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This paper presents a novel hierarchical cooperative control strategy to solve the problems of unbalanced State of Charge (SoC), unreasonable load current sharing, and unstable bus voltage for battery storage system (BSS) in islanded DC microgrid. In the communication layer, a neighbor-to-neighbor communication network architecture is constructed in which each battery storage unit (BSU) only communicates information with neighboring nodes, and a multi-agent consensus algorithm is used to iteratively estimate the relevant average global variables. In the main control layer, the droop coefficient is dynamically adjusted by constructing a fast SoC convergence function to ensure that the load current is reasonably distributed based on the rated capacity of each BSU. In the secondary control layer, a virtual voltage drop equalization controller and an average bus voltage compensation controller are designed to eliminate the impact of line impedance mismatch and compensate the average bus voltage. Furthermore, an optimized factor is designed to accelerate the later stage of SoC convergence. Finally, a DC microgrid experimental platform based on StarSim HIL is built, and the results demonstrate that the proposed strategy can achieve the control objectives of fast SoC balance, proportional load current sharing, and bus voltage stability. [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]

The method used to calculate insertion indexes plays an important role in determining the overall performance of the modular multilevel converter. Direct voltage control, which is the simplest option, results in a large circulating current ripple because this modulation technique does not account for the arm voltage ripples. This led to the development of compensated modulation techniques that compensate for the arm voltage ripples thereby eliminating the circulating current ripple. There are two variants of compensated modulation—closed loop and open loop. The closed-loop version requires measurement of the arm voltages without distortion and delay, which is difficult to achieve in practice. The open-loop method overcomes this challenge by using estimated arm voltages. However, accurate knowledge of the system parameters is needed for effective removal of the circulating current ripples. This is a limitation because the parameters change with time and operating conditions. This paper presents a modified version of the open-loop method, which includes a scheme for correcting parameter errors online. The method estimates the parameters, arm capacitances, and time delay, by using feedback controllers acting on the circulating current ripples. Mathematical derivation of the method, together with its validation using simulation and experimental tests, is presented in detail. [ABSTRACT FROM AUTHOR]

This paper presents a novel grid synchronization method with bumpless start that requires minimal computational load and can selectively track the positive sequence of the grid voltage in an unbalanced and distorted three-phase weak grid. Only 12 floating-point operations are required to obtain the in-phase and quadrature components that define a synchronous frame which tracks the positive sequence of the grid voltage. Contrary to a phase-locked loop (PLL), the presented scheme does not require to measure any ac voltages. Therefore, this sensorless method is particularly suited for weak-grid conditions, in which the voltage at the point of common coupling may contain significant noise and can experience larger deviations from the grid voltage (compared to a strong grid) due to the voltage drop in the weak grid impedance. The error in the estimated phase depends on the accuracy of the plant model. If the LCL filter parameters are known, then both the proposal and a PLL-based scheme result in the same steady-state error. Experimental results show the advantages of the proposal compared to a moving average filter PLL. [ABSTRACT FROM AUTHOR]

This paper proposes solutions to the problem of voltage sags caused by distributed generators anti-islanding protection. This is a recently identified power-quality (PQ) concern resulting from the increasing penetration of distributed generators into distribution networks. This problem has been previously investigated by the authors and it was revealed that it can seriously affect PQ indices as well as distributed generators reconnection procedures. However, no solutions have been formerly proposed or investigated. In this context, this paper investigates several potential solutions to be adopted by the utility and/or by the DG owners and presents a thorough evaluation of their main advantages and disadvantages. Besides this investigation, simple step-by-step procedures for assessing the effectiveness of each solution are described. The presented studies and procedures will guide the utility engineers in decision-making process associated with the choice of reasonable mitigation techniques to this problem. [ABSTRACT FROM PUBLISHER]

This paper describes the application of GUM supplement 1 and supplement 2 in the intercomparison procedure of a group of four dc voltage reference standards. The results obtained are presented and according to these, the choice of a restriction is suggested to improve the uncertainty and avoiding the strong correlation of the estimated values. [ABSTRACT FROM AUTHOR]

The output voltage of power electronic converters is a very important quantity for dynamic control of power electronic systems. In electrical drives without electromechanical position or speed sensor, the terminal reference voltage is used to obtain the rotor position. Dead-time effects and semiconductor voltage drop lead to distortion in the actual output voltage and degrade the control performance when the back electromotive force magnitude is low. Thus, for stable low-speed operation, output voltage sensing becomes necessary. Due to the switching nature of power electronic systems, this is not a trivial task, especially when instantaneous measurement of the terminal voltage is required. In this paper, an instantaneous switching-period average voltage sensing technique is proposed that utilizes a combined approach of oversampling and filtering. Based on the theoretical analysis of the sampling- and filter-induced measurement distortion, a general solution for an optimal filter design is derived. The additional sensing circuit is integrated into the low-side gate driver of the converter outputs. This paper includes details about the hardware implementation and extensive verification measurements. [ABSTRACT FROM AUTHOR]

This paper summarizes an instability monitoring and protection method based on local identification of global voltage instability (LIVES) and applies it to an IEEE test system for voltage stability and security assessment. The identification method (LIVES-alarm) is tested with both detailed and Quasi-Steady-State simulation tools, and two protection schemes, based respectively on reverse Load Tap Changer operation (LIVES-restore) and firm load shedding, are evaluated and compared using the unstable scenario provided with the test system. The effect of load sensitivity to voltage is also investigated. [ABSTRACT FROM PUBLISHER]

