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This study investigates the thermal performance of various counterflow air-to-air heat exchangers under unbalanced flow conditions, aiming to enhance the efficiency of heat recovery systems. Mechanical ventilation with heat recovery is critical in energy-efficient buildings to reduce heat loss, which can reach up to 60% in air exchange processes. This research focuses on the effects of flow imbalance on the heat transfer efficiency of three specific heat exchangers: two commercially available models (Recair Sensitive RS160 and Core ERV366) and a custom 3D-printed prototype (GV PROTO). Experimental tests measured temperature efficiency under both balanced and unbalanced flow conditions, with results indicating that flow imbalance significantly impacts thermal efficiency. Among the exchangers, the RS160 displayed the highest temperature efficiency, maintaining performance better than the others as flow rates increased. The results of the study show that even small differences in the thermal efficiency of different heat exchangers under balanced airflow conditions transform into significant differences under unbalanced conditions. These findings contribute to a better understanding of how real-world ventilation imbalances affect heat exchanger performance, offering insights to optimize energy efficiency in ventilation systems. [ABSTRACT FROM AUTHOR]

In grid-connected photovoltaic (PV) systems, power quality and voltage control are necessary, particularly under unbalanced grid conditions. These conditions frequently lead to double-line frequency power oscillations, which worsen Direct Current (DC)-link voltage ripples and stress DC-link capacitors. The well-known dq frame vector control technique, which is effective under normal conditions, struggles with oscillatory component management in unbalanced grid conditions. To address this issue, this paper presents an advanced control approach designed for grid-connected PV inverters. The proposed approach is effective at reducing oscillations in the DC-link voltage at double the grid frequency, thereby enhancing system stability and component longevity. This method introduces a feedback control method designed to regulate oscillatory components that appeared within the dq frame and suppress the DC-link voltage oscillations under imbalance conditions, including single line-to-ground (SLG) faults. Additionally, the control scheme incorporates a maximum power point tracking (MPPT) controller to optimize PV efficiency. Comprehensive simulations demonstrate the effectiveness of this method in maintaining sinusoidal current injections and stabilizing DC-link voltage during unbalanced grid conditions. Simulation results show that the control scheme effectively stabilizes DC-link voltage, maintains balanced grid current, and ensures constant active power under various conditions, including SLG faults and solar irradiance changes. [ABSTRACT FROM AUTHOR]

This study focuses on a backstepping control (BSC) of a three-phase four-leg pulse width modulation (PWM) rectifier under balanced and unbalanced load and grid conditions. The proposed BSC for the three-phase four-leg PWM rectifier (3P-4LR) connected to the grid is build up using three independent backstepping controllers based on Lyabonov theory to control simultaneously DC voltage and input currents. As a result, unit power factor, stable DC-bus voltage, sinusoidal input 3P-4LR currents with a lower harmonic content and lower zero-sequence and natural currents can be accurately achieved. Not only that, robustness against voltage grid fluctuations, load and filter inductance variations can also be carried out. The effectiveness, superiority, good dynamic, and steady-state performances of the proposed BSC based 3P-4LR have been validated by numerical simulations compared to the conventional proportional-integral control under balanced and unbalanced load and grid conditions. [ABSTRACT FROM AUTHOR]

In view of the spatial and temporal imbalance of residents' travel demands and challenges of optimal bus capacity allocation, in this paper the grand station express bus scheduling mode is introduced in the direction of heavy passenger flow during peak hours. Coordinated scheduling combining whole-journey and grand station express buses is adopted, and the station correlation calculation model is used to determine the optimal stops of the grand station express bus. Thus, a two-way bus scheduling optimization model for peak passenger flow is established with the goal of minimizing the total cost of passenger travel and enterprise operation. Finally, the nonlinear inertia weight dynamic cuckoo search algorithm is selected for the model's solution, and the established scheduling optimization model is solved by combining basic data such as the study line's bus Integrated Circuit (IC) card data. The effectiveness of the model is verified through a comparative study and evaluation of the solution. [ABSTRACT FROM AUTHOR]

In this article, a resistance-emulating control strategy without grid voltage sensors is proposed for three-phase voltage source rectifiers under unbalanced grid conditions. It consists of angle estimator, coordinate correction, resistance-emulating, common-mode resistance (CMR) and the differential-mode resistance (DMR) regulation, extended state observer, and other critical steps. In this scheme, the CMR regulates the average power, and the DMR controls the ripple power caused by the unbalanced supplies. Three different control objectives including constant active power, symmetrical input currents, and constant reactive power are achieved by regulating the DMR. Since the grid voltage sensors and the information of the grid-side circuit parameters are not required, the cost is reduced and the robustness and reliability of the scheme is improved. Experiments verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

