variables and hence one of the four controllers is redundant in conventional schemes. Further, the current dynamics of a DSTATCOM converter are coupled through converter pole voltages in the natural reference frame. This leads to cross-coupling in the sliding variable equations with respect to four manipulated input variables. Considering the above points, in this paper, the current due to Thevenin’s equivalent load neutral-point voltage is considered as a fourth independent controlled variable and the corresponding system dynamic equations are presented. To get a decoupled feature, a new sliding surface function is structured. The performance of a DSTATCOM with the proposed control scheme under various operating conditions is validated through a detailed simulation study and experimental results, obtained from a laboratory prototype of four-leg DSTATCOM. [ABSTRACT FROM AUTHOR]

This paper investigates the Multilevel Current Source Converter (MCSC) STATCOM. It proposes a new method of modulation. The new modulation method produces less THD and di/dt at the terminals of the MCSC STATCOM when compared to the widely used Phase-Shifted Carrier SPWM (PSC-SPWM). To ensure the inductors' currents remain balanced, a Current Balancing Algorithm (CBA) is presented. The CBA does not require any controls. It relies on assigning optimum switching states to each CSC module in real-time. In addition, two control strategies are presented in this paper. The first one assumes the STATCOM to be a SISO. Therefore, it only requires one PI controller. However, the gains for the controller must be changed if the STATCOM was to transition from capacitive mode to inductive mode, or vice versa. The second control strategy overcomes this problem by implementing a TITO integral state feedback controller. The contributions of this paper are validated via simulation and experimental results. [ABSTRACT FROM AUTHOR]

This paper proposes an adaptive model of modular multilevel converter (MMC) for electromagnetic transient (EMT) simulations. The model is applicable to MMCs with arbitrary numbers of half-bridge and full-bridge submodules. The proposed design includes average value model, arm equivalent model, and detailed equivalent model. It allows smoothly transitioning from one model to another during time-domain simulations depending on the desired accuracy and execution time constraints. Modifications required in conventional MMC models to achieve smooth transitions are presented in the paper. Time-domain initialization methods are developed for each constituting model, including initialization of the appropriate control system blocks. Validity and effectiveness of the proposed adaptive model is demonstrated using EMT simulations of 401-level MMC-HVDC system. [ABSTRACT FROM AUTHOR]

The effect of reduction in the terminal voltage of a grid-tied inverter together with minimization in total harmonic distortion (THD) of terminal voltage and current, in the presence of dead-band in the switching remains always challenging for the researchers. With high switching frequency, this issue increases by many folds. The grid-tied inverter used for harmonic compensation acts as a shunt active power filter (SAPF). The implementation of the dead-band elimination technique is based on output current polarity detection, suffers from multiple zero-crossing due to the generation of high frequency switching harmonics during SAPF operation. This paper deals with a dead band elimination methodology together with harmonic compensation using SAPF under non-linear and unbalanced loading conditions. Here, SAPF uses a dual fundamental component extraction (DFCE) scheme with modified complex coefficient filter (MCCF) to extract fundamental components for reference current generation to implement an indirect current control PWM scheme. MCCF extracts fundamental components even in the presence of dc offset and unbalancing at the input. For incorporating dead-band elimination in control, exact polarity detection is done separately through SAPF output current estimation using sensed load current, and modified CCF computed reference source current. Since the estimated SAPF output current contains no high-frequency switching harmonics, the dead-band elimination technique can be correctly implemented. To synchronize the zero-crossing of the estimated and actual SAPF current, an adaptive delay control is used. The proposed MCCF based control logic’s performance together with the dead-band elimination technique is validated for SAPF operating under non-linear, unbalance, and dynamic load conditions. The proposed dead-band elimination scheme based SAPF model is developed and simulated using MATLAB/ SIMULINK. The simulation results show a significant reduction in source current harmonics due to the elimination of odd harmonic injection into the system by incorporating dead-band elimination in SAPF operation. The real-time simulation of the proposed method is also validated using OPAL-RT OP4510 real-time platform. [ABSTRACT FROM AUTHOR]

This paper introduces an averaging control strategy of the phase-unit losses of modular multilevel converters (MMCs). The quantitative analysis of the MMC phase-unit loss distribution under unbalanced ac conditions is carried out by investigating the individual component of the MMC arm currents. It shows that the unbalanced arm currents will result in unbalanced phase-unit losses. To address this issue, this paper proposes a predictive closed-loop-control-based zero-sequence voltage injection method that results in much averaged loss distribution under unbalanced ac conditions. The essence of the new method is that it adjusts the three-phase sinusoidal references so that the unbalanced arm currents are fully utilized to average phase-unit losses. This paper shows the effectiveness of the new approach by electromagnetic transient simulations on PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

A parallel hybrid modular multilevel converter (PHMMC) belongs to the class of modular multilevel converters, which have become potential candidates for high-voltage direct-current (HVDC) transmission systems. Due to the circuit topology of a PHMMC, the dc bus voltage contains low-order harmonics and cannot be fully regulated at a constant dc voltage. The dc bus voltage, if not properly controlled, leads to improper power transfer and increases the magnitude of dc current ripple on the dc transmission line. This paper proposes a zero-sequence voltage injection (ZSVI)-based model predictive control (MPC) strategy to control the dc current/power flow and simultaneously minimize the dc current ripple. The proposed strategy takes advantage of a cost function minimization technique to determine and inject the optimal zero-sequence voltage components into the dc- bus voltage of a PHMMC system. This paper derives a discrete-time dynamic model of the dc transmission-line current and, correspondingly, develops a predictive model. The predictive model is used to inject the appropriate amount of zero-sequence voltage components to the dc bus reference voltage waveform. Compared with the existing triplen harmonics injection method, the proposed ZSVI-MPC strategy improves the performance of a PHMMC system in terms of minimization of the dc current/voltage ripple. Performance of the proposed strategy for a 21-level PHMMC-based HVDC station system is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment. The reported results demonstrate superior performance of the PHMMC-HVDC station operating based on the proposed ZSVI-MPC strategy, under various operating conditions, as opposed to the existing triplen harmonics injection method. [ABSTRACT FROM AUTHOR]

