A SINGLE STAGE SPV SYSTEM UTILIZING AN ADAPTIVE MANAGEMENT SCHEME FOR

MITIGATING LOAD HARMONICS WITH NEURAL NETWORKS JAKKULA SARITHA1 M.RAGINI2

1Pg scholar, Department of EEE (EPS), Avanthi Institute of Engineering & Technology, Hyderabad, TS, India 2Assistant Professor, Department of EEE, Avanthi Institute of Engineering & Technology, Hyderabad, TS, India

ABSTRACT

This project presents a single stage solar photovoltaic (SPV) system utilizing an adaptive control scheme with Neural Network controller. The SPV system includes SPV array, voltage source converter (VSC), ripple filter, nonlinear loads and a distribution network. The proposed SPV system feeds the active power to the distribution system, provides an effective use of solar PV array and mitigation of load harmonics currents. The SPV system with an incremental conductance (INC) based control scheme is used for obtaining the maximum power from the SPV array and an adaptive control scheme to control the switching pulses of the VSC. In addition, it utilizes a SPV feed-forward loop to improve the dynamic response and reduces the burden on the proportional-integral (PI) controller by regulating DC bus voltage. Simulation results are presented to validate the control, design and response of SPV system with various states. In the proposed system PI controller replaced by NN controller for better accuracy.

Key words: SPV , VSC, Harmonics, Neural Networks 1. INTRODUCTION Rapid increasing trend of electric energy demand in the world and depletion of fossil fuel reserves motivate power production from renewable sources [1]. The renewable energy sources based distributed generation (DG) system contributes the minimization of greenhouse gas emissions and undesirable environmental impacts caused thereby. An increased dependence on renewable energy resources is, therefore, exhibited in the recent years. Over 13.5% of global energy demand at present is met through renewable energy sources. The grid integrated single stage and two stage solar photovoltaic (PV) systems are reported in [2]. The growing interest has been exhibited in grid integrated PV system where the need for small scale energy storage device is waived off. The standalone PV system, on other

hand, demands for a battery storage system for its smooth and continuous operation. The operation and modelling of SPV system are discussed. The harvesting of energy from the fossil fuels to fulfil the demand of electricity for operating the large induction motor drives, trains, cranes and automobiles industries which results in atmospheric pollution by changing the carbon cycle and emits dangerous greenhouse gases. PV cells or modules provide DC power from the sun which is further transformed into AC or can be utilized direct to recharge the batteries. The PV array requires extraction of maximum power for real time implementation from water heating to electricity production. The PV system with grid integration feeds the acquired solar power to the grid without any need for small scale or large scale energy storage devices. On the other hand, the standalone PV system [6] necessitates the battery requirement at the night time which makes it expensive in comparison with the grid integrated PV systems.

The grid supportive PV systems with different converter topologies are reported. Whereas, voltage source converter (VSC) is still favorable converter over the other converters for SPV system because of its simplicity and wide range of operating frequencies. In the grid integrated PV systems, the distribution network is used as an electricity storage system where the energy is stored and then it can be re-purchased. Therefore, grid integrated PV systems do not require any physical energy storage and hence the cost of the system is reduced. In addition, losses and complexity are also minimized by utilizing a single stage PV configuration. Moreover, a PV system operating in islanded and grid integrated mode are discussed. The utilization of single stage PV system is proposed by many researchers. Moreover, the different single stage topologies are presented. Reduction of one stage (as compared to two stages) decreases the cost of the SPV system.

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Due to excessive initial installation cost of solar PV array, the aim is to obtain maximum output from the installed plant for an existing capacity. To complete the target of harvesting peak energy output from the PV sting numerous schemes are reported. Here, an incremental conductance based scheme is utilized for the purpose of maximum power point tracking (MPPT). Solar PV array is operated at MPP point when an increased conductance of solar array is identical to the actual conductance of solar array.

