For a class that's rather far outside of my background, I am reading a paper about supply-demand balance in an islanded microgrid. The active power demand of the whole system can be met with renewables, but the authors' control scheme can't balance reactive power, so they say that the conventional generators have to take care of it:

Thus, the active power reference of the SG is set to zero, and the SG [synchronous generator] only produces reactive power for voltage regulation.

Is that a thing? From dispatch and power flow exercises I'm used to having a minimum active power constraint on the generators.

How do you make, say, a diesel generator produce P=0 Q≠0?

  • \$\begingroup\$ Diesel or not, it's only the SM on the output which does the job. You under- or overexcite the field winding. \$\endgroup\$ – winny Apr 20 '18 at 12:14

The characteristics of synchronous machines operated as synchronous condensers are presented in most electric machine courses and texts. The V-curves for a synchronous machine are shown below. Ie is the rotor field excitation current and Is is the stator current. The various V-shaped curves show the relationship between stator current and field current for various levels of output power. The bottom curve shows Ie vs. Is for zero power and describes condenser operation. With over-excitation, the machine behaves as a condenser (capacitor). With under-excitation, the machine behaves as an inductor. The intersections of the more vertical curve with the V curves indicate the condition for power factor = 1.0. Curves for additional values of lagging and leading machine power factor are often drawn to the left and right of the 1.0 pf curve. Note that the machine current is limited to a value below the current shown for rated power shown by the top curve.

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    \$\begingroup\$ Thanks. What do the different curves represent? \$\endgroup\$ – nengel Apr 20 '18 at 13:49
  • \$\begingroup\$ I edited my answer to add more information about what the curves represent. \$\endgroup\$ – Charles Cowie Apr 20 '18 at 16:31
  • \$\begingroup\$ How do you set the power factor? I tried to google it but the sources I found just talk about it as an intrinsic characteristic of a circuit... \$\endgroup\$ – nengel Apr 21 '18 at 5:20
  • \$\begingroup\$ Pf is an intrinsic chariacteristic of a load circuit. With a synchronous condenser, pf is set by adjusting the DC field excitation. The machine contributes leading reactive voltamperes to offset load lagging reactive VA. \$\endgroup\$ – Charles Cowie Apr 21 '18 at 14:28

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