Showing papers in "Iet Renewable Power Generation in 2010"

Renewable energy sources and frequency regulation: survey and new perspectives

Abstract: As the use of renewable energy sources (RESs) increases worldwide, there is a rising interest on their impacts on power system operation and control. An overview of the key issues and new challenges on frequency regulation concerning the integration of renewable energy units into the power systems is presented. Following a brief survey on the existing challenges and recent developments, the impact of power fluctuation produced by variable renewable sources (such as wind and solar units) on system frequency performance is also presented. An updated LFC model is introduced, and power system frequency response in the presence of RESs and associated issues is analysed. The need for the revising of frequency performance standards is emphasised. Finally, non-linear time-domain simulations on the standard 39-bus and 24-bus test systems show that the simulated results agree with those predicted analytically.

Adaptive perturb and observe algorithm for photovoltaic maximum power point tracking

Abstract: The optimisation of energy generation in a photovoltaic (PV) system is necessary to let the PV cells operate at the maximum power point (MPP) corresponding to the maximum efficiency. Since the MPP varies, based on the irradiation and cell temperature, appropriate algorithms must be utilised to track the MPP. This is known as maximum power point tracking (MPPT). Different MPPT algorithms, each with its own specific performance, have been proposed in the literature. A so-called perturb and observe (P&O) method is considered here. This method is widely diffused because of its low-cost and ease of implementation. When atmospheric conditions are constant or change slowly, the P&O method oscillates close to MPP. However, when these change rapidly, this method fails to track MPP and gives rise to a waste of part of the available energy. An adaptive P&O method is proposed in this study that has faster dynamics and improved stability compared to the traditional P&O. The MPPT algorithm was set up and validated by means of numerical simulations and experimental tests, confirming the effectiveness of the method.

Supporting high penetrations of renewable generation via implementation of real-time electricity pricing and demand response

Abstract: The rollout of smart meters raises the prospect that domestic customer electrical demand can be responsive to changes in supply capacity. Such responsive demand will become increasingly relevant in electrical power systems, as the proportion of weather-dependent renewable generation increases, due to the difficulty and expense of storing electrical energy. One method of providing response is to allow direct control of customer devices by network operators, as in the UK `Economy 7` and `White Meter` schemes used to control domestic electrical heating. However, such direct control is much less acceptable for loads such as washing machines, lighting and televisions. This study instead examines the use of real-time pricing of electricity in the domestic sector. This allows customers to be flexible but, importantly, to retain overall control. A simulation methodology for highlighting the potential effects of, and possible problems with, a national implementation of real-time pricing in the UK domestic electricity market is presented. This is done by disaggregating domestic load profiles and then simulating price-based elastic and load-shifting responses. Analysis of a future UK scenario with 15 GW wind penetration shows that during low-wind events, UK peak demand could be reduced by 8-11 GW. This could remove the requirement for 8-11 GW of standby generation with a capital cost of £2.6 to £3.6 billion. Recommended further work is the investigation of improved demand-forecasting and the price-setting strategies. This is a fine balance between giving customers access to plentiful, cheap energy when it is available, but increasing prices just enough to reduce demand to meet the supply capacity when this capacity is limited.

Comparison between CFD simulations and experiments for predicting the far wake of horizontal axis tidal turbines

Abstract: The actuator disc is a useful method for parameterising a tidal stream turbine in a solution of the Reynolds-averaged Navier–Stokes equations. An actuator disc is a region where similar forces are applied to a flow as would be imposed by a turbine. It is useful where large-scale flow characteristics are of interest, such as the far wake, free surface effects, or installation of multi-turbine arrays. This study compares the characteristics of the wake of an actuator disc, modelled using a steady solution to the Reynolds-averaged Navier–Stokes (RANS) simulated equations, with the k–ω shear stress transport (SST) turbulence model, to experimental data measured behind discs of various porosities. The results show that the wake of the experimental and modelled discs has similar characteristics; in both model and experiment, velocity in the near wake decreased as thrust coefficient increased. However, the near wake region in the experiment was shorter than simulated in the model because of near wake turbulence. This, combined with lower ambient turbulence levels in the model, meant that the far wake recovered further downstream, while showing similar overall trends in velocity and turbulence intensity.

