Analysis of Vacuum Circuit Breaker Operation Over-voltage Mechanism and Suppression Measures

Analysis of Operational Overvoltage Mechanism of Vacuum Circuit Breakers and Suppression Measures CHEN Ye, YANG Hao (Jiangxi Water Conservancy Planning and Design Institute, Nanchang 330029, China) In recent years, vacuum circuit breakers are fireproof, explosion-proof, small in size, and light in weight. Due to its short arc time and long contact life, it has been widely used in the medium voltage field of electrical engineering. However, the only drawback to the vacuum circuit breakers that have been trapped over the years has been the issue of overvoltage. Because the arc extinguishing ability of the vacuum circuit breaker is extremely strong, the overvoltage caused when the circuit breaker breaks causes the insulation breakdown of the high voltage motor, the malfunction of the circuit equipment and even the damage of the circuit breaker, and the high voltage switch cabinet burning and other accidents occur frequently.

However, as long as the cause of the overvoltage can be analyzed, an appropriate overvoltage protection device can be used safely and safely.

1 Analysis of operation over-voltage generation mechanism There are three types of over-voltage generated by the vacuum circuit breaker breaking high-voltage motor, namely, over-voltage cut-off, multiple over-combustion over-voltage, and three-phase simultaneous over-current blocking.

1.1 Cut-off Over-voltage Vacuum circuit breakers have excellent arc extinguishing performance. When they are switched off, small-current vacuum arcs can be extinguished before zero-crossing. Because the current is abruptly interrupted, the cut-off current stays in the inductance winding (L) of the high-voltage motor or transformer. This residual electromagnetic energy must be charged to the stray capacitance (C) of the winding and converted into electric field energy (1/2CU2). Excluding the residual voltage between the arc extinguishing power supply voltage on the capacitor C, (since the voltage lags the current, this residual voltage is generally small), at this time, the inrush current voltage value generated due to energy conversion is the damping coefficient and the cutoff current value, wave The product of these three impedances, the oscillation frequency depends on the wave impedance.

The level of the over-voltage cut-off is related to the size of the intercept value Ic of the vacuum circuit breaker. The larger the Ic value, the higher the overvoltage value.

Shutdown only occurs when breaking a small current. Tests have shown that the greater the steepness of the large current, the smaller the interception value. When the breaking current exceeds a certain value (several hundred amps), shutoff does not occur in general.

The oscillation frequency of magnetic energy and electrical energy conversion is very high and can reach several thousand Hz. The oscillation frequency is determined by the loop parameters. A high frequency must be accompanied by a high voltage steepness.

Relatively over-voltage and phase-to-phase over-voltage are allocated according to relative capacitance and inter-phase capacitance. Generally, the relative overvoltage is 2/3 of the phase-to-phase overvoltage. In this case, it also exists when multiple over-ignition over-voltages and three-phase simultaneous over-voltage over-voltages occur.

The smaller the motor and transformer capacity, the higher the overvoltage. On the one hand, this is because the breaking current is small; on the other hand, because the capacitance in the loop is small and the inductance is large, the wave impedance is large. Similarly, breaking open-load transformers is dangerous. Because the impedance of the loop is small when the low-voltage side of the transformer is loaded.

The level of over-voltage cut-off has a random nature. The so-called vacuum circuit breaker shutoff level is the average of many tests. Descriptive features should use probabilistic statistical methods.

1.2 Multiple over-ignition over-voltages When the momentary moment when the vacuum circuit breaker contacts are separated, if the current value is in the zero-crossing point, the arc extinguishes immediately, because at this moment, the opening distance of the contacts is small, so the faster recovery voltage will be increased. The gap breaks down and reignites to generate a high-frequency current. If the amplitude of the high-frequency current is greater than the instantaneous value of the power-frequency current, the zero point of the high-frequency current will appear again. The inherent high-frequency power capability of the vacuum switch will make the arc extinguish again. Then, a new electromagnetic oscillation occurs again to break through the contact gap. Due to the high-frequency arc extinguishing and the charge and discharge of the power supply to the charged capacitor in the reverse direction, multiple re-ignitions can result in more than 4 times overvoltage. Immediately after the contact opens, the closer the current value is to zero, the higher the overvoltage value. The main characteristics of this over-voltage are: over-voltage over-rejection voltage over the amplitude of the over-voltage, high-frequency (megahertz) wave head is very steep, so that the over-voltage motor windings concentrated in the first end of the stator winding lead Insulation between the entrances near the entrance poses a serious threat.

