Is the frequency of civil alternating current in China fixed at any time?

The frequency of alternating current is 50Hz, which is exactly what we want. But in fact, the frequency of the alternating current we use changes, but China has certain regulations on this change. The lower limit error shall not exceed 2.5Hz(47.5Hz) and the upper limit shall not exceed 2Hz(52Hz). There is still a big gap between this standard and foreign standards, but it still cannot be done in practice. You can use an oscilloscope to observe at different time periods. The frequency of alternating current varies.