ASEAN Virtual Instrumentation Applications Contest 2006
Power Quality Monitoring System utilizing Periodogram and Spectrogram Analysis Techniques 2006 ASEAN Virtual Instrumentation Applications Contest – Academic
Author(s): Abdul Rahim Abdullah, Lecturer, Kolej Universiti Teknikal Kebangsaan Malaysia (KUTKM), Malaysia Norhashimah Mohd Saad, Lecturer, Kolej Universiti Teknikal Kebangsaan Malaysia (KUTKM), Malaysia Product(s): National Instruments (NI) USB-6009 Data Acquisition (DAQ) Card National Instruments (NI) Measurement Studio Development Systems Professional Edition The Challenge: Power Quality is very important in power-supply system. Any slight disturbance in the power-line can cause losses up to millions in operating factories and businesses. Thus, monitoring and analysis of power-line waveforms are very important to overcome the power quality problems. The Solutions: A system has been developed which allows real-time measurement, monitoring and analyzing of power quality waveforms. It integrates the signal acquisition system for real-time monitoring of power quality waveforms and then employs signal processing techniques that can identify any power quality problems accurately for corrective and preventive actions. Abstract: Any disturbance in the power-line can cause disruption in manufacturing process or services provided. Under worst case conditions results in equipment failure and subsequent increase in cost of operation. Monitoring and analysis of power-line waveforms are essential to provide assessment of the power quality. A system capable of monitoring and analyzing the power-line waveforms has been developed to overcome the problems. Signal analysis techniques such as periodogram and spectrogram are employed to analyze power-line voltage and current variations. The heart of the system is the National Instruments (NI) USB-6009 Data Acquisition (DAQ) Card for automatically acquiring the power-line voltage and current waveforms and a graphical user interface (GUI) was developed on a personal computer using the National Instruments (NI) Measurement Studio Development Systems Professional Edition to monitor, analyze and store the power quality data efficiently. Introduction: Power Quality is the availability of pure sinusoidal voltage and current waveforms at 50 Hz (frequency power-line in Malaysia) without any sags or spikes at the incoming point of the supply system. Power quality problem occurs as a result of nonstandard voltage, current or frequency deviation that results in a failure or a disoperation of end-use equipment [1]. For the past 20 years, awareness of the power-quality problem has greatly improved, with a report from Business Week (1991) stating that spikes, sags and outages cost the US nation Singapore 1800-226 5886 | Malaysia 1800-887710 | Thailand 662-278 6777 |
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ASEAN Virtual Instrumentation Applications Contest 2006 US$26 billion in downtime. Contributing to this cost is lost time, lost production, lost sales, delivery delays and damaged production equipment [2]. Another problem in power-line system is power factor. Power factor is the measure of the useful power that can be utilized from its supply. However, the actual value varies depending on the loading conditions. For Malaysian industry and business, power factor must be set greater than 0.85. Penalty is levied if value falls below the limit. Therefore, there is a need to understand and improve the power quality problems. Various researchers have implemented real-time power quality measurements [3]-[5]. Spectrum estimation is included in [4] while the wavelet transform employed in [5] for analysis and compression. However, the previous methods available for power quality monitoring have limitations and cannot detect all the variations in power quality problems. The proposed system consists of real-time monitoring of power measurements such as power factor, real power, apparent power, frequency, voltage (rms) and current (rms). In addition, signal analysis techniques such the periodogram power spectrum and spectrogram time-frequency analysis are performed as a combination to analysis and identify power quality problems such transients and harmonics. Figure 1 shows the block diagram of power quality monitoring system.
