How To Prevent Overvoltage In Sf6 Circuit Breaker Operating Mechanism

1. There are many reasons for the internal overvoltage triggering of the electrical system:

1. There are both power frequency overvoltages caused by line parameter matching and operating overvoltages caused by arc reignition during switch operation;

2. In addition, there are overvoltages caused by load cutoff of inductive loads and resonant overvoltages caused by inductor-capacitor series connection. Accidents caused by internal overvoltages, especially operating overvoltages, occur from time to time;

3. According to statistics, the general power frequency overvoltage will not exceed 2 times the phase voltage, the operating overvoltage caused by cutting off the unloaded line and the overvoltage caused by intermittent arcs will not exceed 3.5 times the phase voltage, and the ferromagnetic resonance overvoltage will not exceed 3 times the phase voltage.

4. However, actual operating experience has shown that accidents often occur when several overvoltages are superimposed on each other, and the overvoltage multiple is sometimes as high as 7 to 8 times the rated phase voltage.

2. Operation overvoltage:

In a 6-35 kV neutral point non-directly grounded system, when the load is started or stopped or an accident occurs, the arc between the sf6 circuit breaker operating mechanism contacts reignites, the operating state changes suddenly, causing the electromagnetic energy between the capacitor and the inductor to convert to each other, resulting in an oscillating overvoltage, that is, an operation overvoltage.

(1) Motor starting and closing overvoltage:

Theoretically, when the motor is closed and started, the overvoltage generated at the motor end is: where: is the instantaneous value of the closing voltage; z: is the motor shock wave impedance; Z: is the cable shock wave impedance. Generally, Z = 100-5000 Q, z = 20-50 Q, so when the motor is closed and started, the overvoltage generated at the motor end can reach 2 times the phase voltage.

For sf6 circuit breaker spring mechanism, pre-breakdown often occurs before the contacts close, and the arc can burn and extinguish dozens of times. This pre-burning overvoltage has a large amplitude and a steep wavefront, which can pose a great threat to the insulation of the motor. The motor can be equipped with a motor-type overvoltage protector when it generates overvoltage.

(2) Overvoltage of the motor in the starting state:

Operation experience shows that when disconnecting the inductive load, the inductive current is forced to be cut off when it is not at zero, which is the so-called current cutoff, causing the arc burning between the switch contacts to be very unstable, and the waveform produces high-frequency oscillation. The sudden current generated in the inductive circuit will induce a very high voltage.

1) Current cutoff overvoltage Since the vacuum circuit breaker has good arc extinguishing performance, when disconnecting a small current, the vacuum arc will extinguish before crossing zero. Since the current is suddenly cut off, the energy retained in the inductive winding of the motor must charge the stray capacitance of the winding and convert it into electric field energy. For motors and transformers, especially when no-load or small capacity, it is equivalent to a large inductor, and the circuit capacitance is small, so a large overvoltage will be generated. It can generate very high overvoltage, but due to the loss and breakdown of the contacts and circuits caused by a certain resistance, it has a considerable inhibitory effect on the overvoltage value. However, this inhibitory effect is limited and cannot eliminate the overvoltage. Therefore, especially for inductive loads, when using vacuum circuit breakers as operating elements, overvoltage protectors should be installed. There are many types of overvoltage protectors, and users can choose according to their own protection objects.

2) Three-phase simultaneous disconnection overvoltage:

Three-phase simultaneous disconnection overvoltage is due to the fact that when the circuit breaker first disconnects the phase arc gap to produce reignition, the high-frequency current flowing through the phase arc gap causes the power frequency current in the remaining two phase arc gaps to quickly pass zero, causing the unbroken phase to be cut off, and a similar large-level interception phenomenon is generated in the remaining two phase arc gaps, thereby generating a higher operating overvoltage. The generated overvoltage is added to the insulation between phases. Three-phase disconnection overvoltage is prone to occur when disconnecting small and medium-capacity motors or light loads, so a three-phase combined overvoltage protector should be installed.

3. Resonance overvoltage:

For complex electrical systems, it is composed of a series of oscillation circuits with different self-oscillation frequencies. The oscillation condition is that the inductive reactance and the capacitive reactance are equal, that is, 1, or ∞L=, so the resonant frequency (natural self-oscillation frequency) fo =. When switching operations are performed or single-phase grounding of the system occurs, the power waveform will cause some changes due to the transient process, and the non-sinusoidal power waveform contains a series of harmonics. When one of the self-oscillation frequencies in the circuit is exactly equal to one of the power supply harmonic frequencies, a resonant overvoltage of this frequency will occur. Resonance is a stable phenomenon, and the duration of the resonant overvoltage may be very long. Once it occurs, it often causes serious consequences.

Ferromagnetic resonance overvoltage

Under normal circumstances, the inductance in the circuit is greater than the capacitance, but for some reason, the inductance voltage increases, the inductance is magnetically saturated, the inductive reactance decreases, and the inductive reactance is equal to the capacitive reactance, or even the inductive reactance is less than the capacitive reactance to form a phase reversal, causing ferromagnetic resonance, exciting a continuous high-amplitude ferromagnetic resonance overvoltage, and the ferromagnetic resonance overvoltage will not exceed 3 times the phase voltage.

Practice shows that most of them are between 1.5 and 2 times. Ferromagnetic resonance can be fundamental resonance, high-order harmonic resonance, and sub-harmonic resonance. Although the amplitude of the overvoltage generated by this resonance is not high, because the overvoltage frequency is often much lower than the rated frequency, the core is in a highly saturated state. Its manifestation may be an increase in relative voltage, excessive excitation current or low-frequency swing, causing insulation flashover, arrester explosion, high-value zero-sequence voltage component generation, virtual grounding phenomenon and incorrect grounding indication. In severe cases, it may also induce protection malfunction or overcurrent in the voltage transformer causing TV to burn out. Although the 10kV is equipped with a primary detuning device, the primary detuning device will not work until the resonance occurs. Ferromagnetic resonance can last for a long time, but due to the action of the primary detuning device, the resonance duration is very short, but it cannot eliminate the resonance from the source.

Whether it is operating overvoltage, linear resonance, or ferromagnetic resonance, overvoltage protectors and primary detuning devices should be installed on the 10kV bus side.