The service life of a transformer is determined by the condition of its oil–paper insulation. A modified Cole–Cole model was introduced to describe the frequency domain spectroscopy (FDS) curves based on the classic Debye relaxation model and thereby accurately determined the ageing state of the insulation of a power transformer using FDS. Oil–paper insulation samples with different ageing states are prepared under laboratory conditions, and a characteristic parameter is extracted to quantify their ageing states after a detailed investigation of the frequency domain dielectric properties of the samples. Results reveal that the modified Cole–Cole model, including the α relaxation components, dc and hopping conductivity components, can be employed to effectively model the frequency dielectric characteristics of oil–paper insulation. Furthermore, the extracted characteristic parameter from this model, τα, can be utilized in the quantitative evaluation of the ageing states of transformer insulation. A case example is discussed to demonstrate the applicability of the proposed technique in the condition assessment of field power transformers.

Quantitative evaluation of ageing condition of oil-paper insulation using frequency domain characteristic extracted from modified cole-cole model

The epoxy resin insulation in saturable reactors in a HVDC transmission converter valve is subjected to a bipolar exponential decay pulse voltages under normal operating conditions and its aging mechanism is still unclear. In this paper, an electrothermal aging experiment of epoxy resin was carried out under bipolar exponential decay pulse voltages at high temperature. The dielectric properties of epoxy resin before and after aging were measured using frequency-domain spectroscopy. To quantitatively study the aging characteristics, the double-relaxation Cole-Cole model was used to fit the dielectric spectra. As a result, an exponential function relationship between the relaxation time constant and the aging time of epoxy resin under various aging conditions was obtained. This relationship can be used as an empirical formula to evaluate the degree of aging. Furthermore, a comparison of the aging life characteristics of epoxy resin under pulse and sinusoidal voltages with the same peak value was performed. The results show that the electrothermal aging rate of epoxy resin under a pulse voltage is lower than under a sinusoidal voltage. The effect of the measurement temperature on the insulation life evaluation of epoxy resin was also analyzed.

Electrothermal aging characteristics of epoxy resin under bipolar exponential decay pulse voltage and its insulation life evaluation based on Cole-Cole model

SUMMARY

Frequency domain spectroscopy (FDS) is employed to evaluate the moisture content and aging state of oil-paper insulation in power transformers. Oil-paper insulation samples with different moisture content and aging states were primarily prepared in the laboratory. A method was proposed based on a study of the influence of the aging state and moisture content on the frequency domain dielectric property of oil-paper insulation. This method distinguishes the influence of moisture content and aging state on FDS through the imaginary part of complex relative permittivity (er″). Relevant characteristic parameters that characterize the moisture content and aging state of oil-impregnated pressboards were also extracted. Results reveal that the extracted characteristic parameters are sensitive to moisture content in the range of 10−1 Hz to 101 Hz and in the range of 10−3 Hz to 10−1 Hz for the aging state. An obvious exponential relationship exists between the maximum values of the characteristic parameters in the given frequency ranges and degree of polymerization and moisture content. The new method can be utilized for a non-destructive condition assessment of oil-paper insulation of transformers. Copyright © 2014 John Wiley & Sons, Ltd.

Investigation of characteristic parameters for condition evaluation of transformer oil-paper insulation using frequency domain spectroscopy

The presence of moisture in the oil–paper insulation in a transformer drastically accelerates the aging process of the insulation and reduces the service life of the transformer. To analyse the influence of moisture on the dielectric response of the oil–paper insulation and to precisely determine the moisture content of the transformer by frequency-domain spectroscopy, the modified Cole–Cole model is introduced to fit the frequency versus dielectric responses of the oil–paper insulation samples with different moisture contents, and the influence of the variation in moisture content on the parameters in the modified Cole–Cole model is investigated. Results show that the modified Cole–Cole model parameters, including the α and β relaxation components as well as the DC and hopping conductivity components, can be used to effectively fit the frequency versus dielectric response curve of the oil–paper insulation. The characteristic parameters τα and τβ in this model are found to be sensitive to changes in the moisture content. They can be used to quantitatively evaluate the moisture content in a transformer insulation system. A case study is presented to verify the effectiveness of the proposed technique in assessing the moisture content of onsite transformers. © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Moisture determination of the main insulation in a transformer by frequency-domain spectroscopy

Under the deregulation of generation market in China, all distributed generators will particular in electric power bidding. Therefore power purchase cost optimization (PPCO) problem has been getting more attention of power grid Company. However, under the competition principle, they can purchase power from several of power plants, therefor, there exist continuous and integral variables in purchase cost model, which is difficult to solve by classical linear optimization method. An improved differential evolution algorithm is proposed and employed to solve the PPCO problem, which targets on minimum purchase cost, considering the supply and demand balance, generation and transfer capability as constraints. It yields the global optimum solution of the PPCO problem. The numerical results show that the proposed algorithm can solve the PPCO problem and saves the costs of power purchase. It has a widely practical value of application. Keywords-Power market; Power purchase cost optimization; Improved Differential Evolution; Adaptive parameter

http://www.ijeacs.com/Files/Other/Vol02-Issue07/Solving-the-Power-Purchase-Cost-Optimization-Problem-with-%20Improved-DE-Algorithm.pdf

Solving the Power Purchase Cost Optimization Problem with Improved DE Algorithm

Water of oil paper insulation in transformer will accelerate its insulation ageing, which is the key factor for its failures Therefore, it is necessary to estimate water content of solid insulation for guaranteeing a safety and reliable operation of power transformers The microstructure of insulation molecules can be varied by water, which affects the conductivity and polarization properties Return voltage measurement based on dielectric response theory can detect variation of dielectric properties of insulation material The paper analyze dielectric response properties of oil-paper transformer in detail, and describes the relationship between water content and polarization characteristic quantities, finally, reveals the mechanism of diagnosing moisture content of solid insulation in transformer using return voltage measurement The moisture content have a great influence on polarization characteristic quantities, especially, the relaxation time is an important characteristic quantities for assessing solid insulation condition

Mechanism of assessing moisture in transformer insulation by return voltage method

Under the deregulation of generation market, all distributed generators will participate in electric power bidding; therefore purchase cost problem has been getting more attention of power companies. However, under the competition principle, they can purchase energy from several of power generation companies, so there exist continuous and integral variables in purchase cost model. It is difficult to solve the problem using traditional optimization method. The differential evolution algorithm has been successfully applied to a wide range of applications, mainly in solving continuous nonlinear optimization problems. Therefore, this paper provides a new method based on differential evolution algorithm for purchase cost optimization, and has been successfully applied to solving the problem. The simulation results show that differential evolution was successfully validated for eight distributed generators, which has significant advantages both in global convergence speed and convergence precision compared with improved GA.

Huajun Ran

Papers

Mechanism of assessing moisture in transformer insulation by return voltage method

Application of differential evolution algorithm in purchase cost optimization