Analysis To Vegetable Oil-Paper Insulation State Of Transformer Using Fractional Zener Model

The oil impregnated paper transformer is in the core position in the operation of the power system.Therefore,ensuring the safe operation of the transformer is an important basis for maintaining the stable operation of the power system.At present,The liquid insulating medium used in the traditional oil-immersed transformer is mineral insulating oil,which is difficult to be biodegraded.When leakage occurs,it will cause great damage to the stable operation of the power system,at the same time,it will also pollute the water source and living environment,accompanied by huge economic losses.That urges people to find a new type of liquid insulating material that can replace mineral insulating oil.Vegetable insulating oil has the advantages of high ignition point and 98% degradation rate,which can be used as a substitute of mineral insulating oil.However,in the process of operation,it is aging gradually under the action of various stresses,which leads to the decline of its mechanical properties and insulation properties.In order to study the aging rule and insulation diagnosis method of the vegetable oil-impregnated paper insulation system,this paper introduces the concept of fractional element and fractional calculus,and proposes a parameter identification method based on chaos Gray Wolf algorithm(CGWO)for the oil-impregnated paper insulation fractional Zener model.This method can directly reflect the information carried by the frequency domain spectrum of oil-paper insulation,and obtain the parameters of fractional Zener model by using the complex permittivity obtained by the frequency domain spectroscopy method(FDS).Firstly,the gray wolf optimization algorithm that introduces chaotic sequences is used to identify the fractional Zener model parameters of oil-paper insulation samples with different aging degrees.And take the samples aged for 28 days as an example,the coincidence of real and imaginary part of complex permittivity and the measured value is calculated.It is as high as 95% and 99% respectively.Compared with the original gray wolf algorithm and genetic algorithm,chaotic Gray Wolf algorithm has the advantages of accurate initial position,fast convergence speed and so on.At the same time,it is also shown that the fractional Zener model can characterize the aging process of vegetable oil impregnated paper insulation.The CGWO was used to solve the model parameters to obtain the values of shape parameter ?,polarization parameter ?b and relaxation time ? in the fractional Zener model related to the complex permittivity of FDS.It is found that there are effective rules between them and aging degree respectively.By mathematical fitting,it was found that there is a power function relationship between the parameter Av that characterizes the aging state of the vegetable oil impregnated paper insulation test sample and the relaxation time τ,shape parameter ? and polarization parameter ?b respectively;there is a higher exponential function relationship between the parameter Ck that characterizes the aging state of the vegetable oil impregnated paper insulation test sample and shape parameter ?,and an excellent power function relationship between Ck and relaxation time ? and polarization parameter ?b respectively.In order to make the frequency domain spectroscopy more accurate to evaluate the aging state of vegetable oil impregnated paper insulation system,this paper performs the temperature-dependent frequency domain dielectric spectroscopy tests were carried out on unaged and aged 28-day samples.First of all,the test results show that: the higher the test temperature is,the larger the complex permittivity of the vegetable oil paper insulation system is,and the real part is only distinguished in the low frequency range of 10-4~10-1Hz,while the imaginary part is different in the full frequency range of 10-4~103 Hz.Therefore,a temperature frequency translation method based on the characteristic frequency points of the real part of the complex permittivity is proposed.Secondly,when?′ is at f=10-4 Hz,the absolute value of the curve change rate ∣λ(f)∣ increases as a power function with the increase of the test temperature.And the influence of temperature on the frequency spectrum is solved quantitatively.The reliability and effectiveness of the temperature reduction method and insulation evaluation method are verified by examples.It is helpful for the quantitative study of oil paper insulation state of transformer and the diagnosis of transformer insulation state in practical engineering.