Overvoltage refers to the phenomenon that the rms value of the AC voltage rises at the power frequency, exceeding 10% of the rated value, and the long-term voltage fluctuation phenomenon lasts longer than 1 minute; the occurrence of overvoltage is usually the result of load switching. Occurs when the inductive or capacitive load is turned on or off during normal use.
The abnormal voltage rise over the working voltage in the power system under certain conditions belongs to a kind of electromagnetic disturbance phenomenon in the power system. The insulation of electrical equipment is tolerant to the operating voltage for a long time, and must be able to withstand a certain range of overvoltage, so as to ensure the safe and reliable operation of the power system.
Overvoltage is divided into external overvoltage and internal overvoltage.
External overvoltage is also called lightning overvoltage and atmospheric overvoltage. Caused by the discharge of thunderclouds in the atmosphere to the ground.
Internal overvoltage: An overvoltage caused by a change in the internal operating mode of the power system. There are power frequency overvoltage, operating overvoltage and resonant overvoltage.
1. Atmospheric overvoltage
Caused by direct lightning strike, it is characterized by short duration and strong impact, which is directly related to the intensity of lightning stroke, and has nothing to do with the voltage level of the equipment. Therefore, the insulation level of systems below 220KV is often determined by preventing atmospheric overvoltage.
2. Power frequency overvoltage
It is caused by the capacitive effect of the long lines and the sudden change of the operation mode of the power grid. It is characterized by long duration, low overvoltage multiple, and generally not dangerous to the equipment insulation, but it plays an important role in determining the insulation level of ultra-high voltage and long-distance transmission. Common power frequency overvoltage can be divided into the following types:
a. No-load long-line capacitance effect
b. Asymmetric short circuit grounding
c. Load rejection overvoltage
3. Operating overvoltage
The voltage caused by line fault, no-load line switching, isolating switch operating no-load bus, operating no-load transformer or other causes in the system, can be prevented by high-performance arrester.
Operating overvoltage occurs during system operation and fault, which is characterized by randomness. In the most unfavorable case, the overvoltage multiple is high. The insulation level of 330kv and above ultra-high voltage system depends on the operating overvoltage.
4. Resonant overvoltage
It is the overvoltage caused by the resonance of power supply, capacitance and other energy storage components with power supply frequency in some wiring modes. The generation is related to the operation state, parameters, or magnetic circuit with iron equipment. It is characterized by high overvoltage multiple and long duration.
Cause of Voltage
The sudden change of circuit state and electromagnetic state in power system is the root cause of overvoltage. Studying the causes of various overvoltages in power system, predicting their amplitude, and taking measures to limit them are the premise to determine the insulation coordination of power system, which is of great significance for the manufacture of electrical equipment and the operation of power system.
Regardless of the external overvoltage or internal overvoltage, it is affected by many random factors. It is necessary to combine with the specific conditions of the power system, obtain data through various methods such as calculation, simulation and on-site measurement, and use the probability and statistical method to predict the overvoltage.
For the cause of overvoltage, the power system must take protective measures to limit the overvoltage amplitude. For example, lightning protection line, lightning arrester, reactor, switch contact and parallel resistance are installed to properly implement insulation coordination and ensure safe operation of the power system.