Track II.: Applications using IntelligentTechniques in Electrical & Energy Systems Single Phase-Single Stage Z-Source Solar PVInverter AbstractRecentlydeveloped Z-source inverters have the capability to provide buck and boostfunctions to the preliminary stage of inverters. The current research presentedin this paper proposes a Single Phase-Single Stage (SP-SS) Z-Source SPVinverter topology.
The designed system, not only raises the voltage coming fromthe solar photovoltaic arrays, but also capable of tracking the maximum power forvarying level of irradiance. In this paper difference between single stage anddual stage system is presented. The results are obtained in order to analyse theTotal Harmonic Distortion (THD) and DC component present in the output ACcoming from the inverter.Keywords:Photovoltaic(PV), Single Phase-Single Stage (SP-SS), reliability, THD1. INTRODUCTIONToget a pollution free environment renewable energy resources comprises of solar,wind, hydro etc. are the best available options for generation of electricalpower. Over a period, solar energy for generating the electricity became aviable and feasible option due to its availability and abundance in nature.
Solarenergy is harnessed using solar panels for extracting power which requires highinitial investment. Since, the availability of solar is there only during daytime, therefore to help people during night time storing of electricity duringday through electrical storage or batteries are required. Utilizing the solarpower to its maximum and the efficient design of the inverter are the keyfactors in reducing the cost. PV array’s I-V characteristic is non-linear innature 1 and the movement of big blue marble known as earth, encompassing thesun entails the operation of MPPT 2.
Standalonesystem can attain maximum power point tracking, but it requires battery bank tostore the power. For grid connected system multistage systems have beenreported in the literature. Most of such multistage systems comprises of twostages.
Figure 1(a) represents the conventional two stage system 3 forconditioning the power generated through PV and then feeding it into the grid.In the first stage, the harvested solar power boosts up and also tracks themaximal solar power. In the second stage, DC power generated by the solar arrayis inverted to good quality AC power.
Generally, boost type DC-DC converter orbuck-boost are used in the first stage. The above two stages are time testedand gives good output result. This configuration has some drawbacks too, thatis, increased number of parts, lowering in efficiency and reliability, also thelarge size and high cost are added to it. So, we could attain a situationdepicted in Figure 1(b) i.e.
to lower down of the power processing stages. (a) (b)Figure1.Grid integrated solar photovoltaic topologies: (a) traditional double-stage structure and (b) single-stage structureTo reduce themultistage systems, and problems associated with it, to single stage systemsthere are two simple solutions:1) Byusing a step-up transformer with the H-bridge inverter.2) Byconstructing an array which produces large voltage followed by a H-bridgeinverter.Theabove stated solutions can come into use, but they also suffer from hitches too4. In the first case, by adding atransformer similar to the frequency of the grid, the system will become bulkyand costly and also losses will increase.
In the second case, whileconstructing PV array we would get large DC voltage which results in hot-spotsduring partial shading leads to safety problem and probability of leakagecurrent to flow between the system and ground. But in both the cases we must takecare of maximum power point tracking. 5 Toovercome these problems the perfect choice is to reduce the processing stagesbetwixt the DC generation and the grid as shown in Figure1(b). Such compactsystems are in demand now days, which offers high reliability and performancewith reduced cost, weight and size. As number of processing stage are less, soit is easy to integrate the module and the devices used in the power stages arealso minimized.
In this research paper, a lone stage has been presented, whichcan be used both with grid connected and standalone system. 6 In 7 author haspresented a refined a topology which is single stage and also with boostingcapability. This topology is consists of dual boost converter which areperforming in a supportive way and are interdependent.
Although the topology isnot best suitable for PV applications, then also the circuit is satisfactoryfor lone stage topology for grid linked PV applications. In this all thecomponents are hard switched in one go, leading to huge frequency, making thestructure prone to EMI issues and form more switching losses.In 8 author hasproposed the topology by taking the idea from the Zeta & Cook converterconfiguration. This topology is an improved one as it has an advantage of usinglesser count of semiconductor devices also avoids the drawback of concurrenthuge frequency procedure of all the semiconductor devices. Switch S1 operatesat higher frequency during the negative half cycle, during the same periodswitches S2 & S4 are kept open (ON). Power transferred during the positivehalf cycle depends on the principle of buck n boost. At the same time, switchS2 is operated during negative half cycle at a higher frequency, althoughswitches S1 & S3 are kept open (ON). Therefore Power transferred meanwhilethe negative half cycle follows the boost principle.
Main advantage of thepresented topology is, of having low switching losses. During the two halfcycle converter leads to asymmetrical operation which is the major drawback ofthe topology.In 9 half bridgebuck-boost inverter topology had been proposed by the author. Asymmetricaloperation at the time of two half cycles is the drawback of the topology.
Switching losses and conduction losses are minimal adds an advantage to thetopology, since usage of two power devices is found during the half cycle.Since switches are minimal in number which operates at higher frequency, sothey help in increment in the reliability and low EMI. In 10 reform ofhalf-bridge buck-boost topology known as isolated flyback configuration hasbeen proposed by the author. The mentioned topology consists of a transformerfor isolation and three power devices are used.