Various configurations and control schemes have been implemented in the literature for mitigation of PQ issues caused by nonlinear loads. The balanced nonlinear loads have only positive sequence components. However, compensation of unbalanced nonlinear loads is challenging task as they have negative and zero sequence components. The negative sequence component has same magnitude as positive sequence but opposite in polarity. However, zero sequence component flows through the neutral of the power system if any unbalancing occurs. Kumar et. al. [18] have proposed the distribution static compensator (DSTATCOM) system which works in both modes current control mode (CCM) and voltage control mode (VCM) thereby mitigating the current and voltage based PQ issues, respectively making the system interactive to loads and grid supply. During last few years, the research on the new control schemes have been going on to improve the performance of the system, such as a damped second order generalized integrator (DSOGI) scheme and model predictive controller. DSOGI based control algorithm is proposed along with the damping factor for extraction of fundamental component of load current. SOGI algorithm is implemented as a building block for the orthogonal signal generator (OSG). Moreover, OSG is utilized as feedback loop to estimate the error free phase and frequency of input signal. A third integrator is added to mitigate the DC component completely from the input load current. Here, a damping factor in SOGI is utilized to improve the steady state error and dynamic response of PV system under connection and disconnection of loads. However, dynamic response because of climate (insolations, temperature) change is not reported in it.

The VSC works as a compensator which also feeds the extracted PV energy to the distribution system. Moreover, it is utilized for power factor improvement, harmonics reduction and load balancing. The reported techniques are focused for evaluation of reference current from the polluted load current. Moreover, synchronous reference frame theory (SRFT) control approach to control the grid inverter is reported which is focused to reduce the high switching ripples. The fuzzy logic based scheme for grid interfaced SPV system is reported which is focusing on generation of PWM signals for the grid integrated inverter. However, power quality issues because of nonlinear loads are not considered in it. For efficient and reliable performance of SPV system, it is a challenge to make an accurate, fast, robust and well executed control scheme. In the proposed work, an adaptive control scheme is proposed to control the switching action of the grid tied VSC, for improving the dynamic response and to minimize the dependency on the proportional integral (PI) regulator of PV system. An adaptive digital scheme is presented. Whereas, reported literature is only focused on fixed frequency reference current generation and unbalanced utility voltage conditions.

2. LITERATURE SURVEY

2.1 M. Mirhosseini, J. Pou and V. G. Agelidis, Single and Two-Stage Inverter-Based Grid-Connected Photovoltaic Power Plants With Ride- Through Capability Under Grid Faults:Grid-connected distributed generation sources interfaced with voltage source inverters (VSIs) need to be disconnected from the grid under: 1)excessive dc-link voltage; 2)excessive ac currents; and 3)loss of grid-voltage synchronization. In this paper, the control of single- and two-stage grid-connected VSIs in photovoltaic (PV) power plants is developed to address the issue of inverter disconnecting under various grid faults. Inverter control incorporates reactive power support in the case of voltage sags based on the grid codes (GCs) requirements to ride-through the faults and support the grid voltages. A case study of a 1-MW system simulated in MATLAB/Simulink software is used to illustrate the proposed control. Problems that may occur during grid faults along with associated remedies are discussed.

