Operating overvoltage

In the 6~35kV neutral point indirect grounding system, when the load starts or stops or an accident occurs, the operating state changes suddenly due to the reignition of the arc between the switch contacts, causing electromagnetic energy between the capacitor and the inductive component. Interconversion, an oscillating overvoltage occurs, that is, an operating overvoltage is generated.

(1) Motor start closing over voltage

Theoretically, when the motor is turned on, the overvoltage generated by the motor end is in the formula, which is the instantaneous value of the closing voltage; z is the shock wave impedance of the motor; Z is the shock wave impedance of the cable . Generally, Z=100～5000Q, z=20～50Q, so when the motor is turned on, the overvoltage generated by the motor end can reach 2 times of phase voltage. For vacuum switches, pre-breakdown often occurs before the contacts are closed. The arcing and extinction of the arc can reach dozens of times. This pre-combustion overvoltage has a large amplitude and the wave front is steep, which can cause insulation of the motor. A big threat, the motor generates an overvoltage and can be equipped with a motor type overvoltage protector.

(2) Motor starting state opening overvoltage

The operating experience shows that when the inductive load is disconnected, the inductive current is forced to be cut off at the zero point, so-called interception, which causes the arc combustion between the switch contacts to be unstable, the waveform generates high-frequency oscillation, and the abrupt current generated in the inductance loop Will induce a very high voltage.

1) Intercepting overvoltage Since the vacuum circuit breaker has good arc extinguishing performance, when the small current is disconnected, the vacuum arc is extinguished before zero crossing. Since the current is suddenly cut off, the energy that is retained in the inductive winding of the motor or the like is inevitably charged to the stray capacitance of the winding and converted into electric field energy. For motors and transformers, especially for no-load or small capacity, it is equivalent to a large inductor, and the loop capacitance is small, so a large overvoltage is generated. It can generate a very high overvoltage, but due to the loss of a certain resistance in the contacts and the loop and the breakdown, the overvoltage value is considerably suppressed. However, this suppression is limited and cannot eliminate overvoltage. Therefore, especially for the inductive load when using the vacuum circuit breaker as the operating element, an overvoltage protector should be installed. There are many types of overvoltage protectors, and users can select according to their own protection objects.

2) The three-phase simultaneous disconnection of the over-voltage is caused by the high-frequency current flowing through the phase arc gap caused by the circuit breaker first breaking the phase arc gap, causing the power frequency current in the remaining two-phase arc gap to rapidly cross zero. The unbroken phase is then severed, resulting in a similarly large level of interception in the remaining two phase arc gaps, resulting in a higher operating overvoltage. The resulting overvoltage is applied to the insulation between the phases. Three-phase breaking overvoltage is easy to occur in the case of breaking small and medium-capacity motors or light loads, so install a three-phase combined overvoltage protector.

Resonant overvoltage

For complex electrical systems, consisting of a series of oscillatory circuits with different natural frequencies, the oscillation conditions are equal to the inductive reactance and capacitive reactance, ie one, or ∞L=, thus the resonant frequency (inherent natural frequency) fo = . A non-sinusoidal power supply waveform contains a series of harmonics due to a change in the power supply waveform during a transient process when switching operations occur or when the system is single-phase grounded. A resonant overvoltage at this frequency occurs when one of the natural frequencies in the circuit is exactly equal to one of the harmonic frequencies of the power supply. Resonance is a stable phenomenon, and the duration of the resonant overvoltage can be very long. Once it occurs, it often has serious consequences.

Ferromagnetic resonance overvoltage, under normal circumstances, the inductance in the circuit is larger than the capacitance, but for some reason, the inductor voltage is increased, the inductance is magnetically saturated, the inductive reactance is reduced, the inductive reactance is equal to the capacitive reactance, and even the inductive reactance is less than the capacitive reactance. Forming a phase reversal, causing ferromagnetic resonance, excitation produces a continuous higher amplitude ferromagnetic resonance overvoltage, ferromagnetic resonance overvoltage will not exceed 3 times the phase voltage, practice shows that most are 1.5 to 2 times between. Ferromagnetic resonance can be fundamental wave resonance, higher harmonic resonance, and fractional harmonic resonance. Although the amplitude of the overvoltage generated by this resonance is not high, but the overvoltage frequency is often much lower than the rated frequency, the core is in a state of high saturation, and its expression may be that the relative ground voltage is increased, the excitation current is too large, or the frequency is low. Swing, causing insulation flashover, lightning arrester bursting, high-value zero-sequence voltage component generation, illusory grounding and incorrect grounding indication. In severe cases, it may also induce protection malfunction or overcurrent in the voltage transformer causes TV burnout. Although the 10kV is equipped with a harmonic elimination device, the primary harmonic elimination device will only work after the resonance occurs. The ferromagnetic resonance can last for a long time, but the resonance duration is short due to the action of the primary harmonic elimination device. It is not possible to eliminate resonance from the source.

Whether operating overvoltage, linear resonance, or ferromagnetic resonance, an overvoltage protector and a harmonic elimination device should be installed on the 10kV bus side.

In the power system, the load starts and stops frequently, and the overvoltage is easily generated when the operating overvoltage, especially the vacuum switch operation. Since the circuit is a complex circuit composed of inductors, capacitors and resistors, it can be combined into a series of oscillator circuits with different natural frequencies. Non-sinusoidal power waveforms contain a series of harmonics due to transient changes in the power supply waveform during switching operations or other anomalies. A resonant overvoltage at this frequency occurs when one of the natural frequencies in the circuit is exactly equal to the harmonic frequency of the power supply. Once an overvoltage occurs, it often has serious consequences. Therefore, in a neutral point ungrounded system, in addition to a neutral eliminator at the neutral point of the high voltage winding of each group of voltage transformers, in order to effectively limit the arc grounding overvoltage and eliminate the ferromagnetic resonance, it should also be Appropriate overvoltage protectors are installed in the corresponding parts to limit various overvoltages. In addition, the selection of equipment is also very important. If the selection is not suitable and the quality is problematic, even if the best measures are taken, it is difficult to avoid accidents.