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Recently, there has been a push to integrate renewable energy system (RES) into grid‐connected load system in enhancing reliability and reducing losses. However, integrating these systems introduces power quality (PQ) issues, especially with non‐linear, critical, and imbalanced loads. Addressing this, a hybrid mantis search‐reptile search algorithm (HMS‐RSA) combined with a unified power quality conditioner (UPQC) to mitigate PQ problems related to current and voltages in RES systems. In other words, the UPQC, enhanced by fractional order proportional integral derivative controller parameters tuned using the proposed HMS‐RSA assists in enhancing the power quality. The approach has been validated by connecting a non‐linear load to the system, which typically creates PQ issues. The proposed method is implemented in MATLAB/Simulink and their performance is analysed in three scenarios, such as sag, swell, and disturbance, and the total harmonic distortion is evaluated to quantify improvements in PQ. Finally, the proposed method is compared with existing approaches, such as ant colony optimization (ACO), artificial bee colony optimization (ABC), and bacterial foraging optimization (BFO). The method also outperforms ACO, ABC, and BFO in terms of convergence speed and effectiveness in mitigating PQ issues. [ABSTRACT FROM AUTHOR]

The efficiency of the power‐to‐hydrogen (P2H) alkaline electrolyzer (AE) is the primary factor constraint on its large‐scale application. Decreasing the specific energy consumption (SEC) of AE is positive to enhancing the efficiency of P2H system. However, the factors influencing the SEC of AE are yet to be fully revealed. In this paper, the influence of power quality and stack temperature on the SEC of AE is explored. Firstly, the models of double star rectifier and PWM current‐type rectifier are established. And the mathematical model on the SEC of rectifiers is proposed. The influence of power quality on the SEC is analyzed by comparing the current and voltage waveforms under four types of rectifiers. Secondly, the model of AE that considers the stack temperature is proposed. Unlike the existing simplified models, the proposed model can assess the SEC considering changes in stack temperature. Finally, the results show that the SEC with the 6‐pulse rectifier is 10.99% and 11.98% higher than that with the double‐star rectifier and PWM current‐type rectifier. The stack temperature is inversely related to the SEC of AE. This study provides a reference for reducing the SEC and enhancing the efficiency of the P2H system. [ABSTRACT FROM AUTHOR]

The increasing energy consumption and changing load variations place a significant burden on the sophisticated utility grid, which affects the source's dependability and quality. Researchers examining modern power networks must focus on those factors to prevent grid breakdowns. Superior power quality (PQ) is still intended to make sure everything runs smoothly under diverse organizations. The main cause of PQ problems, however, is now renewable energy used in power electronic converters that are integrated into the electrical grid. Even though new, better solutions are still being developed, adhering to international standards have been strongly advised. Consequently, microgrid clusters powered through renewable energy and incorporating multiple structures in an urban area has been proposed in this paper. This increases the dependability of the power sources by managing the energy that is available inside the cluster instead of having it focused around the utility grid. Additionally, a deep belief network model based on Improved Dwarf Mongoose Optimization is recommended for regulating the inverter by generating optimal pulse‐width modulated signals that increases the quality of the power supplies. When compared to other conventional techniques, the suggested technique possesses less real power and reactive power setting duration of 0.8 ms and 0.75 ms, respectively. [ABSTRACT FROM AUTHOR]

The primary goal of grid‐connected microgrids is to control the active and reactive power, which is reachable by the inner current control loop in the control structure of power converters. However, when facing unbalanced conditions, the inner current control loop implemented in the dq frame does not function properly. In such conditions, the popular current control loop malfunctions since there is an oscillatory coupling between harmonic components. Therefore, in this study, a new harmonic detector based on decoupled double synchronous reference frame within the current control loop is proposed in which the oscillatory coupling between harmonic components is eliminated, and the overall performance of the power converter control system is significantly improved. The performance of the precisely developed mathematical models is verified by Matlab simulations, and the simulation results confirm the accuracy and proper operation of the proposed strategy. [ABSTRACT FROM AUTHOR]

The primary goal of grid‐connected microgrids is to control the active and reactive power, which is reachable by the inner current control loop in the control structure of power converters. However, when facing unbalanced conditions, the inner current control loop implemented in the dq frame does not function properly. In such conditions, the popular current control loop malfunctions since there is an oscillatory coupling between harmonic components. Therefore, in this study, a new harmonic detector based on decoupled double synchronous reference frame within the current control loop is proposed in which the oscillatory coupling between harmonic components is eliminated, and the overall performance of the power converter control system is significantly improved. The performance of the precisely developed mathematical models is verified by Matlab simulations, and the simulation results confirm the accuracy and proper operation of the proposed strategy. [ABSTRACT FROM AUTHOR]

In recent times, multilevel inverters (MLIs) are gaining popularity for grid integration of distributed power generation sources. In this paper, a proportional‐resonant (PR) controller based on current control logic is proposed for a single‐phase eleven‐level inverter topology, enabling the integration of distributed power generation sources into the grid. The presented topology utilizes eight two‐quadrant switches and three dc sources to produce an eleven‐level output voltage. The level‐shifted pulse width modulation (LS‐PWM) scheme is used for providing switching signals to the inverter under grid‐connected operation. Simulation and real‐time results are presented for the eleven‐level inverter to validate the presented current control logic under the grid‐connected mode of operation. The hardware‐in‐loop emulator (Typhoon HIL‐402) is used for taking real‐time results of the presented grid‐connected topology. The total harmonic distortion (THD) of the eleven‐level inverter voltage and grid current is 12.10% and 0.46%, respectively. The efficiency of the inverter while supplying 2 kW active power to the grid is 98.4%. [ABSTRACT FROM AUTHOR]

Buea is connected to the south interconnected grid (SIG) of Cameroon. But the power supply is unreliable, not accessible to all and of very low quality. In addition, solid municipal waste generated in Buea is poorly collected by Hysacam and dumped at Musaka with no further valorization. There is an acute need to improve on the quantity, reliability and quality of the power supplied to inhabitants of Buea as well as to improve on the environmental management system of the town. In this study a project is proposed to design a stand‐alone 1000‐kW biogas power plant from the codigestion of organic fraction of solid municipal waste with microalgae (Chorella Sorokiniana) grown in tubular photo bioreactor (PBR) in Bomaka‐Buea. As far as electrical energy production is concerned, the aforementioned anaerobic biological treatment unit will produce almost 9311.88 MWh/y. Assuming an average electricity consumption of 3 to 4 MWh/y/household the annual produced electrical energy from the anaerobic digestion (AD) unit could supply almost 3000 to 4000 households in the Municipality of Buea. So by treating the 39% of annual organic waste produced in Buea it can cover between 31.3% and 31.8% of Buea municipality's electrical energy demand. With a levellized cost of electricity at 0.265 $/kW and a net present value of 25.3million dollars if this energy is sold to the national grid then the annual profit of energy production will be around $4,188,888/y (assuming a selling price of 0.36 $/kWh) that is valid now in Cameroon. [ABSTRACT FROM AUTHOR]

