Design and control of LCL filter interfaced grid connected solar photovoltaic (SPV) system using power balance theory

In this paper solar photovoltaic (SPV) system connected to the utility grid is designed and simulated. The utility grid and SPV system are coupled with current controlled voltage source converter (VSC) and LCL filter. The design of LCL filter, MPPT algorithm and power quality improvements are discussed and simulation results are shown for the performance analysis of grid-coupled PV system under different load condition. The system is controlled through power balance theory method. The principle behind the control implementation is to evacuate the solar power generated during the daytime and the reactive power demand for the load should be supplied by the PV. The grid coupled system consists of SPV system, dc–dc boost converter, maximum power point tracking (MPPT), voltage source converter (VSC), LCL filter, different loads and three phase utility grid. This system is capable of eliminating harmonic and load balancing by supplying unbalanced current from the PV as a compensator. The system is simulated with 10 kW SPV array using indirect current control scheme.     

Performance investigation of linear and nonlinear controls for a fuel cell/supercapacitor hybrid power plant

In this paper, linear proportional–integral (PI) and nonlinear flatness-based controllers for dc link stabilization for fuel cell/supercapacitor hybrid power plants are compared. For high power applications, 4-phase parallel boost converters are implemented with a switching interleaving technique for a fuel cell (FC) converter, and 4-phase parallel bidirectional converters are implemented with a switching interleaving technique for a supercapacitor converter in the laboratory. As controls, mathematical models (reduced-order models) of the FC converter and the supercapacitor converter are given. The prototype small-scale power plant studied is composed of a PEMFC system (the Nexa Ballard FC power generator: 1.2 kW, 46 A) and a supercapacitor module (100 F, 32 V, based on Maxwell Technologies Company). Simulation (by Matlab/Simulink) and experimental results demonstrate that the nonlinear differential flatness-based control provides improved dc bus stabilization relative to a classical linear PI control method.     

A control scheme for improving the efficiency of DFIG at low wind speeds with fractional rated converters

This study proposes a scheme for extending the low speed range of operation of a Doubly Fed Induction Generator (DFIG) without down grading the efficiency. Also, only fractional rated converters are employed. The technique involves two operational modes for the generator. When the rotor speed is between 70% and 130% of the synchronous speed, the machine is operated in the normal Doubly Fed Induction Generator (DFIG) mode and when the rotor speed falls below 70%, it is operated in Stator Short Circuited (SSC) mode. The switch-over from the DFIG mode to the SSC mode is carried out at a threshold speed to maintain the efficiency of generator with the same fractional rated converters. The computer simulations on a typical DFIG (250 kW) in Matlab/Simulink environment illustrate that the range of efficiency improvement is from zero to 15%. Further, the experimental results on a 2.3 kW DFIG set up are also illustrated to demonstrate the efficacy of the scheme.     

Effect of varying concentration and temperature on steady and dynamic parameters of low concentration photovoltaic energy system

Low-concentration photovoltaic (LCPV) system has huge potential for further cost reduction of solar photovoltaic (PV) power as compared to flat panel PV. The dependence of steady state and dynamic parameters on concentration and temperature is crucial to extract maximum power from solar photovoltaic system. This article aims to present the effect of varying concentration and temperature on steady state and dynamic parameters of LCPV system under actual test conditions (ATC). The rate of change in I_SC with solar irradiation i.e., dI_SC/dG is found as 0.25 A/W assuming ≈±1 °C change in module temperature. The effect of temperature on inherent material properties responsible for photo-conversion efficiency is studied using impedance spectroscopy technique. A linear response of series resistance of LCPV module is observed with respect to change in module temperature, i.e. dR_S/dT from 297 to 333 K is in the range of 1.15–1.20 Ω with a rate of 1 mΩ/K. From real-time analysis of LCPV system open-circuit voltage found decreasing from 21 to 20.6 V with temperature coefficient of voltage ≈−0.061 V/K. The dynamic resistance has a positive coefficient of module temperature i.e., dr_d/dT given by 0.49 Ω/K.     

