Search

In this paper, we present a data-driven secondary controller for regulating to some desired values several state variables of interest in an inverter-based power system, namely, electrical frequency, voltage magnitudes at critical buses, and active power flows through critical lines. The secondary controller is based on online feedback optimization, leveraging the learned sensitivities of changes in the state variables to changes in inverter active and reactive power setpoints. To learn the sensitivities accurately from data, the feedback optimization has a built-in mechanism for keeping the secondary control inputs persistently exciting without degrading its performance or compromising system operational reliability. To ensure safe and reliable operation, we present an approach based on Gaussian process regression that, by making an inference about the modeling uncertainties not accounted for in the sensitivity-based prediction model, allows the controller to correct the predictions and find safe control actions, for which the prediction errors are more likely to be small. The feedback optimization also utilizes the learned power-voltage characteristics of photovoltaic (PV) arrays to compute DC-link voltage setpoints so as to allow the PV arrays to track the power setpoints. To learn the power-voltage characteristics, we separately execute a data-driven approach that fits a concave polynomial to the collected power-voltage measurements by solving a sum-of-squares (SoS) optimization. We showcase the secondary controller using the modified IEEE-14 bus test system, in which conventional energy sources are replaced with inverter-interfaced DERs. [ABSTRACT FROM AUTHOR]

The uncertain nature of renewable energy resources (RERs) and fast demand response lead to recurring voltage violations in the power systems, which causes frequent transformer tap shifting and capacitor switching. Therefore, this paper resorts to the static var compensators (SVCs) to manage the bus voltages based on the multi-agent deep reinforcement learning (MA-DRL) algorithm. The proposed scheme includes several system agents and SVC agents to collaboratively adjust the injected reactive power to restrict the bus voltages within the normal range. All the agents are trained centrally and executed separately, which requires minimum communication cost. The IEEE 14-bus system, IEEE 300-bus system, and China 157-node urban power grid are used to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The penetration of photovoltaics (PVs) has been increasing in active distribution networks (ADN), which leads to severe voltage violation problems. PV inverters can provide fast and flexible reactive power support and are now allowed to participate in the voltage regulation process. This paper proposes a real-time combined central and local Volt/Var control (VVC) strategy to mitigate voltage violation problems while minimizing the network power loss. Based on the historical PV and load data, the load flow and optimal power flow are performed in the centralized controller (CC) to obtain multiple voltage and optimal power settings for each PV system. The local controller (LC) then generates voltage control curves with these optimal scatters. To improve the voltage control effect, a novel 3-Dimension voltage control curve is proposed considering both the measurements of node voltage and PV generation. Moreover, a data-driven deep convolution neural network is designed and trained to generate optimal local voltage control curves without prior knowledge of specific curve functions. The proposed approach is tested on the IEEE 33-bus distribution system and simulation results verify the effectiveness in voltage control compared with the optimal Q(V) and Q(P) method. [ABSTRACT FROM AUTHOR]

This paper proposes a real-time coordinated scheduling method for active distribution networks (ADNs) with soft open points (SOPs) and plug-in electric vehicles (PEVs) via multi-timescale framework under uncertainties. Specifically, this method is achieved with day-ahead pre-scheduling and intra-day corrective control stages by coordinating various flexible resources at different timescales. The day-ahead stage is designed to reduce operational cost, regulate voltage profile and avoid risk exposure through joint scheduling of traditional devices, SOPs and PEVs on hourly basis. In intra-day corrective control stage, an hour is further divided into two timescales. The slow-timescale scheduling (STS) aims to corrective coordination of SOPs and across-time-and-space energy transmission of PEVs, and nested within the STS, the fast-timescale scheduling optimally coordinates the active & reactive power of SOPs and PV inverters to handle fast voltage fluctuations as well as against real-time uncertainties. The formulated three models are all transformed into second-order cone programming problems via sample weighted average approximation (SWAA), linearization and conic relaxation, which can be thus efficiently solved. Case studies based on three modified distribution systems (including two IEEE test systems and one actual distribution network) are performed to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Utility engineers, professionals from regulatory agencies, manufacturers and researchers have discussed if high-frequency distortions produced by rooftop photovoltaic generators are a general power quality concern for distribution systems. In this context, this paper presents the results of an extensive research to investigate the emission and propagation characteristics of such high-frequency distortions. Analytical and theoretical analyses, computer simulations, laboratory experiments and field measurements revealed that high-frequency distortions emitted by rooftop photovoltaic inverters are produced with a voltage source characteristic. As a result, the equivalent grid impedance limits and attenuates significantly the high-frequency distortions injection and propagation into the distribution systems. Moreover, the front-end, low-pass filters found in many home appliances also contribute with such attenuation by sinking part of the high-frequency distortions. In summary, the results indicate that these distortions are unlikely to become a generalized power quality concern, so that the front-end filters commonly available for such devices, when properly designed, are enough to deal with this issue. [ABSTRACT FROM AUTHOR]

