Factors affecting the choice of Insulation system for extruded HVDC Power Cables.

8/18/2019 Factors affecting the choice of Insulation system for extruded HVDC Power Cables.

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2. Temperature requirements.

The maximum electric field strength of HVDC cable insulation depends not only of the appliedvoltage, but also upon the conductor temperature and the resulting maximum temperature differenceacross the insulation wall. Before entering into a discussion of the properties of possible insulationsystems, we will therefore discuss design parameters related to maximum expected temperatures.

The temperature difference across the insulation wall is given by the following equation providedcylindrical symmetry in the heat transfer:

∆T=(P/2πλ i)ln(b/a) (1)

Here b and a are the outer and inner radius of the insulation wall respectively, P is the conductorlosses per unit length of the cable [W/m] and λ i is the heat conductivity of the insulating materialwhich is 0.29W/Km for polyethylene.

It is common practice that cables are designed for maximum conductor losses, P, lower than 30 W/mand that the value of b/a hardly ever exceed 2.5. With these upper limits it is found that ∆T≤15K. - Inmost practical cases the cable will have to be designed for a lower temperature difference across the

insulation wall, perhaps closer to 10K.

The maximum temperature of the insulation will in addition depend on the temperature drop to thesurroundings. Considering a buried cable the temperature drop in the soil is then given by:

∆Ts=(P/2πλ s)ln(2H/c) (2)

where λ s is the heat conductivity of the soil, typically 1W/Km or higher, H the burial depth and c isthe outer radius of the cable. The value of ln(2H/c) will hardly ever exceed 4. It is then found that thetemperature drop, ∆Ts , in the soil is ≤19K.

With reasonable assumptions concerning the temperature drop in the different layers of the cable andthe temperature of the surroundings, it seems clear that the maximum temperature of the cable

insulation will be in the range of 55-65 °C. Thus all thermoplastic polyethylene materials (e.g. LDPEand HDPE) can endure the temperatures to be expected in HVDC cables. Such materials offer certainadvantages as crosslinking by-products as well as scorch formation in the extruder are avoided.

3.Conduction and space charge formation. In general the following equation can be used to calculate the field distribution in a dielectric:

div D=div(εε0E)=ρ (3)When a DC voltage is applied to a specimen, the insulation is initially free of space charges (ρ=0) andthe initial field distribution is determined by the variation of the permittivity inside the insulation.When a DC voltage has been applied for a long time, no space charge accumulation take place and thefollowing equation is valid:

div j=div(σE)=0 (4)This means that the field distribution is determined by the variation of the conductivity inside theinsulation. -Consequently, if the permittivity and the conductivity vary differently, a space chargemust be established in order that both equations are satisfied. This manifests itself as a transientcurrent (polarisation current) in the external circuit, which lasts until both equations are satisfied. Thecurrent is thereafter assumed to attain a constant (steady state) value denoted the leakage current.

In the special case that both the permittivity and the conductivity are constant throughout theinsulation, no polarisation current will flow in the external circuit (except for a short duration currentcaused by polarisation of the particles constituting the insulating material). The current will thenimmediately attain the steady state value.

In a homogenous medium the permittivity is usually constant. This means that if a polarisation currentof some duration is measured, it is an indication of the conductivity not being constant. On the other


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hand, the absence of a polarisation current indicates that the conductivity is constant throughout theinsulation.

The polarisation current may either be caused by a separation of charge formed inside the insulation,i.e. without any supply of charge carriers from the electrodes, or by charge carriers injected from theelectrodes. The longer distance charge carriers need to be transported, the longer will the transientcurrent last. Thus a space charge close to one of the electrodes, formed by injection of charge carriers,may give rise to a polarisation current of relatively short duration even when the conductivity is low.

