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Transactions on Engineering and Sciences Vol.2, Issue 8, August 2014 ISSN: 2347-1964 (Online) 2347-1875 (Print) Harmonic Analysis of 230 V AC Power Supply Using LPC2138 Microcontroller Jeena Joy1 AmalrajP.M.2 Aswin Raghunath3 Nidheesh M.N.4 Vinu Joseph5 1Asst.Prof, 2,3,4,5UG Dept. of EEE, Mar Athanasius College of Engineering, Kothamangalam. Student, Dept. of EEE, Mar Athanasius College of Engineering, Kothamangalam. Abstract: The presence of harmonics in power system is a major concern to power engineers for many years. With the increasing usage of nonlinear loads in power systems, the harmonic pollution becomes more serious. One of the widely used computation algorithm for harmonic analysis is Fast Fourier Transform (FFT). In this project, a harmonic analyzer using FFT is being implemented on ARM7 core processor LPC2138 [1]. The supply voltage of 230V, 50 Hz is stepped down to 6V using a voltage divider in order to match the power rating of the processor input. The output from the processor is displayed on a graphic LCD, which is interfaced to the output ports of the processor. Keywords: Harmonic Analysis, FFT I.INTRODUCTION In recent years, the electrical harmonic pollution is getting more and more serious with the far-ranging use of nonlinear components in electric and electronic devices. The presence of harmonics in power system is a major concern to power engineers for many years. Harmonic Pollution increases industrial plant downtimes and power losses. Harmonic of an electrical signal is the content of the signal whose frequency is an integer multiple of the fundamental system frequency. The cause of harmonic in a power system is from highly nonlinear devices. In power system, harmonic analysis is use to determine the impact of harmonic producing load on a power system.[1] This technique is widely used for system planning, equipment design, troubleshooting, etc. The effect of harmonics on power system can be in the form of power efficiency reduction, overheating in wire, ageing of electrical insulation, etc [2]. Several algorithms have been proposed on harmonic analysis and Fast Fourier transform (FFT) is the most widely used computation algorithm. FFT is an efficient algorithm used to compute Discrete Fourier transform (DFT). DFT uses a finite set of discrete-time sample of an analogue signal and produces a finite set of discrete-frequency magnitude spectrum values [3]. This paper focuses on implementation of 240V, 50Hz power supply harmonic analysis on LPC2138 using FFT technique. II. HARMONIC CALCULATION USING FFT ALGORITHM Basically, the computational problem for the DFT is to compute the sequence {X (k)} of N complexvalued numbers given another sequence of data {x(n)} of length N, according to the formula = , 0 ≤ ≤ − 1 = ⁄ To compute all N values of the DFT requires N 2 complex multiplications and N 2-N complex additions. Direct computation of the DFT is basically inefficient primarily because it does not exploit the symmetry and periodicity properties of the phase factor WN. In particular, these two properties are: ⁄ !"!#"! ∶ % = − &!'#(')' "!#"! ∶ % = FFT algorithms, exploit these two basic properties of the phase factor and is another method for calculating the DFT [4]. While it produces the same result as the other approaches, it is incredibly more efficient, often reducing the computation time by hundreds. The computation of the N-point DFT via the decimation-in-time FFT requires only (N/2)log2N complex multiplications and Nlog2N complex additions.Let us consider the computation of the N = 2v point DFT by the divide-and conquer approach. Wesplit the N-point data sequence into two N/2-point data sequences f1(n) and f2(n), corresponding to the even-numbered and odd-numbered samples of x(n), respectively, that is, * = 2 * = 2 + 1, = 0,1,2, … . , /2 − 1 Thus f1(n) and f2(n) are obtained by decimating x(n) by a factor of 2, and hence the resulting FFT algorithm is called a decimation-in-time algorithm [5].The computation of N = 8 point DFT is performed in 38 Techscripts Transactions on Engineering and Sciences Vol.2, Issue 8, August 2014 ISSN: 2347-1964 (Online) 2347-1875 (Print) three stages, beginning with the computations of four two-point DFTs, then two four-point DFTs, and finally, one eight-point DFT. The combination for the smaller DFTs to form the larger DFT is illustrated in Figure 1. Figure 1: Three stages in the computation of an N = 8-point DFT Figure 2: Conventional butterfly diagram Similar to this case in this project 10 stage butterfly analysis is required for a total input of 1024 samples. A conventional 3 stage butterfly diagram is shown as in figure 2 [6]. 