which circumstance is to use cascade control vs three element control method?

I want to know why the standard solution in north america and europe is called three element control. why not adopt the cascade PID loop?

thank u very much!

 

The standard solution does use cascade PID loops. Here is an excellent reference:

http://www.neboiler.com/boilerbook/Neb-BB 4-03 Sec 6 - Control Guidelines.pdf

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Three element control is a confusing name because it is usually just cascade control (level and flow loops) with a feedforward (the third element, e.g. steam flow).

The advantage of the feedforward is that the flow control loop is given disturbance information before it is seen significantly in the level response. Note that you should delay a steam flow feedforward to control of a steam drum to compensate for shrink and swell.