This paper investigates the applicability of averaged-value models (AVMs) for modular multilevel converters (MMCs) operating in a voltage-sourced converter-based-high-voltage dc (VSC-HVDC) grid. The AVM models are benchmarked by comparison with a detailed electromagnetic transient model of the grid, including a fully detailed MMC model. Analysis results show that the AVM is only effective as long as the capacitors are large enough to maintain nearly constant voltage across each MMC submodule. This paper also shows that previously developed MMC averaged models are not able to accurately simulate the transients under dc fault conditions. This paper introduces topology changes to a previously proposed averaged model that results in much improved simulation for such conditions. This paper also shows that the model can be used effectively to study HVDC grids with significant time savings. [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]

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]

Several methods have been proposed and investigated so far on mitigation of very fast transient overvoltages (VFTO) in gas-insulated switchgear (GIS). The state-of-the-art methods are primarily based on dissipation of the energy associated with electromagnetic waves that the VFTO originate from and are composed of. Present paper reports on an alternative concept of VFTO mitigation based on the principle of controlling voltage conditions preceding voltage breakdown in SF6 gas that leads to VFTO generation. The paper introduces different control algorithms and shows how the algorithms can limit VFTO maximum value and total number of voltage breakdowns during operation of the GIS disconnector. The concept is applied for mitigation of VFTO in ultra-high voltage GIS. As the study case, an 1100 kV test set-up is used as recently reported for Wuhan (China) GIS station, with the disconnector characteristics obtained from 1100 kV development tests. [ABSTRACT FROM PUBLISHER]

Operational performance in the low-voltage distribution network (LVDN) can be undermined by its inherent unbalances, which may become worse as the penetration of rooftop solar continuously increases. To address this issue, load balancing via phase-reconfiguration devices (PRDs), which can change phase positions of residential customers as required, provides a cost-efficient option. However, most reported approaches to control PRDs require that demands of all residential customers are available, which are not viable for many LVDNs without smart meters or advanced metering infrastructure (AMI) installed. To bridging the gap in this field, this paper proposes a novel method to control PRDs purely based on measurable data from PRDs, and its controller. Based on limited information, sensitivity analysis in the network with PRDs is studied, followed by the optimization model that comprehensively considers operational requirements in the network. Moreover, slack variables are introduced to the model, and penalized in the objective function to assure either a strategy that is secure or with minimized violations can always be provided. The model is a challenging mixed-integer non-convex programming (MINCP) problem, which is reformulated as an efficient solvable mixed-integer second-order cone programming (MISOCP) based on exact reformulations or accurate linear approximations. Simulations based on two modified IEEE systems, and a real system in Australia demonstrate that an efficient strategy can be provided to mitigate unbalances in the network. [ABSTRACT FROM AUTHOR]

This paper proposes a model predictive control (MPC) method for modular-multilevel-converter (MMC) high-voltage direct current (HVDC). To control the MMC-HVDC system properly, the ac current, circulating current, and submodule (SM) capacitor voltage are taken into consideration. The existing MPC methods for the MMC-HVDC system utilize weighting factors to configure the cost function in combinations of the SM capacitor voltage balancing algorithm, ac current control, and circulating current control. Because all combinations of the switch states are considered in order to minimize the cost function, their possible combinations increase geometrically according to the increase of the level of the MMC, which is a significant disadvantage. This paper proposes a new MPC method with a reduced number of states for ac current control, circulating current control, and the SM capacitor voltage-balancing algorithm. The proposed cost functions are divided into three types according to their control purposes. Each cost function determines the minimum number of states for controlling the ac current, circulating current, and SM capacitor voltage. The efficacy of the proposed controlling method is verified through simulation results using PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

This paper proposes a startup sequence for a modular multilevel converter (MMC) HVDC-connected offshore wind farm (OWF) and demonstrates how it can be simulated using an electromagnetic transient-type tool. The objective is to analyze inrush currents of transformers and cables (HVAC and HVDC). Modeling accuracies and computational performances are examined using three existing modeling methods for the MMCs. [ABSTRACT FROM PUBLISHER]

With the application of ultra-high-voltage (UHV) technology, how to reduce the construction cost of the UHV substations is becoming an important and emergent issue. In this paper, two kinds of one-column-varistor arresters are used to replace conventional four-column-varistor arresters for busbars or transformers or both of them. The switching overvoltage and lightning intruding overvoltage are calculated to evaluate the feasibility of using one-column-varistor arresters in UHV substations. At the same time, the energy absorbed by and current flowing through the arresters, and the thermal stability of these one-column-varistor arresters are analyzed to ensure their safe operations. It is shown that conventional one-column-varistor arresters can be used for busbars and transformers in UHV substations to replace conventional four-column-varistor arresters and could highly reduce the construction cost of the UHV substation, but the safety of the substation will remain the same. [ABSTRACT FROM PUBLISHER]