10 stage butterfly analysis is chosen according to the sampling theory wherein ‘sampling frequency must be twice the maximum frequency component’. Maximum frequency that is displayed is 500Hz for digital implementation 512Hz is used [7][8][9][10]. III.LPC2138 MICROCONTROLLER The LPC2138 microcontroller is based on a 16/32 bit ARM7TDMI-S™ CPU with real-time emulation and embedded trace support, that combines the microcontroller with 32 kB, 64 kB, 128 kB, 256 kB and 512 kB of embedded high speed Flash memory [11]. A 128-bit wide memory interface and a unique accelerator architecture enable 32-bit code execution at maximum clock rate. For critical code size applications, the alternative 16-bit Thumb® Mode reduces code by more than 30 % with minimal performance penalty. Due to their tiny size and low power consumption, these microcontrollers are ideal for applications where miniaturization is a key requirement, such as access control and point-of-sale. With a wide range of serial communications interfaces and on-chip SRAM options of 8/16/32 kB, they are very well suited for communication gateways and protocol converters, soft modems, voice recognition and low end imaging, providing both large buffer size and high processing power. Various 32-bit timers, single or dual 10-bit 8 channel ADC(s), 10-bit DAC, PWM channels and 47 GPIO lines with up to nine edge or level sensitive external interrupt pins make these microcontrollers particularly suitable for industrial control and medical systems [12]. IV.CIRCUIT CONFIGURATION The power supply section is to supply Vcc for the controller and graphic LCD. A 230V supply from the mains if fed to a step down transformer, 12-0-12, the signal will get downsized. This is then rectified using a full wave rectifier and smoothened using a capacitor. The output thus obtained is regulated using LM7805 IC to get a 5V DC supply .The LPC2138 has a header of its own to convert the input to 3.3V which is its working voltage. In the voltage section of the circuitry provides the input for voltage harmonic analysis. 39 Techscripts Transactions on Engineering and Sciences Vol.2, Issue 8, August 2014 ISSN: 2347-1964 (Online) 2347-1875 (Print) The signal to be analysed is fed to the voltage transformer, 6-0-6. The output of transformer will be 12V peak-to-peak. This has to be converted into 3.3V for matching the requirements of the controller. For safe working, signal is made to 2V peak-to peak using voltage divider. Capacitor is used to prevent the disruptions on AC side by the DC bias given. Signal is then clamped to a reference of 1.7V using a voltage bias and identical clamping resistors. Output so obtained can then be given to the ADC channel of LPC2138 for harmonic analysis. Figure 3: Circuit of power harmonic analyser. Current section aims at providing input for current harmonic analysis. A 20:5A Current Transformer (CT) is used. Wire carrying the input current forms the primary whereas the secondary is a wire of many turns wound around a silicon former. A current which is reduced but proportional to the input current is produced at the output of the CT. The voltage drop by the current across the resistor which is very small is amplified using a differential amplifier. Input to the inverting terminal is the small voltage drop and that to the non-inverting terminal is 1.75V. Microcontroller LPC2138 is the brain of this project. The voltage and current signals to be analyzed are fed to the two channels of ADC which are pin 27 and 28 respectively. Reset operation is achieved by using a reset button connected to the pin 25 of the controller. Channels of ADC can be switched by using the ISP (pin 15). When pin 15 is grounded graphic LCD displays current harmonics and voltage harmonics otherwise. Graphic LCD is having a resolution of 128*64. The output is displayed on a graphic LCD which is interfaced to the controller through pin 35 to 48 as shown in figure 3. V.RESULT The power harmonic analyzer was used to test the harmonics present in the normal electric supply and in the output of the ups. Results obtained are as shown in the figure 4. Output of the analyzer is being displayed on the graphic LCD. It is a plot between FFT magnitude and frequency. For each of the frequency samples the corresponding FFT magnitude is found out using implemented FFT algorithm. Figure 4: Harmonics in normal supply and in UPS Using the harmonic analyser THD at the college hostel at different time during three days were taken, table 1 shows the reading and a graph is plotted with the variation of THD with time. Figure 5 shows the THD VS Time curve graph. By analysing the graph it can be seen that THD rises during the time interval 7.00 pm to 9.00 pm as the usage of non-linear loads like fan, lamps and computers is higher. 40 Techscripts Transactions on Engineering and Sciences Vol.2, Issue 8, August 2014 ISSN: 2347-1964 (Online) 2347-1875 (Print) Table.1 THD during different days DAYS TUESDAY WEDNESDAY THURSDAY 8:00 AM 15 13 14 12:00 PM 13 13 14 4:00 PM 16 15 14 8:00 PM 25 27 28 Figure 5: THD VS Time Curve VI.CONCLUSION With the proliferation of non-linear loads, the issues of power harmonics are more apparent than ever. Controlling and monitoring industrial system designs and their effects on utility distribution systems are a potential problem for the industrial consumer, who is responsible for complying with the IEEE 519-1992 recommended practices and procedures. Industrial facilities should include a system evaluation, including a harmonic distortion analysis, while planning facility construction or expansion. Vendors of non-linear loads, such as variable frequency drives, can provide services and recommend equipment that will reduce harmonics in order to comply with the revised IEEE 519-1992 guidelines. Hence a power harmonic analyzer is presented using LPC2138 ARM7 core processor. This harmonic analyzer is capable of analyzing harmonics in a single phase supply and represents the harmonics as a frequency spectrum. The proposed system provides a harmonic analyzer at much lower cost. 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