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+//Chapter 12
+//Example 12.7
+//page 475
+//To calculate critcal clearing angle
+clear;clc;
+
+Xd=0.25; //direct axis transient reactance of the generator 
+Xl1=0.5; Xl2=0.4; //reactances of transmission line
+E=1.2; //voltage behind transient reactance
+Xinf=0.05; //reactnce before infinite bus
+V=1; //infinite bus voltage
+Pm=1; //mechanical input to the generator
+delta=0:1:180;
+
+//Normal operation (prefault)
+X1=Xd+(Xl1*Xl2/(Xl1+Xl2))+Xinf; //equivalent reactance between sending ened and receiving end
+//Power angle equation before the fault is
+Pe1=(E*V/X1)*sind(delta);
+//prefault operating power =1.0pu
+delta0=asin(1/max(Pe1));
+printf('Normal Operation (prefault):\n');
+printf('X1=%0.3f PU\n',X1);
+printf('Pe1=%0.1fsin(delta)\n\n',max(Pe1));
+
+//during fault there will be no power transfer
+Pe2=0;
+printf('During Fault:\n');
+printf('Pe2=%d\n\n',Pe2);
+
+//Post fault operation(fault cleared by opening the faulted line)
+X3=Xd+Xl1+Xinf;
+Pe3=(E*V/X3)*sind(delta);
+delta_max=%pi-asin(Pm/max(Pe3));
+//from A1 and A2, we solve A1=A2
+deff('[y]=fx(delta_cr)',"y=1.5*cos(delta_cr)+delta_cr-1.293-Pm*(delta_cr-delta0)");
+delta_cr=fsolve(0.45,fx);
+printf('Post fault operation(fault cleared by opening the faulted line):\n');
+printf('X3=%0.1fPU\n',X3);
+printf('Pe3=%0.1fsin(delta)\n',max(Pe3));
+printf('Delta_cr=%0.4f rad =%0.2f deg',delta_cr,delta_cr*180/%pi);
+plot(delta,Pe1,delta,Pe3,delta,Pm*ones(1,length(delta)));
+legend('Pe1=2.3sin(delta)','Pe3=1.5sin(delta)','Pm=1');
+title('Power angle Diagram for example 12.7');
+xlabel('delta (in degrees)----->');
+ylabel('Electrical output (Pe)----->');
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