Here I need help with AGC related research paper. I have certain confusions about tertiary control... Why do we use tertiary control loop in automatic generation control of Power system? Because primary control of frequency controls the dropping characteristics,Active Power-frequency,and Reactive power-voltage droop characteristics and secondary control loop is used to maintain the reference power order, bringing the frequency back to nomial value and regulation of inter-area power flow. Then why secondary control can't do the economic dispatch process? Why we need the tertiary control loop which is very slow process in AGC? What is the benefits of tertiary control over secondary control when both processes are almost same? Does tertiary control loop change reference power order or regulate frequency to maintain grid level control manually? Please explain briefly.