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2.2 J.T. Bialasiewicz, “Renewable Energy Systems with Photovoltaic Power Generators: Operation and Modeling, A substantial increase of photovoltaic (PV) power generators installations has taken place in recent years, due to the increasing efficiency of solar cells as well as the improvements of manufacturing technology of solar panels. These generators are both grid-connected and stand-alone applications. We present an overview of the essential research results. The paper concentrates on the operation and modeling of stand-alone power systems with PV power generators. Systems with PV array-inverter assemblies, operating in the slave-and-master modes, are discussed, and the simulation results obtained using a renewable energy power system modular simulator are presented. These results demonstrate that simulation is an essential step in the system development process and that PV power generators constitute a valuable energy source. They have the ability to balance the energy and supply good power quality. It is demonstrated that when PV array- inverters are operating in the master mode in stand-alone applications, they well perform the task of controlling the voltage and frequency of the power system. The mechanism of switching the master function between the diesel generator and the PV array-inverter assembly in a stand-alone power system is also proposed and analyzed. 2.3W. Libo, Z. Zhengming, and L. Jianzheng, “A single-stage three-phase grid-connected photovoltaic system with modified MPPT method and reactive power compensation: Single-stage grid-connected photovoltaic (PV) systems have advantages such as simple topology, high efficiency, etc. However, since all the control objectives such as the maximum power point tracking (MPPT), synchronization with the utility voltage, and harmonics reduction for output current need to be considered simultaneously, the complexity of the control scheme is much increased. This paper presents the implementation of a single-stage three-phase grid-connected PV system. In addition to realize the aforementioned control objectives, the proposed control can also remarkably improve the stability of the MPPT method with a modified incremental conductance MPPT method. The reactive power compensation for local load is also realized, so as to alleviate grid burden. A DSP is employed to implement the proposed MPPT controller and reactive power compensation unit. Single-stage grid-connected photovoltaic (PV) systems have advantages such as simple topology, high efficiency,

etc. However, since all the control objectives such as the maximum power point tracking (MPPT), synchronization with the utility voltage, and harmonics reduction for output current need to be considered simultaneously, the complexity of the control scheme is much increased. This paper presents the implementation of a single-stage three-phase grid-connected PV system. In addition to realize the aforementioned control objectives, the proposed control can also remarkably improve the stability of the MPPT method with a modified incremental conductance MPPT method. The reactive power compensation for local load is also realized, so as to alleviate grid burden. A DSP is employed to implement the proposed MPPT controller and reactive power compensation unit. 3. PROPOSED SYSTEM

3.1 INC Based MPPT Approach The maximum power extraction from the PV array is ensured using INC based MPPT algorithm as shown in Fig.1. The point where the slope of (Ppv) versus (Vpv) curve is zero means the ratio of dPpv /dVpv is zero. Moreover, toward the left or right, the MPP ratio of PV power to PV voltage is altered. In single stage SPV system, DC voltage is adjusted in accordance with maximum power point of PV array. The PV array voltage and current are sensed for INC based MPPT method. On the basis of an INC approach, reference SPV voltage is evaluated which is named as reference DC bus voltage. The MPP can be tracked by comparing the instantaneous conductance to the incremental conductance. The algorithm is simple and efficient. In single stage SPV system, the SPV voltage is identical to the VSC capacitor voltage. The output of MPPT scheme is Ppv and reference DC bus voltage.

Fig. 1 three-phase PV inverter system.

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Where Vdc*(r) and Vdc*(r-1) are updated and past samples of reference DC link voltage.

3.2CONTRL SCHEME OF PROPOSED SCHEME WITH NN CONTROLLER

The adaptive control scheme of PV system is presented in Fig.2. The SPV feed-forward term, load currents and point of interconnection voltages are utilized for generation of grid reference currents. The feed-forward loop from PV array is estimated as,

Where Vt is amplitude of the PIC voltage. This loop reduces the burden on PI controller.

Fg 2 Proposed control system

The VSC switching algorithm which includes the salient control signals of terminal voltage (Vt), unit templates (wpa, wpb, and wpc), loss component (Iwp), feed forward component of PV (Ipvf) and the reference grid currents (isa*, isb*and ic* ). For execution of proposed control

technique, it is necessary to evaluate the voltage unit vectors. The PIC line voltages (vsab, vsbc) are sensed to estimate phase voltages. The magnitude of point of interconnection phase voltages as

The quadrature unit vectors are evaluated with the help of in phase unit vectors as

For realization of adaptive control scheme, frequency and magnitude of the (ifLa) must be expressed correctly. Comprehensive equations of second loop for extraction of fundamental is express as follows,

Similarly, extracted magnitude of load current of first section is formulated as,

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However, the fundamental component is evaluated as,

