Studies on Polypropylene Laminated Paper (PPLP) for

High Tc Superconducting Power Cables S. Thadela1, Ipsita Das1, B. Nageshwar Rao2, J Sundara Rajan2 and V. V. Rao1

1Applied Superconductivity Laboratory, Cryogenic Engineering Centre, IIT, Kharagpur, West Bengal - 721302, INDIA2Central Power Research Institute, Bangalore - 560080, India

9-P2-159

High Tc Superconducting (HTS) power cables are being developed for efficient transfer of

electrical energy in power transmission. These cables offer unique advantages such as reduced

size, zero joule loss and transmission at lower voltages, when compared with conventional copper

cables for the same power rating. During the manufacture of such HTS cables, a semi-synthetic

Polymeric insulation material namely Polypropylene Laminated Paper (PPLP) is used. This PPLP

is fabricated by inserting polypropylene film between two layers of kraft paper and is used for

electrical and thermal insulation as well as for external sheathing of HTS cable. Further, this

polymeric insulation is required to operate at cryogenic temperatures (65-77K) and high voltages

(220 kV) without losing its mechanical strength and flexibility. The present paper reviews the

recent developments on such PPLP, its properties (thermal, mechanical, electrical) at cryogenic

temperatures and its suitability for Cold Dielectric (CD) HTS power cables.

HTS cables have predominant applications in energy production, energy storage, power

transmission and distribution systems because of the advantage of achieving large power delivery

with negligible electrical loss in comparison with conventional power cables. These HTS cables

must be cooled with cryogens such as Liquid Nitrogen (LN2) during the operation, to retain the

superconductivity of HTS tapes [1-5]. The improvement of HTS cables is believed to be made

possible by using an appropriate insulation material in order to minimize the current loss during

transmission of electrical current [6-10].Therefore, it is necessary to study the behavior of electrical

insulation at LN2 temperatures to evaluate not only the short-term characteristics but also long-term

reliability for HTS cable applications. Usually the electrical insulation system of HTS cables

comprises of LN2 and Polypropylene Laminated paper (PPLP). The PPLP, an insulator composed

of a polypropylene (PP) film sandwiched between Kraft-papers, has low dielectric loss, high

breakdown strength at cryogenic temperatures and is considered as a good candidate for HTS

power cable applications. However PPLP contains some gaps between insulating layers that could

become a source of Partial Discharge (PD). Hence it is necessary to investigate the influence of PD

which is generated in butt gaps of the insulation layers on breakdown strength of PPLP.

Figure 3 : Polypropylene

Laminated Paper [12]

Figure 1: Design of a single phase

Cold Dielectric HTS cable [11]

Figure 4: Factors affecting the

insulation of HTS cable

26th International Cryogenic Engineering Conference, ICEC 26 – ICMC, 7th – 11th March 2016, New Delhi - India

Due to its well known dielectric property such as breakdown strength, low loss, etc., PPLP material

is now widely used in long HTS power cables throughout the world as substitute insulation instead

of the conventional Kraft paper/XLPE. Recent studies reported that PPLP material maximized the

full potential of the power cables in power transmission due to its superior insulation properties

compared with the conventional Kraft paper.

MECHANICAL CHARACTERISTICS

Figure 5: Variation of volume resistivity as a

function of Temperature [8]

Figure 9 : Butt Gap in PPLP [16]Figure 10 : AC Breakdown voltage by variation of

distance between Butt gaps [16]

Figure 11: Weibull plot of AC Breakdown strength of PPLP [15]

Figure 12: Characteristics of tan δ of various Insulations [15]

The High Temperature Superconducting (HTS) cable with PPLP dielectric is one of the

promising ways for handling huge electric power in the future. At 77 K, the mechanical

properties of PPLP such as the elastic modulus, yield and fracture strengths are more compared

to those at 300 K, but displacement (% elongation) is less than that at 300 K. Also the

mechanical properties of PPLP at MD showed much stronger strength and elastic modulus than

that at CD. At higher temperatures, a remarkable decrease in the breakdown voltage of PPLP

was seen because polypropylene(PP) has a melting point of 403 K. With an increase of distance

between the butt gaps, an increase in the Breakdown voltage of PPLP was seen. The dielectric

loss tangent (tan δ) of PPLP increased with the application of high electric stress due to the

retardation of partial discharges inside the butt gaps of the insulation. These results play an

important role in designing optimal operating electrical insulation in the cold dielectric design of

HTS power cables.

