Voltage Endurance Coefficient Research Articles

Impact of step voltage parameters on accuracy of evaluating XLPE insulation DC voltage endurance coefficient

AbstractThe voltage endurance coefficient n of cross‐linked polyethylene (XLPE) insulation is an important indicator for the analysis of insulation failure mechanism. Accurately obtaining the value of n can provide technical support for the reliability evaluation of cable operation. The constant stress method and the step stress method are used to obtain the values of n, but the test parameters can influence the accuracy of the obtained n values. Two methods for selecting test parameters are proposed. The equivalence between the two methods is established, providing theoretical support for the accurate evaluation of the n value of solid insulation materials by using the step stress method. The accuracy of the evaluation results is verified by analyzing the test data. The test results show that the nstep = 13.04 evaluated by the step stress test with the proposed parameter selection method is consistent with the test result of constant stress nconstant = 12.83.

Understanding and predicting the DC voltage endurance coefficient of cross-linked polyethylene insulation under thermal ageing

Voltage endurance coefficient (VEC) is a parameter used in the design of withstand test and insulation thickness of cross-linked polyethylene (XLPE) cables. Thermal ageing is inevitable in operation of cable, but the thermal ageing effect on the VEC is rarely considered. In this study, the trend of VEC of XLPE films under DC voltage during thermal ageing is investigated by 120 and 135°C thermal ageing experiments. The results show that, after 14 days of 120 and 135°C thermal ageing, the VEC decreases from 20.74 to 7.05 and 3.93, respectively. To understand the effect of thermal ageing on the VEC, the VEC is described as a function of the free energy increment which is proportional to the degree of polymerisation (DP) and the electric field. A simulation calculating the free energy increment of several molecular chains is conducted to confirm the proportional relationship between the free energy increment and the DP, the electric field. Based on the function of the VEC, a predicting method for the VEC under thermal ageing is developed. The experimental results of 120 and 135°C thermal ageing are used to confirm the preciseness of the proposed predicting method.

Method of selecting step stress test parameters for XLPE insulation DC voltage endurance coefficient

Cross-linked polyethylene (XLPE) insulation DC voltage endurance coefficient (VEC) is an important basis in the selection of DC cable insulation design and factory test parameters. Studies on the degradation of XLPE insulation performance under DC electric field can also provide technical support for the reliability evaluation of cable operation. The step stress method as an efficient electrical life test method is widely used to obtain VEC. The parameters of step stress method present considerable influences on VEC. An analytical method based on the trajectory of the damage curve and equivalence between step stress test and constant stress test within zone division is proposed in this paper. According to the proposed method, several sets of step stress tests with different test parameters have been carried out. The accuracy of the test results has been confirmed via analyzing test data. The experimental results show that the VEC (n=12.0) obtained by the step stress test whose parameters are selected by proposed method has good equivalence with the result (n=12.2) under constant-stress method.

Evaluation of voltage endurance characteristics for new and aged XLPE cable insulation by electrical treeing test

The voltage endurance coefficient of solid insulating materials can be determined by electrical treeing tests. The measured value is influenced by the test conditions, and its effectiveness remains to be further demonstrated. In this study, samples were made from the XLPE insulation of three 110 kV AC cables, two new and one operated for 16 years. In treeing tests performed with a pin-plane electrode system, the tree initiation times were measured under both progressive and constant voltage, and voltage endurance coefficients were determined by the residual voltage method. The material properties of XLPE were measured using the FTIR, DSC and TSC methods. For the same sample, the voltage endurance coefficient measured with a 10 μm needle electrode was larger than that measured with a 5 μm electrode, indicating the influence of space charges on the electrical field around the needle tip. For the insulation samples of the two new cables, the measured voltage endurance coefficients were different, and the difference became more significant when the rise rate of the progressive voltage decreased. The superior resistance to electrical aging of the insulation with a higher voltage endurance coefficient was confirmed via material property measurements. For the insulation samples of the aged cable, along with a decrease of the progressive voltage rise rate, the tree initiation voltage increased, which was opposite to the test results obtained for the new cable insulation samples. The voltage endurance coefficient could not be measured using the residual voltage method because the trees always initiated at the rising edge of the constant voltage, except for a low value measured at the lowest constant voltage. The abnormal performance of the aged cable insulation might be the result of increased defects and impurities caused by operation aging, which enhanced the space charge effect, especially in the pin-plane electrical field of the electrical treeing test.

