Gas Density Switches for High Voltage SF6 Circuit Breakers
Click here for Solon SF6 Gas Density Switches
What is an SF6 gas density switch?
These devices are used to provide protection to high voltage circuit breakers against a loss of the dielectric insulating gas, sulphur hexafluoride (SF6). Since the gas is contained in a vessel of a fixed size, any change in temperature will affect the pressure within the tank. The density switch must determine if any SF6 gas has been lost even with constantly changing tank pressure. It accomplishes this by altering the set points to follow an SF6 isochor (density level). If a combination of temperature and pressure fall below predetermined levels, the switch will alarm or lockout depending on the action required.
How does an SF6 density switch work?
By measuring pressure and temperature, a density level can be inferred.
The measurement of the pressure is straightforward. The pressure in the vessel is applied to a flexible all metal bellows sensing element. The force produced by the bellows actuates a switch mechanism which is counterbalanced by a calibrated spring. This alone would be a simple pressure switch. The addition of a temperature input is what tells the switch to alter the set points so that it follows the appropriate SF6 isochor. The temperature may be input in several ways:
Remote Bulb temperature sensing. A remote bulb assembly consists of a capillary tube with a bulb on one end and a flexible metal bellows on the other. The assembly is filled with a fluid that will expand and contract depending of the temperature. The expansion and contraction of the fluid will cause the bellows to move and add or subtract a load to the density switch mechanism. It is the change in load that alters the set points and forces the density switch to follow a specific isochor. The advantage of the bulb style mechanism is that the bulb can be placed remotely from the density switch itself.
Intrinsic temperature sensing. This refers to any sensing method that is contained within the switch housing. Typically it consists of a bi-metal component that will add or subtract a load on the density switch mechanism in the same manner as a remote bulb. These units are typically more compact and less costly but the mounting location is more important since the unit needs to change temperature at a similar rate to the circuit breaker tank.
The density levels vary depending on the dielectric level required, the higher the dielectric level required, the higher the pressure required for any given temperature. Decreased temperatures or increased pressure will eventually cause the liquification of the gas. Liquification is accompanied by a decrease in density and the density switch must be capable reacting to the decrease by alarming at the proper level. Some applications make use of mixed gases of SF6 and nitrogen to order to find a balance between low temperature operation and density level.
Click here for Solon SF6 Gas Density Switches
Solon has been producing SF6 gas density switches since the mid 1970’s originally for General Electric. Since that time we have designed and manufactured hundreds of different versions for OEM use. A few of our current customers include:
Areva
Siemens PT&D
ABB Power T&D
Mitsubishi Electric Power Products Inc.
GE Hitachi HVB
Pennsylvania Breaker
While the majority of our manufacturing is for OEM use, we also produce many specials for use by utilities. Just a few of customers include:
Con-Ed NY
Southern Company
Southern California Edison
First Energy
Niagara Mohawk
Allegheny Power
Arizona Public Service
BC Hydro
Trench Electric
Merlin Gerin
Cooper Power
Gulf States Power
British Columbia Hydro & Power Authority
Below is a more comprehensive explanation of high voltage circuit breakers and the heart of the breaker, the interrupting chambers.
High-voltage circuit-breakers have greatly changed since they were first introduced about 40 years ago, and several interrupting principles have been developed that have contributed successively to a large reduction of the operating energy. These breakers are available for indoor or outdoor applications, the latter being in the form of breaker poles housed in ceramic insulators mounted on a structure.
Current interruption in a high-voltage circuit-breaker is obtained by separating two contacts in a medium, such as SF6, having excellent dielectric and arc quenching properties. After contact separation, current is carried through an arc and is interrupted when this arc is cooled by a gas blast of sufficient intensity.
Gas blast applied on the arc must be able to cool it rapidly so that gas temperature between the contacts is reduced from 20,000 K to less than 2000 K in a few hundred microseconds, so that it is able to withstand the transient recovery voltage that is applied across the contacts after current interruption. Sulphur hexafluoride is generally used in present high-voltage circuit-breakers (of rated voltage higher than 52 kV).
In the 1980s and 1990s, the pressure necessary to blast the arc was generated mostly by gas heating using arc energy. It is now possible to use low energy spring-loaded mechanisms to drive high-voltage circuit-breakers up to 800 kV.