Towards the establishment of maximum PV generation limits due to power quality constraints

The solar irradiance dependent level of harmonic distortion due to PV generation is investigated through a combination of experimental and simulation studies. The expected growth of connection densities of PV systems in distribution networks coupled to the environmental conditions and in particular the fluctuations in solar irradiance can lead to undesirable variations of power and supply quality. In order to be able to predict the harmonic pollution due to PV generation, measurements of power quality indices were carried out at the output of a PV system for a period of 2 weeks. Then, a typical distribution system was modeled using the statistically obtained outcome of the measurements from the PV system. From the simulation of the chosen distribution system we obtained results for voltage distortion and categorized them based on solar irradiance levels. The procedure was then validated using the EN50160 analysis approach. The most extreme case, the high solar irradiance condition, was subsequently used for the simulation of different concentration scenarios, in order to assess the concentration limits set by power quality indices and the results are presented.     

A new optimal unified power flow controller placement and load shedding coordination approach using the Hybrid Imperialist Competitive Algorithm-Pattern Search method for voltage collapse prevention in power system

As power systems become more complex and heavily loaded, voltage collapse has become one of the most destructive events in modern power systems leading to blackouts in electric utilities worldwide. Voltage collapse is mainly caused by operating power systems at lower stability margins due to a surge in electric power demand. This paper presents an optimal unified power flow controller (UPFC) placement and load shedding coordination approach for voltage collapse prevention in N − K (K = 1, 2 and 3) contingency condition using Hybrid Imperialist Competitive Algorithm-Pattern Search (HICA-PS). ICA is the main optimizer of the proposed algorithm while pattern search is applied to further fine tune the results of the ICA. To show the effectiveness of the proposed approach in preventing voltage collapse in complex power systems, we implemented it on the New-England 39 bus power system. Its performance was also compared to that of some classical optimization techniques. Decrease in load shedding amounts, continuity of energy supply and voltage collapse prevention is the main positive features of the proposed approach.     

Microcontroller and solar power based electrical energy management system for renewable energy applications

“Microcontroller and solar power based electrical energy management system for renewable energy applications” has its own distribution, controlling and monitoring system for the amount of electrical energy (kW h) and this system is based on smart energy metering. This hardware project measures the electrical energy (kW h) as well as controls the amount of electrical energy from supply to load during day and night automatically. It is also equipped with the control of alternative source of electricity (solar energy). The measurement can be performed up to ten lac units and controlling can be up to two lac units. In the controlling part, the mains supply is automatically tripped and the project provides the alternative power supply in the absence of the mains supply by using the renewable (solar) energy source, when the energy consumption from the mains supply exceeds the preset limit.     

Operating point stability analysis of SMIB power system equipped with high PV penetration

This paper presents the operating point stability analysis of a Single Machine Infinite Bus (SMIB) power system equipped with high Photovoltaic (PV) penetration. The detailed dynamical model of the synchronous generator in dq reference frame is considered including the dynamics of the damper windings and Automatic Voltage Regulator (AVR). A DC–DC buck–boost switch mode converter is placed as an intermediate stage between the PV array and the inverter. The main function of this implementation is to inject the voltage corresponding to the PV generator Maximum Power Point (MPP) by automatic adjusting its duty cycle. The PV array is designed to provide a maximum output power of about 0.78 pu at the full solar irradiance. The nonlinearity of the output characteristics of the PV generator is taken into account. Operating point stability analysis is performed by extracting the eigenvalues of the linearized model around the operating point at different solar irradiances. System response after successive step changes on the synchronous generator input mechanical power at three solar irradiances based on the complete nonlinear dynamical model is investigated. For given synchronous generator input parameters, the response of the system after successive step changes on the solar intensity is addressed. It is found that high PV penetration via DC–DC buck–boost converter and DC–AC inverter is practically possible, experiences a stable operating point and can withstand successive step changes on system parameters in case of practical solar irradiance levels. All numerical simulations are conducted using MATLAB software package by building the code required.     

Direct power control of a multilevel inverter based active power filter

A cascaded H-bridge multilevel inverter based active power filter with a novel direct power control is proposed in this paper. It can be directly connected to medium/high voltage power line without using the bulky transformer or passive filter. Due to the limited switching frequency (typically below 1 kHz) of high-power solid-state devices (GTO/IGCT), multiple synchronous/stationary reference frame current controllers are reviewed and derived. Based on this, a novel current controller is proposed for harmonic current elimination and system power factor compensation. Furthermore, a synchronous/stationary hybrid structure can be derived with fundamental de-coupling control. The instantaneous reactive power theory and synchronous reference frame based control are compared based on mathematical models. A direct power control concept is then derived and proposed. It is equivalent as the hybrid synchronous/stationary frame current controller, but has a simpler implementation. It has clear physical meaning and can be considered as a simplified version of the hybrid frame current controller. Simulations on a 4160 V/1.2 MVA system and experimental results on a 208 V/6 kVA laboratory prototype are presented to validate the proposed active power filter design.     