As no comprehensive theoretical model exists for conduction in polymeric materials we have hereused the following common empirically based mathematical model to describe the field andtemperature dependency of the conductivity:

σ=σ0exp[α(T-T0)(E/E0)γ ] (5)

where σ is the electric conductivity, T the temperature and E the field strength, α and γ are constantscharacterising the material in question, while T0, E0 and σ0 are reference values.The advantage of this relation is that the field distribution can be found analytically for both planar

and concentric cylinder configurations [2], assuming a homogeneous insulation. For cable insulationwith inner and outer radius a and b respectively, the following equations can be used to determine thesteady state field distribution: The field strength at the conductor is:

Ea= [ ]1)/( −k abakU

(6)

and at radius r:

E(r)=Ea(r/a)k-1 where k=

( ) ( )1

/ln/

+

−+

γ

α γ abT T ba (7)

For a planar configuration the following equations apply:

E(x)= E(0)exp[-(α/(γ+1))(T(x)-T(0)] (8)The field strength Eh at the hot side of the insulation then becomesEh=Ecexp[-(α/(γ+1))∆Ti] (9)

Where Ec is the field strength at the cold side of the insulation and ∆Ti is the temperature differenceacross the insulation.

It can be seen from the equations above that the steady state field distribution in cable insulation isuniquely determined by the temperature difference, assuming that the conductivity is a function oftemperature and field strength only. If it is also a function of the position, a calculation of the fielddistribution requires a knowledge that is not easily obtained experimentally. To get an impression ofthe sensitivity to such variation it may be assumed that γ is constant throughout the insulation, whileσ0 is a function of the position, equation (9) is modified to:

E(x)= (σ0(0)/ σ0 (x))1/γ+1E(0)exp[-(α/(γ+1))∆Ti] (9b)To determine E(0) in this equation, σ0(x) must be known from either theory or experiments. This isnot an easy task, but if the variation in conductivity is confined to a small part of the electrode gap,equation (9b) can be used to judge the field enhancement caused by a local change in conductivity.

4. Experimental.

The results presented here are obtained in our laboratories over a number of years. Some of themeasurements are made on test specimens with a planar configuration. The insulation was made byfirst extruding a tape which was subsequently formed in a hot hydraulic press. Some of the specimenswere equipped with electrodes of cable screen material (semiconductor) and some with metallicelectrodes. The semiconductive electrodes were made by tape extrusion and hot pressing. The

specimens were then assembled by hot pressing and crosslinking in the press when this was required.Metallic electrodes were made by vacuum deposition of evaporated material. One of the electrodeshad a Rogowski profile to avoid field enhancement at the edge of the electrodes. In the case of


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conductivity measurements, guard electrodes were applied to avoid the effect of parasitic currents.Most objects were made with Low Density Polyethylene (LDPE), either thermoplastic with andwithout antioxidant (Santanox) or crosslinked. The crosslinked specimens could be heat treated toremove volatile reaction products prior to the measurements.

Conductivity measurements were also performed on five cable specimens made of different polyethylene based materials with semiconducting screens as electrodes. They are denoted cable 1-5.Space charge measurements were performed using the pulsed electro-acoustic method (PEA) where anarrow voltage pulse is applied to the test object. The resulting electric field exerts a pulsed force oncharges that may exist in the insulation creating an acoustic wave that can be detected at theelectrodes. A detailed description of this method can be found in the literature, e.g. in [3].

4.1. Results from conductivity measurement.

The general experience is that the results of conductivity measurements are dependent on electrodematerial, thickness of the insulation, morphology etc. Results published in the literature are oftenobtained under different test conditions, which make it difficult to make use of them for comparisonsand to draw conclusions. The same is to some extent true for some results obtained in our own

laboratories.

4.1.1. Time variation of the transient current.

Figure 1 shows the polarisation current for a cable 1-5 at 60 °C The results for cable 1 are publishedin [4], while the results for cables 2-4 are not published elsewhere. Cable 4 has a very lowconductivity, and due to noise problems the low leakage current could not be recorded. Cable 2 showsa low and nearly constant dc current. Cable 3 exhibits a decay that lasts about 100 seconds followed

by a variation that indicates the beginning of a slow fluctuation of the current, probably reflecting aslow redistribution of space charge inside the specimen. Cable 5 shows a similar feature except for ashorter decaying transient immediately after voltage application.

Figure1. Polarisation current ofcables 1-5 at 60°C and average fieldstrength of 4 kV/mm.

This general trend of leakage currents that rapidly attaining nearly stable values has also beenconfirmed by other measurements at higher electric stress using planar objects made of ordinaryXLPE cable materials [3]. The absence of a long lasting polarisation current is an indication thatconductivity variations may be moderate, except for regions close to the electrodes where spacecharge is injected during the first few seconds after voltage application.

