Why a voltage drop at the busbar's generator?

Question

I was testing the effects of distributed generators on our grid. To do so, I inserted a PV generator of 90 MW PURELY ACTIVE (power factor = 1) as in the picture. I performed a quasi-dynamic simulation, and so I added hourly trends to the loads and to the PV generator (on a sunny day). I noted that the voltage at the busbar where the generator is connected, during 11 a.m./15 a.m. decreases instead of increases. This is very strange because the voltage should increase there. I tried to change the output power of the generator with a smaller one and this phenomenon does not occur. The same is not present when I change the power factor (0,9 for example) where the voltage always increases whatever is the output power. Why does that happen? (be careful that in the picture, the generator on node A1 is disconnected and in all the MV lines there is no other generator, the PV one in the picture is the only one connected). In the last image, you see in green the voltage trend with the generator connected and in red the one without the generator.

Answer 1

There is a reactive drop in the transformer (\$I^2X\$) and system equivalent that you may forgetting about. That reactive power is all coming from the system (when you have gen pf at unity) and will tend to drop bus voltage.

Example: Assume a generator connected to a system by a GSU transformer that has a leakage reactance of \$X=0.1\$ pu and current through it is \$I=0.75\$ pu. The reactive power drop in the bank is \$I^2X=0.056\$ pu. If your generator is only providing real power, then this reactive must be supplied from the system.

So, sketch out a simplified version of your system, and recognizing that \$I_G=\frac{{S_G}^*}{{V_G}^*}\$ you can calculate the var drop in your transformer/lines/system equivalent and then think about where that reactive power must come from if your generator is not providing it. When you start adding reactive output from your generator you will see the bus voltage start going up (all other things constant).