Research Of The Signal Generator For Fault Location In Non-solidly Earthed Network

In our country, the non-effectively earthed neutral mode used mostly in 6～35kV medium voltage distribution network adopts the neutral point unearthed mode or neutral point earthed via Peterson coil. The neutral point high resistance grounding mode is also used occasionally. In distribution system, the main faults are the single phase-to-grounding faults, which account for more than 70% of the total faults. When the single phase-to-grounding fault occurs in the non-effectively earthed neutral system, the short circuit current won't be too high and the three-phase line voltage is still symmetrical, so the system can work normally for some time. However, following the fault, the non-fault phase voltage will increase and it will cause arcing fault. If the power lines work under the arc over-voltage for a long time, the line insulation will fail and serious phase-to-phase faults will follow. So when the single phase-to-grounding faults occur, the fault line and fault point must be found out and cleared immediately.There are lots of fault line selection methods for single phase grounding in distribution network. The signal injection method based on the information of the signal to choose the fault line and the fault location is applicable to the non-effectively earthed neutral systems of different structures. Its performance is independent of the zero sequence current. The accuracy and reliability of the signal injection method is determined by the signal's quality. Therefore, according to the actual situation, it's necessary to design the signal generator meeting the requirements and analyze the influence of signal's frequency and injection mode on the accuracy of fault line selection.In this paper, an analysis of the signal's selection principle and detection method is made on the base of the theory of the signal injection method. Three common signal detection methods are compared and the influence of transition resistance is analyzed by circuit equivalent and mathematical method. In view of the low power output, the low stable, the bad anti-jamming ability and the imperfect protection measures of the common signal generator, a new type of signal generator with perfect performance is studied and developed. High steady power source and PWM modulation technology are introduced in this signal generator to improve the output, the stable and the anti-jamming ability. The special designed filtration and block circuit provides good protection. The administration module withμPSD3334D at its core is designed to achieve voltage detection, generator startup and signal switching integration. It also can determine the fault phase and switch the signal automatically or manually.Some experiments were done on the simulation test power system which was set up based on the practical situation to test the injected signal and the detection effect of the signal under laboratory conditions. The experiments are carried on by potential transformer injection mode and parallel compensation capacitor neutral point injection mode. The dedicated digital detector was used to detect the signal. The data was piled up and the analysis of the detection effect was made. The content combines the actual needs of the power system closely and has theoretical and practical value.