HV circuit breaker technology: A horse of a different color
For most of my life, my main experience with circuit breakers has been resetting the kitchen or bathroom Ground Fault Circuit Interrupters (GFCIs) when they occasionally trip. As a result, although I’ve experienced first-hand the tremendous improvement in other electronic components over the years – actually, decades – in everything from op amps to microcontrollers and even passives, I just naively assumed that circuit breaker (CB) technology was a sleepy backwater somehow unaffected by progress. Out of sight, out of mind, I suppose. Feel free to roll your eyes at this point…..
Recently, I had an opportunity to do a little research on CB technology, and it really opened my eyes. It’s no surprise to many of you, I’m sure, but the field has moved on in the last 30 years.
Most designers (even low-voltage types like me) have an idea of what a circuit breaker is and what it does; here’s a quick definition:
“A mechanical switching device capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying for a specified time and breaking currents under specified abnormal circuit conditions such as those of short circuit.” – IEC 60050
Seems pretty simple, right? But circuit breakers are used in a large number of situations with vastly differing voltage and current requirements. And when you’re talking high voltages, a whole new set of considerations comes into play.
“High voltage” in this context refers to voltages of 72.5kV or above, as defined by the International Electrotechnical Commission (IEC). Primary applications for HV circuit breakers are in electricity generation and transmission: overhead transmission lines, bus transfer switching, generator switching, transformers, capacitor banks, protection, etc.
An ideal circuit breaker should act as an ideal conductor in the closed position, and as an ideal insulator in the open position; a real-world circuit breaker must carry its rated voltage and current when closed, and withstand its rated voltage when open.
At low voltages, of course, merely opening the contacts is adequate to stop the flow of current given that the dielectric strength of air is 3,600 V/mm, but HV circuit breakers require special techniques. When the contacts open, voltage stress across the initially small gap breaks down the insulating medium, causing an electric arc, which provides a high-current, and low voltage path between the contacts. As the gap widens, the arc will lengthen unless the dielectric strength of the gap exceeds the voltage required to maintain it, which is lower than the initiation, or strike, voltage.
Figure 1: Arc produced by 500kV switch failure (source: Pinterest)
In an AC circuit breaker, the voltage drops to zero twice per cycle, automatically extinguishing the arc unless the voltage exceeds the strike voltage. DC voltages do not have this property, so their arcs are therefore harder to extinguish.
The resistance along the arc creates heat, which ionizes more molecules in the insulating medium; continued arcing is a function both of the potential difference between the contacts, and the concentration of ionized particles.
Arc quenching (extinction) can be achieved by increasing the contact separation, but since HV systems might require an impractically large gap, HV circuit breakers also use methods to deionize the arc gap by cooling the arc or removing the ionized particles from the space between the contacts.
HV Circuit Breaker Design Techniques
HV circuit breakers use a number of different insulating media, and specialized design techniques to extinguish arc in the most efficient manner.
Table 1 shows a comparison of the different media.
Table 1: Insulating Media Comparison
Figure 2: Breakdown voltages of different insulating media vs. gap distance (Source: ABB)
SF6 Circuit Breaker Construction
Two of the key goals in HV circuit breaker designs are a reduction in operating energy; and rapid extinction of the arc.
Sulfur Hexafluoride (SF6), as shown in the table above, has many desirable properties as an HV insulating medium. It’s electronegative and has a strong tendency to absorb free electrons.
The principle of operation of an SF6 circuit breaker is as follows: as the breaker contacts are opened and an arc is formed, the contacts are surrounded by a high pressure flow of SF6 gas. The gas captures the conducting free electrons in the arc to form relatively immobile negative ions. This loss of conducting electrons in the arc quickly builds up enough insulation strength to extinguish the arc.
A gas blast applied to 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 appears across the contacts after current interruption.
When interrupting a fault current, heat generated from arcing will compress the SF6 in the compression cylinder, shown as #6 in figure 3. As a result, highly pressurized gas traveling through the nozzle will extinguish the arc. Self-blast technology is more efficient and requires less operating energy than other arc extinguishing methods.
Here’s a detailed description of this mechanism in the Siemens’ 3AP circuit breaker, used for voltages up to 245 kV.
Figure 3: SF6 circuit breaker operation (source: Siemens)
Description of operation
The current-conducting path of the circuit breaker consists of the contact carrier (1), the base (6) and the movable contact cylinder (5). When the CB is closed, the current flows via the main contact (4) and the contact cylinder (5).
During the opening operation, the main contact (4) opens first, and the current commutates to the still closed arcing contact. Next, the arcing contact (3) opens up and an arc is formed between the contacts.
At the same time, the contact cylinder (5) moves into the base (6) and compresses the SF6 gas located there. This gas compression creates a gas flow through the contact cylinder (5) and the nozzle (2) to the arcing contact, extinguishing the arc.
When it comes to interrupting the high current caused by a fault condition such as a short-circuit, the SF6 gas is heated up considerably at the arcing contact by the higher arc energy, compressing the gas in the contact cylinder.
As the circuit breaker continues to open, this increased pressure initiates a gas flow through the nozzle (2), extinguishing the arc. In this case, the arc energy itself is used to interrupt the fault circuit breaking current, so the energy does not have to be provided by the operating mechanism.
Although this type of circuit breaker technology is currently the dominant solution for very high voltages, since SF6 is such a potent greenhouse gas, environmental considerations are forcing designers to look at other gases.
Alstom, for example, has introduced a range of 100% SF6-free circuit breakers employing nitrogen as the insulating gas. Rated voltage is 72.5kV; it remains to be seen whether they’ll be able to extend the voltage and make a dent in SF6 dominance.