Development Of A Fieldbus Instrument For Power Harmonic Analysis

With the wider application of the modern power electronics on the power network, harmonic is getting more and more serious. Various electronic control devices in the industrial field are very sensitive to harmonic, so it is very important to monitor parameters on the power network, especially harmonic. Now, Distributed Control System (DCS) based on the fieldbus technology has been applied to the industrial field widely. Developing a fieldbus instrument, which can monitor the real-time parameters on the power system, becomes more and more necessary.The paper provides a digital fieldbus instrument, which can monitor parameters on the power network, with 16-bit embedded micro-controller MC9S12D64. The paper mainly includes three portions: firstly, discussing the algorithm of power parameters, especially studying the harmonic analysis algorithm based on Fast Fourier Transform (FFT); secondly, explaining the hardware circuit principle and software design in detail; and at last, testing the instrument strictly and analyzing the measurement error.The major work of the paper is summarized as follows:The power parameters measured by the instrument include the Voltage, Current, Power, Energy, Power Factor, etc. The algorithm of these parameters is based on Root Mean Square. Discrete Fourier Transform (DFT) algorithm needs enormous data computation, and causes some quantization errors easily. Decomposition-in-time, radix-2 FFT algorithm is more efficiently in harmonic analysis. Because voltage and current signals are sampled alternately in hardware design, the algorithm can be optimized further. The algorithm by using which two N-point real sequences can be computed by one N-point FFT is presented. The computation can be reduced obviously by this way.Micro-controller is selected according to the FFT algorithm, and the hardware circuit is designed. Signal conditioning circuit converts the input signals from power network into AC signals which range 0～5V (Volt), and transmits the signals to an A/D convertion module formed in the micro-controller. Micro-controller tracks the frequency on power network timely in order to adjust the sampled interval time. ATmega32 communicates with MC9S12D64 by SPI module and can transmit data to the computer monitoring software. Ferroelectric memory can store the setting parameters and data measured by the instrument. Character Liquid Crystal Display (LCD) module can display more power parameters on one screen. The instrument has some output boards so that it can communicate in different fieldbus network, such as CAN bus and PROFIBUS.Program is designed in assembly language to satisfy the accuracy of sampled data. Traditional instrument calibrates the zero-point value and full-scale value by hardware correction circuit. However, this instrument can realize this function by software, which can avoid the impacts of external physical environment. This paper studies the interrupt response machnism of MC9S12D64 and describes a priority scheme that can be used to schedule the interrupts based on the assigned priority. Five-level priority scheme is implemented successfully. The FFT algorithm program is optimized to reduce the computation of micro-controller and to improve the accuracy.This paper makes full use of the function blocks and memory resources of the micro-controller MC9S12D64, and realizes harmonic analysis with FFT algorithm. This instrument can track the power parameters on the power network fully and can transmit data to computer quickly and reliably.