- Email: [email protected]
Space charge measurements by the Thermal Step Method : results in some polymers
Journal of
ELECTROSTATICS ELSEVIER
Journal of Electrostatics 40&41 (1997) 283-288
Space charge measurements by the Thermal Step Method : results in some polymers S. MALRIEU, J. CASTELLON Laboratoire d'Electrotechnique de Montpellier Universit6 Montpellier II (Sciences et Techniques du Languedoc) Place Eugene Bataillon, Case Courrier 079 34095 MONTPELLIER CEDEX 5 FRANCE 1. I N T R O D U C T I O N Properties of several materials submitted to high electric fields are studied in this paper using the Thermal Step Method (TSM). The aim is to prove that certain materials present space charges since the process of manufacturing and to test their electrical state after poling. The Thermal Step Method, which makes it possible to find the charge density in solid dielectrics, has already been used on polymers, such as polyethylenes and PVC. In this study, we used it for the first time to characterise new materials having interesting properties for industrial use, such as polyetherimides (PEI) (High temperature polymers) and epoxyde systems filled with alumina, silica or Wollastonite. Space charge distributions for three different high temperature PEI's and for three filled epoxyde systems, before and after application of stresses, are presented in this paper. No other space charge measurement method had been used to test epoxyde systems before. The results allow us to optimise the insulating structures for a particular application, and show fundamental differences between the materials. The measurements also revealed the presence of space charge from the very manufacturing process of these materials, as well as the possibility for all these systems to accumulate space charge. 2. T H E T H E R M A L
STEP METHOD
2.1. Principle The Thermal Step Method was set up by A. TOUREILLE in 1987 [1]. It allows the determination of the remnant electric field and of the distribution of the remnant charge density in an insulating sample. The method is based on the measurement and the digital processing of the current resulting from the variation of the electrode image charges when a temperature wave (step) crosses the sample. The measurement of the current is made on a hydraulic bench. A negative temperature step is created by the circulation of a cold liquid (glycol at -10°C) in a radiator adjoined to the sample's geometry (flat shaped, cylindrical, cone shaped...). In figure 1 we present the 0304-3886/97/$17.00 © Elsevier Science B.V. All rights reserved. PI1 S0304-3886(97)0005 I-X
284
S. Malrieu, J Castellon/Journal of Electrostatics 40&41 (1997) 283-288
diagram of a bench used to measure the space charge distribution in flat samples. The principle is similar for cylindrical (cable-type) or conical samples. 2.2. T h e o r e t i c a l a s p e c t s o f the m e t h o d
Let us consider an insulating plate of thickness d and of area S, placed between two electrodes of abscissas x-=0 and x=d. A charge Qi to the abscissa xi induces on the electrodes (by total influence) two image charges Qil and Qi2 such that : Qil=-(D-x)Qi/d and Qi2=-x Qi/d
O)
The electrodes are in short-circuit through a low impedance measuring device. T*C
X4 Figure I. Hydraulic bench for space charge measuremeat on flat samples
Z=]l Z-d
Figure 2. Diagram of the principle
The system is balanced. If a thermal step is now applied on the electrode of abscissa x=0, the diffusion of heat in the sample causes a dilatation or a contraction of the material (depending on the sign of the thermal step) and a variation of its permittivity (fig 2). The small displacement of the charge Qi and the variation of the permittiv/ty modifies the balance of the influence charges on the electrodes, and therefore leads to the appearance of a measurable current in the external circuit. We can extend this principle to a charge density p(x) in the insulator. We can consider that the following hypotheses are correct : - The charge density is homogeneous in the directions parallel to the surface area. In this condition, p depends only on the abscissa x, The Poisson equation gives : p(x)=sdE(x)/dx (2) where E(x) is the distribution of the electric field• A. TOUREILLE [2] proved that the current measured in the external circuit is of the form : tD fT(x,t) 1 8C I(t) -- -otCj E(x) ~ d x with
I
3.RESULTS OF PEI The PEI 1 is a high performanc~ thermoplastic material. The combination of high temperatures resistance, low moisture absorption and excellent dielectric properties makes this PEI a superior choice for a broad range of electrical applications.
