UFC 3 -520-01
June 10, 2002
4-1.8
Basic Impulse Insulation Levels (BIL ).
4-1.8.1 The transformer winding BIL is the design and tested capability of its
insulation to withstand transient overvoltages from lightning and other surges. The
rated BIL usually increases with nominal voltage.
4-1.8.2 A 30 kV BIL is usually acceptable for system voltages up to 5 kV and 60 kV
BIL is usually acceptable for system voltages up to 15 kV. Higher BIL levels can be
applied in locations in which transient overvoltages are expected due to nearby lightning
strikes; 60 kV BIL and 95 kV BIL i s recommended in this case for 5 kV and 15 kV,
respectively.
4-1.8.3 Do not specify lower BIL levels solely because surge protection has been
installed.
4-2
LOW VOLTAGE TRANSFORMERS.
4-2.1
Transformers having a primary voltage of 600 volts or less for the supply of
lower voltages should be of the self-cooled, ventilated dry-type. Do not locate ventilated
dry-type transformers in environments containing contaminants including dust,
excessive moisture, chemicals, corrosive gases, oils, or chemical vapors. Transformers
should be designed for floor or wall mounting.
4-2.2
Three-phase transformers with three-phase legs on one core, and with delta
input windings and wye output windings are preferred.
4-2.3
Transformers should have a per unit impedance i n the range of 3 percent to 6
percent. Unless required for some specific design requirement, lower impedance
transformers should not normally be used because of the higher downstream short
circuit currents. If a lower impedance transformer is used, perfo rm an evaluation for the
impact of the higher short circuit current on downstream devices. Transformers with an
impedance higher than 6 percent should not normally be used because of the lower
efficiency and higher voltage regulation unless the design eva luation establishes a
specific need for the higher impedance.
4-2.4
Transformers located within buildings where noise is of concern, such as
hospitals or administrative facilities, must have a noise-level rating appropriate for the
application. Vibration isolators should be provided to minimize sound transmission to
the building structural system.
4-2.5
Transformers should not be operated in parallel because the resulting
interrupting duty requirements placed upon protective devices will increase the
installation cost for such an arrangement. Also, transformers operated in parallel are
subject to circulating currents, unless the impedances are carefully matched. In those
few cases where parallel operation is unavoidable, provide detailed rationale supporting
the proposed arrangement as part of the design analysis.
4-6
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