How to size cables in high-power drive systems?
Most common power connection in between drive system components is a cable. In this post we look at some factors to consider when sizing cables in high-power medium voltage variable frequency drive (MV VFD) systems.
Cable basics
Let’s start with some basics first. Power cables are available in standardized cross sections, either expressed in mm² (millimetre square, used in Europe, most of Asia and many other regions) or using the AWG (American wire gauge). When talking about high-power drives, the cross sections typically range from 185 mm² to 400 mm². However, there are cases with even large cross sections: 500 mm², 630 mm² or even 800 mm². Depending on the transmitted current, multiple cables shall be laid in parallel (see further in cable sizing section).
The cables can be either single core or three core. Three core cable incorporates three cores corresponding to three phases within one cable, arranged in a triangle formation.
The insulation material is either cross-linked polyethylene (XLPE) or ethylene-propylene rubber (EPR). In the past, polyvinylchlorid (PVC) had been used as well but it is seldom the case for new installations.
Insulation levels of cables are standardized as well. They depend on the regions and their common practices. In Europe, it is common to express the insulation level in the format U0/U where U0 is the phase to ground (RMS) voltage and U is the phase to phase (RMS) voltage.
Cable construction
The most inner part of the cable is the core. That is the actual conductor which carries the current. Core material is either copper or aluminium. Copper is a better conductor in terms of resistance and losses. The relative resistivity of copper is about 1.58-times lower than aluminium. For example, a 400 mm² copper cable has AC resistance of 0.0569 Ω/km. Aluminium cable with the same cross section and same reference temperature has an AC resistance of 0.089 Ω/km. In case of using aluminium cables, pay attention to material compatibility.
The conductor is insulated by a layer of insulation. Insulation material is mostly cross-linked polyethylene (XLPE) or ethylene-propylene rubber (EPR).
Cables used in VFD systems shall often be shielded. The shield (also called screen) is typically realized as a thin layer of copper, either in form of a foil (tape) or a wire. The latter is recommended in case of longer cables. The shield has several functions: to minimize the electromagnetic field outside of the cable, to homogenize the potential and (especially in VFD applications) to provide a path for the common mode current.
Cable can be armored or not. The armor provides additional mechanical protection, resistance against moisture, chemicals or corrosive substances and also helps to reduce the electromagnetic field in the vicinity of power cables (similar like the shield but less effective). Armor is often realized using steel wires. In case of VFD cables a strategy is needed how to earth the armor.
Finally, outer jacket (sheath) provides mechanical protection against damages and water ingress.
Between conductor, insulation and shield there are semi-conducting layers to optimize the transition from one material to the other and to homogenize the field. They shall prevent electric stress concentration and help to supress the partial discharge activity.
Cable designation
The way how to designate the cable is a bit complicated. There is a type code to describe the specific type of cable. The code contains information about insulation voltage class, conductor material, insulation material, number of cores and their cross section, type of armor (if applicable) and additional special features (e.g. flame retardant cable, UV resistant material, fire retardant etc).
Example:
N2XS(F)2Y 12/20 1×300/25
- N: standardized cable (referring to DIN VDE 0276 620)
- 2X: XLPE insulation
- S(F): copper wire screen with copper tape
- 2Y: outer sheath of polyethylene (PE) material
- 12/20 kV insulation level
- 1×300: single core with 300 mm² cross section
- 25 mm² cross section of the screen
VFD specific considerations
When it comes to VFD applications, there are additional points to be considered.
i. Increased insulation level
Power cables inside the drive system are mostly exposed to non-sinusoidal voltages. There might be additional voltage peaks, the waveform on the motor side contains voltage steps with high dv/dt and there is a common mode voltage that increases the effective phase to ground peak voltages.
In most cases a simple rule makes sense:
- Cables between transformer and VFD shall have the same insulation level as the transformer secondary windings.
- Cables between inverter and motor shall have the same insulation as the stator of the motor.
That is easy to remember and consistent approach.
ii. Presence of harmonics
The load current flowing through the cable may be distorted and thus containing harmonics. Those components of higher frequency cause additional heat losses. Thus, the maximum loadability of the cable needs to be reduced compared to the reference case when loaded with a pure sinusoidal current. This is expressed with a certain factor – called rating factor or derating (factor).
For example, a specific VFD has a harmonic spectrum that asks for 5% reduction of fundamental current compared to a sine wave. Then we talk about rating factor 0.95 or 5% derating.
iii. Limitation of cable length
Power cables in VFD systems are usually limited in length. That applies especially for motor cables, i.e. cables between VFD inverter and the electric motor. Practically every VFD topology has a certain limit – the question is where/how much.
- High performance VFDs with superior dynamics are more restrictive. The maximum motor cable length is typically 200 – 300 metres.
- VFDs with less dynamics and more motor friendly waveform allow longer motor cables. The limit is usually between 500 and 1’000 meters depending on the VFD model and optional adaptations of the output filter.
- Voltage source inverters (VSI) with an output sine filter can support cable of several thousand meters. The limitation is a consequence of maximum allowed voltage drop.