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Procedia, vol. 45, pp. 293-296, 2013.

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cable," Applied Superconductivity and Electromagnetic Devices, International Conference on, Chengdu, pp. 64-

67, 2009.

[9] M. Ieda, "Dielectric Breakdown Process of Polymers," IEEE Transactions on Electrical Insulation, vol. EI-15,

no. 3, pp. 206-224, 1980.

[10] M. Nagao et al., "Dielectric breakdown mechanism of polypropylene laminated paper in liquid

nitrogen," Electrical Insulation and Dielectric Phenomena (CEIDP), 2011 Annual Report Conference, Cancun,

pp. 419-422, 2011.

[11] Carrollton, Georgia HTS Triax Project, available on http://www.supercables.com/carrollton.html ,2009

[12] M.J. Dedicatoria, J.R.C. Dizon, H.S. Shin, K.D. Sim, "Establishment of CTE measurement procedure for

PPLP at 77 K for HTS power cables using double extensometers," Journal of the Korea Institute of Applied

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[13] A. Gorospe and H.S. Shin, "Mechanical property evaluation of PPLP at cryogenic temperature",

Superconductivity and Cryogenics, vol. 14, No. 4, pp. 16-19, 2012.

[14] J.W. Choi, H.G. Cheon, J.H Choi, H.J. Kim, J.W. Cho, and S.H. Kim, “A Study on Insulation Characteristics

of Laminated Polypropylene Paper for an HTS Cable”, IEEE Transactions on Applied Superconductivity, vol. 20,

no. 3, pp. 1280-1283, 2010.

[15] Young-Seok Kim, Dong-Soon Kwag, Hae-Jong Kim, Jeon-Wook Cho, Ki-Chul Seong and Sang-Hyun Kim,

"Research on insulation design of 22.9-kV high-Tc Superconducting cable in Korea," IEEE Transactions on

Power Delivery, vol. 20, no. 2, pp. 554-559, 2005.

[16] Sci Alexander, M Andrev, S.Y. Kim, I.H.Lee, D.W.Kim and D.S.Shin, "The Effect of Butt gaps on Dielectric

Strength of Taped Insulation in Superconducting Cable”, Journal of the Korea Institute of Applied

Superconductivity and Cryogenics, vol. 5, no.1, pp. 128-132, 2003.

Authors would like to thank Central Power Research Institute (CPRI), Bangalore for their

financial support to this work.

Property Cable paper PPLP PI LDPE

Resistivity (Ω.m) 3*1013 2.91*1014 5.77*1014 2.97*1014

Permittivity 2.3 2.39 2.21 2.36

Loss tangent angle - tanδ <10-4 <10-4 <10-4 <10-4

Dielectric

Strength

AC (kV) 66-77 ~150 190.13 133.98

Impulse (kV) 113-118 250 329.5 271.5

Initial voltage of PD (kV) 22 ~50 80 68.5

Tensile strength (MPa) 36 30.1 120.9 17.7

Cost (relative) Low High Very High Higher

PPLP – Poly Propylene Laminated Paper; PI – Primary Insulation(enamel based) ; LDPE – Low Density Poly Ethylene

Table 1 : Characteristics of various Electrical Insulation materials [8]

Usage of PP with Kraft Paper

The insulating material of PP combined with Kraft paper has excellent properties, compared to

Kraft-paper such as high porosity that provides the opportunity of impregnating in cryogenic

liquids. The significance of the impregnation is the potential to increase the voltage level for the

same insulation thickness, or decrease the insulation thickness and reduce the size of equipment.

