Grid-Interactive Multifunctional Single-Phase PV-Battery System Under Abnormal Grid Conditions

Grid-Interactive Multifunctional Single-Phase PV-Battery System Under Abnormal Grid Conditions

B. P. Sai Surya Teja 1*

Department of Electrical and Electronics Engineering, Annamacharya Institute of Technology & Sciences, Rajampet, – 516126, India

B.Siva Sankar Reddy 2*

Department of Electrical and Electronics Engineering, Annamacharya Institute of Technology & Sciences, Rajampet, – 516126, India

K. Sri Charan 3*

Department of Electrical and Electronics Engineering, Annamacharya Institute of Technology & Sciences, Rajampet, – 516126, India

1. Vamsi Krishnam Raju 4* Department of Electrical and Electronics Engineering, Annamacharya Institute of Technology & Sciences,

   Rajampet, – 516126, India

   Suresh Srinivasan 5*

   Department of Electrical and Electronics Engineering, Annamacharya Institute of Technology & Sciences, Rajampet, – 516126, India

   Abstract:- Renewableenergybased distributed generators (DGs) play a dominant role in electricity production, with the increase in global warming. This article deal with the single- phase grid interactive multifunctional solar PV system with continuous power transfer capability. However, preserving uninterrupted power and to enhance power quality is completely challenging owing to the non-linear load. This proposed model consists of two stages; PV-array and boost converter with a battery and a bi-directional converter integrated with DC link and second stage is a VSC which mitigates the harmonics and effective power utilization. In standalone mode, maintaining the magnitude and waveform almost as ideal case. In this proposed system a PI controller is used to furnish the DC-link voltage to a constant value. A feedforward control is proposed in solar PV to improve the dynamic response of the system. A self-adjustable step based control is introduced for the VSC to estimates the real power reflection portion of the load current in order to illustrate the features of the PV-battery system.The MATLAB/SIMULINK simulation results confirm the concept in terms of flexible conversion, high power density, low leakage currents as well as controllable power flow even under abnormal grid condition.

   KeywordsVoltage source converter, MPPT, Power Quality, Feed forward.

   1. INTRODUCTION

To meet the growing energy demand, reduce carbon dioxide emissions and cope with environmental concerns, renewable energy is expected to play a key role in the future [1-3]. This clean energy can be integrated at transmission level as well as at distribution level. At transmission level, wind energy is the main driver, particularly in Europe. For example in Northern Europe, one can witness many offshore wind farms connected to the main transmission grid via AC or DC links. Due to their intermittent nature and the fact that these wind farms

are connected to the grid via power electronics converters, the power grid becomes more vulnerable and subject to instability.

Establishing a multi stage conversion system with separate general DC-DC and DC-AC converters and developing stand alone multi-port configurations[3]-[9] are two methods to perform the interlinking conversion. Compared to the former solution, standalone hybrid topologies bring more benefits (e.g., increased reliability, higher power density and lower system cost due to the reduced number of conversion stages), and they posses more flexibility. For instance, the split-source inverters were introduced in [6],[7] and to enhance the compactness, efficiency, flexible power flow and voltage boosting. This is a troublesome challenge when applied in PV systems. To lower the leakage currents, transformer-less-stand alone converters [8],[9] can be employed, yet lacking bidirectional power flow capability . Additionally, due to adopting of a dual-buck inverter, large AC filter inductors are required, leading to a relatively low power density that contradicts with the benefits of standalone hybrid converters.

