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I am trying to understand how the grid works. Lets say we have some gas turbines, where we have a combination of air and fuel being used to turn turbine blades which in turn drives a generator to generate lets say three phase power. This 3-phase power is essentially 3 sinusoidal voltages out of phase with each other by 120 degrees. Now lets say that the load demand on the grid were to increase, then we need more power, so probably more air and fuel are used to make the turbines spin faster. What is the effect of this on the three phase power being generated. My understanding is that the three phase power will be 3 voltage waveforms each at 120V RMS and out of phase by 120 degrees with a frequency of 60 or 50 Hz, depending on where you live and it this three phase power should remain constant. So how is more power being generated at the turbine/generated affecting this output voltage. I don't understand the relationship between the power generated and voltage.

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    \$\begingroup\$ No - more air and fuel is not used to make the turbines spin faster. When more load is placed on the generators the turbines would slow down, so to counteract this more air and fuel is used to keep the turbines spinning at the same speed. \$\endgroup\$ – brhans Aug 21 '20 at 23:49
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    \$\begingroup\$ Some wind turbines must absorb power to get started to synchronous speeds and until wind overcomes stiction. Think of all generators as voltage controlled DC switches with inverters to simulate N stepped sine and current sensing with phase error into grid impedance to generate power factor of desired input, normally PF=1. Then Ohm’s law applies with the linear source as function of Vin to grid. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 22 '20 at 0:00
  • \$\begingroup\$ @Tony Stewart Sunnyskyguy EE75, so the place that the OP Edmund Blackadder lives should not be in North America and indeed has power plants generating 3 phase electricity。 I need to google again to clear my EE mind. \$\endgroup\$ – tlfong01 Aug 22 '20 at 2:31
  • \$\begingroup\$ fong... this applies to any grid 1 ph 3 phase and 400 Hz... obviously you must sync to frequency and phase of voltage to inject current slightly higher in voltage by current sensing. Edmund is a Rowan Atkinson character comic. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 22 '20 at 2:33
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For one thing, turbine plants use much higher voltages than 120. 4140V or 12kV are more typical for a large turbine, with that voltage stepped up further for long lines distribution.

For another, a turbine like you describe would be only for a backup generator. Large turbines for a municipal-size grid use steam heated by the fuel, be it natural gas, coal or nuclear fission.

Anyway, what happens with any generator linked to the grid is that they vary the torque to match the load, while keeping RPM constant. This is because the generator's waveforms not only need to be the same frequency, but the same phase.

For example, windmills in groups run at the same RPM together, varying their power output by feathering the props and thus varying their torque, but all in lockstep with the grid frequency and phase.

Think of it like what happens when your car climbs a hill at a constant speed: the engine RPM doesn't change, but to maintain speed you have to give it more accelerator to increase the torque, and thus, the power (power being basically torque x RPM.)

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  • \$\begingroup\$ Actually large generating plant now does work the way the OP describes - driven by a gas turbine. Then, a second turbine uses steam ... heated by the hot exhaust gas from the first turbine. Coal can't do that, which is why coal can no longer compete. \$\endgroup\$ – Brian Drummond Aug 22 '20 at 11:18
  • \$\begingroup\$ Ok I sought of get what you are trying to say. But I'm looking at the unit commitment problem where I need to determine how much power must be generated to meet some load and I'm trying to understanding what goes on at an electric utility in the operation of the electric grid. When I solve the UC problem lets say I have 5 generators generating 100MW each at some point in time, what is the relationship with this power being generated and the voltage delivered to the customer. I am really confused with how the power generated is some voltage delivered to the customer. \$\endgroup\$ – Edmund Blackadder Aug 22 '20 at 19:32
  • \$\begingroup\$ The utility varies the current delivered to the load depending on demand, while keeping voltage within margins. \$\endgroup\$ – hacktastical Aug 22 '20 at 19:55

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