Heard of oil-filled cable? They are less popular than the other wires we use for typical connections.
Voids in the current-carrying conductors can harm the cables’ electrical characteristics.
And this is where oil-filled cables come into play. They also affect the dielectric stress and overall working temperature of the wires.
Today, we bring you an elaboration of how oil-filled cables help fix these issues in detail. Check it out.
Table of Contents
- What is an Oil-Filled Cable?
- Advantages and Disadvantages of Oil-Filled Cables
- Types of Oil-Filled Cable
- Sorts of Cable Oils
- How to Oil-filled Cable Breakdown Strength Test Samples?
- Summary
What is an Oil-Filled Cable?
As per its name, it’s a typical cable featuring a pressured low-viscosity oil in the cable sheath.
The role of the oil is to fill in the voids in the cable, which features oil-impregnated paper.
This paper is imperative in maintaining adequate dielectric strength of the transmitting system.
The most common types of oil for this application include branched nonyl benzene and linear decyl benzene.
Nonetheless, these alkylations haven’t been used for long, as manufacturers applied mineral oils in the past.
But the former mineral oils weren’t as good as the alkylates because of the following reasons:
- They lacked the low viscosity synonymous with alkylates.
- Also, they didn’t adequately absorb the water generated by cellulose aging.
Typical applications of oil-filled cables include long power transmissions.
Also, they are common in scenarios where it’s impossible to use aerial cables.
For instance, they are handy in undersea cable transmissions and underground cable uses.
Advantages and Disadvantages of Oil-Filled Cables
A High Voltage Copper Wire Cable Cross Section.
Advantages of Oil-Filled Cables
- An oil-filled cable features significantly less thermal resistance/high working temperature. It, in turn, boosts the cable’s current carrying capacity.
- Also, their excellent cable insulation is imperative in ensuring relatively high dielectric stress.
- Compared to conventional solid cables, oil-filled types have significantly better impregnation.
- Again, this cable allows you to perform an impregnation even after the sheathing process.
- Where size is an issue, oil-filled cables are better. They are significantly smaller than solid-filled cables. Why? Because of their smaller dielectric thickness.
- It is outright to identify a defect in oil-filled cables from oil leakages. Also, the probability of an earth shield is relatively minimal with oil-filled cables.
- These are the go-to type of cables and have an extensive temperature range.
Disadvantages of Oil-Filled Cables
A Typical Solid Filled Cable Surface.
- They are sophisticated in make-up, translating to a higher acquisition cost than the typical cables.
- Also, they are usually short and thus may not be too suitable for long-distance transmission.
- Again, since they contain oil, they’re prone to causing leakages. Hence, you must have automatic signaling equipment when you use these cables.
- Fourthly, expertise is necessary when laying these cables. Also, you need trained personnel to maintain them.
Types of Oil-Filled Cable
An Oil-Filled Cable Cross Section.
We have three main types of these cables; we’ll look at them in detail below.
Self-Contained Oil-filled Cable
It is characterized by oil-filled ducts with high pressure and a typical cable strength of 180 kV/cm.
This duct has a 12mm diameter. Also, the cables have free space between the core, which is imperative for free oil flow.
This oil further reinforces the wire insulation, thus boosting its conducting capacities.
Next to the cable’s lead sheath is a conductor with a cross-sectional area of approximately 150 square millimeters.
Its current carrying capacity is between 110 to 220 kV. Its upsides are as follows:
- Thanks to its oil duct feature, it has a small conductor, and its installation process is straightforward.
- Also, this is the cheapest oiled-filled cable and requires no pump. Rather, it applies an oil tank as the oil repository.
Flat Type Oil Filled Cable
Its make is as follows:
- The cable has three horizontally laid insulated cores and features no filter material.
- Also, it has metallic tapes and winding wires to reinforce its lead sheaths.
- Further, these bands feature flutes to ensure the cable is highly flexible and can navigate bends.
It barely forms voids due to its excellent oil expansion and contraction mechanism.
When the temperature is loaded, there’s a significant rise in its temperature.
In turn, this prompts the expansion of the oil.
A reduction in temperatures leads to a shrinkage of the loaded oil, and this mechanism inhibits voids formation.
Also noteworthy is that it features an oil storage tank in the cable route to curtail thermal changes.
Hence, when the cable is in loading mode, it yields cable heat. In turn, the oil moves from the storage tank, which shields void creation.
Pipes Types Oil Filled Cables
It’s typically a pipe with three separate paper-insulated screen cores.
The pipe has an insulation oil under high pressure that prevents void formations.
Also, the oil is a typical insulator by expelling heat from the cable.
You can easily tell this cable from the others, as this one lacks a conductor oil duct.
Sorts of Cable Oils
Different cable oil Grades.
There are four main cable oils; the most appropriate type depends on the application.
Other key considerations include the voltage and the cable design.
Cable Oil KM-22
Here are the remarkable features and properties of the cable:
- It has excellent dielectric properties, such as a low dielectric loss tangent.
- Also, it’s remarkable for its highly stable electrical features even under prolonged heating.
- It lacks additives thanks to its selective solvent purification procedure during manufacturing. Also, it is the ideal oil for the impregnation of 1-35 kV voltage power cables.
- It has no mechanical impurities and excellent insulating strength at 25 ± 10 °С.
Cable oil C-220
Thanks to its medium viscosity, this is the go-to cable for height voltage cable impregnation.
Its primary makeup material is air oil which is renowned for its high purity thanks to the dewaxing method.
Also, it features a relatively low freezing point.
In application, this oil is essential for high-pressure oil-filled cables.
But note that since it lacks aromatic hydrocarbons, it differs from transformer oil.
Viscous Cable Oil P-28
As per its name, this bright stock base oil is highly viscous.
It comes from the acid-cleaning process of the Surakhan selective crude oil.
This process yields a viscous oil concentrate, which makes the viscous cable oil.
Also, it cannot withstand alkaline cleaning as it is primarily derived via an acid-cleaning procedure.
It’s ideal for high-voltage power line transmission.
Oil MH-2
Unlike the type above, this lowly viscous oil exhibits excellent dielectric properties.
How to Oil-filled Cable Breakdown Strength Test Samples?
This test’s essence is to determine if there are impurities in the oil that can affect its efficacy. Here’s how to do it:
- Put a power cable oil into an oil cup with two electrodes. Ensure that there’s a separation distance between the electrodes.
- Apply a voltage to the electrodes and read the breakdown voltage on the multimeter. Repeat this test five times, and if there’s a remarkable difference in the values obtained (25%), repeat the experiment.
- Take the average value of the five tests; this is the oil’s breakdown strength.
Summary
Like the current carrying capacity, the cable insulation type significantly determines its functionalities.
Primarily this is the fundamental reason to use oil-filled cables.
So why do we insist on them? Their resistance to thermal changes during electricity transmission is top-notch.
Also, although they’re quite pricey, they reduce the dielectric loss tangent of the system.