This paper proposes interlinking conversion architecture as a promising candidate for RES integration into hybrid grids. It performs well in terms of high reliability, ease of execution, and operational flexibility. The proposed architecture is attained by replacing the power device of the boost converter with an active switch and a voltage source inverter (VSI). In addition, it employs a symmetrical impedance network that is beneficial to system efficiency, leakage current suppression and power density. A dedicated modulation scheme is exemplified, which can further improve the power quality and flexible control [10] while maintaining efficiency

There are numerous MPPT strategies present in the literature. Conventional technique, Soft computing

based technique, optimization based techniques viz. particle swarm optimization (PSO), flower pollination technique are few well known MPPT strategies which are present in the literature. This manuscript discuss the technical prospects and its pros and cons of each type [7]. The type of MPPT strategies are chosen based on the application and the handiness of the data with the user. P&O technique is the conventional technique with simple structure and easy implementation. P&O method are generally preferred for low rating application. It works on the principle of altering the firing pulse every cycle based on the obtained power. The P&O method observes the power and changes the firing angle value based on the previous cycle. The P&O methodology is the simplest MPPT strategy but fails to follow the rapid variations of solar irradiance the modelling of solar is furnished in the next chapter.

II PHOTOVOLTAIC SYSTEM MODEL

Schematic diagram of a 3-phase grid-associated system is demonstrated in figure1. The considered PV framework contains of a PV cluster, a dc-interface capacitor C, a 3- stage inverter, and a filter inductor L and is associated with the network with voltage ea, eb, and ec.

2.1 Photovoltaic Cell and Array Modeling

PV cell may be a straight forward contact diode which changes over solar oriented light into electrical energy. Figure 2. Demonstrates the same circuit diagram of a PV cell which comprises of a light-produced current source IL, an equal diode, a shunt obstruction Rsh, and a series resistance Rs.

Figure 1: Three-phase grid-connected PV system

Figure 2: Equivalent circuit diagram of PV cell

In figure 2, ION is the diode current, which can composed as

Where = q/AkTC, k= 1.3807 ×1023 J.K1 is that the Boltzmanns steady, q= 1.6022 ×1019 C is that the charge

of electron, A is that the contact ideality factor whose worth is between1 and 5, Is is the immersion current, and vpv is that the yield voltage of PV cluster, which during this case is that the voltage across C, i.e., Vdc. Presently, by applying Kirchhoffs current law (KCL) in figure 2, the output current ipv produced by PV cell,

The light-produced current IL relies upon the sun based illumination which may be connected by the following condition

Where Isc is the short-out current, TC is that the cells operating temperature in Kelvin, s is that the sun powered light, ki is the cells short-circuit current coefficient, and Tref is that the source temperature of the cell. The cells immersion current Is differs with the temperature consistent with the subsequent condition [19]:

Where Eg is the band gap force of the semiconductor utilized in the cell, and Irs is the reverse immersion current of the cell at reference temperature and sun based illumination. Since the yield voltage of PV cell is exremely low, variety of PV cells are assembled serially so as to get higher voltages

Figure 3 demonstrates an electrical identical circuit outline of a PV exhibit, where Ns is that the quantity of cells serial and Np is that the quantity of modules in equal. during this case, the cluster ipv are often represented as

1. SYSTEM STRUCTURE WITH CONTROLLER

   Figure 4. System structure of PV-battery-based grid interactive system

   The grid-interactive multifunctional PV-battery system is illustrated in Fig. 4. This system comprises four major parts. The first one is solar energy conversion system (SECS) consisting of two stages, a boost converter and a VSC. The second one is a battery storage with a bidirectional converter. The third one is load coupled at PCC and the fourth major part is a single phase distribution grid. It consists of an interfacing inductor and a ripple filter. To obtain the crest power from a PV array, MPPT control is used with a boost converter.

   The boost converter output is given to the dc-link of VSC. Extracted solar PV energy is given to the grid by the VSC of PV-battery system that also improves the quality of power at single-phase utility network. A full- bridge VSC having four switches (IGBT) with interfacing inductor is tied to the single-phase utility. Although to reduce the switching ripples in PCC voltage, an R-C filter is tied at PCC. The use of battery storage suppresses the fluctuation in output power of the PV array and supports the distribution network at peak load demand hours. Under outage of utility grid and low or zero power generation from the PV array, it supplies the energy to meet the emergency load demand. For import/export power to/from the grid, a back-to-back static transfer switch (STS) arrangement is used.

