Study On Current Leakage Protection In 6kV Coal Mine Power System

In our country, the non-effectively earthed neutral mode used mostly in 6～35 kV medium voltage distribution networks underground mine adopts the neutral point unearthed mode or neutral point earthed via arc suppression coil as well as high resistance. In distribution system, the main fault type is the single-phase to earth fault, which accounts for more than 80% of the total faults. When the single-phase to earth fault occurs in the non-effectively earthed neutral system, the fault current it's not too high and the three phase line to line voltages are still symmetric, so the load can be in normal operation for a period of time. Since the distributed capacitance of cable line is larger than overhead line's, at the same time, the arc suppression coil(ASC) is not input in time following the fault, as a result, arc or spark releasing will bring great risks to mine safety. With the increase of the distributed capacitance, grounding arc can not be able to go out automatically and cause the arc over-voltage for a long time, even develop into a serious systemic accident. So when fault occurs, the fault line and fault point should be found out and cleared immediately so as to ensure the safety of underground mine.There are lots of faulty line selection methods for earth leakage protection, 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 widely. Because its performance is independent on the zero sequence current. The accuracy and reliability of the signal injection method are determined by the injected signal's quality and fault status, including frequency and amplitude. Therefore, according to the actual situation, it's necessary to analyze the influence of the injected signal's frequency, injected mode, the transition resistor and distributed capacitance on the accuracy of fault line selection. It's difficult to communicate between protection unit and host because of the specificity of protection unit installation, so it is very essential to search for a new solution on single-phase to earth fault line.In this paper, a new selective earth leakage protection device is proposed and developed on the basis of the theory of the signal injection method, it can complete the single-phase to earth fault line and trip automatically. It is made up of injected control device and distance protection unit. The injected signal's selection principle and detection method are made according to the actual situation of mine and the influence of transition resistance and distributed capacitance is analyzed by circuit equivalent, mathematical method and system simulation. The administration module withμPSD3334D at the device core is designed to achieve voltage detection, generator startup and signal switching integration automatically. The filtration and block circuit designed provides good protection to the device. The high-precision analog filtering and digital filtering module is also designed to get accurate injected signal which is weak in the primary system as well as design the distance protection unit according to the principle of distribution protection.In order to get the result of the injected signal's detection and extraction, set up a laboratory model for testing. According to the actual situation, set up the simulation test system and construct test data source using MATLAB. Carry on fault location, calculate the transition resistor and distributed capacitance which are used to insulation monitoring and auto-tuning of the arc suppression coil, using injected signal which is controlled to inject by the selective earth leakage protection device when single phase to earth fault occurs. Extract the voltage and current of injected signal in bus, using impedance method to achieve fault location, at the same time, correct the impedance method when transition resistor and distributed capacitance are too large to ignore. Computation of this algorithm is very small while the precision is high verified by system simulation using PSCAD. The content combines the actual needs of the mine distribution networks closely and has theoretical and practical value.