Multiobjective genetic algorithm solution to the optimum economic and environmental performance problem of small autonomous hybrid power systems with renewables

Abstract: The overall evaluation of small autonomous hybrid power systems (SAHPS) that contain renewable and conventional power sources depends on economic and environmental criteria, which are often conflicting objectives. The solution of this problem belongs to the field of non-linear combinatorial multiobjective optimisation. In a multiobjective optimisation problem, the target is not to find an optimal solution, but a set of non-dominated solutions called Pareto-set. The present article considers as an economic objective the minimisation of system's cost of energy (COE), whereas the environmental objective is the minimisation of the total greenhouse gas (GHG) emissions of the system during its lifetime. The main novelty of this article is that the calculation of GHG emissions is based on life cycle analysis (LCA) of each system's component. In LCA, the whole life cycle emissions of a component are taken into account, from raw materials extraction to final disposal/recycling. This article adopts the non-dominated sorting genetic algorithm (NSGA-II), which in combination with a proposed local search procedure effectively solves the multiobjective optimisation problem of SAHPS. Two main categories of SAHPS are examined with different energy storage: lead-acid batteries and hydrogen storage. The results indicate the superiority of batteries under both economic and environmental criteria.

Analytical modelling of partial shading and different orientation of photovoltaic modules

Abstract: Analytical modelling of a photovoltaic (PV) power system for studying the effects of partial shading and different orientation of PV modules is presented. The proposed analytical model, although limited to the case of series PV modules and composed of complicated non-linear implicit functions, allows several important electrical characteristics of a PV system, such as I - V curve, open-circuit voltage, short-circuit current, maximum power and reverse voltage, to be investigated and presented in two- and three-dimensional graphs to provide in-depth physical interpretation of the issue.

Working towards frequency regulation with wind plants: Combined control approaches

Abstract: A pitch angle controller and a rotor speed controller are proposed for wind plant output active power adjustment. Combining the turbine inertial control with pitch angle control provides a better outlook for long-term control. The control schemes are developed for wind generators to equip them with the capability to participate in restoring grid frequency after a disturbance. With the proposed controllers, the wind plant has a higher flexibility to operate more like a synchronous generator and is able to output either larger or smaller amounts of power as required so as to contribute to grid frequency restoration. The controllers are tested on a four-bus test system for verification of the grid frequency performance under varying system dynamic conditions.

Experimental characterisation of flow effects on marine current turbine behaviour and on its wake properties

Abstract: Experimental results of tests carried out to investigate the hydrodynamics of marine current turbines are presented. The objective is to build an experimental database in order to validate the numerical developments conducted to characterise the flow perturbations induced by marine current turbines. For that purpose, we used a tri-bladed horizontal axis turbine. The work is dedicated to measuring the behaviour of the system and to characterising the wake generated by the turbine. The efficiency of the device is quantified by the measurement of the thrust and the amount of power generated by the rotor for various inflow conditions, whereas the wake is characterised by Laser Doppler Velocimetry. Particular attention is paid to the flow characteristic effects on the performance of a 0.70 m diameter turbine. The load predictions on the structure and the measured performance of the turbine over its working range of currents and rotational speeds are presented. The results showed that this kind of turbine is sensitive to the quality of the incoming flow. The turbulence intensity effects on turbine behaviour and on its wake are also characterised in order to study how the far wake decays downstream and to estimate the effect produced in downstream turbines.

Efficient control scheme of wind turbines with doubly fed induction generators for low-voltage ride-through capability enhancement

Abstract: As the penetration of wind power in electrical power systems increases, it is required that wind turbines remain connected to the grid and actively contribute to the system stability during and after grid faults. This study proposes an efficient control strategy to improve the low-voltage ride-through (LVRT) capability in doubly fed induction generators (DFIGs). The proposed scheme consists of passive and active LVRT compensators. The passive compensator is based on a new crowbar arrangement located in series with stator windings. It considerably reduces the rotor inrush current at the instants of occurring and clearing the fault. The active LVRT compensator is realised through rotor voltage control and reduces the oscillations of electrical torque, rotor instantaneous power and DFIG transient response during the voltage dip. The proposed LVRT control not only reduces the peak rotor fault current, but also minimises the rotor instantaneous power oscillations, and consequently limits the dc-link voltage fluctuations. At the end, results of theoretical analyses are verified by time-domain simulations.

Analysis of demand response and wind integration in Germany's electricity market

Abstract: Demand response represents an additional option for reserve capacity as first market experiences have demonstrated. An analysis for Germany shows capacities up to 3 GW and costs starting at 30 euro/MWh in the industrial sector, 8 GW in the commercial sector and more than 20 GW in the residential sector including night storage heating. Simulations of the German power system showed that using these potentials together with improved wind power predictions can limit the additional balancing costs in Germany to below 2 euro/MWh feed-in by wind turbines with 48 GW wind power in 2020.