2. BOOST CONVERTER

The boost converter is utilized to "venture-up" an info voltage to some increased phase, needed by a heap. This specified capacity is attained by reserving power in an inductor and contributing it to the heap at a maximum level voltage. This short note features a portion of the more normal traps when utilizing support controllers. These incorporate higher attainable results I and V, cut-off and fundamental design issues. The references toward the finish of this Para give magnificent diagrams of the activity of a boost controller; and ought to be counseled if the per user is curious about with the fundamental activity of this kind of converter.

Figures 5, shows improved forms of both the boost converters. Just the force stage is appeared; a total controller requires more hardware to direct the result. We will begin by taking a gander at the buck. Note that single side of the inductor is associated with yield hub. Since no DC current can course through the yield capacitor, the complete burden current moves

through the inductor. The alternative side of the inductor is linked to the regular hub in middle of the MOSFET and diode.

Figures 6 & 7 shows that the inductor and MOSFET current in CCM. On off chance that we maintain a strategic distance from the, little three-sided swell, it is easy to see that the pinnacle MOSFET current is virtually similar as the heap current. This makes it simple for the controller producer to define the utmost burden current that the controller can flexible. Not with standing of the i/p or o/p V, the MOSFET can be measured for the most noteworthy burden current.

Likewise, as far as possible the present limits are often arranged just above this utmost value. So, the utmost MOSFET current rating of a buck is that the greatest burden current rating. This is the utmost load I for this gadget. This isnt the situation for a lift converter. Note from figure 5, that the inductor is associated from the info supply to the normal hub between the MOSFET and diode. Along these lines the height MOSFET current is presently almost adequate to the info current, not the heap current. We will see that the info I relies upon the info and yield Vs of the converter.

The boost regulator remains rated supported the utmost MOSFET current but this doesn't represent the utmost load current, like the buck. The easiest approach to measure the information I of a boost controller is to utilize

the energy balance condition. For a DC/DC converter, the i/p and o/p energies are only the merchandise their particular Is and Vs. Adding the 3-sided ripple current, we show up at below equation 6.

of the PV board to follow up the Sun if conceivable. A control system that controls the voltage or current to realize maximum energy is required. This is accomplished utilizing a Power point tracker algorithm to trail the utmost force.

V MAXIMUM POWER POINT

Under steady irradiance and cell temperature, the working purpose of a PV cluster is chosen by the convergence of the Ipv-Vpv characteristic and therefore the load characteristic as demonstrated in figure 8. The heap trademark is represented by a line with the slope M = 1/R = Iload/Vload. The system operating point moves along the Ipv-Vpv characteristic function of the PV panel from B to A because the load resistance increases from 0 to infinity. Position C is that the peak power working point. At now, the world under the Ipv-Vpv characteristic function, this is often like the utmost output power. If the load resistance is just too high, the operating points would be within the CA regions. If the load resistance is just too low, the operating points would be within the CB regions.

From the trademark of I-V and P-V bends of photovoltaic modules, its demonstrated that there was a specific point for the most extreme power (PMPP). This point is explained as the most extreme force point with the ideal voltage Vmpp and the optimal current Impp. Now, the whole PV framework ought to work with the utmost proficiency and produce its highest elevated yield limit.

The photovoltaic cell I-V trademark is nonlinear and transfers along with illumination and temperature. The area of the MPP isnt known yet should be found. Distinctive MPPT techniques have been figured it out. They differ in intricacy, sensors needed for the voltage or current, intermingling velocity, and cost, scope of viability and usage equipment. The three primary classifications of MPPT calculations are model-based calculations, preparing based calculations and searching algorithms.

