Search

Aging of the composite insulator threatens the safety of power grid. A portable nuclear magnetic resonance (NMR) sensor with an adjustable ‘clamp’ structure is constructed for the noninvasive measurement of the aging status of the composite insulator. Two cylindrical permanent magnets are used to establish static magnetic field B0. Magnets can be adjusted to adapt to different thickness of sheds. An “8-shaped” plane radio frequency (RF) coil is used. Composite insulator samples of different service time are measured. Their nuclear magnetic resonance (NMR) echo decay signal was collected, and the decay signal was fitted with an exponential function. After this processing the transverse relaxation time (T2) was obtained. For comparison, the relative permittivity of the samples is measured by an Agilent 4294A. The result in demonstrates that an increase in the aging level of composite insulator results in a decrease of T2. The trend of transverse relaxation time T2 is the same as that of the relative permittivity. The field test results show that NMR sensor can be used to detect the aging status of composite insulator shed. The T2 obtained by our portable NMR sensor can reliably be used as an index to reflect the aging status of composite insulator.

In order to investigate the causes of joint breakdown caused by high voltage cables partial discharge, the five kinds of cable body defects: the insulation shield fracture stripping irregular, the main insulation of the insulation shield fracture being cut, the insulation shield fracture has steps, the main insulation is inserted with the metal tip, and the main insulation having air holes, as well as the two kinds of defects in the accessories: the dislocation of the stress cone and the impurity in the stress cone for 110kV XLPE insulated cable joint, the simulation were made. Then the boost test was carried out in the high pressure test hall, and the test samples were monitored by the partial discharge detection system based on the broadband pulse current method. The analysis of the partial discharge characteristics of various typical defects shows that controlling the installation quality of the cable body and accessories during installation is the direct means to reduce the breakdown risk of the transition joints.

The flashover of polluted insulators is a serious threat to the safe operation of overhead power lines. The leakage current on surface of insulators is closely related to the developing process of pollution flashover, affected by many factors, such as wetting degree, system voltage and structures of insulators. At the same time, the leakage current is easy to be monitored on-line continuously. Hence, as one of on-line monitoring methods for polluted insulators. Based on artificial pollution tests under different relative humidity (RH) and salt deposit density (SDD), a method incorporates S-transform and support vector machine (SVM) for evaluating safety condition of polluted insulators is proposed. Firstly, the approach utilizes S-transform to extract the amplitude Ih, phase θ and total harmonic distortion (THD) of leakage current. Then the I h , θ and THD are as characteristic parameters to put into SVM, the output parameters of SVM are fuzzifled into four fuzzy subsets for evaluating safety condition of polluted insulators. Lastly, the performance of SVM is validated by 8 pairs of testing samples, and the influences of SDD and RH on the performance of insulators are discussed. The results show that, the leakage current becomes inductive when strong local arcs occur, decreases with the increase of RH, and Ih has no relation with SDD when the RH is low but increases with the increase of SDD when RH is high. THD of leakage current increases slowly with the increase of RH when the pollution is light, but decreases firstly and then increases with the increase of RH when the pollution is middle or heavy, THD take the minimum at the RH of about 80%.