4.1.2. Variation with temperature and field strength under isothermal conditions.

Figure 2 and 3 show the conductivity of cables 1 to 5 as a function of temperature and field strengthrespectively with an applied voltage of 20 kV. In the figures are also shown the strait lines that give a

best fit to equation (3). As can be seen, the α-values lay mostly between 0.1 and 0.158 except forcable 5 that at 25°C has given a current value that deviates from a straight line which may be drawn

through the remaining measured values. Measurements on planar objects have usually given valueswithin the same region. It should be noted, however, that the observed values of α are different fordifferent electrode materials. An example of this is given in figure 4 [5]. The tests were carried out on

Polarization current at 60ºC, E av =4kV/mm

1,00E-10

1,00E-09

1,00E-08

1,00E-07

1 10 100 1000 10000 100000 1000000

t i me [ s ]

c u r r e n t [ A ]

cable2

cable3

cable4

cable5

cable1


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the same insulating material, but with different electrode materials. Semiconductive electrodes gaveα≈0.14, while aluminium electrodes gave α≈0.09. An effect, which can be due to different voltagedrop close to the electrodes or to different densities of injected charge from the electrodes. Thematerials used in these tests is the same as that used in cable 2, figure 2 and 3. Even if the results arecorrected for being performed at different field strengths, it is clear that the thin planar objects give

conductivity about one order of magnitude higher than that of the cable. The reason for this is notclear, but may be due to differences in morphology that is known to have a strong influence upon theconductivity. As can be seen from figure 3 the variation with the field strength was nearly the samefor all the examined cables. The γ values were found to be between 1.18 and 1.3.

Figure. 2. Conductivity of cables 1-5 as afunction of temperature at the average fieldstrength of 4 kV/ mm.

Figure.3. Conductivity of cables 1-5 as afunction of the average field strength at atemperature of 60°C.

Figure 4. Conductivity as a function oftemperature for planar specimens made of acrosslinked polyethylene. i) Semiconductiveelectrodes, 0.9 mm thick insulation. ii)Vacuum deposited Al electrode, 0.4 mm thickinsulation. Measurements made at 19kV/mmand 12,5 kV/mm, respectively

4.2. Results from space charge measurements.

Most published results of space charge densities are made under isothermal conditions. In this reportwe will also present results from space charge measurements with the purpose of determining whatfield enhancements space charge may give rise to in case of a temperature gradient across theinsulation.

4.2.1. Field enhancement due to space charge formation under isothermal conditions.

Sanden [3] used the PEA method to show that the space charge formation in planar XLPE specimensunder isothermal conditions depends considerably on electrode material, voltage poling time and onwhether the specimen is degassed to remove volatile reaction products from the crosslinking process.From a practical point of view the question is if the space charges lead to substantial fieldenhancements, particularly near the electrodes. The PEA method allows measurement of the fieldstrength at the electrode/insulation interfaces. This is done by first poling the specimen and measurethe space charge profile immediately after short-circuiting. In addition to the space charges in the bulk

of the insulation, a surface charge is formed at the electrode/insulation interface to compensate for theeffect of space charges in the insulation, often denoted a mirror charge. Due to limited geometricalresolution this surface charge will appear as a distributed space charge. By applying a voltage of


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opposite polarity the surface charge can be made to disappear. The field strength at which thishappens is then equal to the field enhancement at the electrode that is caused by the remaining spacecharges in the bulk of the insulation.

Figure 5 show that the field enhancement depends on the poling field, but even at 70 kV/mm it doesnot exceed 20% neither at the cathode nor at the anode. From a practical point of view this is rathermoderate and confirms that the conductivity close to these semiconductive electrodes does not deviatemuch from that of the bulk of the insulation. The space charge measurements also confirm that theconductivity variation within the bulk of the insulation is moderate.

Figure 5. Field enhancement at the electrodes of 1mm thick planar XLPE objects withsemiconductive electrodes at 40°C. Poling time24 hours.

Figure 6. Measured space charge profile in a planar specimen of crosslinked polyethylene 0.9mm thick with semiconductive electrodes, andwith a temperature difference across the

insulation of approximately 13.5C.

