When XLPE was advertised to us, it was always said that dielectric constant of XLPE (2.4) is noticeable less than that of paper impregnated insulation (4.2). Hence, we had the impression that the capacitance of modern cables (XLPE) would less than that of the previous generation of cables (paper). Accordingly, this means that after switching from paper to XLPE, we expected to get lower capacitive currents in our cable networks:
➡️ charging current of cables in normal mode;
➡️ ground fault current in a 6-35 kV network with an isolated neutral.

It turns out that there is nothing like that! There is no decrease in capacitive currents. Instead, we have to admit that the capacitive currents, have hardly changed at best, and in the worst case, they have increased (!!!) up to 1.5-2.0 times (by up to +100%). Why is that?

The problem with the capacitance lies in the following:
✅ the capacitance of the cable depends not only on the dielectric constant (capacitance is proportional to permittivity), but also on dimensions of the cable (insulation thickness, core cross-section);
✅ it is necessary to distinguish between the capacitance of the positive (C1) and zero sequence (C0).

With the transition to XLPE, four things happened at once:
1️⃣ cables began to be made phase-screened, and the mutual capacitance Cm between phases disappeared for them, which led to a fact that C1=C0+3Cm decreased to the level of C0;
2️⃣ reduced dielectric constant (from 4.2 to 2.4), which decrease C;
3️⃣ reduced insulation thickness, which INCREASE C;
4️⃣ increased core cross-section (to invest to the future), which INCREASE C.

So, we can see that there are factors for capacitance to decrease and to increase at the same time. The table shows calculation example for a three-core 10 kV, 3×240 mm2 cable that take into account factors 1️⃣-2️⃣-3️⃣. Although we expected a noticeable reduction in capacitances, we see:
👉 the positive sequence capacitance (C1) for XLPE is only a little less than for paper (-12%);
👉 the zero sequence capacitance (C0) for XLPE turned out to be significantly higher than for paper (+61%).

Why is this important? Because:
✅ the capacitance of the positive sequence (C1) gives us the charging current of the cable, which is important for choosing a circuit breaker and means of compensating for reactive power;
✅ the zero-sequence capacitance (C0) gives us a capacitive component of a ground fault current, which is important for setting up protections in networks with an isolated neutral and selecting the inductance of the reactor in the neutral of the network (Peterson coil).

Be careful when replacing paper insulation with XLPE. Be ready to find capacitive currents significantly increased. Good XLPE may do bad.

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