This study presents a three‐phase four‐wire grid‐tied solar photovoltaic (SPV) energy conversion system with a reduced ordered generalised integrator frequency locked loop (ROGI‐FLL) based control for the extraction of fundamental grid voltages. The reference grid currents are extracted by using the proposed filtered‐s least mean square (FsLMS) current control for voltage source converter (VSC) switching. This VSC is capable of performing multifunctions like compensating harmonics of non‐linear loads, the grid currents balancing, mitigation of neutral current and it stabilises the DC link voltage. The SPV array generates the active power for feeding loads and to the grid. Matlab environment is used to develop the model of the proposed scheme and simulation results of the system are validated with test results on a developed prototype. Test results have shown the good responses at different environmental conditions. Moreover, a comparative study is performed with ROGI‐FLL controller and other controls at the grid voltages unbalancing, DC offset and the grid voltages distortion conditions. This FsLMS‐based control has shown fast convergence, less oscillations and low settling time as compared with other controls at load unbalancing. Simulated and test results have satisfied the objective of this system and it meets the IEEE‐519 standard. [ABSTRACT FROM AUTHOR]

Matching of the colossal energy demand with carbon‐free resources originates a global challenge to produce and innovate advanced energy technologies that generate clean and friendly energy supplies without threatening our environment. Nevertheless, with the increase in the penetration from renewable energy resources like solar and wind farms many technical issues are presented typically; power stability, power quality, and frequency fluctuations. The aforementioned problems stem from the intermittent nature of renewable resources which makes the generated power unpredictable and requires controlling and smoothing before dispatching. Conventionally low pass filter (LPF) is used with a controlled energy storage system to smoothen the fluctuated power due to its simplicity, but a delay problem increases with higher values of the filtering the time constant. This study suggests a smoothing controller‐based fuzzy logic that determines the value of the filtering time constant based on the present solar power ramp rate to avoid the battery overworking, which results in the predetermined configuration of the LPF. The presented simulation results prove the smoothing performance of the proposed controller and compare its performance to the LPF during clear, normal, and cloudy irradiance profiles. [ABSTRACT FROM AUTHOR]

The digitally controlled inverter is widely applied to the photovoltaic (PV) plant, however, the effects of inverter digital time delay on the harmonic characteristic of PV system which directly influences the power quality and even the stability of the system, have not been investigated yet. This study investigates the harmonic characteristic of a grid‐connected large‐scale PV system based on the equivalent Norton model of the system with considering the digital time delay of the inverter. According to the system equivalent model, the series harmonic resonance circuit and the parallel harmonic resonance circuit are divided to study the impacts of digital time delay in the cases of the inverter output harmonic exist and the grid voltage harmonic exit, respectively. Then, the harmonic resonance mechanism of a grid‐connected large‐scale PV system is studied and the effects of digital time delay on the harmonic characteristic of system output current is quantitatively analysed by defining the harmonic magnification coefficient. It turns out that frequency and magnitude of harmonic magnification points in the grid‐side current of the system are increased as the digital time delay increases. Reference to the actual 20 MWp grid‐connected PV system, a simulation model is built to verify the theoretical analysis results. [ABSTRACT FROM AUTHOR]

As residential PV generation penetrates in the low‐voltage distribution network (LVDN), the unbalance issue may be intensified due to the asymmetry of generations/loads in different phases. Phase‐reconfiguration device (PRD), which can reconfigure connected phases of residential customers, provides an effective method to address this issue. Noting that although the benefit brought by PRDs can vary if they are placed at different locations in the network, little literature has been reported on this topic. To bridge the research gap, this paper presents a novel method to optimally place PRDs in an LVDN aiming at minimizing the power unbalance running through the distribution transformer. The problem considers both installation, operational constraints, and boils down to a challenging mixed‐integer non‐convex programming problem, which is then reformulated as an efficient solvable mixed‐integer linear programming problem. Moreover, operational constraints are relaxed with slack variables penalized in the objective function, which makes sure a feasible solution is always available without or with minimal operational violations. Case studies based on a modified IEEE system and a practical system in Australia demonstrate that an efficient strategy can be provided to address the unbalance issue while improving the network's power supply qualities. [ABSTRACT FROM AUTHOR]

The full controllable electronic device based railway feeder station offers better power quality and more flexible configurations than conventional transformer based stations. This study investigates a modular multilevel converter (MMC)‐based static frequency converter station with renewable energy access. Wind power generation is coupled into the station via DC link of the back to back converter. The dynamic single‐phase traction load and intermittent renewable generation bring double frequency oscillation and large deviation problems to the DC link voltage. Special design considerations and control schemes are proposed for the MMC to stabilise DC link voltage by controlling the total number of total inserted modules. The proposed control scheme resolves the voltage oscillation issue caused by single‐phase load and reduces the DC link voltage deviation under 10 MW step change. A series of device‐based simulations validate the control scheme which realises a reliable coupling interface for connecting the renewable generation to the DC bus. [ABSTRACT FROM AUTHOR]

In railway traction power supply, co‐phase system with hybrid power quality conditioner (HPQC) is capable of tackling the power quality issues caused by single‐phase traction loads. To further reduce the overall carbon emissions in railway systems, this paper considers to integrate the renewable energy with railway power supply, which however leads to a more complicated system to model, design and control. This paper first investigates its modelling aspect. To reduce the operating capacity of HPQC while addressing the power unbalance, optimal design of the compensation scheme for co‐phase system is formulated as a multi‐objective optimization problem which is then solved by the nondominated sorting genetic algorithm‐II (NSGA‐II). To eliminate the impact of errors arising from imperfect predictions of the loads and renewable power, a hybrid optimal compensation control is proposed, yielding full and optimal compensations. Comprehensive simulation studies, considering three operation modes covering variable traction loads, renewable and regenerative braking power, are conducted. The simulation results confirm the validity of the proposed optimal compensation scheme, achieving an average of more than 20% reduction in HPQC capacity compared to the full compensation scheme. Meanwhile, the power quality requirement is satisfied, even in the presence of real‐time prediction errors. [ABSTRACT FROM AUTHOR]

