Hybrid Solar-Wind Energy System with MPPT using Cuk-Sepic Fused Converter

Hybrid Solar-Wind Energy System with MPPT using Cuk-Sepic Fused Converter

P. Ponmani , PG Student,

Power Electronics and Drives, Karpagam University ,Coimbatore, India

Ms. S. Divya Priya ,

Assistant Professor,

EEE Department, Karpagam University ,Coimbatore, India.

Abstract- Prominence of Non-conventional energy sources is ever increasing in order to protect our earth. This paper presents a standalone hybrid Wind/photovoltaic power generation system. Based on the availability of energy sources, this configuration allows the two sources to supply the load either individually or simultaneously. The proposed design has the Cuk and Sepic converters combined together which eliminate the need of additional input filters to filter out the high frequency harmonics. Maximum Power Point Tracking (MPPT) technique used is Modified Incremental conductance algorithm due to its high tracking accuracy and precise control under rapidly changing atmospheric conditions. Simulation results were obtained using MATLAB/Simulink .

Keywords:DCDC converter, Incremental conductance (IncCond), Maximum Power Point Tracking (MPPT), Cuk converter, SEPIC converter

1. INTRODUCTION

   The conventional energy sources are depleting and relying on the non-conventional energy sources is the only way to meet global energy crisis in order to preserve the earth for the future generations. Solar energy and wind energy has become the least expensive renewable energy technology in existence and has attracted the interest of scientists and educators over the world [1]. Photovoltaic cells convert the energy from sunlight into DC electricity. PVs are advantageous over other renewable energy sources such as they give off no noise and require practically no maintenance [7] and green house emissions are reduced[1]- [3]. The presence of solar and wind is unpredictable since they depend on the weather conditions like insolation (incident solar radiation) levels, shadows by birds, clouds and temperature. Standalone solar photovoltaic and standalone wind systems have been promoted around the globe on a comparatively larger scale. These independent systems cannot provide continuous source of energy, as they are seasonal.

   1. Hybridization of Renewable energy sources:

      Hybrid renewable energy systems (HRES) are becoming popular for remote area power generation applications due to advances in renewable energy technologies and subsequent rise in prices of petroleum products. In order to satisfy the load demand, one or more renewable energy sources can be combined together. A wind and solar hybrid system can supply more stable power than a single wind or PV source [2]. The solar and wind energies are

      complementing nature. By integrating and optimizing the solar photovoltaic and wind systems, the reliability of the systems can be improved and the unit cost of power can be minimized. Hybridizing solar and wind power sources provide a realistic form of power generation and is able to satisfy the load demand. By hybridizing the solar and wind sources power generation can be improved. The prime aim of this paper is to extract the maximum amount of power from sources such as wind, Solar depending on the availability and by incorporating Maximum Power Point Tracking techniques.

   2. MPPT Methods:

      Maximum Power Point Tracking technologies are used in renewable energy systems to extract the maximum power output to the load under varying conditions of solar insolation and temperature. All MPPT methods follow the same goal which is maximizing the PV array output power by tracking the maximum power on every operating condition. Different techniques used to obtain Maximum Power are Perturb and Observe, Incremental conductance, Fractional Short Circuit Current, Fractional Open Circuit Voltage, Fuzzy Control, Neural Network Control etc. Among all the methods Perturb and observe (P&O) and Incremental conductance are most commonly used because of their simple implementation, lesser time to track the MPPT and several other economic reasons. P&O algorithm cannot compare the array terminal voltage with the actual MPP voltage, since the change in power is only considered to be a result of the array terminal voltage perturbation and due to this P & O fail under rapidly changing atmospheric conditions [9]. MPPT fuzzy logic controllers have good performance under varying atmospheric conditions [8]; however, the inherent drawback of this method is that its effectiveness is highly dependent on the technical knowledge of the engineer in computing the error. Incremental algorithm overcomes the above mentioned disadvantages and is successfully used in tracking the maximum power under uniform insolation where only one maximum power point (MPP) exists in the power against- voltage (PV ) curve. However, in partially shaded condition where there are multiple MPPs in the P-V curve, the conventional algorithms are unsuccessful in identifying the global MPP (GMPP) among the local MPPs (LMPPs), therefore reducing the overall efficiency of the PV system [10].Concerning the multiple-peak issue during partial shading, several solutions have been proposed by modifying