In this paper, a cascaded H-Bridge (CHB)-multilevel converter (MC)-based photovoltaic (PV) system with no voltage or current sensors at the dc-side is proposed. Eliminating the dc-side sensors simplifies the hardware, leading to lower cost and higher reliability of the PV system. A novel scheme estimating the capacitors’ voltages from the ac output voltage of the inverter is developed. The scheme allows replacing all dc-side voltage sensors by a single-voltage sensor at the ac-side of the converter. Furthermore, the dc current sensors, conventionally required for the maximum power point tracking (MPPT), are also eliminated. Instead, the outputs from the capacitors’ voltage control systems are utilized for the MPPT. The effectiveness of the proposed dc-side sensorless system is experimentally demonstrated on a 2-kW single-phase seven-level CHB converter-based PV system. [ABSTRACT FROM PUBLISHER]

Converter control is an important aspect of voltage-source-converter high-voltage dc (VSC-HVDC) based multi-terminal dc grids. Variants of power drooped against dc voltage control have previously been proposed for this purpose. However, there is at this time no clear methodology available on how to select the size of the droop constants for distributing a power mismatch, following, e.g., a converter trip, on several converters. This paper proposes a methodology for establishing droop constants for a variant of power drooped against dc voltage called pilot voltage droop control. The methodology is derived from the basic control laws in a straight forward fashion and the accuracy is exemplified by time domain simulations in Digsilent's Powerfactory platform. [ABSTRACT FROM AUTHOR]

The multi infeed interaction factor (MIIF) index was introduced by CIGRE as a measure of inter-inverter interactions in multi-infeed HVDC systems, which include only line-commutated converter based HVDC (LCC–HVDC) links. The MIIF is although valid among LCCs in hybrid multi-infeed HVDC systems, but not among voltage-source converters (VSCs) as well as between LCC and VSC when the VSC operates under the constant ac voltage control mode. In this paper, a multi-infeed voltage interaction factor (MVIF) index is firstly proposed as the quantitative measure of inter-inverter interactions in hybrid multi-infeed HVDC systems. Compared to the MIIF, the MVIF is unified since it is applicable regardless of inverter types and control modes. Moreover, the quasi-steady-state power-flow model of hybrid multi-infeed HVDC systems is utilized as the mathematical framework to develop the analytical calculation method of the MVIF. This lends a theoretical basis for the efficient evaluation of inter-inverter interactions. Furthermore, the parametric dependence of the MVIF is analyzed to provide an exhaustive perspective of inter-inverter interactions. This offers a useful tool to aid planning and operation of hybrid multi-infeed HVDC systems. Finally, dynamic simulations conducted on the PSCAD/EMTDC program validate the MVIF and its analytical calculation method. [ABSTRACT FROM AUTHOR]

This paper puts into perspective the state-of-the art knowledge on bearing currents and diagnosis tools, which rely on shaft voltages and bearing currents phenomena. Today, shaft signals measurements are progressively included in predictive health monitoring of large turbo and hydro generators. In addition, bearing fault diagnosis through shaft signals have recently gained some attention to improve reliability and estimation of the remaining useful life in electrical drives. The main objective of this paper is to provide diagnosticians with a broad overview of the current trends in fault diagnosis through shaft signals. Therefore, the study presents a critical review of the measurements methods, advantages, and disadvantages of fault diagnosis methods, and suggests future possible enhancements for bearing health monitoring through shaft signals. [ABSTRACT FROM AUTHOR]

Conventional single-phase power-factor-correction (PFC) rectifiers with active power decoupling capability typically require more than three active switches in their circuits. By exploring the concept of power-buffer cell, a new single-stage PFC rectifier with two active switches, one inductor and one small power-buffering capacitor is reported in this paper. The proposed converter can achieve high-power factor, wide output voltage range, and power decoupling function without using electrolytic capacitor. Additionally, an automatic power decoupling control scheme that is simple and easy to implement is proposed in this paper. The operating principle, control method, and design considerations of the proposed rectifier are also provided. A 100-W prototype with ac input voltage of 110 Vrms and a regulated dc output voltage ranging from 30 to 100 V has been successfully designed and practically tested. The experimental results show that with only a 15 μF power-buffering film capacitor, the proposed converter can achieve an input power factor of over 0.98, peak efficiency of 93.9%, and output voltage ripple of less than 3%, at 100-W output power. [ABSTRACT FROM PUBLISHER]

This paper presents an extended electromotive-force-based discrete-time sliding-mode observer (DSMO) for rotor position/speed sensorless control of interior permanent-magnet synchronous machines (IPMSMs). Without using voltage sensors to measure IPMSM terminal voltages, the reference voltages generated by the vector control system are used as inputs for the DSMO. However, due to the inverter dead-time effect, the mismatch between the reference voltages and actual terminal voltages will degrade the performance of DSMO. In this paper, the periodically oscillating rotor position estimation error caused by the dead-time is first analyzed. Then, a dead-time compensation scheme is proposed to mitigate this position estimation error. The proposed DSMO with the dead-time compensation scheme is validated on an IPMSM control system used in heavy-duty, off-road, hybrid, and electric vehicles. [ABSTRACT FROM AUTHOR]