The access of distributed generations (DGs) in the traction power supply system (TPSS) can not only make use of the renewable energy near the railway route, but also improve the condition of weak external power supply. In order to quantify the effect when DGs access TPSS, this study proposes a method for evaluating the power supply capability and quality of TPSS which considers the access of DGs. First, the method fully considers the randomness and uncertainty of the power provided by DGs and that consumed by electrical locomotives. A comprehensive model of TPSS with the access of DGs is built by means of the stochastic model, and then the index system for evaluating the power supply capability and quality of TPSS is formulated. Finally, the Monte Carlo simulation is applied to calculate the probabilistic load flow to obtain the evaluation indices. The proposed method is verified by simulating a typical TPSS in a remote mountain area. The results show that the method can quantify the power supply capability and quality of the system after the access of DGs. It can provide an intuitive conception of the power supply condition for design and operation personnel. [ABSTRACT FROM AUTHOR]

Back‐to‐back converter based railway traction power supply system (TPSS) can eliminate neutral sections in the traction side and improve power quality in the grid side, but it still has some drawbacks such as low reliability, difficulty in accepting large‐capacity renewable energy, and power mismatches. In this study, a double‐layer DC/AC TPSS with renewable integration is proposed to address these challenges and to improve system performance. The proposed topology breaks the limit of back‐to‐back structure and enables more flexible free energy flow. A top‐down system design method is proposed in this study. Firstly, the characteristics of the proposed TPSS for integration with renewable power are described and compared with the traditional back‐to‐back topology. Secondly, a DC droop controller and a AC droop controller are designed for DC layer grid and AC layer grid, respectively, to control the power flow in each layer. The traditional AC droop control is based on the inductive transmission impedance, but the resistance of traction transmission line cannot be ignored. Thus, a modified droop control strategy with the consideration of line resistance is also proposed in this study. Subsequently, the voltage control strategy for the single modular multilevel converter is designed to track the reference signal from the upper droop controller. Finally, a general double‐layer DC/AC TPSS is designed from bottom to top, and the simulation results confirm that the proposed TPSS with renewable integration is capable of delivering desirable performance. [ABSTRACT FROM AUTHOR]

The large‐scale wind energy conversion systems (WECSs) based on doubly‐fed induction generators (DFIGs) are very popular in recent years due to the numerous technical and economic benefits. With the increasing penetration level of wind energy, the latest grid codes require the DFIG‐based WECSs to remain connected to the grid under grid fault scenarios and deliver the required reactive power into the grid. However, the direct connection of the stator of the DFIG to the grid makes it prone to grid disturbances, especially to voltage sag. This study proposes a modified demagnetisation control strategy to enhance the low‐voltage ride‐through (LVRT) capability of the DFIG under grid faults. The proposed control strategy is implemented in a coordinated approach by using the existing demagnetisation control and the addition of an external resistance in the stator side of the DFIG. The demagnetisation control damps the direct current component of the stator flux and the external resistance accelerates the damping of the transient flux by decreasing the time constant and hence, enhancing the LVRT capability of DFIG. The effectiveness of the proposed control strategy is demonstrated under both symmetrical and asymmetrical grid faults simulated system through MATLAB/Simulink®. The comparative results justify the merits of the proposed methodology. [ABSTRACT FROM AUTHOR]

In a DC microgrid, the anti‐interference ability of bus voltage can easily become vulnerable due to low inertia caused by a large number of power electronic converters. This has already challenged the system stability and practicability in DC girds. In this study, a flexible virtual capacitance (FVC) control strategy with multiple constraints (MCs) is proposed to guarantee the security and stability of the DC microgrid, where the system stability, dynamic characteristics, actual operation requirements and realisability are considered as MCs. The FVC controller, which mainly modifies the reference current in voltage source converter by flexibly regulating virtual capacitance value, is given to provide inertial support and improve voltage quality. The stable operating boundary calculation based on small signal model and key control parameters design are further introduced into the control loop. This can ameliorate the dynamic response of the microgrid and obtain the better engineering practicability. Finally, experiment tests on a controller‐level hardware‐in‐the‐loop simulation platform are carried out to verify the validity of the proposed strategy. [ABSTRACT FROM AUTHOR]

As residential PV generation penetrates in the low-voltage distribution network (LVDN), the unbalance issue may be intensified due to the asymmetry of generations/loads in different phases. Phase-reconfiguration device (PRD), which can reconfigure connected phases of residential customers, provides an effective method to address this issue. Noting that although the benefit brought by PRDs can vary if they are placed at different locations in the network, little literature has been reported on this topic. To bridge the research gap, this paper presents a novel method to optimally place PRDs in an LVDN aiming at minimizing the power unbalance running through the distribution transformer. The problem considers both installation, operational constraints, and boils down to a challenging mixed-integer non-convex programming problem, which is then reformulated as an efficient solvable mixed-integer linear programming problem. Moreover, operational constraints are relaxed with slack variables penalized in the objective function, which makes sure a feasible solution is always available without or with minimal operational violations. Case studies based on a modified IEEE system and a practical system in Australia demonstrate that an efficient strategy can be provided to address the unbalance issue while improving the network's power supply qualities. [ABSTRACT FROM AUTHOR]

The digitally controlled inverter is widely applied to the photovoltaic (PV) plant, however, the effects of inverter digital time delay on the harmonic characteristic of PV system which directly influences the power quality and even the stability of the system, have not been investigated yet. This study investigates the harmonic characteristic of a grid-connected large-scale PV system based on the equivalent Norton model of the system with considering the digital time delay of the inverter. According to the system equivalent model, the series harmonic resonance circuit and the parallel harmonic resonance circuit are divided to study the impacts of digital time delay in the cases of the inverter output harmonic exist and the grid voltage harmonic exit, respectively. Then, the harmonic resonance mechanism of a grid-connected large-scale PV system is studied and the effects of digital time delay on the harmonic characteristic of system output current is quantitatively analysed by defining the harmonic magnification coefficient. It turns out that frequency and magnitude of harmonic magnification points in the grid-side current of the system are increased as the digital time delay increases. Reference to the actual 20 MWp grid-connected PV system, a simulation model is built to verify the theoretical analysis results. [ABSTRACT FROM AUTHOR]