Brief history
The first patents on the use of SF6 as an interrupting medium were filed in Germany in 1938 by Vitaly Grosse (AEG) and independently later in the USA in July 1951 by H.J. Lingal, T.E. Browne and A.P. Storm (Westinghouse). The first industrial application of SF6 for current interruption dates back to 1953. High-voltage 15 kV to 161 kV load switches were developed with a breaking capacity of 600 A. The first high-voltage SF6 circuit-breaker built in 1956 by Westinghouse, could interrupt 5 kA under 115 kV, but it had 6 interrupting chambers in series per pole. In 1957, the puffer-type technique was introduced for SF6 circuit breakers where the relative movement of a piston and a cylinder linked to the moving part is used to generate the pressure rise necessary to blast the arc via a nozzle made of insulating material (figure 1). In this technique, the pressure rise is obtained mainly by gas compression. The first high-voltage SF6 circuit-breaker with a high short-circuit current capability was produced by Westinghouse in 1959. This dead tank circuit-breaker could interrupt 41.8 kA under 138 kV (10,000 MV-A) and 37.6 kA under 230 kV (15,000 MV-A). This performance were already significant, but the three chambers per pole and the high pressure source needed for the blast (1.35 MPa) was a constraint that had to be avoided in subsequent developments. The excellent properties of SF6 lead to the fast extension of this technique in the 1970s and to its use for the development of circuit breakers with high interrupting capability, up to 800 kV.
The achievement around 1983 of the first single-break 245 kV and the corresponding 420kV to 550 kV and 800 kV, with respectively 2, 3, and 4 chambers per pole, lead to the dominance of SF6 circuit breakers in the complete range of high voltages.
Several characteristics of SF6 circuit breakers can explain their success:
- Simplicity of the interrupting chamber which does not need an auxiliary breaking chamber;
- Autonomy provided by the puffer technique;
- The possibility to obtain the highest performance, up to 63 kA, with a reduced number of interrupting chambers;
- Short break time of 2 to 2.5 cycles;
- High electrical endurance, allowing at least 25 years of operation without reconditioning;
- Possible compact solutions when used for GIS or hybrid switchgear;
- Integrated closing resistors or synchronized operations to reduce switching over-voltages;
- Reliability and availability;
- Low noise levels.
The reduction in the number of interrupting chambers per pole has led to a considerable simplification of circuit breakers as well as the number of parts and seals required. As a direct consequence, the reliability of circuit breakers improved, as verified later on by CIGRE surveys.
Thermal blast chambers
New types of SF6 breaking chambers, which implement innovative interrupting principles, have been developed over the past 15 years, with the objective of reducing the operating energy of the circuit-breaker. One aim of this evolution was to further increase the reliability by reducing the dynamic forces in the pole. Developments since 1996 have seen the use of the self-blast technique of interruption for SF6 interrupting chambers.
These developments have been facilitated by the progress made in digital simulations that were widely used to optimize the geometry of the interrupting chamber and the linkage between the poles and the mechanism.
This technique has proved to be very efficient and has been widely applied for high voltage circuit breakers up to 550 kV. It has allowed the development of new ranges of circuit breakers operated by low energy spring-operated mechanisms.
The reduction of operating energy was mainly achieved by the lowering energy used for gas compression and by making increased use of arc energy to produce the pressure necessary to quench the arc and obtain current interruption. Low current interruption, up to about 30% of rated short-circuit current, is obtained by a puffer blast.
Self-blast chambers
Further development in the thermal blast technique was made by the introduction of a valve between the expansion and compression volumes. When interrupting low currents the valve opens under the effect of the overpressure generated in the compression volume. The blow-out of the arc is made as in a puffer circuit breaker thanks to the compression of the gas obtained by the piston action. In the case of high currents interruption, the arc energy produces a high overpressure in the expansion volume, which leads to the closure of the valve and thus isolating the expansion volume from the compression volume. The overpressure necessary for breaking is obtained by the optimal use of the thermal effect and of the nozzle clogging effect produced whenever the cross-section of the arc significantly reduces the exhaust of gas in the nozzle. In order to avoid excessive energy consumption by gas compression, a valve is fitted on the piston in order to limit the overpressure in the compression to a value necessary for the interruption of low short circuit currents.
Self-blast circuit breaker chamber (1) closed, (2) interrupting low current, (3) interrupting high current, and (4) open.
This technique, known as “self-blast” has now been used extensively since 1996 for the development of many types of interrupting chambers. The increased understanding of arc interruption obtained by digital simulations and validation through breaking tests, contribute to a higher reliability of these self-blast circuit breakers. In addition the reduction in operating energy, allowed by the self blast technique, leads to longer service life.
Double motion of contacts
An important decrease in operating energy can also be obtained by reducing the kinetic energy consumed during the tripping operation. One way is to displace the two arcing contacts in opposite directions so that the arc speed is half that of a conventional layout with a single mobile contact.
The thermal and self blast principles have enabled the use of low energy spring mechanisms for the operation of high voltage circuit breakers. They progressively replaced the puffer technique in the 1980s; first in 72.5 kV breakers, and then from 145 kV to 800.
Copyright (c) 2008 Solon Manufacturing Company. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".
Circuit breaker, http://en.wikipedia.org/w/index.php?title=Circuit_breaker&oldid=246778859 (last visited Oct. 24, 2008).
