One element that plays a vital role in delivering the energy generated by wind farms is cable. Wind turbines – nacelle, tower and base – comprise a variety of approximately 50 – 100 cables. This includes power transmission and distribution as well as control and communications cables.
Transformer location and generator dictate cables
The basic composition of a turbine consists of cables used within the nacelle, which connect to the tower cable. The top section of the tower cable is free-hanging and is known as a loop cable. It’s called a loop cable because it has to loop around inside the nacelle as turns into the wind. It then transports power down into the lower section of the tower, where the cable connects to the sides, before connecting to the grid.
The cables used are a mixture of low voltage (LV) and medium voltage (MV) cables. Their configuration within the turbine depends on the type of generators (LV or MV) and their design, and where the transformer is located, as it can be installed inside the nacelle, at the middle/bottom of the tower or, in some cases, outside the tower.
When the transformer is located within the nacelle, a medium voltage (MV) cable rated at 36 kV is usually used to connect the cable from the transformer to the distribution point at the tower base.
In contrast, when the transformer is located at the middle/bottom of the tower, a LV cable (0.6 to 1 kV) is used to connect the LV generator in the nacelle to the transformer. Sometimes, both LV and ‘low’ MV cables are installed as loop cables between the nacelle and the tower (DFIG generator). The transformer steps the voltage up to MV (20 – 36 kV) before sending it to the grid.
There is, however, a current trend in the market to use systems with the transformer located in the nacelle (mainly for onshore). The driver behind this trend is the need for turbines to deliver more power, hence operating at higher voltages.
The four general types of cables within a turbine
There are typically two main kinds of cable used within the nacelle. This includes: low voltage (LV), up to 105ºC flexible cables, and medium-voltage 180ºC single core cables.
The LV power cables are designed for flexible applications (good bending radius), possible contact with aggressive chemicals, ozone resistance, and to withstand high temperatures up to 90/105ºC, and extreme low temperatures up to -40ºC. Therefore they usually have a special rubber insulation and sheath (or compounds with similar physical performances).
There are also MV 180ºC single core cables (silicone insulated), designed to carry high levels of current in extremely hot conditions. These cables are mostly used as output connections from the winding bars of Class H generators and current converter cabinets.
The MV/LV flexible cables (power and control) used directly inside the nacelle, and which go down from the top part of the tower (loop cables) need to be light and flexible (to withstand torsional stress) because they must be able to withstand up to four full turns in either direction – after that the operator will intervene to rotate the nacelle back to its starting position. They come in 1-, 3-, or 4-core versions, depending on the specific power transmission requirements.
These cables are oil-abrasion-, UV- and ozone-resistant, and handle temperatures ranging from -40°C up to 90°C. Increasingly there is a demand for LS0H (Low Smoke halogen free) material for the insulation and sheath.
The cables found within the wind turbine tower comprise LV loop rubber cables; MV loop rubber cables and LV fixed installation cables.
LV cables are used to transport electricity from the generator to the transformer. As with MV/LV flexible leads, these cables are also resistant to oil, abrasion and so on.
LV fixed installation cables are usually installed along the wall of the tower, to ensure efficient energy transmission.
The MV loop rubber cables need similar performances to the LV loop rubber cables, and can handle voltages up to 35 kV to provide direct connections – without any intermediate junction point – between a nacelle-based transformer and the switchgear at the base.
Aside from power cables, the systems within the turbines require sophisticated control, electronic and data transmission cables, and fibre-optic cables to help manage them.
Control cables can comprise anything from 2 – 100 cores, depending on their application. They are flexible, mainly shielded (for EMC protection) and are used to carry small currents (ranging from voltages of 300 V to 1 kV) and low frequency signals that are used to control the motor drive, or the generator for braking, positioning and optimising rotor speeds.
Electronic and data transmission cables are used to control all electronic and mechanical devices. They comprise thermoplastic modified 2 – 5 core sensor multicore and multipair cables, which are used to measure wind speed, temperature and performance parameters. Two-core Fieldbus cables are used in parallel with power cables to digitally control all electronic and mechanical devices, while 2-core Profibus cables deliver up to 12 Mbits for complex control services, and data transmission cables offer Industrial Ethernet speed. Increasingly, all cables are EMC shielded.
Fibre-optic cables are used to assure high data transmission capacity for monitoring and control.
The role of testing – loop cables
All cables within the turbine have to undergo international certification testing. Within the wind power sector it is vital that cables used are able to meet UL and CSA standards.
‘Loop cables’ in particular have to undergo stringent case-by-case testing according to the system they will be installed in. This requires stringent torsion and strength testing.
Testing usually occurs on 10-metre lengths of cable. To ‘pass’ the test, the cable manufacturer and the OEM agree on the number of cycles with a minimum rotational angle per metre that the cable needs to be able to withstand during the wind turbine’s service lifetime – typically 20 years. Usually the aim is to be able to achieve 2000 cycles minimum during the test.
Cable installation within turbines
When cable is supplied to OEMs for use in turbines they can either be supplied according to the required lengths, ready for installation, or on drums, and cut to length later. Alternatively, OEMs use harness makers to prepare cable lengths for them, ready for installation into various positions within the turbine.
There is, however, also a trend for OEM manufacturers to ask for kits from cable manufacturers that include cable that is pre-cut to the various lengths of the tower, that are already stripped on one or both sides of the cable, and which have the appropriate connectors, to help speed up the installation process on site.
When manufacturing a turbine, the nacelle is pre-assembled in the factory and this is where all the internal wiring is handled, but when the turbine’s tower is constructed it is done so in stages. This is because a typical tower can extend up to 140 metres in height (the average is 90 metres), making it practically impossible to transport and install the tower in one go. Therefore, towers are split into four or five sections, approximately 20 – 25 metres in length. Within these sections the appropriate cables can be preinstalled.
The preinstalled cable within the tower sections are usually 50 – 60 centimetres longer on either side of that particular piece of installed cable. This makes it easier to install and connect cable and tower sections together. The reason behind the additional length is because it is cheaper and safer in the long run to cut cable shorter and connect it together than to add additional pieces of cable in those instances where the cable lengths are, initially, too short.
Ideally the industry would prefer to cut and install cable according the precise lengths set out in plans. Unfortunately the sector is not at a stage where it can be this precise but cable manufacturers are working on solutions for the near future.
Cable is also preinstalled into the tower so that both time and money can be saved during the installation process – specially trained electrical installers have to be enlisted to connect up the cables. Using their services can be costly depending on the amount of time taken. As the role of renewables, especially wind power, increases it will become even more critical for OEMs and utilities to install cost efficient cable and accessory solutions that adhere to the strictest safety and performance standards.
Thibaut Zumsteeg is global marketing manager for wind energy at Nexans.