Matching of the colossal energy demand with carbon-free resources originates a global challenge to produce and innovate advanced energy technologies that generate clean and friendly energy supplies without threatening our environment. Nevertheless, with the increase in the penetration from renewable energy resources like solar and wind farms many technical issues are presented typically; power stability, power quality, and frequency fluctuations. The aforementioned problems stem from the intermittent nature of renewable resources which makes the generated power unpredictable and requires controlling and smoothing before dispatching. Conventionally low pass filter (LPF) is used with a controlled energy storage system to smoothen the fluctuated power due to its simplicity, but a delay problem increases with higher values of the filtering the time constant. This study suggests a smoothing controller-based fuzzy logic that determines the value of the filtering time constant based on the present solar power ramp rate to avoid the battery overworking, which results in the predetermined configuration of the LPF. The presented simulation results prove the smoothing performance of the proposed controller and compare its performance to the LPF during clear, normal, and cloudy irradiance profiles. [ABSTRACT FROM AUTHOR]

The full controllable electronic device based railway feeder station offers better power quality and more flexible configurations than conventional transformer based stations. This study investigates a modular multilevel converter (MMC)-based static frequency converter station with renewable energy access. Wind power generation is coupled into the station via DC link of the back to back converter. The dynamic single-phase traction load and intermittent renewable generation bring double frequency oscillation and large deviation problems to the DC link voltage. Special design considerations and control schemes are proposed for the MMC to stabilise DC link voltage by controlling the total number of total inserted modules. The proposed control scheme resolves the voltage oscillation issue caused by single-phase load and reduces the DC link voltage deviation under 10 MW step change. A series of device-based simulations validate the control scheme which realises a reliable coupling interface for connecting the renewable generation to the DC bus. [ABSTRACT FROM AUTHOR]

In railway traction power supply, co-phase system with hybrid power quality conditioner (HPQC) is capable of tackling the power quality issues caused by single-phase traction loads. To further reduce the overall carbon emissions in railway systems, this paper considers to integrate the renewable energy with railway power supply, which however leads to a more complicated system to model, design and control. This paper first investigates its modelling aspect. To reduce the operating capacity of HPQC while addressing the power unbalance, optimal design of the compensation scheme for co-phase system is formulated as a multi-objective optimization problem which is then solved by the nondominated sorting genetic algorithm-II (NSGA-II). To eliminate the impact of errors arising from imperfect predictions of the loads and renewable power, a hybrid optimal compensation control is proposed, yielding full and optimal compensations. Comprehensive simulation studies, considering three operation modes covering variable traction loads, renewable and regenerative braking power, are conducted. The simulation results confirm the validity of the proposed optimal compensation scheme, achieving an average of more than 20% reduction in HPQC capacity compared to the full compensation scheme. Meanwhile, the power quality requirement is satisfied, even in the presence of real-time prediction errors. [ABSTRACT FROM AUTHOR]

Back-to-back converter based railway traction power supply system (TPSS) can eliminate neutral sections in the traction side and improve power quality in the grid side, but it still has some drawbacks such as low reliability, difficulty in accepting large-capacity renewable energy, and power mismatches. In this study, a double-layer DC/AC TPSS with renewable integration is proposed to address these challenges and to improve system performance. The proposed topology breaks the limit of back-to-back structure and enables more flexible free energy flow. A top-down system design method is proposed in this study. Firstly, the characteristics of the proposed TPSS for integration with renewable power are described and compared with the traditional back-to-back topology. Secondly, a DC droop controller and a AC droop controller are designed for DC layer grid and AC layer grid, respectively, to control the power flow in each layer. The traditional AC droop control is based on the inductive transmission impedance, but the resistance of traction transmission line cannot be ignored. Thus, a modified droop control strategy with the consideration of line resistance is also proposed in this study. Subsequently, the voltage control strategy for the single modular multilevel converter is designed to track the reference signal from the upper droop controller. Finally, a general double-layer DC/AC TPSS is designed from bottom to top, and the simulation results confirm that the proposed TPSS with renewable integration is capable of delivering desirable performance. [ABSTRACT FROM AUTHOR]

The access of distributed generations (DGs) in the traction power supply system (TPSS) can not only make use of the renewable energy near the railway route, but also improve the condition of weak external power supply. In order to quantify the effect when DGs access TPSS, this study proposes a method for evaluating the power supply capability and quality of TPSS which considers the access of DGs. First, the method fully considers the randomness and uncertainty of the power provided by DGs and that consumed by electrical locomotives. A comprehensive model of TPSS with the access of DGs is built by means of the stochastic model, and then the index system for evaluating the power supply capability and quality of TPSS is formulated. Finally, the Monte Carlo simulation is applied to calculate the probabilistic load flow to obtain the evaluation indices. The proposed method is verified by simulating a typical TPSS in a remote mountain area. The results show that the method can quantify the power supply capability and quality of the system after the access of DGs. It can provide an intuitive conception of the power supply condition for design and operation personnel. [ABSTRACT FROM AUTHOR]

In a DC microgrid, the anti-interference ability of bus voltage can easily become vulnerable due to low inertia caused by a large number of power electronic converters. This has already challenged the system stability and practicability in DC girds. In this study, a flexible virtual capacitance (FVC) control strategy with multiple constraints (MCs) is proposed to guarantee the security and stability of the DC microgrid, where the system stability, dynamic characteristics, actual operation requirements and realisability are considered as MCs. The FVC controller, which mainly modifies the reference current in voltage source converter by flexibly regulating virtual capacitance value, is given to provide inertial support and improve voltage quality. The stable operating boundary calculation based on small signal model and key control parameters design are further introduced into the control loop. This can ameliorate the dynamic response of the microgrid and obtain the better engineering practicability. Finally, experiment tests on a controller-level hardware-in-the-loop simulation platform are carried out to verify the validity of the proposed strategy. [ABSTRACT FROM AUTHOR]

