XLPE-insulated MV cables

Why single-core XLPE cables dominate new MV lines

Much of Ukraine's existing 6–10 kV cable network was built with three-core cables with impregnated paper insulation in an aluminium or lead sheath — for example ААБл (AABl) or ААШв (AAShv) types. For new lines and reconstruction, designers now mostly specify three single-core cables with cross-linked polyethylene (XLPE) insulation, laid in trefoil or flat formation.

There are several reasons:

• Higher operating temperature. XLPE insulation permits a higher continuous conductor temperature than paper, so a cable of the same cross-section can carry more current.
• No oil and simpler installation. There is no impregnating compound that can drain on sloping routes; joints for XLPE cables are made with heat-shrink or cold-shrink technology following standardised instructions.
• Easier transport and laying. A single-core cable is lighter and more flexible than a three-core cable of equivalent cross-section, a longer length fits on a drum, and so there are fewer joints along the route.
• Moisture resistance. A polyethylene sheath and longitudinal water sealing protect a buried cable well.

Cable construction, layer by layer

Whatever the manufacturer, a single-core XLPE MV cable has a similar sequence of layers. From the centre:

1. Conductor — most often aluminium (the letter «А» at the start of the designation), stranded and compacted. A copper conductor is used when more capacity is needed with a smaller cross-section.
2. Conductor semiconducting layer — evens out the electric field at the conductor–insulation interface.
3. Cross-linked polyethylene insulation — the main insulating layer; its thickness depends on the cable's voltage class.
4. Insulation semiconducting layer — confines the field to the insulation; in many cables it strips without heating, which makes fitting accessories easier.
5. Metallic screen — copper wires, often with copper tape. It carries capacitive and fault currents and makes the cable safe to touch.
6. Water-blocking tapes — in sealed cables they swell on contact with water and limit the spread of moisture along the cable.
7. Outer sheath — polyethylene or PVC compound, depending on the laying conditions.

Each layer matters when installing joints: the jointer removes them one by one to the lengths given in the instructions, keeping the prepared cable clean and correctly shaped. The fault-free life of the line depends largely on the quality of this work.

How to read a cable designation

In Ukrainian catalogues a cable designation is made up of letters describing the construction from conductor to sheath. A common system looks like this:

Letter	Meaning
А	aluminium conductor (no letter — copper)
Пв	cross-linked (vulcanised) polyethylene insulation
Е	screen of copper wires and copper tape
г	longitudinal sealing with water-blocking tapes
П	polyethylene sheath
В	PVC compound sheath
у	in some manufacturers' designations — reinforced (thicker) sheath
нг	flame-retardant version (usually for cables with a PVC sheath)

So АПвЕгП (APvEgP) is a cable with an aluminium conductor, XLPE insulation, copper screen, longitudinal water sealing and polyethylene sheath. АПвЕВ (APvEV) has the same type of construction but with a PVC compound sheath and, as a rule, no longitudinal sealing.

Some catalogues use a system in which the copper screen is not marked by a separate letter, since it is a mandatory element of an MV cable. The same cable for the same purpose is then designated, for example, АПвП (APvP) (aluminium conductor, XLPE, polyethylene sheath), АПвПг (APvPg) (with longitudinal sealing) or АПвПу (APvPu) (with a reinforced sheath). The presence and cross-section of the screen are then shown by the figures after the conductor cross-section.

After the letters come the number and cross-section of the conductors, the screen cross-section and the voltage class, e.g. АПвЕгП 1×240/35-10 or АПвЕгП 1×240/35 6/10 kV. The order differs between manufacturers, so before ordering cable and accessories the designation is checked against the data sheet.

Conductor and screen cross-sections

The notation 1×240/35 mm² means a single-core cable with a 240 mm² main conductor and a 35 mm² copper screen. The conductor cross-section determines the permissible continuous current and voltage drop of the line, as well as its thermal withstand to short-circuit currents. The screen cross-section determines what fault current the screen can carry until the protection trips.