In this topology the belief ofbuck-boost is used. The present scheme is blameless enough, but can be appliedonly to low power system, practically up to 500W, due to the curb on the chargeof the principal inductance of the flyback transformer. Some more losses arethere in the system due to the availability of transformer, keeping this inmind it provides isolation at PV panel and the grid side. In 11 author hasproposed a single stage power conversion topology.
The mentioned configurationin this paper is said to be full bridge configuration which works on theprinciple of the buck-boost. During non-negative half cycle semiconductorswitch S1 remains in ON state, while semiconductor switch S4 operates at ahigher frequency. For turning OFF, the path offered is through the switch S2and diode D1. The major drawback of the presented topology is, usage of morenumber of power electronic devices at a particular instant of time, thereforeresults in higher conduction losses.
In 12 author hasproposed grid interacted single stage topology. The above mentioned topologyworks on the principle of flyback with mutually coupled coils for the negativehalf cycle of grid voltage. One of the major drawbacks of the proposed topologyis that, it can only be used for applications of lower power rating.
Due tothis conduction and switching losses are higher as well as they operate is asymmetricalmanner which is another disadvantage of the proposed system.In 13 usingbuck-boost principle a single stage system has been presented by the author.The current topology is not intended for PV with grid connected system. In theproposed topology system is modelled using five switches, out of which threeswitches operates at a high frequency which is the limitation of the saidtopology, further leads to higher switching losses and EMI concern.
2. PROPOSED TOPOLOGY Figure 2. Z-source inverter block diagram Block diagram shownabove states the proposed topology. This is a single stage topology in whichboosting of DC power and inversion of DC power to AC power is done in singlestage. This AC power can be used to fed grid as well as feeding to theresidential load.
2.1 PV MODULEThebackbone of every solar photovoltaic system is solar photovoltaic array.Presently, the irresistible majority of photovoltaic modules are crystallinesilicon, which is manufactured from the second supreme ample element availableon this planet. PV cells combining together to form PV module.
PV modulesabsorb photons present in sunlight and convert them to electricity. Singlediode model is used to model a PV cell. Accord between the distinctive voltageand current of the equivalents circuit model of PV module. 14 Figure3. Single diode model of solar cell- –) – =0 (1) + =0 (2)= Light Generated current. =Current across diode.= Voltage across diode. (?) = Series Resistance.
(?) = Shunt Resistance.(V) and (Amp) are the module output current andvoltage. Operating equation of the PV module = – { (+) – 1} – +)/(Amp) represents the saturation currentof PV module. PV module temperature (T) is noted down in kelvin. For optimumperformance of the system panel must be operated near to maximum power point.2.2 Z-SOURCE TOPOLOGY · Due to its uniquecircuit topology Z-source functions as buck-boost inverter while avoiding theuse of conventional converter bridge.
· It is an exclusiveX-shaped converter system known as Z-source impedance network that is link withthe power source and the inverter circuitry.· Z-source is resistantto short circuit which occurs on branches and at the opening of the circuitry.· It shields the actionof decreasing and increasing of voltage in a single step.· Promote resistant toEMI distortionsand failure switching.· Enhances power factor,scale down common mode voltage and harmonic current.· Without the use of anyadditional circuit, it provides ride through during voltage sag.· Has low in-rush currentcompared to voltage source inverter.· Provide a reliable,highly efficient single, and low cost stage for buck and boost conversion.
· Relativelysimple start up. Figure4. Z-source network arrangement Figure 5. Circuit diagram of the proposed topology3. SIMULATIONBelow shown is the Simulink diagram of asingle phase single stage z-source SPV inverter.
A constant value is fed as aninput acting as a DC source. Combination of inductor (L) and capacitor (C)forms the z-source network which is acting as boosting circuit for this system.The system comprises of four semiconductor switches i.
e. MOSFETS. LC Filteringcircuit is used to remove out the harmonics. Parallel RLC load is attached inthe circuit. Further, in this section the simulationoutputs have been depicted.
The output if the solar PV array depends on theirradiation and temperature of sunlight which directly affects the generation.Figure6. MATLAB Simulink diagram of SP-SSFigure 6 shows the Simulink diagram ofsingle phase-single stage Z-source SPV inverter. Simple PWM technique is used in this systemto generate the PWM pulses. Figure 7 shows the PWM signals fed to thesemiconductor switches. Figure 7. PWM signals fed to the MOSFET switches S1,S2, S3, S4Figure8. Z-source Inverter output withoutfilter From figure 8 the output of Z-sourceinverter without putting filter is shown.
Figure 9 depicts the sinusoidaloutput voltage signal. Figure9. Z-source Inverter output with filter Figure10. FFT Signal Figure 10 shows the selected signals forcalculating the total harmonic distortion.
Figure11. THD Component Figure12. DC componentFrom figure 11, it is shown that theline voltage harmonic distortion is 2.70% which is in the permissible limit ofthe IEEE standard. Figure 12 depicts about the DC componentthat is 0.001184 which is being fed through the PV system.
4. CONCLUSIONThis research paper gives a clear ideaabout the single phase- single stage PV system, where DC power is converted inAC power in a single power processing stage. This work has been publicizedthrough MATLAB simulink. A single stage topology with new and amendedtechniques has been proposed. The topology proposed is easy to access, simpleand symmetrical in nature. The main benefit of the system is the betterefficiency, and high reliability due to reduced number of switches andswitching losses. It is finally concluded that single stage system offersbetter THD than dual stage system.
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