Güneş pili dizisinin maksimum gücünün bilgisayar kontrollü izlenmesi ve fotovoltaik-pompa uygulaması
Computer controlled maximum power tracking of solar cell array and the photovoltaic pump application
- Tez No: 14354
- Danışmanlar: PROF.DR. M. KEMAL SARIOĞLU
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1991
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 45
Özet
ÖZET Güneş pili, evirici, asenkron motor, santrifüj su pompası ve kontrol biriminden oluşan sistemde; Güneş pilleri tarafından üretilen DC gerilim, evirici ile üç faz sinusoidal gerilime dönüştürülerek asenkron motor beslenir ve çalıştırılan su pompası düşük seviyedeki suyu daha yüksek bir seviyeye basar. Kontrol birimi, maksimum güç izleme olarak belirtilen güneş pilinde üretilen gücün tamamının yük tarafına aktarılmasını sağlar. Kontrol birimi pil akım ve geriliminden örnek alarak kontrol programı ile işledikten sonra ürettiği kontrol işareti ile eviricinin çıkış frekansını değiştirir. Bu da motorun hızının değişmesini, dolayısıyla motorun yükününde değişmesiyle pilden maksimum gücü çekecek yükün ayarlanmasını sağlar.
Özet (Çeviri)
SUMMARY COMPUTER CONTROLLED MAXIMUM POWER TRACKING OF SOLAR CELL ARRAY AND THE PHOTOVOLTAIC PUMP APPLICATION Since the world 's conventional sources of energy are diminishing fast, other types of energy such as photovol taic energy, water energy and wind energy offer promising alternative energy sources. Since the solar energy free of cost, pollution free and abundant, it is most one of all other sources. Terrestrial photovoltaic system technology has matured dramatically during the past decade. Many photovoltaic systems are economically viable for today's applications. Most of the photovol taic systems sold today are designed to provide power to remote, grid-independent or standalone applications such as telecommunications, vaccine refrigeration, lighting, battery charging and water pumping. Another application sector is the grid connected system. In this work, a photovoltaic water pump system consisting of a photovol taic source, a filtering capacitor and a computer con trolled PWM inverter and induction motor driven water pump has been considered. The voltage obtained from the array was converted to three phase sinusoidal voltage by a static inverter which supplies induction motor. To use solar array efficiently. Maximum Power track ing was chosen as a main control strategy. The object of maximum power tracking is to take out the electric power from the solar cell array as much as possible. While this is occuring, the motor currents are observed to prevent any over loads and the control signal ' s limited whenever necessary. From the point of view of the ra tings of the induction motor used, the maximum power point of the solar cell array is also the optimum power point of induction machine. The solar cell is a semiconductor device that con verts the solar insulation directly to electrical energy. The cell is a nonlinear device and can be represented by the I-V terminal characteristics. Fig. 1, or by an appro ximate electrical equivalent circuit as shown in Fig. 2. The solar cell is an electrical cell of low level voltage and power, therefore the cells are in series and in parallel combinations in order to form an array of the desired voltage and power levels. The I-V equation of a single cell is given by: I = Ii_ - Is ( exp[ (q/nkT) (V + Rsl) - 1 ] ) (1)where It- is the light generated current, I» is the satu ration current, kT/q is the thermal voltage, n is per fection factor, R«. is the series resistance of the cell. For each characteristics curve there is optimum operating point with respect to be transferee!. I I.I I I 1000W/m'.47°C (NOCT3) Fiq.l. Characteristics of a solar cell ( SM55 ) ?WW»- o Re Fiq. 2. Solar cell equivalent circuit VIThe most common var i able -speed induction motor drive comprises a DC power source, an inductor/capacitor DC link filter, a voltage source inverter, an induction motor and some form of control system (Fig. 2). The DC source may or may not he capable of accepting regenerated energy. Figure 3. drive Voltaae source inverter fed induction motor To convert the DC voltage into three phase sinu soidal voltage, a voltage-fed static inverter was used. Inverter -fed induction motor drives are complex system whose dynamic behaviour is difficult to predict. Un stable behaviour is common in this drive, and misunderstandings sometimes occur as the source of the instabilities. Inverters use switching technics to convert DC to AC and as a consequence, their outputs are rich in harmonics. In addition, most digital methods of waveform production are