The capacitor condition monitoring is an important issue for the reliable operation of modular multilevel converters (MMCs) in high-voltage applications. The numerous capacitors make the monitoring for the capacitor condition complex and computation-consuming. This paper proposes a hierarchic capacitor condition monitoring strategy to assure the reliability of high-voltage MMCs. In the strategy, the switching times of the switching signals are counted to detect the condition of the capacitor in each submodule (SM). Then, the subsequent capacitance calculation method is only conducted to the abnormal capacitors indicated by the above condition detection algorithm. Meanwhile, the variations of the measured and reference capacitor voltages are employed to calculate the capacitance in the proposed method. The condition detection algorithm shrinks the scope where the abnormal capacitors locate, which avoids calculating the total capacitances in MMCs. In the proposed capacitance calculation method, the complex operations are further saved. Both the two measures reduce the computation burden effectively. The implementation of the whole monitoring strategy is simple and fit for high-voltage MMCs. Moreover, the strategy has no adverse influence on the normal operation of MMCs. The effectiveness of the proposed strategy is verified by the simulation studies with the professional tool Matlab/Simulink. [ABSTRACT FROM AUTHOR]

This paper proposes a new modulated model predictive control method for the control of modular multilevel converters (MMCs) in voltage source converter-high voltage dc systems. The proposed method retains the advantages of the conventional finite control set-model predictive control methods by programing the nonlinear properties of the MMC into the design calculations while minimizing the ac line and circulating current ripples and steady-state error by generating modulated switching signals with a fixed switching frequency. In this paper, first, the predictive modeling of the MMC is provided. Next, the proposed control method is described. Then, the application of the proposed method to an MMC system is detailed. Finally, experimental results from an MMC system connected to a three-phase grid are provided to validate the theoretical outcomes. [ABSTRACT FROM AUTHOR]

Different from typical circuit breakers, breakers used to switch AC filters in convertor stations have to operate frequently according to the real-time operation mode and daily load changes. Therefore, the performance of filter breakers decreases very quickly, which leads to frequent accidents. Both preinsertion resistors and controlled switching devices can suppress switching transients. In this paper, a comparative analysis of whether a controlled switching device and preinsertion resistor should be adopted is carried out using PSCAD. The transient is calculated as the combined result of the switching angle and preinsertion resistor value, and the optimal configurations are proposed. After determining the resistor value and whether a controlled switching device should be included, the reference voltage of the arrester should be reevaluated. [ABSTRACT FROM AUTHOR]

This paper presents a combined control scheme that consists of two complementary control strategies, namely hierarchical control and autonomous control to handle contingencies in a multi-terminal high voltage direct current (MTDC) system. The proposed hierarchical control considers three optimal targets, i.e., equal or proportional power sharing, minimizations of transmission loss and total DC voltage variation. The primary control layer implements the droop control for voltage source converters (VSCs). While in the secondary control layer, voltage reference setpoints of the VSCs in droop control scheme are calculated by the proposed optimal power flow method with the equal or proportional power sharing as equality constraints. As the hierarchical control relies on communication system, the autonomous control for power sharing can be activated during communication system malfunctioning or rapid power variation caused by renewable energy sources. The combined control strategy can greatly increase the reliability of the MTDC system. The simulation results of a five-terminal MTDC network conducted in the MATLAB/Simulink software verify the performance of the proposed combined control strategy. [ABSTRACT FROM AUTHOR]

High penetration level of single-phase solar PVs in low voltage distribution systems has raised concern about the Voltage Unbalance (VU) level. Real-time phase reconfiguration using Dynamic Switching Devices (DSD) has been proposed as a cost-effective solution to mitigate the excessive level of Voltage Unbalance. DSDs are fast power electronics devices that could change the phase connection of a solar PV with minimum disturbances. Most of the proposed methodologies for controlling DSDs are based on a central controller and the assumption of availability of Advanced Metering Infrastructure (AMI) and a communication system. In this paper, we propose a distributed control methodology to control DSDs based on the measurement from the local node only and using the sensitivity of nodal voltage to power injection. Each DSD decides on phase swapping if it rectifies the unbalance index of the local node. We show that decreasing the Voltage Unbalance Factor (VUF) of individual nodes will improve the unbalance situation of the whole feeder. The proposed scheme does not require a communication system nor a central controller. The result of simulations confirms the efficacy of the proposed methodology. [ABSTRACT FROM AUTHOR]

VSC-HVDC systems are being increasingly employed in the power systems. The recently installed HVDC systems have a power capacity similar to traditional power plants. Hence, they are expected to have a similar behaviour as traditional synchronous generators during faults in AC grid, within their limits of course. Recent grid codes require HVDC converter stations to incorporate fault ride-through (FRT) capabilities in order to avoid HVDC converter station disconnection from AC grid for certain fault characteristics. In this paper, two FRT mechanisms are suggested for the two converter stations of an HVDC system. One FRT mechanism is added to the DC voltage control loop of the master converter station, while the other FRT mechanism is added to the active power control loop of the slave converter station. The objective is to ensure the stable operation of the HVDC system during faults that may occur in AC grids located on both sides of the HVDC system. The performance and stability of the suggested FRT mechanisms are tested considering the pre-fault power flow direction and all possible types of balanced and unbalanced faults. Simulation results confirm the effectiveness of the FRT mechanisms and revealed the critical modes during FRT operation. [ABSTRACT FROM AUTHOR]

