Generator set - generator——Four
- Aug 25, 2018 -

Active and reactive control

Generators operating in the power system have both active and reactive loads. The change in active current affects the speed and frequency of the generator. The change in reactive current affects the voltage of the generator. In order to ensure the stability of the generator's frequency and voltage, the input power and excitation current of the generator must be adjusted in time. So the question is how to adjust it?


Active power regulation - that is, the adjustment of the prime mover input power.



When the synchronous generator is connected in parallel with the infinite grid, when the generator has just been put into the grid and has not sent the active load to the grid, suppose we ignore the no-load loss of the generator, then the generator is in the state of “empty connection” on the grid. If it is necessary to send active power to the grid, it is necessary to increase the input power of the generator, that is, increase the opening of the turbine valve in the thermal power plant, or the opening of the turbine gate in the hydropower plant or the wind speed of the wind power plant, etc. The output of the big prime mover increases the moment of the prime mover.


At this time, due to the increase of the torque acting on the generator shaft, the generator rotor will be accelerated, and the position of the generator main magnetic pole will gradually advance. As the main pole leads, the generator excitation potential (Ė0) will be ahead of The terminal voltage (grid voltage Ù), correspondingly, the power angle (δ) and electromagnetic power (Pm) will gradually increase, so that the input power and output power will gradually restore balance and keep running at the new working point. As shown below

image.png

In the actual regulation process, the adjustment of active power needs to pay attention to several problems:


(1) The speed of increasing the active power should be in accordance with the relevant regulations or the manufacturer's requirements. The speed at which the active power is increased should not be too fast, otherwise it will adversely affect the structure of the generator. Of course, the speed of active power growth should not be unreasonably limited, which will delay the power supply time, especially in the case of accidents.


(2) The adjustment of active power should pay special attention to the coordination problem of the input power of the prime mover, especially for thermal power plants or thermal power stations. It is necessary to pay full attention to the adjustment of the boiler's steam supply capacity and heat load to make it coordinated. Sometimes the generator's active power is growing too fast, the boiler's steam supply capacity can't keep up, and the pressure drops rapidly, causing a forced downtime.


2. Adjustment of reactive power - that is, regulation of generator excitation. The excitation of the synchronous generator has three states, namely:


(1) Normal excitation: that is, the total output power of the generator is active power. That is, cosφ=1, at which time the exciting potential of the generator is E.


(2) Over-excitation: Increasing the excitation of the generator so that it exceeds “normal excitation” is called over-excitation.


(3) Under-excitation: Reduce the excitation of the generator so that it is less than “normal excitation” is called under-excitation.


Similarly, there are several issues to be aware of during the adjustment of the useless power:


(1) Since the generator is operated in parallel with the infinite grid, the grid voltage is regarded as a constant value, and the adjustment of the excitation does not cause a voltage change.


(2) Adjusting the excitation can adjust the reactive power, but only the reactive power can be adjusted, but the active power cannot be changed.


(3) When adjusting the excitation, please pay attention to not exceed the allowable value of the excitation power supply or the generator excitation winding to avoid overheating of the excitation winding of the exciter and the generator.


(4) Pay attention to the generator armature current and keep it from exceeding the apparent power of the generator to prevent overcurrent of the generator armature winding.