Products and Devices
4.6 Instrument Transformers
205Siemens Energy Sector t Power Engineering Guide t Edition 7.0
4
the housing; i.e., only the rupture disc is released. Gas escapes,
but the complete transformer remains intact and no explosion
occurs.
Most reliable insulation properties
SF6 gas is the main insulation medium between high-voltage
and earth potential. A stable quality can be guaranteed by the
use of SF6 gas according to IEC 60137 (2005) / ASTM 2472 D
and the fact that this inert gas shows no ageing even under the
highest electrical and thermal stresses. The insulation properties
remain unchanged throughout its lifetime. All of these features
guarantee an operation period over many years without any
control of the insulation condition.
Full functional security and monitoring
The guaranteed SF6 leakage rate is less than 0.5 % per year. The
gas pressure can be checked on site or by means of a remote
control device, i.e., a densimeter with contacts for remote
control. In the case of loss of SF 6 pressure, the transformer still
operates at rated pressure.
Environmentally beneficial under extremely severe conditions
SF6 gas is absolutely safe for humans. It bears no ecologically
toxic potential and its decomposition products have no delet-
enous effects on the environment, e.g., groundwater pollution.
This SF6 gas insulation medium allows easy waste management
of the transformers. Furthermore, the hydrophobic features of
the composite insulator result in problem-free service even
under saline fog or polluted conditions. As a long-term benefit,
the change of cores or windings, even after years, can be real-
ized easily for new requirements like additional metering.
Current transformers
All Trench current transformer (CT) designs are based on “head
type” construction. CTs are available with either oil (fig. 4.6-2) or
SF6 gas dielectric systems (fig. 4.6-3).
Features of oil-immersed type
tȋ Low weight and minimum oil volume
tȋ Excellent seismic performance as a consequence of the
optimized design of flanges, vast choice of porcelain strengths
and their interconnection and low weight
tȋ Available for the full voltage range of 72.5 kV up to 550 kV and
full current range of few Amperes up to 5,000 A with multiple-
turn primaries for small primary currents. Ratio change
available either on primary side or secondary side
tȋ Short, symmetrically arranged low-reactance bar-type primary
conductor permits higher short-circuit currents up to 80 kA
and avoids large voltage drop across the primary winding
tȋ Excellent control of internal and external insulation stresses
through the use of a proprietary finely graded bushing system
tȋ Hermetically sealed by stainless-steel metallic bellows and
high-quality gaskets
tȋ Uniformly distributed secondary windings guarantee accurate
transformation at both rated and high currents
tȋ Essentially unaffected by stray external magnetic fields
tȋ stable accuracy over life-time
Fig. 4.6-1: 800 kV gas-insulated
current transformersFig. 4.6-2: 550 kV oil-immersed
current transformers
Fig. 4.6-3: 420 kV gas-insulated current transformers
Top Related