This paper deals with the estimation of parameters of a Thevenin equivalent in an AC grid. We consider the practically relevant case of measurements coming from a grid-connected inverter. Our focus is on non-intrusive measurements, as we postulate and later experimentally observe transients that occur in normal operation as the primary source of information. We notice that even during quiet, after work hours there are typically enough variations in the grid to enable successful convergence of our procedure. For completeness, we also consider the artificial case of no external variations and show that injection of very small current perturbations suffices for convergence. Our procedure is based on real-time monitoring of the estimated determinant of the regression matrix. Regression matrix properties are also used to evaluate the integrity of the estimates and to initiate a brief sequence of perturbation in rare cases of the exceptionally restful grid. The estimation algorithm is simple enough to be implementable on standard industrial controllers, yet robust and reliable in terms of speed of convergence. [ABSTRACT FROM AUTHOR]

Directional, phase-selection, and distance elements of a relay are prone to misoperation in the presence of inverter-based resources (IBRs). To address these problems, modifications of relay elements as well as new control schemes for IBRs have been proposed in the literature. These methods focus mainly on addressing the misoperation of only one of the above relay elements. The available solutions that are based on a new control system focus only on the negative-sequence current loop. Thus, they work only for unbalanced faults. This paper proposes a comprehensive dual current control scheme for IBRs during all types of faults such that all of these relay elements operate correctly. The proposed method controls the IBR to operate as a voltage source behind a virtual impedance in the positive-sequence circuit, and a single impedance in the negative-sequence circuit, thereby emulating the phase angles of a synchronous generator's (SG's) fault current. The proposed method also maximizes the active, and reactive current generation of the IBR during faults, and emulates the incremental sequence current magnitudes of an SG. The proposed method does not require altering the relay elements. Moreover, correct operation of the relay elements is achieved regardless of the fault type, resistance, and location. The effectiveness of the proposed method is verified through not only simulation studies, but also hardware-in-the-loop testing of commercial relays. [ABSTRACT FROM AUTHOR]

Large-scale integration of renewable generation, usually interfaced to the network through power electronics, has led to drastic changes in power system dynamics. This paper presents novel insights into stability properties of such systems. For that purpose, a high-fidelity dynamic model of a generic low-inertia power system has been developed. The full-order, state-of-the-art control schemes of both synchronous and converter-based generators are included, with the latter differentiating between grid-forming and grid-following mode of operation. Furthermore, the dynamics of transmission lines and loads are captured in the model. Using modal analysis techniques such as participation factors and parameter sensitivity, the most vulnerable segments of the system are determined and the adverse effects of timescale coupling and control interference are investigated. More precisely, this work characterizes the maximum permissible penetration levels of inverter-based generation as well as the nature of the associated unstable modes and the underlying dynamics. Finally, potential directions for improving the system stability margin under different generation portfolios are proposed for several benchmark systems. [ABSTRACT FROM AUTHOR]

To analyze the grid integration effect of a virtual synchronous generator (VSG), this paper proposes a simple power system modeling method. It is shown that for a specific disturbance, by simplifying the original network to a radical n generators single-load model, its state-space small-signal model can be obtained using a unified formula, as long as the single generator infinite bus model of each generator is given. Case studies based on the IEEE 9-bus test system as well as the New York New England test system show high efficiency of the proposed modeling to represent slow power and frequency deviations. Modal analyses give insight into how system poles are synthesized by individual generator poles, and how different designs of VSGs affect the system oscillation modes. Potential errors due to the topology simplification and the measures to avoid them are discussed. The time-domain results and modal analyses highlight some important features of VSG, which are overlooked in previous studies. [ABSTRACT FROM AUTHOR]

The replacement of directly connected synchronous generators with power electronics interfaced generation has led to a decrease in system's inertia posing a significant challenge on frequency dynamics. In isolated systems with reduced inertia predefined limits for renewable penetration and primary reserves are frequently set for dynamic security purposes. This approach might not ensure dynamic security or can prove conservative in certain conditions. Furthermore, these approaches rarely consider the capabilities of inverter based renewable generation to provide frequency services. In this paper, a data driven approach, based on optimal classification trees is proposed to extract, from a detailed dynamic model of the system, the constraints for a frequency dynamic unit commitment formulation. Hence, both dynamic security and optimal exploitation of renewable and conventional units for power production and frequency support can be achieved. The advantages of the proposed method compared to conventional and state of the art approaches in frequency security are validated through dynamic simulations on a realistic model of Rhodes island and IEEE 118. Uncertainties in load demand and renewable generation are dealt by a robust optimization method. Its economic performance, computational overhead and modelling complexity is compared to a stochastic approach. [ABSTRACT FROM AUTHOR]

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

In this paper, we present two methods to identify the admittance of a 2.3-MVA commercial grid-following inverter through frequency-domain data and time-domain data. In addition to the well-known harmonic injection method to obtain the admittance at frequency points followed by vector fitting, we adopt a step response-based method where the step response data of the converter are collected and their $s$ -domain expressions are obtained. In turn, the $s$ -domain admittance model of the converter is found. The two methods can be used for cross validation. Step responses and frequency-domain responses obtained from the two methods are compared. Results show that the time-domain data-based method leads to an admittance comparable with that from the frequency-domain measurement in the range less than 60 Hz. In addition, two insights are obtained from the frequency-domain admittance measurements. First, when the $dq$ -frame is aligned to the point of common coupling (PCC) voltage, the per unit DQ-domain admittance directly reflects operating conditions. Second, existence of negative-sequence control can be detected via sequence-domain admittances. [ABSTRACT FROM AUTHOR]

