siemens power engineering guide 7e 205

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