Small-signal stability of ac power systems is able to be assessed by measuring their terminal impedance, and a transformerless series injector is very attractive for wideband measurement of load impedances. A typical implementation of the transformerless series injector is paralleled H-bridge (PHB) converters, where dc voltage imbalance will occur when system current magnitude is larger than a critical point, and thus the operation region of the series injector is seriously restricted. To address this issue, this paper presents a deep analysis on the mechanism behind dc voltage imbalance for PHB-based series injector, and it is revealed that a positive feedback exists in the dc voltage balancing loop under high system current magnitude. Then an enhanced control scheme is proposed to balance dc voltages in full system current range, where a reactive component is injected in the output voltage while reactive circuiting current is introduced among H-bridge converters for redistributing active power. Furthermore, the system behavior with the proposed scheme is comprehensively analyzed and the reference design for the injected reactive component magnitude in the output voltage is offered. Finally, the imbalance mechanism and the proposed scheme are validated by simulation and experimental results. [ABSTRACT FROM AUTHOR]

A method for the seamless transition of three-phase inverters switched between grid-tied and stand-alone modes of operation is presented in this paper. In this method, only the inverter current and voltage sensors are utilized, and no control over the grid-side static transfer switch is needed. The presented method contains two strategies for grid-tied-to-stand-alone and stand-alone-to-grid-tied transitions. In the stand-alone-to-grid-tied transition strategy, a novel algorithm is presented for estimating the grid angle nearly instantaneously, which allows the three-phase inverter to respond very quickly if the grid and point-of-common-coupling voltages are out of phase. This fast response allows the inverter to effectively eliminate the transient overcurrent that would normally occur if it was connected to the grid without first being synchronized. The fast response also allows the inverter to return to normal operation very quickly after such an event. The strategy for the seamless transition from grid-tied to stand-alone mode is also presented. These strategies have been verified through experiments, and the results are presented in this paper. [ABSTRACT FROM PUBLISHER]

Generic electrical simulation models of wind power generation have been developed as standards, such as the IEC 61400-27-1, to be used by wind industry, system operators, and academia for power system stability studies. In this paper, the IEC type 3 wind turbine model with wind turbine level voltage controller and with wind power plant level power factor controller is validated based on field measurements from a 52-MW wind power plant. In addition to the validation of the IEC type 3 wind turbine and wind power plant controller models, a comparison of the validation approaches, which are the full grid and play-back simulation, is provided together with a survey of the existing validation studies and recommendations for future modeling and validation tasks. The implemented IEC models are tuned to match the measurements accurately and the validated values for the control parameters of the reference wind power plant model are given. [ABSTRACT FROM PUBLISHER]

Phase-shifted carrier modulation is an industry standard in its application to multilevel H-bridge converters. The major advantage of this scheme over level-shifted and space vector modulation schemes is its inherent ability to evenly distribute losses between semiconductor devices. However, phase-shifted schemes suffer from harmonic degradation when operated at the low carrier frequencies required in high-power high-level number applications. This paper develops a model predictive modulation (MPM) scheme that achieves superior harmonic performance, compared to phase-shifted modulation, while ensuring equal semiconductor loss distribution. With respect to complete-enumeration-based predictive techniques, the proposed modulation significantly reduces the number of times the system cost function is evaluated. Reducing the number of calculations by several orders of magnitude ensures computational feasibility. Simulation and experimental results are presented, which confirm correct operation of the modulation strategy. [ABSTRACT FROM PUBLISHER]

A multilevel H-bridge StatCom inherently contains redundancy in the available switching states. This paper develops a variation on the typical model predictive control scheme which is able to exploit this redundancy to simultaneously balance the H-bridge capacitor voltages, provide excellent current reference tracking, and minimize converter switching losses. The scheme consists of a dead-beat current controller that has been integrated with heuristic models of the voltage balancing and switching loss characteristics. The integration of a pulsewidth modulation scheme is also described. Simulation and experimental results are presented that confirm the correct operation of the control and modulation strategies. Comparison with traditional control and modulation schemes is provided in terms of the key performance indicators associated with multilevel H-bridge StatComs. [ABSTRACT FROM AUTHOR]

When fast voltage control is used in a power factor control, the ripple of the dc link causes current harmonics. This paper proposes a method to reduce the harmonic content of a fast voltage loop in a power factor correction, which can both produce low distortion during steady-state operation and fast recovery from a load transient. The performance is achieved by using a Takagi–Sugeno-type nonlinear controller for dc-link voltage control. The proposed nonlinear control is compared with linear proportional–integral (PI) control, which is tuned to meet the standardized regulations for the harmonic content in a 3-kW single phase power supply. The results show that the presented nonlinear voltage controller can maintain the dynamic performance of the linear control with reduced current harmonics. It is also shown that the used nonlinear control method requires only marginally more complex control algorithm compared to commonly used linear PI control. [ABSTRACT FROM AUTHOR]

In this paper, a regeneration protection solution is proposed to address the dc-link overvoltage issue and the unequal power sharing in the parallel uninterruptible power supply (UPS) systems. First, a dc-link voltage protection control strategy is proposed to protect the inverter against the excessive dc-link voltage that may trigger the protection mechanism of the UPS system. In addition, an active power-sharing control strategy by regulating the virtual resistance is proposed to solve the circulating current caused by UPS regeneration issue. Finally, the feasibility of the proposed regeneration protection solution is verified by experimental results from the parallel UPS system prototype. [ABSTRACT FROM AUTHOR]

This paper focuses on a fully distributed peer-to-peer control scheme for voltage regulation and reactive power sharing of multiple inverter-based distributed energy resources (DERs) in ac microgrids. The proposed peer-to-peer control strategy is fully distributed enabled through broadcast gossip communication, where each DER unit only requires local voltage and current measurement from its own and some (but not all) nearby neighbors for the voltage and reactive power sharing control. Employing the broadcast gossip communication protocol with attractive scalability and reliability properties, the control inputs can be updated to restore the voltage magnitudes at the point of common coupling to a desired value ensuring an accurate reactive power sharing for each DER. Since the proposed distributed controllers are implemented on local DERs, the central controller and hierarchy are no longer required. Accordingly, the microgrid system stability is preserved in the peer-to-peer requirements of line switches, which in turn, enables a plug-and-play operation of DERs and their robustness against microgrid topology change scenarios. Simulation studies in a modified IEEE 34-bus test network demonstrate the effectiveness and applicability of the proposed control strategy. [ABSTRACT FROM AUTHOR]