AC–DC line-commutated converters (LCCs) are widely utilized in existing and emerging ac–dc power systems. Analysis of such systems under balanced and unbalanced conditions is often carried out using electromagnetic transient (EMT) simulation programs. Therein, the detailed switching models of LCCs are often the computational bottlenecks for system-level studies. Recently, the parametric average-value models (PAVMs) have been developed to achieve fast and efficient simulations of switching converters. In this paper, the PAVM methodology is extended to consider operation of LCCs under unbalanced conditions in the ac network. This is done in the extended PAVM by formulating the ac-side harmonics in positive and negative sequences as well as the dc-side harmonics (i.e., ripples) with respect to the ac network imbalance. The new PAVM is validated experimentally and by simulations, and is demonstrated to be accurate in reconstructing the ac and dc waveforms under unbalanced conditions in the ac network. Meanwhile, the proposed PAVM is computationally much faster than its detailed switching model counterpart. [ABSTRACT FROM AUTHOR]

The reliability of supply in distribution networks can be improved by islanding (parts of) the network as autonomous microgrids in case of a contingency. Microgrids are commonly fed by inverter-based distributed energy resources, which causes transient stability challenges due to the lack of inertia and strongly coupled dynamics. The transient stability of inverter-based microgrids is conventionally analyzed by time-domain analysis. However, energy-function based transient stability analysis has advantages over time-domain analysis, as it provides a stability result for all initial conditions and allows uncertainty to be taken into account with the domain of attraction. Microgrids are generally unbalanced, however models for unbalanced energy-function based transient stability analysis are lacking in the literature. In this paper, coupled sequence components models of distributed energy resources and load devices in the $dq$ reference frame are proposed, and experimentally validated with unbalanced voltage transients. The validated models are used for energy-function based transient stability analysis of an unbalanced inverter-based case study microgrid. Finally, the impact of unbalanced connection of devices on transient stability is analyzed. The results indicate that the proposed models accurately represent the behavior of physical devices and that the unbalanced connection of devices significantly impacts the transient stability of inverter-based microgrids. [ABSTRACT FROM AUTHOR]

In the rapidly changing domain of hybrid AC/DC urban distribution networks, this research unveils a groundbreaking method for the restoration of three-phase unbalanced systems by astutely harnessing the unique potential of DC line interconnections. At the heart of this innovation lies the synthesis of symmetrical Second Order Cone Programming (SOCP) with a sophisticated topology search technique, a union that offers a precise depiction of complex three-phase power flow with network restoration while simultaneously accelerating computational processes. Building upon this foundation, our approach places significant emphasis on the utilization of adaptable DC power control, coupled with the optimal deployment of mobile energy storage systems (MESSs), to ensure a harmonized power balance during critical interruptions. These strategies converge to prioritize the restoration of vital loads, especially those with high weighting factors, thereby significantly augmenting the network's resilience, particularly in contexts vulnerable to disasters. The corroborative numerical results, as delineated in our study, highlight the distinct advantage and effectiveness of our methodology over prevailing practices in fortifying grid resilience against serious adversities. • Novel restoration method for hybrid AC/DC unbalanced three-phase networks unveiled. • Synthesis of SOCP and topology search improves computational processes. • Adaptable DC control and mobile energy storage utilization ensure power harmony. • Strategy significantly reduces recovery time, boosting load restoration capabilities. • 75.23% resilience index increase with DC link connections integration. [ABSTRACT FROM AUTHOR]

This article proposes a novel capacitor-voltage balancing strategy for the double-Y modular multilevel converter static synchronous compensator operated under unbalanced current conditions of the system. The proposed method, based on zero-sequence voltage injection, manipulates the current distribution between the converter arms to minimize the converter ratings. The proposed balancing strategy is compared with the classical approach, based on controlling the circulating dc currents. For the same unbalanced conditions, the proposed strategy leads to lower energy rating for the converter as compared with the classical one, whereas the semiconductor ratings remain the same in both strategies. Theoretical findings are verified through experimental results. [ABSTRACT FROM AUTHOR]

This paper presents a general model to carry out a small signal stability analysis in electrical power systems (EPSs) that operate in unbalanced conditions. The classic traditional approach is based on a phase representation where it is supposed that the triphasic electric grid does not suffer any variations during its operation. With the presence of unbalances in transmission lines and loads, it is necessary to develop a general model that allows answering the needs and challenges with which modern electric systems must deal. The present work firstly intends to address the three-phase representation of an EPS, including the controllers. The proposed model is applied to a classical test system, a three machine-nine bus system, considering all the dynamic and algebraic variations associated with angular stability analysis. The proposed approach to small signal stability analysis shows results that differ from the classical analysis. The results are backed up with time domain simulations, and therefore, these results could be used in the calculation of the controllers that operate in unbalanced multimachine systems. [ABSTRACT FROM AUTHOR]

