system for cables
System has been developed to measure directly space charge profiles on
Cables . The system has been deviced for quality control and diagnostic
purposes. The main feature is the sensibility of the system, required to
achive consistent measurements on thick specimens. Used for
factory and on site (off line) measurements, the system detects quantities
related to water tree and insulation degradation processes, providing a
bulk diagnosis of the cable.
for space charge measurements on cables is realized according to the idea
reported in . As sketched in Fig. 2, it consists of a HVDC generator, a
pulse generator and the measurement system (amplifier and piezoelectric
sensor). A voltage pulse is applied between outer semicon and ground,
unlike the conventional PEA method for flat specimens. A decoupling
capacitor is not needed because the cable itself is used for this purpose.
cell for cables.
system for enamelled wires
The measurement of space charges through the
PEA system developed for this application constitutes a new useful tool to
evaluate and compare the electrical properties of the enameled wires used
for motor windings. The comparison of different materials through space
charge measurements shows that significant
differences regarding space charge trapping properties can be introduced
modifying enameled wire insulation.
This PEA System has been developed to measure directly
space charge profiles on enameled wire insulation. The main feature is the
high spatial resolution necessary for specimens with thickness between 15
and 50 μm.
scheme of the new PEA system developed with the purpose to measure space
charge accumulation on enameled wires is reported in Figs. 3 and 4 . This
system is realized starting from the PEA version for power cable described
in the previous section .
circuit diagram is reported in Fig. 3. A front
view of the test cell realized is shown in Fig. 4. The enameled wire under
test can be correctly positioned on the aluminum ground plate through the
central fixing screw on the support structure (note that the dimensions
are not in scale).
main difference with respect to the other PEA systems developed in the
past is the specimen to be tested. Till now systems have been developed
for flat specimens or thick power cables. The PEA measurement on enameled
wires presents specific features and difficulties. Due to the small size
of the wire (diameter of about 1 mm) the contact with the aluminum ground
plate may be unsatisfactory. Moreover, the small thickness of the enameled
insulation (30-40 mm)
requires the use of very short pulse and very thin transducer, in order to
achieve a good spatial resolution needed for such thin insulation.
Fig. 3 Circuit diagram of the PEA measuring cell
Fig.4: Front view of the PEA measuring cell
Fig. 6: Charge pattern windows. The color scale is represented on the right side
of the window. For this example the maximum positive value is 50 C/m3 and
the maximum negative value is -50 C/m3.
Quantities for diagnostic evaluations can be
derived from space charge profiles under field and in depolarisation, such
as space charge accumulation threshold characteristic (Fig. 7),
depolarisation characteristic (Fig. 8), apparent trap-controlled mobility
and trap depth distribution characteristics (Figs. 9-10) [9-11].
Fig 7: Space charge accumulation threshold characteristic. The
threshold field, ET, and the rate of space charge accumulation,
b, are indicated.
Fig. 8: Example of depolarisation characteristic. Material LDPE
after poling at 60 kV/mm
Fig. 9: Apparent mobility plot obtained from depolarisation
characteristic of Fig. 8.
Fig. 10: Trap depth distribution obtained from apparent
mobility of Fig. 9.
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space charge measurements and accelerated life tests", IEEE Trans. on
Dielectrics and Electrical Insulation, Vol. 7, no. 3, pp. 322-328, June
Montanari, "Extraction of information from space charge measurements and
correlation with insulation ageing", invited paper, IEEE CSC, pp. 178-184,
Tours, France, July 2001.
Cavallini, D. Fabiani, G. Mazzanti, G.C. Montanari, "A general model for
life estimation of cables under dc stress with voltage-polarity inversions
accounting for space-charge effects", IEEE ISEIM, pp. 449-452, Himeji,
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Cavallini, D. Fabiani, G. Mazzanti, G.C. Montanari, "Life model based on
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voltage-polarity inversions", IEEE Trans. on Dielectrics and Electrical
Insulation, Vol. 9, no. 4, pp. 514-523, August 2002.
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electroacoustic method", IEEE Trans. on Dielectrics and Electrical
Insulation, Vol. 5, no. 3, pp. 433-439, June 1988.
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the effect of additives for high-voltage polymeric cables through space
charge measurements on cables and films", IEEE CEIDP, pp. 456-459,
Kitchener, Canada, October 2001.
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the estimation of apparent mobility and trap depth as markers for
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trap-controlled mobility evaluation in insulating polymers through
depolarisation characteristics derived by space charge measurements",
Journal of Applied Physics, 2003.
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