Cristina Stancu | University Politehnica of Bucharest (original) (raw)
Papers by Cristina Stancu
IEEE Transactions on Components, Packaging and Manufacturing Technology
A rapid method based on activation energy values for the lifetime assessment of PP films used as ... more A rapid method based on activation energy values for the lifetime assessment of PP films used as dielectrics for capacitors is proposed. The activation energy is determined from a nonisothermal measurement made by differential scanning calorimetry and an aging test at a single elevated temperature. The use of the onset temperature of the exothermal peak is proposed to evaluate the activation energy of the degradation of the PP film. Four types of PP film capacitors were thermally aged at 100 °C for different aging times. For each aging time, the capacitance was measured at different frequencies. According to the standards, two end-of-life criteria (2% and 5%) were imposed for the reduction in the capacitance. The parameters of the lifetime line <inline-formula> <tex-math notation="LaTeX">$a$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$b$ </tex-math></inline-formula> were determined, and finally, the lifetime values were calculated. The novel proposed method is effective in terms of both energy and manpower costs compared with the current method, which uses three aging temperatures.
Electrotehnica, Electronica, Automatica
This paper presents a study on the influence of the environmental factors (respectively, ambient ... more This paper presents a study on the influence of the environmental factors (respectively, ambient temperature and solar radiation) on hot spot temperatures and the consumed and remaining lifetimes of power transformers. In this regard, 15 identical 105 MVA distribution transformers are considered, loaded with the same load, and operating in 15 areas of Romania (with different climatic characteristics). Using the equations recommended by IEC 60076-7 (modified in the case of the presence of solar radiation) the values of oil temperature, hot spot temperatures and consumed and remaining lifetimes in two days from July 2020 are calculated, both in the presence and in the absence of solar radiation. It is shown that the values of the hot spot temperature and of the consumed lifetimes differ from one locality to another and that they increase with the increase of the ambient temperature and the intensities of the solar radiation. Finally, it is shown that the results obtained can be used t...
In this paper an experimental study regarding the structure and electrical properties of low dens... more In this paper an experimental study regarding the structure and electrical properties of low density polyethylene composites filled with neodymium particles is presented. The analysis of the samples structure shows that the neodymium particles appear as clusters and the distance between them decreases if the filler content increases. The DC measurements show that the electrical conductivity values increase with the temperature and filler content. Using the absorption currents, dielectric response functions of composites f(t) are determined and the complex permittivity values are calculated and compared with experimental values. Good correlation between calculated values and experimental ones was found for frequencies lower than 100 Hz. Variations of the complex permittivity components with the neodymium content, temperature and frequency are also presented.
Materials, methods & technologies, 2016
After an analysis of the structure and properties of the DC joints insulation components, the gen... more After an analysis of the structure and properties of the DC joints insulation components, the generation mechanisms and the measurements methods of the space charge in DC multilayer joints are presented. Then, the equations used to calculate the charge accumulated at the homogeneous interfaces of the joints and the electric field are given. To calculate the space charge density and the electric field, experiments on flat samples (XLPE and reductionEPDM) are performed and their dielectric properties are measured. Finally, it is shown that the charge accumulation leads to local enhancements of the electric field and to degradation and lifetime reduction of joints.
Przegląd Elektrotechniczny, 2009
In the present paper the results concerning the structural modifications and dielectric propertie... more In the present paper the results concerning the structural modifications and dielectric properties of polyethylene insulations taken from a medium voltage cable (20 kV) subjected to 120 thermal cycles between −35 °C and + 100 °C are presented. The structural modifications were investigated by thermal analysis methods (TG, DTG and DTA), showing that during the thermal cycling the degree of cross-linking of polyethylene increases and the degree of crystallinity decreases. Dielectric modifications of polyethylene due to thermal cycling were determined by using the dielectric spectroscopy method in range 10 <sup>−2</sup> ÷ 10<sup>6</sup> Hz, for 30 °C but also for 100 °C. After 120 thermal cycles the results obtained show that tgδ for 30 °C increases slowly for frequencies higher than 100 Hz and substantial for frequencies lower than 100 Hz. This increase is more pronounced for 100 °C (up to 100 %) especially for frequencies between 10<sup>−2</sup> ÷ ...
