Switching overvoltage is a kind of overvoltage caused by operation in power system.The operation here includes normal operation such as closing and opening of no-load line, and also includes abnormal fault such as line passingIntermittent arc grounding。Operating overvoltage isInternal overvoltageClass of.[1]
Cause
Announce
edit
The reason for switching overvoltage is that there are inductance and capacitance of energy storage elements in the power system. When normal operation or fault occurs, the circuit state changes, which leads to the transition process of oscillation. In this way, overvoltage exceeding the normal operating voltage may occur in the system, which is called switching overvoltage.During the transition process of oscillation, the magnetic field energy of the inductance and the electric field energy of the capacitor are converted to each other.At a certain moment, the magnetic field energy stored in the inductance will be transformed into the electric field energy in the capacitor, so there will be switching overvoltage several times of the system voltage in the system.[1]
Common voltage
Announce
edit
Common switching overvoltage in power system includes:
(1) Intermittent arc grounding overvoltage of neutral insulation system;
(2) Opening overvoltage of no-load line;
(3) No load line closing overvoltage;
(4) Cut off the overvoltage of traction transformer.[1]
characteristic
Announce
edit
Operating overvoltage has the following characteristics:
(1) The duration is relatively short.Although the duration of switching overvoltage is longer thanLightning overvoltageLong, but longer thanPower frequency overvoltageIt is much shorter, usually in a few milliseconds to tens of milliseconds.The switching overvoltage exists in the transient transition process. When there is power frequency voltage rise at the same time, the switching overvoltage appears as a transient oscillation process superimposed on the power frequency overvoltage, which can make the amplitude of the switching overvoltage reach a higher value.
(2) Since the magnetic field energy in the inductance and the electric field energy in the capacitance both come from the system itself, the operating overvoltage amplitude has a certain multiple relationship with the system phase voltage amplitude.The switching overvoltage for calculation when selecting insulation specified in relevant regulations of China is shown in the table.
(3) The amplitude of switching overvoltage is related to various factors of the system, and has strong statistics.Among various factors that affect switching overvoltage, system wiring and circuit breaker characteristics play an important role.In addition, many factors affecting the switching overvoltage, such as the closing phase of the closing overvoltage, have great randomness, so the specific amplitude of the switching overvoltage also has great randomness, but the probability of the occurrence of switching overvoltage with different amplitude obeys a certain regular distribution, which is the statistical characteristics of the switching overvoltage.It is generally believed that the amplitude of switching overvoltage is approximately distributed in a normal distribution law.
(4) The importance of the display of various switching overvoltages varies according to the voltage level of the system.In the neutral point insulation system with lower voltage level, single-phase gap arc grounding overvoltage is the most noticeable.For the system with higher voltage level, with the direct grounding of neutral point, switching off no-load transformer and opening overvoltage of no-load line are more prominent.In EHV system, the closing overvoltage of no-load line has become an important switching overvoltage.
(5) Switching overvoltage is one of the bases to determine the insulation level of power system.The higher the system voltage level is, the higher the amplitude of switching overvoltage will be. On the other hand, due to the continuous improvement of arrester performance in the high voltage level system and the continuous improvement of atmospheric overvoltage protection, switching overvoltage plays a more and more important role in determining the insulation level of the power system.In EHV system, switching overvoltage will gradually play a decisive role in the insulation selection of some equipment.[1]
Avoidance measures
Announce
edit
No load line closing and reclosing operation overvoltage
When the no-load line is closed, the closing overvoltage will be generated due to the line inductance capacitance oscillation.When the lines are overlapped, the electromagnetic oscillation process is aggravated due to the high power supply potential and the existence of residual charges on the lines, which further increases the overvoltage.Therefore, the circuit breaker shall be installed with closing resistance to effectively reduce the closing andReclosing overvoltage。
The no-load line closing, single-phase reclosing, successful and unsuccessfulThree phase reclosingOvervoltage distribution, including phase to ground and phase to phase statistical switching overvoltage of line receiving terminal.
Breaking no-load transformer andShunt reactorSwitching overvoltage of
The switching overvoltage caused by forced arc extinction when the circuit breaker breaks the inductive current of these equipment shall be determined according to the circuit breaker structure, circuit parameters, transformer (shunt reactor) wiring and characteristics and other factors.This switching overvoltage can generally be limited by the arrester installed between the circuit breaker and the transformer (shunt reactor).For the transformer, the lightning arrester can be installed on the low-voltage side or high-voltage side, but if the neutral point grounding mode of the high-voltage and low-voltage power grids is different, the low-voltage side should adopt magnetic blowValve type arrester。When the lightning arrester may act frequently, the circuit breaker with high opening resistance should be used.
Line asymmetric fault opening and oscillation splitting operation overvoltage
The connection between the sending and receiving ends of the power grid is weak. If the asymmetrical fault of the line causes opening, or the power grid is disconnected in the oscillation state, the asymmetrical fault of the line will open or the oscillation disconnection overvoltage will occur.To predict the opening overvoltage of the asymmetric fault of the line, the condition that there is a single-phase ground fault at the receiving end of the line can be selected. When opening, the power angle difference of the potential at the sending and receiving ends of the line should be selected according to the actual situation.The circuit breaker with opening resistance can reduce the line asymmetric fault opening and oscillation splitting overvoltage.When this condition is not met, the arrester installed on the line shall be used for restriction.[2]