Unbalance or asymmetry in the distribution network is a well-known power quality issue. In the modern active distribution system, with the increasing penetration of renewables, this phenomenon becomes more pronounced. In the context of microgrids (MGs), several works have been proposed for the management and mitigation of the unbalance, for both the sharing of unbalanced load and maintaining the voltage quality in the islanded mode and for the control of distributed generators in the grid-connected mode during unbalanced conditions. This study comprehensively reviews, summarises, and classifies the various strategies of the unbalance mitigation techniques for the islanded and grid-connected modes of operation for three-phase MGs and presents the possible challenges and avenues for future investigations on the topic. [ABSTRACT FROM AUTHOR]

Compared with the well-known modular multilevel cascaded converter with single-star bridge-cells (MMCC-SSBC), the hybrid cascaded converter (HCC) has the advantage of the lower component count of power transistors and dc capacitors. Thereby, this paper explores the HCC in grid applications and proposes a hierarchical control scheme, which allows the HCC to operate under unbalanced grid conditions and power imbalances among all cells. The HCC is further compared with the MMCC-SSBC under these unbalanced conditions to evaluate its performance. The laboratory test results are provided to verify the proposed control scheme and the comparison. The operational flexibility and reliability of the HCC are also discussed. [ABSTRACT FROM AUTHOR]

Parallel photovoltaic inverters system can increase the power rating and efficiency. However, the zero-sequence circulating current (ZSCC) will be generated under unbalanced conditions. Therefore, this paper proposes a ZSCC control scheme for the M-paralleled (M ≥ 3) inverters under unbalanced grid voltage and unbalanced filter parameters. Based on the analysis with the proposed generalized model of parallel inverters, it is observed that the ZSCC of the ith inverter is influenced by the output currents, filter parameters, duty cycles and ZSCCs of the ith and first inverters. To eliminate these adverse effects on the ZSCC control loop, a decoupled controller consisting of modified input type PI-quasi-resonant and the feedforward controller is proposed. The modified input type PI-quasi-resonant controller mitigates the effects of the output currents, filter parameters, and ZSCCs, meanwhile, the feed-forward controller is employed to eliminate the difference of duty cycles. Compared with the conventional PI control method, the ZSCC is effectively suppressed with the proposed scheme, thus improving the quality of the currents. Moreover, the reliability of the paralleled system is improved because such scheme only needs the information of the ith and the first inverters. Simulation and experimental results obtained from three-paralleled inverters validate the effectiveness of theoretical analysis and the proposed method.

In the modern renewable energy system, recent years have seen a rise in the share of power being generated through photovoltaic (PV) plants. In the Danish power system, PV plants are mostly integrated in the medium- and low-voltage networks which are usually operating under unbalanced conditions. Furthermore, the increasing number of power-electronic-based equipment affects the grid during faults through their contribution to the fault current. So far studies of PV plants in unbalanced conditions are based on computational simulations, which have limitations in representing reality. Therefore, this study investigated the performance of a three-phase PV inverter under unbalanced operation and fault conditions. The inverter is tested in stable power system operation and during grid support situations through frequency response and reactive power control. All experiments are carried out using an experimental laboratory platform in PowerLabDK. The key outcomes from this study includes the correlation between positive sequence component of voltage and reactive power, active power and current under unbalanced operation, the frequency response dependence on positive sequence voltage, and the fault current contribution from PV inverter during different fault conditions.

The presence of distributed generators (DGs) based on renewable energy is a fact in the electrical grid. However, DGs based on renewable resources such as photovoltaic panels and storage systems lack inertia, which is used by synchronous generators to compensate oscillations in the electrical grid. Thus, virtual inertia is introduced via a virtual synchronous generator (VSG) scheme. Although VSG is widely used, its robustness cannot be ensured since it employs proportional–integral–derivative‐type (PID‐type) controllers, which are sensitive to parameters variations. Furthermore, PID‐type controllers are designed assuming balance conditions and the negative sequence components produced during unbalanced conditions are not considered. This paper proposes a robust VSG topology working under unbalanced conditions. A sliding mode control (SMC) algorithm named super‐twisting (ST) is integrated into the control loop of VSG providing insensitivity to matched disturbances/uncertainties and finite‐time convergence. Since the design of the ST algorithm considers the presence of negative‐sequence components, it is not necessary a modification of the control loop before, during, or after the fault. The method to compute the ST control gains and the stability test using Lyapunov are provided. The VSG scheme is tested via simulations, where voltage sags are applied to generate the unbalanced conditions. [ABSTRACT FROM AUTHOR]