Part I of this paper presents a cylindrical two-layer insulation model and two methods (A and B) ... more Part I of this paper presents a cylindrical two-layer insulation model and two methods (A and B) for determining the space charge density in the insulation (dual insulation) of dc cable joints. The method A is based on the material parameters (electrical conductivity and permittivity) and the method B on electroacoustic pulses. The values of the electrical conductivity for different temperatures and electric fields are computed using some experimental results and equations specific to each material. The equations used to compute the electrical conductivity, the electrical field and the superficial space charge densities at the interfaces between the joint's insulation layers are also presented. Finally, the values and the variations of the electrical conductivity with the electric field and temperature are presented and analyzed.
This paper deals with the influence of the nature and the electrical properties (conductivity and... more This paper deals with the influence of the nature and the electrical properties (conductivity and permittivity) of the DC joints insulations components on space charge accumulation and electric field repartition at the XLPE/EPR and XLPE/EPDM joints insulation interfaces. The results show that the conductivity and permittivity have lower values for XLPE than for EPDM and EPR samples. Also, the charge accumulated at the XLPE/EPDM interface has lower values (with two orders of magnitude) than those on XLPE/EPR samples. Concerning the electric field, its values are more than twice higher on XLPE/EPR samples than on XLPE/EPDM ones. Finally it is shown that the use for DC joint insulations manufacture of XLPE/EPDM is more benefic, than XLPE/EPR.
This paper presents a study concerning the dependence of power cables polyethylene's conducti... more This paper presents a study concerning the dependence of power cables polyethylene's conductivity on temperature and electric field. In the first part the experimental results obtained on flat samples of XLPE are presented and analyzed, for an electric field between 5 and 20 kV/mm and temperatures between 30 and 70 C. Next, the values of the conductivity are calculated using known empirical equations, and the results are compared with the experimental ones. Finally, a new equation is proposed for calculating the polyethylene’s conductivity with closer results to those obtained experimentally.
re and " e and dielectric loss factor tgδ for various electric field frequencies and at diff... more re and " e and dielectric loss factor tgδ for various electric field frequencies and at different temperatures. The results show that the dielectric properties are heavily influenced by the filler content with very high registered values in the case of low frequency electric fields and high temperatures. It can be ascertained that samples having a polypropylene matrix have superior electric properties than those having a polyethylene matrix. Finally, the opportunity for utilization within low voltage and industrial frequency electrical engineering applications of the tested composite materials is shown.
Degradation of polymeric insulation systems of electrical equipments increases the space charge d... more Degradation of polymeric insulation systems of electrical equipments increases the space charge density and the concentration of electrical dipoles. By consequence, the maximum values and the slopes of absorption/resorption (A/R) currents can change with insulation systems ageing. In this paper, an analysis of the nature of the A/R currents and the importance of their components, especially the polarization current and the current given by the space charge, is presented. The experimental study concerns the A/R currents measurements of plane samples (made from CALMICAGLAS tapes), virgin and thermally accelerated aged. The obtained results show that the ageing process produces an increase of the values and a decrease of shapes of the A/R currents. Finally, the possibility of estimating insulations ageing state and lifetime from A/R currents measurements is discussed. Keywords—Insulation Systems, Absorption/Resorption Currents, Ageing, Lifetime.
This paper deals with the thermal, electrical and combined thermo-electrical ageing of some XLPE ... more This paper deals with the thermal, electrical and combined thermo-electrical ageing of some XLPE and EPR flat samples. The samples were aged for different ageing times and electric fields. Before and after ageing, the electrical properties (e.g. electrical conductivity and permittivity) were measured. The physical modifications of the specimens were analyzed, also. Combined stresses (electrical and thermal) lead to significant variations for the specimens properties, compared to the variations caused by uni-factor stresses. For the electrical and thermo-electrical stresses, color changes of the specimens and thermal breakdown occur.
In this paper, the residual electric field computation in a cable insulation in the presence of w... more In this paper, the residual electric field computation in a cable insulation in the presence of water trees and space charge is presented. Considering that, in treed areas, the variations of the permittivity and the space charge density are known, the thermal step current Ic(t) is computed and the curves Im (t) and Ic(t) are compared. The results show an important increase of residual electric field for some dimensions and concentrations of the water trees (up to 10 MV/m). Finally, the influence of the residual electric field on electrically aged cable insulations is analyzed.