A new transformer-less three-level grid-tied inverter is proposed in this article. By employing the switched-capacitor (SC) modules, a new topology is derived. The proposed topology has some features which are high efficiency, the capability of voltage boosting without utilization of any additional boost stage, and suppressing the leakage current by using common grounded strategy. In this article, a peak current control is employed to generate the gate pulses of power semiconductor switches and adjustment of both active and reactive powers. By doing so, the grid injected current is controlled and adjusted to have a proper quality. The operation modes, design consideration, and a comparison with some recently proposed grid-connected topologies are done through this paper. Finally, the performance and feasibility of the proposed inverter are confirmed with a 500 W experimental prototype. [ABSTRACT FROM AUTHOR]

The benefits of intermediate couplers (IC) in inductive power transfer (IPT) systems are well documented in the literature. However, the lack of guideline rules makes the selection process of the passive components and operation mode difficult. This paper presents an iterative design methodology based on the equivalent mathematical model of the system to determine the optimal magnetic coupler (MC) dimensions and resonant tanks parameters that operates the system in a quasi constant current mode. It was found that the transconductance function of a series-series (SS) with an intermediate coupler exhibits a quasi-load independent characteristic when the reflected reactance from the receiver side is null. The design methodology then combines the self and mutual coupling profiles of the MC, obtained from a finite element analysis (FEA) tool, with the system specifications and constraints to find all possible solutions. A set of selection metrics are proposed to identify the optimal solution. This work also presents an identification method of the load and mutual inductances based only in the information available from the transmitter side. The proposed method uses the steady-state equations and does not require any form of communication to the receiver side. An experimental prototype was built and assembled in a converted combustion vehicle with efficiency of 89 $\%$. [ABSTRACT FROM AUTHOR]

This paper presents a new multi-objective control strategy for inverter-interfaced distributed generation (IIDG) to ensure its safe and continuous operation under unbalanced voltage sags. The proposed control strategy can effectively improve the low voltage ride through (LVRT) capability, reduce active power oscillations, and limit overcurrent simultaneously, which are marked as the most important control objectives of IIDG during unbalanced voltage sags. The advanced voltage support scheme, which utilizes positive sequence component, is firstly proposed to maximize the LVRT capability of IIDG during unbalanced voltage sags. Then, to ensure the safety of IIDG, the active power oscillation suppression and current limitation algorithm are designed individually. Based on the control algorithms of such objectives, the multi-objective control method, including scenario classification and reference current determination, is then presented to achieve such three objectives under various system conditions simultaneously. Finally, case studies and evaluations based on MATLAB/Simulink are carried out to illustrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The strength is an important concept in planning studies for voltage stability analysis of hybrid multi-infeed HVDC (HMIDC) systems, which include both VSC- and LCC- HVDC links. However, the accuracy of earlier strength indices is very limited because they are developed by the empirical reasoning rather than rigorous theoretical derivation. In this paper, a virtual single-infeed model of HMIDC systems is firstly established utilizing the reduced-order equivalent nodal assessment of the inverter bus current-voltage equations. While incorporating the inter-inverter interactions, the virtual model is a strict dual-bus equivalent of the original complicated multi-infeed model. This enables easier and simpler analysis to be implemented. Secondly, the virtual model is employed to propose the hybrid multi-infeed interactive effective short-circuit ratio (HMIESCR) strength index for HMIDC systems by the rigorous theoretical derivation. Compared to earlier indices, the HMIESCR can depict the voltage stability much more accurately. This is because the critical HMIESCR (CHMIESCR) indicative of the stability limit is explicitly and solely evaluated to 1.5 according to the PV sensitivity based stability criterion, while the critical values of earlier indices are ambiguous. Finally, case studies by extensive numerical simulations on the classical hybrid four-infeed HVDC system validate the HMIESCR along with good robustness. [ABSTRACT FROM AUTHOR]

Historically, distribution system planning studies mainly focused on steady state and quasi-steady state analysis, with limited attention paid to dynamic analysis. This paper develops three-phase, electromechanical models for both grid-forming and grid-following inverters, and integrates them into an open source, three-phase distribution network solver, thereby enabling dynamic simulation of large-scale, three-phase unbalanced distribution systems with high penetration of inverter-based DERs. The proposed inverter models are validated against electromagnetic simulation and field test data from the CERTS/AEP microgrid testbed, and simulated in an islanded 5252 node distribution system in the GridLAB-D simulation environment. Simulation verifies the effectiveness of the proposed inverter models for large-scale distribution system analysis. Study results show that compared to traditional grid-following inverters, the high penetration of grid-forming inverters can improve the voltage and frequency stability of islanded distribution systems. [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]