Smart grid technologies are expected to increase system efficiency and reliability using advancement in automation, communication, computation, and optimization. Several control algorithms have been developed or enhanced for operation and control of smart distribution grid. In recent years, volt/var control (VVC) algorithms have been proposed for energy saving in distribution systems due to advancement provided by smart grid automation and control. VVC helps to lower distribution feeder voltage to decrease energy consumption. However, quantification of this impact is hard due to several external factors such as weather and end users' behavior. On the other hand, existing evaluation methods of VVC algorithms are either not suitable for long-term study or intrusive. This paper addresses these issues by proposing a new technique to estimate “distribution system demand without smart grid implementation” utilizing “data measured from system with smart grid implementation.” Interdependence of voltage and load is considered based on estimated load characteristic. Our key contribution is developing a method for quantifying the real-time energy saving of smart grid automation and control focused on VVC and implementing in an electric utility in eastern Washington, USA. The algorithm is developed using both commercial and open-source software. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is an emerging and highly attractive multilevel converter topology for high-voltage and high-power applications. This paper proposes the control method of parallel-connected modular multilevel converters (parallel-MMCs), which assumes that the multiple MMCs are directly connected at both ac and dc sides to effectively enhance the power rating as expected. Two key problems were first solved for the parallel-MMCs under the normal operation conditions: voltage balancing of submodules and mitigation of circulating currents, where the novel transformed third-order harmonic resonant controller in the synchronous reference frame was employed to mitigate the dominant second-order and fourth-order circulating currents and a sixth-order harmonic resonant controller is used to attenuate the zero-sequence sixth-order circulating current existed in all phase currents per MMC. Considering the high risk of switches fault in the parallel-MMCs, the fault-tolerant operation schemes were then proposed in this paper to address the major concerns of open-circuit and short-circuit switch fault in a submodule, respectively. Carefully controlling the healthy submodules and the corresponding phase arms, the parallel-MMCs can successfully maintain their balanced capacitor voltages and mitigate the circulating currents with the qualified output waveform obtained. In addition, the parallel configuration of MMCs provides the unique solution for the short-circuit switch fault operation which was seldom discussed in the published literature works with respect to the MMC fault-tolerant operation schemes. MATLAB simulations and the constructed experimental prototype have verified the performance of the proposed control strategy. [ABSTRACT FROM AUTHOR]

Filters for switching ripple attenuation are essential at the input, and sometimes at the output for certain applications, for the deployment of matrix converters (MCs). Due to the absence of inertial elements in the MC structure and the consequent tight input–output coupling, the filter parameters significantly affect its dynamic behavior. This paper presents an exhaustive filter design method for synchronous applications of the MC in power systems. Apart from the usual considerations of ripple attenuation, voltage regulation, reactive current loading, and internal losses, this paper also addresses additional constraints which may be imposed by requirements of dynamic performance and reliable commutation. Rigorous analytical justification of each design step is provided and the sequential design process is summarized. Relevant experimental results are presented to validate the proposed design tool. [ABSTRACT FROM PUBLISHER]

With the rapid development of photovoltaic (PV) technology, more utility-scale PV systems are being integrated into rural areas where abundant and low-cost land is available. In these areas, step-voltage regulators (SVRs) with an open-delta configuration are widely used for voltage regulation. However, since SVRs were initially designed for tackling slow and smooth load variations, frequent PV output fluctuations can cause excessive tap changes. Moreover, rural communication facilities are normally expensive and underdeveloped, so only limited parameters are recorded with low resolution and reliability. Thus, much needed tap information cannot be properly monitored in a cost-effective way. In contrast, a downstream load center or utility-scale PV site is commonly well equipped with modern sensors and data acquisition systems, which provide the possibility for tap evaluation. Therefore, this paper proposes a new remote tap position estimation approach to accurately assess the upstream tap positions of open-delta SVRs from downstream measurements. Furthermore, the proposed approach is firmly validated through field testing under the support of the local utility and PV plant owner. This novel method not only enables reliable tap position monitoring with high precision for utilities to research PV–SVR interaction, but also provides valuable information to PV owners for correctly analyzing downstream voltage performance. [ABSTRACT FROM AUTHOR]

Three-phase phase-modular isolated power factor correction rectifiers are an interesting alternative to phase-integrated three-phase rectifiers as matrix-type phase modules allow a single-stage isolated energy conversion between the three-phase mains and a dc bus. Therefore, this paper presents a phase-modular isolated matrix-type rectifier which can be connected to the mains either in star (Y) or delta ( \Delta) configuration, enabling a wide input voltage range. Additionally this allows to select the voltage and current stresses of the phase module switches according to the used semiconductor technology, for example {\text{650 V}} Si or GaN devices could be used in rectifiers powered from the {400} or \text480 V\text{rms} mains. A detailed analysis of the operating principles and switching behavior of the converter is presented, showing that zero voltage switching can be achieved in the phase modules. Additionally a third harmonic current injection concept is proposed which allows an up to {15} \% higher output voltage in \Delta -mode. The concepts are validated with measurements taken on a {\text{7.5}}\;{\text{kW}}, {\text{400}}\;{\text{V}} dc output voltage prototype converter achieving 97.2 \% efficiency and a total harmonic distortion of < 2 \% at rated power. [ABSTRACT FROM AUTHOR]

