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The turbines are Francis type which means they keep spinning until the water flow is stopped. When a generator is disconnected from the power system without fi

The turbines are Francis type which means they keep spinning until the water flow is stopped. When a generator is disconnected from the power system without first removing the power from the shaft then power in is greater than power out and the result is an acceleration of the generator. Generators are built to be able to withstand a certain maximum rotational speed without flying apart for a certain duration of time before the bearings get too hot. Of course it can operate at the normal speed forever, but the rated over speed might be twice as fast for 10 minutes. So when the power lines are cut the turbines and generators speed up until the flow of water is stopped. Pictures of the Mosul dam show four surge tanks, one for each turbine, so they can probably turn off the water in 5-10s. Pelton turbines are used for higher head and the flow can be maintained without turning the turbine by deflecting the water away from the runner. I've commissioned about a dozen hydro electric power plants and opening the breaker to simulate cutting the transmission line while making 10, 25, 50,75,100% of the power is standard procedure.

xg15 10y ago
Thank you very much for the expert information. but that still doesn't make sense to me. The article states they have two control gates and a spillway. (one gate previously defunct but now repaired) So, shouldn't it be possible to block off the turbines and open the control gates instead, so that the total flow stays (roughly) the same? Moreover, if the dam is indeed that easy to destabilize, letting ISIS control the load seems equivalent to handing them a big red "trigger flood" button for use at their discretion. Sure, right now it's in their interest to keep the dam intact - because they still control many parts of Mossul and need the electricity. But "scorched earth" is not exactly a new concept and if the operation in Mossul continues as planned and ISIS is driven out, they could decide to cut the lines on their end to deliberately cause mechanical failures.
- snowwindwaves 10y ago
  I haven't re-read the article, but I recall the control gates were at the bottom of the dam. Traditionally the spillway is at the top of the dam and on one portiona there are gates which are operated hydraulically or by winches that open to allow water to pass underneath and run down a specially shaped (ogee shape) face of the dam. If for some reason the water level behind the dam gets too high there can be a section of spillway with no gates that will allow water to flow around the dam without the dam being overtopped in an uncontrolled fashion. The mosul dam is 113 m high and the article said they were operating the dam with the water level 30m below the normal full level. Probably the sill of the spillway is only 10 or less below the normal operating level, because if the dam isn't full why would you want to spill. The control gates at the bottom of the dam could have been primarily for allowing sediment to be sluice out from behind the dam or to allow the reservoir to be drained. There is no guarantee the two control gates could pass as much flow as the turbines. Who knows how they were sized. As I tried to explain before, cutting the power lines shouldn't damage any equipment. It effectively happens all the time due to natural causes, be they tree branches falling on power lines, birds, squirrels, etc. Flow through an orifice (sluice gate, control gate, turbine, etc) varies with the square root of the pressure of water (the head), so a side effect of operating the dam with 30 m less head (30% less!) is that the turbines would be operating outside of their normal range and the control gates would pass less flow than they would at the nominal head. There was a terrible accident at a dam in russia that was caused by operating the turbine outside of the normal range of pressure, and poor maintenance (loose bolts holding the turbine down...): https://en.wikipedia.org/wiki/2009_Sayano%E2%80%93Shushenskaya_power_station_accident https://en.wikipedia.org/wiki/2009_Sayano%E2%80%93Shushenska...

xg15 10y ago

Thank you very much for the expert information. but that still doesn't make sense to me. The article states they have two control gates and a spillway. (one gate previously defunct but now repaired) So, shouldn't it be possible to block off the turbines and open the control gates instead, so that the total flow stays (roughly) the same? Moreover, if the dam is indeed that easy to destabilize, letting ISIS control the load seems equivalent to handing them a big red "trigger flood" button for use at their discretion. Sure, right now it's in their interest to keep the dam intact - because they still control many parts of Mossul and need the electricity. But "scorched earth" is not exactly a new concept and if the operation in Mossul continues as planned and ISIS is driven out, they could decide to cut the lines on their end to deliberately cause mechanical failures.

snowwindwaves 10y ago
I haven't re-read the article, but I recall the control gates were at the bottom of the dam. Traditionally the spillway is at the top of the dam and on one portiona there are gates which are operated hydraulically or by winches that open to allow water to pass underneath and run down a specially shaped (ogee shape) face of the dam. If for some reason the water level behind the dam gets too high there can be a section of spillway with no gates that will allow water to flow around the dam without the dam being overtopped in an uncontrolled fashion. The mosul dam is 113 m high and the article said they were operating the dam with the water level 30m below the normal full level. Probably the sill of the spillway is only 10 or less below the normal operating level, because if the dam isn't full why would you want to spill. The control gates at the bottom of the dam could have been primarily for allowing sediment to be sluice out from behind the dam or to allow the reservoir to be drained. There is no guarantee the two control gates could pass as much flow as the turbines. Who knows how they were sized. As I tried to explain before, cutting the power lines shouldn't damage any equipment. It effectively happens all the time due to natural causes, be they tree branches falling on power lines, birds, squirrels, etc. Flow through an orifice (sluice gate, control gate, turbine, etc) varies with the square root of the pressure of water (the head), so a side effect of operating the dam with 30 m less head (30% less!) is that the turbines would be operating outside of their normal range and the control gates would pass less flow than they would at the nominal head. There was a terrible accident at a dam in russia that was caused by operating the turbine outside of the normal range of pressure, and poor maintenance (loose bolts holding the turbine down...): https://en.wikipedia.org/wiki/2009_Sayano%E2%80%93Shushenskaya_power_station_accident https://en.wikipedia.org/wiki/2009_Sayano%E2%80%93Shushenska...