      the conventional algorithms. As reported in [5], Inc Cond algorithm is altered to to track the GMPP . P&O algorithm is widely used to track the MPPs and GMPP .However, under rapid-varying solar irradiation, the P&O algorithm may fail to track the MPPs accurately [10]. In this paper, a modified Inc Cond algorithm is proposed which effectively utilizes the periodic P-V characteristics of partially shaded condition described. This modified algorithm was successfully demonstrated in tracking the GMPP under varying loads and weather conditions and in addition it eliminates the sensor circuitries at the output of dcdc converter.

   3. Fused converter Topology:

   A simpler multi input structure is proposed[5] that combine the sources from the DC-end while still achieving MPPT for each renewable source and the structure proposed by [5] is a fusion of the buck and buck-boost converter. This requires passive input filters to remove the high frequency current harmonics injected into wind turbine generators [6].In this paper, an alternative multi-input rectifier structure is proposed for hybrid wind/solar energy systems. The Cuk and SEPIC converters are combined together in the proposed system. The circuit operating principles will be discussed in this paper.

2. PROPOSED MULTI INPUT CONVERTER TOPOLOGY:

   DC-DC converters can be used as switching mode regulators to convert an unregulated dc voltage to a regulated dc output voltage. The regulation is normally achieved by Pulse Width Modulation at a fixed frequency using the switching devices like BJT, MOSFET or IGBT. Now-a-days, MOSFETs are used as a switching device in low voltage and high current applications. It may be noted that, as the turn-on and turn-off time of MOSFETs are lower as compared to other switching devices, the frequency used for the dc-dc converters using it (MOSFET) is high, thus, reducing the size of filters as stated earlier. Many consider the basic group to consist of the three: BUCK, BOOST and BUCK-BOOST converters. The CUK, essentially a BOOST-BUCK converter, may not be considered as basic converter along with its variations: the SEPIC and the zeta converters.

   1. Cuk Converter:

      The Cuk converter is a type of DC-DC converter that has an output voltage magnitude that is either greater than or less than the input voltage magnitude[11]

      Fig.2.1.Circuit diagram of the Cuk converter

      It has the capabilityfor both step up and step down operation. The output polarity of the converter is negative with respect to the common terminal. This converter always works in the continuous conduction mode. The uk converter operates via capacitive energy transfer. When M1 is turned on, the diode D1 is reverse biased, the current in both L1 and L2 increases, and the power is delivered to the load. When M1 is turned off, D1 becomes forward biased and the capacitor C1 is recharged [10].

   2. SEPIC Converter

      The SEPIC officially stands for Single-Ended Primary Inductance Converter. However, the unofficial interpretation is more descriptive: Secondary Polarity Inverted Cuk. SEPIC is a type of DC-DC converter allowing the voltage at its output to be greater than, less than, or equal to that at its input. The SEPIC converter was developed primarily to have step up/down capability without inverting the polarity of the regulated output voltage [11].It is similar to a buck boost converter. It has the capability for both step up and step down operation. The output polarity of the converter is positive with respect to the common terminal [10]

      Fig.2.2.Circuit diagram of the SEPIC converter

      The SEPIC converter was developed to have step up/down capability without inverting the polarity of the regulated output voltage [7].It has also been shown that both converters have advantages over the boost converter in terms of the conducted Electromagnetic interference (EMI) noise [9]. This is because all inductive components can be integrated on the same core, and with careful magnetic design can offer very low levels of inductor current ripples [7]. This condition is very desirable, particularly for the DCM operation, because the generated EMI noise is minimized, reducing input filtering requirements. The proposed MPP tracking is achieved by connecting a pulse- width-modulated (PWM) dc/dc Cuk converter between a solar panel and a load or battery bus and the SEPIC converter is connected between wind and a load.

