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Control Scheme for the Reduction of Peak Current and Current Harmonics in Grid Connected Photovoltaic Inverters

Jinu John1, Alan M George2
  1. M.Tech Scholar, Department of EEE, ASIET, Kalady, Kerala, India1
  2. Assistant Professor, Department of EEE, ASIET, Kalady, Kerala, India2
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International Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering

Abstract

Peak current and current harmonics are main problems associated in a grid connected photovoltaic system under unbalanced voltage sag. During the voltage sag, the photovoltaic source should operate with increasing converter currents to maintain the injection of the generated power causes peak current in the system. There are number of grid fault controllers are used in voltage sag condition. Using these controllers, considering the condition of minimum peak current causes injection large current harmonics distortion during sag voltage. In this paper present a control scheme used to minimize peak current and current harmonics distortion, which can be implemented in unbalanced voltage sag condition.

Keywords
Voltage sag, Peak current, Photovoltaic (PV) inverter, gridfault, Current harmonics

INTRODUCTION
Grid-connected photovoltaic power systems are power systems energized by solar energy which are connected to the utility grid. Grid-connected PV power systemsconsist of PV panels, MPPT, inverters, power conditioning control units and grid connection equipment. Solar energy gathered by photovoltaic solar panels and delivery to a power grid, it must be conditioned for use, by a grid-connected inverter
Photovoltaic distributed generations have dc output voltage, so they should connect to grid via power electronic converters. The operation of power electronic interfaces during extreme voltage disturbance is harmful and can damage power electronic converters. According to IEEE defines power quality as:“The concept of powering and grounding electronic equipment in a manner that is suitable to the operation of that equipment and compatible with the premise wiring system and other connected equipment”.Voltage sag has become one of the major power quality concerns in recent years. As applications of power electronics in the different sectors grow rapidly, these causes sensitive loads easily interrupted by voltage sags and the resulting losses are significant

GRID CONNECTED SYSTEM
During voltage sag, one or more phase voltages at the point of common coupling (PCC) are reduced and the currents supplied by the PV source must be increased in order to maintain the same amount of injected power as in nominal conditions. Thus, the PV system can be viewed as a high-reliability current source that must inject all the generated power to the grid. The source should be able to override the large currents caused by temporary voltage sags, and continue feeding the grid according to the standards. Theseover currents, which can double the nominal values, can damage the power systems and switches as well as the interconnection lines, unless the source is disconnected. Fig: 1 shows the block diagram representation of grid-connected three-phase PV inverter.
During the voltage sag, the source should operate with increasing converter currents to maintain the injection of the generated power. This undesired operation may result in system disconnections due to over currents.The quality of power is mainly governed by practices and standards on voltage, frequency, and harmonics. In particular, PV systems should have low current harmonic distortion to assure that no adverse effects are caused to other equipmentconnected to the utility grid. The common practice is to accept a maximum total harmonic distortion (THD) of 5% at rated inverter output.
The different methods based on the reference current generation in grid fault controllers are; the 1st approach is the instantaneous unity power factor control (IUPFC), which must be used when the injected current i is required to follow exactly the voltage v. The second approach, the average unity power factor control, is implemented to avoid the appearance of current harmonics that increase the current THD in a grid connected system. Third, the instantaneous positive-sequence control (IPSC) gives an injected current that only follows the positive voltage sequence v+. Fourth, the average positive-sequence control (APSC) provides a current that follows the averaged positive-voltage sequence, avoiding the appearance of current harmonics in the system. And fifth, the positive negative sequence compensation control (PNSCC) gives a constant injected active power and low current THD [3].
From the above five grid fault controllers, obtain a generalized method for reference current generation;
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The unbalance factor n Eq.(2) is calculated on the basis of positive and negative sequence voltage. The optimal values of α and β are given using interpolation in a look-up table .The value of α is varied from-1 to1 and β varies from 0 to 1Used with Eq.(1) to obtain the reference current ir[4].