4.2.2 Field enhancement due to a temperature difference across the insulation gap.

Tests were made using a planar specimen of the same materials as in figure 4 to measure the spacecharge profile and the electrode field strengths with an anode temperature of 30.5 °C and a cathodetemperature of 44°C, resulting in a temperature difference of 13.5°C across the insulation. It should benoted, however, that the temperature measurements are encumbered with considerable uncertainty.The space charge profile is shown in figure 6 for a poling time of 24 hours. The electrode fieldstrengths were found to be Ean=1.3*Eav at the “cold” anode and Eca =0.8*Eav at the “warm” cathode.The test arrangement used for these measurements is shown in [5]. The test procedure has, however

been modified to improve the measurement of the temperature difference across the insulation, whichis complicated by poor thermal contact between the semiconducting and the metallic electrodes.

With α=0.13 and γ=1.6 as measured under isothermal conditions, the field strengths were calculated by means of equation (8) to be 1.37*Eav and 0.7*Eav respectively. Even though the numericalagreement could have been better, it appears that the calculations based on conductivity measurementsgive a reasonably good picture of the steady state field distribution in the presence of a temperaturegradient in the insulation. But it must be kept in mind that there are several sources of error,

particularly in the temperature measurements, and as pointed out previously, the calculated values are based upon an assumption of constant material properties across the insulation. Our results are not inagreement with results published in [6] where no effect of the temperature difference was found. Thereason for this disagreement is not clear.5. Calculated values of electrode field strengths in cables.

As stated earlier the field distribution (and the space charge density) can be calculated when theconductivity is known (as a function of temperature and field strength), provided that the conductivitydoes not vary substantially across the electrode gap. Results presented above show that α, γ and ∆T


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vary within the following approximate limits: α≈0.1-0.15, γ≈1.0-1.6 and ∆T≈10-15K. - The maximumelectric field relative to the average field strength are then calculated for the most unfavourable andthe most favourable combination of these values. The results are presented in figure 7 with the ratio ofthe inner and outer radius of the insulation, b/a, as the variable.

Figure7.Calculated maximum field strengths,shown as multiples of the average appliedstress versus the dimension ratio b/a.Curve 1. Initial field at the conductor.Curve 2. Steady state field strength at theouter electrode (α=0.15, γ=1.0 and ∆T=15K)Curve 3. The field strength at the conductorimmediately after a polarity reversal (α=0.15,γ=1.0 and ∆T=15K)Curve 4. Steady state field strength at theouter electrode, (α=0.1, γ=1.6 and ∆T=10K)Curve 5. The field strength at the conductor

immediately after a polarity reversal (α=0.1,γ=1.6 and ∆T=10K)

If Voltage Source Converters (VSC) are used the direction of the power flow may be changed without polarity reversal of the service voltage. Then the slowly decaying field strength Ear will not appear.The steady state field strength at the outer electrode (curve 2 and 4) as well as the Poisson fieldstrength (curve 1) are quite moderate and should represent no problem.

In long distance HVDC transmission systems utilising thyristors and Line Commutated Converter(LCC) technology, polarity reversal is required to change the direction of the power flow. The DCstresses associated with polarity reversal must then be taken into account (curve 3 and 5). Knowledgeof the long term DC withstand voltage of extruded insulation is limited, but tests in [7] indicate veryhigh values. The importance of the DC stress may therefore be rather indirect; meaning for instancethat the insulation becomes more vulnerable to impurities, that the design of joints and terminationsmay cause greater problems etc.

6. The influence of irregularities in the insulation.

Irregularities are hard to avoid completely in extruded insulation. They may appear as particlecontaminants or cavities. To improve the longevity of AC cables considerable effort has been made toavoid conductive particles and cavities. Insulating particles are less serious in AC cables as long asthey are well bonded to the surrounding insulation. In DC cables even insulating particles may causelocal field enhancement as they are likely to have a different conductivity/permittivity ratio than thatof the surrounding insulation.