Harmonics have been considered as one of the major issues in future power grids. With the increasing demand in advanced control functions, power electronic converter interfaced distributed generators (DGs) are expected to perform harmonic compensation when necessary. It has been demonstrated in a number of studies that DG converters operating in voltage-controlled mode can be easily configured to realize voltage harmonic compensation utilizing naturally embedded voltage control loop. While for DGs operating in current-controlled mode (CCM), such function was rarely studied. Considering that CCM is commonly used in renewable energy based generators and energy storage systems, it has certain significance to achieve the same function with CCM converters. Aiming at such objectives, this paper proposes a voltage detection based harmonic compensator (HC) for CCM converters. The novelty and main advantages of the proposed method include the following. It realizes seamless interface of HC with inner fundamental current control loop. Compared with a conventional active power filtering method, it does not require remote load harmonic current measurement since it is local voltage detection based. Compared with a conventional voltage detection based method, it offers better performance because of directly harmonic voltage regulation. Experimental results are presented to demonstrate the effectiveness of the method. [ABSTRACT FROM PUBLISHER]

Owing to the development of highly efficient electric power converters, the number of studies on the construction of low-voltage dc (LVDC) systems is gradually increasing. To use LVDC distribution systems, voltage regulation methods to compensate for the voltage drop and to limit the voltage unbalance are essential. In a bipolar LVDC distribution system, a voltage drop and voltage unbalance could occur because of the load current and the variation in the amount of power supplied to the poles. However, not enough research has been conducted on voltage regulation methods for LVDC distribution systems. To reduce the voltage drop and the voltage unbalance, this paper proposes a voltage regulation method based on the neutral to line drop compensation (NLDC) method, which employs a modified LDC to calculate the sending-end reference voltage. In the NLDC method, the neutral current and neutral line impedance are taken into consideration to compensate for the neutral line potential fluctuation and voltage drop on the pole. The next sending-end voltage is determined by checking the voltage unbalance factor, and it is adjusted to maintain it within the allowable voltage unbalance factor. A voltage regulation algorithm and a bipolar LVDC system model are implemented using the Electromagnetic Transients Program. [ABSTRACT FROM PUBLISHER]

In this paper, the real-time operation and the harmonic analysis of isolated and nonisolated dc microgrid are investigated. The microgrid features hybrid energy sources with various loading schemes. This hybrid dc microgrid was supplied by an internal generator and the ac utility grid. A supercapacitor bank functions by acting as a power buffer to satisfy the high power requirements of the dc microgrid. In order to properly manage the energy and prevent the ac grid power pulsation, a current–voltage control technique based on master–slave control concept is proposed. In this technique, the supercapacitor bank operates as the master and controls the dc bus voltage. The other converters are working in current control mode to share the required power. The controllers of the converters are equipped with hysteresis voltage control, which monitors the entire supercapacitor bank. Additionally, an analog hysteresis voltage protection circuit board was designed and implemented to protect the supercapacitor cells from overvoltages that can occur due to uneven charge distribution during the fast charging processes. The protection circuit, which was developed and built here, was experimentally tested, and its error was analyzed. Various operating modes based on different power sharing patterns were defined. The experimental test was carried out for both galvanically isolated and nonisolated dc grid systems. The results show that the proposed energy management algorithm properly shares power and control the voltage. In addition, depending on the power sharing pattern, the isolation of the dc microgrid significantly affects the harmonic content of the current. [ABSTRACT FROM AUTHOR]

Conservation voltage reduction (CVR) is widely adopted by utilities for peak demand reduction and energy savings through reducing the voltage level of the electrical distribution system. This paper presents an in-depth review on implementing and assessing CVR. The methodologies to quantify CVR effects are categorized into comparison-based, regression-based, synthesis-based and simulation-based methods. The implementation strategies for voltage reduction are classified into open-loop and closed-loop methods. The impacts of emerging smart-grid technologies on CVR are also discussed. The paper can provide researchers and utility engineers with further insights into the state of the art, technical barriers and future research directions of CVR technologies. [ABSTRACT FROM PUBLISHER]

In order to address the load sharing problem in islanding microgrids, this paper proposes an enhanced distributed generation (DG) unit virtual impedance control approach. The proposed method can realize accurate regulation of DG unit equivalent impedance at both fundamental and selected harmonic frequencies. In contrast to conventional virtual impedance control methods, where only a line current feed-forward term is added to the DG voltage reference, the proposed virtual impedance at fundamental and harmonic frequencies is regulated using DG line current and point of common coupling (PCC) voltage feed-forward terms, respectively. With this modification, the impacts of mismatched physical feeder impedances are compensated. Thus, better reactive and harmonic power sharing can be realized. Additionally, this paper also demonstrates that PCC harmonic voltages can be mitigated by reducing the magnitude of DG unit equivalent harmonic impedance. Finally, in order to alleviate the computing load at DG unit local controller, this paper further exploits the band-pass capability of conventionally resonant controllers. With the implementation of proposed resonant controller, accurate power sharing and PCC harmonic voltage compensation are achieved without using any fundamental and harmonic components extractions. Experimental results from a scaled single-phase microgrid prototype are provided to validate the feasibility of the proposed virtual impedance control approach. [ABSTRACT FROM AUTHOR]

