What is the best method to handle the excess load of coal generation plant, required to deliver its full capacity to a portion of a distribution utility's load, when almost 50% of the said is out? The coal generation is embedded to the distribution utility. I need to prevent the coal generation to export power to the national Grid. I have thought of ways to handle this excess power, but I still need suggestions. Thank you!

  1. Use Dump load, and convert excess power to heat.
  2. Use Large-scale electricity storage for future use.
  • \$\begingroup\$ A decrease in load on a grid-less generator with no control loop will cause the plant generator to rotate faster for the same heat input. In general there is a generator speed governor that uses the "sloop" rating of the generator to determine the operating point based on changes in speed. With an "infinite" synchronized grid a change in load will cause very little frequency drift (fraction of a Hz). A large change in load for a generator off grid will be detectable and has other implication including reactive load (magnetic inertia) and backfeeding the generator, so please clarify the system \$\endgroup\$ – crasic Aug 13 '15 at 3:07
  • \$\begingroup\$ What is important, if this is a single generator (off grid) is the response time of the generator, if you can handle Cycle frequency changes you could tolerate the higher distribution frequency while the generator ramps down, otherwise you need a load (storage or heat dump) to dissipate the inertia of the system (total inertia including reactive, mechanical, and heat) \$\endgroup\$ – crasic Aug 13 '15 at 3:09
  • \$\begingroup\$ Since this is a solved problem in the real world I will assume this is for education purposes. I would read this discussion as I found it helpful in understanding these concepts. \$\endgroup\$ – crasic Aug 13 '15 at 3:13

There are a few issues at play here.

First is the concept of a synchronized electrical grid, using the "infinite grid" approximation, in most cases a reduction in load on an electrical system will be imperceptibly distributed across all the generators in the grid. The load that a specific power plant sees is a managed load (managed in the context of the entire grid) and the system may respond to a large decrease in a number of ways outisde of the realm of a single generating station. If half of the entire load on the national grid drops out suddenly then you have bigger issues to deal with than a single coal generating plant going offline.

A generator will have a speed governor and a droop rating for the given prime mover (ratio of speed at full load vs no load) . A decrease in generator load will in general increase the rotational speed of generator from steady state, the speed governor will command the power plant to produce less torque (less heat or less steam into turbine) and decrease the rotational speed. In a synchronized grid setting this process is "clamped" - meaning that if the generator represents a small fraction of the grid power it will be unable to increase its frequency to the new steady state and will go out of sync with the grid if this doesn't happen reasonably quickly (or if it is a large fraction of power generated, take the whole grid out of sync)

What this means is if the load on your generator is suddenly decreased (say local sector load dump) it must compensate by decreasing prime mover torque output or dump power into a quick turn-on load to avoid risk losing synchronization with the grid

That being said power =/= energy. If your governor and prime mover have quick response (say seconds) in reducing generator output then 1s at 50MW is 50MJ, or approximately 13 W-hrs of energy. This is not a huge amount of energy in a power plant context and can easily be dissipated as heat through dump loads. If you need to divert 50MW for hours, this is a large amount of energy that will be better served using energy storage. You will need to determine the dynamic properties of the coal plant and prime mover to determine the best course of action.

E.G. if the turbine has a steam reservoir then the heat output of the coal is buffered and you are more likely to have quick control over prime mover output independent of the heat produced by the coal fire. Given enough physical energy storage (as heat in the reservoir) to ride out the load dump or buffer the turn down delay then excess power loads of some sort will be enough depending on the expected response. If the requirement is that "coal burner operates at louvres=100% for 100% of the time" Then yes, you need more energy storage, the most efficient being upstream of the generator (heat diversion) because conversion to mechanical force, then EMF, then any electrical storage then back to mechanical and EMF will be much less efficient overall, but I'm sure that this has been solved worldwide with much less handwaving and more hard economic numbers so you will have to read the literature.


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