What Causes Active Power Variation of Generator Set

The active power fluctuation of the generator set has a great impact on the operation stability of the generator set. The stability margin of the active power fluctuation of the generator in late phase operation is relatively large. When the generator is in phase leading operation, especially in deep phase leading operation, the stability margin of the generator set will be greatly reduced, which is easy to cause the generator set to lose stability and trip, which seriously threatens the safe operation of the unit and power grid. 

This article analyzes the reasons for the power fluctuation of the generator set, and takes targeted preventive measures to ensure the stable operation of the generator set.

1. Influence of reactive power on active power

Analysis on the relationship between active power and reactive power of generator

The generator excitation device is analyzed without automatic adjustment in the whole process of generator prime mover power, and the operator only adjusts the generator active power, which is equivalent to keeping the generator potential E and reactance constant. The active power and power angle of the generator are sinusoidal, and the reactive power and power angle are cosine.

In the formula, E0, U is generator potential and terminal voltage; X1 is synchronous reactance of generator; δ is Electromagnetic power angle of generator.

It can be seen that the more active power increases, the more reactive power decreases. When a certain power angle is reached, the generator changes from late phase operation to forward phase operation, absorbing inductive reactive power from the system. The power angle increases with the increase of phase depth, and the deeper the leading phase is, the smaller the excitation current is. Therefore, in order to avoid excessive phase advance of the generator, it is necessary to increase the excitation current while increasing the active power of the generator, so as to reduce the advance depth and ensure the constant terminal voltage.

2.Influence of unit frequency regulation, regulating stage pressure and primary frequency regulation on active power generation

1)The pressure of the regulating stage of the unit is in the same position as the fluctuation of the active power. In the time cycle, the pressure of the regulating stage of the unit is 20 minutes ahead of the fluctuation of the active power.

2)The unit frequency is in phase with the active power fluctuation, and the unit frequency fluctuation is 0.1s ahead of the active power fluctuation in the time cycle

3)The active power fluctuation of the unit is caused by the primary frequency regulation action. In principle, the primary frequency regulation operates by the speed of the generator.

It can be analyzed that the change of starting generator speed causes primary frequency regulation action. The result of primary frequency regulation action command is reflected in the change of unit regulation stage pressure, and then reflected in the change of unit frequency and active power.

3.Influence of grid frequency on active power

It can be seen from the waveform diagram that the frequency fluctuation amplitude of the power grid is much smaller than that of the generator set, and the periodicity is poor and almost irregular. Therefore, the influence of grid frequency on active power fluctuation can be eliminated.

Through the above analysis, the influence of reactive power, excitation regulation system, network frequency and other factors on active power fluctuation can be eliminated. It is concluded that the active power fluctuation of the generator set in the plant is due to the increase of generator set speed, the primary frequency regulation acts, at the same time, because the opening of the high-speed regulating valve of the steam turbine is at the inflection point, the characteristic curve at the inflection point is very unstable. When the primary frequency regulation acts, the opening command received by the regulating valve in this area is in the sending state.

4.Prevention and countermeasures

The generator set is located at the end of the power grid, with long double circuit outgoing lines and an important power support point. In view of the current reduction of industrial power consumption, reactive power overflow of the power grid and high system voltage, in order to ensure the voltage quality, the power grid will require the unit to operate in phase, and the single machine operation has limited ability to absorb reactive power and adjust system voltage. In combination with the active power fluctuation of the unit, the following preventive measures are formulated: 

(1) The current operation status has a great impact on the stability of the unit. For the large power grid system, the regulation performance of generator set is limited. Therefore, it is required that the leading phase capacity of the unit should be appropriate when the system voltage is too high, and the deep leading phase operation should be avoided as far as possible.

(2) Due to the nonlinear regulation of the speed regulating valve, there is an inflection point at about 70% of the comprehensive valve position. At this point, if various parameters of the control system change (including primary frequency modulation action), the control divergence may be caused at this point, which may lead to a significant change in the opening of the regulating valve and lead to the fluctuation of active power. In view of this situation, the current measure is to avoid the inflection point of the regulating valve.