In this paper a study regarding the influence of water trees on the values and repartition of the... more In this paper a study regarding the influence of water trees on the values and repartition of the electric field in power cables insulations is presented. Experiments were carried out on flat samples under the circular crown shape, taken from some high voltage cables insulations. In these samples water trees were developed and thermal parameters, water content from treed areas, dimensions and water trees density were measured. Using the dimensions of water trees, water content in treed areas and thermal properties, the electric field computation was done. The results show that the electric field values are strongly modified due to the development of water trees.
Magnetic polymer composite materials have very good rheological (that are characteristic for poly... more Magnetic polymer composite materials have very good rheological (that are characteristic for polymers used as matrix) and magnetic properties superior to those of polymers (due to their magnetic filler) making them useful in many power applications (electromagnetic shielding, permanent magnets etc.). In this paper the results of an experimental study regarding the manufacture and characterization of some composites which have low density polyethylene (LDPE) as matrix and neodymium (Nd) and neodymium-iron-boron (Nd-Fe-B) as filler are presented. The manufacture process (with mass content between 0 and 15 %) and their structure (obtained by electronic and optical microscopy) are presented. It is shown that the samples are inhomogeneous and isotropic and the filler particles form clusters of variable dimensions and distances between them. The rheological behavior of composites was analyzed and more rapid melting was observed for composites compared to neat polyethylene Then, the first ...
Materials, methods & technologies, 2015
In this paper a study regarding the influence of Neodymium (Nd) powder content on the electric an... more In this paper a study regarding the influence of Neodymium (Nd) powder content on the electric and magnetic properties of low density polyethylene composites is presented. The distribution of the particles in the polymeric matrix was determined by Scanning Electron Microscopy (SEM) and optical microscopy (OM). A relative uniform dispersion of the particles in LDPE was observed. The variations of the electrical conductivity and real and imaginary complex permittivity parts with the electric field frequency, the temperature and the filler content were studied. Neodymium filler determines important increases of real ' er and imaginary " er permittivity but also the conductivity components of samples, and their magnetically properties i.e. remnant magnetization, maximum magnetization and magnetic susceptivity. The increase of temperature determines, generally, a decrease of the real part of permittivity but a frequency range for each sample when an increase of ' er with tem...
IEEE Transactions on Components, Packaging and Manufacturing Technology
A rapid method based on activation energy values for the lifetime assessment of PP films used as ... more A rapid method based on activation energy values for the lifetime assessment of PP films used as dielectrics for capacitors is proposed. The activation energy is determined from a nonisothermal measurement made by differential scanning calorimetry and an aging test at a single elevated temperature. The use of the onset temperature of the exothermal peak is proposed to evaluate the activation energy of the degradation of the PP film. Four types of PP film capacitors were thermally aged at 100 °C for different aging times. For each aging time, the capacitance was measured at different frequencies. According to the standards, two end-of-life criteria (2% and 5%) were imposed for the reduction in the capacitance. The parameters of the lifetime line <inline-formula> <tex-math notation="LaTeX">$a$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$b$ </tex-math></inline-formula> were determined, and finally, the lifetime values were calculated. The novel proposed method is effective in terms of both energy and manpower costs compared with the current method, which uses three aging temperatures.
Electrotehnica, Electronica, Automatica
This paper presents a study on the influence of the environmental factors (respectively, ambient ... more This paper presents a study on the influence of the environmental factors (respectively, ambient temperature and solar radiation) on hot spot temperatures and the consumed and remaining lifetimes of power transformers. In this regard, 15 identical 105 MVA distribution transformers are considered, loaded with the same load, and operating in 15 areas of Romania (with different climatic characteristics). Using the equations recommended by IEC 60076-7 (modified in the case of the presence of solar radiation) the values of oil temperature, hot spot temperatures and consumed and remaining lifetimes in two days from July 2020 are calculated, both in the presence and in the absence of solar radiation. It is shown that the values of the hot spot temperature and of the consumed lifetimes differ from one locality to another and that they increase with the increase of the ambient temperature and the intensities of the solar radiation. Finally, it is shown that the results obtained can be used t...