Electromagnetic launch (EML) system needs pulse power supply (PPS) as the energy source for discharging in EML experiments. The EML system’s work effectiveness is decided by the PPS’s parameters, such as the charging precision of the voltage, the energy density of the energy storage module, and the charging rate. As the development of EML technology, the experiment needs more and more energy and the number of energy storage module is increasing rapidly. However, the output power of the charging power supply cannot hold up the requirement energy at the same time. So, how to charge the numerous modules rapidly and accurately becomes a key problem to be solved. In this paper, the load is divided to several groups and a controller to manage the multigroups of load is designed, with the function of voltage feedback and isolation driving. The design of the isolation drive technology is introduced, which include the design of the pulse signal amplification and shaping circuit, the isolation pulse transformer, the drive circuit, and the optical fiber communication circuit. The voltage feedback system is also introduced and the control strategy of the multigroup loads’ switchover and voltage measuring method is designed. The digital signal process controller that used to achieve these functions is introduced and the controller is applied in the charging experiment for multigroups of load and the results prove that the controller can be used effectively. [ABSTRACT FROM PUBLISHER]

This paper investigates the influence of the axial magnetic field (AMF) arc control on voltage distribution (VD) of double-break vacuum circuit breakers (VCBs) in order to gain balanced VD, and thus enhance the breaking capacity. The interaction of vacuum arc plasma, postarc current (PAC), and AMF is discussed. We conduct the breaking test of double-break VCBs with externally applied AMF of a different pulsewidth, magnitude, and a different moment. The PAC, postarc charge, and the VD are measured to analyze the relationship between the AMF arc control and the VD. The optimal AMF arc control and the relationship between the AMF magnitude and the unbalanced voltage ratio are gained when the grading capacitors were 400 pF. The experimental test showed that the proper AMF arc control can improve the unbalanced VD. The mechanism is that the proper AMF arc control can make the postarc charge small and stable, so the unbalanced postarc charge, which caused the unbalanced VD is limited. [ABSTRACT FROM AUTHOR]

The flying-capacitor-based multilevel converter is one of the well-known breeds of the multilevel power converters. This paper proposes a new flying-capacitor-based multilevel converter to minimize the number of flying capacitors (FCs) and power switches. The advantage of the proposed FC-based multilevel converter in comparison with the conventional flying-capacitor multicell converter is that it needs fewer FCs. Also, in comparison with the stacked multicell converter, the proposed multilevel converter requires fewer semiconductor switches. In order to balance the voltage of the FCs in proposed multilevel converter, a new active voltage balancing method which is fully implemented using logic-form equations is presented. The proposed voltage balancing method measures output current and FC voltages to generate switching states to produce the required output voltage level, as well as balance the FCs voltages at their reference values. The output voltage of the proposed multilevel converter controlled with suggested active voltage balancing method can be modulated with any pulse-width-modulation (PWM) method, such as phase-shifted-carrier PWM or level-shifted-carrier PWM. Simulation results and experimental measurements of proposed FC-based multilevel converter are presented to verify the performance of the proposed converter, and its novel switching and modulation strategy, which is based on the active voltage balancing method. [ABSTRACT FROM PUBLISHER]

In this paper, a method is presented for removing the risk associated with the testing and development of novel drive system topologies, prototype electrical machines, advanced control system strategies, or a combination of the aforementioned without using any real motors/generators. The test platforms for low-power machines are relatively inexpensive and accessible; however, as power levels increase into the upper kilowatt and megawatt range, validation of prototype machines and drives becomes costly. The proposed induction machine emulator (IME) platform utilizes the power hardware-in-the-loop concept in conjunction with a high-fidelity machine model and load dynamics. The electrical machine and its load dynamics are simulated with a real-time digital simulator, which generates appropriate control commands to a power electronics-based voltage amplifier that interfaces to a variable speed drive (VSD). Specifically, the current draw is recreated by altering the phase and magnitude of a voltage amplifier connected to a VSD under test via a unique transformer-based LCL-type coupling network. Based on the proposed concept, the use of a multiwinding, tap-changing transformer establishes a truly versatile and universal test platform for a wide range of power levels. In addition, this paper presents a control strategy in the synchronously rotating reference frame in dq coordinates for the power electronic converters in IME operation. Experimental results at the 25-kVA power level validate the feasibility and highly dynamic performance of the proposed test platform. [ABSTRACT FROM AUTHOR]

This paper presents a new synchronous-reference-frame (SRF)-based control method to compensate power-quality (PQ) problems through a three-phase four-wire unified PQ conditioner (UPQC) under unbalanced and distorted load conditions. The proposed UPQC system can improve the power quality at the point of common coupling on power distribution systems under unbalanced and distorted load conditions. The simulation results based on Matlab/Simulink are discussed in detail to support the SRF-based control method presented in this paper. The proposed approach is also validated through experimental study with the UPQC hardware prototype. [ABSTRACT FROM AUTHOR]

This paper describes two subjects, both aimed at the industrialization of Nb3Al conductors. One is the metastable solid-solution-strand restacking method, which is expected to be a low-cost Cu-stabilization method compared with the electroplating technique. In this paper, we mainly focus on recent efforts toward long-length production. The key point here is an improvement of the contact between the Cu tube and the restacked original strands. The second subject is the first attempt at “constant voltage” control in the continuous rapid-heating and quenching operation for 300-m-class precursor wire. With constant current control, changes in the diameter of the wire lead to variations in power per unit volume, so that the processing temperature changes. With constant voltage control, changes in diameter of the wire do not lead to changes in the power per unit volume, so that the processing temperature does not change and thus the uniformity of the longitudinal performance is improved. [ABSTRACT FROM AUTHOR]