Traditionally voltage control in distribution power system (DPS) is performed through voltage regulating devices (VRDs) including on load tap changers (OLTCs), step voltage regulators (SVRs), and switched capacitor banks (SCBs). The recent IEEE 1547-2018 from March 2018 requires inverter fed distributed energy resources (DERs) to contribute reactive power to support the grid voltage. To accommodate VAR from DERs, well-organized control algorithm is required to use in this mode to avoid grid oscillations and unintended switching operations of VRDs. This paper presents two voltage control strategies (i) static voltage control considering voltage-reactive power mode (IEEE 1547-2018), (ii) dynamic and extensive voltage control with maximum utilization of DER capacity and system stability. Further, effective time-graded control is implemented between VRDs and DER units to reduce the simultaneous and negative operation. The proposed voltage control strategies are tested in a realistic 140-bus southern California distribution power system through extensive time-domain simulation studies. The results show that voltage quality in a distribution system is effectively achieved through the proposed voltage control strategies with a significantly reduction in the number of switching operations of VRDs. In addition, proposed voltage control strategies increase reliability and security of the DPS during unexpected failures. [ABSTRACT FROM AUTHOR]

The high voltage direct current transmission system based on modular multilevel converter (MMC-HVDC) technology is now studied and used widely. But traditional half bridge topology of sub module (HBSM) of MMC can't block dc link fault current because of the freewheeling effect of diodes. In order to solve this problem, firstly, this paper improves HBSM topology, a series connected double sub module (SDSM) is designed and dc link fault current blocking capability is realized without tripping ac circuit breaker, so MMC-HVDC with this improved topology can immediately rebuild dc link voltage and resume power transmission. Secondly, the self starting and fault disposing procedure are also designed. At last, simulations in the software PSCAD/EMTDC are carried out to verify its effectiveness and feasibility. [ABSTRACT FROM AUTHOR]

Active front-end (AFE) rectifiers are becoming widely used in medium-to-high-power adjustable speed drives (ASDs) to achieve regenerative operation and meet the energy efficiency and harmonic requirements. The typical control methods used with AFE rectifiers include voltage-oriented control (VOC), direct power control (DPC) and virtual-flux-based methods. This paper presents a dynamic average-value model (AVM) of the AFE rectifier system which is based on the voltage-source converter (VSC) operated with DPC. The developed AVM and the modeling methodology presented in this paper highlight the specifics of the hysteresis bang-bang type DPC that is used to track the real and reactive power references. The proposed AVM of the AFE rectifier system is demonstrated to significantly reduce the computing effort while accurately capturing the fast transient performance of the system. [ABSTRACT FROM AUTHOR]

This paper proposes a transformerless dc–dc converter, which features a large voltage ratio, for medium-voltage dc (MVDC) applications. The proposed converter combines a boost converter with a series-parallel resonant converter which enables soft switching of the power switches and diodes. Further, the power switches and diodes of the proposed converter experience lower voltage stresses compared to the other topologies. The proposed converter is modular and, therefore, permits series and/or parallel connection of multiple units for the desired voltage and/or power ratings. This paper discusses the principles of operation, analysis, and design of the proposed converter. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed converter. [ABSTRACT FROM AUTHOR]

Current-transformer (CT) saturation causes severe distortion in the measured current waveform which may lead to maloperation of the protective devices. This paper proposes a low-cost, power-electronic device to prevent the CT from saturation. The proposed compensator is inserted in series with the relay in the CT secondary circuit and acts as a controlled voltage source (CVS). The proposed CVS generates a time-varying voltage to cancel the voltage developed across the CT burden; therefore, the CT magnetic flux remains almost constant and undistorted during the power system transients. It will be shown that this device can precisely compensate fault current, inrush current, and other probable transient currents despite its simplicity. The proposed device can be employed to compensate the already in-service CTs connected to nondigital and digital relays. Comprehensive computer simulations are carried out to validate the effectiveness of the proposed compensator. The performance of a sample compensator of this type is validated through special high-current laboratory experiments (carried out over several hundred amperes up to 1.6 kA), and the obtained results illustrate the capability of the proposed compensator to prevent CT saturation. [ABSTRACT FROM PUBLISHER]

The no-load curve of the dielectric recovery strength of the ultra-fast mechanical vacuum switch (MVS) changing with time is the theoretical foundation to optimize the turn-off strategy of the hybrid direct current circuit breaker with load commutation switch. To avoid the shortcomings of the classic synthetic circuit, a novel test circuit is proposed to measure the no-load dielectric recovery curve of the MVS in this paper. In the test circuit, a succession of pulse voltages is automatically applied on the MVS, and multiple breakdowns of the MVS occur in one test. Therefore, the dielectric strength of the MVS at multiple time points is obtained, and the envelope of the applied voltage on the MVS during one test can describe the dielectric recovery curve of the MVS; as a result, the test workload could be reduced dramatically and the measurement is more authentic. By changing test conditions, including the rate of rise of the test voltage and the operating speed of the MVS, no-load dielectric recovery characteristics are studied. The research has shown that the dielectric recovery speed of the MVS is irrelative with the rate of rise of the test voltage and it is directly proportional to the opening speed of the MVS. [ABSTRACT FROM AUTHOR]

This paper presents an accurate steady-state initialization method for electromagnetic transient (EMT) models of modular multilevel converters (MMCs). The proposed method uses steady-state arm voltages and currents to initialize internal electrical variables and control systems of the converter, including capacitor voltage and gating signals of each sub-module. In addition to typical voltage source converter upper level control, MMC specific controls are also considered, including circulating current control, dc ripple control, and capacitor voltage balancing algorithm. EMT simulations on 101-level and 401-level MMC-HVDC systems confirm the accuracy of the proposed method. [ABSTRACT FROM AUTHOR]

