Design of Hybrid Solar Wind Energy System in a Microgrid with MPPT Techniques

Received Dec 9, 2017 Revised Mar 20, 2018 Accepted Mar 27, 2018 DC Microgrid is one feasible and effective solution to integrate renewable energy sources as well as to supply electricity. This paper proposes a DC microgrid with enhanced Maximum Power Point Tracking (MPPT) techniques for wind and solar energy systems. In this paper, the PV system power generation is enhanced by introducing a two-model MPPT technique that combines incremental conductance and constant voltage MPPT algorithms. Also, for the Wind Energy Conversion System (WECS) with pitch angle controlling technique, an Optimal Power Control MPPT technique is added. The Space Vector Pulse Width Modulation technique is introduced on grid side converter to improve the supply to the grid. The performance of proposed system is analyzed and the efficiency obtained with these methods is enhanced as compared with the previous methods. Keyword:


INTRODUCTION
The rapid depletion of conventional sources is increasing the demand for the use of alternating energy sources. Mainly wind and solar with their abundance and high potential gained the attention of the world. There are different concepts and technologies developed from years in order to utilize the available sources. Both wind and solar power system generations change with varying weather conditions. To best utilize the sources each are provided with some techniques. There are different MPPT techniques [1][2][3][4][5] generally being used in PV systems. The Perturb & Observe (P&O) [6], [7], the Incremental Conductance (INC), Open Circuit Voltage, Short Circuit Current are generally used MPPT techniques and each has its own drawbacks. The P&O produce oscillations around the Maximum Power Point (MPP) even under steady state illumination. The INC method is efficient and accurate even under varying weather conditions but its controlling circuit is complex than P&O. The OC voltage and SC current method are simple but not efficient for high energy generation units.
In PV system to improve the performance at any weather condition, the two-model MPPT technique is introduced. From the generally used techniques, any two techniques are combined based on the climatic conditions of an area and of all combinations; the Incremental Conductance and Open Circuit Voltage combination is efficient and accurate for all weather conditions [8], [9].
The Wind Energy Conversion System (WECS) has different concepts in terms of generators, fixed and variable speed wind turbine generators. Here, it is provided with a Permanent Magnet Synchronous Generator (PMSG), a pitch angle controlled wind turbine and also with an MPPT technique that acts in accordance with the wind speed. The PMSG is a variable speed wind turbine generator which is much 731 efficient than Doubly Fed Induction Generator of the same type. The wind and solar power that was generated is integrated at the microgrid. The dc power is boosted and then converted to ac using an inverter. To control the power at the inverter end, it is provided with Space Vector PWM technique that eliminates the distortions in output voltage and improves the efficiency. The novelty of this paper is to improve the power generation of both the PV system and WECS with enhanced techniques .The contributions of the paper include (1) Analysis of different MPPT techniques for a PV system and to design a two-model MPPT technique. (2) Development of a WECS with PMSG [10][11][12] and deploying the pitch angle controlling technique and an Optimal Power Controlling MPPT technique. (3) Integrating the PV system with WECS and implementing SVPWM technique [13][14][15] on the inverter side in order to improve the performance of output power.
The paper is organized in such a way that Section 2 discusses the design of the PV system, different MPPT methods used in the PV system and the design of two-model MPPT technique. Section 3 contains the modeling of WECS with MPPT technique. In Section 4, the integration of PV system with WECS and the SVPWM technique are discussed. The simulation results are shown and discussed in Section 5. Section 6 is the conclusion of the paper.

DIFFERENT MPPT METHODS FOR PV SYSTEM
The PV module is first modeled in Mat lab based on the following equations. With change of irradiation and other specifications of the module, the output varies. With the specifications stated above, the maximum output from the PV system is about 3W. To maintain the system at its maximum production, the Maximum Power Point Tracking (MPPT) techniques are introduced. The most generally used MPPT techniques are as:

Perturb & observe (P&O) MPPT Method
The P&O MPPT algorithm is a hill climbing technique in which, the operating voltage of the PV system is perturbed by the change in voltage (dv) is carried out according to change in power (dp) such that the variation of both the quanties need to be in the same direction. Figure 1 shows the flowchart of this MPPT algorithm. The drawback of this method every MPPT cycle the array terminal voltage is perturbed, resulting in a power loss in the PV system due to heavy oscillations of output voltage at the MPP. Furthermore, it sometimes fails to find the MPP.

Incremental Conductance MPPT Method
In the second method, the PV system voltage is regulated according to the voltage corresponding to maximum power based on conductance (I/V) and instantaneous conductance (dI/dV).The PV system voltage is increased or decreased by verifying the positive or negative variation of dP/dV respectively.Inorder to track the voltage corresponding to maximum power.The flow chart of the incremental conductance method as shown in Figure 2. The advantages of the INC algorithm are of its high stability and fine control of the system with rapid change of atmospheric conditions.

Short Circuit Current MPPT Method
The Short circuit current or constant current algorithm is a simple MPPT technique and its flowchart is as shown in  The relation between short circuit current and current corresponding to maximum power seems to be simple. However, the design of gain K 2 is complex. Typically the value of gain K 2 lies between 0.78 & 0.92.

Two-Model MPPT Method
The two-model MPPT can be designed with different techniques that are available. But, the most beneficial and efficient for the rapid changes is the combination of incremental conductance and Open Circuit Voltage MPPT techniques. The flowchart of proposed two model MPPT algorithm is shown in Figure 5 and Simulink model of two model MPPT method is shown in Figure 6. Here G t is the threshold value of irradiation value and G n is the irradiance value at particular value.

WIND ENERGY CONVERSION SYSTEM
The wind energy is the one which is much available after solar energy and is a promising source of electrical energy. The kinetic energy of the wind is used to rotate the turbines that generate mechanical energy. The generators convert the mechanical energy to electrical energy.

INTEGRATION OF PV SYSTEM TO WECS
The PV system and the WECS are integrated at a point and it is supplied to an inverter to generate AC output. On the inverter side, SVPWM technique is provided to improve the quality of the power. Figure 9 shows the simulink block diagram of the proposed microgrid. The Space Vector PWM is preferred over sinusoidal PWM method for variable frequency drive applications. It improves the quality of the generated output. One important advantage of this technique is it reduces the Total Harmonic Distortion (THD) created by the rapid switching. Figure 10 show the space vector trajectory.  From the research results, integration of solar wind generation with MPPT and SVPWM generates greater voltage than without MPPT and SVPWM (Table 3).

CONCLUSION
This paper conferred a microgrid with an improved output from both PV system and WECS. The PV system output is improved with the Two-Model MPPT than the generally used P&O MPPT model and the WECS output also enhanced with a pitch angle controlled wind turbine model and an MPPT technique. The pitch angle control technique is actuated for high wind speeds and the MPPT algorithm is for driving the rectifier to track maximum power point. Also, the quality of AC output of the microgrid is much improved with the introduction of SVPWM technique on the inverter side. The improvement is well observed through the simulink results.