The Fifth Harmonic Fault Line Selection Method Based On Chaos Detection Principle

The neutral point of main transformer in our nation usually adopts NUS or NRS forthe3~66kV distribution network. The short-circuit fault current can form a small current looponly through the ground capacitance or impedance when a single-phase ground occurred insuch grid, so it is called the small current grounding system.When a single-phase ground faultoccurs, the biggest advantages of using small current grounding systems are that thesymmetry of the system voltage would not be destroyed and the fault current is too small tocause a great harm for the power supply equipment. The electricity regulation allows faultlines or equipment continues to run1to2hours and does not affect the normal power supply.If the single-phase grounded fault would not be timely processed, it may cause insulationweaknesses which would be breakdown to become the phase short-circuit, and affect theuser’s normal use of electricity since the voltage of non-fault phase increases. At the sametime, the arc grounding will cause the over-voltage of whole system, and damage theequipment and the safty of system operation, even become a permanent phase-to-phase fault.Therefore, the fault line must be correctly and timely detected to minimize the damage whenthe single-phase ground fault occurs. The existing methods of fault line selection can bedivided into steady-state fault line selection and transient signals fault line selection.Steady-state signals last for a long time, but it is too tiny to be extracted, and creates obstaclesto the fault line selection.The present situation of the fault line selection method in the small current system andthe disadavantages of the traditional fifth harmonic selection principle is deeply researched inthis paper. The steady-state characteristics of the single-phase ground fault in the neutralnon-effectively grounded system and the weak signal detection principle based on the chaoticDuffing model are respectively analyzed. The small current single-phase ground model andchaotic Duffing oscillator detection model are analysed, and the fifth harmonic wave ofzero-sequence current in the state-state is regarded as the fault characteristic physical quantities. The full-wave signal of fault zero sequences current is took as the periodicdisturbance, which is added into the Duffing oscillator model. Then the Duffing system ismade into the critical chaotic state, and the different direction of the fifth harmonic ofzero-sequence current in fault line and the normal line to let this Duffing model in differentstate are used to find the fault line. The probability distribution of the time-domain waveformfrom Chaos’ x wave is caculated by the symbol sequence information entropy method, so thatwhether the Duffing system is in chaotic state or cycle state is identified to select a fault line.At last, this paper uses Matlab software to simulate the application of experiments to verifythe reliability of this method.