How does a power plant's increased output translate to the Volts, Amps and Watts on the power lines?

Question

In the context of a power plant, that wants to output more electricity.

If the formula for Watts is V*A=W, does Voltage or Current get increased to supply more Watts?

Answer 1

How does a power plant increase load?

Power plants do not increase load, the customer's served control the load by turning things on and off or adjusting loads to use more power. A power plant connected to a grid can supply a larger share of the load. Viewed simply, the plant does that by increasing its internal voltage to be slightly above the voltage at the grid connection. The grid voltage remains constant, but the current and power supplied by the power plant under consideration increase.

I know the formula for wattage is V*A=W.

For an AC power plant, that is not true. The phase relationship between the voltage in current must be considered. That means that W = V * A * pf, where pf (power factor) is a number between 0 and 1 that accounts for the phase relationship. A power plant could increase power delivered without increasing current by increasing pf.

Answer 2

A power plant increases amount of fuel burning, which is also power. It increases power when it sort of lags a little bit in terms of frequency. A small phase difference indicates that the plant doesn't generate enough and a control loop adds more gas.

The voltage in the generator is very tightly related to speed (EmF). This is important, because by convention the frequency is constant (50 or 60 Hz, depending on jurisdiction) and is physically tied to the speed of generator rotation.

So the thing that changes the most to compensate for added power is current, which is tightly related to torque applied by steam on the turbine.

Things are different with solar, but behind the electronics, which makes it behave the same.

Answer 3

For a synchronous generator, Active Power is mainly related to Rotor Speed, and Reactive Power is mainly related to Terminal Voltage

The reactance of the power system is typically much larger than the resistance, so the equations for Active and Reactive Power can be written:

$$P=3\frac{|E||V|}{X}sin\delta$$ $$Q=3\frac{|V|}{X}(|E|cos\delta-|V|)$$

where \$\delta\$ is the rotor angle.

If you burn more fuel, you can increase the turbine's mechanical power and create more rotor torque in the generator, which increases your rotor angle and delivers more electrical active power onto the grid. Other generating units will respond to this increase in speed by decreasing their speeds if on AGC (Automatic Generator Control), but your generator whose speed/load increased initially will stay at the new higher load setpoint even after the frequency recovered. Increasing \$\delta\$ increases \$P\$.

Since \$\delta\$ is typically kept fairly small for stability purposes, \$sin(\delta)\$ changes more than \$cos(\delta)\$, so reactive power will not change as much as active power when \$\delta\$ changes.

If you increase the amount of DC Current through your rotor windings by increasing your exciter current, the terminal voltage of your stator (\$E\$) will increase relative to your system voltage (\$V\$). This is the main way Reactive Power is controlled on the grid. Unlike for Active Power where some generators are permitted to set their loads manually while connected to the grid, all generating units in North America are expected to use AVR (Automatic Voltage Regulation) while connected to the grid.

Answer 4

How does a power plant increase load?

The question should be 'How does a power plant handle an increase in load?'

So do power plants that generate electricity increase voltage or do they increase amps to increase watts?

An increase in load is met by an increase in prime mover input power to maintain the generator speed that would satisfy the constant voltage and frequency requirements of the consumer.

The result is increased current at constant voltage and frequency.