Optimal participation and risk mitigation of wind generators in an electricity market

Abstract: Forecasted output of wind electric generators (WEGs) in a 24-h horizon has large uncertainties. These uncertainties pose a challenge while computing optimal bids necessary for participating in the day-ahead unit commitment process (DACP) thus limiting their integration and success. This study proposes a new optimal participation strategy for a WEG that employs an energy storage device (ESD) for participating in the DACP. The WEG is modelled to function as a price-taker. The proposed formulation has two objectives: (a) maximise returns from the market considering the best forecast; and (b) minimise risks considering the forecast uncertainties. Risk in the participation strategy is quantified by computing expected energy not served (EENS). The multiobjective mixed integer linear programming formulation is transformed into a fuzzy optimisation problem and solved. Through suitable examples, the ESD is shown to play two important roles: (a) it helps to shift wind energy produced during hours with low marginal prices to those hours with higher marginal prices by appropriately storing and releasing it. This shift can be forward or backward in time. (b) The second crucial role played by ESD, upon minimising EENS, is to maintain an energy reserve akin to spinning reserve such that the risk of the optimal participation schedule is the least.

Improved SMS islanding detection method for grid-connected converters

Abstract: Islanding detection is a mandatory function for grid-connected converters. The popular slip mode frequency shift (SMS) and auto phase shift active islanding detection methods are investigated and an improved (IM)-SMS strategy is proposed in this study. In the proposed method, additional phase shift is introduced to help in stimulating the action of the islanding detection and the algorithm is simplified as well. When the utility grid is disconnected, the algorithm keeps the frequency of the converter output voltage deviating until the frequency protection relay is triggered. The working principle of the method is introduced and the guidance of parameters selection and optimisation is also provided. The islanding detection performance is evaluated through theoretical analysis and verified by digital simulation and experimental results. The IM-SMS method exhibits features of simplicity, easy implementation and high reliability.

Modelling tidal energy extraction in a depth-averaged coastal domain

Abstract: An extension of actuator disc theory is used to describe the properties of a tidal energy device, or row of tidal energy devices, within a depth-averaged numerical model. This approach allows a direct link to be made between an actual tidal device and its equivalent momentum sink in a depth-averaged domain. Extended actuator disc theory also leads to a measure of efficiency for an energy device in a tidal stream of finite Froude number, where efficiency is defined as the ratio of power extracted by one or more tidal devices to the total power removed from the tidal stream. To demonstrate the use of actuator disc theory in a depth-averaged model, tidal flow in a simple channel is approximated using the shallow water equations and the results are compared with the published analytical solutions.

Reliability assessment of a wind-power system with integrated energy storage

Abstract: This study looks into reliability assessment and components rating of a wind-power system with integrated battery energy storage. The system can potentially be used in remote electrification projects to mitigate the reliance on diesel generators. A reliability assessment method has been proposed in this study, based on a combination of the traditional analytical and simulation-based approaches, to enable calculation of reliability indices, required battery capacity and power rating, and power rating of the power-electronic converter of the wind-power units. The proposed method is easy to implement in the MATLAB software environment, takes into account the units forced outage rate (FOR), and also permits modelling of the grid-connected mode.

Coordinated reactive power control for facilitating fault ride through of doubly fed induction generator- and fixed speed induction generator-based wind farms

Abstract: This study proposes a coordinated control strategy for adjacent fixed speed induction generator (FSIG)- and doubly fed induction generator (DFIG)-based wind farms whereby the reactive power output of the DFIG is prioritised over active power after a fault on the network, in order to boost the FSIG terminal voltage and improve its fault ride-through capability. The simulation results show that the stability margin of an FSIG can be significantly greater when there is extra reactive power compensation available from a DFIG in the vicinity. It is also seen that the increase in FSIG active power output because of higher voltage can compensate for the reduction in DFIG active power. The effects of different reactive power levels and coupling impedance on stability improvement and active power generation are also investigated. It is shown that an optimal reactive power limit for the DFIG should be chosen which takes account of both the increase in stability and also of the active power generation from the DFIG and FSIG wind farms.

