Instruction Manual
© GE Grid Solutions
6
2.
DESIGN AND CONSTRUCTION
2.1.
Inverted construction and
primary circuit
The active part of the OSKF current
transformer is located above the
porcelain insulator. The advantages of
this construction concern the primary
circuit conception. The primary circuit is
short, rigid and straight, minimizing
heating.
In
addition,
it
bears
electrodynamics forces more easily,
allowing high level performance, relating
to electrodynamics and thermal effects,
in a presence of short-circuit currents.
The primary bar (simple or multiple
ratios) with magnetic circuits are
centered to assure a distributed
magnetic flow in uniform way to avoid
local saturation. This arrangement
allows also better accuracy in transient
conditions.
2.2.
Secondary circuit
The ring-shaped magnetic circuits are
manufactured either from very high
permeability grain oriented steel or from
nickel alloy. In order to achieve a
satisfactory
transient
behavior,
protection cores can also be provided
with gaps.
OSKF may have metering and protection
cores with different loads and different
accuracy classes. Secondary turns are
evenly wound around the circumference
of the ring-shaped cores. Changing ratio
is available by means of taps on the
secondary or primary side.
All secondary windings are enclosed in
an aluminum box which protects them
against high frequency disturbances
while acting as a low voltage shielding
electrode.
This
aluminum
box
is
extended up to the base by an aluminum
tube which contains all secondary leads.
The
cross
section of
the
earth
connections is designed to support
defect currents.
The bottom of the hermetic and
watertight secondary terminal box is
closed by a fitted plate, with or without
cable gland depending on customer
requirements.
2.3.
Paper-oil insulation
Insulation
between
primary
and
secondary circuits is made by multiple
layers of insulating papers impregnated
with mineral oil.
To protect the oil from the environment,
the CT head is closed by a metallic
bellows. Due to its wave shape
construction, the bellows adapts to the
changes in oil volume without producing
any increase or decrease in internal
pressure. This bellows resists aging and
corrosion.