MV cable conductors come in standard sizes — 50, 70, 95, 120, 150, 185, 240 mm² and larger; screens are usually 16, 25, 35 or 50 mm². The designer chooses the specific values based on the load, laying conditions (depth, neighbouring cables, ducts, soil), fault currents at the connection point, DSO requirements and future network development.

Cable voltage classes

A cable's insulation class is written as U₀/U, where U₀ is the voltage between conductor and screen and U is the phase-to-phase network voltage. For networks built in Ukraine, the most common are:

Cable class U0/U	Network
3.6/6 kV	6 kV networks
6/10 kV	10 kV networks — the most common case in distribution grids
12/20 kV	20 kV networks
18/30 kV and 20/35 kV	35 kV networks and internal MV networks of large power plants

In Ukraine, 6–35 kV networks operate mainly with an isolated or compensated neutral, in which a single-phase earth fault may persist for some time until it is located. Under these conditions the insulation of the healthy phases is subjected to increased voltage, so the designer or the DSO may require a higher cable class than the nominal network voltage formally suggests. The final cable class is set by the design and the operator's technical requirements.

Which cable goes where

Type	Typical laying conditions
АПвЕгП, АПвПг	in the ground (trenches), in ducts, including HDD crossings, in wet soils; the polyethylene sheath resists moisture and mechanical stress during pulling
АПвЕгПу, АПвПу	difficult routes and soils, long pulls where a reinforced sheath is needed
АПвЕВ, АПвЕВнг	cable structures, switching station and substation rooms, tunnels and cable bridges where lower sheath flammability matters; in the ground only if the manufacturer and the design allow it

A polyethylene sheath propagates fire, so in cable structures and inside buildings such cables are either not used or fire-protection measures are provided. This is one of the issues the designer resolves at the ground–structure boundary, for example at the cable entry into a switching station.

Cable laying — what we pay attention to

• Depth and protection. The PUE sets the minimum laying depth of cable lines (for lines up to 20 kV — 0.7 m below finished grade, deeper under streets and squares) and requirements for protection against mechanical damage. The specific values and protection method are set by the design.
• Bending radius. Not less than the manufacturer's permitted value — for single-core MV cables it is usually 15 or more outer diameters. Too sharp a bend damages the insulation or screen.
• Pulling force. When a cable is pulled mechanically, the force is monitored so as not to exceed the conductor's permissible value. We use a hydraulic winch with pulling-force control.
• Temperature. The manufacturer specifies the minimum temperature at which the cable can be unreeled and laid without pre-heating.
• Bedding, backfill and marking. The cable is laid on a layer of sand or fine soil, covered with it and marked with warning tape or protective elements according to the design.
• Ducts. At road and utility crossings the cable is laid in ducts whose diameter allows it to be pulled freely without damaging the sheath.
• Cable ends. Ends awaiting jointing are closed with sealed caps to keep moisture out of the cable.
• Screen earthing. The earthing scheme for single-core cable screens (both ends, or single-point bonding or cross-bonding on long lines) is set by the design, since it determines the screen currents and cable heating.

Cable accessories

The cable itself is only half of the line. The other half is the accessories that connect drum lengths and terminate the cable at equipment:

• straight joints — connect two lengths of the same cable type; on a three-phase line of single-core cables a set of three joints is installed at each joint location;
• transition joints — connect a new XLPE cable to an existing paper-insulated cable, which is common in the reconstruction of urban networks;
• indoor terminations — in switching station and substation panels;
• outdoor terminations — on poles where the cable line changes to an overhead line, together with surge arresters;
• separable (T-type) connectors — for compact SF6-insulated switchgear and some transformers.

Accessories are selected for the specific cable construction: conductor cross-section, diameter over insulation and screen type. Each kit comes with the manufacturer's instructions giving the stripping lengths, the insulation cleaning method and the screen connection — following them is decisive for the joint's service life.

After laying and fitting the accessories, the line undergoes acceptance tests — see the article From design to acceptance for details. How we build cable lines is described on the MV and LV cable lines page, and the terms are in the glossary.