In this paper an experimental study regarding the structure and electrical properties of low dens... more In this paper an experimental study regarding the structure and electrical properties of low density polyethylene composites filled with neodymium particles is presented. The analysis of the samples structure shows that the neodymium particles appear as clusters and the distance between them decreases if the filler content increases. The DC measurements show that the electrical conductivity values increase with the temperature and filler content. Using the absorption currents, dielectric response functions of composites f(t) are determined and the complex permittivity values are calculated and compared with experimental values. Good correlation between calculated values and experimental ones was found for frequencies lower than 100 Hz. Variations of the complex permittivity components with the neodymium content, temperature and frequency are also presented.
Materials, methods & technologies, 2016
After an analysis of the structure and properties of the DC joints insulation components, the gen... more After an analysis of the structure and properties of the DC joints insulation components, the generation mechanisms and the measurements methods of the space charge in DC multilayer joints are presented. Then, the equations used to calculate the charge accumulated at the homogeneous interfaces of the joints and the electric field are given. To calculate the space charge density and the electric field, experiments on flat samples (XLPE and reductionEPDM) are performed and their dielectric properties are measured. Finally, it is shown that the charge accumulation leads to local enhancements of the electric field and to degradation and lifetime reduction of joints.
Przegląd Elektrotechniczny, 2009
In the present paper the results concerning the structural modifications and dielectric propertie... more In the present paper the results concerning the structural modifications and dielectric properties of polyethylene insulations taken from a medium voltage cable (20 kV) subjected to 120 thermal cycles between −35 °C and + 100 °C are presented. The structural modifications were investigated by thermal analysis methods (TG, DTG and DTA), showing that during the thermal cycling the degree of cross-linking of polyethylene increases and the degree of crystallinity decreases. Dielectric modifications of polyethylene due to thermal cycling were determined by using the dielectric spectroscopy method in range 10 <sup>−2</sup> ÷ 10<sup>6</sup> Hz, for 30 °C but also for 100 °C. After 120 thermal cycles the results obtained show that tgδ for 30 °C increases slowly for frequencies higher than 100 Hz and substantial for frequencies lower than 100 Hz. This increase is more pronounced for 100 °C (up to 100 %) especially for frequencies between 10<sup>−2</sup> ÷ ...
Part I of this paper presents a cylindrical two-layer insulation model and two methods (A and B) ... more Part I of this paper presents a cylindrical two-layer insulation model and two methods (A and B) for determining the space charge density in the insulation (dual insulation) of dc cable joints. The method A is based on the material parameters (electrical conductivity and permittivity) and the method B on electroacoustic pulses. The values of the electrical conductivity for different temperatures and electric fields are computed using some experimental results and equations specific to each material. The equations used to compute the electrical conductivity, the electrical field and the superficial space charge densities at the interfaces between the joint's insulation layers are also presented. Finally, the values and the variations of the electrical conductivity with the electric field and temperature are presented and analyzed.
This paper deals with the influence of the nature and the electrical properties (conductivity and... more This paper deals with the influence of the nature and the electrical properties (conductivity and permittivity) of the DC joints insulations components on space charge accumulation and electric field repartition at the XLPE/EPR and XLPE/EPDM joints insulation interfaces. The results show that the conductivity and permittivity have lower values for XLPE than for EPDM and EPR samples. Also, the charge accumulated at the XLPE/EPDM interface has lower values (with two orders of magnitude) than those on XLPE/EPR samples. Concerning the electric field, its values are more than twice higher on XLPE/EPR samples than on XLPE/EPDM ones. Finally it is shown that the use for DC joint insulations manufacture of XLPE/EPDM is more benefic, than XLPE/EPR.
This paper presents a study concerning the dependence of power cables polyethylene's conducti... more This paper presents a study concerning the dependence of power cables polyethylene's conductivity on temperature and electric field. In the first part the experimental results obtained on flat samples of XLPE are presented and analyzed, for an electric field between 5 and 20 kV/mm and temperatures between 30 and 70 C. Next, the values of the conductivity are calculated using known empirical equations, and the results are compared with the experimental ones. Finally, a new equation is proposed for calculating the polyethylene’s conductivity with closer results to those obtained experimentally.
re and " e and dielectric loss factor tgδ for various electric field frequencies and at diff... more re and " e and dielectric loss factor tgδ for various electric field frequencies and at different temperatures. The results show that the dielectric properties are heavily influenced by the filler content with very high registered values in the case of low frequency electric fields and high temperatures. It can be ascertained that samples having a polypropylene matrix have superior electric properties than those having a polyethylene matrix. Finally, the opportunity for utilization within low voltage and industrial frequency electrical engineering applications of the tested composite materials is shown.
