Cable or busbar? Which one is better?
In this article we will have a closer look at the two common power connections in drive systems. Is cable or busbar more suitable? The answer depends on several factors.
Power connections
1. Power cable
Power cables are used for power distribution or transmission. In this article the wording power cable refers to a medium voltage cable that interconnects main electric components. The fundamental components of a power cable are:
– electric conductor
– insulation system
– protective jacket / sheath
Modern medium voltage power cables have quite complex and sophisticated construction. However, the above three components are always part of a power cable. In addition, the power cables may have an electrostatic screen (or shield) which is often a requirement in VFD applications.
Cable parameters
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Cable connection is very versatile as cables are available is various cross sections and insulation classes, allow bending, can be laid in different environment etc.
Cable dimensioning
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2. Busbar and busduct
Busbar or busduct is another solution for power connection. Busbar is a solid structure with limited elasticity. It can be realized up to large cross section for highest current ratings.
Busbar can be used as blank (without any insulation layer) or insulated. Internal connection in clean area is often done with blank busbar whereas external connections can be done with blank or insulated busbar.
Insulated busbars are mostly realized with resin coating. A typical example of a commercial product is e.g. DURESCA® (DU = durable, RES = resin, CA = casting). The product is available up to 170 kV and current of 8’000 A. Up to certain size a full cross section is used. For higher current rating a tube type is used. The product is touch-safe and corrosion protected.
Busbar parameters
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Factors to consider
There are multiple factors that play a role in the decision process between a power cable and a busbar:
- Current and voltage rating
- Overall length of the power connection
- Routing of power connection
- Environmental factors and ambient conditions
- Installation responsibility
Hence the decision is mostly project specific. Despite that there are cases where only one solution might fit (e.g. busbar for connection between large generator and step-up transformer, cable for connection between inverter and motor with longer distance and complex routing).
Suitability of each solution
In many cases the decision between power cable and busbar is straight forward. This section presents typical examples.
1. Use of power cables
Power cable is ideally suitable in one of the following situations:
– Larger physical distance between components to be interconnected
– Exact distance not known or difficult to control (with uncertainty)
– Interface of two environments (e.g. indoors/outdoors)
– Laying in the ground
– Generally when flexibility is required (typically moving parts)
– When insulation is required due to safety (accessible areas)
2. Use of busbar or busduct
Busbar is a natural choice in situations such as:
– Mostly for shorter power connection between the components
– Internal connection inside a product with exactly known distances
– High current rating (that would require too many parallel cables)
– Very limited requirements on flexibility (rigid connection)
– Areas not accessible when the equipment is energized (unless using insulated busbar)
– Turn key type of projects where the whole system or subsystem is in responsibility of one supplier
Comparison
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Table 1: Comparison of power cable and busbar
Summary
Power cable and busbar system are the ultimate means to transfer power between system components of a variable speed drive system. Each option has its specific advantages as well as drawbacks. The choice depends on multiple factors such as distance between components and connection length, current and voltage rating, ambient conditions, type of installation, available space and so on.