Direct current offset controller for transformerless single-phase photovoltaic grid-connected inverters

Abstract: Limitation of direct current (DC) injection into the alternating current (AC) network is an important operational requirement for grid-connected photovoltaic (PV) systems. One way to ensure that this requirement is met is to use a power transformer as interface to the AC network. But this adds costs, mass, volume and power losses. In a transformerless system, the inverter forming part of the PV system has to be operated so that the DC content in its output current is below the specified limits. Ideally, there should be no DC at the output of the inverter, but in practice, in the absence of special measures, a small amount of DC current is present. A technique for elimination of the DC-offset is proposed. It is based on the sensing of the DC-offset voltage at the output of the inverter. A sensor output is used to drive a feedback system designed to control the operation of the inverter so that the DC-offset is forced to stay within the acceptable limits. A mathematical model for the DC-offset controller is derived. A design procedure, based on the model, is proposed for the controller. The results of tests performed on a system of 1 kW nominal rating provide validation for the mathematical model.

Integration of large-size photovoltaic systems into the distribution grids: a p-q chart approach to assess reactive support capability

Abstract: This study addresses the issue of the control of both active (P) and reactive (Q) power flows injected by grid connected photovoltaic (PV) units participating in the provision of the reactive power compensation ancillary service. A model is derived allowing to define a sort of `capability chart` for the PV unit, that is, the set of points in the P-Q plane which, at steady state, can be reached by properly operating the control system and without exceeding the physical limits of all the involved devices. To this aim, a basic algorithm is developed in the MATLAB environment relying on a simplified description of the system, which neglects the harmonics injected by the voltage source converter inverter used to connect the PV unit with a medium voltage radial distribution grid. The proposed model is used to evaluate all the possible working points in the P-Q chart; a proof of the stability of those points, using the well-known Lyapunov methods, is also annexed. Finally, simulations with the electromagnetic code PSCAD-EMTDC are performed in order to assess the validity of the approximate results and to improve the developed basic algorithm.

Probabilistic evaluation of transient stability of a power system incorporating wind farms

Abstract: This study presents a stochastic-based approach to evaluate the probabilistic transient stability indices of a power system incorporating wind farms (WFs). In this context, investigations have been conducted on a hypothetical test system representing a typical power system taking into consideration the uncertainties of the factors associated with the practical operation of a power system, namely fault type, fault location, fault impedance, fault clearing process, system parameters and operating conditions and high-speed reclosing process. The effects of the WF sizes and locations on the overall system stability have been investigated. Moreover, this study presents stochastic models for the wind turbine as well as the spring constant of the reduced two-mass shaft model of the wind generator. The time-domain simulations are obtained using the electro-magnetic transient programme.

Design and implementation of a hybrid regenerative power system combining grid-tie and uninterruptible power supply functions

Abstract: A hybrid regenerative power system including photovoltaic (PV) and wind powers and combining the functions of the grid-tie system and uninterruptible power supply (UPS) for critical load applications is presented. The proposed system employs six-arm converter topology with three arms for the rectifier-inverter, one arm for battery charging/discharging and two arms for power conversion of the PV module and wind turbine generator. The operation modes include the grid-tie mode and the UPS mode depending on the grid status. A power balance control scheme is presented, which can reduce the grid power and utilise the regenerative power in the most effective way for fulfilling the two requirements of a three-stage charging of the battery and no interruption of the load. Also, the PV and wind powers can be utilised with priority in order to provide the flexibility for adapting to local circumstances. A single-phase 1.2-kW/110-V system is designed and implemented, and the effectiveness of the proposed system and control methodology are verified with some experimental results.

Improved current regulation of three-phase grid-connected voltage-source inverters for distributed generation systems

Abstract: Stationary frame linear proportional integral (PI) control has been widely used for current regulation of three-phase voltage-source inverters for a long time, but it has disadvantages of steady-state errors for ac quantities regulation. An attractive solution to eliminate these errors is P+Resonant (PR) control. However, one of its limitations is slow transient response in the startup. So far, no literature has explicitly explained the reason for its slow response phenomena. Therefore this study reveals, for the first time, a clear description of its nature by using classical control theory. To improve the current regulation performance of PR control, a novel proportional complex integral (PCI) control is proposed in this study. Unlike PR control based on the internal model principle, the proposed PCI control is derived from the concept of infinity gain to achieve zero steady-state error. A straightforward well-formulated design procedure for the PCI controller is presented based on system bandwidth criterion. In addition, practical issues such as control delay effects and digital roundoff error mitigations are discussed. Theoretical analysis, continuous-domain simulations in the MATLAB/Simulink environment and discrete-domain digital control experiments based on TMS320F2812 DSP are carried out from the viewpoint of steady-state and dynamic performance comparisons among stationary frame PI, PR and PCI control strategies. In agreement with theoretical analysis, simulation and experimental results indicate that PCI control is the best solution among three strategies to achieve fast and accurate current regulation for three-phase grid-connected voltage-source inverters.