When voltage source converters (VSCs) are subjected to disturbances or faults, they may not be able to perform as per the desired control objective, which presents a significant challenge to system operations. One well-known situation is when the converter is subjected to an unbalanced ac voltage with a negative-sequence fundamental voltage injection. Under this condition, the VSC may generate undesired double-frequency ripples at the dc voltage and uncharacteristic harmonics at the ac current. This paper presents a new two-stage control method suitable for VSCs used in high-power applications. In the first stage, a simple proportional controller is used to suppress overshoots during the transients. Then, the iterative controller is activated to finely adjust the modulation indices and firing angles of the converter to compensate the uncharacteristic harmonics. To verify the performance of the proposed control method, the control algorithms are implemented in a 10 MVA VSC system and validated by means of PSCAD/EMTDC and hardware-in-the-loop simulations. The results show that the state variables converge within a few iterative steps, and the transient response is sufficiently fast. The proposed method is shown to perform better than conventional methods for attenuating and eliminating the uncharacteristic dc and ac harmonics caused by imbalances in the grid voltage. [ABSTRACT FROM AUTHOR]

Distribution system possesses high resistance to reactance ratio and unbalanced load profile. Introduction of power electronic devices such as solar photovoltaic (PV) inverter in the distribution system leads to power imbalance and unregulated voltage profile at the point of common coupling (PCC) because these devices having low‐voltage ride through feature remain connected to grid during fault and inject balanced current to unbalanced fault. Also, high PV integration may increase the voltage at PCC beyond its desired limit. To overcome such unbalanced conditions and to maintain voltage at PCC, a positive, negative and zero sequence‐based current controller with reactive power compensation is proposed in this work. The sequence controller controls the sequence currents to their reference command. DC‐link voltage regulator, reactive power compensator and PCC voltage regulator decide the references for sequence currents. The proposed reactive power compensator uses both positive sequence voltage at PCC and reactive power supplied by the filter capacitor to generate the reactive power reference value. Feedforward and feedback actions are involved in the controller to produce switching pulses for inverter. The proposed method is tested in OPAL‐RT for real‐time validation using IEEE 13 bus distribution system model and found to be accurate. [ABSTRACT FROM AUTHOR]

Highlights • Power control strategies for PV systems during unbalanced conditions are compared. • Energy storage requirements of the converter during unbalanced conditions are analytically computed. • Dc-link capacitors thermal stress is evaluated during reactive power injection in voltage sags. Abstract Unbalanced voltage sags are common phenomenon in power systems which affects the performance of grid-connected photovoltaic (PV) systems. The unbalanced voltage conditions results in double-line frequency power oscillations. These oscillations increase the dc-link voltage ripple leading to higher stresses in dc-link capacitors. In this case, the literature presents many power control strategies in order to improve the performance of PV systems during unbalanced conditions. Nevertheless, a deep comparison of these strategies has been missing. This work presents a complete analysis of the main power control strategies proposed in the literature. The energy storage requirements of the converter are analytically deduced and some guidelines for the dc-link design are presented. Furthermore, a case study based on a 11 kW PV system is used to compare the performance of the most interesting strategies in terms of dynamic behaviour, power oscillations and dc-link voltage ripple. Furthermore, the dc-link capacitors stress is discussed, considering the characteristics of reactive power injection, which is required by the modern grid codes. The power losses and the thermal stresses in dc-link capacitors are evaluated and the worst operational scenario is obtained for each power control strategy. [ABSTRACT FROM AUTHOR]

Shunt active power filters (APFs) are used to address power quality issues such as harmonic distortion and power system imbalance. Many control systems for shunt APFs demand measurements of voltages and currents for the determination of instantaneous real and reactive power signals. From these signals, the reference currents of the shunt APF are calculated. A method of control for the APF shunt that only measures currents is proposed in this paper. The control method includes a modified technique to extract the positive and negative sequence components in the time domain. The reference currents in the shunt APF are calculated by the negative sequence components of the measured currents. Simulation and experimental results confirm the efficiency of this control scheme under ideal and non-ideal conditions.

This paper presents a general model to carry out a small signal stability analysis in electrical power systems (EPSs) that operate in unbalanced conditions. The classic traditional approach is based on a phase representation where it is supposed that the triphasic electric grid does not suffer any variations during its operation. With the presence of unbalances in transmission lines and loads, it is necessary to develop a general model that allows answering the needs and challenges with which modern electric systems must deal. The present work firstly intends to address the three-phase representation of an EPS, including the controllers. The proposed model is applied to a classical test system, a three machine-nine bus system, considering all the dynamic and algebraic variations associated with angular stability analysis. The proposed approach to small signal stability analysis shows results that differ from the classical analysis. The results are backed up with time domain simulations, and therefore, these results could be used in the calculation of the controllers that operate in unbalanced multimachine systems.

