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.

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  • \$\begingroup\$ This is solar? Could it be related to MPPT? \$\endgroup\$
    – Aaron
    Nov 10, 2021 at 14:52
  • \$\begingroup\$ @Aaron yes it is solar. Why should it be related to MPPT? \$\endgroup\$ Nov 10, 2021 at 15:16

1 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).

  • \$\begingroup\$ what do you mean with ''that reactive is coming from the system'? And another thing is that with reactive power injection (pf=0.9) the voltage always increases at the busbar because we have a higher voltage drop towards the top of the line, which means a voltage increase at the busbar as I expected. I don't understand why this is not true with PF=1 \$\endgroup\$ Nov 10, 2021 at 17:26
  • \$\begingroup\$ You are putting power into the grid, that requires current. Current flowing through a transformer results in a reactive drop because the transformer is predominantly all inductance. You forced your generator to only put out real power, so the reactive requirement for the transformer must come from the system. \$\endgroup\$ Nov 10, 2021 at 17:53
  • \$\begingroup\$ but the current does not come from the external grid but from the generator that is too high. The current through the transformer is always low or if it becomes high because the generator power becomes so high it would cause an upward voltage drop on the transformer, and so a voltage increase on the MV busses and not a voltage drop as I got \$\endgroup\$ Nov 10, 2021 at 22:35
  • \$\begingroup\$ Think about it in terms of \$P\$ and \$Q\$ flows & drops. I added example to my answer. \$\endgroup\$ Nov 11, 2021 at 0:15
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    \$\begingroup\$ @relauman357 I checked it, moving to a small value the reactance of the transformer that phenomenon was not still present! You are right! Thank you very much, you gave me a better way to think about the electric world! \$\endgroup\$ Nov 11, 2021 at 14:32

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