PP impregnated with Kraft paper combines high electrical breakdown strength, low dielectric

loss, low dissipation factor and low relative dielectric permittivity at LN2 temperature (77K).

Major reason for using PPLP as electrical insulation, is its cost to performance ratio. Hence it is

considered as a good electrical insulating material for HTS power cable applications.

Insulation thickness required for the HTS cable is calculated for three test voltages:

acac

min AC ac

Vt r exp 1

E *M *r

where tac is the insulation thickness required, Vac is the ac withstand test voltage, EminAC is the

minimum breakdown strength, Mac is the ac conversion coefficient and r is the outer radius of the

conductor

2. Insulation requirement for Impulse withstand test:

imp 1 2 3

imp

minimp imp

V *L *L *Lt r exp 1

E *M *r

where Vimp is the impulse withstand voltage, Eminimp is the minimum impulse breakdown strength,

L1, L2, L3 are the impulse degradation coefficient, impulse temperature coefficient and the impulse

design margin respectively.

3. Insulation requirement for PD Inception test:

m 1 2 3pd

minpd AC

V *K *K *Kt r exp 1

E *M *r

where Vm is the system voltage, Eminpd is the minimum PD Inception strength and K1, K2, K3 are

the AC degradation coefficient, AC temperature coefficient, and the AC design margin

respectively.

Amongst the three obtained insulation thicknesses, it is reasonable that the final radial thickness

chosen must be the maximum among the three .

In addition to its dielectric properties, PPLP should also possess

superior mechanical properties at LN2 temperatures that is necessary

for the insulation winding process.

Alking B. Gorospe et al.[13] presented the results of Mechanical Properties of PPLP at

cryogenic temperature with different orientation (MD and CD). It was found that the mechanical

properties of the PPLP material varied and greatly depended on the orientation against the applied

load. The failure stress (tensile strength) of the PPLP material along MD was larger than that along

CD, but its elongation was significantly smaller in both ambient and cryogenic temperatures mainly

due to its fibre orientation. Compared with the cases at RT, the failure stress and Young’s modulus

of the material at 77 K increased almost twice. However the elongation of the material was

significantly decreased at cryogenic temperature especially in the case of CD. The failure stress at

both cryogenic and room temperatures depended upon the orientation of the sample to the load

application.

Since most of the fibres in Kraft papers are aligned on MD, its mechanical properties are

superior in the MD orientation as compared with the CD one. The drastic reduction of the

elongation value of CD sample from room temperature to cryogenic temperature can be addressed

to the behavior of polypropylene (PP) which is the dominant material in this direction. At cryogenic

temperature, the PP transits from “rubbery” to “glassy” state exhibiting a higher tensile load and a

smaller elongation.

Material

OrientationFailure Load

Failure

Stress

Elongation

at break

(mm)

Youngs

Modulus

(GPa)

Yield

Strength

(MPa)

At Room Temperature

MD 108 59 2 5.4 39.3

CD 69 37.9 5.6 3.1 21.8

At LN2 Temperature

MD 169 95.1 0.8 13 -

CD 114 71.5 0.9 9.3 55.9

Table 2 : Mechanical characteristics of PPLP [13]

Effect of Butt gap on the Electrical Insulation:

In the design of insulation for HTS cables, it is necessary to set small gaps between insulating

layers in the insulation system called Butt-gaps. If these gaps were not placed properly in PPLP, the

PPLP layers would have been torn while bending the cable. These butt gaps may become the

source of Partial Discharge (PD) [6]. In other words, PD may occur and then generate gas-filled

thermal bubbles in LN2. Prolonged PD activity erodes the solid insulation surface and can

eventually result in electrical breakdown. Besides, this generates heat which makes the

refrigeration resistance in cryogenic temperature. Hence it is important to investigate the effect of

butt gap on electrical properties of cable insulation at cryogenic temperature.

