Space Charge Measurements

The increasing request for energy quality and, thus, insulation system reliability, associated with significant steps in the manufacturing of new insulating materials, are pushing strongly the research on new techniques for the diagnosis of insulation ageing.
Space charge observation is becoming the most widely used technique to evaluate polymeric materials for dc-insulation applications, particularly high-voltage cables.
It is already well assessed in fact,  that the presence of space charges is the main problem causing premature failure of high-voltage dc polymeric cables and, indeed, the main reason preventing from a rapid diffusion of such kind of cables [1-4]. Moreover, it has been shown that insulation degradation under service stresses can be diagnosed by space charge measurements. However, quantities are still lacking that can help to summarize and interpret the huge amount of data resulting from space charge measurements, and that are also associated with the electrical performance of the insulation.
The pulsed electroacoustic analysis (PEA) can be used for space charge measurements under dc or ac fields.
The PEA method is a non-destructive technique for profiling space charge accumulation in polymeric materials. The method was first proposed by T. Takada et al. in 1985 [5].
A series of high-voltage pulses of very short time length is applied to an insulation specimen interposed between two electrodes. Each pulse produces an electric force displacing internal charges and generating  pulsed acoustic pressure waves in correspondence of each charge layer in excess with respect to neutrality. The resultant pressure pulse is detected by a piezoelectric transducer, so that the charge distribution in the specimen under test can be obtained from the output voltage of the transducer. The analysis of space-charge profiles is restricted to one dimension.

LIMAT can perform PEA measurements on:

  • flat specimens of insulating material

  • high voltage cables

  • enameled wires for machine windings (innovative system compoletely developed at LIMAT)

PEA measurement system for flat specimens

The most simple PEA system is able to detect the inner space charge in a polymeric flat specimen with thickness up to 0.5 mm.

A complete PEA system (Fig. 1) is composed by:

  • a high voltage generator (AC or DC)

  • a voltage pulse generator

  • the PEA cell

  • a digital oscilloscope

  • a personal computer

  • a GPIB card to interface computer and oscilloscope.

 

Figure 1 shows more in detail the components of the PEA cell (note, in particularly, the presence of a decoupling capacitor, C, between high voltage supply and pulse generator circuits).

The resistor, R, is used in order to reduce the discharging current in the case of specimen breakdown.

A film of semiconductive material is generally interposed between the upper brass electrode and the specimen, in order to reduce the difference of sonic impedance between two different materials and to allow a good propagation of the pressure wave generated by voltage pulse.

The specimen is fed by an high-voltage power supply. A voltage pulse, with an amplitude of 500 V, 10 ns width, is applied to the specimen through a coupling capacitor. The pressure wave propagating through the insulation down to the lower aluminum ground electrode reaches the piezoelectric transducer. The piezoelectric transducer (PVDF) generates a voltage signal (PEA output signal) proportional to the pressure wave propagating through it. The weak voltage signal generated by the PVDF is then amplified by two large-band amplifiers, is sent to an oscilloscope and recorded by a personal computer, connected with the oscilloscope through an IEEE-488 bus. A PMMA adsorber is located under the piezoelectric transducer in order to avoid reflections which can disturb the PEA output signal.

Fig.1: PEA cell scheme for flat specimens.

 

PEA measurement system for cables

This PEA System has been developed to measure directly space charge profiles on Cables [6]. 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.

The system for space charge measurements on cables is realized according to the idea reported in [7]. 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.

 

   

Fig.2: PEA cell for cables.

 

PEA measurement 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.

A 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 [8]. This system is realized starting from the PEA version for power cable described in the previous section [6].  

The 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). 