With the increasing integration of PV generation in AC microgrids, it is challenging to keep the stability of system frequency due to the intermittent and stochastic nature of PVs. Thus, in order to reduce the investment and maintenance costs of storage systems, the electric utility has shown increasing interest in calling on PVs to provide frequency regulation services. In this paper, a novel sliding mode control (SMC) based adaptive power point tracking (APPT) control strategy is proposed to provide bi-directional primary frequency regulation (BPFR) of an AC microgrid. In this strategy, in order to restrain the variation of frequency, the sliding mode surface is adaptively regulated to provide an adaptive power reserve. Thus, the PVs will have a power headroom to regulate the frequency both up and down through releasing the reserved power or increasing the reserves of the PV systems. Furthermore, the sliding mode surface is adaptively regulated only based on the locally measured frequency of the microgrid without the requirements of communication, detailed PV model, irradiance sensors as well as MPP estimators. Thus, the proposed control strategy shows the advantages of easy to implement and reduces the investment and maintenance costs of installing storage systems or irradiance sensors. [ABSTRACT FROM AUTHOR]

This paper presents an extensive multi-period optimal power flow framework, with new modelling elements, for smart LV distribution systems that rely on residential flexibility for combating operational issues. A detailed performance assessment of different setups is performed, including: ZIP flexible loads (FLs), varying degrees of controllability of conventional residential devices, such as electric vehicles (EVs) or photovoltaics (PVs), by the distribution system operator (DSO) (adhering to customer-dependent restrictions) and full exploitation of the capabilities offered by state-of-the-art inverter technologies. A comprehensive model-dependent impact assessment is performed, including phase imbalances, neutral and ground wires and load dependencies. The de-congestion potential of common residential devices is highlighted, analyzing capabilities such as active power redistribution, reactive power support and phase balancing. Said potential is explored on setups where the DSO can make only partial adjustments on customer profiles, rather than (as is common) deciding on the full profiles. The extensive analysis can be used by DSOs and researchers alike to make informed decisions on the required levels of modelling detail, the connected devices and the degrees of controlability. The formulation is computationally efficient, scaling well to medium-size systems, and can serve as an excellent basis for building more tractable or more targeted approaches. [ABSTRACT FROM AUTHOR]

In 2017 and 2018, bulk power system (BPS) connected solar photovoltaic (PV) inverters tripped after grid disturbances in South California, causing large-scale power loss. One cause of PV tripping is subcycle overvoltage experienced by PV inverters when the grid suffers voltage dip and PVs enter into momentary cessation. This paper examines the underlying mechanism of the subcycle overvoltage dynamics. A dq-frame analytical model is built for a PV grid-integration system with a focus on the grid side converter (GSC) and grid interaction. PV's dc circuit dynamics and controls are ignored. Eigenvalue analysis indicates that two modes cause the subcycle overvoltage. The modes are associated with shunt compensation, grid electromagnetic dynamics, and PV inverter controls. Furthermore, a more insightful explanation of subcycle overvoltage is offered relying on s-domain admittance models. The dq-frame admittance is derived for the PV viewed from the point of common coupling (PCC). A circuit model relying on the derived admittance can demonstrate subcycle overvoltage due to momentary cessation. Impacts of grid strength, phase-locked-loop (PLL) parameters and converter current control parameters are all examined. It is found that the inherent LC dynamics due to shunt compensation and grid inductance are the main dynamics that can cause overvoltage, and momentary cessation excites overvoltage dynamics. [ABSTRACT FROM AUTHOR]

The AC fault ride-through and support capability of line commutated converter based high voltage direct current (LCC-HVDC) have significant influence on the safe operation of power system. The ability of phase locked loop (PLL) to synchronize with the voltage phase under AC faults would affect the recovery performance of LCC-HVDC systems. This paper analyzes the influence of the conventional PLL on the subsequent commutation failure (SCF) of LCC-HVDC systems and points out that the conventional PLL has some problems such as slow dynamic response, significant frequency fluctuation, and coupling of phase and frequency detection under fault conditions. A novel PLL is proposed to address the problems. Cascade delayed signal cancellation operators are combined with a mathematical filter, forming a hybrid pre-filter to extract fundamental positive sequence voltage for the PLL. To reduce the fluctuation of PLL's frequency during AC faults, the estimated frequency is fixed when the fault is detected by a fault detector. As a consequence, the decoupling between phase detection and frequency detection is realized, and the proposed PLL's dynamic response performance is enhanced. Simulation results show that the proposed PLL can provide accurate phase reference for the DC control system and reduce the probability of SCFs of LCC-HVDC system. [ABSTRACT FROM AUTHOR]

In this paper, a novel and non-invasive stator inter-turn short circuit (ITSC) online detection method is presented for an induction machine (IM), driven by a two-level voltage source inverter (2L-VSI) via finite control set model predictive control (FCS-MPC). The main idea of the proposed method is to utilize the controller itself as an observer: under the presence of a fault, the distribution of inverter switching states significantly deviates from the original balanced case. Therefore, by inspecting the inverter switching vectors, which are the outcomes of the FCS-MPC routine's online optimization procedure, a stator fault can be detected efficiently. It is observed that both the zero-vector allocation over the complex plane and the allocation among the active vectors are influenced by the presence of a stator short-circuit fault. The proposed fault detection strategy introduces little to no extra burden for processor and memory. Experimental results verify the proposed method, and inter-turn short circuits of two and three turns are confidently detected and located for a 500 W, two-pole IM. [ABSTRACT FROM AUTHOR]

