Fotovoltaik Panelden Beslenen Bir Buzdolabı İçin Elektronik Kontrol Sistemi Tasarımı

Abstract

Bu yüksek lisans tez çalışmasında PV panelden beslenen bir buzdolabı için verimli enerji depolama ve soğutma sağlayan bir buzdolabı sisteminin kontrol elektroniği tasarımı anlatılmıştır. PV panel, akümülatör, yüksek verimli FDAM içeren bir kompresör sistemi kullanılarak bir sistem tasarımı gerçekleştirilmiştir. Tasarlanan sistemde motorun giriş gücü 100 W ve çalışma hızı aralığı 2200-3600 min-1’dir. Sistem tasarımı açık devre gerilimi 15 V ile 50V arasında olan tipte paneller ve 12V/24V gerilimli aküler bağlanabilecek şekilde yapılmıştır. Belli bir ışınım altında maksimum enerjiyi elde etmek için MPPT algoritmaları kullanarak, PV paneli ortam koşullarına göre en uygun gerilim ve akımda çalıştırmak gereklidir. PV panelinden alınan enerji MPPT algoritması yardımıyla en verimli şekilde aküye aktarılmaktadır. Sistemde yüksek verimli olması ve değişken hızlarda kontrole daha uygun olmasından ötürü fırçasız doğru akım motoru kullanılmıştır. FDAM tarafından sürülen kompresör bir soğutma çevrimi sistemi(kompresör, evaporatör, kondanser vb.) içerisinde buzdolabı kabin içi sıcaklığının gerekli düzeye inmesini sağlamaktadır. Aküden beslenen bir yükseltici DA/DA çevirici katının görevi üç faz motor eviricisi girişindeki DA bara gerilimini istenen değere ayarlamaktır. DA bara geriliminin ayarlanmasıyla motor hızı kontrol edilebilecektir. Üç faz motor eviricisinin görevi ise sensörsüz pozisyon algılama devresinden gelen rotor konum bilgilerine göre motor fazlarını uygun anlarda enerjilendirerek trapezoidal sürüş ilkesine göre FDAM’yi kontrol etmektir. MPPT şarj devresi tasarımında aşağıdaki kriterler göz önünde bulundurulmuştur: • Kullanım esnekliği (farklı akü ve panelleri bağlayabilme), • Günün her saatinde ve her mevsimsel koşulda başarıyla MPPT yapabilme, • Düşük elektromanyetik yayınım ve düşük elektriksel gürültülü bir tasarım, • Uygun maliyet (tek anahtarlı topolojiler üzerinde durulmuştur). PV panelden elde edilebilecek maksimum gücü üretmek için kullanılan algoritmalara MPPT algoritmaları denir. Bu tez kapsamında açık devre algoritması, “boz ve gözle” algoritması, artan iletkenlik algoritması incelenmiştir. Bunların dışındaki algoritmalar ile ilgili bilgiler kaynaklarda mevcuttur. Yükseltici DA/DA çevirici yardımıyla 12/24V seviyesindeki akü gerilimi FDAM hız kontrolü için 35-50 V’luk seviyeye ayarlanmaktadır. Yükseltici çevirici kontrolünde tepe akım kontrol yöntemi kullanılmıştır. Kompresörde tahrik elemanı olarak kullanılan FDAM 6 kademeli trapezoidal sürüş yöntemi ile sürülmüştür. Sensörsüz konum algılama yöntemi kullanılarak FDAM kontrolü gerçekleştirilmiş ve 2200 – 3600 min-1 hız aralığında dolabın soğutma ihtiyacına göre hız değeri regüle edilmiştir.

