Characteristics of SF6 Gas and Circuit Breakers

Since the 1960s, SF6 gas has been successfully used as an insulating and arc-quenching medium in high-voltage switchgear. Today, SF6 circuit breakers have achieved or surpassed other types of breakers in terms of operating voltage levels and breaking performance. They dominate the high-voltage range at levels above 126 kV.

1.Properties of SF6 Gas
SF6 gas possesses exceptional arc-quenching and insulating capabilities unmatched by other mediums. It is the nearly exclusive insulation and arc-extinguishing medium in high, extra-high, and ultra-high voltage applications.

Physical and Chemical Properties:
Under standard conditions, SF6 is a colorless and odorless gas. It can liquefy under normal conditions, requiring temperatures above 45°C to remain in a gaseous state. As a result, SF6 cannot be used at excessively low temperatures or pressures. In high-voltage electrical equipment, the working pressure of SF6 ranges from 0.2 to 0.7 MPa, maintaining its gaseous form.

At room temperature (below 500°C), SF6 is chemically stable and inert, non-flammable, and non-reactive with substances such as water, strong bases, ammonia, hydrochloric acid, and sulfuric acid. However, during circuit breaker operations or internal faults, SF6 can decompose under electric arcs, corona discharge, sparks, or high temperatures, generating toxic by-products like SF₄, SF₂, SOF₂, HF, and SO₂F₂. When mixed with water, these substances form highly corrosive electrolytes that can damage internal materials and cause operational failures. Materials such as aluminum, steel, copper, and brass are largely unaffected, whereas glass, ceramics, and insulating paper are more susceptible to damage. The effects on other insulating materials are minimal. Adequate material selection and structural design can mitigate moisture and corrosion risks. Adsorbents like alumina, quicklime, molecular sieves, or their mixtures can remove moisture and SF6 decomposition products during equipment operation.

Electrical Characteristics:
SF6 gas exhibits excellent insulation properties due to its strong electronegativity, which allows it to capture free electrons and form negative ions. This characteristic accelerates the recovery of dielectric strength in arc gaps. SF6’s arc-quenching capability is approximately 100 times that of air, and its breaking capacity is 2–3 times greater, attributed to its unique thermal and electrical properties.

In an SF6 arc, the core region is highly conductive but has low thermal conductivity, while the outer region has high thermal conductivity and low electrical conductivity. The arc core reaches temperatures of 12,000–14,000 K, concentrating the arc current and reducing arc voltage and power, facilitating extinction. The outer arc column maintains lower temperatures (below 3000 K), aiding in dissipating the core heat. Additionally, the electronegative nature of SF6 gas and its by-products in cooler zones promotes recombination of ions, accelerating dielectric recovery after current zero.

2.Features of SF6 Circuit Breakers

SF6 circuit breakers’ performance derives from the exceptional arc-quenching and insulation properties of SF6 gas. Key advantages include:

High Current Capacity and Longevity: SF6 gas’s high molecular weight and specific heat enhance cooling effects on contacts and conductors, enabling higher rated currents of up to 12,000 A. The contacts can withstand higher temperatures with minimal damage, ensuring long electrical life.

High Short-Circuit Breaking Capacity: SF6’s excellent arc-quenching properties result in short arc durations and the ability to break currents typically exceeding 40–50 kA, with maximum values up to 80 kA.

Low Operating Overvoltage: SF6 gas minimizes current interruption trends around zero-crossing, reducing chopping currents and avoiding overvoltage. Its rapid dielectric recovery ensures reliable fault clearance without restrike, even when breaking capacitive currents, eliminating the need for parallel resistors.

Additional advantages include high reliability, safety, compact size, ease of installation and commissioning, minimal maintenance, and extended maintenance intervals.

However, SF6 circuit breakers have drawbacks:

High Manufacturing Standards and Costs: SF6 breakers demand precise manufacturing and advanced techniques, leading to higher costs—2–3 times those of oil circuit breakers.

Complex Gas Management: SF6 gas requires careful handling, especially under low temperatures or pressures, necessitating advanced recovery, analysis, and testing equipment. Decomposed toxic by-products pose safety risks, requiring precautions against asphyxiation in confined spaces.

Principles and Structure of SF6 Circuit Breakers

The development of SF6 circuit breakers has progressed through several stages, including double-pressure, single-pressure, self-blast, and intelligent secondary technology. Currently, double-pressure breakers are obsolete, while single-pressure breakers dominate at 550 kV and 1100 kV levels. Thermal expansion types are used at 110–245 kV and are advancing towards 420 kV. Intelligent secondary technology integrates electronics, sensors, and computing, enabling intelligent control and state-based maintenance.

1.Single-Pressure SF6 Circuit Breakers

Structure:
Single-pressure SF6 breakers are categorized into pillar and tank types. The arc-extinguishing chamber can feature variable or fixed gaps, with arc-quenching primarily achieved through gas compression, sometimes supplemented by thermal expansion. At voltages above 252 kV, single-pressure breakers are commonly used.

Pillar-type breakers position the arc chamber atop insulating columns, allowing modular voltage scaling by connecting multiple arc chambers. Tank-type breakers place the arc chamber inside a grounded metal tank, insulated by SF6 gas. This configuration offers low profile designs with better seismic resistance but requires more materials, gas, and cost.

2.Self-Blast SF6 Circuit Breakers

Structure:
Unlike compressed-gas designs, self-blast SF6 breakers utilize arc energy to extinguish itself, reducing actuator load and improving reliability.

Working Principle:
Self-blast designs include magnetic-blast and thermal-blast types. Magnetic-blast breakers use magnetic fields to spin the arc, facilitating cooling and extinction. Thermal-blast breakers use arc heat to generate high-pressure gas flows, creating a pressure differential for quenching. They often include auxiliary mechanisms for small current operations.