285
S. Malrieu, ,1.. Castellon/Journal o f Electrostatics 40&41 (1997) 283-288
The PEI 2 offers a very good resistance to chemical products and a relatively low smoke generation on exposure to fire. It is well suited to applications requiring excellent loadbearing capabilities in a continuously high temperature environment. The PEI 3 is composed of 30% glass which gives very good mechanical qualities, and it equally resists to high temperatures.
3.1. Preliminary measures M1 the samples are passed by the assessment bench to verify if their remnant charges, after fabrication, are measurable. These measures allow the evaluation of the various effects of the manufacturing process on the insulators. The three PEI's we studied showed very low remnant charge densities (below 1 mC/mS).
3.2. The poling of the samples In the following, we will call 'poling' the submission of a sample to high voltage. Each sample was placed in a stream room at 125°C and submitted to a continuous voltage of-35kV during 1 hour, 20 hours and 60 hours. Before characterisation by the thermal step method, the test pieces were placed in short circuit for 30 minutes in order to eliminate an eventual conduction current which could have disturbed the measure.
3.3. Measurements on PEI Figure 4 represents the remnant charge density in PEI 1 after 1, 20 and 60 hours of poling. We can note that this material has the tendency to charge progressively. The charge density still remains weak. 0,14
0,035 0,030 -
Clm3
1h .
.
.
.
0,12
C/mS
,:
0,025 -
0,10
0,020 •
0,06
0,015 -
0,06
0,010 -
0,04
0,005
0,02
0,000
0,1~
m
i!
.
.
.
l .
.
.
.
.
h
20h 8Oh
mm -
.o,oos t~ /
-O,OLO
-o,04
-o,o15
-0,06
Figure.4.Space charge density distributions in PEI 1
Figure.5. Space charge density distributions in PEI 2
,,,
Figure 5 shows the remnant charge density in PEI 2 after 1, 20 and 60 hours of poling. It can be seen that this material rapidly charges and reaches a maximum after 20 hours. A part of the charge is evacuated after 60 hours of poling. Nevertheless, its charge density remains high.
286
S. Malrieu, J. Castellon/Journal of Electrostatics 40&41 (1997) 283-288
Figure 6 represents the remnant charge in PEI 3 after 1, 20 and 60 hours of poling. The material charges very quickly and reaches its maximum after only 1 hour of poling, before evacuating a part of its charges. Its rate of charging remains weak.
:1.2_Ic,., '°t
A n
1, .......
0,00 ~
-0,02
1
°"°/c,m3
I
1
5
0 , 0 0 ~
_o,o
"
- 0 0 4 /
Figure.6. Space charge density distributions in PEI 3
.......
mm
:i:i ,
.
~-/.~//:'. k . . ~ - - - - - - ~
mmj
~
Figure.7. Charge density distribution in the three PErs after 60 hours of poling
Figure 7 represents the remnant charge in the three PEI after 60 hours of poling. We can deduce that PEI 1 and 3 have similar remnant charge levels after 60 hours (weaker than PEI 2).
3.5. Analysis of the results The weak capacity ofPEI 1 and 2 to accumulate charges makes them interesting materials, especially for use under strong electric fields and even under high temperatures (125°C). Moreover, destructive breakdown measures proved their good breakdown field (70 kV/mm for PEI 1 and 67 kV/mm for PEI 2). These materials are therefore adapted to heavy duty (strong electric fields, high temperature and even chemical stress). However, they do not support high mechanical stress. The distribution of the space charge in PEI 3 shows its capacity to evacuate the charge. This quality is due mainly to the strong proportion of glass (30%) which PEI 3 contains. Some measures ofelectrieal conductivity made at 125°C confirmed this hypothesis ((~ = 1.1 10-12~ -1,m-1). The measure of dielectric rigidity of PEI 3 reveals a lower breakdown field than the ones ofPEI 1 and 2 (46 kV/mm compared with 67 and 70 kV/mm). This places us in a difficult situation, between a risk of thermal breakdown and a risk of electronic breakdown [4]. The proportion of glass in the material increases the risk of percolation. It seems that a decrease of the glass ratio in the material could improve its electrical properties. 4. R E S U L T S O N F I L L E D E P O X Y D E S Y S T E M S
Note : Between the manufacturing and the experiments, the samples passed several month in non=controlled atmosphere.