5.1 MPPT Controller

For maximum power transfer, the heap ought to be coordinated to the obstruction of the PV board at MPP. Along these lines, to work the PV boards at its maximum power point, the framework ought be ready to coordinate the heap consequently and furthermore change the position

A regulator that trails the utmost energy point locus of the PV array is known as a maximum power point tracking regulator. There are few calculations to trail the maximum power point and a few mutual MPPT calculations have been reviewed. For ideal activity, the heap line must match the PV clusters MPP locus and if the specific burden isnt utilizing the most extreme power, a power conditioner ought to be utilized in the middle of the array and the heap.

1. Model-Based MPPT Algorithm

   Fractional short-circuit current method: This strategy is established on the estimation intermittently of the PV hamper, which is around direct to the current greatest force point. Experimentally, k2 may be a steady somewhere 0.78 and 0.92. When the steady k2 is known, Impp is registered. The PV cluster should be shorted occasionally to calculate Isc.

   Fractional open circuit voltage: Similarly, the Fractional open-circuit V is established on the linear dependence between array voltages at maximum power VMPP with its open circuit voltage Voc.

   MPPK1Voc (8)

   K1 is a steady somewhere 0.71 and 0.78. Voc is calculated by shortly shutting down the force converter. Implementations of those strategies are basic and modest however here is unnecessary power loss and the effectiveness of the PV is extremely low becaus of the inaccurate determination of the constant K1 and K2. The power loss is brought by the necessity to open and close the circuit for estimations.

2. Searching MPPT Algorithm

These algorithms are supported the calculations of the PV model result V and current. Then, it computes the PV power and decides whether the control boundary should be expanded or diminshed. The control boundary might be a reference signal (voltage or current) for a regulator or it is often the obligation proportion for the exchanging signal DC/DC converter.

5.3.1 Perturb & Observe P&O/ Hill Climbing

P&O and Hill climbing utilize an equivalent key methodology. The obligation ratio is that the perturbation in slope climbing, while the V of the PV model is that the perturbation for the P&O. Changing the worth of the obligation cycle makes a change to the present and as outcome, perturbs the voltage array. In figure 9, the V and I

are calculated and the MPPT regulator estimates the voltage reference. The contribution for the controller PI is the distinction of the Vref and Vpv. The voltage managed creates the PWM for the converter.

In figure 9, it can be seen that increasing PV voltage exceeds the capacity of the PV and decreasing the PV voltage reduces the capacity of the PV while working on the left of the MPP. On the privilege of MPP, augmenting the voltage diminishes the power and decreasing the voltage exceeds the capacity. This cycle will be executed in the MPPT regulator to separate the most extreme force point from the PV module. The framework fluctuates around the MPP with this strategy. The cycle of increasing and decreasing can fizzle under quick change in illumination. The framework veers away from maximum power point if the irradiance exceeds out of nowhere. To remedies those issues, improved strategies of perturb and observe are used: decreased perturbation step size, variable step size, three points loads correlation techniques and upgraded examining rate.

Figure 11, demonstrates the flowchart diagram of P&O strategy. First info sources are given are voltage and current and the power is determined from these v & i. The indication of the power decides the duty cycle result of the MPP regulator. Duty ratio is the control variable in simulation. Perturbing the duty ratio of the converter perturbs the PV array current Ipv and inevitably perturbs the PV array voltage Vpv. The starting estimation of the duty cycle and PV power are given. The V and I of the PV clusters are calculated initially then the facility P is measured. If the similarity is negative then increase the duty ratio and note down the present values of v, i, p and duty cycle and rehash the cycle. The scope of the duty cycle is restricted somewhere in the range 0 and 1 to guarantee that the boost will venture up the i/p voltage within cutoff.

The execution of the P&O relies on the inspecting span and the duty-cycle perturbation of the calculation. The exactness, speed of the P&O relies upon the boundaries. The duty cycle step must be picked appropriately. The motions and consistent state losses are diminished by reducing the duty cycle. However, under changes in environmental conditions this regulator gives less productivity.