In this thesis work, the electronic control system design and design method of a refrigerator that is supplied from PV panel have been studied. The designed system includes PV panel, accumulator and a compressor with a high efficiency BLDC (brushless DC) motor. For the system, motor has an input power of 100 W and the controlled speed range is 2200 – 3600 min-1. Open-circuit voltage range for the PV panel is 15 – 50V and design is suitable for both 12V and 24V accumulators. For such an application, it is necessary to use PV panel at the voltage and current that gives maximum power. Using MPPT algorithms, energy supplied from PV panel is transferred to the accumulator with maximum energy efficiency. Having high efficiency and being suitable for using in variable speed applications, BLDC motor was preferred at this study. A compressor is a part of the cooling system (compressor, evaporator, condenser etc.) and it is driven with the BLDC motor. This cooling system ensures the inside temperature of the refrigerator cabin as low as desired. A boost DC/DC converter that is supplied from the accumulator is used for adjusting the DC bus voltage at the input of 3-phase motor inverter. The speed of the BLDC motor is controlled by adjusting the DC bus voltage. On the other hand, three phase motor inverter is used for energizing the motor phases at the right instances according to the rotor position information coming from sensorless position detection circuit. As a result, BLDC motor is controlled with six-step trapezoidal control method. Below are the criteria for the MPPT charger circuit design: • Flexibility in using(being compatible with various kind of accumulators and PV panels), • Successful MPPT operation during the whole day and all climate conditions, • Low electromagnetic radiation and low electrical noise, • Cost-effectiveness (single switch topologies were considered) Input impedance of buck-boost type DC/DC converters can be adjusted in range [0,∞) in continuous conduction mode. Consequently, regardless of the ratio between input voltage and output voltage of the converter, a successful MPPT operation can be done and converter can run the PV panel from open-circuit point to short-circuit point. Therefore, buck-boost type DC/DC converter topologies were considered for MPPT charger circuit. Classical buck-boost converter and Zeta converter has a controlled switch at the input side. Thus these converters have high input current ripple, harmonic and differential mode electromagnetic noise. On the other hand, Ćuk and SEPIC topologies are better in harmonic and noise performance because of the input inductor. In battery supplied systems, peak value of the battery charge current is crucial because high ripple current affects battery life and parasitic losses. Because of the output inductor, ripple of the battery charging current for Ćuk converter is low, whereas SEPIC converter has a discontinuous and high ripple output current because of the diode at the output. As a result, Ćuk converter is the best alternative among other single switch topologies for MPPT charger circuit. With the Ćuk MPPT charger an efficiency of 92,04% has been obtained. MPPT algorithms are used for obtaining maximum power from the panel that is available. At this thesis work, open circuit voltage method, perturb&observe method and incremental conductance method have been studied. There are various other MPPT algorithms in the literature. Open circuit (OC) voltage method adjusts panel voltage to a k multiple of open circuit voltage. But this method is satisfactory only certain conditions because maximum power point voltage depends on irradiance and ambient temperature. On the other hand, OC voltage method is a useful method to approach maximum power point rapidly after power-on. Perturb and observe (PO) algorithm is the method that the PV panel voltage or current is increased and decreased, the PV power is calculated at each time and the direction of change is reversed when the PV power decreases. As a result, PO algorithm is an algorithm of trial and error that the PV power works and oscillates around maximum power point. According to the incremental conductance algorithm maximum power point for the PV panel is located when the instantaneous conductance is equal to the negative value of incremental conductance. When instantaneous conductance is greater than negative of the incremental conductance, this means operating point is at the left of the MPPT point. Then, algorithm will decrease PV voltage. When instantaneous conductance is smaller than negative of the incremental conductance, this means operating point is at the right of the MPPT point. Then, algorithm will increase PV voltage. Incremental conductance algorithm is successful in rapidly varying irradiance conditions. However, it is not immune to noise, then noise reduction will be important in the implementation of this algorithm. At this thesis work, two implementations of MPPT algorithms have been done and MPPT efficiencies have been measured. MPPT efficiency is the ratio of generated PV output power to available PV power under test irradiance and temperature conditions. During MPPT tests, Chroma 62150H400S DC power supply, which has solar panel simulation property, has been used. A model of Kyocera KC85T PV panel has been done and two parallel KC85T panel model has been used in MPPT efficiency tests. At the first implementation, OC method is used after first power-on and approach to maximum power point rapidly. Then, PO algorithm starts to work and find the MPPT point. At this first hybrid MPPT method an MPPT efficiency of 99% has been obtained under constant 600W/m2 