287
S. Malrieu, J Castellon/Journal of Electrostatics 40&41 (1997) 283-288
4.1. Study Before Poling This study makes it possible to obtain information on the presence of space charges from the very manufacturing. Just after their preparation (deposit of 2 electrodes in vacuum), the samples were measured on the thermal step bench. In spite of the low and very noisy signals, we were able to detect a measurable electric field (consequently, greater than 10 V/mm) and a measurable remnant space charge density (greater than 1 mC/m 3) in the 3 tested samples
4.2. Study After Submission to Stress The samples were submitted to a continuous voltage of 10 kV (Anode to plus) during 24 hours, in a stream room at 70°C (this gives an applied electric field of 2.5 kV/mm). Filled epoxyde systems samples :-System filled with Silica GPD1. -System filled with Alumina GPE1. -System filled with Wollastonite GPF1. -measures before poling (fig. 8, 9). -measures after poling (samples short-circuited 30' before measure) (fig. 10, 11).
°
f
½
~4
I
-12 ~
nG~l
I
figure 8. Remnant electric field before poling figure 10. Remnant electric field after stress application
figure 9. Space charges distribution before poling
figure 11. Space charges distribution after stress application
288
S. Malrieu, J Castellon/Journal of Electrostatics 40&41 (1997) 283-288
4.3. Discussion of the Results The results obtained before poling (without an applied electric field) revealed the presence of space charges since the process of manufacturing. In spite of the problems caused by the noise in the measured signals, the magnitude level was respected (fig. 8, 9). The charges appear at the same place both before and after poling, but the amplitudes are significantly increased by the stresses (fig. 9, 11). For the results obtained under no-load conditions(fig. 8), we can notice the remnant electric field circulation is almost nil. From figure 10 (which presents the electric field dislxibution after submission of samples to stresses), we can notice the appearance of a dominant phenomenon of polarization in most of the systems, excepting the GPE1 system (which presents a very low local phenomenon of injection). The space charge level of systems GPD1 et GPF1 are the highest On the contrary, the GPE1 system shows the weakest level of space charge (0.07 C/m 3 maximum). The results we presented reveal fundamental differences between initially comparable materials and give information on the presence of space charges from the very manufacturing process [5]. The most reliable system is the GPEI one (filled with alumina), because it possesses the weakest space charge level. This technique allows us to make the next classification, using the criteria of the space charge level : the GPE1 is the best, followed by the GPF1 and the GPD1. 5. C O N C L U S I O N The Thermal Step Method applied to polyethefimide and filled epoxyde systems samples allowed us to reveal their tendency to accumulate space charge when submitted to strong electric fields witch could lead to electric breakdown [6]. A classification of these materials has since been established thanks to this characterisation technique. 6. R E F E R E N C E S
[I] A.Toureille "Sur une mdthode de ddterminafion de la densitd de charges d'espaee dans le PE", JICABLE 87,pp89-I03, September 1987 [2] A. Toureille,J-P Reboul, P. Merle, "D6termination des densitds de charge d'espace dans les isolantssolidespar la mdthode de l'ondethermique", J. Phys HI I, (1991), I 11-123 [3] A ToureilIe, "Mesures de charges d'espace par la mdthode de l'onde thermiquc: diffdrentestechniques de validationnumdfique et expdrimentales",Journal of Electrostatics, 32 (1994) 277-286. [4] R Coelho and B Aladenizc, "Lcs didlectriques"cd Hermes, table 6-13 i)208. [5] N. Vella, A. Toureille,U. Nilsson, "Study of the origin of space charge created during preparationof polyethylene plaques",Jicable 95. [6] N. Vella, A. Tourcille,N. Zebouchi, T.G. Hoang, "Precursory phenomenon of electric breakdown in polyethylene",REE Power cables and insulatingmaterials science,pp102-106, August 96.
