How to control power factor output of an alternator in Power Plant

We can control power factor in a power plant easily. In a power plant the alternator set is supplying power to the grid independent of load. Thus in this case there is no load connected to make the power factor lagging or leading, we are left to control the power factor.

Modern power plant control system has automatic system of changing the power factor by just a command. But how this happens internally is fact to think about.

  

Say an alternator in a power plant is generating 8.5 MW active power and 6.5 MVAr under .85 power factor. The power factor is the ratio of two power active and reactive. Low power factor means reactive power is high & high power factor means active power is high.

Now say as per demand an operator wants to make the power factor low say .80. The increase and decrease of power factor command is done in SCDA (Supervisory control and data acquisition).  But as an engineer we need to look-see the internal flow of incident happens when alternator is to control the power factor. The basic concept of controlling the power factor is to control the ratio of active power to reactive power.

Concept of Alternator and Prime mover in power plant

The first concept is needed to clear of power generation set or unit. A power generation set consist of two mandatory parts. One is prime mover and another is always common -alternator.

The prime mover is the mechanical system that runs or drives the alternator to rotates its rotor in surroundings of stator. A prime mover is of different types like – diesel engine, gas engine, steam boiler in turbine plant, water flow in hydroelectric plant with DAM.

The power plant is named after the prime mover type like- diesel plant has diesel engine, nuclear power plant has nuclear power to drive boiler to run turbine.

How to control power factor in a power plant

As the power factor is nothing but the ratio of active and reactive power. We can control it by varying either active or reactive power.

To control power factor In a power plant the active and reactive power is adjusted by the prime mover and alternator set.

As 

Power = active power + reactive power.
Active power generates by prime mover.
Reactive power generates by the alternator set.

                                                      Controlling the active and reactive power

01

How active power is controlled in power plant:

One thing only – control the fuel to or torque strength of prime mover! So if you like to increase the active power increase the fuel to diesel engine or increase steam flow to turbine – increasing the torque, and to decrease the active load or power do the reverse that is decrease the fuel to prime mover.

02

How reactive power is controlled in power plant:

One thing only – control the excitation power to rotor circuit! If you like to increase the MVAr or reactive power then increase the excitation current in the excitation system. And do decrease to do the reverse. The excitation current to rotor creates magnetic flux. So by controlling the strength of magnetic flux we are controlling the reactive power.

Thus the operator when commands to control the active power and reactive power, the system itself automatically controls the fuel to prime mover and excitation power to alternator.

                                                          Alternator in a power plant