This article presents a potential wind energy conversion system (WECS) using the recently proposed brushless induction excited synchronous generator (BINSYG), which can operate as a self-excited synchronous-machine. In BINSYG, an induction-machine (IM) and a synchronous-machine (SM) are wound for different number of poles and are embedded in the same machine structure. By operating the SM in self-excited mode, dc-link voltage of grid-side inverter can be controlled smoothly from the stator side of the IM. A set of conditions has to be maintained for stable operation in self-excited mode. This is particularly critical when the machine needs to be operated in wide variation in speed. So far, no stability-study is available to explore the stable-operating-zone, when the machine operates in self-excited mode. Therefore, in this paper stability of self-excited BINSYG is investigated to check suitability of the proposed system for wind energy applications. The study clearly indicates the presence of a critical speed (i.e. a minimum speed below which self-excitation is not possible). The movement of eigen values are presented to show the influence of several parameters on stable behavior of the system. Simulations using PLECS and supporting experimental-results using a laboratory-prototype (configured for 4/6 poles, 5.5 kVA) have been presented. [ABSTRACT FROM AUTHOR]

This article proposes a Virtual State Damping (VSD) plugin to output current controllers of distributed generation (DG) inverters. The VSD is designed to suppress the resonance usually present in LCL-type grid-connected DG systems, providing robustness to the control system against grid impedance uncertainties or variations. Typical active damping techniques apply an equivalence impedance model to design their structures. The proposed VSD has the advantage of not needing to take such analysis into account for its design, i.e., it is not dependent on the LCL filter's physical model. The VSD structure applies the model reference theory, and it does not require additional sensors for its implementation. The paper also presents the VSD design methodology and stability analysis. Real-time domain emulation (RTDE) results and 5kW lab-prototype experimental results demonstrate the effectiveness of the proposed technique. [ABSTRACT FROM AUTHOR]

This article investigates discharge bearing currents in inverter-fed electric motors. Industrial reports indicate that among prevalent bearing failures, bearing currents are one of the influent drivers leading to a variety of tribological issues and premature wear in bearings. While the root-causes of bearing currents have been extensively reported over the years, the response of bearing under voltage stress and the factors that influence bearing endangerment through electric discharges have been far less studied. An online measurement of the bearing voltage is proposed on a motor test-bench to estimate both the discharge activity and the energy of the discharges. The designed test bed allows reproduction of representative bearing currents leading to electrical micropitting. This paper presents extensive measurements results that highlight the critical operating conditions and parameters that worsen the discharge phenomenon inside bearings. Testing protocols are performed to quantify the influence of each parameter separately. While the bearing voltage magnitude play a significant role in the discharge phenomenon, other parameters such as successive startups, bearing temperature, and type of grease deserve appropriate investigations to avoid or mitigate the well-known shortened bearing life under inverter operation. [ABSTRACT FROM AUTHOR]

As compute demands and their large energy costs continue to rise in the datacenter, it is imperative to identify low energy compute solutions. One approach is to investigate device schemes based on collective mode phenomena arising at low temperatures where power dissipation and switching voltage can be extremely low. Additionally, such devices could also have applications in quantum computing where such a low power logic/memory scheme could possibly be integrated onto the qubit plane, which is typically at ∼10 mK temperature. Josephson Junction Field Effect Transistor (JJFET) is a promising candidate for low power operation with zero voltage drop across its drain-source terminals while operating in the superconducting regime. JJFET logic gate is typically realized as a n-type JJFET connected in a common-source configuration with a current source load. This results in high noise margin for logic-1 input but not for both logic-1 and 0. In this paper, we propose an overdamped region, n-type JJFET based digital logic using a cascaded common-source configuration-based design yielding high noise margin for both the logic inputs. DC noise margin sensitivity analysis is performed for the bias current and threshold voltage modulation. The noise margin can be further improved (approaches ideal inverter case) by cascading multiple common source stages yielding higher inverter gain. [ABSTRACT FROM AUTHOR]

Maintaining balanced voltages across distribution networks is becoming more challenging with increasing deployment of single-phase distributed generation and larger single-phase loads. The paper develops a reactive power compensation strategy that uses distributed solar photovoltaic (PV) inverters to mitigate such voltage unbalance. The proposed strategy takes advantage of Steinmetz design and is implemented via both decentralized and distributed control. The latter coordinates PV inverters through a communication network. We demonstrate the performance of the controllers on the IEEE 13-node feeder and a much larger taxonomy feeder (617 nodes and 1196 triplex nodes), and consider different connections of loads and PV systems. Simulation results demonstrate the trade-offs between the controllers. It is observed that the distributed controller achieves greater voltage unbalance reduction than the decentralized controller, but requires communication infrastructure. [ABSTRACT FROM AUTHOR]