The overvoltage value is higher with a certain length of cable connection.

Although the reburning over-voltage average decreases with the increase of the cable length, the cable length increases the accumulated energy, so the probability of over-voltage during repetitive breakdown is also high.

The re-ignition overvoltage that breaks during the motor start-up process is more serious. At this time, due to the low rotor speed, the back-EMF of the stator winding is very small, and the contact gap is easier to break down. However, in a normally operating motor, even if the motor is idling, because the back electromotive force of the motor is high and the function of the power supply voltage is counteracted, no reburning overvoltage occurs.

1.3 Three-phase simultaneous open-circuit overvoltage The high-frequency current generated by the arc extinguishing circuit of the vacuum circuit breaker primary circuit is superimposed on the other uninterrupted two-phase power-frequency currents through the three mutual coupling and the neutral point. This causes the other two-phase arc currents to force zero crossings so that the unbroken phases are cut at the same time. The two-phase cut-off power frequency current is often larger than the cut-off value of the primary phase, which is very similar to the larger level of the shutoff phenomenon, resulting in a higher operating overvoltage than the cutoff overvoltage of the primary phase.

The characteristic that the three-phase breaks the overvoltage at the same time is: Its maximum overvoltage always occurs in the breaking time of the latter two phases. It has little to do with the level of interception of circuit breakers.

In the case of breaking a small or medium-sized motor or a light load, three-phase simultaneous cut-off is prone to occur.

2 Main Measures for Suppressing Operational Overvoltage In order to prevent operating overvoltages from endangering the safety of electrical equipment, various foreign protective measures have been tried in the early days. If the capacitor is connected to the motor outlet in parallel to reduce the overvoltage wave head and reduce the amplitude; in the motor circuit series reactor, in order to reduce the steepness; installed in the motor outlet common valve arrester to limit the over-voltage amplitude. Practice has proved that these are relatively large and some of the insulation is difficult to match and gradually eliminated.

At present, as a control method for operating overvoltage, it can be divided into two categories: one is a method using a low overvoltage vacuum interrupter; the other is a method of setting a protective device.

2.1 Using a low overvoltage vacuum interrupter A vacuum interrupter with a small cut-off value and high-frequency breaking properties of the electrode material is also used. In the case of vacuum contactors that do not require short-circuit breaking performance and vacuum circuit breakers with small breaking capacity, this type of electrode material can be used in VZ-C type contactors and VF-83BZV12, VF-20BZ type vacuum circuit breakers. This type of low voltage overvoltage vacuum interrupter is used. Therefore, neither the overvoltage nor the re-ignition overvoltage nor the voltage at the time of three-phase simultaneous cutoff is cut off.

2.2 Measures to Set Protection Devices The role of the overvoltage protection device of the high voltage motor is to prevent atmospheric (lightning) overvoltage shocks; to limit the overvoltage amplitude of the operation; and to reduce the steepness of the overvoltage steep slope. Commonly used overvoltage protection devices include magnetic blowout arresters, zinc oxide arresters, and RC absorbers.

The magnetic blowout valve arrester is used to protect the AC motor of the corresponding rated voltage from the atmospheric overvoltage. Because it has an internal series gap, it does not have the operation overvoltage protection function, and the steep-wave response characteristics are poor.

Zinc Oxide Arrester has excellent nonlinear volt-ampere characteristics, strong ability to suppress overvoltage (have a certain protection effect on steep wave, lightning and operating overvoltage impact) no continuous wave, resistant to multiple lightning strikes and repeated action operation over-voltage shock Such features as capacity are contemporary more advanced over-voltage protection devices and have been widely used.

The RC snubber (or RC snubber) is connected in parallel with the motor outlet. The resistor and capacitor are connected in series and grounded. It is the most ideal protection device for limiting the overvoltage amplitude of the motor operation and reducing steepness and steepness of the overvoltage.

Resistance and capacitance parameters of RC devices are selected and configured : . Electrotechnician Li Jianguo. Vacuum circuit breakers are used as overvoltage issues for the main switches of medium and small generator sets and water pump motors. Hydroelectricity, 1995. Chief Editor Wang Jimei. Vacuum Switch Theory and Its Application Xi'an: Xi’an Jiaotong University