Study On Online Monitoring For Overvoltage Based On Frequency Measurement Of Capacitive Equipment’s Leakage Current

Power grid overvoltage is the main cause which influences the safe operation ofpower grid. The analysis of why overvoltage accident happened and how it spreaddepends on the real-time waveforms of overvoltages. The improvement of insulationcoordination also needs the characteristic parameters of the overvoltage. On-lineovervoltage monitoring is which can provide the waveforms and characteristic parametersof the overvoltage. High-voltage dividers are the most popular overvoltage sensors inmonitoring systems. However, some problems like the heat dissipation of high-voltagedivider at high voltage level and the like limit its application under the higher voltagegrade. At the meantime, changing the original connection way also brings potential risk topower grid. Nowadays monitoring systems mostly adopt centralized monitoring structure.The distortion of analog signals which are provided by the overvoltage sensors usuallyoccurs in the long distance transmission. So the systems have poor performance ofdisturbance rejection. In this paper a new method is developed to solve these problems.The key technologies of overvoltage online monitoring system are researched deeply. Anew online monitoring system based on frequency-dividing measurement of leakagecurrent of capacitive equipment is designed.The monitoring system in this thesis is mainly composed of voltage sensor,monitoring unit and a monitoring computer. Two mutual inductors induct leakage currentfrom capacitive equipment to the earth. They are designed according to integralmeasuring principle. The sensor has fine characteristic of electric safety, and it doesn’taffect the wiring of power system. Online monitoring device installed near to voltagesensors to capture waveform data from voltage sensor and accomplish the digitization ofwave form signal, system performance of anti-interference is improved by digitalizedtransition. Ethernet communication is accomplished between monitoring device andbackground monitoring computer with high transmission rate and reliability.The thesis gives a detailed analysis on the structure and measure principle of thesensor, and the influencing factors on measuring result. On that basis it describes theimplementation methods of voltage measuring sensor. And applying the Pspice softwareto emulate circuit and by adjusting circuit parameter to ensure fine amplitude-frequencycharacteristic and phase-frequency characteristic of overvoltage sensor prove thetheoretical feasibility of this project. Online monitoring device hardware adopts modular design and mainly comprises imitated regulate module, acquisition module andprocessing monitoring module, the component structures and implementation principlesof each functional module are introduced in this article.At last, performance tests of the sensor and the monitoring system are accomplishedin laboratory. The results show that the sensor which adopts fractional frequency canreveal fine precision and linearity, the data acquisition works fine and the monitoringsystem has high transmission rate and reliability. It can satisfy the requirements ofovervoltage monitoring in power grid.