Kdtyjb

The output I get

The output I want is the same except that the brackets of the Z_bus matrix should stretch to contain phantom content of array environment as follows

Clearly, I can use phantom and define the array I need inside it, but after try to write the visible content bar{mathnormalbold{Z}}_{bus}, the content starts stretching further.

My MWE

documentclass{book}





    % ========== Table packages ==========

    usepackage{tabularx}

% BEGIN_FOLD



        newcommand{arraystretchdefaultl}{1.3}

        newcommand{arraystretchdefault}{ renewcommand{arraystretch}{arraystretchdefaultl} }

        arraystretchdefault



% END_FOLD



    usepackage{booktabs}

    usepackage{makecell}





    % ========== Equations and math packages ==========

    usepackage{amsmath}

    usepackage{amssymb, amsfonts}

    usepackage{mathtools}

    usepackage[defaultmathsizes, italic, eulergreek, symbolre, symbolmisc]{mathastext}





begin{document}



begin{align}

    left[

        begin{array}{c}

            mathnormalbold{V}_{G}

            \

        %

            Xhline{0.25mm}

            V_{R, thinspace i}

        %

            \

            V_{R, thinspace j}

        end{array}

    right] &=

%

    left[

        begin{array}{c !{vrule width 0.25mm}c c}

            bar{mathnormalbold{Z}}_{GG} & bar{mathnormalbold{Z}}_{GR_{i}} & bar{mathnormalbold{Z}}_{GR_{j}}

        \

            Xhline{0.25mm}

        %

            bar{mathnormalbold{Z}}_{R_{i}G} & bar{Z}_{RR, thinspace ii} & bar{Z}_{RR, thinspace ij}

        %

        \

            bar{mathnormalbold{Z}}_{R_{j}G} & bar{Z}_{RR, thinspace ij} & bar{Z}_{R, thinspace jj}

        end{array}

    right]

    %

    left[      

        begin{array}{c}

            bar{mathnormalbold{I}}_{G}

        \

        Xhline{0.25mm}

        %

            bar{I}_{R, thinspace i}

        \

        %

            bar{I}_{R, thinspace j}

        end{array}

    right]

label{eq:op_imp_load_RER_int_op}

\

intertext{where}

bar{mathnormalbold{Z}}_{bus} &=

    left[

        begin{array}{c !{vrule width 0.25mm}c c}

            bar{mathnormalbold{Z}}_{GG} & bar{mathnormalbold{Z}}_{GR_{i}} & bar{mathnormalbold{Z}}_{GR_{j}}

        \

        Xhline{0.25mm}

        %           

            bar{mathnormalbold{Z}}_{R_{i}G} & bar{Z}_{RR, thinspace ii} & bar{Z}_{RR, thinspace ij}

        \

        %

            bar{mathnormalbold{Z}}_{R_{j}G} & bar{Z}_{RR, thinspace ij} & bar{Z}_{R, thinspace jj}

        end{array}

    right]

label{eq:Z_bus_hat_def}

end{align}



end{document}

Here is a solution with saveboxes:

documentclass{book}





    % ========== Table packages ==========

    usepackage{tabularx}

% BEGIN_FOLD



        newcommand{arraystretchdefaultl}{1.3}

        newcommand{arraystretchdefault}{ renewcommand{arraystretch}{arraystretchdefaultl} }

        arraystretchdefault



% END_FOLD



    usepackage{booktabs}

    usepackage{makecell}





    % ========== Equations and math packages ==========

    usepackage{amsmath}

    usepackage{amssymb, amsfonts}

    usepackage{mathtools}

    usepackage[defaultmathsizes, italic, eulergreek, symbolre, symbolmisc]{mathastext}



newsavebox{mybox}

newsavebox{mysbox}

begin{document}



begin{align}

    left[

        begin{array}{c}

            mathnormalbold{V}_{G}

            \

        %

            Xhline{0.25mm}

            V_{R, thinspace i}

        %

            \

            V_{R, thinspace j}

        end{array}

    right] &=

%

    left[

        savebox{mybox}{hbox{(begin{array}{c !{vrule width 0.25mm}c c}

            bar{mathnormalbold{Z}}_{GG} & bar{mathnormalbold{Z}}_{GR_{i}} & bar{mathnormalbold{Z}}_{GR_{j}}