An implementation of these equations is shown in Fig.3. Moreover, these mathematical equations do not have any trigonometric function or voltage control oscillator (VCO) in its formation. Therefore, it is fast as compared to other schemes. Gains G1 and G2 control the behavior of proposed control scheme under steady state and dynamic condition. Artificial Neural Networks are massively interconnected networks in parallel of simple elements (usually adaptable), with hierarchic organization, which try to interact with the objects of the real world in the same way that the biological nervous system does. As a simple element we understand the artificial equivalent of a neuron that is known as computational neuron or node. These are organized hierarchically by layers and are interconnected between them just as in the biological nervous systems. Upon the presence of an external stimulus the artificial neural network generates an answer, which is confronted with the reality to determine the degree of adjustment that is required in the internal network parameters. 4.PROPOSED SYSTEM SIMULATION RESULTS

Fig.3 MATLAB/SIMULATION Circuit diagram of proposed

system

Fig.4 Proposed adaptive control scheme with PI

controller

Fig .5 Proposed adaptive control scheme with NN

controller

Fig.6 Proposed Three-phase grid-side currents in the grid-

connected PV inverters w

Fig.7 single phase voltage

Fig.8 DC link voltage simulation and theretical

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Fig.9 Source Voltage THD% ( THD%=24.73%) OF EXISTING

SYSTEM

Fig.10 Source Voltage( THD%=16.51%) OF PROPOSED

SYSTEM

CONCLUSION It has been validated that the solar PV system using adaptive control scheme with Neural network controller has been found quite suitable for grid supportive system. Moreover, it is able to supply good quality of power to the grid. A new method for reference grid current generation for SPV system has been proposed by controlling the voltage at DC bus and feed-forward loop from MPPT. The NN controller has regulated DC bus voltage and controlled grid reference currents in phase with sensed grid currents. The benefits of NN controllers are1) Its structure massively distributed in parallel 2) Its ability to learn and generalize 3) Response speed high REFERENCES [1] V. Smil, “Power Density: A Key to Understanding Energy Sources and Uses” MIT Press, 2015. [2] M. Mirhosseini, J. Pou and V. G. Agelidis, “Single and Two-Stage Inverter-Based Grid-Connected Photovoltaic Power Plants With Ride- Through

Capability Under Grid Faults,” IEEE Trans. on Sus. Energy, vol. 6, no. 3, pp. 1150-1159, July 2015. [3] J.T. Bialasiewicz, “Renewable Energy Systems with Photovoltaic Power Generators: Operation and Modeling,” IEEE Trans. Ind. Elect., vol.55, no.7, pp.2752-2758, July 2008. [4] W. Libo, Z. Zhengming, and L. Jianzheng, “A single-stage three-phase grid-connected photovoltaic system with modified MPPT method and reactive power compensation,” IEEE Trans. Energy Convers., vol. 22, no. 4, pp. 881–886, Dec. 2007. [5] F. Liu, S. Duan, Fei Liu, B. Liu, and Y. Kang, “A variable step size INC MPPT method for PV systems,” IEEE Trans. Ind. Electron., vol. 55, no. 7, pp. 2622–2628, Jul. 2008. [6] M. Das and V. Agarwal, “Novel High-Performance Stand-Alone Solar PV System With High-Gain High-Efficiency DC–DC Converter Power Stages,” IEEE Trans. Ind. Appl.,vol. 51, no. 6, Nov.-Dec. 2015.

JAKKULA SARITHA Completed her Bachelor of technology (B.Tech-EEE) from Balaji Institute of Technology & Science and Master of Technology(M.tech-EPS) From Avanthi Institute of Engineering and Technology. Her research interest includes Power Electronics, Renewable energy sources, electrical drives etc.

M.RAGINI completed her Bachelor of Technology (B.Tech.) from Scient Institute of Engineering & Technology and Master of Technology (M.Tech.) from Nova College of engineering. Presently, she is working as assistant professor in Avanthi Institute of Engineering & Technology. Her research interest includes Power systems, Power Electronics, electrical drives etc.

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