Assessment of distribution network limits for non-firm connection of renewable generation

Abstract: Before new renewable generators can be connected to the electricity network, it is necessary to carefully evaluate the impact they will have. Firm connection agreements are based on snapshot assessments of the worst-case situation of maximum generation and minimum demand, which restrict renewable capacities despite infrequent occurrence. This work describes how time series of several renewable generation technologies together with demand can be applied to examine the opportunities and challenges offered by non-firm generation connections. It applies optimal power flow to extract maximum energy from available renewable resources while using curtailment of generation to maintain the network within thermal and voltage limits. By way of a case study of potential wind, wave and tidal current development in the Orkney Islands, Scotland, the analysis provides estimates for the degree of curtailment and consequent economic impact a renewable generator operating under non-firm connection may experience. The methods described provide a first-level analysis that could facilitate appraisal of non-firm connections at the planning stage by estimating the consequences of concurrent generation and demand as well as the frequency and duration of necessary curtailments.

Modelling and control of a marine current turbine-driven doubly fed induction generator

Abstract: This paper deals with the modelling and control of a variable speed doubly fed induction generator-based marine current turbine with and without tidal current speed sensor. The proposed maximum power point tracking control strategy relies on the resource and the marine turbine models that were validated by experimental data. The sensitivity of the proposed control strategy is analysed regarding the swell effect because it is considered as the most disturbing one for the resource model. Tidal current data from the Raz de Sein (Brittany, France) are used to run simulations of a 7.5-kW prototype over various flow regimes. Simulation results are presented and fully analysed.

Coordinated output control of multiple distributed generation schemes

Abstract: Candidate strategies for the coordinated output control of multiple distributed generation (DG) schemes are presented. The proposed strategies are underpinned by power flow sensitivity factors and allow real-time knowledge of power system thermal ratings to be utilised. This could be of value in situations where distribution network power flows require management as a result of DG proliferation. Through off-line open-loop simulations, using historical data from a section of the UK distribution network, the candidate strategies are evaluated against a benchmark control solution in terms of annual energy yields, component losses and voltages. Furthermore, the individual generator annual energy yields and generator-apportioned losses are used to assess the net present values of candidate control strategies to DG developers.

Harmonic assessment of variable-speed wind turbines considering a converter control malfunction

Abstract: This study is on variable-speed wind turbines with permanent magnet synchronous generator. Three different drive train mass models and three different topologies for the power-electronic converters are considered. The three different topologies considered are, respectively, a matrix, a two-level and a multilevel converter. A novel control strategy, based on fractional-order controllers, is proposed for the wind turbines. The influence of a converter control malfunction on the harmonic current emissions is studied. The performance of disturbance attenuation and system robustness is ascertained. Simulation results are presented, and conclusions are duly drawn.

Self-excitation and control of an induction generator in a stand-alone wind energy conversion system

Abstract: This study presents a rugged and cost-effective scheme for start-up and operation of a stand-alone squirrel cage induction machine (SCIG) for a wind energy conversion system (WECS). A voltage source converter (VSC) directly interfaces the SCIG with an equivalent dc load network, which could also be the DC link of an inverter. The VSC dc bus is supported by an electrolytic capacitor and the proposed scheme ensures reliable start up with these rudimentary components. The dc bus voltage is ramped to the rated value from a small initial voltage produced by remanent magnetism of the SCIG core. Subsequently, the load is applied. The V/f control paradigm for motoring operation is suitably extended to the SCIG for controlling both voltage build-up and dynamic transients. A controller is specifically designed to maintain constant dc bus voltage under wind speed and electrical load variations. Steady-state machine flux is maintained constant up to the base speed, which maximises machine utilisation and power extraction especially at higher wind speeds. The overall system modelling and analytical control design is presented. The proposed control strategy has been validated through simulation and experimentally verified on a low power (2.2 kW) laboratory prototype.

