Polarization and Depolarization Current (PDC) technique is an effective tool to assess the condition of oil-paper insulation system in power transformers. So far the PDC behaviors of mineral oil-paper insulation have been widely investigated. However, with the increasing number of transformer choosing natural ester as its insulation oil, it is important to investigate the PDC characteristics of natural ester-paper insulation to see whether the PDC technique can also be used to assess the condition of new insulation system using natural ester in transformers accurately. In this research, natural esterpaper insulation sample and mineral oil-paper insulation sample were subjected to thermally accelerated ageing experiment at 110°C for 120 days. The PDC characteristics of natural ester-paper insulation sample and mineral oil-paper insulation sample were compared over the ageing process. A new method for assessing the ageing condition of the oil-paper insulation in terms of the depolarization charge quantity was proposed. Results show that the polarization/depolarization current of natural ester-paper insulation sample is higher than that of mineral oil-paper insulation sample with the same ageing intervals. The depolarization charge quantity of both kinds of oil-paper insulation sample is very sensitive to their ageing conditions. There is an exponential relation between the stable depolarization charge quantity of both kinds of oil-paper insulation sample and the degree of polymerization (DP) of paper. The depolarization charge quantity can be used to predict the ageing condition of oil-paper insulation providing the measurement temperature is kept the same.

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Quantitative analysis ageing status of natural ester-paper insulation and mineral oil-paper insulation by polarization/depolarization current

Optimization of transformer oil blended with natural ester oils using Taguchi-based grey relational analysis

Moisture is one of the most significant parameters that can accelerate ageing in paper/pressboard insulation. In order to understand the impact of moisture and ageing on Frequency Domain Spectroscopy (FDS) measurements, oil impregnated pressboard samples of different moisture contents are prepared and aged at 105°C temperature. FDS measurements on pressboard samples are carried out at different ageing time. Other measurements such as moisture, degree of polymerization (DP), Furan and Dissolved Gas Analysis (DGA) are also performed on oil and pressboard samples. It is observed that FDS measurements on pressboard samples are mostly influenced by the moisture contents. Several other pressboard samples are prepared under different conditions to determine how FDS measurements are affected by moisture, aged oil and ageing of pressboard sample considering each of the parameter individually as well as in different combinations. Results suggest that FDS measurements are sensitive to the change in moisture, ageing products and ageing of pressboard (DP) sample. However, impact of ageing products and ageing of pressboard sample (DP) on FDS measurements are smaller in comparison to moisture.

Understanding the impact of moisture and ageing of transformer insulation on frequency domain spectroscopy

Space charge behavior is important in the oil-immersed-paper insulation in converter transformers for HVDC transmission. This paper reports an investigation of the space charge property in samples with two layers of oil-immersed-paper and oil. Based on pulsed electroacoustic (PEA) technique, a waveform recovery method was proposed to get rid of the effects of interface and anisotropy on PEA result, and the space charge distributions in the two-layer samples under different voltage and oil thicknesses were measured. It's found that the charges with the same polarity as oil side electrode were accumulated at the interface and gradually increased with the increase of applied voltage. However, the positive charge charges always appeared in the oil under positive and negative voltages. Moreover, the charges in samples increased with the increase of applied voltage, while the amount of interface charges decreased with the increase of oil thickness. Due to the existence of interfacial charges, the electrical field in oil-immersed-paper was distorted with its maximum close to the interface. It is indicated that due to the different mobility properties of charges in paper and oil and the interface trap state, the interface between paper and oil plays an important role in charge formation and decay.

Space charge behavior in the sample with two layers of oil-immersed-paper and oil

Oil-paper compound insulation has been widely used in power transformers for quite a long time because of its good performance. The paper insulation degrades under the combined stresses of thermal, electrical and chemical stresses during routine operations, resulting in the reduction of its degree of polymerization (DP) and hence its mechanical and electrical strength. Space charges in a dielectric material have a close relation to its electrical performance. Therefore, in this work, the effect of paper-aged states (with different DP values) on the space charge behavior in oil-impregnated paper insulation is investigated, and the pulsed electro-acoustic (PEA) method is used for the space charge distribution measurement. In order to achieve this purpose, new oil-impregnated paper samples are thermally-accelerated aged for different time interval to obtain different DP values, and then the residual oil in each sample is removed, and the Kraft paper samples are dried in a vacuum dry oven. After that, all paper samples are impregnated with new oil, and PEA measurements are performed at room temperature. The experimental results show that, when the external dc voltage is applied on the oil-paper sample, homo-charge injection occurs for all samples. The samples with different aging states exhibit quite different space charge characteristics for both voltage-on and voltage-off conditions. When the aged states are worse, for voltage-on condition, more charges are induced at cathode and anode, as well as more space charges are accumulated in the bulk. For voltage-off condition, the space charge decay rate shows no obvious change for different aged samples.