nonideal in that the output waveform contains some uncertainity or jitter, even for constant demanded frequency. The uncertainity or jitter may stem from the anti-condition overlap delays usual in inverter design or from the use of digital techniquies involving asynchronous inputs. A widely available lar ge-scale integration (LSI) circuit for Pulse Widh Modula tion (PWM) waveform generation, the HEF 4752V, exhibits this form of behaviour. Both harmonics and jitter have been postulated as the cause of observed instability in voltage source of inverter-fed induction motor drives. Control signals which drives the tranzistors were produ ced by the Philips, IC HEF 4752V chip. The main feature of speed control of induction motor using IC HEF 4752V is that it is based on double edge modulation which result in minumum harmonic distortion in line current, minumum torque pulsation and motor losses. In this modulation scheme» each edge of the carrier wave is modulated by a variable time 6, where 5 is pro portional to Sin a J Iq'O 'i «2 *3 *4 «5 '6 Fiqure 4. Double edge modulation HEF 4752V chip, uses three clock signals, FCT clock, VCT clock, RCT clock. FCT controls the inverter output frequency, VCT controls the inverter output voltage and RCT is the reference clock which is used as the timing clock. It is also determines the maximum switching frequency. The carrier wave which is square wave that has duty free ratio of unity at a frequency proportional to the FCT clock frequency, is produced. The PWM is defined by the number of pulses produced in one sine wave period. The pulses widen according to the desired sine function. The FCT clock determines the carrier frequency and VCT clock controls the modulation widths of pulses. Therefore, the FCT clock controls the inverter output frequency and the VCT clock controls the output voltage when the modulated pulses are not merged, the harmonic distorsion is very small. However r.m.s. value of sinus oidal wave is also small. To increase the r.m.s. value, VCT clock frequency is reduced. Hence the same number of carrier pulses are widened more so that the average of each pulse increases, yielding an increase in the r.m.s. value. If there are some merged pulses, the harmonic distorsion increases. To obtain the maximum undistorted sinusoidal voltage with this IC, FCT is adjusted at nominal frequency and VCT clock is doubled. In this vmcase. The r.m.s. value of the output voltage is given by Vac = 0,624.Vt>c (2) Once the VCT clock frequency is adjusted at nominal value and not changed, the output voltage varies linearly with the output frequency. Hence the constant peak torque requirement of the induction motor is provided. In induction motor control. If the ratio of voltage to frequency is kept constant, the flux remains constant and the maximum torque which is independent of frequency can be maintained approximately constant. At a low frequency, the air-gap flux is reduced to the drop in stator impedance and the voltage has to be increase to maintain the torque level. This type of induction motor control is usually known as Volts/Hertz control. If the air-gap flux of induction machine is maintained constant, the torque of the motor will be approximately proportio nal to the slip frequency. The centrifugal pump converts the mechanical power into hidrolic power by creating a pressure difference between two regions in the liquid. The torque on the pump shaft is proportional to the square of the speed. The optimal use of power from solar battery to the load is discussed and the hardware of computer based control system is given in chapter 4. To carry out control strategy for maximum power tracking, DC current and voltage from photovoltaic array have to be measured. Therefore there will be need to use a shunt resistor, operational amplifiers and optocouplers to measure the array DC current and overcome the isolation problem. For array DC voltage measurement, a simple voltage divider can be used to drive the circuit which includes opamp and optocoupler. Measured variables are converted into the digital form by ADC (Analog Digital Converter). Digital values provided input the data for the control program. After processing this data, digital control signals were converted back into analog signals. These analog signals obtained from the DAC (Digital Analog Converters), con trols a VCO (Voltage Controlled Osilator). The FCT clock signal was obtained by VCO output. The block diagram of Maximum Power Tracking controller is shown in Figure 5. IXÜ-S Fiq. 5. The block diagram of Maximum Power Tracking System
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