Degradation of polymeric insulation systems of electrical equipments increases the space charge d... more Degradation of polymeric insulation systems of electrical equipments increases the space charge density and the concentration of electrical dipoles. By consequence, the maximum values and the slopes of absorption/resorption (A/R) currents can change with insulation systems ageing. In this paper, an analysis of the nature of the A/R currents and the importance of their components, especially the polarization current and the current given by the space charge, is presented. The experimental study concerns the A/R currents measurements of plane samples (made from CALMICAGLAS tapes), virgin and thermally accelerated aged. The obtained results show that the ageing process produces an increase of the values and a decrease of shapes of the A/R currents. Finally, the possibility of estimating insulations ageing state and lifetime from A/R currents measurements is discussed. Keywords—Insulation Systems, Absorption/Resorption Currents, Ageing, Lifetime.
This paper deals with the thermal, electrical and combined thermo-electrical ageing of some XLPE ... more This paper deals with the thermal, electrical and combined thermo-electrical ageing of some XLPE and EPR flat samples. The samples were aged for different ageing times and electric fields. Before and after ageing, the electrical properties (e.g. electrical conductivity and permittivity) were measured. The physical modifications of the specimens were analyzed, also. Combined stresses (electrical and thermal) lead to significant variations for the specimens properties, compared to the variations caused by uni-factor stresses. For the electrical and thermo-electrical stresses, color changes of the specimens and thermal breakdown occur.
In this paper, the residual electric field computation in a cable insulation in the presence of w... more In this paper, the residual electric field computation in a cable insulation in the presence of water trees and space charge is presented. Considering that, in treed areas, the variations of the permittivity and the space charge density are known, the thermal step current Ic(t) is computed and the curves Im (t) and Ic(t) are compared. The results show an important increase of residual electric field for some dimensions and concentrations of the water trees (up to 10 MV/m). Finally, the influence of the residual electric field on electrically aged cable insulations is analyzed.
In this paper a study regarding the influence of water trees on the values and repartition of the... more In this paper a study regarding the influence of water trees on the values and repartition of the electric field in power cables insulations is presented. Experiments were carried out on flat samples under the circular crown shape, taken from some high voltage cables insulations. In these samples water trees were developed and thermal parameters, water content from treed areas, dimensions and water trees density were measured. Using the dimensions of water trees, water content in treed areas and thermal properties, the electric field computation was done. The results show that the electric field values are strongly modified due to the development of water trees.
Magnetic polymer composite materials have very good rheological (that are characteristic for poly... more Magnetic polymer composite materials have very good rheological (that are characteristic for polymers used as matrix) and magnetic properties superior to those of polymers (due to their magnetic filler) making them useful in many power applications (electromagnetic shielding, permanent magnets etc.). In this paper the results of an experimental study regarding the manufacture and characterization of some composites which have low density polyethylene (LDPE) as matrix and neodymium (Nd) and neodymium-iron-boron (Nd-Fe-B) as filler are presented. The manufacture process (with mass content between 0 and 15 %) and their structure (obtained by electronic and optical microscopy) are presented. It is shown that the samples are inhomogeneous and isotropic and the filler particles form clusters of variable dimensions and distances between them. The rheological behavior of composites was analyzed and more rapid melting was observed for composites compared to neat polyethylene Then, the first ...
Materials, methods & technologies, 2015
In this paper a study regarding the influence of Neodymium (Nd) powder content on the electric an... more In this paper a study regarding the influence of Neodymium (Nd) powder content on the electric and magnetic properties of low density polyethylene composites is presented. The distribution of the particles in the polymeric matrix was determined by Scanning Electron Microscopy (SEM) and optical microscopy (OM). A relative uniform dispersion of the particles in LDPE was observed. The variations of the electrical conductivity and real and imaginary complex permittivity parts with the electric field frequency, the temperature and the filler content were studied. Neodymium filler determines important increases of real ' er and imaginary " er permittivity but also the conductivity components of samples, and their magnetically properties i.e. remnant magnetization, maximum magnetization and magnetic susceptivity. The increase of temperature determines, generally, a decrease of the real part of permittivity but a frequency range for each sample when an increase of ' er with tem...