Introductions of control system including boosting converter engineering essay

C: UsersljwDesktopQQ截图20130425155331. pngFaculty of Engineering and Design University of BathGroup Activity XX50180

Micro generation scheme-Part 2

Design and Introductions of Control System Including Boosting Converter and MPPT and Inverter System for PVGroup number: 9Student: Jianwei LiStudent number: 106477885MSc Programme: Electrical power systemsDate: 23/4/2013Contents5. Reference ………………………………………….. 14

1. Introduction

1. 1 overview of output control and inverter system

Solar panels or more precisely photovoltaic (PV) arrays which are mainly used to transform solar energy into electricity have been a heated topic in the power market and the energy industry. (Part 1 Group 9) Because the price of PV devices has became more reasonable, people become increasingly more interested in applying PV system in order to save money and make it environment friendly. However the process that transforming the solar energy into our daily used electricity is very complicated and challenging. As shown in Fig. 1 the home PV system that our team designed is mainly consist of four parts the solar array, the home applications, the grid and the battery[1]. The four parts should be connected together in order to make the system be able to not only generate the standard alternating current (AC) for the home applications but also storage and maybe sell the redundant power. My work in our group is to design the connection and transform process of the four different parts. It can be mainly introduced by two parts one is the output control system of solar panel and the other is the inverter system. Among the four parts as shown in Fig. 1 inverter system and control system are the bridges which can achieve the interaction among the installed devices (solar system), customers (home applications) and the Grid. C: UsersljwDesktopQQ截图2013111111425131011. png

Fig. 1 the Diagram of Home PV system

1. 2 Necessity

The solar array cannot produce stable currents and voltages because the output characteristic is significantly influenced by local surroundings such as temperature and illumination.(Part 1 Group 9) If PV arrays are connected to the battery system or the Grid the output voltage should be refined, hence the DC-DC switching boost converter. MPPT is working as the brain of PV system and it will figure out that on what point the solar arrays should work. By adding the MPPT the efficiency of the switch system will be improved up to 93%. Many existing innovative power generation equipments can only produce the DC power such as solar panel. However, plain truth is that home applications and the Grid are all treated as AC system. Hence the inverter system is a must. The report will describe the design of control system including boosting circuit and the technology of maximum power point tracking (MPPT) and the inverter circuit which makes the direct current (DC) generated from the solar array into the AC power.

2. Output Control System

2. 1 DC-DC switching boost converter circuit

2. 1. 1 Purpose of Appling DC to DC boosting converter

Solar panels is a kind of photovoltaic devices and it can only produce electric current when there is enough sun light to enable the development of photoelectric effect. Due to the potential limitations of the material of solar panels and the properties of sun light, the generated voltage is unstable. [2] In the case of a better lighting condition and a appropriate temperature the selected panels can provide the voltage over 20 v. (Part 1 Group 9) However when it is the rainy day and the light condition is poor such solar panels can only produce around 10 v output voltages. Therefore the instability voltage cannot satisfy the requirement of both the home applications and the Grid. Actually the voltage generated by the photovoltaic devices cannot be used directly into any kinds of applications it should be refined firstly hence the boosting circuit.[3] The DC-DC switching boost converter circuit is installed between the solar array and the battery system and the inverter system. When the solar panels delivers unstable and low voltage to the DC to DC boosting converter the voltage can be unified and boosted in order to guarantee real-time solar system can work normally and raise the efficiency of the use of solar energy.[4]

2. 1. 2 The Principle of DC to DC boosting converter

The booster circuit this report adopted is a kind of dc step-up circuit switch. It can produce higher output voltage than the input voltage. The basic circuit diagram is shown in Fig. 2. Assume that MOSFET has been off for a long time, all the components is in the ideal state and capacitor C1 voltage is equal to the input voltage. When the transistor (MOSFET) is conducting, input voltage will flow past the inductor. At this time energy is being stored in the inductor When is break over the. Because of the dc input, the current of the inductance L1 will increase at a certain linear ratio and the ratio is related with the size of the inductor. As the inductor current increases, the inductance is storing energy. when MOSFET cut off , due to inductances’ characteristic that maintaining current, the inductor current will not immediately turn to zero but decreases to slow down to zero from the charging. However at this time the original circuit is disconnected therefore inductance will discharge only through the new circuit. Meanwhile the inductance L1 begins to charge the capacitor C1 and voltages on both ends of the capacitor will rise. as a result the output voltage is higher than the input voltage, the boosting process is completed. Diode D1 is used to prevent the capacitor discharge to ground. [5]C: UsersljwDesktopQQ截图20130425131110. pngFig. 2 the Principle Diagram of DC to DC boosting converter