In this paper, we develop a data-driven framework for controlling distributed energy resources (DERs) in a balanced radial power distribution system with the objective of regulating voltages across the whole system. The objective is to determine optimal DER power injections that minimize the voltage excursions outside a desirable voltage range without knowing a complete model of the power distribution system. To this end, we approximate the nonlinear relationship between the voltage magnitudes and the power injections by a linear model. The parameters of this linear model—referred to as the voltage sensitivities—can be computed using information on the network topology and the line parameters, the values of which will be estimated. Assuming the knowledge of feasible network topology configurations and distribution line resistance-to-reactance ratios, we propose a framework for identifying the true network topology configuration and the corresponding line parameters using only a few measurements of voltage magnitudes and power injections. Utilizing the estimated voltage sensitivities, the optimal DER power injections can be readily determined by solving a convex optimization problem. Due to its data-driven nature, the proposed framework is intrinsically adaptive to changes in system conditions such as unknown topology reconfiguration. The effectiveness of the proposed framework is validated via numerical simulations on the IEEE 123-bus distribution test feeder. [ABSTRACT FROM AUTHOR]

An isolated bidirectional dc–dc converter, which combines parallel-connected two-level converters (TLCs) on the medium-voltage side and a modular multilevel converter (MMC) on the high-voltage side, namely a TLC-MMC converter, is a promising candidate for the interconnection of medium-voltage direct current (MVDC) and HVdc grids. Different from typical phase-shift controlled isolated bidirectional dc–dc converters, transformer currents of a TLC-MMC converter are regulated by the MMC in a closed-loop manner in order to achieve extremely low switching losses and low total device rating of power-semiconductor devices in TLCs. This paper addresses the dedicated operation technique and the voltage-balancing control of the TLC-MMC converter. The MMC in the TLC-MMC converter operates with six-step phase voltages and sinusoidal phase currents. Such an operation makes the MMC present remarkable differences to ac–dc applications. The direct-modulation technique, which is the most-used technique performed to MMCs in line-frequency ac–dc power conversion, has been concluded not applicable to the MMC in a TLC-MMC converter due to the significant sixth-order oscillation resulted in the dc-link voltage. Thus, the indirect-modulation technique is employed, and consequently a closed-loop voltage-balancing control is mandatory for the MMC. Possible measures for voltage balancing have been explored, and effective voltage-balancing control with a simplified implementation is proposed. Validity and effectiveness of the proposed control strategy have been verified by both simulations and experiment using a downscale prototype. [ABSTRACT FROM AUTHOR]

Polymer electrolyte membrane fuel cell (PEMFC), because of faster response, portability and the low operating temperature has found wide usage in applications requiring regulated output voltage. Alongwith voltage regulation, optimum stack efficiency is important for reliable operation. Overall, the maximum efficiency point (MEP) operation and voltage regulation need to be achieved while restricting the PEMFC operation till maximum power point (MPP). This demands operational adaptability of PEMFC over a wider range, to meet the desired levels of voltage regulation, maximum efficiency and MPP operation. However, achieving all the objectives simultaneously is challenging since operating points corresponding to the objectives differ. This paper proposes a control strategy that achieves operation dependent adaptability of PEMFC between the objectives as per the demand. The adaptability in controller tuning for MPP and voltage regulation has been formulated as multi-objective optimization (MOO) problem with a constraint on controller parameters, for allowing the PEMFC operation closer to MEP. The solution of MOO problem provides a set of controller configurations, each achieving different operating scenarios/objectives of the PEMFC system. Compared to the existing works aiming at either of the objectives, the effectiveness of the proposed approach is reflected in terms of simultaneous attainment of all the objectives. [ABSTRACT FROM AUTHOR]

This paper presents a modification of the Vienna rectifier (VR) to obtain more than three voltage levels at the DC line connection, while preserving the unity power factor at the AC grid side. The proposed unidirectional multilevel there-phase rectifier, called the “Negev rectifier”, has a reduced device count and utilizes a unique novel Time Division Multiplexed based control scheme, which makes the challenging N-level VR expansion possible. The Negev rectifier can serve as a grid interface for supplying dc voltage to multilevel inverters with 4 up to 7 voltage levels, eliminating the burden of inverter-side voltage balancing control. The Negev hardware can be constructed by two complementary topologies, making it suitable for either high current or high voltage applications. [ABSTRACT FROM AUTHOR]

A novel method for controlling direct voltage regulated active dc-link capacitance reduction circuits (ACRC) is proposed in this paper. Instead of forcing the dc-link voltage of the power conversion system to follow a constant reference, the ACRC imposes dc-bus voltage in proportion to the integral of its input current. Consequently, it resembles an adjustable value capacitor from the dc-link side. Therefore, as opposed to existing control strategies, the proposed method simultaneously allows the dc-link voltage to reflect the power balance of the system and the pulsating power component to be diverted into significantly reduced auxiliary capacitance via a bidirectional dc–dc converter. As a result, replacing bulk dc-link capacitance by the proposed circuitry does not require any modification of existing grid-connected power conversion system controllers, thus allowing plug-and-play operation. The proposed approach is validated by application to a commercial power factor correction front end by both simulations and experiments. [ABSTRACT FROM AUTHOR]