   3. Block Diagram of Proposed system:

      In this paper, a new converter topology for hybridizing the wind and solar energy sources has been proposed. In this topology, both wind and solar energy sources are incorporated together using a combination of Cuk and SEPIC converters, so that if one of them is unavailable, then the other source can compensate for it.

      Fig.2.3.Block Diagram of the proposed Cuk-SEPC fused converter

      Solar energy source is the input to the Cuk converter and wind energy source is the input to the SEPIC converter. The average output voltage produced by the system will be the sum of the inputs of these two systems.The Cuk-SEPIC fused converters have the capability to eliminate the HF current harmonics in the wind generator. This eliminates the need of additional passive input filters in the system also step-up or step-down the input voltage during startup and overload conditions, lower input current ripple, and less electromagnetic interference (EMI) associated with the DCM topology. These converters can support step up and step down operations for each renewable energy sources. They can also support individual and simultaneous operations. All these advantages of the proposed hybrid system make it highly efficient.

      In SEPIC and Cuk converters, the average voltage across each inductor is zero whilst the average voltage across ,Vc , is as follows.

      For SEPIC, Vc= Vi

      For Cuk converter Vc= Vo

      The converter is operating in discontinuous capacitor voltage (DCV) mode where its input current is kept The features of the proposed topology are: 1) the inherent nature of these two converters eliminates the need for separate input filters for PFC [7]-[8]; 2) it can support step up/down operations for each renewable source (can support wide ranges of PV and wind input); 3) MPPT can be realized for each source; 4) individual and simultaneous

      operation is supported.

      When only wind source is available,(i.e) When M2 is on condition, in the hybrid system, Wind energy will meet the load by a sepic converter operation. The wind energy will produce the Ac power, the Ac power further converted to dc power by using the rectifier. On the other hand, if only the PV source is available, then D2 turns off and D1 will always be on

      and the circuit becomes a Cuk converter. Thus, individual and simultaneous operation is supported depending on the availability of sources.

      continuous. The Cuk and SEPIC converters operating in discontinuous current mode (DCM) can offer a number of advantages, such as inherent PFC function, very simple control, soft turn-on of the main switch, and reduced diode reversed-recovery losses. The SEPIC and Cuk converters offer several advantages in PFC applications, such as easy implementation of transformer isolation, inherent inrush current limitation. The output of DC-DC converters are sent to an external DC-AC inverter to supply AC power to the load. So the new proposed input side converter topology with maximum power point tracking method to meet the load and opt for a grid connected load as well as commercial loads. The implementation of new converter topology will eliminate the lower order harmonics present in the hybrid power system circuit.

   4. Circuit Diagram of Multi-input Cuk-SEPIC converters:

      Fig.2.4.Circuit Diagram of the proposed Multi inputCuk-SEPIC fused converter

   5. Technology Used In Hybrid Energy Systems and their modeling:

   Hybrid renewable energy systems (HRES) usually consists of two or more renewable energy sources used together to provide increased system efficiency as well as greater balance in energy supply. Here the system used is photovoltaic array coupled with a wind turbine This would create more output from the wind turbine during the winter, whereas during the summer, the solar panels would produce their peak output. When sunlight hits the semiconductor, an electron springs up and is attracted toward the n-type- semiconductor. This causes more negatives in the n-type semiconductors and more positives in the p-type, thus generating a higher flow of electricity. This is the Photovoltaic Effect. The power generated from the PV array is the combination of the power derived from each PV module.

