Find Z-bus using Z building algorithm using MATLAB code

[Fatima Hasan]

Homework Statement

Find the Z bus Matrix using Z building algorithm method, using MATLAB code; where the Line data matrix and Bus data are given.

Relevant Equations

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I tried to write to the code, but I got confused in line 46 & 59.Every new added bus increases the dimension of the Z matrix, and since we have 3 busses, the dimension of Z should be [3x3]. I formed a for loop in case of adding a new bus to add a new row and new column (increases the dimension), I kept the limits of b from 1 to max. # of busses (Which is 3) to not add the 4th row and 4th column. However, I do not know how to get the last element of the matrix (which is element (3x3)). So, could someone please help me with?
Here's my code:

clc;
clear all;

         % | From | To | R | X | gsh | B | T|ysh
         % | Bus | Bus | pu | pu | pu | pu | ph-sh
LD= [ 1  2 0 0.8    0 0
             1  3 0 0.4 0 0
             2  3 0 0.4 0 0];
      
r=1.05;
angle=5*(pi/180);
[x,y]=pol2cart(angle,r) ;
t=x+i*y;

fbus=LD(:,1);             % Reading from bus
tbus=LD(:,2);             % Reading to bus
r = LD(:,3);              % Resistance, R...
x = LD(:,4);              % Reactance, X...
gsh=LD(:,5);
B = 1i*LD(:,6);           % Ground Admittance
%T=LD(:,7);
Zl=1/(r+(1i*x));     
%     fb tb Eg  Xg  Zd
BD = [1   0  0  inf 0.2i
      2   0  0  inf 0.4i
      3   1  0  inf 1]
fb=BD(:,1);
tb=BD(:,2);
Eg=BD(:,3);
Xg=1i*BD(:,4);
Zd=BD(:,5);     

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

busses=max(max(fbus),max(tbus));
mb=busses;
%zold=zeros(busses,busses)

%for b=1:busses
%    z(b,b) = Zd(b)  ; %diagonal matrix
%end

for b=1:busses
    z(b,b)=[Zd(b)] %diagonal elements
    %if the new bus isn't connected to GND
    if tb(b)~=0 && b==busses-1 %to not exceed 3 rows and 3 columns
        nrow=z(fb(b),:) %repeat row# (the old bus)
        ncol=nrow'      %new column
        ns=length(z)    %new size
        zz=zeros(ns,ns) %set a zero matrix [ns,ns]
        zz(ns,:)=nrow %add the new row to the zero matrix
        zz(:,ns)      %add the new col to the zero matrix
        z(ns,ns)=Zd(b)+ z(fb(b),fb(b))    %the last element of the modified matrix
                            %= the added Z (Zd) + Z(#of old Bus,#of old Bus)
        zz+z          %add zz matrix which has the new col and new row,
                      %with the diagonal matrix   
    end 
    %special case: the new bus is connected to GND
        if tb(b)==0 && b<busses     %b<busses -> to avoid adding the 4th row and col
        ncol=zeros(b+1,1);          %add a column of zero elements
        nrow=zeros(1,b+1);          %add a row of zero elements
        ns=length(z);               %to get the new size
        zz=zeros(ns,ns); %the last element of the modified matrix
        z(ns,ns)=Zd(b)  %if the bus is connected directly to GND,
                        %the last element is only Zdd (the added one)
        z+zz;
    end
end
z

The result:

However, element (3x3) should be 1+0.2 = 1.2 (Zd (3) + element (1,1) of z matrix = 0.2)

 

[LightHero]

z =1.0000 + 0.2000i 0 00 1.0000 + 0.4000i 00 0 1.0000 + 0.2000i