The characteristics of the residual voltage of a surge protective device (SPD) connected to an overhead distribution line due to nearby natural cloud-to-ground lightning and to rocket-triggered lightning are analyzed in this paper. The residual voltage of the SPD is found to be closely related to the ground potential rise at the grounding terminal of the SPD. For natural lightning occurring 1.8 km to 10.5 km away from the SPD, the ground potential rise (GPR) is too small to influence the residual voltage of SPD. For rocket-triggered lightning with a 40-m distance between the lightning channel and the SPD, the residual voltage of SPD is primarily determined by the voltage induced in the distribution line for return stroke and large M-components process, whereas the influence of the GPR becomes strong in the minor M-components stage. For a 5-m distance between the lightning channel and the SPD, the residual voltage of SPD originates primarily from the GPR during the entire flash, exhibiting the characteristics of reversed polarity. The duration of SPD's residual voltage may be up to the order of milliseconds due to the GPR in the case of the large M-components and a long continuing current process, which may cause damage to the SPD. [ABSTRACT FROM AUTHOR]

This paper proposes a two-level voltage controller for large-scale power systems. The aim of the controller is to maintain a near-optimal voltage profile in the transmission network by coordinating discrete reactive power (var) control devices in the system. The controller is targeted for implementation in electric power transmission utility companies where most of the voltage controls are coordinated by switching of discrete var devices such as shunt capacitor and reactor banks and transformer banks load tap changers (LTCs). By effectively dividing the controller responsibilities between local substation controls and a central coordinator at control center level, the design is aimed at voltage control of large-scale power systems. At local level, the substation controllers maintain their respective substation bus voltages by local power-flow-like computations using mostly local PMU measurements. The central coordinator computes and provides the voltage set-points to the substation controllers and also coordinates by enabling or disabling the local controllers as needed. The controller is being designed towards prototype implementation in Southern California Edison (SCE), starting with local controllers at specific substations in the first stage. It is tested on real-time dynamic models and large-scale power-flow simulations of SCE transmission network. [ABSTRACT FROM AUTHOR]

This paper proposes a decentralized adaptive emergency control scheme against power system voltage instability. Decentralized control architecture is proposed by segregating the system into several local areas or zones based on the concept of electrical distance. Intelligent agents are assigned in each area to monitor the bus voltages and generator reactive powers to detect any threat of voltage collapse and to actuate countermeasures. A novel performance index has been formulated based on the load voltage and generator reactive power violations to identify the severity of disturbance and the risk of system emergency in each area. The coordination of the timing of the countermeasures among the agents is achieved through the formulation of the integral of the performance index. The simplicity and the adaptive nature of the proposed control scheme to provide countermeasures against any disturbance make it useful for real-time application. The robustness of the proposed approach has been validated through several case studies using the New England 39-bus test system and a more realistic Nordic32 test system. [ABSTRACT FROM AUTHOR]

This paper describes a control algorithm based on modified enhanced phase-locked loop (MEPLL) in dynamic voltage restorer (DVR). It compensates distortions and unbalances in the supply voltage along with voltage sag/swell. Three-phase MEPLL extracts the fundamental positive-, negative-, and zero-sequence components from the distorted/unbalanced signals. Further fundamental positive-sequence components are used in the reference load voltage calculations. In addition to track the angle of the input signals similar to the conventional PLLs, the proposed algorithm (MEPLL) offers features of getting fundamental and sequential components in case of distorted or unbalanced grid voltage for all the three phases simultaneously. Optimization approach named as autonomous groups particle swarm optimization, a variant of PSO is used for the calculation of PI-controller gains. The integrated time square error is used as a cost function for optimization of an error between the reference and actual values. This approach of tuning PI gains improves the performance by eliminating the manual process. The proposed control algorithm is implemented in DVR system using MATLAB software and validated in a laboratory environment. The performance shows that the proposed control algorithm gives time effective and satisfactory solution for the unpredictable issues mentioned. [ABSTRACT FROM AUTHOR]

Variable-speed drives have reduced voltage requirements when operating below the base speed. In a modular-multilevel-converter-based (M2C-based) motor drive, it is then possible to operate with reduced voltage in the submodule capacitors, than at the base speed. In this sense, a greater capacitor-voltage ripple can be accommodated, without exceeding the maximum peak-capacitor voltage. This paper presents an analytical investigation for the optimal selection of the average capacitor voltage for M2Cs, when the motor is operating with rated torque, below the base speed. This method does not require any power exchange between the converter arms, so it keeps the conduction losses at the minimum level. Additionally, the method decreases the switching losses, due to the decreased capacitor-voltage level. The overall ratings of the converter remain the same as in the base-speed operation. It is shown that this method can be applied at a speed range between the base speed and down to approximately one third of it, i.e., an operating range that covers the requirements for typical pump- and fan-type applications. The results obtained from the analytical investigation are experimentally verified on a down-scaled laboratory prototype M2C. [ABSTRACT FROM AUTHOR]

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

The kinetic energy stored in the rotating mass of wind turbine generators can provide the effective power quality solution to the inertia-free stand-alone (IFSA) microgrid by improving its stability. In conventional power systems, the power released from the inertia of synchronous generators regulates the system frequency. However, the frequency does not vary much even when a large disturbance or a load change occurs in the IFSA microgrid with only converter-based generators. This paper proposes a new inertia control method for a type-4 permanent magnet synchronous generator wind turbine system in the IFSA microgrid by utilizing the measurement of the dc-link voltage of the converter and its derivatives, not by the system frequency. Several case studies are carried out to evaluate the performance of the proposed inertia control in the IFSA microgrid with electromagnetic transients program-based simulation. Also, its effectiveness is verified with the experimental results from the hardware-in-the-loop test implemented by the controller interface with the digital signal processor, which is connected to the real-time digital simulator. [ABSTRACT FROM AUTHOR]