The examining rate likewise impacts in the calculation, higher the testing rate may cause precariousness. In the event that the PV exhibit tests the V and I excessively fast, at that point most extreme force track will be remembered fondly. The testing time period calculation ought to be set as little as conceivable without causing swaying of the framework and the difference away from the MPP. Something else, the insecurity will decrease the proficiency of the PV

VI. SIMULATION OUTPUTS

To validate the proposed hybrid system in grid connected mode, a Simulink model for 560W photovoltaic structure, with Battery Connection and single MPPT strategy are implemented in MATLAB with DC-DC converter to AC grid of rating 230V, 50HZ network. For validating the efficiency of the suggested controller, the system is validated in the pair of standalone and grid allied system. In standalone system, the input sources PV and battery are allied to DC link and act as input to VSC and the output is connected to non-linear load.

To display the virtue of the initiated controller together with incongruent irradiance is considered in this study as demonstrated in below figures 12. The execution of the PV with BC together by single P&O drew on maximum power point tracker topology is depicted in figure 13. The DC link voltage, current, and energy obtained utilizing single P&O based MPPT technique employed for the composite structure is depicted in figure 13. The P&O technique fails to pursue the pinnacle energy obtained by the solar view of highly variable characteristic of PV irradiance.

The execution of PV and battery structure with Boost converter and MPPT topology is demonstrated. The DC link voltage, current and power obtained established maximum power point tracking technique employed for the

PV and Battery. The MPPT technique not only trails utmost power but also provides better stability during the parameter changes of irradiance. A stable DC link voltage is obtained because of faster convergence speed from MPPT technique. The DC link voltage is the primary necessity of VSC for an efficient

performance.

Figure 13: PV and Battery with Boost Converter DC Link Outcome Voltage, Current and power with Single Perturb & observe Maximum Power Point Tracking.

Above table represents the summary of the initiated PV and Battery with Boost converter and modified single maximum power point tracking controller techniques for varying solar irradiance. For period 0 to 0.5 sec, the developed power is 712.3 W in P&O MPPT method and 716.8W (0.5 – 1) seconds the developed power is 736 W in P&O MPPT method and 742.4 Wand 1 to 1.5 sec, the developed power is 758.8 W in P&O MPPT method. By the above tabular column, its evident that the suggested maximum power point tracking controller gives the satisfactory results collated to the single Perturb & Observe Maximum Power Point controller. [12-14]

6.1 Grid Connected System

To examine the execution of the initiated configuration for grid gleaned application together to validate its performance Photovoltaic and battery are connected. The common DC link voltage obtained from the two different regions based on the input source data is fed to the 1- VSI. By using sinusoidal PWM technique pulses are generated to mastery the voltage source inverter

Figure 14 to 17 represents the Inverter voltage current, load voltage current and grid voltage current.

Figures 18, demonstrate the capacity of the initiated PV and Battery System, under three different load conditions, gleaned on the inverter denouement capability, the demand of the load is met by sharing with the grid. During the period 0 to 0.5 sec, the net inverter output power is 754.1 W, for the same period load demand is 500 W, and then the excessive 254.1 W power is fed to the grid. Similarly for period 0.5 to 1 sec, the demand of the load is 750W but the generated power is 780.4W, then the excessive 30.4 W power is injected the grid and for the period 1 to 1.5 sec.

VII CONCLUSION

Single-phase two-stage PV-battery system with self-adjustable step-based control algorithm for grid- interactive multifunctional topology has been used for injecting PV energy to the single-phase ac utility and feed the power under outage of the distribution network. This also enhances the quality of current at PCC in grid- interactive mode. A self-adjustable step-based control has been introduced for obtaining the peak load current corresponding to fundamental part. The behavior has been evaluated with extensive changes in working states, consisting of load change, weak utility scenarios, and variation in PV irradiance. This PV-battery system adequately eliminates harmonic componnt in utility as well as it decreases the losses in line and enhances the voltage waveforms indirectly at PCC. The PV grid- interactive system performance under steady-state and dynamic conditions has been found satisfactory. This proposed PV and Battery system is designed and modeled in MATLAB/Simulink platform then the results are compared and validated with different case studies

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