Experimental verification of linear generator control for direct drive wave energy conversion

Abstract: It has been shown through modelling and simulation that a linear electrical generator can be effectively controlled to maximise the energy extracted from sea waves. A reaction force control scheme allows the performance of a direct drive wave energy converter to be optimised, which adds to the benefits of low mechanical complexity and high conversion efficiencies in a direct drive system. In this study, reaction force control through experimental verification is presented. The use of a linear generator test rig and electronic hardware to control the phase and amplitude of oscillation are investigated.

Investigations into the performance of photovoltaics-based active filter configurations and their control schemes under uniform and non-uniform radiation conditions

Abstract: With their growing popularity as alternative to conventional fuel, photovoltaics(PV) are being increasingly employed for a wide range of applications. Power quality improvement through a PV-fed active filter (PVAF) or inverter is one such application. A PVAF can supply active and reactive power to the grid when solar radiation is available or control the reactive power during periods of low or no radiation. The advantages of PVAF as an independent active source of compensation, reduced transmission losses etc. over a conventional AF have already been highlighted in the literature. However, not much attention has been paid to the effects of the PVAF configuration on its operational performance. Similarly, the effect of control schemes used to control such filters has also not been investigated. Various configurations of PVAF (viz. string- or multiple-string- or centralised-inverter based) are possible depending upon the configuration of the PV array (source). This study compares various PVAF configurations and their control schemes in terms of their ability to compensate reactive power, harmonics and phase-imbalances caused by unbalanced linear and non-linear loads in a three-phase, four-wire distribution system. Performance parameters such as power output and power quality as described by total harmonic distortion, phase imbalance and neutral conductor current are evaluated under both uniform and non-uniform radiation (e.g. partially shaded) conditions. In particular, results are presented to highlight and compare the effects of partial shading on the control schemes and PVAF configurations. Based on this study, several critical observations and remarks are presented.

Experimental measurements of irregular wave interaction factors in closely spaced arrays

Abstract: Much of the published work concerning the response and power output of closely spaced arrays of heaving wave energy devices concerns behaviour in regular waves only and is based on numerical analysis. To date, limited experimental work has been published and it remains unclear how device interactions predicted in idealised models relate to the response of proposed devices in realistic irregular wave-fields. Experimental measurements of the power absorbed by a small two-dimensional array of heaving devices in both regular and irregular waves in a wide flume are reported. In regular wave conditions, positive interactions (where the average power output of the array exceeds the same number of isolated devices) are measured. These tests indicate that the occurrence of positive interactions is largely dependent on the incident wave period and the performance of adjacent devices. Preliminary tests indicate that float responses tend to be smaller when subjected to short period irregular waves of matching peak frequency and standard deviation of surface elevation. The data presented provide an insight into interactions within irregular wave conditions and forms a basis for evaluating numerical models.

Offshore underwater substation for wave energy converter arrays

Abstract: In this study, the design, construction, deployment and operation of an offshore underwater substation is discussed. The seabed placed substation interconnects three linear generator wave energy converters (WECs) at the Swedish Lysekil wave energy research site. The power from the WECs fluctuates because of their direct-driven topology. The generator voltage has varying electrical frequency and amplitude. To reduce the fluctuations, the individual voltages of the WECs are rectified and the power is added on a common DC-bus in the substation. The voltage is inverted, transformed and power is transmitted to an on-shore resistive load. The substation was retrieved on two occasions since the deployment in the spring of 2009. The functionality of the substation is validated by comparing voltage and current wave forms from Simulink with measured results from laboratory experiments. In addition, a sample of results from real offshore operation is illustrated and discussed. With a proportional-integral-derivative (PID)-regulator in the inverter control, the small fluctuations in the DC-bus voltage could be minimised. However, this would reduce the energy storage capability of the DC-link smoothing capacitors.

Simple and high-efficiency photovoltaic system under non-uniform operating conditions

Abstract: The interest in improving the efficiency of photovoltaic (PV) system has emerged because of increasing the number of home-based or small-scale PV power system. However, the home-based PV system is vulnerable to the non-uniform operating conditions. Under such circumstances, multiple-local maximum power points (MPPs) occur on the power-voltage characteristics and an advanced control algorithm is required to track the global MPP. It is very difficult to provide a sophisticated control algorithm because of the non-linear characteristics of PV system. This study describes the potential to improve the efficiency of PV arrays under non-uniform operating conditions by using the conventional hill-climbing MPP tracking method in total cross tied (TCT) connected PV arrays, in which each group of series connected solar cells that belong to single bypass diode is interconnected. The various scenarios were tested and the results indicate that the efficiency of the proposed system is much higher than that of the same size of series-parallel (SP) PV array configuration.