There is a common misconception that the mains power we use, except for occasional power outages, is continuous and constant, but it is not. As a public power grid, the mains system is connected to tens of thousands of various loads. Some of the larger inductive, capacitive, switching power supplies and other loads not only obtain power from the power grid, but also cause damage to the power grid itself. Affect and deteriorate the power supply quality of the power grid or local power grid, resulting in the distortion of the mains voltage waveform or frequency drift. In addition, unexpected natural and man-made accidents, such as earthquake, lightning strike, open circuit or short circuit of power transmission and transformation system, will endanger the normal supply of electricity, thus affecting the normal operation of the load. According to the tests of electric power experts, the main problems that often occur in the power grid and cause interference or damage to computers and precision instruments are as follows:
1. Power surges: refers to the rms value of the output voltage being 110% higher than the rated value, and the duration is one or several cycles. Surge is mainly due to the high voltage generated by the sudden unloading of the grid when the large electrical equipment connected to the grid is shut down.
2, high voltage spikes (high voltage spikes): refers to the peak value of 6000v, the duration from one ten thousandth of a second to one half cycle (10ms) of the voltage. This is mainly due to lightning strikes, arcing, static discharges or switching operations of large electrical equipment.
3. Switching transients: refers to the pulse voltage with a peak voltage of up to 20,000W, but the duration is between one millionth of a second to one millionth of a second. The main causes and possible damage are similar to high-voltage spikes, but the solution is different.
4. Voltage sag (power sags): refers to the low-voltage state in which the effective value of the mains voltage is between 80% and 85% of the rated value, and the duration is one to several cycles. This problem can be caused by the startup of large equipment, the startup of large electric motors, or the connection of large power transformers.
5, electrical noise (electrical line noise): refers to radio frequency interference (FI) and electrical interference (EFI) and other various high-frequency interference. The operation of the motor, the action of the relay, the operation of the motor controller, the broadcast emission, the microwave radiation, and the electrical storm will all cause noise interference.
6. Frequency variation (frequency variation): refers to the variation of the mains frequency exceeding 3Hz. This is mainly caused by the unstable operation of the emergency generator, or by the power supply with unstable frequency.
7. Continuous low voltage (brownout) means that the effective value of the mains voltage is lower than the rated value and lasts for a long time. The causes include: large equipment startup and application, main power line switching, starting large electric motors, and line overload.
8. Mains failure (power fai1): refers to the situation where the mains is interrupted and lasts for at least two cycles to several hours. The reasons for this are: tripping of circuit breakers on the line, interruption of mains supply, and grid failure.