Ieda et al [8] used a needle-plane arrangement with metallic needles inserted in LDPE to simulate theeffect of conductive contaminants in practical insulation systems. By applying a DC ramp voltage, itwas found that tree inception took place at a voltage that increased with decreasing rate of voltagerise. The calculated field strength at the needle tip exceeded by far expected values of the intrinsic

breakdown strength of LDPE. This behaviour was explained as a result of homocharge injection intothe region surrounding the needle tip. When given enough time to develop, this charge reduces themaximum field strength sufficiently to prevent treeing. This was further supported by experimentswhere the voltage polarity was reversed by applying an impulse voltage of opposite polarity to theapplied DC field. It was then found that tree growth started at a much lower voltage. The effect of thevoltage reversal was more marked the faster the reversal took place. It was even demonstrated thatsimply short-circuiting the test specimen after DC poling was sufficient to cause treeing at a voltagelower than the DC tree inception voltage. The reason for this is supposed to be that the injected charge

needs time to be removed when the test specimen is short-circuited, and that the field originating fromthe injected charge alone is sufficient to cause treeing. Observations, which have been qualitatively,confirmed by Oldervoll [9] and others. In addition to needle experiments Oldervoll also made


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experiments with conductive particles imbedded in the insulation to better simulate particles, as theyare likely to appear in a practical insulation. The time to breakdown in long term ageing experimentsat average field strength of 100 kV/mm was not influenced by the particles, even though their sizeamounted to 15-20% of the insulation thickness.

Thus it is qualitatively well documented that conductive inclusions are particularly harmful in case of polarity reversals and rapid voltage transient. Such transients may occur during faults, even ininstallations with VSC technology. It is therefore of great importance that material handling and themanufacturing process is kept extremely clean, and that routine test procedures are available that candetect dangerous inclusion and defects in cable insulation.

7.Conclusions.

• When typical value of losses and heat transfer is taken into account, the temperaturedifference across the insulation wall is typically limited to 10-15K and the maximumconductor temperature hardly exceeds 55-65 °C. A temperature which can be endured by anythermoplastic polyethylene based materials.

• Provided the properties of the insulation material are constant across the insulation wall, the

steady state field distribution can be calculated by means of empirically based formulae forthe conductivity as a function of temperature. Experimental results indicate that this presumption is reasonably well satisfied. This turns out to be the case even under non-isothermal conditions as verified by comparison with space charge measurements ofspecimens with and without a temperature gradient.

• The modification of the DC stresses caused by space charge formation is moderate and can probably be endured by insulation without harmful irregularities, even when the polarity must be changed to reverse the direction of the power flow.

Bibliography.

[1] G. Evenset: “Cavitation as a Precursor to Breakdown of Mass-Impregnated HVDC Cables”.

Dr.Thesis NTNU 1999[2] I.W. Mcallister, G.C. Crichton, Å. Pedersen: “Charge Accumulation in DC Cables: A

Macroscopic Approach”. IEEE Symp. El. Insulation, June 1994.[3] B.Sanden : “XLPE Cable Insulation Subjected to HVDC Stress. Space Charge, Conduction

and Breakdown strength”. Dr. thesis NTNU 1996.[4] E. Ildstad, F. Mauseth, G. Balog: “Space Charge and Electric Field Distribution in Current

Loaded Polyethylene Insulated HVDC Cables”. ISH 2003 p. 366 and CD.[5] S. Trætteberg, E. Ildstad, R. Hegerberg: “Influence of DC Voltage and Temperature Gradient

on the Distribution of Space Charges in XLPE”. Nordic Ins. Symp. 2003 p 119-126.[6] K.R. Bambery, R.J. Fleming, J.T. Holbøll: “Space Charge Profiles in Low Density

Polyethylene Samples Containing a Permittivity/Conductivity Gradient”. J.of Physics 34(2001) pp 3071-3077.

[7] M.Byggeth, K. Johannesson, C. Liljegren, L. Palmquist, U. Axelsson, J. Jonsson, C.Tørnkvist: “The Development of an Extruded HVDC Cable System and its first Applicationin the Gotland HVDC Light Project”. Jicable Vol 2/2 (1999) pp 538-542.

[8] M. Ieda, M. Nawata: “DC Treeing Breakdown Associated with Space Charge Formation inPolyethhylene”. IEEE Trans. Electr. Insul Vol EI-12 No.1 (1977), pp. 19-25.

[9] F. Oldervoll: “Electrical and Thermal Ageing of Extruded Low Density PolyethyleneInsulation under HVDC Conditions”. Dr. Thesis NTNU 2000.

Factors affecting the choice of Insulation system for extruded HVDC Power Cables.

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