Voltage sags can be symmetrical or unsymmetrical depending on the causes of the sag. At the present time, one of the most common procedures for mitigating voltage sags is by the use of dynamic voltage restorers (DVRs). By definition, a DVR is a controlled voltage source inserted between the network and a sensitive load through a booster transformer injecting voltage into the network in order to correct any disturbance affecting a sensitive load voltage. In this paper, modelling of DVR for voltage correction using MatLab software is presented. The performance of the device under different voltage sag types is described, where the voltage sag types are introduced using the different types of short-circuit faults included in the environment of the MatLab/Simulink package. The robustness of the proposed device is evaluated using the common voltage sag indices, while taking into account voltage and current unbalance percentages, where maintaining the total harmonic distortion percentage of the load voltage within a specified range is desired. Finally, several simulation results are shown in order to highlight that the DVR is capable of effective correction of the voltage sag while minimizing the grid voltage unbalance and distortion, regardless of the fault type.

This paper proposes a generic steady-state modeling and power-flow analysis approach for droop- and isochronously controlled microgrids. The proposed framework adopts symmetrical sequence component models, rather than phase-coordinate models, of microgrid elements. Such approach immensely reduces the power-flow execution time, as it breaks down the system model into independent equation sets with considerably reduced sizes. To render the proposed approach practical and generic, it integrates different types and control schemes of distributed generation (DG), including synchronous generator-based DG (SGDG) and electronically interfaced DG units. Furthermore, it incorporates unbalanced loads and feeders, transformer connections, different load characteristics, and configurations, as well as microgrid droop features. A novel power-flow algorithm based on a modified Newton–Raphson method is proposed to solve for the microgrid steady-state voltage magnitudes, angles, and frequency. The accuracy of the models and algorithm is verified through comparison with detailed time-domain simulations in MATALB/Simulink. Additionally, the proposed approach is shown to outperform the reported Newton-Trust Region approach in generality, accuracy, and performance. Two case studies, incorporating IEEE 123-node test microgrid, are further performed to examine the effectiveness of the proposed approach in solving complicated droop-controlled microgrids, and to examine the behavior of droop-controlled DGs in isochronous microgrids. [ABSTRACT FROM AUTHOR]

Microgrid-interactive converters are increasingly expected to provide auxiliary services, particularly during islanded microgrid operation. Unbalanced, non-sinusoidal pulse width modulation (PWM) references may be required when converters provide auxiliary services, such as negative-sequence compensation. Since PWM references cannot exceed dc-bus voltage defined limits without causing overmodulation, the maximum positive-sequence voltage producible by microgrid-interactive converters may be diminished unless reference waveforms are adjusted. In this paper, a technique for maximizing the linear modulation region by injecting a common mode component into PWM references is proposed for microgrid-interactive cascaded H-bridge (CHB) converters. The technique presented can be implemented in real-time without lookup tables, is controller independent, is compatible with any carrier-based PWM method, enabling its use for symmetric and asymmetric CHB, and enables continued operation after internal fault events when utilized with fault tolerant PWM schemes. Experimental data verify the common mode injection technique. [ABSTRACT FROM PUBLISHER]

Wind turbine generating systems (WTGSs), which are conventionally connected to high voltage transmission networks, have frequently been employed as distributed generation units in today’s distribution networks. In practice, the distribution networks always have unbalanced bus voltages and line currents due to uneven distribution of single or double phase loads over three phases and asymmetry of the lines, etc. Accordingly, in this study, for the load flow analysis of the distribution networks, Conventional Fixed speed Induction Generator (CFIG) based WTGS, one of the most widely used WTGS types, is modelled under unbalanced voltage conditions. The Developed model has active and reactive power expressions in terms of induction machine impedance parameters, terminal voltages and input power. The validity of the Developed model is confirmed with the experimental results obtained in a test system. The results of the slip calculation based phase-domain model (SCP Model), which was previously proposed in the literature for CFIG based WTGSs under unbalanced voltages, are also given for the comparison. Finally, the Developed model and the SCP model are implemented in the load flow analysis of the IEEE 34 bus test system with the CFIG based WTGSs and unbalanced loads. Thus, it is clearly pointed out that the results of the load flow analysis implemented with both models are very close to each other, and the Developed model is computationally more efficient than the SCP model. [ABSTRACT FROM AUTHOR]

This paper provides a novel power flow algorithm as a viable tool to accurately analyze the steady-state behavior of islanded hybrid ac/dc microgrids. The operational philosophy of these networks is sophisticated since both the ac frequency and dc voltage are variable, coupled, and sensitive to the control schemes of the distributed generators (DGs) and interlinking converters (ICs). In order to render the analysis generic, mathematical models are developed for control schemes of the DGs and ICs under unbalanced conditions. The presented models are then incorporated into an optimized power flow formulation and the solution is found using the globally convergent Newton-Trust Region (NTR) method. The algorithm accuracy is verified using detailed time-domain simulations in PSCAD/EMTDC environment. The effectiveness of the introduced power flow algorithm is illustrated via two case studies. The first case investigates parallel operation of droop-controlled ICs and highlights the effects of IC settings on load sharing among subgrids and power-transfer sharing through installed ICs. The second case reveals the significance of analyzing the loading condition for each phase of the DGs rather than for the total three-phase power generation, which is commonly followed in steady-state analysis in the reported literature. [ABSTRACT FROM AUTHOR]

