Photovoltaic Micro-inverter Embedded Control a ...

International Journal of Applied Engineering Research, ISSN 0973-4562 Vol. 10 No.55 (2015) © Research India Publications; httpwww.ripublication.comijaer.htm

Photovoltaic Micro-inverter Embedded Control a Reconfigurable One 1

Senthamilarasan T, 2Lukram Dhanachandra Singh and 3Sivaranjani S and 4Dr. R Rajkamal 1 PG Scholar - Department of EEE, Vel Tech Multitech Dr RR Dr SR College of Engineering, Avadi, Chenna, India. 2 PG Scholar - Department of EEE, Vel Tech Multitech Dr RR Dr SR College of Engineering, Avadi, Chenna, India. 3 Assistant Professor - Department of EEE, Vel Tech Multitech Dr RR Dr SR College of Engineering, Avadi, Chennai. 4Professor - Department of EEE, Vel Tech Multitech Dr RR Dr SR College of Engineering, Avadi, Chennai, India. Abstract In this paper the proposed control scheme is capable of reconfiguring a micro-inverter to function in both grid-linked mode and islanded mode. The chief advantage of this proposed reconfiguration control for micro-inverter is that in grid-linked mode it injects power to the grid by working as a current source in phase with the grid voltage. This is the process method of most commercial grid-connected PV microinverters. The main plan is to provide additional functionality to the micro-inverters to work in the island mode without altering their organize algorithms for grid-linked mode. It is proposed that in island mode, the micro-inverter control is reconfigured by using standard PWM technique in order to properly deliver the power to the loads. The mean of this paper is to illustrate that the projectedreconfiguration control is achievable without hazardous transients for the micro-inverter and the loads. Simulation results are provided to show the feasibility of the planned control strategy. Keywords: Grid-linked mode, island mode, micro-inverter, reconfigurable control.

This includes petroleum products, electricity and coal. This energy is basis for all modernized process includes industries, transports, commercial development and agriculture. This not only used for industrial purpose but also used for house hold works. Those sources that are not commercially available at fixed price in market are called as non commercial energy. Energy which used are stored in different forms. These sources are divided into renewable and nonrenewable energy source. Renewable energy is we can use again and again. Those energy that cannot be recreated within short span is non renewable. Since the India is located near the equator it receives abundant of sunlight throughout the year. The solar energy is used to light up in homes by using solar PV cells. To remove the energy crises solar energy that using the renewable energy is the only way. The reason for using solar as a main renewable source in India is as follows: Solar energy is abundant. All other source of renewable energy has geographical limitations. . The solar energy is available throughout the year. It rectify rural electricity problem It is highly scalable. In recent years, renewable energy becomes one main source of energy. Compare with energy production energy delivery is a major problem in isolated and rural areas. This can be removed by using distributed generation (DG). They are two modes of operation happen during this interface one is grid linked mode and the other is island mode. The Energy coming from renewable energy is connected with grid for transmission purpose in grid linked mode and also fed to the local loads in island mode. A reconfigurable control for inverter is employed here to do this function to provide uninterrupted power supply. Without altering the control algorithm for the grid-linked mode, the inverter can be made to function in both grid connected mode as well as island mode by providing extra functionality to the inverter [11].

1.Introduction In the day to day life energy plays a major role. The amount of energy utilized by a human being shows the development of the country. Due to increased population, civilization the need of energy becomes more day by day. The utilization of fossil fuels like petroleum, coal and natural gas become more so this lets to the depletion of fossil fuels. Thus the rate of energy production and consumption are inversely proportional let to energy crisis. So the only solution is using renewable energy. Energy can be classified into several types namely Primary and Secondary Energy, Commercial and Non-Commercial Energy, Renewable and Non-Renewable Energy. Primary energy are those that obtain from nature. This includes oil, natural gas, coal and biomass. The primary energy can also be obtained from radioactive substance as nuclear energy from the interior of the earth as thermal energy and from gravity as potential energy. Among these the major source of primary energy obtained from coal, natural gas, hydro power, 2.Inverter Modeling and Control Pulse-width modulation (PWM) is the basis for control in petroleum etc. power electronics. The theoretically zero rise and fall time of Secondary energy source are mainly derived from primary an ideal PWM waveform represents a preferred way of driving source. Example: The steam and electricity is obtained from primary source like gas, coal or oil. We can use primary modern semiconductor power devices. With the exception of energy directly. Few source can be used as non energy usage some resonant converters, the vast majority of power purpose, for example natural gas or coal is used in fertilizer electronic circuits are controlled by PWM signals of various plant as feedstock. Commercial energy is the energy sources forms. The rapid rising and falling edges ensure that the semiconductor power devices are turned on or turned off as with definite price available in market.