The large-scale wind energy conversion systems (WECSs) based on doubly-fed induction generators (DFIGs) are very popular in recent years due to the numerous technical and economic benefits. With the increasing penetration level of wind energy, the latest grid codes require the DFIG-based WECSs to remain connected to the grid under grid fault scenarios and deliver the required reactive power into the grid. However, the direct connection of the stator of the DFIG to the grid makes it prone to grid disturbances, especially to voltage sag. This study proposes a modified demagnetisation control strategy to enhance the low-voltage ride-through (LVRT) capability of the DFIG under grid faults. The proposed control strategy is implemented in a coordinated approach by using the existing demagnetisation control and the addition of an external resistance in the stator side of the DFIG. The demagnetisation control damps the direct current component of the stator flux and the external resistance accelerates the damping of the transient flux by decreasing the time constant and hence, enhancing the LVRT capability of DFIG. The effectiveness of the proposed control strategy is demonstrated under both symmetrical and asymmetrical grid faults simulated system through MATLAB/Simulink®. The comparative results justify the merits of the proposed methodology. [ABSTRACT FROM AUTHOR]

This work deals with the implementation, control and operation of a reconfigurable small hydro‐solar photovoltaic (PV)‐battery energy storage (BES)‐based distributed generation system (DGS) working in isolated mode (IM) and grid connected mode (GCM). The DGS consists of a hydro generation‐based on a PMSG integrated with a PV array supported by the BES. It operates in an IM during the grid outage and when the grid is available, it synchronizes and operates in GCM. A frequency adaptive comb filter‐frequency locked loop (FLL)‐based control is adopted to estimate errorless phase angles of the hydro generator and grid voltages for quick and accurate grid synchronization. A third‐order band pass filter‐based control technique is used to extract fundamental component of load currents and determine switching pulses to VSC. The overall multi‐objective control is used to maintain voltage and frequency of the system at the point of common coupling during IM and GCM. The power quality of grid and generator currents are maintained at non‐linear loads. The modified perturb and observe‐based control technique is used to extract maximum power from the PV array and it provides drift‐free operation during the change in insolation with de‐rating capability to protect the BES from overcharging in IM. [ABSTRACT FROM AUTHOR]

This study presents a grid‐integrated solar photovoltaic system (SPVS). The objective of the work is to explore the scope of cascaded H‐bridge multilevel inverter (CHBMLI) in the high infiltration of photovoltaic (PV) in the distribution system. A five‐level CHBMLI is implemented with a balanced power‐sharing and equal switch stress mode, which increases the system power rating with longer life. The power conversion efficiency is elevated by reduced switching frequency, compared to two‐level inverters. The damped second‐order generalised integral (DSOGI) control‐based grid synchronisation is used to critically damp system oscillations and overshoots at dynamic operating conditions. Hence, safer service of lower rating power electronic switches at high power operation is ensured. It also has better bandpass filtering at the nominal grid frequency. A single‐input multiple‐output single‐ended primary inductance converter realises the isolated DC sources for the CHBMLI. It extracts maximum power from the PV array by using centralised incremental conductance algorithm. The SPVS also considers the nonlinear unbalanced loads at the bus of common coupling and provide shunt active filter functionality. A minimum number of voltage and current sensors are used in the configuration. An extensive analysis is carried out to validate the theoretical claims. [ABSTRACT FROM AUTHOR]

In the unfolding‐type interleaved two‐switch flyback inverter (ILTFI) operating in discontinuous conduction mode (DCM), the hybrid control strategy combining the one‐phase DCM and the two‐phase DCM has a significant impact on improving the efficiency under all load conditions. In the situation where the marginal power of this control strategy is increased, the converter may enter CCM. Consequently, the converter cannot track the reference current and the output current quality is notably decreased. To tackle this issue, a novel hybrid control method is proposed which varies the switching frequency based on the output power. The proposed approach is immune to the loss analysis and power losses of the components. Hence, it is independent of components selection and their characteristics. In this case, the smooth transition between the one‐phase and the two‐phase operation modes is guaranteed without affecting the output power quality and the stability of the converter in DCM. The control complexity of the proposed scheme is low and the converter can be easily controlled in DCM. The performance of the flyback microinverter with the proposed hybrid control scheme is verified by the simulation and the experimental results together with the loss analysis. [ABSTRACT FROM AUTHOR]

Zero‐voltage ride through (ZVRT) is the extreme case of a low‐voltage ride through (LVRT), which represents the optimal grid‐connection capability of a wind turbine (WT). Enforcing ZVRT will improve the dynamic performance of a WT under the extreme conditions, and therefore significantly enhance the resiliency of a renewable‐rich power grid. Compared with LVRT, there are few research on ZVRT with no field validation been conducted. The feasibility of the ZVRT requirement, especially the 400‐ms‐level fault‐time condition, is not clear yet. The first system‐level study including the control strategy, transient analysis, hardware enhancement strategy, WT modelling and field test on the ZVRT capability of a Type‐3 WT is presented in this study. Particularly, to meet the challenging technical requirements, the authors proposed a hardware enhancement strategy, which accommodates with the widely‐used LVRT control approach. The holistic strategy is tested under the extreme grid‐connection condition by both simulation and a costly field test. The field test in the 400‐ms‐level fault‐time condition based on an operating WT in a weak power system directly verifies the feasibility and effectiveness of ZVRT for a Type‐3 WT. Furthermore, the critical time window and the key parameter ranges during ZVRT, which significantly affect ZVRT performance, are obtained. [ABSTRACT FROM AUTHOR]

DC microgrids are well known as a proper solution to link different DC sources, such as photovoltaic panels and wind turbines, directly to DC loads. Along with their advantages, they suffer from an imbalance state of charge (SOC) in their energy storage units (ESUs), improper current‐sharing between ESUs, and DC bus voltage deviation. This study proposes a novel control strategy for DC microgrids, which not only balances ESUs SOC and shares current between ESUs proportional to their capacity but also, reduces DC bus voltage deviation significantly. In this way, the same droop curve is defined. Then, three virtual resistances are considered. The first one is set based on the lines resistance and ESUs capacity to share current among the ESUs properly. As the lines resistance changes affect the proportional current‐sharing, the second virtual resistance is intended to relieve this effect. The third one is suggested to balance the ESUs SOC. The small‐signal stability analysis is performed, which demonstrates that the proposed method has stability in all operating conditions. Simulation results in MATLAB/Simulink confirm that with the aid of the proposed control method, not only proper current‐sharing and ESUs SOC balancing are obtained, but also DC bus voltage deviation is effectively diminished. [ABSTRACT FROM AUTHOR]

