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very little relationship to how the Ballistic Breaker works.

Monday, November 12, 2012

ABB ANNOUNCES WORLD'S FIRST CIRCUIT BREAKER FOR HVDC

This announcement appeared on numerous websites on November 8, 2012:
ABB ANNOUNCES WORLD'S FIRST CIRCUIT BREAKER FOR HVDC
Switzerland-based ABB today announced that it has developed the world’s first circuit breaker for high voltage direct current (HVDC), solving what it says has been “a 100-year-old electrical engineering puzzle and paving the way for a more efficient and reliable electricity supply system.” The breakthrough holds promise not just for renewables development but also for all types of generation that nations and regions wish to transmit over long distances, including under large bodies of water. Read More »


There were many followup analyses after ABB's announcement, but the above link to Power Magazine was my favorite, and they got quotes from the right people!

This paper from the 2011 CIGRE conference in Bulogna describes their breaker concept pretty well. Here is a diagram of the breaker:

HERE is a link to the US publication of the PCT application. The high importance that ABB places on this technology is evident from the fact that they also have applied for several other versions of HVDC hybrid circuit breakers.

Their new device appears to fall under the above PCT patent, but the picture (above) from the Bologna CIGRE paper is much better for understanding how it works than the patent document. Although the auxiliary switch shown above is an IGBT, I believe that could also be a more recent innovation: IGCTs (integrated gate commutated thyristors); IGCTs were first introduced in shipboard DC circuit breakers (for 980 volt service) by ABB Marine in 2011. (This paper by Jean-Marc Meyer and Alfred Rufer describes how the hybrid breaker principle can be applied at MVDC, using IGCTs.) The fast electromechanical switch could be of several designs (ABB has several patents in this area; my personal favorite is US patent 6,501,635). 

The 2 to 5 ms delay that ABB cites in operation of their breaker (different numbers quoted in different places; which is quite fast compared to most prior art breakers) is not the time until current stops flowing, but the time at which di/dt changes sign and the current starts being reduced.  This delay is due to the fast mechanical switch, which must open far enough to prevent restriking an arc before the main IGBT array can be opened. The hybrid nature of the commutation differentiates ABB's approach from Lian's US patent 3,534,226, which is also one of the closest prior art patents to my Ballistic Breaker patent application



The ABB device works in this way: there is a parallel circuit between three pathways: in the middle is a series connected IGBT array that is capable of shutting off the HVDC circuit and withstanding an overvoltage higher than the normal line voltage. In this series-connected IGBT array, each IGBT component is shunted through a metal oxide varistor (MOV) that allows current to flow when voltage goes above a selected level. All the component IGBTs of the IGBT array could be switched off simultaneously; in this case there would be a rather large voltage spike as the voltage rises at least 50% over normal voltage to activate the string of series-connected MOVs. Note though that the individual IGBTs can also be switched off sequentially to control the over voltage during breaking of the circuit. (The overvoltage is due to insertion of resistance into the circuit; absorption of inductive energy by the MOVs is not instantaneous, so the voltage rises each time resistance increases, followed by decay. The big headache with any circuit breaker but especially an HVDC circuit breaker, is that the line inductance can vary a lot depending on where the short is located, and the current flowing at the time of the fault can also vary. ABB's method of rapid adaptive switching while opening the circuit was clearly anticipated in Lian's patent from 1970; the switching times for the various IGBTs can be varied to control the switching transients and the times allowed for decay between switching events to squelch the inductive energy in the flowing current in an optimized manner. Altenatively, one can just make a worst case assumption and switch on that basis (this is what happens in a Ballistic Breaker).  Or (most crudely) all the IGBTs in the array can be switched simultaneously, in which case the MOVs control the switching surge (but not very well). 