This paper presents a steady-state model of MMC for the second-order phase voltage ripple prediction under unbalanced conditions, taking the impact of negative-sequence current control into account. From the steady-state model, a circular relationship is found among current and voltage quantities, which can be used to evaluate the magnitudes and initial phase angles of different circulating current components. Moreover, in order to calculate the circulating current in a point-to-point MMC-based HVdc system under unbalanced grid conditions, the derivation of equivalent dc impedance of an MMC is discussed as well. According to the dc impedance model, an MMC inverter can be represented as a series connected R–L–C branch, with its equivalent resistance and capacitance directly related to the circulating current control parameters. Experimental results from a scaled-down three-phase MMC system under an emulated single-line-to-ground fault are provided to support the theoretical analysis and derived model. This new models provides an insight into the impact of different control schemes on the fault characteristics and improves the understanding of the operation of MMC under unbalanced conditions. [ABSTRACT FROM PUBLISHER]

Control structures have critical influences on converter-interfaced distributed generations (DG) under unbalanced conditions. Most of previous works focus on suppressing active power oscillations and ripples of DC bus voltage. In this paper, the relationship between amplitudes of the active power oscillations and the reactive power oscillations are firstly deduced and the hierarchical control of DG is proposed to reduce power oscillations. The hierarchical control consists of primary and secondary levels. Current references are generated in primary control level and the active power oscillations can be suppressed by a dual current controller. Secondary control reduces the active power and reactive power oscillations simultaneously by optimal model aiming for minimum amplitudes of oscillations. Simulation results show that the proposed secondary control with less injecting negative-sequence current than traditional control methods can effectively limit both active power and reactive power oscillations. [ABSTRACT FROM AUTHOR]

Wind turbine generating systems (WTGSs), which are conventionally connected to high voltage transmission networks, have frequently been employed as distributed generation units in today’s distribution networks. In practice, the distribution networks always have unbalanced bus voltages and line currents due to uneven distribution of single or double phase loads over three phases and asymmetry of the lines, etc. Accordingly, in this study, for the load flow analysis of the distribution networks, Conventional Fixed speed Induction Generator (CFIG) based WTGS, one of the most widely used WTGS types, is modelled under unbalanced voltage conditions. The Developed model has active and reactive power expressions in terms of induction machine impedance parameters, terminal voltages and input power. The validity of the Developed model is confirmed with the experimental results obtained in a test system. The results of the slip calculation based phase-domain model (SCP Model), which was previously proposed in the literature for CFIG based WTGSs under unbalanced voltages, are also given for the comparison. Finally, the Developed model and the SCP model are implemented in the load flow analysis of the IEEE 34 bus test system with the CFIG based WTGSs and unbalanced loads. Thus, it is clearly pointed out that the results of the load flow analysis implemented with both models are very close to each other, and the Developed model is computationally more efficient than the SCP model.

The aim of this paper is to investigate the ability of cascaded H-bridge STATic COMpensator in star and delta configuration to exchange negative-sequence current with the connecting grid. Zero-sequence voltage for the star and zero-sequence current for the delta configuration are utilized to guarantee dc-capacitor voltage balancing. A general solution for the required zero-sequence voltage/current for any kind of unbalanced condition is provided. The obtained solutions show that a singularity for both configurations exists under specific conditions, leading to infinite zero-sequence requirement. In case of star configuration, this singularity occurs when the amplitude of the positive- and negative-sequence components of the current output of the converter are equal. Analogously, the singularity for the delta configuration occurs when the amplitude of the positive- and negative-sequence components of the voltage at the converter terminals are equal. This singularity impacts the ratings of the system and imposes a limitation in the utilization of the compensator for unbalance compensation purposes, both in industrial and utility applications. Experimental results are presented to validate the theoretical analysis. [ABSTRACT FROM PUBLISHER]

