4-1
4.1  Introduction
e design of the high-voltage substation must include consideration for the safe operation and
maintenance of the equipment. Switching equipment is used to provide isolation, no-load switching
(including line charging current interrupting, loop splitting, and magnetizing current interrupting),
load switching, and interruption of fault currents. e magnitude and duration of the load currents and
the fault currents will be signicant in the specication of the switching equipment used.
System and maintenance operations must also be considered when equipment is specied. One sig-
nicant choice is the decision of single-phase or three-phase operation. High-voltage power systems are
generally operated as a three-phase system, and the imbalance that will occur when operating equip-
ment in a single-phase mode must be considered.
4.2  Ambient Conditions
Air-insulated high-voltage electrical equipment is usually covered by standards based on assumed
ambient temperatures and altitude. Ambient temperatures are generally rated over a range from −40°C
to +40°C for equipment that is air insulated and dependent on ambient cooling. Altitudes above 1000 m
(3300 ) may require derating.
At higher altitudes air density decreases; hence, the dielectric strength of air is also reduced and
derating of the equipment is recommended. Operating clearances (strike distances) must be increased
to compensate for the reduction in the dielectric strength of the ambient air. Current ratings gener-
ally decrease at higher elevations due to the decreased density of the ambient air, which is the cool-
ing medium used for dissipating the heat generated by the losses associated with load current levels.
However, the continuous current derating is slight in relation to the dielectric derating and in most
cases it is negligible. In many cases, current derating is oset by the cooler temperature of the ambient
air typically found at these higher elevations.
4
High-Voltage Switching
Equipment
4.1 Introduction ......................................................................................4-1
4.2 Ambient Conditions .........................................................................4-1
4.3 Disconnect Switches .........................................................................4-2
4.4 Load Break Switches .......................................................................4-13
4.5 High-Speed Grounding Switches .................................................4-17
4.6 Power Fuses......................................................................................4-17
4.7 Circuit Switchers .............................................................................4-19
4.8 Circuit Breakers ..............................................................................4-22
David L. Harris
SPX Transformer Solutions
(WaukeshaElectric
Systems)
David Childress
David Childress Enterprises

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