summation constraint

 @mohansx 

I am checking
Thank you

I got the solution.
Best regards

When solving the above problem, I am not getting the desired resolution.
Declare:
That belongs to the three Buses.
Set Buses {
     Index: i, j;
}
{b1, b2, b2}

Then the parameters
Parameter Susceptance {
     IndexDomain: (i, j);
}

Variable 
Variable FlowLines {
     IndexDomain: (i, j);
     Range: free;
     Definition: Susceptance (i, j) * (DeltaAng (i) -DeltaAng (j));
}

I don't know where the error is,
Since there is a power flow going from bus 1 to bus 2
bus 2 to bus 3, bus 1 to bus 3 and vice versa.

How can I declare what is blue in the constraints (Image above).
j belongs to set i

If you can support me, thanks.

@Manuel 

You might want to update your variable declaration as below for efficiency. 

Variable FlowLines {

     IndexDomain: (i, j)|i <> j;

     Range: free;

     Definition: Susceptance (i, j) * (DeltaAng (i) -DeltaAng (j));

}

Now, the constraint highlighted in blue can be declared as 

Constraint1 {

    Index domain: i

    sum[j, B(i, j) * (DeltaAng(i) - DeltaAng(j))] + p(i) = D(i) 

}

Now, you say j belongs to set i but the image shows j belongs to omega(i). What does omega(i) represent ? 

In summary
The problem has three nodes or buses, unit 1 is on bus 1 and unit 2 is on bus 2. Demand (triangle) on bus 3.
finally there are three lines or links.
Line 12 means that it is connected from bus 1 to bus 2.
Line 13 means that it is connected from bus 1 to bus 3.
Line 23 means that it is connected from bus 2 to bus 3.
is where the problem is.

Now if I am on bus 1 there are two power flows that go through the lines, the flow that goes from bus 1 to bus 2, from bus 1 to bus 3. The same thing happens if I am on bus 2 now it is the flow that goes from 2 to 1 and from 2 to 3. mathematically it is the suscepatancia by the difference of angles.

so that would be the Omega set
Ωi where i = 1,2,3 are the buses
Ω1 = {2,3}
Ω2 = {1,3}
Ω3 = {1,2}

then I have
the generation units set
the Demand set.
the set Nodes or buses (which has two indexes i, j)

the developed model would look like this:

my other idea is to declare the set Lines as:
{Line1, Line2, Line3} or {1,2,3} or {Line12, Line13, Line23}

but as I tell AIMMS that Line1 goes from node 1 to node 2.
Line2 goes from node 1 to node 3 and line3 goes from node 2 to node three.

It should be noted that the power flow goes in both directions
1 to 2, 1 to 3, 2 to 1, 2 to 3, 3 to 1 and 3 to 2 therefore the 3x3 matrix(3 buses) and the i and j indices.

Finally, as I declare that generator 1 is on bus 1, generator 2 is on bus 2 and the demand is on bus 3.