Studies On New Methods Of Faulted Line Identification In Non-effectively Grounded System

Non-effectively neutral grounded systems is a common practice in the power distribution systems of our country, its single-phase to ground faults received many sight which happen in high frequency. Many methods have been proposed to solve the problem and some equipments developed from them can identify the faulted line preferably in the relative pure systems or when the fault resistances are not great. But for the petroleum and coal industry which the network are polluted seriously, or the fault resistances is relative great, it is difficult to find a satisfied equipment to find the faulted line precisely.In comparison with the effectively neutral grounded systems, in which the single-phase to ground faults will lead the switching out, the non-effectively neutral grounded systems do not need to switch out immediately when the single-phase to ground faults occur. It is benefit for the power supply reliability. With the increasing attention on the power quality, the superiority becomes more notable. If the faulted line can't be selected automatically and precisely after the faults occur, the superiority in enhancing the reliability can't be embodied sufficiently. So in order to enhance the reliability, improve the power quality, exert the advantage of the non-effectively neutral grounded systems, it is very significant to find a good faulted line identification method which can work effectively in all situations.In accordance with the character which a small steady-state fault current is available for identification in non-effectively grounded system, this paper proposes a new faulted line identification method named faulted line identification using controllable short-circuits. Namely when a single-phase to ground occurs, we temporarily converts an non-effectively neutral grounded system into a grounded system through a controlled short-circuit between the system neutral point and the ground, a large transient fault current will appear. This transient current will flow through the faulted line and can be used for identification. By controlling the strength of short-circuit, we can achieve current signals that are large enough to be detected yet small enough not to cause disturbances to the load. In the proposed scheme, a thyristor is placed between the main transformer neutral point and the ground and fired in different ways to achieve the above process. The distinct characteristics of transient current signals and the subtraction method enable the proposed scheme to overcome the formerly faulted line identification method's disadvantages, such as poor intention or characters of single used for faulted line identification, which make the singles difficult to be drown. Results of theoretical analysis, computer simulation and lab experiments have all conformed the effectiveness of the proposed method. It has the potential to be a valid solution to the problem of faulted line identification in non-effective neutral grounding systems.To overcome the difficulty in implementing the proposed scheme of faulted line identification based on controllable short-circuits, an improved scheme is developed. When a single-phase to ground fault occurs to the non-effectively neutral grounded system, a thyristor is used to make the open-loop delta coil of PT controllable short-circuits, it is equivalent to create a short-circuit between the PT primary side neutral and the ground. A transient fault current pulse flows through the faulted line to the ground and can be used to identify the faulted line. The restrict of PT capacity on transient fault current pulse is weakened greatly due to the small RMS of transient pulse. It makes the improved scheme has the same faulted line identification accuracy with formerly scheme, meanwhile reduces the cost and simplifies the local implement. Meanwhile, a new method based on the improved faulted line identification scheme is proposed to mitigate the PT resonance, which is proved by theoretical analysis and computer simulation. The improved faulted line identification scheme based on controllable short-circuits has the potential to be a ideal solution to the problem of faulted line identification and PT resonance in non-effective neutral grounding systems.Basing on the systems identification theory, this paper proposes a new faulted line identification method based on incremental impedance. The method is evaluated using theoretical analysis, computer simulation and lad experiments. This method is innovation and easy to achieve, its physical concept is also clear. It provides an optional idea for faulted line identification. Lab tests is the important method to check the valid of new theory. The lab test platform which is made up of controllable short-circuit creator, transient current pulse detector and simple distribution network lay a foundation for proving the valid of the above three new faulted line identification method. The faulted line identification equipment based on the improved controllable short-circuit has been used on the distribution network, it is very important for the controllable short-circuits faulted line identification to be changed to productivity and serves for power enterprise in the future.