3-Phase 4-leg unified series–parallel active filter system with ultracapacitor energy storage for unbalanced voltage sag mitigation

This paper presents the analysis and design of a 3-phase 4-leg (3P4L) unified series–parallel active filter (USPAF) with ultracapacitor energy storage (UCES) for improving the power quality in three-phase four-wire (3P4W) distribution system. The series and parallel active filter (AF) of 3P4L USPAF system are realized by four-leg voltage source inverters (VSIs) to a common dc-link capacitor. Due to its high power density, the UCES is well-suited to supply high power for short period of time. The objective of this paper is to enhance the unbalanced voltage sag mitigating capability of the 3P4L USPAF system by adding UCES, directly connected in the dc-link. This is achieved by injecting energy from the UCES to maintain the dc-link voltage constant. Thus, the proposed system is capable of mitigating unbalanced voltage sag with zero-sequence component in the source voltage and compensating harmonic, reactive power and unbalanced current of the load in 3P4W distribution systems. The proposed scheme is validated by an experimental prototype with a 1.93 F UCES bank and using dSPACE DS1103 real-time control platform in the laboratory. The experimental results show that the combined system offer improved performance to maintain the load voltage constant at its rated value during unbalanced voltage sag in the supply voltage.     

Optimal design of a robust discrete parallel FP + FI + FD controller for the Automatic Voltage Regulator system

The purpose of this paper is to design a good tracking controller for the generator Automatic Voltage Regulator (AVR) system. A fuzzy logic-based controller that is called Fuzzy P + Fuzzy I + Fuzzy D (FP + FI + FD) controller has been designed optimally and applied to AVR system. In the proposed method, optimal tuning of controller parameters is very important to achieve the desired level of robust performance. Thus, a hybrid of Genetic Algorithm (GA) and Particle Swarm Optimization (PSO) (HGAPSO) technique has been used to find a better fuzzy system control. The motivation for using this hybrid method is to increase disturbance rejection effort, reduce fuzzy system efforts and take large parametric uncertainties into account. The developed FP + FI + FD control strategy leads to a flexible controller with simple structure that is easy to implement. The simulation results have been compared with the conventional Proportional–Integral–Derivative (PID) and fuzzy PID controllers. Three cases of simulation have been performed, case 1: comparing the tracking capability of the controllers, case 2: comparing the disturbance rejection capability of the controller and case 3: evaluating the performance of the controllers assuming that amplifier and exciter system parameters have 50% uncertainty. The simulation results shows that the proposed parallel FP + FI + FD controller has good performance from the perspective of overshoot/undershoot, settling time, and rise time in comparison with both conventional and fuzzy PID controllers.     

Energy management strategy for fuel cell/battery/ultracapacitor hybrid vehicle based on fuzzy logic

In order to enhance the fuel economy of hybrid vehicle and increase the mileage of continuation of journey, a fuzzy logic control is utilized to design energy management strategies for fuel cell/battery (FC + B) hybrid vehicle and fuel cell/battery/ultra-capacitor (FC + B + UC) hybrid vehicle. The models of hybrid vehicle for FC + B and FC + B + UC structure are developed by electric vehicle simulation software ADVISOR which uses a hybrid backward/forward approach. The results demonstrate that the proposed control strategy can satisfy the power requirement for four standard driving cycles and achieve the power distribution among various power sources. The comprehensive comparisons with the power tracking control strategy which is wide adopted in ADVISOR verify that the proposed control strategy has better rationality and validity in terms of fuel economy and dynamic property in four standard driving cycles. Therefore, the proposed strategy will provide a novel approach for the advanced energy management system of hybrid vehicle.     

