| Dielectric dissipation factor (tanδ) is a key parameter in capacitive power equipment insulation on-line monitoring, which can reflect the insulation station of the capacitive equipments such as Bush and CT. However, it is a challengeable task to measure tanδ precisely because many factors such as harmonic, DC offset, signal frequency fluctuation, random noise, ambient temperature and humid introduce considerable error to the measurement of tanδ. Therefore, tanδ on-line measurement methods and devices have been a hot research task for many scholars in the field of power equipment on-line monitoring. Especially, since the 1980's, many devices of this kind have been installed on substations with the improvement of electronic devices and better understanding of signal processing methods. Base on some surveys, however, a quite few of these devices failed to offer useful data because poor stability and precision. To solve these problems, this thesis conducted an intense study, which includes the following contents. In the view of signal processing, harmonic, DC offset, frequency fluctuation and random noise are the main factors that infect tanδ measurement. An all-side analyse is made on the features of different tanδ measurement methods like zero-crossing, correlation function, sine fitting and harmonic analyse. Based on the good computing ability of the processor in this system, we choose harmonic analyse, which have more advantages over the other methods because it makes a better use of the feature of the signal, as the basic method. Harmonic analyse, however, is more sensitive to signal frequency fluctuation than sine fitting method. In this case, if the sampling rate keeps unchanged, integral numbers of sampling data with equal time interval distributing in the range of a signal period can not be satisfied, and then a considerable error with a order of 10-3 will appear. This phenomenon is called non-synchronization sampling. To solve this critical problem, some existing synchronization methods are analyzed and a new one is presented: (1)find the range of two zero-crossing points with the same direction and decide their precise position by interpolation, and then decide the true signal frequency according t the distance between these two points; (2) using this frequency as the selecting frequency and the number of data between these two zero-crossing points as the numerical integrating length to do the DFS; (3) to further improve the precision, replace the rectangle integration with trapezoidal integration. In this way, the tanδ error introduced by non-synchronization can be reduced to the order of 10-6. The additional processing task introduced by this method is very slight and no additional hardware is needed. A distributed on-line monitoring system is developed. The on-site measurement part is based on Digital Signal Processor (DSP) and the upper machine is the computer in the control room. Each on-site part is responsible for the monitoring of one three-phase equipment. All the on-site parts start measuring simultaneously according to the time of the real time clock in each part and thus the results of different equipments are comparable to each other. Measurement results are stored in and displayed on the on-site part and also transferred to the upper computer via RS485 serial bus for further analyzing and longer storage. The utility of DSP simplified the hardware design and enhanced the stability of the system. Analog devices with excellent performance, especially high stability and consistency are used. Further more, some measures, both of hardware and of software, are used to protect the system. Because quite a few of tasks are involved, RTOS (Real Time Operation System) is introduced to the embedded system. In this way, software design is simplified and reliability and real-time operation ability of the system are improved. The program interface of upper computer is programmed with VB6.0 and SQL Server2000 is used as the background database. Besides ordering data from the on-site parts, the upp...
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