t = 0:0.0001:0.3;
f = 50; % Hz
Vm = 1.4; % volt
sag = 0.2*Vm; % voltage sag in 0.1<t<0.2
V = Vm*sin(2*pi*f*t);
idx = 0.1<t & t<0.2;
V(idx) = sag/Vm*V(idx);
N = 100;
Vrms = zeros(1,numel(t));
for k = 1:numel(t)
% When k < N, then k-N+1 < 1, which would be indexing before the
% beginning of the vector V. To handle that situation, use max(1,k-N+1)
% as the starting index. This gives the "rising" RMS at t = 0 seen in
% the plot.
Vrms(k) = sqrt(sum(V(max(1,k-N+1):k).^2)/N);
end
plot(t,V);
hold on
plot(t,Vrms,'--r');