Methods used for evaluation of actual power generating thermal cycles and comparing them

In this study, thermodynamic optimization criteria used for assessing thermal engines are investigated and compared. The Purpose of this is to determine the most advantageous criteria. An irreversible Carnot cycle is analyzed by using five different methods and results are compared. According to calculations, the ecological function criterion (ECF) is defined as the most convenient optimization method. Although, its work output is less than the maximum work criteria and maximum available work (MAW), it has advantageous in terms of entropy generation and first law efficiency. In addition, ecological coefficient of performance (ECOP) and exergetic performance criteria (EPC) values provide minimum entropy generation and maximum efficiency at their maximum, however, their work output is very small. ECF obtains its maximum values at x = 0.488 (377.175 kW) for endoreversible cycle and at x = 0.477 (329.812 kW) for irreversible cycle. For these reasons, ECF is suggested as the best optimization criteria.     

Thermo-economic multi-objective optimization for a solar-dish Brayton system using NSGA-II and decision making

A 100 kW regenerative Brayton heat engine driven by the hybrid of fossil fuel and solar energy was considered for optimization based on multiple criteria. A thermodynamic model of such hybrid system was developed so that the power output, thermal efficiency and dimensionless thermo-economic performance with the imperfect performance of parabolic dish solar collector, the external irreversibility of Brayton heat engine and conductive thermal bridging loss could be obtained. Evolutionary algorithm based on NSGA-II (Elitist Non-dominated Sorting Genetic Algorithm) was employed to optimize triple-objective and dual-objective functions, where the temperatures of hot reservoir, cold reservoir and working fluid, the effectiveness of hot-side heat exchanger, cold-side heat exchanger and regenerator were considered as design variables. Using decision makings, including Shannon Entropy, LINMAP and TOPSIS methods, the final optimal solutions were selected from Pareto frontier obtained by NSGA-II. The results show that there exists an appropriate working fluid temperature to cause optimal solution under each given condition. The comparisons of triple-objective and dual-objective optimization with single-objective optimization indicate that multi-objective optimization can yield the more suitable results due to the lower deviation index from the ideal solution. In the analysis of triple-objective optimization, an expected result is obtained that the optimal values of the power out, efficiency and dimensionless thermo-economic performance of solar-dish Brayton system (68.65 kW, 0.2331 and 0.3077) are 22.6%, 34.9% and 18.4% respectively less than that of convectional Brayton heat engine. Finally, a range of functional relationship between the optimized objectives in Pareto frontier is fitted to provide more detailed insight into the optimal design of solar-dish Brayton system.     

Mitigation of transformer-energizing inrush current using grid-connected photovoltaic system

This paper presents a novel technique for suppressing the transformer-energizing inrush current. This technique is based on existing of a Photovoltaic (PV) generation system. Large magnitude inrush current can occur with certain combination of point of wave energization and residual core flux. The PV system is exploited to produce a magnetic flux in the core of the energized transformer in a negative direction of that produced by the main grid during grid-off. In this paper, the wave-energizing instant is optimally chosen and hence the amount of the residual flux existing in the core is controlled to be ready to sink the energizing effect. The impact of existing of PV system that connected to 280 kV, 60 Hz grid is studied at different energizing instances, different power ratings and different solar irradiances. The method is illustrated by simulation results and validated by harmonic analysis. The optimum energizing instances are explored at different working circumstances. The results at transient and steady states verify that the proposed technique enables the minimization of the inrush current by optimized grid-switching instance.     

Transient and steady-state performance analysis of hybrid powered DC series motor via DC shunt and PV generators with maximum power point tracking

This paper presents a comprehensive transient and steady-state analysis of hybrid powered DC series motor through DC shunt and PV generators. The solar cell generator is interfaced with the system through a DC–DC buck–boost switch mode converter. At full solar irradiance, the PV generator can completely run the DC series motor as its maximum power point is placed at the rated conditions of the DC series motor. As the solar intensity decreases, the shortage of power demanded by the series motor is compensated by the fuel-driven DC generator. To fully utilize the pollution-free PV generator at various solar intensities and, therefore, to reduce the total fuel consumption by the prime mover of the DC shunt generator, the operating point of the solar cells in case of hybridization is adjusted at its maximum power point by automatically calibrating the terminal voltage at the common coupling point of the two generators. The transient analysis comprises step changes in the load coupled to the motor at different solar irradiances and after successive step changes on the solar illuminations for some given fixed loading conditions. The nonlinearities of the PV generator are taken into account along with that of the ferromagnetic material of the two DC machines. The effect of changing the field resistance of the DC generator on the performance of the system is addressed. The steady-state output characteristics of the DC series motor when powered by only PV generator at full solar illumination, in case of hybridization at 0.85 and 0.70 of full solar intensity are outlined and compared.     