irradiance. At the second MPPT test, from 600 W/m2 to 300 W/m2 and from 300 W/m2 to 600 W/m2 a rapidly varying irradiance at each 5 second has been implemented. At this test, an MPPT efficiency of 96,2% has been obtained. At the second implementation, OC method is used after first power-on and approach to maximum power point rapidly. Then, IC algorithm starts to work and find the MPPT point. At this hybrid MPPT method an MPPT efficiency of 98,5% has been obtained under constant 600W/m2 irradiance. At the second MPPT test, from 600 W/m2 to 300 W/m2 and from 300 W/m2 to 600 W/m2 a rapidly varying irradiance at each 5 sec has been implemented. At this test, an MPPT efficiency of 96.6% has been obtained. A boost DC/DC converter is used for boosting 12/24V accumulator voltage to 35-50V. Peak current mode control has been used for the control of boost converter. An efficiency of 91.41% has been obtained for this boost converter with 100W maximum output power. BLDC motors have generally three phase winding in the stator and they have permanent magnet in the rotor. Back-EMF voltages induced in phase windings are trapezoidal for BLDC motors. Brushed DC motors need frequent maintenance due to the arcs caused by brush-commutator structure and mechanical frictions. On the other hand, for BLDC motors commutation action is done electronically instead of mechanical brush-commutator action. Thus, BLDC motors do not require maintenance and do not have risk of arcs. Moreover, compressor contains oil for the protection of mechanical parts of the compressor; arcs that result from mechanical friction of brushes can ignite or cause fire. Therefore, BLDC motor was preferred for this thesis work. When two of the three phases of BLDC motors are energized consecutively, this motor model does not have a difference with simple brushed DC motor. To obtain maximum torque from BLDC motor and to drive efficiently each phase should be energized at the 120 electrical degrees part where the EMF is maximum. In a BLDC inverter there are six semiconductor switches that enable electronic commutation. In six step trapezoidal drive method there are six combinations for two of three motor phase winding energizing. At every 60 degrees two of three phases are energized and there will be a sequence of energizing according to the rotor position. If this action is done in the right sequence, motor runs at desired speed and torque. Speed control of the motor is done via adjusting the input DC bus voltage of the three phase inverter with a boost DC/DC converter, thus there is no need to a high frequency PWM operation in motor inverter. As a result, at the motor inverter, switching frequency of semiconductor switches is equal to the motor electrical commutation frequency. Therefore, switching losses of motor inverter transistors and parallel diode reverse recovery losses are very low. There is a relationship between motor EMF voltage and rotor’s mechanical position. Rotor position information is critical for six step trapezoidal drive. Thus, there are sensored and sensorless position detection methods for BLDC motors. There are various position detection sensors such as Hall effect sensors, optical sensors etc. that generate three sensor signals at the same phase and frequency of back EMF voltage. According to information coming from these sensors, motor inverter switches energize the motor windings. In sensorless position detection methods, by sensing the back EMF voltage on motor terminals, it is possible to detect rotor position without sensors. Generally, there are two kinds of sensorless position detection methods. At the first method, zero crossing points of the back EMF voltages are detected and delayed digitally. As the commutation point has a phase difference of 30 degrees with respect to zero crossing point, commutation signals can be determined with digital delay. At the second type of sensorless methods, phase voltages are filtered first and delayed after filtering. Delaying and/or filtering is done in such a manner that the result exactly gives the commutation point when they are compared to each other or compared with the neutral point. The sensorless position detection method used in this thesis work is belonging to the second group. In this method, delay time is adjusted according to the rotating speed and motor phase voltages are compared to each other. As a result, method gives three position signals that have the same electrical frequency with the back EMF voltage. Square wave position detection signals have a phase difference of 120 degrees to each other and they have 180 degrees on time. Magnitude of back EMF voltage has a linear relationship with the motor speed. Thus, at low speeds motor back EMF voltages are small and not enough to give correct commutation instant. Sensorless motor control methods are not able to detect motor position at low speeds. Then, sensorless position detection algorithms work as follows: firstly, rotor is aligned to a known position. Secondly, during free run interval, motor phases are energized according to six step trapezoidal drive for predetermined time intervals in an open loop manner. After this open loop interval, closed loop sensorless position detection algorithm starts to work and all commutation is done according to the position signals coming from sensorless position detection circuit. As a result, BLDC motor control has three steps: align, free run, closed loop operation. With the stated sensorless position detection and motor control method, motor speed control and drive has been done and motor speed has been regulated to desired reference between 2200-3600 min-1 speed interval.