   Wind turbines are designed to exploit the wind energy that exists at a location. Aerodynamic modelling is used to determine the optimum tower height, control systems, number of blades and blade shape. Wind turbines convert wind energy to electricity for distribution. The wind turbine is the first and foremost element of wind power systems. Wind turbines capture the power from the wind by means of

   aerodynamically designed blades and convert it to rotating mechanical power [6]. This mechanical power is delivered to the rotor of an electric generator where this energy is converted to electrical energy. Electric generator used may be an induction generator or synchronous generator. The mechanical power that is generated by the wind is given

   by

   Where p is the air density (kilograms per cubic meter), Vw is the wind speed in meters per second, A is the blades swept area, and Cp is the turbine-rotor-power coefficient, which is a function of the tip-speed ratio and pitch angle

   .The coefficient of performance of a wind turbine is influenced by the tip-speed to wind-speed ratio, which is given by

   TSR=(WmR/Vw)

   The wind turbine can produce maximum power when the turbine operates at maximum Cp (i.e., at Cp_max). If the wind speed varies, the rotor speed should be adjusted to follow the change.

3. MODIFIED INCREMENTAL CONDUCTANCE ALGORITHM FOR THE PROPOSED CIRCUIT

   A Maximum Power Point Tracking algorithm is necessary to increase the efficiency of the solar panel. There are different techniques for MPPT such as Perturb and Observe (hill climbing method),Incremental conductance, Fractional Short Circuit Current,Fuzzy Control, Neural Network Control etc. Among all the methods Perturb and observe (P&O) and Incrementa conductance are most commonly used because of their simple implementation, lesser time to track the MPPT and several other economic reasons. Under partial shading conditions, multiple peaks are observed in the powervoltage (PV ) characteristic curve of a photovoltaic (PV) array, and the conventional maximum power point tracking (MPPT) algorithms may fail to track the global maximum power point (GMPP). Therefore, this paper proposes a modified incremental conductance (Inc Cond) algorithm that is able to track the GMPP under partial shading conditions and load variation. Inc Cond algorithm is implemented instead of P&O algorithm due to its consistent performance under fast-varying weather conditions. The GMPP is located in the middle of three consecutive peaks.

   The proposed algorithm finds three consecutive peaks and locates the one which has the highest magnitude. If the GMPP is not in the middle of the three peaks, the algorithm continues to explore until the GMPP is in the middle or until the end of the PV curve where the lowest or highest possible voltage is reached. The flowchart is shown below

   Fig.3.Flow chart for the proposed Incremental conductance algorithm

   The proposed algorithm is able to track the GMPP accurately and thus reduces the power losses faced by the conventional algorithm

4. SIMULATION RESULTS

   The MATLAB/Simulink software is used to perform simulation. When Wind source has failed and only the PV source (Cuk converter mode) is supplying power to the load. Figure 4.1 illustrates the system where only the wind turbine generates power to the load (SEPIC converter mode).

   Fig .4.1. Simulation Model for individual Cuk and Sepic mode operations

   Figure 4.3:PV Panel Output Voltage

   Figure 4.4 : PV Panel Output Voltage After Cuk Converter Operation

   Figure 4.2: Simulation Model for Closed loop Hybrid Energy System

   Figure 4.2 illustrates the simultaneous operation (Cuk- SEPIC fusion mode) of the two sources where M2 has a longer conduction cycle.

   Figure 4.5 : Modified Incremental Conductance Output Voltage To Cuk Converter

   Figure 4.6 Wind Output To Sepic Converter

   Figure 4.7: Wind Output After Sepic Converter Operation

   Figure 4.8 :PV Panel closed loop Output Voltage After Cuk Sepic Operation

   Parameters of Cuk converter model

   Parameters of SEPIC converter model

5. CONCLUSION

   In this paper a fused Cuk-SEPIC converter stage for hybrid wind/solar energy systems has been presented. This topology makes continuous power generation possible in order to meet the load demand by allowing the two sources to supply the load either individually or simultaneously. The advantages of proposed converter are eliminating the need of additional input filters to filter out the high frequency harmonics, supports wide ranges of PV and wind input, low input current distortion, low conduction losses and improving the conversion efficiency using Maximum Power Point Tracking algorithm. Simulation results have been presented using MATLAB/Simulink to verify the features of the proposed topology. This system has lower operating cost and finds applications in remote area power generation, constant speed and variable speed energy conversion systems, aerospace industries, electric vehicles, communication equipment and rural electrification.

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