Cable Types, Design, and Associated Insulation Faults. Preface To Hipot Testing

Power cables

Addressing the task of power cable diagnostics and fault location, the energy industry now offers a wide range of tools using various cable testing methods. The choice of the right method and the most appropriate instrument greatly depends on a number of factors, including the test purpose, the type and age of the cable, its environment, and the anticipated cable fault type. Here we will talk about the basic information on power cables, namely their types and typical structure, and will touch upon the associated types of cable errors.

Power cables: general notes on types, design and application

The power cable market can be segmented into three areas based upon the voltage class of the cable: the medium voltage class with cables 6 kV to 69 kV, the high voltage cable class with cables 69 kV to 150 kV, and the extra high voltage with cables greater than 150 kV. The medium voltage cable dominates in the underground cable market segment.

Insulated power cables are used for the transmission and distribution of electricity both for industrial and commercial, and various underground applications.

In a typical medium voltage cable, copper and/or aluminum wires, stranded and/or solid, are used as the conductors. These conductors are covered with an extruded polymeric stress-control layer, often referred to as the permashield or conductor shield, made of semi conductive compounds. The insulation layer immediately surrounds and is fully bonded with the conductor shield. The insulation shield encases the insulation and in some cases may be composed of the same semi conductive material as the conductor shield.

insulation layers

The copper neutral wires are wound around the insulation shield, and are usually covered with a thermoplastic polyethylene jacket, which ensures mechanical protection from the external environment, and also reduces moisture intrusion into the cable, thus preventing a premature cable failure.

There are two basic categories for cables, the extruded dielectric and the laminated style cable. Examples of the extruded dielectric cables will include the cross-linked polyethylene, or polyethylene XLPE or PE style cables, and ethylene propylene rubber (EPR) style cables. In its turn, the paper insulated lead covered (PILC) type would be a representation of the typical laminated style cables.

Aging characteristics: treeing 

Water tree degradation is a major problem for medium voltage extruded dielectric cables, particularly the service aged XLPE and PE style cables. It is perhaps the worst degradation process of the power cable insulation and contributes to the failure of the cable. Now, water trees are formed and grow in the presence of moisture, impurities or contamination, and electric field over time.

            types of water trees

There are generally two types of water trees, namely the bow tie tree and the vented tree.
Bow tie trees are water trees that grow from the insulation outward toward the surfaces of the insulation. These trees grow in the direction of the electric field, in both directions toward the two electrodes, the centre conductor of the cable and the concentric neutral surrounding the cable.

While having a faster growth rate compared to the vented trees, bow tie trees are not capable of growing to large sizes and usually do not grow to a size significant enough to cause a failure in the insulating system.

Vented trees are water trees that grow in the direction of the electric field, from the surface of the polymer inward into the insulating system. Vented trees have a lower initial growth rate as compared to the bow tie tree. However, they are capable of growing right through the entire insulation thickness.

So vented trees are definitely the more problematic of the tree series, leading service age cables to eventual electrical failure or a fault mode.

Now, in the case of extruded dielectric the treeing is a result of water and grass contamination and is referred to as a water tree.

In laminated cables the most common cause of the tree effect is from the drying of the oil and then the burning of the insulating layers of paper. As the insulating layers of paper burn, they leave behind carbon deposits, which are conductive. So in time as the papers begin to burn, leaving behind little carbon deposits, a conductive path is created through the insulation, again, causing a cable failure. This type of treeing is referred to as a carbon tree.

Timely conducted testing procedures can help notice the loss of cable insulation integrity, spot signs of its deterioration caused by aging, and therefore prevent cable failure. The next article will look at a popular method of high voltage cable diagnostics, hipot cable testing, and will discuss its strong and weak points in relation to cable insulation analysis.

Alexei Tiatiushkin
Marketing manager
KharkovEnergoPribor Ltd.
marketing@keppowertesting.uk
http://www.kep.ua