The connection of distributed energy resources (DERs) to distribution feeders can significantly increase the operational flexibility of system operators. Local feedback control schemes (such as Volt/VAr droop curves) are a cheap, scalable, and communication-free solution to control DERs in active distribution grids. However, these controllers can interfere detrimentally with each other when they act on multiple DERs connected to the same grid. We show that even the standardized curves recommended in the most recent grid codes may exhibit an unstable behavior. In this paper, we investigate the stability of local incremental DER control laws in three-phase active distribution grids with balanced, and unbalanced loading, and we bound the resulting rate of convergence. The use of low-pass filters on the DER set-points allows us to achieve closed-loop stability even for high-gain local control laws that would otherwise destabilize the grid. This feature is particularly relevant in data-driven approaches that yield optimal DER local control schemes, often in the form of steep customized piece-wise linear Volt/VAr curves. [ABSTRACT FROM AUTHOR]

Interharmonic is an emerging issue in photovoltaic (PV) systems. It has been revealed in previous studies that the maximum power point tracking (MPPT) control is one potential source for interharmonics. In cascaded H-bridge (CHB) PV inverters, if the MPPT perturbations of the CHB cells are in-phase, the sum of the voltages of all CHB cells will oscillate with a higher amplitude, leading to an amplification of interharmonics delivered to the grid. To address this issue, a phase-shifting MPPT (PS-MPPT) method for interharmonic mitigation is proposed in this paper. By properly shifting the phase-angle of the MPPT perturbation of each CHB cell, the oscillation of the equivalent total dc voltage can be effectively mitigated, and in turn, the interharmonics can be suppressed to a large extent. The proposed PS-MPPT scheme has been demonstrated on CHB PV inverters through simulations and experimens. The implementation for generalized n-cell CHB PV inverters has further been discussed. Moreover, a hybrid PS-MPPT method is also developed to improve the interharmonic mitigation performance. Simulation and experimental results have validated the effectiveness of the proposed method in terms of interharmonic suppression. [ABSTRACT FROM AUTHOR]

The goal of the paper is to develop a hybrid modulation technique to combine space vector modulation (SVM) scheme and synchronous optimal pulsewidth modulation (SOPWM) technique for current source inverter (CSI). SVM provides better dynamic performance, but is unable to produce less total harmonic distortion (THD) on inverter current at low device switching frequency ($< $ 1 kHz). SOPWM provides less THD even at low device switching frequency, however it needs a conversion technique to operate CSI and it needs an additional control technique to improve dynamic performance. Hence to overcome these limitations, this paper develops a single modulation technique called optimal SVM technique that can achieve less THD as well as better dynamic performance. To develop optimal SVM technique, three methods have been proposed in this paper. These three methods do not require an additional conversion technique to operate CSI and they provide less THD even under low device switching frequency of 450 Hz. Experimental results prove that the proposed modulation technique can provide better steady-state performance as well as dynamic performance. [ABSTRACT FROM AUTHOR]

The instantaneous output power of the two-stage single-phase grid-connected photovoltaic (PV) inverter pulsates at twice the line frequency ($2f_{{\text{o}}}$), generating second harmonic current (SHC) in the front-end dc–dc converter and PV panel, which will affect the maximum power point tracking operation and deteriorate the overall conversion efficiency. The generating mechanism of the SHC is analyzed in this paper and it is pointed that in order to eliminate the SHC in the front-end dc–dc converter and PV panel, the voltage loop gain of the front-end dc–dc converter should be high enough at $2f_{{\text{o}}}$. Since there is a −180° phase abrupt at the resonant frequency of the input side filter capacitor and the inductor, the system may be unstable. To cope with this problem, the inductor current feedback active-damping scheme (ADS) is adopted. For further improving the loop gain at $2f_{{\text{o}}}$ , proportional–integral–resonant regulator with ADS (PIR + ADS) is proposed in this paper. Besides, a step-by-step closed-loop parameters design method is presented. Finally, a 3-kW two-stage single-phase grid-connected PV inverter has been fabricated and tested, and the experimental results verify the feasibility of the proposed control schemes. [ABSTRACT FROM AUTHOR]

The photovoltaic (PV) output voltage varies over a wide range depending on operating conditions. Thus, the PV-connected converters should be capable of handling a wide input voltage range while maintaining high efficiencies. This paper proposes a new series resonant dc–dc converter for PV microinverter applications. Compared with the conventional series resonant converter, a dual-mode rectifier is configured on the secondary side, which enables a twofold voltage gain range for the proposed converter with a fixed-frequency phase-shift modulation scheme. The zero-voltage switching turn-on and zero-current switching turn-off can be achieved for active switches and diodes, thereby, minimizing the switching losses. Moreover, a variable dc-link voltage control scheme is introduced to the proposed converter, leading to a further efficiency improvement and input-voltage-range extension. The operation principle and essential characteristics (e.g., voltage gain, soft-switching, and root-mean-square current) of the proposed converter are detailed in this paper, and the power loss modeling and design optimization of components are also presented. A 1-MHz 250-W converter prototype with an input voltage range of 17–43 V is built and tested to verify the feasibility of the proposed converter. [ABSTRACT FROM AUTHOR]

