Chapter # 2                      

ENCLOSURE MATERIAL                      

2.5   Enclosure Losses

          

          Any metallic object in the vicinity of a current-carrying conductor will have induced losses.  All enclosed Busbar Systems have metallic enclosures.  Non-magnetic metallic enclosure, such as Aluminium or stainless Steel will generate eddy current losses.  Magnetic metallic enclosure, such as sheet steel will, in addition to the eddy current losses, generate hysteresis losses.  Metallic segregation, if provided between the phases, will also generate losses and are considered as part of the enclosure losses.  These losses are sometimes referred to as parasitic losses.   Enclosure losses increase with the increase in the conductor current.  Along with the conductor losses, enclosure losses contribute to the temperature rise of the enclosure, which is one of the constraints in the design of an enclosed Busbar System.

          Certain types of enclosed Busbar Systems such as continuous type isolated phase bus generate losses due to intentionally induced circulating currents. 

          These losses will be fed by the source to which the Busbar System is connected.  The magnitude of the enclosure losses as a percentage of total losses depends upon the type of Busbar System. 

          Enclosure losses, in non-segregated & segregated phase bus,  also depend upon the type of construction of the enclosure and earthing scheme.  The enclosure currents are not uniform along the length of the Busduct and can result in increased current densities in certain locations.

          Eddy current losses vary with the conductivity of the enclosure material.  The skin depth of steel is forty times less than that of Aluminium.  It is therefore obvious that a thicker Aluminium enclosure or a thinner steel enclosure will generate fewer losses. 

          Hysteresis losses will decrease with the decrease in the thickness of the steel enclosure.

          The above phenomenon has dictated the choice of the enclosure material for different types of Busbar Systems.

          Where alternatives are acceptable, the selection of the enclosure material will be governed by specific requirements of mechanical strength, and resistance to corrosion provided its temperature at the hot spot under, rated conductor current, meets the specification requirements.  The enclosure may be given a coat of paint for additional environmental protection and improved emissivity.

          Enclosures of isolated phase bus tap-off connections to surge protection voltage transformer cubicle, excitation transformer, and auxiliary transformers run at temperatures that are significantly lower than the specified maximum temperature when operating under rated conductor current.  These connections are more often sized to withstand the short-time current for the specified duration.  The relative size and phase spacing of the tap-off enclosure are small while it is subjected to the largest magnitude of short circuit current in a generator connection.  The enclosure needs to withstand very high electrodynamic forces.  An Aluminium alloy enclosure material with a lower conductivity, in exchange for a higher mechanical strength is in order as long as it meets the specified temperature rise requirements.