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Tram Systems
Show Same Faults
in
In the course of a dialog with Mark Bee, the Doppelmayr representative, City Council Transportation Committee Chair John Liu established two facts – that there have been similar failures over the years, cured by using a reset button on the SCR (Silicon-Controlled Rectifier) drive – and that those failures were replicated during testing that followed in the days after the night-long crisis. Liu: Over the last few years, we have [had] half a dozen disruptions in the SCR drive, the primary drive system. Would you consider that normal – that there’d be half a dozen disruptions of the primary drive system in as many years?
Bee: Well, the SCR drive system is a complex device that has lots of circuits and lots of safety systems, and it’s not unusual that they fault occasionally. If there’s anything outside of normal parameters, those can cause a fault. So it’s not unusual to cause occasional faults. For the most part they can be reset with a button and the system will continue to operate. At another point, Bee told Liu that a list of outages provided to his committee included only those where a service interruption had exceeded 15 minutes. Apparently, there had been other occasions to reset the SCR drive. Liu: And these faults, are they just a phenomenon to be accepted as a matter of reality, or are there measures that can be implemented to minimize these kinds of faults? Bee: Right now, and maybe Herb can respond to this, they are in the middle of an engineering review of the entire Tram system that was started approximately six months or a year ago. They hired a consulting engineer to do a complete review of the system and make recommendations on what kind of upgrades should the Tram have for the next 30 years.
Catherine Johnson (RIOC Vice President for Operations): The directors of RIOC approved a $3.9 million capital plan for the upgrade of the Tramway system. We elected to engage an engineer who would do a survey for us, of state-of-the-art equipment that’s currently available, [and] provide us with a series of options for upgrading the Tram which would assure its continued reliability and safety. We have actually a meeting scheduled for the 17th of May to discuss with him some of his initial findings and recommendations for the corporation. There’s no decision been made yet about how the upgrade will take place; we’re looking at all the options to ensure that we have the best information and the best practices out there as we embark on upgrading the Tram. Liu: In recent years, there have been other instances where the fuses were blown, or there was a fault that disrupted the service. In December of 2003, the fuses were blown, and I don’t know if maybe that was seen as a fluke... In your earlier statements, these faults occur on almost a regular basis. You implied it was just something that a Tram system has to deal with. On the other hand, we’re finding out that the equipment exists to protect against these kind of faults. Johnson assured Liu that such equipment would be part of any Tramway upgrade. Liu: In the last incident, in September of 2005, can you describe what happened there? Bee: Yes. There was an SCR fault in the afternoon, and... the electronic circuit shut itself down. We don’t necessarily know why... The on-duty shift supervisor was not able to figure out how to solve the problem. He froze at the controls and was not able to reset it, so the manager came in and was able to simply reset the SCR by pushing a button and bring the Tram cars in. We replaced that supervisor... We don’t know why the system faulted, but it’s a very complicated electronic device that has lots of protections built in. When it senses any anomaly, it shuts itself off. Most of the time, it can be reset by pushing a button. Liu: OK. So, when the system shuts itself off, there’s no need to find out why it shut itself off? You just push the reset button and let the Tram continue? Bee: If it becomes frequent, we should certainly check it. What happened in this incident is that we thought we had a problem with one component that provides a speed input reference to the drive. That component was changed. At the time we thought that was what caused the problem. Liu: All right, but to this day, which is about eight months from that disruption in September of 2005, you still don’t know what happened that caused the system to shut itself down. Bee: We’re still making a complete investigation of the entire SCR drive system. Liu: And yet, it was OK to, in these eight months, in the intervening eight months, to continue to run the Tram? Now, based on the incident last week, it is not OK to run the Tram. What’s different about what happened last week and what happened in September, 2005? In both instances, the Tram ground to a halt. The SCR system, the drive system, shut itself down. Bee: Well, what’s different is we were able to reset the September fault by pushing a button. We changed the fuses and, a day after the incident, a technician from Europe arrived, a specialist on the system, that next day we were able to duplicate the same problem; a couple of days later, we were again able to duplicate the same fault, so we now have a pattern there. Liu: You were able to duplicate the fault in September? Bee: No, no, we were able to duplicate the one that occurred last week. We’ve had it happen twice more since we’ve been testing it this last week, and we’re still trying to find what the cause is. Liu: It just seems that... I don’t know if that difference is enough for us to understand why, after the September shutdown, it continued and why, after last week’s