Effects of paper-aged state on space charge characteristics in oil-impregnated paper insulation

Dielectric response measurements are non-invasive and promising diagnostic methods that are being gradually used for assessing the insulation and aging condition of transformer oil-paper insulation system. It is known that moisture content in oil-impregnated insulation has significant effect on its dielectric response phenomena, and whereas at present there is few research about the characteristics of aged cellulose without the impact of water. In this work, a series of experiments are designed and carried out in well-controlled laboratory. Accelerated thermal aging is performed on oil-immersed pressboard samples with different time intervals (0, 120, 250 and 400 h). Then these pressboard samples with different aging states are put into petroleum ether to remove the residual oil inside, and dried in a vacuum oven to remove the petroleum ether and moisture. The degree of polymerization (DP) and scanning electron microscope (SEM) are implemented to characterize the aging status of pressboard samples. Two kinds of dielectric response tests, i.e., the polarization and depolarization currents (PDC) and frequency domain spectroscopy (FDS), are performed on each oil/water-removed pressboard sample in vacuum and new-oil-impregnated ambient, respectively. Samples with different aging states show quite different dielectric response characteristics in both ambients. With the aging status more serious, the pressboard sample exhibits higher values of polarization/depolarization currents and complex capacitances. It is confirmed that, besides the influence of water induced by aging, the deterioration of cellulose itself also causes the increasing of pressboard conductivity, and hence greatly affects the PDC and FDS test results. Dielectric response diagnostic techniques have a potential to reflect the aging information of solid insulation.

Investigation on dielectric response characteristics of thermally aged insulating pressboard in vacuum and oil-impregnated ambient

For achieving the aging characteristic parameters of transformer oil-paper insulation, oil impregnated pressboard samples are acceleratedly thermally aged at high temperature (~150°C) in laboratory, and on the same sample before and after being aged, polarization and depolarization current (PDC) tests are performed at different temperatures (30, 40, 50 and 60°C), respectively. Besides analyzing the PDC results, the differential time-domain spectroscopy (DTDS) technique is introduced to investigate the aging status of oil-paper insulation and the influence of measurement temperature. The DTDS parameters such as polarization component Qn, polarization response time τn and line-shape parameter anare extracted with DTDS unscrambling method based on PDC data. It is revealed that DTDS represents similar polarization process and mechanism to PDC, and all the DTDS curves calculated with the obtained parameters (Qn, τn,αn) are in good fit with those measured. By analyzing the effect of temperature and aging, two types of relaxation polarization mechanism are observed, i.e., the interfacial polarizations between oil and paper insulation and inside the pressboard. The polarization components, response speeds and line-shape parameters of both the polarization mechanisms increase evidently on the whole with temperature rising or sample aging. It is demonstrated that, the DTDS theory is suitable for oil-paper insulation material, the DTDS unscrambling method is effective and objective, and such DTDS parameters are sensitive and can quantitatively reflect the changing of temperature and aging condition with regularity. Consequently, the parameters (Qn, τn, αn) can be considered as the novel characteristic parameters for aging condition estimation of oil-paper insulation to upgrade the application effect of PDC curves.

Novel characteristic parameters for oil-paper insulation assessment from differential time-domain spectroscopy based on polarization and depolarization current measurement

Here, we report the molecular self-assembly of hydroxido-bridged {Ln5Ni6} ((Ln3+ = Dy3+, Y3+) metal clusters by the reaction of enantiopure chiral ligands, namely, (R/S)-(2-hydroxy-3-methoxybenzyl)-serine), with NiII and LnIII precursors. Single-crystal diffraction analysis reveals that these compounds are isostructural sandwich-like 3d-4f heterometallic clusters showing helical chirality. Direct current magnetic measurements on {Dy5Ni6} indicates ferromagnetic coupling between DyIII and NiII centers, whereas those on {Y5Ni6} denote that the NiII centers are antiferromagnetically coupled and/or magnetically anisotropic. Magneto-chiral dichroism (MChD) measurements on {Dy5Ni6} and its comparison to that of {Y5Ni6} provide the first experimental observation of intense multimetal site MChD signals in the visible-near-infrared region. Moreover, the comparison of MChD with natural and magnetic circular dichroism spectra unambiguously demonstrate for the first time that the MChD signals associated with the NiII d-d transitions are mostly driven by natural optical activity and those associated with the DyIII f-f transitions are driven by magnetic optical activity.

Magnetic 3d-4f Chiral Clusters Showing Multimetal Site Magneto-Chiral Dichroism.

Converter transformers are suffered from DC voltage, DC voltage with AC voltage and polarity reversed voltage additionally, compared with common power transformers. Under DC voltage it is easy to form space charge in oil-paper insulation. The formation, distribution and mobility of space charge will affect its dielectric performance. What's more the insulation system is easier to fail under polarity reversed voltage. In order to observe space charge behavior in oil-paper insulation under different polarity reversed conditions, space charge measurement is made under different voltage reversed polarity (from positive to negative and from negative to positive) and different voltage reversed time (20s, 30s, 40s and 60s) separately. The pulsed electro-acoustic (PEA) method is used for space charge measurement at room temperature. The impact of voltage reversed polarity and voltage reversed time on space charge characteristics was discussed. Besides the electric field distortion in samples with different polarity reversed conditions is calculated. The experimental results show that polarity reversed conditions have great effect on space charge characteristics.

https://ieeexplore.ieee.org/iel5/6403647/6416166/06416427.pdf

Shi-Qiang Wang

Papers

Investigation on dielectric response characteristics of thermally aged insulating pressboard in vacuum and oil-impregnated ambient

Effects of paper-aged state on space charge characteristics in oil-impregnated paper insulation

Novel characteristic parameters for oil-paper insulation assessment from differential time-domain spectroscopy based on polarization and depolarization current measurement

Magnetic 3d-4f Chiral Clusters Showing Multimetal Site Magneto-Chiral Dichroism.

Space charge behavior in oil-paper insulation under polarity reversed voltage