A high efficiency input/output magnetically coupled interleaved buck–boost converter with low internal oscillation for fuel-cell applications: Small signal modeling and dynamic analysis

Dynamic behavior of DC–DC converters plays a crucial role in stability of renewable energy exploitation systems. This paper presents small signal modeling of an input/output magnetically coupled interleaved buck–boost converter for fuel-cell applications to help the designers with the better understanding of converter dynamics. Aiming to have a continuous converter transfer function for a smooth transition between the operation modes and an improved inner dynamics, a damping network and an input/output coupling have been added to the interleaved structure of well-known cascaded buck–boost converter. Having the same step-up/step-down voltage transfer ratio, smooth transition and improved inner dynamics make this converter quite suitable for renewable energy applications. The paper presents a small signal ac equivalent circuit model of the proposed converter based on state space averaging (SSA) method. Simulation results show remarkable improvements in converter dynamic behavior in both time and frequency domains. Prototype setup of 360 W and 36 V output voltage for a fuel cell with a brand of “FCgen 1020ACS” Ballard Power Systems, Inc. was implemented. Experimental results are presented to verify the theoretical model and its expected merits.     

Design and development of a model-based hardware simulator for photovoltaic array

This paper presents a model based hardware simulator to emulate a photovoltaic (PV) array/module for all operating conditions. For making the model accurate the values of manufacturer dependent parameters pertaining to a PV array are extracted from the published data sheet of the array by a curve fitting based extraction technique. The proposed simulator consists of a microcontroller controlled switched mode DC–DC converter. The mathematical model of the PV array is embedded in the controller with provisions for the user to enter the required ambient conditions. A feedback compensator is implemented to achieve fast response and good stability and to minimize the steady-state error. As a test case to design, develop and test for compliance the published data of 115 W solar panel Shell S115 has been used. The prototype is tested for steady-state and transient conditions. The experimental results of the simulator are presented. The results are compared with the cell characteristics available in literature and compliance is confirmed.     

The impact of single-phase grid-connected distributed photovoltaic systems on the distribution network using P-Q and P-V models

The gradual emergence of photovoltaic (PV) systems as the most common distributed generation interconnected with the electric power system calls for a detailed power flow analysis with different models especially in the evolving unbalanced active distribution network. This paper is an extension of a previous study carried out on the performance evaluation of a 10 kWp grid-connected PV system deployed in a school, with the grid providing a virtual storage and access to upstream markets. For increased adoption of such systems in the unbalanced distribution network, it is pivotal to understand the mode of operation and the type of connection to the system. This article presents an impact analysis of such utility interactive single-phase PV systems distributed on all the single-phase load nodes of the traditional IEEE-13 bus distribution test feeder. The PV distributed generation (PV-DG) can be modelled as constant P-Q or P-V nodes with varied impacts in power flow studies for the unbalanced active distribution network. Results from these models are compared in terms of their impacts on voltage profiles at load buses, voltage unbalance, equipment loading, power losses and the total number of iterations for a converged power flow solution.     

A novel wavelet transform based voltage sag/swell detection algorithm

This paper proposes a novel sag/swell detection algorithm based on wavelet transform (WT) operating even in the presence of flicker and harmonics in source voltage. The developed algorithm is the hybrid of Daubechies wavelets of order 2 (db2) and order 8 (db8) to detect voltage sag/swell with and without positive/negative phase jumps. The hybrid detection algorithm can detect the start and end times of voltage sag/swell with and without phase jumps within 0.5 ms and 1.15 ms, respectively. The performance of the proposed voltage sag/swell detection method is compared with the results of dq-transformation, Fast Fourier Transform (FFT) and Enhanced Phase Locked Loop (EPLL) based voltage sag/swell detection methods. The good robustness and faster processing time to detect balanced and unbalanced voltage sag/swell are provided using proposed method. With the proposed hybrid detection algorithm consisting of db2 and db8 wavelet functions, a robust sag/swell detection is achieved which can give precise and quick response. The performance of proposed hybrid algorithm is validated and confirmed through simulation studies using the PSCAD/EMTDC analysis program.