Real-time Data Acquisition: The hardware development part for the power quality monitoring system is build around the NI USB-6009 DAQ card. The DAQ card allows real-time data acquisition of power-line waveforms. Voltage is measured between 0 to 500 volts. A step-down transformer and voltage divider circuit is connected to the power-line to step-down the input voltage of less then 12 volts. In addition, the step-down transformer acts as isolation between power-line and DAQ card. To allow discrete-time processing of power-line waveforms, sampling is performed continuously at the analog input at sampling frequency of 8 kHz. In this system, current is measured between 0 to 100 amps. Clamp adapter is connected to analog input of the DAQ card. Similar to the voltage waveform, DAQ card will sample the signal and send the data into the computer for processing, monitoring and data-logging purposes. Singapore 1800-226 5886 | Malaysia 1800-887710 | Thailand 662-278 6777 |
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ASEAN Virtual Instrumentation Applications Contest 2006 Software Development in Computer: Power Quality data from DAQ card is transferred to computer for process, display and storage. The data is presented in a graphical-based interface in a form of meter indicators, graphs and texts. The software was developed with NI Measurement Studio Development Systems Professional Edition. The use of NI Measurement Studio gives an advantage for user to build stand-alone software, which the software can be installed anywhere in Windows operating system without supported by other software. Thus, user can monitor the system in a network-based environment or by one control-unit. The basic measurements performed are voltage (rms), current (rms), frequency, power factor, real power and apparent power. Besides that, the periodogram and spectrogram of voltage and current waveforms are also computed. Figure 2 shows the main GUI display that controls and represents the measurement results on the computer.
The first row of the GUI software shows readings for voltage (rms), current (rms), frequency, power factor, real power and apparent power in meter indicators. Simultaneously, real-time voltage and current waveforms are represented on the next row. The periodogram power spectrum for voltage and current waveforms are calculated and shows on the third row of the GUI. It represents the distribution of the signal in frequency domain. Periodogram power spectrum is calculated based on the Fast Fourier Transform (FFT) of the discrete-time waveform. In addition to frequency measurement, the periodogram can detect the presence of harmonics and transients in the power-line waveforms. Data logger is built in the system for automatically store the power quality data into the computer. User can set specific time or duration to log the data into the computer. The data Singapore 1800-226 5886 | Malaysia 1800-887710 | Thailand 662-278 6777 |
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ASEAN Virtual Instrumentation Applications Contest 2006 can be retrieved from a text file (.txt) for further evaluation. Besides periodogram, the spectrogram time-frequency monitoring can be chosen from the software. It is a contour-plot, or 3-dimensional plot of power quality waveforms that representing the signal jointly as a time-frequency distribution. It overcomes the limitation of periodogram that estimates only the frequency contain of signals. The spectrogram gives characterization of power quality problems in term of signal power, time and frequency that allows user to identify any power quality problems. In addition to the system, besides power quality parameters measurement (shown on the first row of the main GUI software), user can also evaluate real-time long-term monitoring of power quality parameters: voltage, current, frequency, power factor, real power and apparent power by choosing the window menu from the main software. Conclusion: The power quality monitoring system is built around the NI USB-6009 DAQ Card and NI Measurement Studio Development Systems Professional Edition. Besides basic power measurements such as voltage, current, frequency and power factor, the system can also perform analysis such as power spectrum and spectrogram. The results are available in real time through a built-in software in computer. Power Quality monitoring shows that the spectrogram gives better characterization of power quality problems compared to power spectrum. The system allows users to identify power quality problems and take the necessary corrective or preventive action. References: [1]. Dugan R.C., Electric Power Quality, 2nd ed. Scottsdale AZ: Stars in a Circle Publication, 1996. [2]. [Khan, A.K., “Monitoring Power for the Future”, PowerEngineering Journal, Vol 15, Issue 2, April 2001, pp 81-85. [3]. Batista, J. Alfonso, J.L. et al, “Low Cost Power Quality Monitor Based on PC”, IEEE Int. Symp. on Ind. Electronics, 2003.(ISIE '03), 9-11 June 2003, pp 323 – 328. [4]. [Lakshmikanth, A. Morcos, M., “A Power Quality Monitoring System: A Case Study in DSP-based Solutions for Power Electronics”, IEEE Trans. on Instrumentation and Measurement, Vol 50, Iss 3, June 2001, pp 724-731. [5]. Ribeiro, M.V., Romano, J.M, et al., “An improved method for signal processing and compression in power quality evaluation”, IEEE Trans. On Power Delivery, Vol 19, Issue 2, April 2004, pp 464-471. For more information, contact: Abdul Rahim Abdullah Lecturer, Faculty of Electrical Engineering, Kolej Universiti Teknikal Kebangsaan Malaysia (KUTKM) Ayer Keroh,74540 Melaka, Malaysia Tel: +6019-6671979 Fax: +606-5552222 Email:
[email protected] Singapore 1800-226 5886 | Malaysia 1800-887710 | Thailand 662-278 6777 |
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