This paper presents a new concept and approach for topology control and switching in distribution systems by extending the traditional circuit switching to laterals and single-phase loads. Voltage unbalance and other key performance indicators including voltage magnitudes, line loading, and energy losses are used to characterize and demonstrate the technical value of optimizing system topology on a per-phase basis in response to feeder conditions. The near-optimal per-phase topology control is defined as a series of hierarchical optimization problems. The proposed approach is applied to IEEE 13-bus and 123-bus test systems for demonstration, which included the impact of integrating electric vehicles (EVs) in the test circuit. It is concluded that the proposed approach can be effectively leveraged to improve voltage profiles with electric vehicles, the extent of which depends on the performance of the base case without EVs. [ABSTRACT FROM AUTHOR]

In a line-commutated converter based high-voltage direct-current (LCC-HVDC) system, the inverter control system usually includes constant extinction angle (CEA) controller, constant current (CC) controller, current error controller (CEC), and voltage-dependent current order limiter (VDCOL). However, the interaction influence of multiple controllers may cause repetitive commutation failure (CF). For this purpose, this paper deeply investigates the mechanism and the influencing factors of repetitive CF caused by controller interaction. It is found that the lower limit value of CEA controller can affect the control switching process and thus the probability of repetitive CF. Besides, the minimum gamma measurement (MGM) unit prevents the system from acquiring the real extinction angle in time, which results in the DC current fluctuation during the recovery process. Moreover, the probability of control-induced repetitive CF is much higher when the HVDC inverter is connected to a weak AC system or the AC fault is not serious but not cleared in time. Based on the above analyses, an improved control method is proposed for recovery performance improvement, which is mainly realized by modifying the lower and upper limits of the CEA controller during recovery. Finally, the simulation and experimental results verify the theoretical analyses and the proposed control method. [ABSTRACT FROM AUTHOR]

The hybrid multilevel voltage-source converter, combining semiconductor switches and power module cells, is a new family for VSC-HVDC applications. The hybrid multilevel converter in parallel arrangement (PHMC), in terms of converter investment, is very adaptable for cable transmission systems. However, the dc voltage of the converter, synthesized by the output voltage of three-phase cell stacks, is strongly coupled with ac voltage variations, which influences the stability of converter operation. This paper proposes a dc voltage compensation strategy for the parallel hybrid multilevel converter. This strategy, implemented by introducing an isolation period, can effectively cope with the dc voltage deviation, especially under ac network faults. Simulation results in PSCAD/EMTDC verify the effectiveness of the proposed strategy under various circumstances. [ABSTRACT FROM AUTHOR]

The three-phase series-connected modular multilevel converter (SC-MMC) is presented as one option for high-voltage direct current (HVDC) tap to reduce the cost of an HVDC station. Compared with conventional three-phase parallel-connected MMC, the number of semiconductor switches in SC-MMC is reduced by one-third to withstand the same dc voltage. First, the paper studies the operating principle of SC-MMC, and then presents an interspacing phase-shifted pulsewidth-modulated method which could improve the waveform of the output ac voltage for a given switching frequency. Next, the control strategy based on the identical real power input of each phase is proposed to balance the dc voltage of each phase under an unbalanced grid condition. The current limit of switching devices is also taken into consideration to protect SC-MMC from overcurrent. Furthermore, a universal zero-sequence current controller is developed as one part of the control strategy. In addition, an innovative method to suppress the third harmonics of ac current is presented. With the third harmonic suppression strategy, the SC-MMC can operate with much smaller SM capacitors without deteriorating the ac current, which contributes to the cost reduction further. Finally, simulation results obtained in PSCAD/EMTDC are provided to validate the proposed control strategies. [ABSTRACT FROM PUBLISHER]

The modular multilevel converter-based unified power flow controller (MMC-UPFC) is able to operate under unbalanced grid conditions with symmetric component decoupling. However, the constraint of the voltage limit of UPFC is not considered and no protection schemes are investigated to protect the UPFC from overmodulation under unbalanced grid conditions. To solve this problem, this paper proposes the cascaded control scheme for MMC-UPFC based on voltage limit control and symmetric component decoupling to balance the ac current of the transmission line. With appropriate transformer connections for MMC-UPFC, the negative- and zero-sequence currents are suppressed by the corresponding inner current loops. Considering the voltage limit of MMC, the operating ranges of UPFC-MMC under balanced and unbalanced grid conditions are investigated in both mathematical derivation and point scanning methods. The results are depicted and analyzed in 3-D vision. Based on the analysis of operating regions, the voltage limit control is proposed to protect the MMC from overmodulation and to maximize the controllable region under unbalanced grid conditions. Finally, the final cascaded control structure is constructed. The simulation results obtained in PSCAD/EMTDC are provided to validate the effectiveness and robust performance of the proposed control strategies. [ABSTRACT FROM AUTHOR]