- Current source inverters (CSI) mostly support longer motor cables. As these drives are less common, it is recommended to check directly with the manufacturer.
However, here is our take:
The maximum cable length given in a technical catalogue or brochure is just an indication. The real allowable cable length depends on additional factors, such as:
- cable cross section
- number of parallel cables
- cable insulation level
To get a project specific limit for maximum cable length, contact the VFD manufacturer.
iv. Cable shield and bonding
Purpose of the cable shield is to minimize electromagnetic interference (EMI) and potential disturbances, both within the drive system (e.g. disturbing measurement and control system) as well as external consumers (loads). Majority of VFDs require shielded cables. Exceptions are drives with a sine filter or multi-level inverters with a very high-quality waveform.
The VFD manufacturer may specify the minimum cross section of the shield to avoid overloading by the return current. Another principal question is how and where to earth the shield. The earthing connection shall be low-ohmic at high frequencies. Cable shield can be earthed on one side (inverter side or motor side) or on both ends. In this context we talk about single bonding and multiple bonding. Each choice has specific pros and cons and shall be very carefully evaluated.
v. Product- and installation restrictions
The cable sizing needs to respect restrictions coming from the product and installation. For example:
- Maximum number of power cables to be connected to the inverter output terminals (i.e. size of the terminal field)
- Maximum amount and cross section of cables to fir into the motor terminal box
- Bending radius in the installation
Cable formation: Single core versus three core cables
Is it better to use single core or three core cables? It depends. If possible, a three core cable with individual shields of each core as well as a global shield is the ideal solution with regards to the electromagnetic field distribution. However, for high values of rated current (focus of this article) a large cross section may be unavoidable. In such case single core cables are mostly used.
Compliance with EMC
Selection of a suitable power cable and its proper installation are key factors to achieve best compliance with electromagnetic compatibility (EMC) regulations and standards.
For example, it is not just about shielded cables but also how is the bonding done (where is bonding applied, what technique is used etc).
Cable sizing in high-power drives
After reading previous paragraphs you shall have some minimum information to proceed with actual cable sizing. It may look following:
- Collect the required input data
- Rated RMS current
- Overload current and duty cycle (if applicable)
- Information about harmonics
- Information about installation (cable laying, formation, grouping)
- Environmental data
- Determine all rating factors for your specific case/project
- Rating factors based on ambient temperature, soil thermal resistivity etc.
- Rating factors based on cable laying (in air, inside cable duct, under the ground,…)
- Rating factors based on number of parallel cables and their arrangement
- Rating factor for the converter operation (harmonics)
- Rating factor to consider higher output frequency (if applicable)
- Select suitable cable type based on requirements (suitable insulation level, cable construction etc) and get the current loading (ampacity).
- Calculate the required cross section and number of parallel cables considering all rating factors
- Multiple combinations exist (e.g. 6 x 400 mm² versus 5 x 500 mm²).
- Compare the variants from economical point of view
- Try to harmonize the cross section of motor cables with power cables between transformer and VFD and between switchgear and transformer
- Check the mechanical integrity (e.g. withstand of short-circuit forces) and voltage drop.
In high-power range of drives with rated motor current of 2’500 A or more, the amount of cables and their cross section become considerable. Especially when such drives are installed in countries with very warm climate where significant environmental derating applies. A typical case is the Middle East region. There you can easily end up with a cable monster.
Let’s take an example of 3’000 A rated motor current, single core cables, copper conductor, cables in an underground duct:
- 10x(3x1x500 mm²)
- 12x(3x1x400 mm²)
Handling of such amount of cables is not easy and requires good engineering and planning. Do not forget to respect the physical limitations of the VFD terminal field or motor and transformer terminal box.
Consequences of large cabling
Consequences of multiple parallel cables with large individual cross section are:
- High purchase cost of cables and high installation cost
- Large openings to accommodate all cables
- Large bending radius -> More space in electric room required
- High weight of cables -> reinforced cable trays, special cable supports
- Derating due to cable laying: The more parallel cables the less ampacity per cable → sort of saturation (an additional cable brings minimum increase of total ampacity)
How to deal with such installations?
- Consider realistic load current. Avoid unnecessary margin stacking (adding margin over margin over margin). In case of overloads, consider the duty cycle and equivalent RMS current instead of peak overload current.
- Prefer cables laid in air instead of ground. When the cables are under ground, better use a ventilated duct than simply buried cables.
- In case of cables partly laid under ground, different cross section can be used for the portion in the ground and for the cable laid in air (linked with point below).
- Respect the limitations (max. number of parallel cables, max. cross section) of terminal fields and terminal boxes of the components (transformer, VFD, motor).
Do you want to know more?
What types of cables are best suitable for VFD applications?
How does skin effect impact the cable sizing?
What to pay attention when dealing with cable shield?
How to find the balance between cable cross section and number of parallel cables?
⇒ Purchase our webinar on cable selection and sizing in high-power VFD applications
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References
[1] Power cable basics, MB Drive Services, November 2021, available online, https://mb-drive-services.com/power-cable-basics/
[2] Long motor cables, MB Drive Services, July 2020, available online, https://mb-drive-services.com/long-motor-cables/
