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  • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 4, April (2014), pp. 104-110 © IAEME 104 HYBRID AC-DC MICROGRID WITH INTELLIGENT LOAD FLOW CONTROL Rajan P. Thomas1 , Akhil S2 , Jithin K K2 , Sanjay S G2 , George Joseph2 1 (Professor, Dept. of EEE, Mar Athanasius College of Engineering, Kothamangalam, India) 2 (UG Student, Dept. of EEE, Mar Athanasius College of Engineering, Kothamangalam, India) ABSTRACT A microgrid consists of cluster of loads and distributed generators that operate as a single controllable system. The microgrid concept introduces the reduction of multiple reverse conversions in an individual AC or DC grid and also facilitates connections to variable renewable AC and DC sources and loads to power systems. This paper proposes a hybrid AC/DC micro-grid which consists of an AC grid and a DC grid which operates in the stand-alone mode. The conventional AC loads are connected to the AC grid whereas photovoltaic arrays with boost converter and DC loads are tied to the DC grid. Battery is connected to the DC bus through a charging/discharging converter to maintain energy requirement when the system operates in the isolated mode. An intelligent load flow control system is incorporated to complement the main system. It enables the user to prioritize his devices as critical/non-critical, and divert power to critical devices when needed. Keywords: Hybrid Microgrid, Modified Sine Wave Inverter, MPPT, P&O Algorithm, Load Monitoring, Soc, Webserver. 1. INTRODUCTION Our electric power system was designed to move central station alternating current (AC) power, via high-voltage transmission lines and lower voltage distribution lines, to households and businesses that used the power in incandescent lights, AC motors, and other AC equipment [1]. Today’s consumer equipment and tomorrow’s distributed renewable generation requires us to rethink this model. Electronic devices (such as computers, fluorescent lights, variable speed drives, and many other household and business appliances and equipment) need direct current (DC) input. However, all of these DC devices require conversion of the building’s AC power into DC for use, and that conversion typically uses inefficient rectifiers. Moreover, distributed renewable generation (such as rooftop solar) produces DC power but must be converted to AC to tie into the building’s INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET) ISSN 0976 – 6545(Print) ISSN 0976 – 6553(Online) Volume 5, Issue 4, April (2014), pp. 104-110 © IAEME: www.iaeme.com/ijeet.asp Journal Impact Factor (2014): 6.8310 (Calculated by GISI) www.jifactor.com IJEET © I A E M E
  • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 4, April (2014), pp. 104-110 © IAEME 105 electric system, only later to be re-converted to DC for many end uses. These AC-DC conversions (or DC-AC-DC in the case of rooftop solar) result in substantial energy losses. One possible solution is a DC microgrid, which is a DC grid within a building (or serving several buildings) that minimizes or eliminates entirely these conversion losses. In the DC microgrid system, AC power converts to DC when entering the DC grid using a high-efficiency rectifier, which then distributes the power directly to DC equipment served by the DC grid. On average, this system reduces AC to DC conversion losses from an average loss of about 32% down to 10%. In addition, rooftop photovoltaic (PV) and other distributed DC generation can be fed directly to DC equipment, via the DC microgrid, without the double conversion loss (DC to AC to DC), which would be required if the DC generation output was fed into an AC system. But implementing a DC system alone in today’s environment is not practical. All the consumer devices available today are designed to operate with AC supply. Introducing a DC grid all of a sudden will result in wastage of resources. The coexistence of AC and DC subgrids in a hybrid microgrid is likely given that modern distributed sources can either be AC or DC [2]. The block schematic representation of the proposed system is given in Fig.1. Fig.1: Block schematic of the proposed AC-DC hybrid system. 2. MAXIMUM POWER POINT TRACKING Maximum power point tracking is a technique that grid connected inverters, solar battery chargers and similar devices use to get the maximum possible power from one or more photovoltaic devices, typically solar panels. Solar cells have a complex relationship between solar irradiation, temperature and total resistance that produces a non-linear output efficiency which can be analyzed based on the I-V curve. It is the purpose of the MPPT system to sample the output of the cells and apply the proper resistance (load) to obtain maximum power for any given environmental conditions. MPPT devices are typically integrated into an electric power converter system that provides voltage or current conversion, filtering, and regulation for driving various loads, including power grids, batteries, or motors.
  • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 4, April (2014), pp. 104-110 © IAEME 106 2.1 MPPT Algorithm In the proposed system, the perturb & observe algorithm is implemented in the MPPT charge controller. In this we use only one sensor, that is the voltage sensor, to sense the PV array voltage and so the cost of implementation is less and hence easy to implement. The time complexity of this algorithm is very less but on reaching very close to the MPP it doesn’t stop at the MPP and keeps on perturbing on both the directions. The flowchart for the P&O algorithm is shown in Fig.2. Fig. 2: Flowchart for perturb & observe algorithm The perturb & observe algorithm states that when the operating voltage of the PV panel is perturbed by a small increment, if the resulting change in power ∆P is positive, then we are going in the direction of MPP and we keep on perturbing in the same direction. If ∆P is negative, we are going away from the direction of MPP and the sign of perturbation supplied has to be changed.
  • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 4, April (2014), pp. 104-110 © IAEME 107 Fig.3: Solar panel characteristics showing MPP and operating points A and B [3] Fig.3 shows the plot of module output power versus module voltage for a solar panel at a given irradiation. The point marked as MPP is the Maximum Power Point, the theoretical maximum output obtainable from the PV panel. Consider A and B as two operating points. As shown in the Fig.3, the point A is on the left hand side of the MPP. Therefore, we can move towards the MPP by providing a positive perturbation to the voltage. On the other hand, point B is on the right hand side of the MPP. When we give a positive perturbation, the value of ∆P becomes negative, thus it is imperative to change the direction of perturbation to achieve MPP. 2.2 The MPPT Circuit TheMPPT circuit is basically a buck converter. The switching element (here, MOSFET) is driven by a driving circuit which is controlled by microcontroller. The circuit is driven at a high frequency of 16kHz generated from the microcontroller. The advantage of using high frequency is that it reduces the size of the inductor and capacitor. This also reduce cost of these circuit elements. In simple terms, the circuit works according to the maximum power transfer theorem. The output resistance or the load resistance is made equal to the characteristic resistance of the solar panel thus transferring maximum power from the panel to the load. This is achieved by varying the duty cycle with which the MOSFET is driven. This is a continuous process so closed loop control is used. So whatever be the climatic conditions or nature of load, the operating point of solar panel is invariably fixed at the maximum power point using this simple circuit. 2.3 Experimental Results A comparison of solar panel output power with and without the MPPT interface is given in Table 1.The test was conducted on an 80W multi-crystalline solar panel connected to a 12V, 40Ah lead-acid battery. The variation in output current is negligible while the voltage varies considerably to track the maximum power point of the panel. Table 1: Power output of 80W solar panel with and without MPPT interface Voltage (V) Current (A) Power (W) without MPPT 12.58 4.30 54.09 with MPPT 17.10 4.20 71.82 Clearly, there is a considerable increase in output power level due to the integration of MPPT stage. The MPPT circuit used for this study is given in Fig. 4. Two hall effect sensors are used to measure the input and output currents of the MPPT circuit. Similarly, voltage divider circuits
  • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 4, April (2014), pp. 104-110 © IAEME 108 measure the voltages. Analog sensor values gets converted to digital values in the inbuilt ADC of the microcontroller. An LCD display is provided to display the instantaneous sensor output values. Fig. 4: MPPT circuit implementing the P&O algorithm 3. POWER ELECTRONIC INTERFACES In grid-tied mode of operation, the main converter is to provide stable DC bus voltage and required reactive power and to exchange power between the AC and DC buses [4]. Several classifications of converter topologies can be done with respect to the number of power processing stages, location of power-decoupling capacitors, use of transformers, and types of grid interface [5]. The main types of converters employed in the project are the DC-DC buck converter, the DC-DC boost converter and the DC-AC inverter. The DC-DC buck converter is used to obtain the optimum charging level of the lead acid battery, while the DC- DC boost converter and the DC- AC inverter are put to use in the output side to obtain the desired output AC. 3.1 Transformerless DC-DC Boost Converter The DC-DC converter can be used to power appliances equipped with switched supply, which only is rectifying and smoothing the mains voltage. Not applicable to power inductive and capacitive loads, i.e. products with conventional transformer, induction motors, etc. It cannot be used for switched power supplies that have conventional auxiliary transformer or that are obtaining aux voltage resistive or capacitive way from the voltage before the rectifier. Most switching power supply operate at 230V DC too, so they can be used along with resistive loads. For loads with higher ripple rectified voltage and without regulation (e.g. energy-saving lamps - CFLs) should be set slightly less than 325V, so that the voltage corresponds to an effective value of the rippling voltage (let’s say 280V). The circuit works as a flyback converter. Supply operates in discontinuous current mode (DCM), which reduces the reverse recovery loss of diodes. Ultrafast diodes are used to rectify the secondary voltage. The current is sensed using a current transformer, because of direct sensing would cause excessive loss. Spikes are dampened by the primary "lossless" snubber with advantage over conventional RCD snubber - much lower power loss (there's no dissipation resistor) and also reduction of dv/dt for MOSFET.The working frequency of the inverter is about 40 kHz. Efficiency is more than 90% at full power.
  • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 4, April (2014), pp. 104-110 © IAEME 109 3.2 Modified Sine wave Inverter With the increasing popularity of alternate power sources, such as solar and wind, the need for static inverters to convert DC energy stored in batteries to conventional AC form has increased substantially. Most use the same basic concept: a DC source of relatively low voltage and reasonably good stability is converted by a high frequency oscillator and step up transformer to a DC voltage with magnitude corresponding to the peak of the desired ACvoltage. A power stage at the output then generates an AC voltage from the higher-voltage DC. There are basically two kinds of DC-AC inverters on the market today. One category is the “pure sine-wave” inverter, which produces sine waves with total harmonic distortion (THD) in the range of 3% (-30 dB) [6]. These are typically used when there is a need for clean, near-sine-wave outputs for medical, instrument and other critical applications. Early techniques for designing these true sine wave inverters incorporated significant linear technology, reducing their efficiency and contributing to their higher cost. The second category consists of relatively inexpensive units, producing modified sine-wave outputs, which could logically be called “modified square waves” instead. They are basically square waves with some dead spots between positive and negative half- cycles as in Fig.4 [6]. Switching techniques rather than linear circuits are used in the power stage, because switching techniques are more efficient and thus less expensive. These inverters require no high-frequency switching, as the switching takes place at line frequency. Their costs are generally in the range of $0.1 per watt. The typical modified sine-wave inverter has a waveform as shown in Fig.5. It is evident that if the waveform is to be considered a sine wave or a modified sine wave, it is a sine wave with significant distortion. Fig.5: Output waveform of the designed modified sine wave inverter in CRO screen Compared with the grid-connected PV inverters with galvanic isolation, the transformerless PV inverters (e.g. full-bridge, NPC and HERIC) have the advantages of lower cost, higher efficiency, smaller size and lower weight [7]. It is suitable for battery powering of mains appliances. It is a switching converter, which contains no bulky, heavy and expensive iron transformer. The advantage of small size and weight, precise output voltage stabilization and among other things, very little quiescent power consumption, which cannot be achieved using the classical transformer [8]. The disadvantage is that the design is complicated. It can be used for most appliances except those capacitive (e.g. with high parallel capacitor) and those that require pure sine for another reason. Not suitable for capacitive limited mini fluorescent night lights and LED light bulbs. For the conventional fluorescent (or discharge) lights is necessary to disconnect the capacitor.
  • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 4, April (2014), pp. 104-110 © IAEME 110 4. INTELLIGENT LOAD FLOW CONTROL WITH WEB BASED INTERFACE The communication layer of an electric power distribution system is vital in converting it into a “smart” one. The hybrid AC-DC microgrid system proposed in this paper incorporates an intelligent web based load monitoring and control system. The webpage is hosted in a microprocessor whose general purpose input output pins are configured to measure the instantaneous power consumption of the consumer. Thus the customer can access this data at any time by logging into the server. Provisions are provided to turn the individual loads ON/OFF remotely as per the power availability. The heart and brain of the load monitoring part is a system on a chip (SoC) named BCM2835 that you can find in many mobile phones. It’s cheap, it’s powerful, and it does not consume a lot of power. In contrast to a typical PC architecture, a SoC integrates a processor (CPU), a graphics processing unit (GPU), and some memory into a single unit. The BCM2835 contains an ARM1176JZ-F processor running at 700MHz, 256MB of RAM, and a GPU named VideoCore IV. A Python web framework called Flask is used to turn the SoC into a dynamic web server. 5. CONCLUSION The proposed AC-DC hybrid system is found to reduce multiple AC-DC conversions and hence result in an increase of overall efficiency. It’s highly adaptable to todays distributed generation platforms and makes the integration of rooftop power production a whole lot easier. The main disadvantage of the system is its inability to exchange power with the utility grid. For a commercially viable system, provisions must be included to synchronize the inverter output to the utility grid. With this, care must be taken to limit the harmonics and frequency variations inside the limits specified by the utility. 6. REFERENCES [1] Rahmanov, N. R., et al. "Combined AC-DC Microgrids: Case StudyNetwork Development and Simulation." International Journal on Technical and Physical Problems of Engineering (IJTPE) 12 (2012): 157-161. [2] Loh, Poh Chiang, Ding Li, and Frede Blaabjerg. "Autonomous control of interlinking converters in hybrid AC-DC microgrids with energy storages."Energy Conversion Congress and Exposition (ECCE), 2011 IEEE. IEEE, 2011. [3] Oi, Akihiro. “Design and simulation of photovoltaic water pumping system”. (Diss. California Polytechnic State University, 2005). [4] Liu, Xiong, Peng Wang, and Poh Chiang Loh. "A hybrid AC/DC microgrid and its coordination control." Smart Grid, IEEE Transactions on 2.2 (2011) [5] Carrasco, Juan Manuel, et al. "Power-electronic systems for the grid integration of renewable energy sources: A survey." Industrial Electronics, IEEE Transactions on 53.4 (2006): 1002-1016. [6] Hahn, James H. "Modified Sine-Wave Inverter Enhanced." Power Electronics Technology 32.8 (2006) [7] Koutroulis, Eftichios, and Frede Blaabjerg. "Methodology for the optimal design of transformerless grid-connected PV inverters." Power Electronics, IET 5.8 (2012): 1491-1499. [8] K. N. Reddy and V. Agrawal, “Utility-interactive hybrid distributed generation scheme with compensation feature,” IEEE Trans. Energy Convers., vol. 22, No. 3, Sep. 2007. [9] Anto Joseph, Nagarajan and Antony Mary, “A Multi Converter Based Pure Solar Energy System with High Efficiency MPPT Controller”, International Journal of Electrical Engineering & Technology (IJEET), Volume 4, Issue 4, 2013, pp. 205 - 212, ISSN Print : 0976-6545, ISSN Online: 0976-6553.