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A friend witnessed the explosion of the 14th street substation. I believe these transformers, about 1 city block in extent, delivered power to most of lower Manhattan. I am guessing the flood water obstructed cooling, and they overheated and exploded spectacularly.

(there is a 4 stack natural gas generating station next door to where the explosion happened, so -maybe- the explosion was not the transformer, but most reports say transformer)

My friend reported: 'The power flickered, then came back on'. Some unknown time later the power went out.

So the question is, could power in the grid fed by these transformers stay on for a while after the explosion, and for how long?

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    \$\begingroup\$ Speculation: there's more than one substation feeding that area of the grid, and the flicker was the system failing over to a different substation. \$\endgroup\$ – pjc50 Nov 1 '12 at 16:13
  • \$\begingroup\$ @pjc50, that seems the most reasonable answer, with the delay being the time the other grid takes to realize it can not handle the load. Only question then is, why does all of LM stay dark? Wouldn't they cut in as much of that other grid as they can? \$\endgroup\$ – Bobbi Bennett Nov 1 '12 at 16:23
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    \$\begingroup\$ It takes time for electrons to read and understand the modifications to the schematic which have just occurred. :) \$\endgroup\$ – Kaz Nov 1 '12 at 18:52
  • \$\begingroup\$ For a moment, I was considering how much energy is stored in the grid; all those transformers' magnitizing current, and even more if there are capacitors. But c'mon, that would only last a second at most, right? \$\endgroup\$ – Bobbi Bennett Nov 1 '12 at 21:59
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    \$\begingroup\$ In addition to the spectacular failure photos, there is some interesting discussion here about failure modes in various parts of the power transmission system: 205.243.100.155/frames/longarc.htm \$\endgroup\$ – mickeyf Nov 2 '12 at 1:50
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Without any more details than what's in your question, here's what I believe happened: (It might be counterintuitive, so to avoid confusion: When a breaker is closed, current can go through it. When a breaker is open, current can not go through it. Also, when a relay trips, it will eventually open one or several breakers (thus cutting the power)).

The flickering:

For some (unknown) reason, the transformer substation exploded. This might have caused a bunch of different faults that may trip nearby relays. My guess would be a three-phase fault, as such faults often result in the highest currents (dependent on grounding). Normally you would only want the closest relay to trip, thus keeping the rest of the grid intact. However, this time the relay is probably of little use, as the substation is blown to pieces. So, other nearby relays will trip breakers in order to isolate the fault.

The relays will normally try to close the breakers again to get the power back up within a matter of (milli) seconds. (Note that even though a relay may trip immediately, it will take about 100 ms for the breaker to actually break the current.)

This is most likely what caused the initial flickering.

  1. Explosion
  2. Relays around the substation trips and opens the breakers and thereby cuts the power to the area
  3. The breakers closes again (except those that must still be open to isolate the fault area).
  4. The power is back on

So, what causes the power to go some unknown time later?

Practically all power systems are operated after the N-1 criterion (or in some cases N-2, N-k). "The N-1 criterion expresses the ability of the transmission system to lose a linkage without causing an overload failure elsewhere." [1] It is however impossible for the transmission system operators (TSO) to comply to the N-1 criterion at all times.

Transformers, lines, cables etc. can handle more than what they are rated for. Transformers can often operate at 50% overload for as much as one hour without taking any damage. Transmission lines can actually be loaded as much as you want. However, as you don't want to risk damage to the equipment, relays are designed to cut the power if the overcurrent lasts too long or gets too high.

enter image description here

The figure above shows a typical relay trip characteristic on a log-log scale. You can find the trip time of the breaker if you know the current. You do that by finding the current of the x-axis, go up and see which value the green curve corresponds to on the y-axis. On the far right the current is very high, 10-1000 x In, where In is the nominal current of the equipment. The horizontal line of the far right is typically at approx 0-100 ms.

The dashed line to the left shows the lowest pick-up value for the relay. This line if typically at 1.2 x In. Since the trip curve is vertical here, any current less than 1.2 x In will never cause a trip.

Between 1.2 and 10 times In, the trip time varies according to the curve shown between the two dashed lines. The rightmost part of the inverse curve is typically at 300 ms, whereas the leftmost part of the curve might be as much as minutes (remember the scale is logarithmic).

Hypothesis:

The failure of the substation causes an overload of (at least one of) the remaining substations feeding Manhattan with power. In this case, the current has probably been slightly above 1.2 x In for one component, thus causing a trip, but with a large time delay. When the first relay trips, another connection will be even more overloaded thus causing another trip, and another one, and another one, eventually cutting all the power to the city.

  1. Slight overload of one (or more) component(s)
  2. Relay trips (and breaker opens) with a large time delay
  3. New component(s) overloaded due to the what happened in 2.
  4. One more trip, and another one ...
  5. Good night Manhattan!
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    \$\begingroup\$ I agree with this hypothesis. Well answered. \$\endgroup\$ – Li-aung Yip Jul 8 '14 at 4:11
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It depends how close you are the transformer and if you are downstream or upstream where if it fails and causes a disturbance and opens keeps the upstream line working or if shutdown for over current.

So the time can be anywhere from 0 to infinity. But if it does fail far away, and you on the grid that is affected by a chain reaction, it can be several seconds with shutdown and followed by a restart and if fault condition is perceived to still be in effect, shutsdown immediately again. The4 phenomena is common because the thresholds for trip on steady power and trip on startup are quite different as startup-surge is normal with incandescent light bulbs taking 10x the current and large motors also taking often more than the breaker rating but for a short period of time.

The algorithm of startup trip current is rather complex and dependant on many factors but safety is paramount. You dont want a short circuit causing a cascade of power transformer overloading, so trip time must be short enough to protect the upstream transformers.

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  • \$\begingroup\$ Thanks for addressing the question. I think you may have missed the situation, it was a pretty big transformer that went. There are clips of the explosion on youtube. Anyway, I now wonder if there isn't a lot of 'spinning' energy in the grid, from all the motors and equipment that don't just stop when the power blows out, they spin down. Just a thought. \$\endgroup\$ – Bobbi Bennett Nov 16 '12 at 21:31
  • \$\begingroup\$ Although it would be possible for the motor in something like an escalator to feed some power into the grid if many people are riding it downward, I don't think spinning motors feed into the grid much. I think a bigger issue is that much of the infrastructure is designed so that it can for short periods of time carry more power than it can carry continuously. For example, a particular transformer which had been operating at 75% of its rated power for a long time might be able to operate at 150% of its rated power for up to five seconds before it overheats enough to require shutdown. \$\endgroup\$ – supercat Nov 17 '12 at 20:24
  • \$\begingroup\$ If a portion of the grid was fed by two such devices and used 75% of the power that both devices together could handle continuously, then if one device were to fail the other device would be able to feed that portion of the grid for five seconds before it had to be shut down. \$\endgroup\$ – supercat Nov 17 '12 at 20:26

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