By controlling the switching of the IGBTs the voltage transient due to the voltage rising high enough to push current through the MOVs is split into many small parts, which can keep the overvoltage quite low if the timing of closing the IGBTs is properly controlled. Collectively, the MOVs must have enough capacity to absorb all the magnetic energy stored in the line, which can be hundreds of MJ, implying that the MOV array weighs a metric ton or so for a long HVDC line. An individual MOV begins to conduct around 1.5X the normal line current, so if all the IGBTs were switched simultaneously the voltage during shutoff could go to ~1.5-2X the normal line voltage. However, by controlling the switching time of the individual IGBTs the voltage switching transient could be much lower. This part of the breaker (the IGBT array) in isolation is the "power electronic breaker" to which Ram Adapa refers in the Power Magazine article cited above. 

The much lower resistance of the single IGBT or IGCT used for commutation means the on-state losses are (according to ABB's CIGRE paper) in the low tens of kW for a 2kA, 320kV HVDC line; if we take that to mean 40kW on-state loss in the commutating IGCT/IGBT, that implies only ~0.005% of transmitted power. This is much better than the on-state loss for an IGBT-based switch in which all the current flows all the time through the entire IGBT array, for which the on-state loss would be ~0.25% of power at full rated power. The ABB breaker is much better than that, and does indeed usher in an era where large interconnected high power HVDC grids (supergrids) can be protected by circuit breakers. I believe ABB's breaker will be more expensive than my Ballistic Breaker or a hybrid Ballistic Breaker, which follows the same idea as ABB's design of using a fast switch to do the first commutation to the device which ultimately opens the circuit, but where the circuit is opened via the electromechanical commutating circuit breaker (which I have been calling a Ballistic Breaker). I welcome ABB's innovation as the first viable HVDC circuit breaker, and I look forward to competing with them.

ABB's approach cuts the on state power losses, because in the on state, most of the power only goes through the low loss low voltage IGCT or IGBT, but makes an even more expensive circuit breaker than a simple IGBT-based switch with MOVs (which Ram Adapa mentioned in the Power Magazine article cited above). This is no doubt a big advance from the prior art DC circuit breaker of US patent 3,809,959 (from ASEA before they joined Brown-Boveri to form ABB; this device is still used in current-source converters around the world), but it is not true to say it is the "world's first circuit breaker for HVDC." It is true to say it is more compact and faster than the prior art methods, though I think the story put out by ABB that faster action (2-5 ms versus 50 ms) is crucial to create a workable HVDC grid is debatable (see this post by Gregor Czisch). Faster acting circuit breakers are indeed needed for multi-terminal HVDC based on VSC converters, but not for the older thyristor-based LCC designs (you can read about that in this excellent review by Professor Franck). The prior art ASEA method is widely deployed in HVDC schemes today, to shut down one leg of a bipole HVDC scheme when needed (so that the other leg can still operate as a monopole with ground return in case of a fault on one leg of the scheme). 

ABB's HVDC circuit breaker may not be cost competitive with an HVDC Ballistic Breaker (my invention, see www.ballisticbreaker.com), once I get funding to build one. The use of power electronics requires liquid cooling, and a high degree of redundancy. If I am correct, the cost of ABB's Hybrid HVDC Breakers will be about 25% of the cost of a VSC AC/DC converter station, or about $35/kW 
(AC circuit breakers at 200kV cost ~ $.15/kW for comparison); this is high enough that it will still be impossible economically to place ABB hybrid HVDC circuit breakers between every set of next neighbor power taps on main lines of the supergrid (which may well carry ~30 GW) in the future. The supergrid needs something much less expensive to make full circuit protection (as is routine in the HVAC transmission grid) feasible economically. The Ballistic Breaker is that device.



FYI, here is a video on the ABB hybrid circuit breaker’s operation:

And here is an interview with Claes Rytoft, head of ABB's Power Transmission Group:

9 comments:

  1. The published information does not yet make it clear which of their 417 issued US patents on circuit breakers actually apply.
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  2. Actually, I think that the IGCT is there primarily to make their device patentable. One could do the same thing, using a mechanical switch alone to commutate over to the IGBT array, but that would not be patentable.

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    It was a great article with nice content. I really enjoyed it while reading it. Thank you!
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  5. Hi
    Thank you for such article, its really helps me to enhance my understanding on circuit breaker.

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