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fast as practically possible to minimize the switching transition time and the associated switching losses. A carrierbased PWM modulator is comprised of modulation signals and carrier. A model of the 3-phase 2-level inverter was implemented, with MOSFET. The fig. 1 shows a model of a typical 3-phase 2-level inverter. The input given to this inverter is 400v DC. In general an inverter is a power electronic device which converts DC power to AC power at required output voltage and frequency level. The controlling of the inverter circuit can be categorized into two types: current control and voltage control. The amplitude and frequency from the output of the inverter is controlled by the network when it is connected to the network and the operating mode of the inverter is referred to as current control mode. During such cases, the voltage is controlled by the inverter. The Phase-locked loop (PLL) control will guarantee the 50 HZ frequency. The converter circuit used is a full-bridge MOSFET-based converter and it inverts the DC power into AC power so that the energy can be transferred from the renewable source to the grid/load. Under the gridconnected operation mode, the grid voltage must be tracked inorder to provide synchronization between the grid and the inverter, the PLL control circuit will track the voltage and it will provide synchronization between the grid and the inverter; but under the islanding mode of operation, the inverter will not track the grid specifications because it is not needed in this mode of operation [2]. The inverter gate pulses are controlled by the grid inverter control block. In order to capture the total control of the grid current, the DC-link voltage is boosted to a higher level more than that of the amplitude of the grid line- line voltage. Thus after achieving this feat the DC-link voltage must be kept constant in order to guarantee the power flow and thus it must

Fig.1. 3-phase 2-level inverter

Fig.2. Control structure for DC/AC converter

be controlled to be kept constant and thus the power flow of the grid side inverter is controlled. The control structure for the DC/AC converter is shown in figure 2. The inverter is controlled by using a current loop which is in a synchronous rotating-frame with a feed-forward load current component added in the reference, completed with the DC voltage control loop in a cascaded manner. The outer loop controller consists of two parts: the phase-locked loop (PLL) and the DC link voltage controller. First the PLL, which is responsible for extracting the fundamental frequency component of the grid voltages and also for generating the corresponding quadrature signals in d-q synchronous reference frame, Ed - Eq, from which the active and reactive power of the grid can be calculated. The second is the DC link voltage controller, which is responsible for monitoring the power control loop [5]. The power control of the PWM inverter is based on the two control loops namely: DC-voltage feedback control loop and the Power detection feed-forward control loop. The voltage controller performs two tasks one is to control the grid converter power flow and another is to maintain the DC link voltage to a certain value. The Proportional-Integral (PI) controller accomplishes the tasks of the current regulation and the DC-link voltage, due to its dynamic behavior and maintaining a steady-state with the power inverter [11]. It is important to underline that the PI controller performances are very sensitive to the change in parameter values, due to its design procedure it is based on the inductor and capacitor values. However, when it is used for specific applications, they are known with reasonable accuracy. Such system can be designed either in frequency domain or the time domain. The figure 3 shows the control structure for the DC/AC converter which has been elaborated in the above sentences. The transfer function of the PI controller is given as CPIc (S) = Kpc (1 + 1/TicS) (1) where Kpc is the Proportional gain Tic is the integral time CPIcis the transfer function of the PI controller The transfer function of the PI controller can be calculated by calculating the values of the Kpcand Tic as : Tic=1/ c.tan() (2) Kpc= - Tic. c2. L / sqrt{1 + (Tic . c)2} (3) where is the phase margin is the bandwidth Therefore by using the equations (2) and(3) we can calculate the transfer function of the PI controller by using it the task of current regulation and DC-link voltage can be achieved. 3.Proposed System A. Grid-connected operation The system in which the photovoltaic (PV) panel generated power is interconnected to the grid which is otherwise more commonly called as the Grid-connected PV system uses the generated power in a more efficient manner. Moreover, the technical aspects along both sides (i.e.) from the PV panel side and also from the grid side should be clarified in order to guarantee the safety and reliability of the PV panel and grid respectively. Therefore by clarifying these technical aspects this system can provide various applications of PV systems [4].