With the development of the MMC-HVDC, the lightweight requirements of the converter valves have become increasingly urgent. The reduction of submodule (SM) capacitance is difficult because the ripple of capacitor voltage cannot be too large, which is one of the key factors that affect the weight and the volume of the MMC. On the basis of the fundamental frequency and double fundamental frequency components are the critical factors for voltage ripples, this paper proposes an arm inductor switching control (AISC) strategy to reduce the voltage ripples of SM capacitor by controlling the inductance value of the bridge arm. The proposed strategy and the existing circulating current suppression controller (CCSC) are researched in terms of mathematical principles. A two terminal DC transmission model is built in PSCAD/EMTDC simulation environment, then the capacitor voltage fluctuation curves of non-capacitance-reducing control, CCSC, and proposed strategy with the same parameters are simulated. Afterwards, an experiment prototype is built to verify the simulation results. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

Recently, a modular multilevel converter (MMC) has become one of the most popular converters in medium/high-power applications. In this paper, a new technique is presented to decrease the computational time of the MMC by using switching state grouping for the predictive controller. The proposed technique has the ability to control the output current, minimize the circulating currents, and balance all capacitor voltages, simultaneously. For a predefined number of MMC submodules, the number available switching states is known. The proposed technique uses two stages of predictive modules. In the first stage, the switching states of the MMC are divided equally into a number of groups, controlled simultaneously by the same number of predictive modules. The optimal number of the switching state groups in the first stage is obtained. In the second stage, only one predictive module is used to obtain the optimal switching state of the MMC without any other complexities or additional sorting controllers. The effectiveness of the proposed control technique has been verified by using MATLAB/SIMULINK software considering steady state and dynamic analysis. The results of the proposed technique are compared with the conventional one in terms of computation time, verifying better performance of the proposed method. [ABSTRACT FROM PUBLISHER]

This paper describes observation of lightning overvoltages in a 500 kV switching station at Kansai Electric Power Co., Japan. Three types of lightning overvoltages, such as back flashover, direct lightning hit to a phase conductor, and lightning-induced voltage have been observed. The lightning overvoltages due to a back flashover were lower than operation voltage of a surge arrester. The observation suggests that induced voltage caused by electromagnetic fields radiated from a lightning channel affects lightning overvoltages in a station. [ABSTRACT FROM AUTHOR]

The emerging voltage source converters (VSCs) such as hybrid cascaded multilevel converters (HCMCs) are promising converter technologies which enable higher converter efficiency, compactness, and DC fault resilience for high voltage direct current (HVDC) transmission. However, the large number of switching events due to the converters’ submodules introduces a heavy computational burden for electromagnetic transient (EMT) simulations. Efficient and accurate EMT-type converter models play a vital role in the control and design of HVDC converters. Several computationally efficient models for the HCMCs are proposed in this paper, including the detailed equivalent model (DEM) and several average value models (AVMs) of varying accuracy. Additionally, a combined model is proposed which allows the users to switch among the DEM and AVMs during dynamic simulations. The proposed models are validated against the DEM for dynamic transients. The simulation results demonstrate good modeling accuracy of the proposed models. The AVMs are also shown to significantly improve simulation efficiency compared to the DEM, which is very desirable especially for HCMCs with a large number of submodules. [ABSTRACT FROM AUTHOR]

Modular multilevel converter based high voltage direct current (MMC-HVDC) transmission is the prospective technology for large-scale wind farms integration. In this paper, a comprehensive coordinated control strategy for DC fault ride-through (FRT) of wind farms connected to the grid through bipolar MMC-HVDC transmission is proposed. To reduce the requirement of breaking speed and capacity of DC breakers, an active current-limiting control method is proposed to limit the rise rate of DC fault current, and the principle of parameter selection is designed. After DC breakers isolate fault, the control mode switching methods of bipolar MMC are designed to make the fault pole MMC operate as a STATCOM to support the AC grid and improve the active power transmission capability of the healthy pole MMC, which can reduce the impact range of unbalanced power between wind farms and converter station. In addition, a coordinated control strategy for wind farms load-shedding considering speed constraints is designed to realize the optimal distribution of load-shedding power among wind turbines, which can regulate unbalanced power together with the healthy pole MMC to realize DC FRT. Finally, the effectiveness and feasibility of the proposed control methods are verified by simulation results. [ABSTRACT FROM AUTHOR]

This paper presents the essential reason for subsequent commutation failures (SCFs) in high voltage direct current (HVDC) systems, and accordingly proposes a novel adaptive extinction angle recovery (AEAR) control strategy to prevent SCFs. Firstly, based on the changes of the inverter control mode after the first CF, the interaction process of inverter control loops is divided into 3 stages and analyzed in detail. It is found that the control aim of DC current is ambiguous in stage 2, causing the output of current error control (CEC) to persist above zero. In stage 3, however, the CEC output drops rapidly, leading to an uncontrolled status of extinction angle and bringing the risk of SCFs. Furthermore, the precondition for the control mode switching is derived. On the basis of that, an AEAR control strategy is proposed. By adopting an adaptive extinction angle reference, the proposed scheme maintains the inverter at the constant extinction angle (CEA) mode after the first CF. Thus, the uncontrolled status of extinction angle can be avoided. The problem of SCFs due to improper inverter control interactions is also able to be fixed at the root. Simulation results show the validity of the analysis and the superiority of the proposed control strategy in preventing SCFs. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC or M2C) is an emerging attractive multilevel topology for medium-voltage high-power applications. The capacitor voltage balancing control and the improvement of output current rating are two challenging issues of MMC. This paper proposed a new parallel-connected diode-clamped M2C (DCM2C), in which each arm consists of two parallel-connected clusters. In this topology, the capacitor voltages can be balanced automatically without any balancing control algorithms. Theoretically, no voltage sensors are needed to measure the capacitor voltages theoretically. Thus, the control of the converter can be simplified greatly. With a simple current-sharing control, an equal current distribution can be achieved between the two clusters. Furthermore, an extended parallel-DCM2C is proposed for larger current applications, and the corresponding current-sharing control method is developed. Experimental results validated the voltage self-balancing capability of parallel-DCM2C and the effectiveness of the proposed current-sharing control. [ABSTRACT FROM PUBLISHER]