This paper proposes a new single-current-sensor (SCS) control method for a permanent-magnet synchronous motor (PMSM) driven by a quasi-Z-source inverter (qZSI). In this method, the sum of two branch currents in the inverter is sampled by an SCS twice for one control period, under actions of two different shoot-through voltage vectors. Then, the three-phase currents of the motor can be reconstructed by these two sample values and can be later used for the current closed-loop control of the PMSM. In this paper, the feasibility of the SCS control method is analyzed thoroughly and a corresponding modulation strategy is given for the qZSI. Compared with the traditional SCS control methods, the new method fully eliminates the measurement dead-zone problem without any additional compensation strategy. The operation range of the PMSM extends and utilization rate of the input voltage improves. In conclusion, the proposed method is easy and convenient to implement, which makes itself a promising application in electrical vehicles. [ABSTRACT FROM AUTHOR]

Motors and inverters are increasingly being used in vehicles. As more vehicle components are being adopted, requirement for safety and installation space are increasing. Growing attention is being paid to the use of six-phase permanent magnet synchronous motors (PMSMs) and their parallel control by two inverters, because of their reduced size and enhanced safety. This paper proposes a pulsewidth modulation (PWM) control algorithm for a six-phase PMSM. In low-voltage vehicle components such as electric power steering, the current is usually acquired with resistors placed under the low-side switching devices. In the proposed control algorithm, the neutral voltage of the two inverters is shifted to either a lower or higher voltage within a certain range to allow for the signal conditioning time delay after the switching device turns on or off, and the time taken for the phase current to rise or settle. The paper proposes a novel PWM control algorithm that ensures that the on-time of the low-side switching devices satisfies the restriction on current sensing with resistors and reduces the current in the capacitor at the same time. The work contributes to reducing the current in a practical configuration involving low-voltage vehicle components. [ABSTRACT FROM AUTHOR]

This paper proposes a new Vernier reluctance machine (VRM) drive for electric vehicle. The key is to incorporate both stator-slot permanent magnets (PMs) and zero-sequence current as the excitation source to establish a parallel magnetic field and interact with armature winding. In this way, two different machines, namely, slot-PM-excited VRM and dc-excited doubly salient machine, are artificially integrated into a single structure, which contributes to enhance the torque density. Moreover, flexible flux control can be realized by regulating zero-sequence current. In this paper, the machine structure and operation principle are introduced, with its feasibility verified by both finite-element analysis and prototype experiments. [ABSTRACT FROM AUTHOR]

This paper presents the design of a multi-MHz inductive power transfer (IPT) system showcasing lightweight and energy-efficient solutions for non-radiative wireless power transfer. A proof of concept is developed by powering a drone without a battery that can hover freely in proximity to an IPT transmitter. The most challenging aspect, addressed here for the first time, is the complete system-level design to efficiently provide uninterrupted power flow while allowing for variable power demand and highly variable coupling factor. The proposed solution includes the design of lightweight air-core coils that can achieve sufficient coupling without degrading the aerodynamics of the drone, and the design of newly developed resonant power converters at both ends of the system. At the transmitting-end, a load-independent Class EF inverter, which can drive a transmitting-coil with constant current amplitude and achieves zero-voltage switching for the entire range of operation, was developed; and at the receiving-end, a hybrid Class E rectifier, which allows tuning for large changes in coupling and power demand, was used. For the demo, the range of motion of the drone was limited by a 7.5 cm nylon string tether, connected between the center of the transmitting-coil and the bottom of the drone. The design of the IPT system, including all the power conversion stages and the IPT link, is explained in detail. The results on performance and specific practical considerations required for the physical implementation are provided. An average end-to-end efficiency of 60% was achieved for a coupling range of 23%–5.8%. Relevant simulations concerning human exposure to electromagnetic fields are also included to assure that the demo is safe, according to the relevant guidelines. This paper is accompanied by a video featuring the proposed IPT system [ABSTRACT FROM AUTHOR]

This paper proposes a novel method to determine and control active and reactive power output of the virtual ocillator control-based inverter in grid-connected mode. An analytical expression is introduced to describe the quasi-steady-state relationship between the parameters of the inverter including its controllers, and the power injection and voltage at the inverter terminals. We introduce a new, complex valued parameter (K) to the conventional virtual oscillator control (VOC) scheme in order to control the inverter output power. We compare analytic and numeric solutions of the above analytical expression with time-domain simulation results, using different K values and line impedances to validate the analytical expression. In the second part of the paper, we design a controller: we use the above expression to derive K for given reference powers and use adaptive integrators to compensate for the remaining errors. We compare this control scheme to a method that uses PI controllers, and also to a simple decoupling method for independent control of active and reactive power. The controller based on the analytical method exhibits very good dynamic and decoupling performance. A single VOC-controlled inverter connected to a nonstiff grid is simulated in MATLAB/Simulink for demonstration, and is also validated using real-time simulations. [ABSTRACT FROM AUTHOR]