ELECTROSTATICS ELSEVIER
Journal of Electrostatics 40&41 (1997) 283-288
Space charge measurements by the Thermal Step Method : results in some polymers S. MALRIEU, J. CASTELLON Laboratoire d'Electrotechnique de Montpellier Universit6 Montpellier II (Sciences et Techniques du Languedoc) Place Eugene Bataillon, Case Courrier 079 34095 MONTPELLIER CEDEX 5 FRANCE 1. I N T R O D U C T I O N Properties of several materials submitted to high electric fields are studied in this paper using the Thermal Step Method (TSM). The aim is to prove that certain materials present space charges since the process of manufacturing and to test their electrical state after poling. The Thermal Step Method, which makes it possible to find the charge density in solid dielectrics, has already been used on polymers, such as polyethylenes and PVC. In this study, we used it for the first time to characterise new materials having interesting properties for industrial use, such as polyetherimides (PEI) (High temperature polymers) and epoxyde systems filled with alumina, silica or Wollastonite. Space charge distributions for three different high temperature PEI's and for three filled epoxyde systems, before and after application of stresses, are presented in this paper. No other space charge measurement method had been used to test epoxyde systems before. The results allow us to optimise the insulating structures for a particular application, and show fundamental differences between the materials. The measurements also revealed the presence of space charge from the very manufacturing process of these materials, as well as the possibility for all these systems to accumulate space charge. 2. T H E T H E R M A L
STEP METHOD
2.1. Principle The Thermal Step Method was set up by A. TOUREILLE in 1987 [1]. It allows the determination of the remnant electric field and of the distribution of the remnant charge density in an insulating sample. The method is based on the measurement and the digital processing of the current resulting from the variation of the electrode image charges when a temperature wave (step) crosses the sample. The measurement of the current is made on a hydraulic bench. A negative temperature step is created by the circulation of a cold liquid (glycol at -10°C) in a radiator adjoined to the sample's geometry (flat shaped, cylindrical, cone shaped...). In figure 1 we present the 0304-3886/97/$17.00 © Elsevier Science B.V. All rights reserved. PI1 S0304-3886(97)0005 I-X
284
S. Malrieu, J Castellon/Journal of Electrostatics 40&41 (1997) 283-288
diagram of a bench used to measure the space charge distribution in flat samples. The principle is similar for cylindrical (cable-type) or conical samples. 2.2. T h e o r e t i c a l a s p e c t s o f the m e t h o d
Let us consider an insulating plate of thickness d and of area S, placed between two electrodes of abscissas x-=0 and x=d. A charge Qi to the abscissa xi induces on the electrodes (by total influence) two image charges Qil and Qi2 such that : Qil=-(D-x)Qi/d and Qi2=-x Qi/d
O)
The electrodes are in short-circuit through a low impedance measuring device. T*C
X4 Figure I. Hydraulic bench for space charge measuremeat on flat samples
Z=]l Z-d
Figure 2. Diagram of the principle
The system is balanced. If a thermal step is now applied on the electrode of abscissa x=0, the diffusion of heat in the sample causes a dilatation or a contraction of the material (depending on the sign of the thermal step) and a variation of its permittivity (fig 2). The small displacement of the charge Qi and the variation of the permittiv/ty modifies the balance of the influence charges on the electrodes, and therefore leads to the appearance of a measurable current in the external circuit. We can extend this principle to a charge density p(x) in the insulator. We can consider that the following hypotheses are correct : - The charge density is homogeneous in the directions parallel to the surface area. In this condition, p depends only on the abscissa x, The Poisson equation gives : p(x)=sdE(x)/dx (2) where E(x) is the distribution of the electric field• A. TOUREILLE [2] proved that the current measured in the external circuit is of the form : tD fT(x,t) 1 8C I(t) -- -otCj E(x) ~ d x with
I
3.RESULTS OF PEI The PEI 1 is a high performanc~ thermoplastic material. The combination of high temperatures resistance, low moisture absorption and excellent dielectric properties makes this PEI a superior choice for a broad range of electrical applications.