DESIGNING OF THE CONTROL SCHEME
The field of overcurrent reduction solutions that based on the control scheme, which avoid any increase in the number of electronic devices. In the selection of a properreference current is proposed to override grid faults and achieve different power quality requirements especially peak current during voltage sag. In this method proposes a controller for a PV inverter that ensures minimum peak values in the grid-injected currents during the voltage sag. This study is based on the generalized algorithm and design method is developed in order to search for the control parameter values that minimize the peak currents.
Basic control block diagram during voltage sags is shown in fig.2 .The inverter should supply a reference current i synchronized with the grid voltage. The detection block is responsible for extracting the positive and negative synchronization sequences from the voltage [1].The reference generation block uses these voltage sequences and the reference power signal from the external dc-link voltage controller Pr to generate the reference current vector ir. As a last stage, current controller produces the pulses to the inverter.
In order to reduce the peak current to its minimum, noninteger values are proposed for α and β when computing the expression of the reference current vector ir .A simple searching algorithm is used offline to find the optimum values of α and β for each unbalance factor n. With each pair alpha, beta for a given unbalance factor, the peak values over a line period are computed. Then for each n, a single pair alpha, beta giving minimum peakcurrent values is stored in a lookup table. When sag is detected, n is computed online .Then the appropriate values of α and β are retrieved from the look-up table to obtain the reference currents. This reference current iris given to the input of resonant current controller. The resonant current control has been extensively employed to reduce the current harmonic distortion in a grid-connected distributed generation applications, including photovoltaic (PV) inverters .In this control uses a proportional-resonant (PRES) compensator to track the fundamental component of the current reference signal and a resonant harmonic (RESH) compensator to attenuate the most important current harmonicsSpace vector modulation signal Sa,Sb,Scpulses to the inverter.

A. Detection of Positive and Negative Sequences
The grid voltage is a crucial issue in order to have full control over the power that is delivered from the DPGS to the grid. In this section, characterization is performed by means of a positive and negative sequence voltage detector based on a second-order generalized integrator.The proposed positive- and negative-sequence detection system that provides an effective solution for grid synchronization of power converters in the presence of grid faults. The calculation of the instantaneous symmetrical components on the alpha-beta reference frame makes it possible to use only two SOGIBPFs.

B. Current Controller
The resonant current control has been extensively employed to reduce the current harmonic distortion in a wide range of grid-connected distributed generation applications, including photovoltaic (PV) inverters.This control usesa proportional-resonant (PRES) compensator to track the fundamental component of the current reference signal and a resonant harmonic (RESH)compensatorto attenuate the most important current harmonics.

SIMULATION MODEL

RESULT ANALYSIS
In order to evaluate the performance of the controller, a deep symmetrical sag with an unbalance factor n was used, and a performance comparison with a reference controller. Fig.6 the output of pv to grid connected system during voltage sag with a time period of .3 to .5 sec. Comparing with previous controller, we can see that overcurrent reduced during LLLG. But presence of current harmonics during the current is high.it about 30.61%.
From the fig.7 it can be analysed that by incorporating the resonant current controller into the existing peak current reduction scheme current harmonics and peakcurrent is reduced. By analysis using resonant controller incorporate with peak current reduction method peak current and harmonics are reduce large extent. Fig.8shows the change in peak current and current harmonics.

CONCLUSION
This paper has presented a controller for a grid connected three-phase PV inverter, which ensures minimum peak values in the grid-injected currents during unbalanced voltage sags. Also it reduces current harmonics during the period of voltage sag where large harmonics are presented due to the reference current injection.
Existing control strategy, peakcurrent reduction using reference current generation method injects grid current which causes large current THD .In existing Control method current THD is greater than 30% even though the peakcurrent is reduced.
To improve the performance of the system resonant current controller incorporated in grid connected PV inverter. By doing so, the current harmonic distortion and peak current can be reduced during voltage sag. The current THD is less than 5%.So when compared to existing peakcurrent reduction method, both current THD and peakcurrent can be reduced and hence it is more preferred.

ACKNOWLEDGMENT
First of all, we thank the Almighty for His enlightening presence and blessings throughout our life and for helping to complete this work successfully. We also like to thank our family and friends for their continuous support and prayers.

Figures at a glance
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References
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