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Exploring HV circuit breaker technology

04 Nov 2015  | Paul Pickering

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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.

Arc formation
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.

Arc extinction
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 deionise 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 specialised design techniques to extinguish arc in the most efficient manner.

The table shows a comparison of the different media.

Table: 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.

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