A multilayer frequency adaptive fundamental signal extractor‐based filter is used for the improvement of power quality of a single phase utility grid interfaced to a wind–photovoltaic (PV) system. This filter is used for the grid synchronisation, serving multiple objectives such as mitigating the lower‐order harmonics, DC offset elimination, load reactive power compensation, and power flow management. As this filtering technique is based on frequency adaptation, it adapts the changes in the grid fundamental frequency. The effectiveness of this filter is studied with simulations using MATLAB Simulink under varying solar irradiance, wind speeds, and loads also validated on the test setup developed in the laboratory. [ABSTRACT FROM AUTHOR]

This study presents a robust Kalman filter‐based multifunctional control strategy, to enable wide‐scale utilisation of the grid‐interfaced solar energy conversion system (SECS). The presented control technique offers multifunctional features such as unity power factor, harmonics mitigation, reactive power compensation along with ride through operation under balanced and unbalanced low‐voltage faults in the distribution grid. Under fault in the grid side area, the DC–DC converter is controlled in a way that it reduces the active power injection and restricts the inverter currents within its maximum current limit. Based on the depth of grid voltage sag, reactive power supply to the grid is adaptively adjusted, as recommended by IEEE 1547‐2018 standard. To minimise the inverter losses, adaptive DC link voltage is introduced, which adapts the DC link voltage in proportion to the grid voltages. Simulation results exhibit the efficacious of the presented robust control strategy in achieving multifunctional features such as harmonics mitigation, grid currents balancing feature and reactive power requirement, under unbalanced line to ground (L‐G) and double line to ground faults. Test results demonstrate the satisfactory performance for diverse operating scenarios such as imbalanced currents in the load side network, SECS to DSTATCOM mode and vice‐versa, and L‐G fault. [ABSTRACT FROM AUTHOR]

Development of control technologies facilitates the integration of renewable energy sources to the grid through power electronic converters. In this study, the inverter acts as shunt active power filter along with the power conversion operation. Various sliding mode control (SMC) techniques namely relay and signum method, hysteresis method and equivalent control method are implemented to control the real, and reactive power flow between Renewable Energy System (RES), load and grid using an inverter. To reduce the complexity, the control law (ueq) for equivalent control method is derived using a simple single‐phase equivalent circuit of a three‐phase voltage source inverter. In this study, the robustness is enhanced by choosing the integral sliding surface. These controllers are used to reduce the total harmonic distortion (THD) and to ensure unity power factor (PF) in the grid during balanced and unbalanced grid voltage conditions even in the existence of nonlinear loads. The SMC using equivalent control function outperforms the other methods in terms of chattering, THD and PF. To illustrate the effectiveness, three different modes are considered: (i) no RES power, (ii) insufficient RES power and (iii) excess RES power. The performance of controllers is validated by the simulation and experimental results. [ABSTRACT FROM AUTHOR]

This work presents a generalised integrator‐based control algorithm for power quality (PQ) amelioration of the grid in the presence of non‐linear load enabling leakage current suppression feature. Owing to the presence of stray capacitance between solar photovoltaic (PV) array and the ground, the variation in common‐mode voltage across stray capacitance leads to potential safety issues, electromagnetic interference, and distortion into the injected grid currents. Furthermore, the PQ of the grid is highly deteriorated under the existence of non‐linear loads. To solve the aforementioned issues, the harmonic compensation controller is presented herein to ensure the unity power factor operation, harmonic compensation, leakage current suppression using the grid‐connected solar PV array system. Simulated results show the effectiveness of the control strategy under various operating scenario. In contrast to the state‐of‐the‐art techniques, the grid currents are attained sinusoidal and balanced even in the presence of unbalanced non‐linear load. Furthermore, the leakage current is suppressed within 300 mA as per the VDE‐00126‐01 standard. The comparative performance explicitly validates the effectiveness of the presented harmonic compensating strategy over the conventional schemes. Test results manifest the response of the solar PV array system for various operating conditions and follows the revised IEEE‐519–2014 sand VDE‐00126‐01 standards. [ABSTRACT FROM AUTHOR]

Asymmetrical faults in the grid cause oscillations in dc‐link capacitor voltage or overcurrent in the converters. In this study, an optimal controller with a fixed structure is introduced based on the command generator tracker (CGT) to restrict the fault currents and increase stability performance under asymmetrical grid faults. For this purpose, first, the integrated dynamical model including grid‐connected converters and dc‐link voltage are introduced. Then, using the optimal control techniques, a CGT is designed to enhance the fault ride through performances. Next, the proposed controller is applied to the system including converter and dc‐link at the source side and the dynamic performances of the closed‐loop system under different fault conditions are determined. Finally, the simulation results are compared with the other conventional controllers and the dynamic performances of the proposed controller are evaluated. The results indicate that the proposed controller provides stability for the system and satisfies allowable constraints of converter current for different grid fault types such as voltage sags, asymmetrical and symmetrical faults. [ABSTRACT FROM AUTHOR]

This article presents dual‐mode control of a single‐stage utility interactive microgrid based on a photovoltaic array and battery energy storage with improved power quality. For protection reasons, grid tied solar inverters are mandatory to shut down at loss of the grid. However, proposed multitasking microgrid is developed to provide energy to critical loads, even at disappearance of grid. The microgrid is integrated the BES and a PV array at DC link of voltage source converter of it to provide the harvested energy to local loads. In islanding, the amplitude and waveform of load voltages are maintained sinusoidal by the voltage controller. The transfer of modes (islanded to grid connected and vice‐versa) is implemented using the synchronisation controller as per the accessibility of the grid. The instantaneous characteristic‐based adaptive filter is utilised to control the microgrid in the grid‐integrated mode. Moreover, a PV feed‐forward term is incorporated in current control for injection of active power to the utility as well as to improve the dynamic response of microgrid. The phase detection controller estimates phase, frequency of utility and load voltages accurately and capability of disturbance rejection is improved. The effectiveness of PV‐BES‐based microgrid is validated experimentally. [ABSTRACT FROM AUTHOR]