shutdown, it has been discontinued, at least temporarily, we hope. So, in both cases, it was the SCR primary drive system that failed, and the only thing that was different last week was that the backup system was not able to engage, as well... But the main problem, it seems, is not with the backup system – of course, we have to find out why the backup system failed – but the main problem is why the primary system failed. In fact, according to this chart, the failure of the primary drive on April 18, 2006... September, 2005, a fault in the SCR drive; December, 2004, a fault in the SCR drive; September 20, 2002, a fault in the SCR drive; December 30, 1999, a fault in the SCR drive. I think something’s wrong with the SCR drive. [Laughter, applause, from the audience] Maybe there’s something fundamentally wrong with that SCR system. Johnson: Certainly we’d be looking at the primary power drive system. But I also want to state that when you look at it in isolation of the Tram system... We transport a million people a year on the Tram, and a huge number – I can’t think of the number off the top of my head – daily and annually that that Tram runs. What we’re looking at here is no different than when my furnace goes and the electrical controls in my furnace trip, and I go down and I hit the reset button. It’s an electrical system. So a fault in an SCR drive doesn’t mean that there’s a problem with the SCR drive. It’s doing what it should do in order to protect the safety of the passengers in those Tram cars. The Tram, by and large, has an outstanding safety record over its 30 years. We’ve had one instance where we had to engage the rescue cabins, and I think that we’d all be hard-pressed to find one other major form of transportation in this country that can claim the same high safety record as the Tramway on Roosevelt Island. Liu: I appreciate that [but] I don’t think that most people taking the Tram, particularly those 68 people who were stuck above the Manhattan skyline that day, would appreciate a comparison to the furnace breaking down. I understand what you’re saying – that it’s just resetting – but the consequences are far different than the furnace breaking down in someone’s house. Berman: You’re absolutely right, that’s precisely the reason why there has to be an extensive inquiry as to what went wrong and what we have to do to repair it, and that’s precisely what’s going on now. Liu then turned the questioning over to Councilmember Jessica Lappin, who represents Roosevelt Island, who took Bee back to something he’d said earlier:
Lappin: You discussed faults that happened over the last week... Were those created by you or did they happen spontaneously while you were testing. Bee: They happened during testing that we have been performing, and we’re getting the identical fault indications that we had last week. Lappin: So you weren’t artificially creating that situation. It arose two more times. Bee: That’s correct. Lappin: And were there surges? I mean, how were those being duplicated? Bee: Well, we were running at the same basic point on the line when the motor switches from being a motor to being a generator is where we had these faults, so we were at one point taking current from the line and, [at] another point putting current into the line, and the electronics are very complicated to do all this... A power surge is one thing that can affect an SCR, but the SCR by itself can cause its own problem. If something misfires, it can blow fuses, even if there’s no disturbance on the power grid. So the SCR itself can cause a fault that blow fuses regardless of what’s happening on the Con Edison supply. Lappin: It sounds like something apart from Con Ed. You mentioned this before. The power that you get from the grid, there’s [also] power that you generate internally. And you just said the Tram goes from being a motor to being a generator, so are you saying that the Tram generates its own power? Bee: Well, that’s how it decelerates. When it wants to slow down and the motor that’s driving it turns into a generator and it’s making electricity now – it’s putting electricity back into the power grid, and that’s how you slow the Tram down. There’s three levels of braking – mechanical braking systems beyond that, but the primary way is the electric motor to slow the Tram down, much like if you have a hybrid car, you’re using an electric motor to slow the car down. It’s the same principle. Lappin: Is it possible that the Tram itself is generating the power that’s causing these surges? Bee: Yes. Here, Lappin may have hit pay dirt. Both in September and on April 18, the SCR drive "faulted" just after the eastbound cabin cleared tower #2, the highest point. Regular riders are familiar with the point where the cabins pass, just west of tower #2. When, moments later, the eastbound cabin clears tower #2, each cabin is going "downhill." Gravity gets involved, and the motor driving the Tram switches from being a motor, using power, to a generator, feeding electricity back into the grid. It appears at least possible that testing will reveal the SCR control system is having trouble making that transition under certain load conditions. (The eastbound cabin is more heavily loaded at home-bound rush hour. On April 18, there were 47 passengers on the way to Roosevelt Island, and 21 headed toward Manhattan.) Lappin: Is it possible that the Tram itself is generating the power that’s causing these surges? Bee: Yes. Lappin: And that would be something that we would fix how? Bee: Well, once we know the exact cause, we will replace the faulty components. And we’re still investigating that.
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