The 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

 

Exemples of PEA measurement results

Indipendently of the measurement system used (flat specimen, calbes, etc.), one of the most important PEA results consists of profiles of space charge accumulated in the insulation bulk, from which the inner electric field distribution is obtaineable. 
The figure 5 shows examples of space charge accumulation profiles for an enamelled wire during poling (Fig. 5A) and after voltage removal (Fig. 5B). After 20 s from the voltage application (Fig. 5A) we see a flat zone between the two peaks representing the electrodes, which means that injected charges have not reached yet the insulation bulk. One hour after, a large amount of negative charge, injected by the electrode, accumulates in the insulation bulk, which causes the apparent shifting of the negative electrode peak visible in Fig. 5A, as well as the increase of anode charge. This is, again, confirmed by the profile under volt-off reported in Fig. 5B, which shows clearly the presence of bulk negative space charge, which decreses as the time elapses.

Fig. 5: Space charge profiles at two different poling times (A) and space charge profiles at two depolarization times (B). Enameled wire insulation. 

Another important information that can be carried out from PEA measurement is the colored pattern relevant to the spatio-temporal evolution of the net charge during polarization and depolarization at different fields (Fig. 6). Each representation has three dimensions: thickness, time and charge. Thickness is the Y axis (cathode and anode are indicated), time is the X axis (non linear scale), and the 3rd dimension is charge, represented by a color scale. All the values exceeding the maximum level, positive or negative, are associated with the color white. This can happen, in particular, at the electrodes, where the applied voltage leads to highest values of charges (capacitive).

Charge packets during polarisation and trapped charge decay during depolarisation are observable in Fig. 6.

  

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.

 

References 

[1] G.C. Montanari, D. Fabiani, "Evaluation of dc insulation performance based on space charge measurements and accelerated life tests",  IEEE Trans. on Dielectrics and Electrical Insulation, Vol. 7, no. 3, pp. 322-328, June 2000.

[2] G.C. Montanari, "Extraction of information from space charge measurements and correlation with insulation ageing", invited paper, IEEE CSC, pp. 178-184, Tours, France, July 2001.

[3] A. 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, Japan, November 2001.

[4] A. Cavallini, D. Fabiani, G. Mazzanti, G.C. Montanari, "Life model based on space-charge quantities for HVDC polymeric cables subjected to voltage-polarity inversions", IEEE Trans. on Dielectrics and Electrical Insulation, Vol. 9, no. 4, pp. 514-523, August 2002.

[5] T. Maeno, T. Futami, H. Kusibe, T. Takada and C. M. Cooke, "Measurement of spatial charge distribution in thick dielectrics using the pulsed electroacoustic method", IEEE Trans. on Dielectrics and Electrical Insulation, Vol. 5, no. 3, pp. 433-439, June 1988.

[6] G. Perego, F. Peruzzotti, G.C. Montanari, F. Palmieri, G. De Robertis, "Investigating 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.

[7] N. Hozumi, T. Takeda, H. Suzuki, T. Okamoto, “Space charge behavior in XLPE cable insulation under 0.2-1.2 MV/cm dc fields”, IEEE Trans. on DEI, Vol. 5, no. 1, pp. 82-90, February 1998.                                             

[8] D. Fabiani, G.C. Montanari, F. Palmieri, "Space charge measurements on enameled wires for electrical machine windings", CWIEME 2001, pp.110-115, Berlin, Germany, June 2001.

[9] G. Mazzanti, G.C. Montanari, J.M. Alison, "A space-charge based method for the estimation of apparent mobility and trap depth as markers for insulation degradation. Theoretical basis and experimental validation", IEEE Trans. on Dielectrics and Electrical Insulation, Vol. 10, no. 2, pp. 187-197, April 2003.

[10] G. Mazzanti, G.C. Montanari, F. Palmieri, J. Alison, "Apparent trap-controlled mobility evaluation in insulating polymers through depolarisation characteristics derived by space charge measurements", Journal of Applied Physics, 2003.

[11] G. Mazzanti, G.C. Montanari, F. Palmieri, "Quantities extracted from space-charge measurements as markers for insulation ageing", IEEE Trans. on Dielectrics and Electrical Insulation, Vol. 10, no. 2, pp. 198-203, Aprile 2003.