An increasing number of single-phase loads and renewable energy resources (RESs), such as single-phase rooftop PV units, are unevenly distributed in low voltage (LV) distribution networks. This exacerbates unbalanced conditions in the network which in turn adversely affects the power quality, stability and reliability. This paper presents a comprehensive review of the state-of-the-art techniques for alleviation of the unbalanced conditions in both three-wire and four-wire LV three-phase distribution networks. Technical issues arising from the unbalanced conditions are highlighted. Several definitions of the unbalance indicators and standards that are widely used for unbalance analysis are also given. Furthermore, recent unbalance compensation techniques based on three devices; namely, power electronic converters, electric vehicles (EVs) and energy storage systems (ESSs) are critically reviewed. It is found that the power electronic converters with suitable control strategies and management of EV chargers and plug-in electric vehicles (PEVs) have been intensively studied so far. However, based on the literature survey, there is currently no work on applying cutting-edge methods, such as the distributed cooperative control strategy and multi-agent deep reinforcement learning (MADRL), to utilize distributed single-phase ESSs to mitigate the unbalanced conditions. Therefore, key challenges, possible unbalance mitigation techniques and future research directions are recommended and discussed. [Display omitted] • Techniques for alleviating unbalanced conditions in LV systems are reviewed. • Three devices with recent techniques for compensation are presented. • Indicators and standards for analysis of unbalanced conditions are provided. • 1-phase ESSs is recommended with new strategies for future research directions. [ABSTRACT FROM AUTHOR]

Voltage sags can be symmetrical or unsymmetrical depending on the causes of the sag. At the present time, one of the most common procedures for mitigating voltage sags is by the use of dynamic voltage restorers (DVRs). By definition, a DVR is a controlled voltage source inserted between the network and a sensitive load through a booster transformer injecting voltage into the network in order to correct any disturbance affecting a sensitive load voltage. In this paper, modelling of DVR for voltage correction using MatLab software is presented. The performance of the device under different voltage sag types is described, where the voltage sag types are introduced using the different types of short-circuit faults included in the environment of the MatLab/Simulink package. The robustness of the proposed device is evaluated using the common voltage sag indices, while taking into account voltage and current unbalance percentages, where maintaining the total harmonic distortion percentage of the load voltage within a specified range is desired. Finally, several simulation results are shown in order to highlight that the DVR is capable of effective correction of the voltage sag while minimizing the grid voltage unbalance and distortion, regardless of the fault type. [ABSTRACT FROM AUTHOR]

Active distribution networks (ADNs) are capable of mitigating phase imbalance caused by various operational conditions, including uneven growth of single-phase and intermittent distributed energy resources (DERs), incurring financial losses or costly infrastructure reinforcements. In this paper, the research gap for a flexible phase imbalance mitigating solution is addressed by proposing a multi-terminal phase-changing soft open point (PC-SOP). It is explored in detail on balancing the power flows and compared with other different types and ways of connection (including two-terminal and conventional). Then operational strategies based on different cases are presented for imbalance mitigation. Semidefinite programming (SDP) relaxation is utilized to convert the original non-convex nonlinear model into an SDP model which can be solved efficiently by commercial solvers. Two case studies demonstrations are conducted on IEEE 13-node and 123-node three-phase networks. It is found that multi-terminal PC-SOPs can minimize power losses by between 5.56 % and 28.98% and have better voltage control (all buses operate in the allowed voltage range [0.94, 1.10]) and less PV curtailment (reduced by at least 6.31 MW/24 h and 0.63 MW/24 h for the two test networks separately) when compared to conventional SOP technologies. • Propose a multi-terminal phase-changing soft open point to reduce phase imbalance. • Symmetrical semidefinite programming solves optimal power flow efficiently. • Power losses are minimized by between 5.56 % and 28.98%. • Achieve effective voltage regulation in the allowed voltage range [0.94, 1.10]. • PV curtailment is reduced by at least 6.31 MW/24 h and 0.63 MW/24 h in case studies. [ABSTRACT FROM AUTHOR]

This paper presents the analysis and control of a multilevel modular converter (MMC)-based HVDC transmission system under three possible single-line-to-ground fault conditions, with special focus on the investigation of their different fault characteristics. Considering positive-, negative-, and zero-sequence components in both arm voltages and currents, the generalized instantaneous power of a phase unit is derived theoretically according to the equivalent circuit model of the MMC under unbalanced conditions. Based on this model, a novel double-line frequency dc-voltage ripple suppression control is proposed. This controller, together with the negative- and zero-sequence current control, could enhance the overall fault-tolerant capability of the HVDC system without additional cost. To further improve the fault-tolerant capability, the operation performance of the HVDC system with and without single-phase switching is discussed and compared in detail. Simulation results from a three-phase MMC-HVDC system generated with MATLAB/Simulink are provided to support the theoretical analysis and proposed control schemes. [ABSTRACT FROM AUTHOR]