In this paper, a generalized droop control (GDC) is proposed for a grid-supporting inverter based on a comparison between traditional droop control and virtual synchronous generator (VSG) control. Both the traditional droop control and VSG control have their own advantages, but neither traditional droop control nor VSG control can meet the demand for different dynamic characteristics in grid-connected (GC) and stand-alone (SA) modes at the same time. Rather than using a proportional controller with a low-pass filter, as in a traditional droop control, or fully mimicking the conventional synchronous generator parameters in a VSG control, the active power control loop of the GDC can be designed flexibly to adapt to different requirements. With a well-designed controller, the GDC can achieve satisfactory control performance; unlike a traditional droop control, it can provide virtual inertia and damping properties in SA mode; unlike a VSG control, the output active power of an inverter with GDC can follow changing references quickly and accurately, without large overshoot or oscillation in the GC mode. Moreover, given specific controller parameters, the GDC can function as both a traditional droop control and a VSG control. The GDC's controller parameter design is more intuitive and flexible, and this paper provides a distinct design process. Finally, the effectiveness of the proposed control method is validated by the simulation and experimental results. [ABSTRACT FROM AUTHOR]

Phase-to-neutral voltage or neutral-to-virtual neutral voltage zero-crossing points (ZCPs) detection method is usually used for sensorless brushless dc motor commutation control. Unfortunately, neither of them can be realized in lower speed range. In this paper, a simple commutation point detection method is proposed based on detecting inactive phase terminal to dc-link midpoint voltage. It eliminates the requirement of neutral wire or virtual neutral voltage and provides an amplified version of back electromotive force at the ZCPs which makes the lower speed range detection possible. As the speed increases, commutation point error is enlarged due to the low-pass filter. Utilizing the symmetry of the terminal to midpoint voltage, the phase error can be corrected. However, due to the nonlinear relationship between the detected voltage difference and phase error, it is difficult to regulate the error fast and robustly. Therefore, a novel phase regulator based on fuzzy neural network is proposed in this paper with simple structure and learning ability. The validity of the proposed ZCPs detection method and commutation instant shift correction method are verified through experimental results. [ABSTRACT FROM AUTHOR]

The huge untapped energy from ocean waves is one of the ideal solutions for world energy crisis. This paper proposes a linear-generator-based direct power conversion plant model to generate the electrical energy from ocean waves. The linear generator output is of variable magnitude and variable frequency in nature. In this paper, this variable output is converted to fixed dc voltage by using a diode bridge rectifier and a dc-link capacitor. The linear generator along with a diode bridge and a capacitor forms a wave power unit. These wave power units can be connected in series and parallel to meet the power ratings. A reliable multilevel inverter is fed from the wave power units to ensure uninterrupted tapping of power from this abundant renewable energy source. The reliable inverter operation is analyzed under open- and short-circuit faults in switches by reconfiguring the carrier waves of sine pulsewidth modulation during fault conditions. A fault detection algorithm using a single sensor at the output is also proposed to detect the faults in the switches of the proposed reliable inverter. The proposed plant model with a linear generator and a reliable inverter is simulated using ANSYS MAXWELL and MATLAB/Simulink. The plant model is tested using a programmable ac source for emulating the linear generator and the developed prototype of the diode bridge rectifier and the reliable multilevel inverter. [ABSTRACT FROM AUTHOR]

This paper introduces a compensation strategy to deal with both current and voltage measurement errors in the three-phase pulsewidth modulated rectifier system. The dc offset and scaling errors in the voltage and current measurements cause the injection of undesired dc and unbalanced currents into the three-phase input current and subsequently lead to voltage ripple at the dc output voltage. This paper proposes a compensation scheme for current measurement error where the dc offset and scaling errors in the current measurement are estimated from the characteristic of the dc output voltage ripple combining with simple band-pass and low-pass filters. Meanwhile, an advanced current controller designed with a proportional integral plus two resonant controllers tuned at the fundamental grid frequency $({{\omega _s}})$ and $2{\omega _s}$ in the synchronous (d–q) reference frame is suggested to reject the impact of the dc offset and scaling errors in the voltage measurement. The proposed compensation method is developed without the need of extra hardware circuit, sensor, or precise information of system parameters so that it can be considered as more cost-effective and robust solution. The effectiveness of the proposed solution is verified by experimental results. [ABSTRACT FROM AUTHOR]

Electric grid codes are expected to change in near future to accommodate an increased number of distributed generation units in the distribution power system without impairing its power quality. It is desired that the generators remain connected during voltage sags and provide ancillary services, such as voltage and reactive power control, ensuring the operational stability of the power system. This paper explores how the existing strategies for active and reactive power injection impact the operation of grid-tied inverters in terms of required power, current flowing, and reduction of active power delivery during the voltage sags. Such inputs are relevant to properly size converters for operation under fault events. In addition, this paper contributes to devise: 1) constant peak current control, 2) constant active current control, and 3) constant average active power control strategies for three-phase four-wire grid-tied inverters considering the natural (abc) reference frame. The design and implementation of the investigated power injection strategies are discussed, and their effectiveness and technical viability are analyzed through dynamic computational simulations under symmetrical/asymmetrical voltage sags, and variation of the short-circuit ratio. [ABSTRACT FROM AUTHOR]