Megger DELTA 4000, Reference Manual

Page: 23 / 48

ZM-AH02E DELTA 4000 23
3 TESTING POWER SYSTEM COMPONENTS
Bushing test tap
A connection to one of the conducting layers of a capaci-
tance graded bushing for measurement of power factor and
capacitance values.
Capacitance (of bushing)
(1) the main capacitance, c
1,
of a bushing is the capacitance
between the high-voltage conductor and the voltage tap or
test tap.
(2) the tap capacitance, c
2
, of a capacitance graded bushing
is the capacitance between the voltage tap and mounting
flange (ground).
(3) the capacitance, c, of a bushing without a voltage or test
tap is the capacitance between the high-voltage conductor
and the mounting flange (ground).
Capacitance graded bushing
A bushing in which metallic or non-metallic conducting
layers are arranged within the insulating material for the
purpose of controlling the distribution of the electric field
of the bushing, both axially and radially.
Cast insulation bushing
A bushing in which the internal insulation consists of a
solid cast material with or without an inorganic filler.
Composite bushing
A bushing in which the internal insulation consists of sev-
eral coaxial layers of different insulation materials.
Compound-filled bushing
A bushing in which the radial space between the internal in-
sulation (or conductor where no internal insulation is used)
and the inside surface of the insulating envelope is filled
with insulating compound
Creep distance
The distance measured along the external contour of the
insulating envelope which separates the metal part operat-
ing at line voltage and the metal flange at ground voltage.
Insulating envelope
An envelope of inorganic or organic material such as a ce-
ramic or cast resin placed around the energized conductor
and insulating material.
Internal insulation
Insulating material provided in a radial direction around
the energized conductor in order to insulate it from ground
voltage.
Major insulation
The insulating material providing the dielectric, which is
necessary to maintain proper isolation between the ener-
gized conductor and ground voltage. It consists of internal
insulation and the insulating envelope(s).
Oil-filled bushing
A bushing in which the radial space between the inside
surface of the insulating envelope and the internal insula-
tion (or conductor where no internal insulation is used) is
filled with oil.
Oil-impregnated paper insulated bushing
A bushing in which the internal insulation consists of a
core wound from paper and subsequently impregnated with
oil. The core is contained in an insulating envelope, the
space between the core and the insulating envelope being
filled with oil.
Resin-bonded paper-insulated bushing
A bushing in which the internal insulation consists of a
core wound from resin coated paper. During the winding
process, each paper layer is bonded to the previous layer
by its resin coating and the bonding is achieved by curing
the resin. Note: a resin bond paper-insulated bushing may
be provided with an insulating envelope, in which case the
intervening space may be filled with another insulating
medium.
Resin impregnated paper-insulated bushing
A bushing in which the internal insulation consists of a
core wound from untreated paper and subsequently im-
pregnated with a curable resin.
Solid bushing
A bushing in which the major insulation is provided by a
ceramic or analogous material
Non-condenser bushings
Non-condenser bushings include the following designs:
solid porcelain, gas-filled hollow shell bushings (porcelain
or epoxy shells). Solid porcelain bushings were used exclu -
sively in early electrical systems, but it became apparent that
there was a voltage limit to the application of these solid
porcelain bushings. Solid porcelain bushings were utilized
up through 23kV, but after that point alternative insulation
mediums had to be employed. The next step in bush-
ing construction used other materials between the metal
conductor and the solid porcelain shell. Some of the early
materials included oil, asphalt, & air. These designs worked
well, but given the ever increasing voltages of the world’s
developing electrical systems, it became apparent that ever
increasing diameter bushings would be required. These
large diameter bushings were impractical for an industry
determined to construct smaller apparatus. A new solution
had to be found. That solution was condenser bushings.
Today, our new sf
6
gas breakers are equipped with hollow
shell bushings, constructed of either porcelain or epoxy,
which are filled with sf
6
gas.
Condenser bushings
The major goal of condenser designed bushings is to
reduce the physical size of the bushing. This compaction al-
lows not only for a smaller bushing, but also a smaller host
apparatus (i.e. oil circuit breaker or transformer).
Condenser bushings allowed for this compaction by plac-
ing the foil condenser layers at varying intervals during the
winding of the paper core, which resulted in uniform volt-
age stress distribution axially throughout the bushing. Addi-
tionally, varying the lengths of the foil layers provided even
voltage distribution along the upper and lower ends of the
bushing. The incorporation of condenser layers in bushings
provided both radial and axial voltage stress control, which
resulted in smaller compact bushings. The condenser layers