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To provide interconnection between the PV panel and the grid the key circuit which is needed is the inverter, which plays a key role in these type of systems, because the PV panel provides DC power which cannot be fed into the grid because the grid can only be fed AC power so in order to interconnect the grid and the PV panel we must convert the DC power into suitable AC power and this conversion is achieved with the help of the inverter which converts DC power generated by the PV panel into AC power [3]. Inverter is very important to ensure the safe and reliable operation of these types of systems. The inverter should also provide elevated quality of power to the AC function systems with a sensible cost. To achieve with this feat, up to date technologies of power electronic devices are used for PV based inverters. With the help of the power electronic switching device and connecting them in full-bridged or half bridged configuration the inverter circuits are developed now a day’s and the still this circuit is to be controlled in order to produce high power and low harmonics[5]. This goal is achieved by controlling the gate terminal of the power electronic device with the help PWM (Pulse Width Modulation) technologies, which guarantees the conversion with high power quality and also low harmonics since the PWM technique will hold back lower order harmonics.

PHASE SEQUENCE MATCHING: The Phase sequence of the PV scheme should match with the phase sequence of the corresponding network. If a three phase system is used the three phases should be separated by an angle of 120 degree from each other for both the PV panel and the grid. FREQUENCY MATCHING: The normal operating frequency of the grid is 50 HZ in our country. The frequency of the PV scheme should be same as the grid (i.e.) it must be 50 HZ. If the frequency of the PV scheme is larger than the frequency of the grid but within the margin of acceptable level then the synchronization is possible. On the other hand if the frequency is less than that of the grid then the synchronization is not possible. VOLTAGE MATCHING: The voltage of the PV scheme and the grid should be same as each other. The voltage matching plays an key role in synchronizing the PV scheme and the grid. There are several ways for designing a Grid linked PV scheme, each way depends upon some requirements likewise to store the power from the panel when the PV scheme is not supplying power to the grid the systems are designed with battery and alternatively if the power is not stored instead it is supplied to the grid always it is possible to design the system without battery, and also if it is needed to boost the voltage level from the output of the PV panel to a suitable level grid interconnection design the system with transformer otherwise the system designed is without the transformer. Most we are designing a Grid linked PV scheme without a battery due to some advantages. Also the use of a transformer in the proposed system if want to boost the voltage from the output of the PV panel to a certain level and feeding it to the grid this part is optional in our design. Then the important circuit/device is called as inverter which is used for converting the DC power

Fig.3. Block diagram for Grid Connected System

TABLE I. GRID SPECIFICATION The PV panel will produce the electrical energy more competently during the day period in which the sun energy will be available. During the night period PV panel will not produce any kind of electrical energy. In the grid connected systems, the electrical energy will be provided by the grid during the period when the PV panel cannot produce electrical energy. Throughout the day time, the PV panel will produce the electrical energy and feds the energy into the grid. For this purpose we are using an inverter to convert the DC power from the PV panel into AC power to supply the power to the grid and also transformer which is used to boost the voltage from the output of the inverter to a level which is needed by the grid. The size of the Grid linked PV systems can vary significantly. The overall set up consists of a PV panel, Inverter, Transformer (optional), Protective devices and grid and mechanism essential for cabling and mounting which is shown in the fig.4. To interface the PV panel with the grid first the PV panel is synchronized with grid by satisfying the required conditions. If the synchronization is not done correctly then the power from the PV panel cannot be fed into the grid. Hence satisfy the following conditions, the power from the PV panel to the grid is supplied. The conditions are as follows:

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No. of Phases

3

Voltage Rating

230 Volts AC

Frequency

50 HZ

TABLE II. INVERTER SPECIFICATION Input DC Voltage

230 V

Output AC Voltage

230 V

Output Frequency

50 HZ

No. of Phases Type Efficiency Total Distortion

3 PWM (for suppressing harmonics) Almost 90-93%

harmonic