Acids generated and influence on electrical lifetime of natural ester impregnated paper insulation

Acids have attracted an increasing interest from researchers for the standard evaluation of transformer oil, especially in natural ester. This study aims to investigate the types of generated acids in natural ester under hydrolysis, oxidation, and accelerated thermal aging tests with varying paper moistures. The oxidation and hydrolysis degrees were evaluated by acid number and kinematic viscosity, and acids were distinguished from aging condition hydrolysis of oil paper insulation by combining pH values. Meanwhile, the electrical lifetime of oil-paper insulation with seven types of acid were analyzed by step-stress test. The two-parameter Weibull distribution was used to analyze the mean lifetime of specimens statistically. Experimental results reveal that the majority of acids generated from hydrolysis and the main acids are long-chain fatty acids of natural ester impregnated paper insulation. Furthermore, the electrical lifetime models of oil-paper insulation with different types acid were established, and the voltage endurance coefficient with seven types of acids were also obtained. The electrical lifetime test indicated that acids can reduce the electrical lifetime of natural ester impregnated paper insulation and that the effect of low-molecular-weight acids on insulation lifetime is approximately 2–3 times higher than high-molecular-weight acids.

A novel method for the insulation thickness design of HV XLPE cable based on electrical treeing tests

Electrical treeing endurance behavior of XLPE cable insulation was investigated by application of AC voltages on specimens with a pin-plane electrode system. The needle inserted has a tip radius of 10 μm, and the pin-plane distance is 2 mm. The relation between applied voltage and tree initiation time, when plotted in a log-log coordinate, could be approximated by three straight lines, and an electrical threshold stress of about 100 kV/mm was deduced from it in the low field region. The voltage endurance coefficient obtained from tree initiation time increased with higher cumulative probability in the constant voltage method, but altered little with the ramping-up rate of step voltages applied in the step-constant voltage method. The treeing breakdown time gave a smaller value of this coefficient, compared with tree initiation. The XLPE insulation of a taping mould joint was tested to be more voltage withstanding than a cable body, for its higher voltage endurance coefficient, 18.6 to 13.8, and slower tree growth, 0.53 to 0.62 mm in 20 minutes. The electrical field for cable insulation design could be connected with tree initiation stress by a field enhancement factor, and this allowed the insulation thickness of a 110 kV XLPE cable to be determined with the value of 12 to 13 mm. It is shown that the insulation design of HV XLPE cables could be safely achieved by the characteristic parameters determined from electrical treeing test.

Comparison of electrical aging tests on epr-insulated minicables and ribbons from full-sized EPR cab

The results of electrical endurance tests performed on ribbons cut from full-sized EPR cables are compared with those obtained by tests performed on cable models, having the same insulation as the full-sized cable. It is shown that the voltage endurance coefficient estimated on the basis of the short-term life test data has approximately the same value for cable models and ribbons, according to both inverse-power and exponential models. However, the surface roughness resulting from cutting the specimens can affect the result at high stresses.

Long-term electrical performance and life model fitting of XLPE and EPR insulated cables

The results of life tests performed on XLPE and EPR cable models are presented and discussed. The performance of the two insulating materials when subjected to electrical stress are compared on the basis of characteristic parameters derived by the best-fitting life models. It is found that while EPR is well described by linear models, XLPE data fit curvilinear life models, showing a tendency to electrical threshold. Hence the fundamental parameter for XLPE characterization is the threshold value, while the voltage endurance coefficient should be used to characterize EPR.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

General Equation of the Decline in the Electric Strength for Combined Thermal and Electrical Stresses

In this paper the equation for the decrease in the electric strength with time for combined thermal and electrical stresses is obtained, using the following assumptions: (a) the theory of total aging treated as a cumulative quantity, (b) the electric strength is a basic property for aging evaluation of an insulation, and (c) the life model, already proposed by the author for combined stresses is valid. The model is based on the inverse-power law for electrical aging, the Arrhenius relationship for thermal aging, and a linear dependence of the voltage endurance coefficient on thermal stress. The equation thus obtained represents a synthesis of all the main relationships used in electrical, thermal, and combined-stress endurance, and, in addition, includes a new model, that of the electric strength variation due to thermal stress. The comparison with the experimental curves shows a good agreement with the theory for both thermal and electrical stresses.