Study On Measuring System Of Water Tree In XLPE Cables Based On Measurement Of Harmonic Distortion In Loss Current

Since brought into service in the sixties, XLPE cable has become the main kind of cable in middle and high voltage transmission system for its outstanding performance. But water tree in XLPE cable insulation is the main factor that will affect security and reliability of XLPE cable.There are some methods to diagnose water tree aged situation in off-line condition. But what these methods can be developed into online detection technique is relatively few. Method of DC component and method of harmonic component are the most feasible methods which will be developed into online diagnosis technique. And the method of DC component is used successfully for online diagnosis in Japan.The purpose of this paper lies in studying the possibility of eliminating effects of harmonics in power supply through using signal processing technology and developing the method of harmonic component into the online detection technique that can be used actually. The content of studying in this paper includes analyzing measuring theory of the method of harmonic component through computer simulating and basic experiments; designing a measuring system for the measurement of harmonics of loss current under the harmonics interference from power supply; designing the parts of the hardware and the parts of the software of the measuring system; studying the non-linearity compensation method of the measuring system and the revising technology of the system.It is discovered that the harmonics of loss current in XLPE cable insulation aged with water tree is caused by the non-linearity of the aged insulation. And the DC component in leakage current is resulted from the polar effect of the relationship between conductance and voltage after XLPE cable insulation aged with water tree. It is possible to diagnose the water tree situation by using the method of harmonic component measurement. Moreover, the measuring sensitivity is independent of the length of XLPE cable in good condition. The measuring system in this paper balances capacitance current in the first loop and establishes the foundation for the measurement of harmonics of loss current. The information of the interference of harmonics from power supply is provided, and the harmonics of loss current with interference is obtained from the current comparator. After the interference of harmonics in power supply was suppressed by software, the harmonics of loss current in XLPE cable insulation caused by water tree aging can be obtained.In this paper, a measuring system for the measurement of harmonics of loss current under the harmonics interference from power supply is developed, and offers some experimental foundation for the further study about the application of the measuring system in the engineering field.