This study aims to propose optimised and robust current controller for the transformerless grid‐integrated photovoltaic inverter. Challenge of ensuring improved power quality while minimising the DC‐component under non‐ideal grid condition is at the forefront in the photovoltaic system. In this context, an advanced current controller is designed and developed in the stationary reference frame with proportional‐integral (PI) plus four‐vector PI (4VPI) controllers. The PI‐controller modulates the shape of fundamental current to follow its reference induced from outer DC‐link loop. Meanwhile, 4VPI controllers are tuned at the multiple of grid frequencies including third‐(3ω0), fifth‐(5ω0), seventh‐(7ω0), and ninth‐(9ω0) to introduce infinite gain and thereby alleviating stationary errors in the current loop of the photovoltaic inverter. The most distinguishing feature of the proposed controller is its inherent control robustness, and therefore, it operates satisfactorily against control delay time and frequency excursion. Traditionally, cumbersome trial‐and‐error techniques have been largely employed to adjust the PI‐VPI controller gains. Nonetheless, tuning of these parameters has made many practitioners switch‐off from employing the proposed controller. Given this, the particle swarm optimisation algorithm is proposed. Finally, the practicality of the proposed controller is verified through comprehensive set of simulation and experimental tests conducted on a laboratory‐developed prototype using dSPACE‐1104. [ABSTRACT FROM AUTHOR]

In this study, a control algorithm based on non‐linear adaptive second‐order Volterra filter (NLVF) is utilised to operate the voltage‐source converter (VSC). The main role of VSC is to supply compensating current to improve the power quality in wind‐based off‐grid distributed power generation system. This off‐grid system consists of three‐phase induction generator, VSC and non‐linear loads. The proposed control algorithm is adaptive as well as non‐linear in nature. The characteristic of Volterra series is exploited to generate the reference source current for VSC operation. The NLVF estimates the active and reactive weights of fundamental load current for reference current generation. It mitigates the power quality problems created by disturbances in non‐linear load or wind oscillations on off‐grid system. The solution of power quality problems such as the presence of harmonics in supply current, load unbalance and reactive power compensation is obtained. Nevertheless, voltage and frequency control are also achieved. The VSC is used as a shunt compensator in the system. The entire off‐grid system is designed and validated through the laboratory prototype development. The objective of the system is achieved and the steady state and dynamic performance is found satisfactory. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) have the advantages of high‐power density and small‐harmonic distortion because of their modularity and flexibility, thus providing a new avenue for research into scalable superconducting magnetic energy storage (SMES) in renewable energy generation. This study presents coordinated control for a three‐phase four‐wire (3P4W) MMC‐based SMES system under unbalanced voltage applications. First, the positive‐ and negative‐sequence mathematical models and the port‐controlled Hamiltonian with dissipation model of the 3P4W MMC‐SMES system are established by introducing an additional path for zero‐sequence current. Second, a multi‐objective passivity‐based control strategy that can effectively improve the power quality and system robustness and eliminate both double‐frequency active and reactive power fluctuations or double‐frequency active power fluctuation and negative‐sequence current is proposed. The simulation results based on MATLAB/Simulink demonstrate the effectiveness of the proposed topology of the SMES and its control strategy. [ABSTRACT FROM AUTHOR]

This work proposes a design method for robust state feedback H∞ grid current controller applied to single‐phase grid‐connected photovoltaic (PV) systems. The proposed method achieved high system reliability even dealing with grid distorted voltages and other power quality issues. The robustness of the H∞ controller was verified in the face of parametric uncertainties at the first‐order output filter, and also a satisfactory disturbance rejection performance was achieved. The proposed design is based on linear matrix inequalities associated with constraints involving H∞ optimisation and a pole placement analysis on the continuous‐time domain. A discrete‐time analysis is also presented in order to indicate the simplicity of its implementation by a microcontroller or similar devices. Based on the presented results and in order to validate the proposed controller, its robust performance was compared with two linear and well‐known control methods: a classical PI controller, in terms of robustness; and a robust H∞ resonant controller, regarding the power quality at the connection point with the grid‐utility. Although the conclusions were listed regarding a low voltage PV grid‐interactive multilevel power converter, the proposed H∞ control design (and its concepts) can be applied to a great variety of renewable energy resources systems. [ABSTRACT FROM AUTHOR]

This work presents an improved phase‐locked loop (IPLL)‐based control for grid‐integrated photovoltaic (PV) system (GIPVS). It is used to extract amplitude, frequency, and phase angle of distorted load currents to control PV‐grid‐interfaced voltage source converter (VSC). The VSC controls active and reactive powers at unbalances and distortions in the grid voltages. The control algorithm of VSC includes power feed‐forward term, DC‐link voltage, and grid voltages. The feed‐forward term improves the dynamic performance of a GIPVS and enhances the utilisation of VSC. Three‐phase IPLLs extract the in‐phase components of three‐phase load currents at utility frequency. Further, these in‐phase components of load currents are used for estimation of reference‐grid currents. The proposed IPLL‐based control algorithm offers good dynamic response even under unbalanced load condition. The efficient working of IPLL‐based control algorithm is realised by extracting maximum power from PV‐grid system and through its power‐quality improvement. The perturb and observe (P & O) control algorithm is used for optimum utilisation of a PV array. A developed prototype of PV‐grid system is used to implement an IPLL‐based control algorithm under different operating conditions. [ABSTRACT FROM AUTHOR]

This study presents a new microgrid topology that uses a bidirectional interleaved converter performing a power interface between DC buses in a hybrid microgrid allowing for both grid‐connected and islanded modes. The authors propose a new control strategy and controllers' design method aiming at achieving a high‐performance dynamic response regarding the converter load and generation disturbance rejection capability. In the grid‐connected mode, the interleaved converter operates in the buck mode providing a high‐power‐quality DC microgrid voltage. In the islanded mode, the interleaved converter operates in the boost mode and it is responsible for regulating the DC link of the back‐to‐back converter that connects the main grid to the AC microgrid. A detailed mathematical model is presented to obtain a MIMO system that takes into account the system's disturbances to analyze both stability margins and disturbance‐rejection response. Simulations of the proposed topology are carried out in PSCAD/EMTDC in a microgrid operating in grid‐connected and islanded operation modes. Experimental results are provided in order to validate the proposed control tuning method. [ABSTRACT FROM AUTHOR]

Here, the impacts of biogas power plants and photovoltaic (PV) power plants on the power quality in the distribution network are presented and analysed. The biogas power plant's influence on the current harmonics is explored, as a synchronous generator powers the biogas power plant, and the PV plants are connected to the distribution network via the inverter. All the analyses are based on actual measurements that are performed in the last 10 years. Results of the analysis of these numerous measurements were a challenge for investigation how large share of PV systems in distribution network with non‐linear loads influence on THD from one side and how biogas power plant with a synchronous generator can improve THD from the other side. In order to investigate interaction of the biomass power plant and PV power plants on the power quality in distribution network, computer model was developed in DIgSILENT Power Factory where PV power plant and household were modelled as a source of current harmonic injections and biomass power plant was modelled as synchronous machine, as a source of voltage harmonics. Analysis of harmonic behaviour in different scenarios is examined through the performed simulations. [ABSTRACT FROM AUTHOR]