The large number of switching elements in the modular multilevel converter (MMC) is a challenging problem when modeling the MMC-HVDC systems for the computation of electromagnetic transients. The modeling complexity increases even further when a multiterminal (MT) MMC-HVDC system is used to integrate offshore wind farms (OWFs) with power-electronics-based wind energy converters, such as doubly fed induction generators (DFIGs). This paper compares modeling accuracy and computational performances for various combinations of MMC and OWF models. Onshore and offshore ac fault simulations are performed for an OWF system composed of DFIG-type wind turbines and connected to a practical ac grid through an MT MMC-HVDC system. The OWF system model includes the detailed representation of the offshore collector grid and the associated overcurrent protection. The offshore MMC controls include an offshore fault current limiter and fast OWF power generation reduction-based fault-ride-through function. [ABSTRACT FROM PUBLISHER]

This paper proposes a new fault ride-through (FRT) configuration for a fixed-speed induction generator-based–wind turbine (FSIG–WT). It utilizes a nine-switch converter (NSC) to enable shunt and series compensation in response to the system dynamics and grid faults. The steady-state control strategy and transient-management scheme are developed to ensure proper performance in steady-state, dynamic operation as well as enhanced FRT capability of the FSIG–WT. In addition to that, the developed control strategy tackles the balanced and unbalanced operation of the electric grid with an enhanced positive- and negative-sequence compensation control scheme. The recently reported grid synchronization scheme, namely, the frequency-locked loop (FLL) is implemented to achieve fast phase detection and transient response. Consequently, the performance of the FLL has been compared with the multiple reference–frame–phase-locked loop (MRF–PLL) to demonstrate its superior performance for enhancing transient response. The proposed topology, FLL, and control strategies are verified through a comprehensive simulation study to demonstrate the enhancement of the transient response and FRT capability of FSIG-WT in adherence to grid-code requirements. [ABSTRACT FROM PUBLISHER]

A special stress for circuit breaker is the occurrence of evolving fault or parallel switching condition. These switching cases are not standardized by international standards, but may occur in service and may create problems for the breaker. In the past, it was a problem mainly with oil circuit breakers and some types of air-blast circuit breakers but lesser known with SF6 circuit breakers. In the meantime, with upcoming new switching technologies along with changed operating duties, some doubts exist that these switching cases may become an issue again. This paper explains in detail the background of these stresses and presents a method to evaluate the capability of SF6 circuit breakers for such situations. The theoretical evaluation involves network calculations, gas flow simulations, and dynamic dielectric analysis. It is important to note here that several tests have been done in the past for evolving and parallel switching faults for both puffer as well as self-blast circuit breakers with the knowledge available at that time. These test results are used to validate the outcome of the theoretical evaluation which will help in better prediction of the circuit breaker behavior before performing actual tests. It can be concluded that theoretical simulation may, in majority of the cases, serve as sufficient proof to show feasibility for such cases. [ABSTRACT FROM AUTHOR]

The modular multilevel converter is the state-of-the-art topology for voltage source converter HVDC. Despite its advantages, this converter handles large internal low-frequency energy ripples, and the capacitance that supports these dynamics is a key design parameter that affects the operating region of the converter. Different strategies can be found in the literature to increase the feasible region of operation of the converter. Nevertheless, they are typically open loop in nature and use precalculated control references. This paper presents an alternative based on model predictive control that steers the system through optimal control trajectories that are calculated online. This provides feedback and corrective control action in real time. The predictive controller used for this purpose is presented and a linear time-varying approximation is used to reduce the computational burden of the algorithm. The feasible boundaries of the converter are sought and the final performance of the control algorithm is evaluated through detailed simulations using a switching model of the converter. [ABSTRACT FROM AUTHOR]

This paper aims to present a passive island detection technique for synchronous distributed generation (DG). The technique is based on frequency oscillation estimation in order to distinguish the islanding from other events that may occur in distribution systems. The island detection uses a small window to estimate the oscillation frequency, obtaining faster responses than the existing methods which use larger windows to estimate the damping and frequency of oscillation. The algorithm performance has been tested considering different generation and load scenarios, including short circuits, load, and capacitor switching, DG outage, and islanding. The technique is reliable since it does not trip for a nonislanding event; the island detection time is less than 40 ms and its nondetection zone is less than 1.6% of the DG nominal power. The proposed method has been compared with one of the most common algorithms used in practice—the rate of change of frequency. The results show that the algorithm based on frequency oscillation detection performs better than the ROCOF, and its mathematical simplicity is adequate for practical relay implementation. [ABSTRACT FROM PUBLISHER]

