Şebeke bağlantılı tek fazlı mikro eviricinin tasarlanması ve gerçekleştirilmesi
Design and realisation of grid connected single phase microinverter
Yazar: EMRE ÇELİK
Danışman: YRD. DOÇ. DENİZ YILDIRIM
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Elektrik Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering

Özet:
Yenilenebilir enerji kaynakları kullanımının her geçen gün artış gösterdiği gözlemlenmektedir. Bu artışın başlıca sebeplerine baktığımız zaman;  fosil yakıtların her geçen gün artan enerji ihtiyacını karşılayamayacak olması, buna bağlı olarak artan enerji maliyetleri, çevre kirliliği ve güvenlik etkenleri olarak sıralayabiliriz. Bu sebeple yenilenebilir enerji kaynaklarından özellikle de rüzgar ve güneş ön plana çıkmaktadır. Halen gelişme aşamasını tamamlamamış olan bu sistemler üzerinde yoğun çalışmalar devam etmektedir. Bu tezin kapsamında yenilenebilir enerji kaynaklarından bizi ilgilendiren kaynak güneş enerjisidir. 
Güneş enerjisi incelendiği zaman bu enerji türünün üzerinde de halen çalışmalar yapıldığı gözlemlenmektedir. Çalışılan ve öne çıkan başlıca konular güneş panellerinin verimi, panellerden alınan enerjinin maksimum seviyeye çıkarılması, panellerin ürettiği doğru akım (DA) türünde ki enerjinin alternatif akıma (AA) çevrilmesi, elde edilen ve çevrilen enerjinin tüketicilere güvenli ve doğru bir şekilde aktarılması şeklinde sıralanabilir.
Özellikle de bu tez kapsamında üzerinde çalışılan konu güneş panellerinden elde edilen doğru akımın alternatif akıma çevrilmesi ve çevrilen bu enerjinin şehir şebekesine veya şebeke bağlantısız bağımsız olarak tüketiciye doğru bir şekilde aktarılmasını kapsamaktadır. Diğer bir ifadeyle çalışma bir eviricinin tasarlanması ve gerçeklenmesidir. Bahsi geçen çalışma büyük güçlü merkezi bir evirici olmayıp kendine göre bir çok avantajı bulunan her bir güneş paneline bağımsız olarak takılan ve her bir panelin kendisinden alternatif akım elde etmemizi sağlayan mikro eviricidir.
Yapılan çalışma; güneş panellerinden doğru akım şeklinde ki enerjinin alınması ve bu enerjinin yüksek seviyelerde tekrar bir doğru akıma çevrilmesi, yüksek seviyelerde ki doğru akımın alternatif akıma çevrilmesi, çevrilen alternatif akımın filtrelenmesi, elde edilen uygun alternatif akımın şebeke bağlantısız yükler için kontrolünün yapılması ve yük üzerinde ki gerilimin sürekli olarak şebeke şartlarında tutulması, şebeke bağlantılı çalışma şekli için şebeke gerilim ve frekans değerlerinin izlenmesi ve uygun ve güvenli enerji aktarımının yapılması, sistemin çalışması esnasında güneş panellerinden alınan enerjiden maksimum oranda faydalanmak için maksimum güç noktası takibinin (MPPT) yapılması ve bütün bu hususların tek bir mikro denetleyici tarafından kontrol edilmesini içermektedir.
Bu amaçla yapılması planlanan mikro eviricinin tasarım aşaması ve simülasyonları aşama aşama  yapılmış olup, deneysel olarak da yapılan çalışmaların doğruluğu ve uygulanabilirliğinin görülebilmesi amacıyla sistem gerçeklenmiştir. Gerçeklenen sistem şebeke bağlantısız durumlar için başarılı bir şekilde gerilim regülasyonu yaparken şebeke bağlantılı çalışma durumunda ise şebekeyle senkronize olarak ulusal elektrik şebekesine akım aktarımı sağlnamıştır. Son olarak teorik ve pratik çalışmalar karşılaştırılıp gözlemler, sonuçlar, farklılıklar ve öneriler sunulmuştur.

Abstract:
The day by day increase in using renewable energy is being observed. The main reasons could be explained the fossil fuel's being not able to meet the rapid growing energy demand, increased energy costs, environmental pollution and safety factors caused by this deficiency. The solar and wind from the renewable energies are the prominent ones. The detailed studies keep going on the systems whose development process still continues. This study deals with solar energy source within the sources of renewable energy.
Solar energy usage is increasing regularly each year. Especially, according to the technological improvements on multilayer cells, photovoltaic  panels'  efficiencies are increased, therefore amortization and investment expenses are reduced which effected solar energy industry in a positive way. 
Today, amongs the countries, the best ratio of solar energy production to total energy cunsumption belongs to İtaly with 7.9 percent, afterwards Greece with a ratio of  %7.6 and Germany is the third with %7 in the world. On the other side, according to the installed capacity, ranking is totally different. Germany has the largest installed solar energy capcaity with 38GW which is followed by China. China has 28GW solar energy production capacity, lastly the third is Japan with a 23GW installed solar energy capacity. Unfortunately Turkey is at the beginning of solar energy industry. Right now, Turkey has 71.2MW total installed solar power capacity that fulfills the %0.1 of total energy demand. However it is worthwhile to state that, %56 of the installed capacity is settled up in 2014. While the total capcity of Turkey is 31.2MW till 2014, it reached 71.2MW by 2015.  So it can be exctracted that an aggressive solar market growing will be held in Turkey next decades..