Sci Alexander et al. [16] investigated the effect of Butt gap on the electrical Insulation properties

for a HTS cable. It was observed that the Breakdown voltage increases with an increase of distance

between butt gaps.

ELECTRICAL CHARACTERISTICS

Dielectric breakdown of PPLP

Dielectric breakdown information is one of the design parameters taken into consideration to

determine the thickness of electrical insulation for the desired voltage level. The dielectric

breakdown data of pristine (Kraft) and PP impregnated paper were analysed by J.W. Choi et al.

[14] using Weibull and log-normal distribution functions, to better estimate a design value. The

probability of having a breakdown at 0.1% using the log-normal distribution is 17.25 kV/mm.

Repeating the same analysis for the PP impregnated paper; improved values for the breakdown

characteristics were obtained. These values indicate that PP impregnation improves the dielectric

breakdown design values by 55%-62% [10,14].

Figure 6: Charge Density dependence on

applied DC voltage [8]

From Figure 5, it is seen that the volume resistivity of PPLP is about 1016 Ω-cm at LN2

temperature. As the temperature is increased, a linear decrease in the volume resistivity was seen.

At room temperature, the volume resistivity of PPLP is about 1014 Ω-cm. As shown in Figure 6, the

space charge density gradually increased, as the applied voltage increased from 2 to 12 kV. Also

there is a decrease in the space charge density at LN2 temperature, compared to room temperature.

Figure 7 : Temperature dependence of

breakdown voltage of PPLP [15]

Figure 11 shows a typical Weibull plot of the AC breakdown strength of PPLP. From the figure,

the calculated breakdown strength at 50% probability is found to be 65 kV/mm and the shape

parameter is found to be 21.7. However, the minimum breakdown strength was set at 52 kV/mm

for the cable design to ensure the stability of cable insulation. The impulse breakdown strength

of PPLP was investigated and it was observed that the breakdown strength at 50% probability is

135 kV/mm and the shape parameter is 8.47.

The probability density function of a Weibull random variable is given by:

It has been found that the dielectric loss tangent (tan δ) of PPLP is 1.16*10-3 at 77 K. This is due

to the influence of PPLP material parameters such as thickness, density and component ratio of

PP to Kraft paper in PPLP. The Electric stress dependence of tan δ in liquid nitrogen is shown in

Figure 12, with the data of Kraft and Tyvek insulations for comparison. Though the tan δ of

PPLP is lower than that of Tyvek and Kraft insulations at a given temperature, it increases with

an increase in electric stress.

Where, k > 0 is the shape parameter and λ > 0 is the scale parameter of the distribution

1. Insulation for AC withstand: It is calculated using the relation

The temperature dependence of breakdown voltage of PPLP is shown in Figure 7. At low

temperatures, Breakdown voltage is relatively insensitive to temperature, but at higher

temperatures the breakdown voltage decreases remarkably. At LN2 temperature, PPLP is in the

form of glass and hence the electronic mechanism of breakdown process is dominant; therefore an

increase in breakdown strength is seen at this temperature [6]. PP has a melting point of 403 K,

thus, breakdown voltage decreases rapidly at this temperature range. Hence, the possibility of PD

at LN2 temperature is lower than at room temperature (RT) and the dielectric Breakdown strength

of PPLP at LN2 temperature is higher than at RT.

Figure 8 shows the pressure dependence of PD inception stress of PPLP. It is observed

that the PD inception stress increases as the pressure of LN2 increases, but as the pressure of LN2

exceeds 3 kg-f/cm2, the PD inception stress becomes saturated. A lot of bubbles exist in liquid

nitrogen at atmospheric pressure due to which the PD inception stress is low because partial

discharge takes place in the bubbles. However, the occurrence of the bubbles is reduced by

increasing the LN2 pressure.

Figure 8 : Pressure dependence of PD

Inception Stress of PPLP [15]

Figure 2: Design of a single phase

Warm Dielectric HTS cable [11]