In this study, to compensate voltage unbalance and harmonics and improve load sharing, a distributed hierarchical control scheme is proposed for islanded microgrids. In the proposed scheme, a method is proposed, which can improve the voltage quality of sensitive load bus by determining the amount of effort made by each distributed generation (DG) and taking their rated power constraints into account for compensation. Moreover, this method is capable of improving the load sharing among DG units adaptively using circulating currents rule. Compared to the previous proposed methods, the method proposed in this study does not need a central controller, and communication is only needed between adjacent DG units. Furthermore, in this scheme, the contribution of each DG unit in compensation will be in proportion to its rated power, and none of them will be overloaded. Also, to compensate the error of load current sharing components, there is no need to have any information about impedance of DGs feeders and load type. To show the efficiency of the proposed scheme, simulation studies are performed on a typical islanded microgrid in MATLAB/Simulink environment. [ABSTRACT FROM AUTHOR]

In well‐designed photovoltaic (PV) power plants, such as in typical utility‐scale PV plants, operational conditions are quite stable and homogeneous apart from the fast irradiance transitions caused by cloud shadings. These partial shading events cause fast power fluctuations leading even to stability and quality problems in power networks. Fast non‐homogeneous irradiance transitions cause also mismatch losses in PV generators and the occurrence of multiple maximum power points (MPPs), which appear in a wide voltage range of the PV generator. In consequence, the global MPP can fluctuate in a wide voltage region causing possible MPP tracking problems and power losses. This article presents a study of the behaviour of the global MPP voltage of various PV arrays during irradiance transitions caused by clouds. The global MPP voltage, its rate of change and the differences in voltage and available energy between the global MPP and the MPP with the highest voltage of the PV generators have been analysed comprehensively, for the first time, by using the characteristics of around 8000 measured irradiance transitions. The global MPP voltage was found to vary between 28 and 112% of the nominal MPP voltage of the array. [ABSTRACT FROM AUTHOR]

Unbalanced grid faults are the most severe perturbations which degrade the performance of grid‐connected inverter interfaced renewable energy sources (IIRESs). In this regard, this study proposes a novel control strategy that provides optimum use of the power capability of the grid‐connected IIRES under unbalanced grid fault conditions. One of the major contributions to previous methods is to propose a fast and robust improved Virtual Park (VP) sequence extractor to implement the reference current generator (RCG). The dynamic response of the grid‐connected IIRES has also enhanced thanks to the proposed VP sequence extractor. The computing of maximum active power injection and simultaneously amplitude of phase currents is embedded into a new algorithm based the RCG to meet the aimed control objectives. The controllability and minimisation of active and reactive power oscillations are also formulated and carried out with only one control parameter. Another contribution of this study is to prevent overcurrent issues with peak current limitation control. Reference for active power is online computed based on the maximum current injection at inverter capacity for overcurrent protection. The validating effectiveness of the proposed solution is performed by a comprehensive set of cases to show the shortcomings of previous similar studies. [ABSTRACT FROM AUTHOR]

This study presents the development of Legendre‐based functional neural network algorithm for shunt compensation in photovoltaic (PV)‐based grid‐connected system. The controller is developed for improving power quality (PQ) and the compensator is controlled to work in current control mode. It injects the requisite compensating current depending on the nature of the load current. The compensator is also interfaced with PV source and the controller design incorporates its contribution too. Some of the PQ problems studied include curtailment of harmonics, providing necessary reactive power, power factor improvement and so on. Results under distorted grid, varying solar irradiation and variety of loads have been presented. The proposed algorithm is designed using non‐linear functional Legendre expansion of load current and has not been used for compensation or PQ problem alleviation till date. Both simulation and experimental results verify that the proposed algorithm performs far better than the adaptive popular backpropagation multilayer perceptron neural network, recurrent neural network and non‐adaptive conventional synchronous reference frame theory based techniques. [ABSTRACT FROM AUTHOR]

This study presents a novel approach for ensuring the power quality and reliability of supply required for low‐voltage networks with distributed energy sources. For this purpose, a control strategy which enables active network uninterrupted transfer to islanded mode and continues its operation when the quality of supply from the utility grid is insufficient is proposed. Islanded operation is initiated based on an immunity curve, which defines the tolerance of the considered system for different voltage disturbances. The proposed strategy is based on expanding the functionality of the energy storage system and the application of a central controller. Moreover, it does not interfere with the method for control of energy sources. An original mechanism was developed and implemented to reserve power and energy in the ESS, enabling the switch to islanded mode at any time. A simulation model of the considered network was developed using the PSCAD/EMTDC platform to verify the proposed strategy. Simulation results illustrating the functioning of the state of charge storage control mechanism and demonstrating the flexible operation of the network in grid‐connected and islanded modes are provided. The results clearly confirm the effectiveness of the proposed control strategy for improving the quality and reliability of power supply. [ABSTRACT FROM AUTHOR]

DC Grid does not transmit reactive power, nor does it offer frequency and phase issues. This results in malfunction of conventional islanding detection methods (IDMs) that rely on the above quantities. At present, the DC IDMs proposed in literatures are mostly active power injection methods based on DC/DC converters. However, in a multi‐photovoltaic (PV) DC system, the synchronization of the injection of converters is difficult to guarantee, which may lead to detection failure. This paper proposes an impedance measurement IDM for multi‐PV DC micro grid based on the analysis of high‐frequency impedance model of power electronic devices. The impedance of DC converters is constant at high frequency and the harmonic impedance of the system changes significantly after islanding, so that islanding operation can be accurately detected. The harmonic impedance change directly reflects the structural changes of the system, and its calculation requires less disturbance intensity, which avoids the requirement of synchronous injection. Therefore, the method uses a DC/DC converter for injection, without the need for additional injection equipment. The method has little influence on power quality and theoretically has no none detection zone. Simulation results show that the method can perform accurately islanding detection in multi‐PV DC micro grid. [ABSTRACT FROM AUTHOR]