2. 1. 3 Main Parameters

The main parameters (see Fig. 2) are needed for the designing of the DC to DC boosting converter and they are introduced and described as following[5]: Input voltage and original current andOutput voltage and output current andWorking frequencyInductance and capacitanceDuty ratioThe drop voltage of the rectifierCurrent changesFirstly the Duty ratio can be calculated. In each switching cycle, when converter is working stably, the increase of the inductor current during conduction time equal to the decreases during the shut off time: Secondly can be chosen according to the following expression: Since the capacitance is a kind of ceramic capacitor hence the value of the capacitor can be calculated:

2. 2 Maximum Power Point Tracking (MPPT)

2. 2. 1 Necessity of Appling MPPT

MPPT is a kind of solar control technology and it is an upgraded version of traditional solar charging and discharging controller. It have been mentioned in this report that DC to DC boosting converter is used to transform the unstable electrical energy generated from solar panel to a stable and reliable power source. Nevertheless the output voltage and current of solar arrays are also influenced by the lighting condition temperature and array angle. Therefore it is necessary to control the operating points and to tack the maximum power of the solar array in order to make sure both the DC to DC boosting converter and the solar panel are working in high efficiency condition. [6]MPPT is working as the brain of PV system and it working together with DC to DC converter to make real-time detection of the power generated from the solar panels and to refine the electricity for the further usage and process. Protection system of the PV panels also needs the output analysis of MPPT. (Part 5 Group 9)In addition, because of the high initial investment and limited life span of solar arrays, the use of MPPT technology is a necessary for the customers to draw the best working power point from the PV system. Usage of MPPT will result in high performance of the PV system and the improvement of the efficiency of PV system will increase the life span of solar devices hence save not only energy but also money. In conclusion MPPT used in solar photovoltaic system coordinate the progress of different parts including the solar panel, battery system and load and the Grid hence MPPT get its name the brain of a photovoltaic system.

2. 2. 2 The Principle of MPPT

The principle of MPPT is mainly based on the output characteristics of the photovoltaic arrays. So the characteristics of the photovoltaic arrays should be briefly introduced here. The solar array is a kind of nonlinear equipment and according to its equivalent circuit diagram (sees Fig. 3), it can be represented as a current source model. Fig. 3. the Equivalent Circuit Diagram of Solar CellBased on the equivalent circuit diagram of solar cell the output characteristic equation can be obtained: [6]The main parameters in the output characteristic equation of solar cell are introduced as following: [6], it refers to the photocurrent density and it associated with the intensity of sun light., it is the inverse saturation current., it refers to the electronic charge, and its value is., it is the Boltzmann Factor and the value is., it refers the surface temperature of solar panels. is the output voltage and is the output current of solar array. The output characteristic equation of solar cell is able to describe the main features of the solar panels. Based on this equation the report gives the characteristic curves under mainly two variable quantities the solar radiation intensity and the temperature. Fig. 4 shows the characteristic under (a) different solar radiation intensities and (b) different temperature conditions respectively. The figure shows that under a certain light and temperature, solar cell has a maximum power output point. When the condition of light and temperature varies, utilization efficiency of solar cell will change.[7] To make it more simple this report combined the tow different curvy in to one named characteristic PV array power curve shown below in Figure 6. It is noted that in Fig. 6 under the given conditions, there is certain voltage value corresponding to the a peak value for the output power and increase and decrees the voltage will both result in the decline of output power. Therefore the problem that solved by MPPT is to automatically find the peak condition at which solar panels can run to obtain the maximum power output under a certain irradiance and temperature.[8]P-U curve under same temperature but different illumination conditionP-U curve under same illumination but different temperature conditionFig. 4. the Characteristic Curves of PV ArraysQQ截图20130421203816Fig. 5. the Characteristic PV Array Power Curve

2. 2. 3 MPPT Control Strategy

According to the output characteristic equation and the characteristic curves it can be obtained that the output characteristic of solar array is nonlinear. Ignoring the power loss of transformer itself, can be concluded that when working on the maximum power point the PV should satisfy the following equation:(a)Where stands for the input resistance of the photovoltaic cell, equivalents to internal resistance of PV battery and and refer to the Buck – boost converter duty cycle and the SPWM inverter modulation ratio respectively. In order to effectively reduce the output harmonic values, the inverter using SPWM modulation (which will be introduced in part 3). Assuming the output voltage and current is and . On the maximum power point there is:(b)Where refers to the photovoltaic battery output voltage value. Hence combined formula (a) and (b) it can be obtained that: From the above expression, it is obvious that PV system can achieve maximum power tracking control by real-time adjusting of duty cycle of DC to DC converter and SPWM modulation of the inverter. Because there are two control variables, during the process of the entire control, it can take advantage of both the variables to avoid the excessive changes of duty ratio. Many algorithms can achieve MPPT control including hill climbing method, incremental conductance method, fractional open-circuit voltage, neural network and ripple correlation control. [8] This report apply incremental conductance method to control the PV system.

