Analyze the causes of frequency instability of hydro generator

AC frequency is not directly related to the engine speed of hydropower station, but it is indirectly related.

No matter what type of power generation equipment, it is necessary to transmit electric energy to the power grid after generating electric energy, that is, the generator needs to be connected to the grid for power generation. After being connected to the grid, it is connected with the power grid as a whole, and the frequencies everywhere in the power grid are exactly the same. The larger the power grid, the smaller the frequency fluctuation range and the more stable the frequency is. However, the frequency of the power grid is only related to whether the active power is balanced. When the active power emitted by the generator set is greater than the active power of electricity consumption, the overall frequency of the power grid will rise, and vice versa.


Active power balance is a major topic of the power grid. Because the power load of users is constantly changing, the power grid should always ensure the power generation output and load balance. The important purpose of hydropower station in the power system is frequency modulation. Of course, the super large-scale hydropower of the Three Gorges is mainly used for power generation. Compared with other types of power stations, hydropower stations have inherent advantages in frequency modulation. The water turbine can quickly adjust the speed, which can also quickly adjust the active and reactive output of the generator, so as to quickly balance the grid load, while thermal power and nuclear power adjust the engine output much slower. As long as the active power balance of the power grid is good, the voltage is relatively stable. Therefore, hydropower stations make a great contribution to the frequency stability of the power grid.

At present, many small and medium-sized hydropower stations in China are directly under the power grid. The power grid must have control over the main frequency modulation power plants, so as to ensure the stability of power grid frequency and voltage. Simply put:
1. The power grid determines the speed of the motor. We now use synchronous motors for power generation, that is, the change rate is the same as that of the power grid, that is, 50 times in one second. For a thermal power plant generator with only one pair of electrodes, it rotates 3000 revolutions per minute. For the generator of hydropower plant with n pairs of electrodes, it rotates 3000 / N per minute. The water turbine and generator are generally connected together through some fixed ratio transmission mechanism, so it can be said that it is also determined by the power grid frequency.
2. What role does the water regulation mechanism play? Adjust the output of the generator, that is, the power sent by the generator to the power grid. Usually, a certain power is required to keep the generator up to its rated speed, but once the generator is connected to the grid, the speed of the generator is determined by the grid frequency. At this time, we usually assume that the grid frequency remains unchanged. In this way, once the power of the generator exceeds the power required to maintain the rated speed, the generator sends power to the grid and absorbs power on the contrary. Therefore, when the motor generates power under heavy load, once it is disconnected from the, its speed will quickly increase from the rated speed to several times, which is prone to flying accidents!
3. The power generated by the generator will in turn affect the grid frequency, and hydropower units are usually used as frequency modulation units because of the relatively high regulation rate.

Post time: May-17-2022

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