Guidelines

At this point, additional information from DNV-GL recommendations and class regulations are sorted thematically. Repetitions should be avoided and not all statements of all recommendations need to be reproduced:

IEEE 43 Recommended Practice for Testing Insulation

Resistance of Electric Machinery

IEEE 98 Standard for the Preparation of Test Procedures for

the Thermal Evaluation of Solid

IEEE 1580 Recommended Practice for Marine Cable for Use on

Shipboard and Fixed or Floating Facilities

DNV-RP-J301 Subsea Power Cables in Shallow Water Renewable

Energy Applications

ITU-T G.976

Test methods applicable to optical fibre submarine cable systems

NEK 606

Cables for offshore installations - halogen-free and/or mud resistant -- Technical specification

TB 493

Non-destructive water-tree detection in XLPE cable insulation

According to recommended practice IEEE 43 (IEEE 43, p. 14) water should have a conductivity of not more than 0.25 µS/cm. It is therefore recommended to use deionized water to prevent chemical reactions due to the contents of the water. In relation to the selection of artificial seawater, the chemical activity should be determined.

The insulation resistance value varies exponentially with temperature. IEEE 43 (IEEE 43, p. 10) also points out the difference between the temperature dependence of the resistivity in metals and non-metallic materials, especially in good insulators. In metals, a higher temperature leads to a thermally excited movement that reduces the mean free path of electron movement and thus increases electron mobility and resistivity. In the case of insulators, on the other hand, an increase in temperature

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supplies heat energy. This releases additional charge carriers and reduces the specific resistance. This temperature change affects all components of the measured current, with the exception of the capacitive current.

Figure 13 Correction of insulation resistance(KT) for insulation systems (IEEE 43, p. 11)

Figure 13 shows temperature correction factors for insulators called "thermoplastic" (asphalt) and

"thermosetting" (epoxy or polyester).

𝑅_𝐶 = 𝐾_𝑇 ∙ 𝑅_𝑇 (2-9)

with

RC Insulation resistance (in megohms) corrected to 40 °C

KT Insulation resistance temperature coefficient at temperature T °C RT Insulation resistance (in megohms) at temperature T °C.

Recommended practice IEEE 1580 (IEEE 1580, p. 20) presents an adaptation of the method to determine insulation resistance as a function of temperature, which is summarised here. To determine the temperature correction factor, two samples with defined dimensions in length (60 m) and thickness of the insulation (1.14 mm) are immersed in a water bath. The temperature of the water bath is pre-selected, so that stable insulation resistance values are measured in the calibrated area of the measuring instrument at the lowest water bath temperature. The water bath should be equipped with heating, cooling and circulation systems. The ends of the samples must be at least 0.6 m above the water surface to avoid surface leakage currents. Before setting the water temperature to 10.0 °C, the samples should remain in the water for 16 hours at room temperature.

The DC resistance of the metal conductor should be measured at suitable intervals until the temperature remains constant for at least five minutes. The insulator can then be viewed at the same temperature as the water. Each of the two specimens shall be subjected to successive increasing water temperatures of 10.0 °C, 16.1 °C, 22.2 °C, 27.8 °C and 35.0 °C then decreasing again to 27.8 °C, 22.2 °C,

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16.1 °C and 10.0 °C. The insulation resistance values shall be measured at each temperature after the equilibrium between insulator and water temperature has been established.

The measured values at a specific temperature are averaged for the two samples. These four mean values and the mean value of the individual values at 35.0 °C are applied to logarithmic paper. A continuous curve (usually a straight line) is to be drawn through the five points. The value of the insulation resistance at 15.6 °C, which is the normal temperature in IEEE 1580, can then be read from the graph.

The resistivity coefficient C for a temperature change of 1 °C shall be calculated to two positions after the decimal point. To do this, the insulation resistance read at 15.1 °C from the graph is divided by the insulation resistance at 16.1 °C. The temperature correction factor M required to correct to the standard test temperature of 15.6 °C is calculated from the following formula:

𝑀 = 𝐶^{(𝑡−15.6)} (2-10)

with

t Actual test temperature in °C C Resistivity coefficient

This formula has been used to generate a table of temperature correction factors (Fehler!

Verweisquelle konnte nicht gefunden werden.).

Table 2 Temperature correction factor M for adjusting insulation resistance to 15.6 °C (IEEE 1580, pp. 49–50)

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The same principles are used to perform the insulation resistance test in recommended practice IEEE 98 (IEEE 98, p. 9), but with the limitation that the correction factor is investigated for each test method to assess the thermal resistance of thermally aged solid electrical insulating materials (EI) in air. The insulation resistance should be measured after one minute with a direct current at an applied voltage of not less than 100 V nor more than 500 V. The conductor is connected to the negative pole. For water the calculation, according to IEEE 1580, is performed in a modified form. If the insulation resistance test is performed in water or air at a temperature other than 15.6 °C, multiply the measured value by the correction factor M shown in Fehler! Verweisquelle konnte nicht gefunden werden.. This results in the following formula for the insulation resistance (IEEE 1580, p. 92):

𝑅 = 𝐾 𝑀 log₁₀(𝐷

𝑑) (2-11)

with

R Insulation resistivity [MΩ ⋅ km]

K Insulation resistance constant (from Table 12, Table 13, or Table 14) (MΩ⋅1000 ft) M Temperature correction factor to 15.6 °C

D Diameter over the insulation d Diameter under the insulation

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Besides the change of characteristics over time, DNV-GL also considers the persisting age of the specimen. “Testing of aged cables with voltages higher than nominal can increase the risk of insulation breakdown” (Recommended Practice DNV-RP-J301, p. 128).