Power cable basics
In this short series we talk about power cables. Naturally, we focus on cables in VFD applications, particularly on cables between inverter and motor. Before diving into specific topics related to VFD operation let’s spend some time looking at power cables in general. Welcome to power cable basics.
Drive system components, such as transformer, VFD and motor (eventually also others) must be electrically connected when the system is in operation. The connection can usually be done using either power cables or busbars. While busbars are very common inside the VFD itself, for external connections power cables prevail. VFD applications pose specific requirements on the power cables. These requirements will be described in more detail within this series. However, for a better understanding, this article deals with power cable basics. It is a foundation that we will build on.
1. Construction of power cable
The power cable construction depends on the manufacturer’s design and purpose of cable (application). Despite of that we can identify common elements that each power cable has:
– Electric conductor
– Insulation system
– Protective jacket / sheath
1.1 Conductor
Conductor is the working part of the cable that enables flow of current. Copper (Cu) or aluminum (Al) are used as conductor material. You have certainly heard the term core which is the conductor of one phase. Standard power cables are available either as single core cables or three core cables.
There are various types of conductor structure. Note that these structures might be called slightly different depending on manufacturer. For small cross sections of the core a round single conductor is often used. As the cross section increases a round multi wire core construction is typically used. Those wires may be stranded or compressed.
Next, a segmental (sector) shaped core is used. The core is divided into several segment-shaped conductors. 4 to 7 segments (sectors) are typically used. Those segments are insulated from each other by semi-conductive or insulating tape. Segmental conductor is also called “Milliken”. It is typically used for large cross sections (e.g., 1200 mm2 and above for aluminum, 1000 mm2 and above for copper).
Finally, a segmental stranded enamelled conductor structure is used. It is suitable for largest cross sections (typically 1600 mm2 and above). It is a segmental type of conductor where approx. 2/3 of the wires are enamelled. By doing so, the proximity effect is almost eliminated and skin effect is considerably reduced.
1.2 Insulation system
Insulation is crucial for proper function of the cable. The choice of insulation material depends on the system voltage and cable application.
Medium voltage cable mostly use either PVC (polyvinyl chloride) or XLPE (cross-linked polyethylene). In high voltage range XLPE is the most common insulation material. In some applications, for example submarine cables, EPR (ethylene propylene rubber) is preferred due to particular advantages.
1.3 Shielding
Power cables are available without or with an electrostatic screen (shield)¹. Specifically in VFD applications screened cables are often required (see manufacturer’s recommendation).
Screen has three basic functions: The main function is to eliminate electric field outside of the cable (Faraday cage effect). The secondary function is a radial barrier to protect the insulation system against humidity (“waterproofing” relevant especially for high voltage cables). Finally, the third function is an electric conductor for zero-sequence current. The screen can be realized as a wire or a tape (foil). For shorther cables foil screen is a good choice, also with respect to higher frequencies. For longer cable installations a wire screen is tendentially preferred. Each consutruction has specific advantages and drawbacks. We will come to that in more detail when talking specifically about VFD cables and associated challenges.
¹ Terms such as shield and screen are being used.
1.4 Jacket
Cable jacket (also called outer jacket or sheath) provides mechanical protection of the cable and protection against ambient (moisture, UV radiation, aggressive environment etc). It also insulates the metallic screen from the ground and protects it against corrosion. Material used for jacket is PVC (polyvinyl chloride), polyethylene (PE), polypropylene (PP) etc.
2. Design of power cables
2.1 Cable cross section
When talking about cross section it is referred to the cross section of core. In Europe and most parts of Asia the cross section is expressed in mm². In United States, the unit area is commonly expressed in circular mil (cmil). This is very small unit so for practical reasons kcmil is used (1 kcmil ≅ 0.507 mm²). Cables are typically manufactured with standardized cross sections, such as 25, 35, 50, 70, 95, 120, 185, 240, 300, 400, 500, 630, 800, 1000 etc mm².
2.2 Insulation level
Practically all cable manufacturers use standardized insulation classes for the cable design. The cable insulation level is characterized by the rated phase-to-ground voltage, rated phase-to-phase voltage and the highest permissible voltage. Typical format of the insulation level is U0/U (Um), for example 6/10 (12) kV.
U0 … rated phase-to-ground voltage
U … rated phase-to-phase voltage
Um … highest permissible voltage
3. Cable parameters
3.1 Resistance
Cable datasheets often specify both DC resistance and AC resistance at nominal frequency (50 Hz or 60 Hz). The specific resistance is given in Ω/km and depends on the cross section of the core as well as operating temperature.
For higher frequencies (>> 50 Hz) the skin effect shall be considered. Resistance values of power cables with aluminum and copper conductor are illustrated in Tables 1 and 2. The values are valid for 50 Hz power frequency.
3.2 Inductance
Cable inductance (or inductive reactance) depends among others on cable layout. The datasheets normally state reactance values for trefoil and flat cable configuration. Specific inductance is mostly given in mH/km.
3.3 Capacitance
Cable capacitance is affected by the cross section of the cable, the insulation level (i.e. insulation thickness) and cable geometry. The higher the insulation level the lower the specific capacitance. Specific cable capacitace is typically given in μF/km (micro farrad per kilometer). The catalogues often state susceptance value as well. Susceptance (B) is reciprocal of capacitive reactance and the unit used for power cables in usually μS/km (micro siemens per kilometer).
Cable can be modelled as a π-element or a series of π-elements. The number of elements depends on the total cable length and maximum frequency of interest.
Note: The parameters are not constant but frequency dependent. Sign up for Premium subscription to learn more about this and many other topics.
4. Cable dimensioning
Cable is dimensioned based on its “nominal” current rating and multiple derating factors considering the laying (in air/underground), the ambient or soil temperature, soil thermal resistivity, laying depth, number of cables and their arrangement etc. In VFD applications there is an additional rating factors considering the thermal effect of harmonics.
For detailed dimensioning rules and recommendations refer to ABB switchgear manual or manufacturer’s guidelines.
5. Design lifetime
Power cables are designed for a long lifetime ranging 30 to 40 years or even more. The service life is influenced by the used materials, construction, production methods and operating conditions. The insulation system is mainly affected by temperature and voltage. As a rule of thumb, a long-term increase of operating temperature by 8 to 10°C reduces the service life by 50%. Similarly, an increase of operating voltage by 8 to 10% above the design value reduces the service life by half.
6. Cable installation
Several aspects related to cable installation shall be considered:
- cable laying
- bending radius
- cable support
- maximum pulling force
- dynamic forces
- cable bonding
This section is available for our premium subscribers.
Renowned cable manufacturers
Selected world-class cable manufacturers are listed below (alphabetically):
♦ Brugg Cables, https://bruggcables.com/
♦ Nexans, https://www.nexans.com/
♦ NKT cables, https://www.nkt.com/
♦ Prysmian Group, https://www.prysmiangroup.com/en
Do you want to learn more? Sign up for one of our premium plans!
References
[1] ABB switchgear manual
[2] Long motor cables, https://mb-drive-services.com/long-motor-cables/