285
S. Malrieu, ,1.. Castellon/Journal o f Electrostatics 40&41 (1997) 283-288
The PEI 2 offers a very good resistance to chemical products and a relatively low smoke generation on exposure to fire. It is well suited to applications requiring excellent loadbearing capabilities in a continuously high temperature environment. The PEI 3 is composed of 30% glass which gives very good mechanical qualities, and it equally resists to high temperatures.
3.1. Preliminary measures M1 the samples are passed by the assessment bench to verify if their remnant charges, after fabrication, are measurable. These measures allow the evaluation of the various effects of the manufacturing process on the insulators. The three PEI's we studied showed very low remnant charge densities (below 1 mC/mS).
3.2. The poling of the samples In the following, we will call 'poling' the submission of a sample to high voltage. Each sample was placed in a stream room at 125°C and submitted to a continuous voltage of-35kV during 1 hour, 20 hours and 60 hours. Before characterisation by the thermal step method, the test pieces were placed in short circuit for 30 minutes in order to eliminate an eventual conduction current which could have disturbed the measure.
3.3. Measurements on PEI Figure 4 represents the remnant charge density in PEI 1 after 1, 20 and 60 hours of poling. We can note that this material has the tendency to charge progressively. The charge density still remains weak. 0,14
0,035 0,030 -
Clm3
1h .
.
.
.
0,12
C/mS
,:
0,025 -
0,10
0,020 •
0,06
0,015 -
0,06
0,010 -
0,04
0,005
0,02
0,000
0,1~
m
i!
.
.
.
l .
.
.
.
.
h
20h 8Oh
mm -
.o,oos t~ /
-O,OLO
-o,04
-o,o15
-0,06
Figure.4.Space charge density distributions in PEI 1
Figure.5. Space charge density distributions in PEI 2
,,,
Figure 5 shows the remnant charge density in PEI 2 after 1, 20 and 60 hours of poling. It can be seen that this material rapidly charges and reaches a maximum after 20 hours. A part of the charge is evacuated after 60 hours of poling. Nevertheless, its charge density remains high.
286
S. Malrieu, J. Castellon/Journal of Electrostatics 40&41 (1997) 283-288
Figure 6 represents the remnant charge in PEI 3 after 1, 20 and 60 hours of poling. The material charges very quickly and reaches its maximum after only 1 hour of poling, before evacuating a part of its charges. Its rate of charging remains weak.
:1.2_Ic,., '°t
A n
1, .......
0,00 ~
-0,02
1
°"°/c,m3
I
1
5
0 , 0 0 ~
_o,o
"
- 0 0 4 /
Figure.6. Space charge density distributions in PEI 3
.......
mm
:i:i ,
.
~-/.~//:'. k . . ~ - - - - - - ~
mmj
~
Figure.7. Charge density distribution in the three PErs after 60 hours of poling
Figure 7 represents the remnant charge in the three PEI after 60 hours of poling. We can deduce that PEI 1 and 3 have similar remnant charge levels after 60 hours (weaker than PEI 2).
3.5. Analysis of the results The weak capacity ofPEI 1 and 2 to accumulate charges makes them interesting materials, especially for use under strong electric fields and even under high temperatures (125°C). Moreover, destructive breakdown measures proved their good breakdown field (70 kV/mm for PEI 1 and 67 kV/mm for PEI 2). These materials are therefore adapted to heavy duty (strong electric fields, high temperature and even chemical stress). However, they do not support high mechanical stress. The distribution of the space charge in PEI 3 shows its capacity to evacuate the charge. This quality is due mainly to the strong proportion of glass (30%) which PEI 3 contains. Some measures ofelectrieal conductivity made at 125°C confirmed this hypothesis ((~ = 1.1 10-12~ -1,m-1). The measure of dielectric rigidity of PEI 3 reveals a lower breakdown field than the ones ofPEI 1 and 2 (46 kV/mm compared with 67 and 70 kV/mm). This places us in a difficult situation, between a risk of thermal breakdown and a risk of electronic breakdown [4]. The proportion of glass in the material increases the risk of percolation. It seems that a decrease of the glass ratio in the material could improve its electrical properties. 4. R E S U L T S O N F I L L E D E P O X Y D E S Y S T E M S
Note : Between the manufacturing and the experiments, the samples passed several month in non=controlled atmosphere.