Abstract: This paper proposes a new adaptive control algorithm for a three-phase current-source shunt active power-line conditioner (APLC) operating under unbalanced and distorted network conditions. This control scheme aims at compensation of network’s reactive power, elimination of active power’s oscillating components, compensation of network current and voltage harmonic contents resulting in sinusoidal waveforms, and equilibrating the drawn power from the source evenly between the three-phases. Unlike many of the existing methods, the proposed strategy does not require any coordinate transformations or complicated calculations. The reference signals for the hysteresis-band current controlled voltage-source converter (HBCC-VSC) are generated by passing the measured current and voltage signals through two layers of modified adaptive notch filters (ANFs). To ensure superb performance and minimum total harmonic distortion (THD) level of the power system, parameters of the HBCC-VSC are obtained using differential evolution (DE) optimization algorithm. The proposed strategy is simple, easily implementable, and robust against uncertainty or variations of power system parameters and loads. The effectiveness of the proposed control scheme is validated by simulation results of a selected network under various load and power system conditions. [Copyright &y& Elsevier]

This work presents a control strategy of a dynamic voltage restorer (DVR) to improve the stability in wind farms based on squirrel-cage induction generators. The DVR controller is tailored to work under unbalanced conditions, which allows overcoming most faults in the power grid. The proposed strategy is capable of balancing voltages at wind farm terminals obtaining several advantages. Firstly, negative-sequence currents are eliminated; thus, overheating, loss of performance, and decreasing of generator useful life are avoided. Secondly, by nullifying negative-sequence voltages, 2\omega pulsation in the mechanical torque is prevented, reducing high stress in the turbine mechanical system, especially in the gearbox. The proposed control strategy for the DVR is compatible with farms which already have installed a static VAR compensator. Several scenarios and disturbances, carefully chosen to provide a realistic assessment, have been tested showing the adequacy of the proposed arrangement and controllers. [ABSTRACT FROM PUBLISHER]

This paper proposes a control strategy for the autonomous (islanded) operation of a four-wire, electronically-coupled distributed generation (DG) unit which can feed a highly unbalanced load, e.g., due to the presence of single-phase loads. In the grid-connected mode, the power-electronic interface of the DG unit enables the exchange of real and reactive power with the distribution network, based on the conventional dq-frame current control strategy. The current control scheme is disabled subsequent to the detection of an islanding event, and the proposed controller is activated. The proposed control strategy utilizes: i) an internal oscillator to maintain the island frequency and ii) a feedback control system to regulate the island voltage. The proposed control strategy provides a set of balanced three-phase voltages for the load, despite the load imbalance and parameters uncertainties. The proposed control strategy also guarantees robust stability, fast dynamic response to disturbances, and zero steady-state error. A stability analysis is also carried out to determine the robust stability margin of the closed-loop islanded system. Effectiveness of the proposed control strategy is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment and verified based on laboratory experiments. [ABSTRACT FROM AUTHOR]

This paper proposes a unified mathematical model of a voltage-sourced converter (VSC) based on positive and negative synchronously rotating frames under both unbalanced grid supply and imbalanced input impedances. Based on the developed model, a new definition of instantaneous reactive power is introduced, and an accurate mathematical model of the instantaneous active and reactive powers at different locations of the VSC is derived. To avoid the use of band-trap filter for decomposing the positive- and negative-sequence dq components of current and voltage, a new control scheme with multifrequency proportional-resonant controller in the stationary αβ frame is employed to improve the steady state and dynamic response under generalized unbalanced operation conditions. Simulation and experimental studies with various factors were conducted and compared under different types of unbalanced conditions. The simulated and experimental results demonstrate that the proposed control scheme in the stationary αβ frame provides sufficiently steady-state tracking capability for ac input current, and a better transient response in compensation for wide-range generalized unbalanced operation conditions. [ABSTRACT FROM AUTHOR]

This paper discusses the operation of a distribution static compensator (D-StatCom) under unbalanced conditions when the compensator exchanges also negative-sequence currents with the network. Unbalanced conditions, for example, occur when the D-StatCom is used for the compensation of unbalanced loads or in the case of an unbalanced network voltage. A new control algorithm based on the D-StatCom mathematical model for unbalanced conditions is presented. The algorithm enables effective operation of D-StatCom with independent control of the positive- and negative-sequence currents. Furthermore, the problem of low-frequency harmonics generation is solved with an appropriate modulation of the switching function enabling the use of a relatively small capacitor on the dc side. The developed control algorithm is tested by means of digital simulation. [Copyright &y& Elsevier]

Shunt active power filters (APFs) are used to address power quality issues such as harmonic distortion and power system imbalance. Many control systems for shunt APFs demand measurements of voltages and currents for the determination of instantaneous real and reactive power signals. From these signals, the reference currents of the shunt APF are calculated. A method of control for the APF shunt that only measures currents is proposed in this paper. The control method includes a modified technique to extract the positive and negative sequence components in the time domain. The reference currents in the shunt APF are calculated by the negative sequence components of the measured currents. Simulation and experimental results confirm the efficiency of this control scheme under ideal and non-ideal conditions. [ABSTRACT FROM AUTHOR]