The standard alternative-arm converters (AAC) with the sweet spot or short overlap operation have faced a challenge of balancing submodule (SM) capacitor voltages, where this issue has not been paid much attention so far. In this paper, a new control strategy for the SM-capacitor voltage balancing of the AAC applied in high-voltage dc transmission systems is proposed. The suggested control technique is composed of three regulators, where a control for the average voltage of all SM capacitors, the voltage balancing among the converter arms, and the voltage balancing of the individual SM capacitor are performed. Based on the operation principle of the standard AAC, which activates only three of six converter arms simultaneously, the energy balancing of the three activated arms is performed every one sixth of the fundamental cycle by the converter common offset voltage. Band-stop filters with the central frequencies at the fundamental, second-, and third-order components of the line frequency are applied to obtain the average values of the SM capacitor voltages for the feedback controllers. The simple proportional and integral controllers are employed for this scheme and the voltages of SM capacitors are mostly balanced during the steady states and kept closely to the rating with overshoot of less than 7% under the transient conditions. The proposed control method is verified by simulation results for a 33-level AAC under both normal and fault conditions. [ABSTRACT FROM AUTHOR]

Future multi-terminal dc networks are envisioned for the large-scale integration of renewable energy sources into today's power systems. An essential aspect for the reliable operation of these systems is the fast and selective dc-side fault handling and separation of faulted lines. The objective of this paper is the development and evaluation of a multi-terminal dc protection strategy based on fault-blocking converters (e.g., full-bridge based modular multi-level converters) and high-speed switches. Both a suitable fault control and a new high-speed switch design for a fast separation of faulted dc lines are elaborated. The novel concept is intended to reduce the requirements on the high-speed switches compared to dc circuit breakers and to ensure a fast restoration of the active power transmission capability, as well as the dc voltage. The fault-clearing strategy is analyzed using electromagnetic transient (EMT) simulations under variation of the fault location, type, and resistance. The investigations are carried out in a cable-based multi-terminal HVdc system with full-bridge modular multi-level converters in symmetrical monopole configuration. [ABSTRACT FROM AUTHOR]

This paper presents an efficient hybrid approach for volt/var control in distribution systems, with switching limits on taps and shunts. Since the problem is nondeterministic polynomial hard and combinatorial with huge solution space, metaheuristic techniques, despite being computationally expensive, have been used. The proposed approach combines the strengths of a gradient technique and a metaheuristic technique. In the first phase, the primal-dual interior point method (PDIPM) treats the discrete variables as continuous. These are then discretized using PDIPM-based sensitivities. These two solutions provide heuristic bounds on controls which are used in the second phase by a metaheuristic algorithm. The bounds aid in limiting the number of controls' combinations. Results for two systems have been obtained which ascertain the potential of the proposed approach over the conventional metaheuristic technique. These also indicate approximately 30-35 times increase in speed with the proposed algorithm compared to the conventional one. [ABSTRACT FROM AUTHOR]

In addition to supplying electrical power to the transmission grid, nuclear generating stations rely on the grid to provide power into the facility to support critical equipment during upsets. Although backup power supplies exist, such as diesel generators, the offsite power circuits are considered to be the “preferred power supplies” and must be kept at a high state of readiness. During certain operating conditions, the nuclear unit's generator is utilized to help support the voltage of the nearby high-voltage transmission system. If the offsite power circuits are served from the same high-voltage system, events that would result in tripping of the generator could also cause a depression of voltage on the offsite circuits due to loss of this voltage support. Nuclear operators must ensure that post-trip voltage levels are adequate to support critical equipment, such as emergency pumps and valves, during extreme postulated events, such as the rupture of reactor coolant system piping. Nuclear engineers utilize analytical models of their generating station electrical distribution system to determine the switchyard voltage level needed during various scenarios, but usually rely on outside organizations, such as transmission system operators and planners, to predict the change in switchyard voltage as a result of a nuclear generator trip. These organizations utilize complex models of the transmission system to make this determination. This paper proposes a simplified approach to predicting post-trip switchyard voltage utilizing data that do not rely heavily on outside organizations. [ABSTRACT FROM AUTHOR]

The current-carrying capability of dc lines is limited by their thermal and electric stress limits. Thus, the line current must be maintained within the permissible operational region to protect the lines from damage. In a dense dc grid, control over each line current cannot be achieved without including additional control devices. In this paper, a dual H-bridge current flow controller (2B-CFC) is used to manage the dc grid line power flow by providing dc voltage compensation in series with dc lines. A centralized hierarchical control system is proposed to coordinate the operation between multiple CFCs. A novel voltage-sharing control scheme is demonstrated. It is shown that such a scheme reduces the workload on a single CFC by sharing the required control voltage between multiple CFCs, and, in addition, can be used to avoid control conflicts among active CFCs during communication failure. An experimental platform consisting of a three-terminal dc grid and small-scale 2B-CFC prototypes has been developed to validate the concepts. For completeness, the CFC performance has been analyzed for overload conditions and when no communication exists. Small-scale dc circuit breakers have been developed to study the CFC performance under a pole-to-pole fault. [ABSTRACT FROM PUBLISHER]

Voltage source converter (VSC) based high voltage direct current (HVdc) transmission draws more attention in recent years for the grid connection of large-scale offshore wind farms (WF). This paper proposes nine switch converter (NSC) based configuration and control strategy for the VSC-HVdc connected offshore WFs to enhance the fault ride through operation of the system. The configuration allows having both shunt and series interfaces to the onshore grid, which allows the isolation of the faulty part of the network, continuous power delivery to the healthy portion, and significant reduction of the fault current. Reduced switch count NSC is utilized at the onshore station to provide simultaneous shunt and series compensation to the electric grid. The proposed control strategy ensures full power evacuation and the dc-link voltage regulation during network disturbances. The comparative analysis between the proposed and conventional VSC-HVdc systems carried out in MATLAB/ Simulink environment revealed the former's enhanced transient performance. [ABSTRACT FROM PUBLISHER]