2. 2. 4 Achievement of Appling MPPT

If the group project applies MPPT in the micro generation system (PV system) the following advantages can be achieved: [8]Appling MPPT can significantly improve energy conversion efficiency reaching 95%. Varied load control mode enhances the flexibility of monitoring system. Real-time monitoring can improve the system response speed. Using temperature compensation method can adjusted discharge parameters automatically resulting in improvement of the life span of the battery system. (Part 3 Group 9)

3. Inverter system

3. 1 The overview of power inverter

Power generated from the solar panel will be optimized by the output control system (including DC to DC converter and MPPT). Both the output voltages form the solar arrays and control systems are DC voltages. However, almost all the home applications apply the AC power and the power system and the Grid are both based on the AC power. A power inverter or electrical power converter that transforms direct current or voltage into alternating current or voltage and the converted AC power can be refined at any frequency or any required current and voltage value. Therefore the inverter is a kind of transformer that can make the DC power use for any AC applications by using appropriate technology switching circuits, and control strategy.[01] With the improvement of power electronic technology power inverter will have a bigger stage for its necessity and good performance.

3. 2 The classification and principle of power inverter

Power electronic equipment is booming from the last century and the inverter also has a great development from the 1950s. [9] Hence there are many kinds of inverters appearing on market. The classification of the inverter varies according to the different usage of it. This report has briefly introduced some mainstreams: Based on the output of inverters there are two kinds modified square wave and sine wave inverters. According on whether the system has battery system the inverter can be divided into three kinds including stand-Alone inverter, intertie inverter and multifunction inverter. For the PV system, there are two parts needed to convert DC to AC. Hence a multifunction inverter is selected. (Part 4 Group 9)Based on the different source of inverter it can be divided into voltage source inverter and current source inverter. The micro generation system should connect to the Grid we care more about the quality of the output voltage such as the harmonic wave and frequency. Therefore this report should choose the inverter mainly based on the output of the inverter hence the sine wave inverters. Aiming to introduce the principle of the inverter in a simple way this report chooses the single-phase half-bridge inverter as an example. Although the practical inverter system is rather more complicated than the single phase inverter the basic principle is the same. Single phase half bridge inverter shown in Fig7 is a kind of square wave inverter. Two transistors and two diodes are used to make up the half-bridge circuit. Transistors one (T1) and transistors two (T2) take turns to open circuits as a result the input DC voltage () will be changed to the three level voltage or the square wave. Changing of the switching frequency of T1 and T2 can change the frequency of the alternating voltage output. The output voltage in the opposite direction of current flow will go through the diode D and the diode provides a reverse current path hence the diode D is a must. The function of diode is to guarantee the current will not be interrupt as a result in this circuit it is called fly-wheel diode (FWD). [9]Fig. 8. the Single-phase Half-bridge InverterFurther promotion is the three-phase full-bridge inverter see the Fig. 8. The principle is the same with half bridge though the full bridge has the batter output. C: UsersljwDesktopQQ截图20130425102901. pngFig. 8. the Three-phase Full-bridge Inverter

The Principle of Sinusoidal Pulse Width Modulation (SPWM)

It have been mentioned above the PV system should have a better sine wave output so SPWM is a good way to achieve it. SPWM method a kind of more mature PWM method now is widely used in industry area. SPWM is based on the theory of sampling control pulse equivalent principle: narrow pulse has the equal impulse but different shapes added on inertia unit, the effects are basically the same. (See Fig. 9)C: UsersljwDesktopQQ截图20130425111100. pngFig. 9. the Rectangular Pulse Equal to Sine WaveAs shown in Fig. 10 by controlling the on-offs of power electronic devices it can modulate the width of pluses into the sine sinusoidal regular. As a result the area of the pulse voltages equal to the area of expected sinusoidal wave. Hence obtain the high quality output. By changing the frequency and amplitude of the modulation wave it can achieve the output voltage in different frequency and amplitude. C: UsersljwDesktopQQ截图20130425112145. pngFig. 10. the Principle Diagram of SPWM


Energy sources such as the coal, the natural gas and the oil are exhausting day by day. The green power including sola is regarded as one of the very promising renewable source in the future developing. As described in this report the control and inverter system is the most significant part in such PV system. Control system including boosting circuit and the MPPT is the brain of the whole system. It can overcome the limitations (unstable output) of solar panels and improve the efficiency of PV system. The problem is that how could the DC power generated by the PV arrays can be used in the daily home applications and connected with the Grid. The inverter system will solve this problem perfectly. This report has described the principle of inverters and introduced different kinds of inverters and how to choose suitable inverter. It is true that the development of control and inverter system is the key to guarantee the application of PV system. In the further I think the integrated board which combines the three technologies (DC converter, MPPT and inverter) together will be the main direction.