287
S. Malrieu, J Castellon/Journal of Electrostatics 40&41 (1997) 283-288
4.1. Study Before Poling This study makes it possible to obtain information on the presence of space charges from the very manufacturing. Just after their preparation (deposit of 2 electrodes in vacuum), the samples were measured on the thermal step bench. In spite of the low and very noisy signals, we were able to detect a measurable electric field (consequently, greater than 10 V/mm) and a measurable remnant space charge density (greater than 1 mC/m 3) in the 3 tested samples
4.2. Study After Submission to Stress The samples were submitted to a continuous voltage of 10 kV (Anode to plus) during 24 hours, in a stream room at 70°C (this gives an applied electric field of 2.5 kV/mm). Filled epoxyde systems samples :-System filled with Silica GPD1. -System filled with Alumina GPE1. -System filled with Wollastonite GPF1. -measures before poling (fig. 8, 9). -measures after poling (samples short-circuited 30' before measure) (fig. 10, 11).
°
f
½
~4
I
-12 ~
nG~l
I
figure 8. Remnant electric field before poling figure 10. Remnant electric field after stress application
figure 9. Space charges distribution before poling
figure 11. Space charges distribution after stress application
288
S. Malrieu, J Castellon/Journal of Electrostatics 40&41 (1997) 283-288
4.3. Discussion of the Results The results obtained before poling (without an applied electric field) revealed the presence of space charges since the process of manufacturing. In spite of the problems caused by the noise in the measured signals, the magnitude level was respected (fig. 8, 9). The charges appear at the same place both before and after poling, but the amplitudes are significantly increased by the stresses (fig. 9, 11). For the results obtained under no-load conditions(fig. 8), we can notice the remnant electric field circulation is almost nil. From figure 10 (which presents the electric field dislxibution after submission of samples to stresses), we can notice the appearance of a dominant phenomenon of polarization in most of the systems, excepting the GPE1 system (which presents a very low local phenomenon of injection). The space charge level of systems GPD1 et GPF1 are the highest On the contrary, the GPE1 system shows the weakest level of space charge (0.07 C/m 3 maximum). The results we presented reveal fundamental differences between initially comparable materials and give information on the presence of space charges from the very manufacturing process [5]. The most reliable system is the GPEI one (filled with alumina), because it possesses the weakest space charge level. This technique allows us to make the next classification, using the criteria of the space charge level : the GPE1 is the best, followed by the GPF1 and the GPD1. 5. C O N C L U S I O N The Thermal Step Method applied to polyethefimide and filled epoxyde systems samples allowed us to reveal their tendency to accumulate space charge when submitted to strong electric fields witch could lead to electric breakdown [6]. A classification of these materials has since been established thanks to this characterisation technique. 6. R E F E R E N C E S
[I] A.Toureille "Sur une mdthode de ddterminafion de la densitd de charges d'espaee dans le PE", JICABLE 87,pp89-I03, September 1987 [2] A. Toureille,J-P Reboul, P. Merle, "D6termination des densitds de charge d'espace dans les isolantssolidespar la mdthode de l'ondethermique", J. Phys HI I, (1991), I 11-123 [3] A ToureilIe, "Mesures de charges d'espace par la mdthode de l'onde thermiquc: diffdrentestechniques de validationnumdfique et expdrimentales",Journal of Electrostatics, 32 (1994) 277-286. [4] R Coelho and B Aladenizc, "Lcs didlectriques"cd Hermes, table 6-13 i)208. [5] N. Vella, A. Toureille,U. Nilsson, "Study of the origin of space charge created during preparationof polyethylene plaques",Jicable 95. [6] N. Vella, A. Tourcille,N. Zebouchi, T.G. Hoang, "Precursory phenomenon of electric breakdown in polyethylene",REE Power cables and insulatingmaterials science,pp102-106, August 96.