This thesis, in particular, contains transferring the direct current to alternating current and transferring this converted energy to national grid or independent consumers in a safe way. In other words the study is a design of an inverter and realizing it. The study is not on a large – central inverter instead on a micro inverter which can be installed on each solar panel, which maintains to gather alternating current from itself and which has many advantages as well. 
In general, micro inverters are produced  at a power range of 100W to 300W and they have some advantages over the central large capacity inverters. First of all, in a situation of malfunction or  shade, the whole systems that work with central inverters are effected. However, in a system which works with micro inverter, the malfunction individually effects the single photovoltaic panel which has the problem in spite of the whole system.  
Additionaly, due to their low power levels micro inverters are safer than central inverters and they do not need active cooling systems which means more quiet plants. Recently, by developments of soft switching techniques more efficient systems could be produced. However, compared to the central inverters, they are a little bit more expensive systems.
 According to some resources it is extremely likely that production expenses will be reduced much more due to widespread usage and mass production. Because; in spite of using a 20kW central inverter 80 unit 250W micro inverter must be used. In other words; more unit production means less component costs.
The study will be detailed. But to mention in general the study contains these steps; getting energy as direct current from solar panels and transferring this energy again as direct current at high voltage levels; converting the high voltage leveled direct current to alternating current; filtering the alternating current; checking the appropriate current for grid disconnected loads and keeping the voltage on the load appropriate for grid conditions; monitoring the grid voltage and frequency values and realizing proper-safe energy transferring for grid connected study; fulfilling maximum power point tracking  (MPPT) for benefitting from the energy at the maximum level coming from the solar panels during system run and controlling all these steps only by one single microcontroller. 
At the conversation stage of low direct current to high level direct current, push-pull circuit topology is used. It is also determined as the most convenient circuit topology for the range of power that is needed for this study which is around 250W, besides it provides galvanic isolation. Even though any soft switching technique isn't used,  %92 efficiency is achieved from the implemented circuit design for rated power.
 The high level direct current is converted to alternating current after it is subject to "H" bridge and filter combination that is triggered with unipolar pwm technique whose switching losses are lower. LCL filter is designed as filter topology which is a third order filter, it has better harmonic attenuation and allows less volume for the same amount of attenuation compared to L and LC filter topologies. Efficiency of implemented H bridge and LCL filter combination obtained as % 94.5. As a whole micro inverter system efficiency which is propotion of generated photovoltaic panel power to delivered alternating current power is %88.
Control stage of the inverter involves micro controller, current and voltage sensors that are located at the input and output sides. To keep galvanic isolation chain, both current-voltage sensors and sensor supplies are used isolated.
 In this study, synchronization issue is solved as monitoring and following grid voltage in real time that is different from many conventional inverter's synchronization algorithm which is based on phase locked loop(PLL). 
Following grid voltage and regulating the injected current according to that voltage requires fast digital analog conversion stage and unlike phase locked loop algorithm it does not have to use past grid frequency data for regulating upccoming sinus cycles. Drawback of phase locked loop is little phase misleadings because of using previous frequency data for regulating current sinus signal. 
For benefitting from the photovoltaic panel as possible as, the micro inverter is fitted up with maximum power point tracking(MPPT) algorithm because the current-voltage charachteristic is different from most direct current sources. As the photovoltaic panel delivers current to load, panel voltage tends to collapse. In other words, PV panel's current and voltage are inversely propotional quantities. As the PV panel delivers more current it produces more power. However, increasing the current injection does not always raise the delivered power, because PV panel voltage is decreasing at the same time.
 While the power term is multiplication of current and voltage, the aim should be maximize the current-voltage multiplication rather than just current injection. For a certain condition there is a certain amount of current that maximize delivered power, but as you can guess environmental conditions are changing dynamically at any time, so considering a fixed current for maximum power harvesting is not a logical approach because aging, moisture, temperature, radiation and such kind of weather situations that effects power generation of PV panel are always changing. Therefore MPPT algorithm must consider the situation.
The MPPT algorithm is based on perturbation and observation, after applying a little perturbation to the system the resulted effect is observed, if the perturbation provides harvesting more power from photovoltaic panel, the perturbation direction is kept till decresing the harvested power, when a decresing is relaized the direction of perturbation is changed and so on. 
The term perturbation means increasing or decreasing current injection physically by regulating duty ratio of inverter. The controller which operates behind the control and MPPT algorithm is propotional and integral (PI) type controller. The designed and implemented micro inverter makes voltage regulation successfully for off grid operation, while it achives synchronized current injection to national grid for grid tie operation.
During a malfunction or a state that is beyond the micro inverter's capacity and specification, means of power and potential, a relay that is located at the output stage seperates the inverter and load side safely, after separation it enters stand by mode. This circumstance occurs when low input voltage level and excessive output current level detection.
For the aim of realizing all the above mentioned; the design and simulation processes are done step by step and the system is actualized for the purpose of observing the accuracy and practicability of the studies. In accordance with these; the theoretical design process, calculated parameters, simulation outcomes and comparasions are presented. For inconsistent results; essential revision are made and the process is repeated until observing consistent results. Based on gathered data and simulation results, the implementation process is served and carried out step by step, for each part, calculated and simulated results are compared with implemented outcomes. The differences between theorical and experimental data are presented and discussed.
In summary, from beginning to completion a grid connected micro inverter is designed, simulated and implemented whose all workload is controlled by a single micro controller. Successful operation is ensured and supported by outcomes and results, then proposals are offered accordingly.