Products and Devices 4.6 Instrument Transformers 205 Siemens 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 SF 6 gas is the main insulation medium between high-voltage and earth potential. A stable quality can be guaranteed by the use of SF 6 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 SF 6 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 SF 6 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., groundwat er pollution. This SF 6 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 curr ent transformer (CT) designs are based on “head type” construction. CT s are available with either oil (fig. 4.6-2) or SF 6 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 primar y 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 transformers Fig. 4.6-2: 550 kV oil-immersed current transformers Fig. 4.6-3: 420 kV gas-insulated current transformers