2 Responses

1. Electrical energy is a ‘just in time’ commodity – it is produced at the same instant that it is created. That means that mechanical power is continuously being converted to electrical power, and if the electrical output is forced to go to zero because the plant is disconnected from the grid, then the mechanical power must also be shut down. Shutting down the mechanical system is more complicated that disconnecting the plant from the grid – if the plant relies on a steam cycle, then valves must be mechanically closed to stop the flow of steam, and that takes much longer than disconnecting the plant electrically. During that period of time, the generator will overspeed (mechanical energy continues to go into the generator shaft, but electrical energy is no longer being taken out, so the net unbalance in energy is converted into acceleration). And when the steam valves close, that creates another problem in that the boiler will continue to produce steam, so what generally happens is that another valve must open to allow steam to exhaust to atmosphere. The bottom line is that suddenly disconnecting the plant from the grid creates a series of severe mechanical stresses that the plant has to be designed to withstand.

   Gas turbine plants are easier to shut down – all that has to happen is that the fuel valve closes to shut off the flow of combustion gas. There will be some transient temperature stresses that the plant must withstand, but they aren’t as severe as the pressure transients in a steam plant.

   Hydro plants are very challenging to shut down because when they are disconnected from the grid, something must be done to stop the flow of water through the turbine. And because of the tremendous inertia involved in a stream of water, it can take many seconds to stop that flow of water. Some hydro plants have bypass valves that allow water to flow around the turbine. Others have diversion chambers to take that flow of water.

   You used the term ‘melt down’. That’s a totally incorrect concept. “Melt down” is a concept that applies ONLY to nuclear power plants. The vast majority of power plants are NOT nuclear, and therefore the concept of ‘melt down’ doesn’t apply. When those power plants shut down, they go directly to a safe standby mode Operators then choose whether to leave them in that condition, or shut them down completely.

   Nuclear power plants also go to a safe stand-by mode, but because their reactors continue to generate heat for a very long time, they require power to be able to sustain that safe mode. “Melt-down’ occurs when a plant is shut down, and when auxiliary power is not available to support the active cooling required when they are shut down.

2. When the main circuit breakers open or sudden loss of load happens there is a rapid sequence of reactionary events that will happen. First an over speed trip of the generator followed by a rapid increase in boiler pressure which can cause its safety valves to lift and blow off steam. At the same time the automatic boiler controls will cut the firing rate and reduce feed water flow to the boiler.

   DETAILS:
   The amount of mechanical effort it takes to spin a generator is directly proportional to the amount of electrical load placed on that generator. The more electrical energy that is produced the more steam that is required to spin that generator at the proper speed. That means that the throttle of a generator is opened up quite a bit when that generator is connected to the electric bus(grid).

   Suddenly removing that electrical load also removes that mechanical resistance to spinning the generator. But with the throttles opened up they cannot close fast enough to keep the speed of the generator from suddenly increasing. Thus you will most likely encounter a sudden over speed condition which will cause a mechanical safety device(over speed trip) to activate and close the throttles.

   The demand for steam is thus suddenly reduced which causes an imeadiate increase in boiler pressure. The increased pressure causes drum water level to shrink as steam bubble get compressed while at the same time the feed water regulator cuts back on feed water flow. The increased pressure also causes the firing rate to be reduced by reducing fuel pressure to the burners. Excessive pressure build ups int the boiler will trigger mechanical safety valves to lift blowing off steam. And sudden and excessive reduction to firing rates can cause the flames to be blown out by the forced draft fans. Manually cutting out extra burners can help prevent flame out.

   All of this happens in a relatively quick amount of time measured in 10’s of seconds. Within a few minutes the plant can achieve a stable firing rate at the reduced load. In essencevthe plant is at an idling firing rate.

   For a nuc plant its a little more complicated as the nuclear reaction doesn’t respond as quickly as a gas or oil fired boiler but the principles are pretty much the same. Reduced load means that the extra heat must be dumped or wasted. Rather than turning a generator excess steam can be dumped to the main condenser( at least some of it) and safeties on the secondary loop can lift dumping excess steam to atmosphere( not radioactive). That volum of excess steam dumped means that cool water can be added to the secondary loop to make up for it loss and that cool water will help cool the primary loop. Meanwhile the reactor rods would have dropped cutting the reaction but the core will remain hot for a much longer time. So it now becomes a matter of cooling the primary loop with additional cooling flow and using secondary loop control measures to waste energy rather than usecthat energy to spin avturbine. Remember that the power plant can still generate electricity for use with in its facility to run its auxilliary systems so some load is still available that can be used to absorb the energy from the boiler or reactor which ever the case may be.