        \

            Xhline{0.25mm}

        %

            bar{mathnormalbold{Z}}_{R_{i}G} & bar{Z}_{RR, thinspace ii} & bar{Z}_{RR, thinspace ij}

        %

        \

            bar{mathnormalbold{Z}}_{R_{j}G} & bar{Z}_{RR, thinspace ij} & bar{Z}_{R, thinspace jj}

        end{array})}}savebox{mysbox}{hbox{(bar{Z}_{bus})}}

        vphantom{usebox{mybox}}hspace{dimexpr(dimexprwdmybox/2)-dimexpr(wdmysbox/2)}usebox{mysbox}hspace{dimexpr(dimexprwdmybox/2)-dimexpr(wdmysbox/2)}

    right]

    %

    left[      

        begin{array}{c}

            bar{mathnormalbold{I}}_{G}

        \

        Xhline{0.25mm}

        %

            bar{I}_{R, thinspace i}

        \

        %

            bar{I}_{R, thinspace j}

        end{array}

    right]

label{eq:op_imp_load_RER_int_op}

\

intertext{where}

bar{mathnormalbold{Z}}_{bus} &=

    left[

        begin{array}{c !{vrule width 0.25mm}c c}

            bar{mathnormalbold{Z}}_{GG} & bar{mathnormalbold{Z}}_{GR_{i}} & bar{mathnormalbold{Z}}_{GR_{j}}

        \

        Xhline{0.25mm}

        %           

            bar{mathnormalbold{Z}}_{R_{i}G} & bar{Z}_{RR, thinspace ii} & bar{Z}_{RR, thinspace ij}

        \

        %

            bar{mathnormalbold{Z}}_{R_{j}G} & bar{Z}_{RR, thinspace ij} & bar{Z}_{R, thinspace jj}

        end{array}

    right]

label{eq:Z_bus_hat_def}

end{align}



end{document}

It could be better to put this functionality inside a newcommand of course. Feel free to ask me for this if this way seems ok to you.

Feel free to use wider rules (I don't think you should). I removed mathastext that yields a painful result.

documentclass{book}

usepackage{amsmath,bm}



newcommand{bigarraystretch}{renewcommand{arraystretch}{1.3}}



begin{document}



begingroupbigarraystretch

newcommand{Zbus}{%

  begin{array}{@{} c | c c @{}}

    bar{bm{Z}}_{GG} & bar{bm{Z}}_{GR_{i}} & bar{bm{Z}}_{GR_{j}} \

    hline

    bar{bm{Z}}_{R_{i}G} & bar{Z}_{RR,ii} & bar{Z}_{RR,ij} \

    bar{bm{Z}}_{R_{j}G} & bar{Z}_{RR,ij} & bar{Z}_{R,jj}

  end{array}%

}

begin{align}

left[begin{array}{@{}c@{}}

  bm{V}_{G} \

  hline

  V_{R,i} \

  V_{R,j}

end{array}right]

&=

begin{bmatrix}

vphantom{Zbus}

ooalign{hphantom{$Zbus$}crhfil$bar{bm{Z}}_{mathrm{bus}}$hfilcr}

end{bmatrix}

left[begin{array}{@{} c @{}}

  bar{bm{I}}_{G} \

  hline

  bar{I}_{R,i} \

  bar{I}_{R,j}

end{array}right]

label{eq:op_imp_load_RER_int_op}

\

intertext{where}

bar{bm{Z}}_{mathrm{bus}} &= left[Zbusright]

label{eq:Z_bus_hat_def}

end{align}

endgroup



end{document}

The idea is to save the contents of the Zbus matrix and to use ooalign to superimpose the symbol over the horizontal space taken by the matrix, with the vertical space before it to set the size of the brackets.